- Email: [email protected]
Experimental studies on heat and mass transfer in tubular generator for R134a-DMF absorption refrigeration system
International Journal of Thermal Sciences 61 (2012) 118e128
Contents lists available at SciVerse ScienceDirect
International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts
Experimental studies on heat and mass transfer in tubular generator for R134a-DMF absorption refrigeration system P. Balamurugan, A. Mani* Refrigeration & Airconditioning Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600 036, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 September 2011 Received in revised form 8 March 2012 Accepted 5 June 2012 Available online 20 July 2012
Experimental investigations have been carried out to study heat and mass transfer during desorption of Tetrafluoro ethane (R134a) from R134a and Dimethyl formamide (DMF) solution in a tubular generator. Absorption refrigeration system has been built with brazed plate heat exchangers as condenser, absorber, evaporator and solution heat exchanger and with stainless steel concentric tubes as the generator. Effects of operational parameters viz., solution two phase Reynolds number, driving temperature ratio, driving pressure ratio, solution initial concentration on generator performance are analyzed. Desorption ratio, Sherwood number and Nusselt number increase as the solution Reynolds number, solution initial concentration, driving temperature ratio increase whereas these parameters decrease as the driving pressure ratio increases. Finally a correlation for Nusselt number and Sherwood number are proposed based on the experimental studies. Ó 2012 Elsevier Masson SAS. All rights reserved.
Keywords: Desorption R134a DMF Tubular generator Heat transfer Mass transfer
1. Introduction In the vapour absorption refrigeration system, even though they are traditionally used, ammonia/water suffers from toxicity, corrosion and rectification requirements. Water/lithium bromide encounters limitation of evaporator temperature, vacuum pressure in the components and possibility of crystallization. From the literature [1e3], it is observed that the above disadvantages associated with the traditional working pairs have prompted researchers in the past, to search for alternative working fluids. Though R22-organic solvent based absorption refrigeration systems have been extensively studied by Fatouh [4], Karthikeyan et al. [5] and Sujatha et al. [6], HCFCs along with CFCs, are being phased out by Montreal and other International Protocols. Hence attention was shifted towards HFCs as an alternate to CFCs and HCFCs. They are not destructive to the ozone layer though they have a slight effect on global warming. R134a is the commonly used HFC refrigerant. The Kyoto protocol during 1997 has put R134a as one of the green house gases. However there is no phase out date for this refrigerant and it is expected to be highly used in refrigeration industries. Nezu et al. [7] examined the possibility of testing R134a as a refrigerant in VARS with various organic solvents and showed
* Corresponding author. Tel.: þ91 44 22574666; fax: þ91 44 22570509. E-mail address: [email protected] (A. Mani). 1290-0729/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.ijthermalsci.2012.06.001
that the R134a-DMA and the R134a-DMF systems are considered attractive as the working-fluid pairs for the absorption refrigeration system than other R-134a-absorbent combinations. Yokozeki [8] compared the performances of various refrigeranteabsorbent pairs in a VARS cycle, by the use of equations of state and concluded that R134a-DMF and R134a-DMA systems exhibited better performance, compared to other R134a-absorbent combinations. Also the circulation ratio is less and COP is more for R134aDMF system compared to R134a-DMA system. Mani [9] carried out experimental studies on R134a-DMF based compact vapour absorption refrigeration system with plate heat exchangers and reported that this system could be very competitive for applications ranging from 10 C to 10 C, with heat source temperature in the range of 80 Ce90 C and with cooling water as coolant for absorber and condenser in the range of 20 Ce35 C. Generator, one of the crucial components of the vapour absorption refrigeration system, influences the system performance significantly. In the generator, desorption of refrigerant vapour takes place from refrigeranteabsorbent solution by simultaneous heat and mass transfer phenomena. A detailed heat and mass transfer study of desorption in a generator will contribute to the improvement of absorption refrigeration technologies. Bennett and Chen [10] studied forced convective boiling with aqueous ethylene glycol solutions and developed a correlation to predict the heat transfer coefficient. This correlation improved the understanding of flow boiling of binary mixtures. Charters et al. [11]
P. Balamurugan, A. Mani / International Journal of Thermal Sciences 61 (2012) 118e128
Nomenclature A Cp Di Dc F G H Hfg h L m M Nu P Re Sh Sc q Q T x X
heat transfer area, m2 specific heat, J kg1 K1 inner diameter of inner tube, m diffusion coefficient, m2 s1 friction factor mass flux, kg m2 s1 enthalpy, J kg1 latent heat of vaporization, J kg1 heat transfer coefficient, W m2 K1 tube length, m mass flow rate, kg s1 mass transfer coefficient, kg m3 s1 Nusselt number pressure, Pa Reynolds number Sherwood number Schmidt number heat flux, W m2 heat transfer rate, W temperature, C vapour quality liquid concentration
measured heat transfer coefficients experimentally for H2OeLiBr boiling on an electrically heated vertical copper tube. A correlation was proposed to predict the experimental data for pool boiling heat transfer on surfaces of platinum, copper and brass. Matsuda et al. [12] carried out experiments on generator in a H2OeLiBr absorption refrigerating machine using a vertical falling-film type of stainless steel column. The measured evaporation rate decreased with reducing pressure and increasing concentration of LiBr in the falling liquid. Varma et al. [13] measured heat transfer coefficients during pool boiling of H2OeLiBr solutions over horizontal stainless steel tubes. Heat transfer coefficient of lithium bromideewater solution was found to be slightly smaller than that of pure water. Also average heat transfer coefficient decreased as the concentration of LiBr increased. Rivera et al. [14] studied heat transfer in forced convective boiling experimentally for NH3eH2O and NH3eLiNO3 mixtures in a vertical tube. Correlations were proposed to predict the experimental local heat transfer coefficients. Rivera et al. [15] analyzed NH3eH2O, NH3eLiNO3 and H2OeLiBr in saturated upward flow nucleate boiling in a uniformly heated vertical tube. It was observed that the highest values of average heat transfer coefficients were obtained for NH3eH2O mixture. Local and average heat transfer coefficients in saturated nucleate boiling were measured by Rivera et al. [16], for H2OeLiBr mixture in a uniformly heated vertical tube, which was the generator of a solar absorption refrigeration system. It was observed that the average heat transfer coefficients increased for the mixture with an increase in the heat flux and with decrease in solution concentration and temperature difference between tube wall and fluid. Inoue et al. [17] conducted experimental study for heat transfer during nucleate pool boiling on the horizontal heated wire in mixtures of NH3eH2O. The heat transfer coefficients in the mixtures were less than those in single component substances. An applicability of existing correlations to the experimental data was discussed. Arima et al. [18] measured boiling heat transfer coefficient of NH3eH2O mixture on a horizontal heated surface. Heat transfer coefficients were compared with existing correlations and a revised correlation was proposed to predict them. Roriz et al. [19]
119
Subscripts abs absorber av average d desorption eq equilibrium gen generator hw hot water i inlet l liquid o outlet ph preheater s solution ss strong solution tp two phase v vapour wallin inner wall ws weak solution Greek symbols 3 effectiveness h mass transfer efficiency q non dimensional temperature m dynamic viscosity, Pa-s r density, kg m3
compared the overall performance of the absorption system and the heat transferred in the generator with that of a system with falling film desorber and thus proved the feasibility of a compact generator. Khir et al. [20] performed an experiment on forced convective boiling heat transfer of NH3eH2O mixtures inside a vertical tube. Experimental data were compared with correlations of Bennett and Chen [10], Mishra et al. [21], etc. Taboas et al. [22] compiled experimental data available in the open literature on nucleate boiling of NH3, H2O and NH3eH2O mixture. The experimental data were compared with three correlations available in the literature for pool boiling of mixtures. A new correlation was proposed which satisfactorily predicted the experimental data. Francisco Taboas et al. [23] carried out experiments to measure heat transfer coefficient and pressure drop for NH3eH2O mixtures under flow boiling conditions in a vertical brazed plate heat exchanger, at different operating conditions. Balamurugan and Mani [24] developed a numerical model on heat and mass characteristics of R134a-DMF based tubular generator based on ColburneDrew formulation and validated by comparing with the results available in literature. Numerical analysis of forced convective boiling of refrigeranteabsorbent solution in a stainless steel tube in tube generator has been carried out by Balamurugan and Mani [25]. Using NH3eH2O, the model has been validated and compared with the experimental results in the literature. In the present experimental work, heat and mass transfer parameters are measured during desorption of R134a from R134aDMF solution in the tubular generator. Vapour absorption refrigeration setup has been built using brazed plate heat exchangers for other major heat and mass transfer components. The performance of tubular generator is evaluated using dimensionless parameters. The influence of solution two phase Reynolds number, driving temperature ratio, driving pressure ratio and solution initial concentration on desorption is presented and examined. 2. Experimental setup Fig. 1 shows the schematic diagram of experimental setup. This consists of a stainless steel tubular generator, condenser,
120
P. Balamurugan, A. Mani / International Journal of Thermal Sciences 61 (2012) 118e128
T22 T5
T6
NV1
T8
Condenser
R134a/DMF solution line
P4
L2
BV5
Refrigerant line Water line
T21
BV6
BV8 Evaporator
MS S1
P2
T18
T7
Liquid Refrigerant Receiver
T4
P3
T9
L1
S5 BV7
BV4 T15
BV9 Chilled water pump
Gas separator
Vapour Generator
T20 S3
BV12
S4
T14 T12
T13 P6
BV2
BV15
BV16 T2 Online denstiy meter BV13 Preheater Hot water simulator
T17
Hot water T16 pump
BV3
Absorber
Cooling load simulator
P7 NV2
Solution heat exchanger
S4
T19
BV10 T10 T1
S L
Ball valve Needle valve BV1 Flow meter Level gauge
T
Temperature gauge
P
Pressure gauge
SG
Absorber Tank
Sight glass
L3
Cold water pump
P5
T11
BV11
Cooling water simulator
T3 P1 BV14
Solution Pump
Fig. 1. Schematic diagram of tubular generator experimental setup.
evaporator, absorber and solution heat exchanger, solution pump, cooling water simulator, hot water simulator, cooling load simulator, instrumentation and valves. Table 1 shows the specifications of major components used in the setup. The system consists of refrigerant circuit and solution circuit. In refrigerant circuit, R134a vapour coming from gas separator is condensed in the condenser and accumulated in the receiver. Heat of condensation is removed by the cooling water. Liquid R134a from the receiver is expanded through throttle valve and evaporated in the evaporator. Chilled water from the simulator acts as the heat load. In solution circuit, R134a vapour from the evaporator is absorbed by the weak DMF solution in the absorber. Integral enthalpy of solution is removed by water from cooling water simulator. Strong solution collected in the absorber tank is pumped through solution heat exchanger and solution preheater to the bottom of inner tube of generator. Tubular generator which is used for the present study consists of two concentric tubes as shown in Fig. 2. Hot water is supplied as heat source through generator annulus in counter flow direction. R134a vapour boiled off in the generator is separated in the gas separator. Weak solution remaining in the gas separator is sent through solution heat exchanger and pressure reducing valve to absorber for absorption. A picture of the present experimental setup is shown in Fig. 3. Hot water simulator consists of a hot water tank insulated with glass wool, electric heaters, pump, flow meter, PT100 sensor, PID temperature controller, contactor, piping and valves. It supplies hot water to the generator. Cooling water simulator consists of R22 based vapour compression refrigeration (VCR) circuit, a cooling water tank insulated with expanded polyethylene (EPE) sheets,
electric heaters, pump, flow meter, PT100 sensor, PID temperature controller, contactor, piping and valves. VCR circuit consists of a hermetically sealed reciprocating compressor, an air cooled condenser, a thermostatic expansion valve and cooling coil. Cooling water simulator supplies cooling water to absorber and condenser. Cooling load simulator consists of a chilled water tank insulated with expanded polyethylene sheets, electric heaters, pump, flow meter, PT100 sensor, PID temperature controller, contactor, piping and valves. It supplies water as heat load to evaporator and maintains a constant desired value of chilled water temperature at evaporator inlet. This system also includes various measuring devices such as temperature sensors, pressure sensors, flow meters and online density meter fitted at suitable locations as shown in Fig. 1. All these measuring instruments are calibrated. Thirty two numbers of calibrated coppereconstantan thermocouples are used as temperature sensors with a measurement uncertainty up to 0.5 C. Ten thermocouples have been fixed to the tube as shown in Fig. 2, to measure the wall temperature and the bulk solution temperature along the generator. Seven numbers of piezoeelectric type pressure transducers are used as pressure sensors with a measurement uncertainty up to 4.6%. Metal tube rotameters are used to measure the flow of solution, liquid refrigerant and hot water with a measurement uncertainty up to 5% and glass rotameters are used to measure the flow of cooling water and cooling load with a measurement uncertainty up to 2.5%. An online density meter is used to measure the density of strong and weak solutions with a measurement uncertainty of 0.3%. Concentrations of strong and weak solutions are evaluated from the measured density values
P. Balamurugan, A. Mani / International Journal of Thermal Sciences 61 (2012) 118e128 Table 1 Specifications of major components. Tubular generator Type Fluid circuit Material Inside tube Outside tube Tube length Evaporator Type Fluid circuit Fluid direction Heat transfer area, m2 Length, m Width, m Height, m No. of plates No. of effective plates No. of passes Condenser Type Fluid circuit Fluid direction Heat transfer area, m2 Length, m Width, m Height, m No. of plates No. of effective plates No. of passes Absorber Type Fluid circuit Fluid direction Heat transfer area, m2 Length, m Width, m Height, m No. of plates No. of effective plates No. of passes Solution heat exchanger Type Fluid circuit Fluid direction Heat transfer area, m2 Length, m Width, m Height, m No. of plates No. of effective plates No. of passes Solution pump Type Capacity Discharge pressure Plunger diameter SPM Online density transmitter Tube inner diameter Tube length (Inlet to outlet) Measuring range Measuring repeatability Accuracy in the adjusted range Temperature range Pressure range Flow rate (water) Pressure transducers Type Measuring range Temperature Input Output Accuracy Response time
Tube in tube Tube side: solution Annulus: hot water SS 316 ID: 20 mm, OD: 23 mm ID: 30 mm, OD: 33 mm 1000 mm Brazed plate heat exchanger Cold side: R134a, hot side: water Countercurrent 0.16 0.064 0.077 0.207 14 12 1
121
Table 1 (continued ) Material Flow meters Type Flow rate Operating pressure End fitting material Float material Tube material Tube taper length Accuracy Type Flow rate Operating pressure End fitting material Float material Tube material Tube taper length Accuracy Type Flow rate
SS316L Glass rotameter Minimum: 0.025 m3 h1, maximum: 0.25 m3 h1 2 105 Pa (abs.) SS316 SS316 Borosilicate glass 250 mm þ/1% on F.S.D Glass rotameter Minimum: 0.018 m3 h1, maximum: 0.18 m3 h1 2 105 Pa (abs.) SS316 SS316 Borosilicate glass 250 mm þ/1% on F.S.D Metal tube rotameter Minimum: 0.010 m3 h1, maximum: 0.104 m3 h
Operating pressure End fitting material Float material Tube material Tube taper length Accuracy Type Flow rate Operating pressure End fitting material Float material Tube material Tube taper length Accuracy Type Flow rate Operating pressure End fitting material Float material Tube material Tube taper length Accuracy
11 105 Pa (abs.) SS316 SS316 SS316 102 mm þ/1% on F.S.D Metal tube rotameter Minimum: 0.002 m3 h1, maximum: 0.02 m3 h1 11 105 Pa (abs.) SS316 SS316 SS316 102 mm þ/1% on F.S.D Metal tube rotameter Minimum: 0.033 m3 h1, maximum: 0.33 m3 h1 2 105 Pa (abs.) SS316 SS316 SS316 102 mm þ/1% on F.S.D
1
Brazed plate heat exchanger Cold side: water, hot side: R134a Countercurrent 0.16 0.064 0.077 0.207 14 12 1 Brazed plate heat exchanger Cold side: water, hot side: R134a þ DMF Countercurrent 0.1 0.055 0.077 0.207 10 8 1 Brazed plate heat exchanger Cold side: R134a þ DMF, hot side: R134a þ DMF Countercurrent 0.37 0.059 0.094 0.324 18 16 1 Metring pump 0.050 m3 h1 40 105 Pa 25 mm 100 6.6 mm 500 mm 0 to 3000 kgm3 5 102 kg m3 2 101 kgm3, 0.1% 0e100 C 0e50 105 Pa 0.1e0.5 m3 h1 Piezoresistive 0e16 105 Pa absolute 15e100 C 8e30 V DC 0e5 V DC (3 wires) 1% of full scale Maximum: 250 m Sec
using HBT (HankinsoneBrobsteThomson) equation used by Reid et al. [26]. Readings from all these instruments and sensors are recorded continuously by connecting them to a data acquisition system and a computer. 3. Experimental procedure Initially the refrigerant and solution circuit are separated by closing the valve between (i) gas separator and condenser and (ii) evaporator and absorber. Hot water simulator and cooling water simulator are started. Hot water is circulated through the annulus of the generator at a temperature higher than that to be maintained in the generator. Cooling water is circulated through absorber and condenser at a temperature lower than that to be maintained in the respective components. Cooling load simulator is operated by circulating water through evaporator. Water temperature in the chilled water tank is maintained constant by operating the heaters equivalent to cooling capacity of system. Solution pump is started to circulate strong solution through the inner tube of generator. Level of weak solution collected in the gas separator, level of strong solution in the absorber storage tank and pressure in each component of solution circuit are monitored continuously. When pressure in the gas separator becomes higher than that in the condenser, the valve between them is opened to allow refrigerant vapour to get condensed in condenser. Level of liquid refrigerant
122
P. Balamurugan, A. Mani / International Journal of Thermal Sciences 61 (2012) 118e128
Weak solution out
225
Hot water in
1000
225
Thermocouple
225
225
Thermocouple
Hot water out Strong solution in φ 20 φ 23 φ 30 φ 33 All dimensions in mm Fig. 2. Schematic diagram of tubular generator.
accumulated in the liquid refrigerant receiver is monitored. After collecting sufficient amount of liquid refrigerant, it is allowed through the throttle valve to enter the evaporator. Flow rate of weak solution and liquid refrigerant are adjusted to maintain steady flow in the system. When all the readings of pressure transducers, thermocouples, flow meters and level gauges remain constant over a period of time, steady state readings are recorded in the computer. Water flow rates in the hot water simulator, cooling water simulator and cooling load simulator are maintained at the desired constant values. Experimental runs are repeated for different operating conditions. While shutting down the system after the end of the experimental test, solution circuit and refrigerant circuit are again isolated by closing the valves. 4. Data reduction Experimental tests are conducted in tubular generator by varying the operational parameters within the range as given in Table 2. The range of generator temperature for the present study has been chosen to suit the hot water temperature that could be obtained by harnessing solar energy. Experimentation with varying parameters has been planned based on a preliminary thermodynamic analysis carried out on the vapour absorption refrigeration system using R134a-DMF pair. Experimental runs are repeated for different operating conditions. In every run of the experiment, the system is allowed to run for a couple of hours to attain steady state
Fig. 3. Tubular generator experimental setup.
conditions. Calculations are carried out using the recorded values at these conditions. The following assumptions are considered for the calculating the heat and mass transfer parameters: (i) As the temperature of hot water varies nearly linearly along the generator, heat flux imposed on the test section is assumed to be uniform over the entire length of generator; (ii) Bulk solution temperature is considered to be constant in a given cross section of generator; (iii) Generator is assumed to have uniform thermal conductivity across its wall; (iv) Thermodynamic equilibrium exists in the test section; (v) As the distance between preheater and the generator is very short, the exit quality of preheater is assumed as the entry quality for the generator. From the measured values of pressures, temperatures and compositions of the solution, enthalpies of solution at the inlet and outlet of generator are found using thermodynamic equations for mixtures. The dimensionless operating parameters, which are used for evaluating performance of the generator, driving pressure ratio (Pgen/Pabs) and driving temperature ratio (Tgen/Tabs), besides desorption ratio, Nusselt number and Sherwood number, vapour quality, heat and mass transfer effectiveness, friction factor are
Table 2 Operating range of parameters. Solution flow rate Generator pressure Solution initial concentration Generator temperature Condenser temperature Evaporator temperature Hot water flow rate Hot water temperature
0.02e0.05 m3 hr1 650e1000 kPa 0.58e0.76 kgkg1 78e90 C 15e30 C 5e13 C 0.08e0.33 m3 hr1 82e98 C
P. Balamurugan, A. Mani / International Journal of Thermal Sciences 61 (2012) 118e128
found using the derived quantities from the experimental data. Thermodynamic and physical properties of R134a-DMF used are obtained from experimental correlations from Reid et al. [26], Nezu et al. [7] and Yokozeki [8]. Wall temperature and solution temperature at core are nondimensionalized using Eq. (1).
q ¼
ðT Tice Þ ðTsteam Tice Þ
(1)
Heat energy supplied is obtained from energy balance on hot water side of generator as in Eq. (2).
Qhw ¼ mhw Cphw Thw;i Thw;o
immediately to the tubular generator for desorption, the exit condition of preheater is assumed to be the same as the entry condition of generator.
xph;o ¼
xgen;o ¼ xgen;i þ
(2)
(3)
Volumetric mass transfer coefficient is obtained from the log mean concentration difference (LMCD) and the mass of refrigerant vapour desorbed as in Eq. (5).
md M ¼ p 2 Di L LMCD 4
(5)
Desorption ratio (md/mss) is defined as the ratio of mass of refrigerant vapour desorbed to the total mass flow rate of solution flowing through the generator. Local heat transfer coefficient is calculated from the wall and bulk solution temperatures and the heat flux:
Q ¼ hðTwallin Ts Þ q ¼ A
(6)
Inner wall temperature is found from the measured outside wall temperature by radial heat conduction. Average heat transfer coefficient is found from the local heat transfer coefficients at five thermocouple stations.
hav ¼
h1 þ h2 þ h3 þ h4 þ h5 5
(7)
Preheater is used in the experimentation to evaluate (Yan et al. [27]) the solution vapour quality at the entry of the generator. Heat transferred from the preheater is calculated from the voltage supplied and the resistance of the electric heater. In the preheater, heat is transferred to the solution both to rise its temperature (sensible heat transfer) and for evaporation of refrigerant in the solution (latent heat transfer). Knowing the amount of heat transferred in the preheater, vapour quality at the exit of preheater could be evaluated from the energy balance across the preheater as given in Eqs. (8)e(11).
Qph ¼ Qph;sensible þ Qph;latent
(8)
Qph;sensible ¼ ms Cps Ts;ph;o Ts;ph;i
(9)
Qph;latent ¼ ms Hfg xph;o
(10)
Above equations are rearranged to get the vapour quality at the exit of preheater. As the solution from the preheater is sent
Qgen ms Hfg
! (12)
Xs;i Xs;o ¼ Xs;i Xeq;s;o
(13)
Friction factor associated with the flow boiling of R134a-FMF solution is found (Incropera and Dewitt [28],) from the correlation
f ¼ (4)
(11)
Mass transfer effectiveness is calculated from 3
Xeq;s;i Xs;i Xs;o Xeq;s;o LMCD ¼ Xeq;s;i Xs;i ln Xs;o Xeq;s;o
! 1 Qph Qph;sensible ¼ xgen;i ms Hfg
Quality of refrigerant vapour at the exit of generator is found from the energy balance on the solution side in the generator as given in Eq. (12).
While heat gained by solution across the generator is determined from the enthalpy difference and solution mass flow rate as
Qs ¼ mss ðHi Ho Þ:
123
1
! (14)
2 0:79lnRetp 1:64
The uncertainties in the measured quantities namely temperature, pressure, flow rates, and density are determined from the minimum value of the measured output and accuracy of the instruments. The uncertainties in derived quantities namely desorption ratio, concentration, Sherwood number, Nusselt number, heat transfer effectiveness, mass transfer effectiveness, quality and friction factor have been determined using the formula proposed by Kline and McClintock [29]. Table 3 gives the results of the uncertainty analysis. 5. Results and discussion Using the equations given in previous section, performance of R134a-DMF tubular generator is deduced, for the range of operating conditions. Effect of the solution Reynolds number, driving temperature ratio, driving pressure ratio and solution initial concentration on desorption ratio, Nusselt number, Sherwood number, heat and mass transfer effectiveness, vapour quality, friction factor, etc, are plotted and discussed. Fig. 4 depicts the effect of driving temperature ratio on non-dimensional wall temperature, solution temperature and local Nusselt number along the generator. With the increase in driving temperature ratio, which is realized by the increase in temperature of simulated hot water
Table 3 Estimated uncertainties. Measured quantities Solution flow rate Pressure Temperature Density Hot water flow rate Derived quantities Desorption ratio Concentration Sherwood number Nusselt number Mass transfer effectiveness Heat transfer effectiveness Quality Friction factor
5.2% 4.6% 0.5 C 0.3% 2.8% 7.9% 2.7% 7.0% 4.7% 4.6% 7.5% 8.0% 13.5%
P. Balamurugan, A. Mani / International Journal of Thermal Sciences 61 (2012) 118e128
Driving temp.ratio = 4.05 Driving temp.ratio = 3.70
1
0.9
600 Twall ----- Ts Nu
400
0.8
200 Driving Temp.ratio = 3.60 0
0.7 0
0.2 0.4 0.6 0.8 Non-dimensional length of generator
1
Fig. 4. Variation of non-dimensional wall temperature, solution temperature and local Nusselt number for different driving temperature ratios.
supplied to the generator, heat transfer rate to the wall and subsequently to the solution increases, resulting in higher wall and solution temperatures and the Nusselt number. For the uniform heat flux imposed on the generator, difference between the wall temperature and solution temperature along the generator decreases, causing the local Nusselt number to increase. However, as the driving temperature ratio increases, reduction in difference between wall and solution temperatures is more pronounced. Hence, increase in Nusselt number is steep at latter half of the generator which may be due to the increased utilization of quantum of heat to boil refrigerant from the solution. Fig. 5 shows the variation of average Nusselt number, Sherwood number and desorption ratio with solution Reynolds number for various driving temperature ratios. Heat transfer rate increases due to increased solution flow rates, as more heat need to be supplied at higher desorption rates. As the Reynolds number increases, quantity of solution available for desorption is more, resulting in higher mass generation of R134a vapour. Heat transfer rate also increases as the driving temperature ratio increases resulting in higher generation of refrigerant vapour. Hence, increase in heat transfer rate and mass transfer rate results in the increase in Nusselt number and Sherwood number respectively, with solution Reynolds number and driving temperature ratio. However, as the Reynolds number increases, even though the mass generation of R134a vapour increases, increase in Nusselt number with Reynolds number slows down, as solution approaches the maximum limit of
300 Nusselt number Sherwood number ---- Desorption ratio
150
5000
Sherwood number
Nusselt number Desorption ratio x 2000
Driving pressure ratio = 1.91 Hot water Reynolds number = 4960 Initial concentration = 0.67 10000
0 8000
0.36
15000
600
450
proportional desorption for a given operating condition. This effect is more pronounced at higher driving temperature ratios, revealing that the increase in Nusselt number is not infinite. As the Nusselt number and Sherwood number increase, for the reasons explained above, desorption ratio increases with the increase in Reynolds number and driving temperature ratio. However, as the increase in Nusselt number with Reynolds number is not significant, desorption ratio also flattens at higher values of Reynolds number and driving temperature ratios. Fig. 6 shows the effect of solution Reynolds number and driving temperature ratio on exit vapour quality and concentration difference across the generator during desorption of R134a in the tubular generator. At higher driving temperature ratios and Reynolds number, the solution undergoing desorption is subjected to increased heat transfer rate, probably because of higher temperatures, resulting in the generation of higher quantity of R134a vapour. This increases the refrigerant vapour quality at the exit of the generator. However at higher Reynolds number, though the heat transfer rate and desorption rate increase, effective desorption decreases which results in a higher value of weak solution concentration at the exit of generator. Thus the difference in concentration is less for higher solution Reynolds number. At higher driving temperature ratio, for a given solution flow rate, both heat transfer and desorption rates are higher. Weak solution concentration at the exit is lower and hence difference in concentration increases. Thus the concentration difference between the entry and exit of generator increases with driving temperature ratios and decreases with Reynolds number. Variation of heat transfer effectiveness and mass transfer effectiveness with solution Reynolds number for different driving temperature ratios is depicted in Fig. 7. Heat transfer effectiveness increases with the increase in driving temperature ratio due to the increase in heat transfer rate. However with the increase in Reynolds number, though heat transfer rate is high, heat transfer effectiveness decreases because the ratio, of heat transferred to solution flow rate, and the utilization of heat decreases as the Reynolds number increases. Hence effective heat transfer decreases at higher Reynolds number. Also for the reason explained above for Fig. 6, at higher Reynolds number, though the heat transfer rate and mass generation rate are higher, effective desorption decreases which results in a higher value of weak solution concentration at the exit of generator. This causes an increase in the gap between the actual weak concentration and the equilibrium weak concentration at the exit of generator. Hence, the tendency of weak solution at the exit of generator to get closer to equilibrium concentration decreases as
Driving temperature ratio = 4.05 Driving temperature ratio = 3.70 Driving temperature ratio = 3.60 0 13000
18000
23000
28000
Two phase Reynolds number Fig. 5. Effect of two phase Reynolds number on Nusselt number, Sherwood number and desorption ratio for different driving temperature ratios.
Hot water Reynolds number = 4960 Driving pressure ratio = 1.91 Initial concentration = 0.67
0.6
0.27 0.4 Concentration difference ---- Exit vapour quality 0.18
0.2
Exit vapour quality
Retp = 10700 Driving pressure.ratio = 1.9 800 Re = 3140 hw Xsi = 0.7
Concentration difference across generator
Local Nusselt number
1000
Non-dimensional temperature
124
Driving temperature ratio = 4.05 Driving temperature ratio = 3.70 Driving temperature ratio = 3.60 0.09 8000
0 13000
18000
23000
28000
Two phase Reynolds number Fig. 6. Effect of two phase Reynolds number on concentration difference across generator and exit vapour quality for different driving temperature ratios.
P. Balamurugan, A. Mani / International Journal of Thermal Sciences 61 (2012) 118e128
0.85 ---- Mass transfer effectiveness Heat tranfser effectiveness
0.6
0.8 Driving temperature ratio = 4.05 Driving temperature ratio = 3.70 Driving temperature ratio = 3.60
0.5 8000
0.75 13000
18000
23000
0.8 Hot water Reynolds number = 4700 Driving temperature ratio = 3.18
Exit vapour quality
0.7
0.8
0.6
0.6 ---- Exit quality Concentratrion difference
0.4
0.2
0.2 Driving pressure ratio = 1.72 Driving pressure ratio = 2.03
0 8000
28000
0.4
Concentration difference across generator
0.9
Hot water Reynolds number = 4960 Driving pressure ratio = 1.91 Initial concentration = 0.67
Mass tranfser effectiveness
Heat transfer effectiveness
0.8
125
0 13000
18000
23000
Two phase Reynolds number
Two phase Reynolds number Fig. 7. Effect of two phase Reynolds number on heat and mass transfer effectiveness for different driving temperature ratios.
Fig. 9. Effect of solution Reynolds number on exit vapour quality and concentration difference across generator for different driving pressure ratios.
the Reynolds number increases whereas it increases as the driving temperature ratio increases, for a given solution flow rate. Hence mass transfer effectiveness decreases as the Reynolds number increases and increases as driving temperature ratio increases. Fig. 8 depicts the variation of Nusselt number, Sherwood number and desorption ratio with Reynolds number at different driving pressure ratios. At lower driving pressure ratios, the solution enters the generator at a lower temperature resulting in a higher temperature gradient for a given driving temperature ratio and solution flow rate. This increases the heat transfer rate which results in increase in the mass generation rate of R134a vapour, with decrease in pressure ratio. Thus increased heat and mass transfer rates results in increased Nusselt number and Sherwood number, respectively, with decrease in pressure ratio. As explained earlier, as the Reynolds number increases both Nusselt number and Sherwood number increase respectively. However the rate of increase slows down at lower values of driving pressure ratios due to the solution approaching the saturation limit of effective desorption, as explained earlier with reference to Fig. 5. As the Nusselt number and Sherwood number are higher, desorption ratio increases at lower pressure ratio and at higher Reynolds number respectively. However, the rate of increase in desorption ratio with Reynolds number is not eternal, as it decreases with decrease in pressure ratio, due to the similar behaviour of Nusselt number and Sherwood number with Reynolds number at lower pressure ratios. Fig. 9 shows the effect of driving pressure ratio on exit vapour quality and concentration difference across the generator. The
vapour quality and the difference between strong and weak solution concentration increase as the driving pressure ratio decreases due to increase in heat transfer rate for a given solution flow rate. Fig. 10 depicts the effect of solution initial concentration on desorption ratio, exit vapour quality and concentration difference between the strong and weak solution of the generator. As the solution concentration increases, equilibrium inlet temperature of solution decreases resulting in increasing the temperature potential for higher heat transfer to solution. This causes both Nusselt number and Sherwood number to increase, at higher concentration. Thus desorption ratio increases as solution initial concentration increases. As explained earlier, the rate of increase in desorption ratio is limited at higher values of Reynolds number, as the effective desorption decreases. For the reason explained above, the vapour quality and the concentration difference increase with the increase in solution initial concentration due to the increase in heat transfer rate to the solution in the generator. Fig. 11 shows the effect of initial concentration of solution on the heat and mass transfer effectiveness during flow boiling of R134aDMF solution in the tubular generator. Heat transfer effectiveness increases with the increase in solution initial concentration due to the increase in heat transfer rate. However, as the solution concentration increases, equilibrium inlet temperature and pressure of solution decrease, resulting in lower weak solution equilibrium concentration at the exit of the generator. Thus at higher initial concentration, though difference between strong and weak solution actual concentration is higher, there exists a gap between
Hot water Reynolds number = 4700 Driving temp. ratio = 3.18
12000
---- Desorption ratio Nusselt number Sherwood number 140
4000
Driving pressure ratio = 2.03 0 13000
18000
0.3
23000
Two phase Reynolds number Fig. 8. Effect of solution Reynolds number on Nusselt number, Sherwood number and desorption ratio for different driving pressure ratios.
Initial concentration = 0.70 Initial concentration = 0.58
0.4
0.3 0.2 0.1
Driving pressure ratio = 1.72
50 8000
Exit vapour quality Desorption ratio
8000
230
0.4 Sherwood number
Nusselt number Desorption ratio x 1000
320
Hot water Reynolds number = 4400 Driving temperature ratio = 3.02
Exit quality Concentration difference
---- Desorption ratio 0 8000 13000 18000 Two phase Reynolds number
0.2
Concentration difference
0.5
0.5
0.1 23000
Fig. 10. Effect of solution Reynolds number on desorption ratio, exit vapour quality and concentration difference across generator for different solution initial concentration.
126
P. Balamurugan, A. Mani / International Journal of Thermal Sciences 61 (2012) 118e128
3.6
Heat transfer effectiveness ---- Mass transfer effectiveness
0.7
0.83
0.6
0.81 Initial concentration = 0.70 Initial concentration = 0.58
0.5 8000
Fig. 11. Effect of solution Reynolds number on generator heat and mass transfer effectiveness for different solution initial concentration.
the exit weak solution actual concentration and equilibrium concentration, as the gradient between the strong solution inlet concentration and the exit weak solution equilibrium concentration is still higher reducing the mass transfer effectiveness. Hence as solution concentration increases, mass transfer effectiveness decreases. Fig. 12 shows the effect of solution Reynolds number and driving temperature ratio on two phase friction factor associated with flow boiling of R134a-DMF in the tubular generator. At higher Reynolds number, mass velocity of solution undergoing desorption is higher, thus causing a decrease in friction factor. As driving temperature ratio decreases, mass generation of vapour decreases resulting in lower mean vapour quality. This increases average two phase density between the entry and the exit of the generator causing an increase in friction factor. Thus friction factor decreases with the increase in Reynolds number and the driving temperature ratio. The effect of driving pressure ratio on friction factor is depicted in Fig. 13. At lower pressure ratios, mass generation of R134a vapour is more, contributing for decreased two phase density. This causes a decrease in friction factor at lower pressure ratios as shown in Fig. 13. 6. Correlation for Nusselt number A correlation for Nusselt number has been proposed as a function of two phase Reynolds number, Martinelli parameter and Boiling number.
2.4 8000
2.3 8000
Driving pressure ratio = 1.91 Hot water Reynolds number = 4960 Initial concentration = 0.67
18000
23000
Fig. 13. Effect of solution Reynolds number on friction factor at different driving pressure ratios.
b Nu ¼ a Retp ð1=Xtt Þc ðBoÞd
(15)
where
1=Xtt ¼ Lockhart Martinelli parameter 0:5 0:1 rl mv x 0:9 ¼ rv ml 1x Bo ¼ Boiling number ¼
(16)
q GHfg
(17)
In the present work, Eq. (15) is used as the generic form of correlation to correlate the experimental heat transfer coefficients for R134a-DMF mixture. Based on the present experimental data, values of the constant a and indices b, c and d are obtained. In Fig. 14, experimental Nusselt numbers are compared with the Nusselt numbers predicted using the proposed correlation. The proposed correlation is able to predict more than 93% of the data within 25% deviation. The correlation obtained for R134a-DMF boiling with 25% error band is given by:
0:188 Nu ¼ 3:848 104 Retp ð1=Xtt Þ0:116 ðBoÞ0:469
(18)
The above correlation is valid in the range of Retp ¼ 8100 to 27,800; 1=ctt ¼ 0.95 to 14.54; Bo ¼ 4.05 104 to 3.4 103. 450 Nu = 3.848 x 104 (Retp) -0.188 (1/Xtt)-0.116(Bo)0.469
400 Predicted Nusselt number
-2
Friction factor x 10
2.7
13000
Two phase Reynolds number
Driving temperature ratio = 3.60 Driving temperature ratio = 3.70 Driving temperature ratio = 4.05
2.9
2.5
2.8
Driving pressure ratio = 1.72
0.79 23000
18000
Two phase Reynolds number
3.1
3.2
Driving pressure ratio = 2.03
13000
3.3
Hot water Reynolds number = 4700 Driving temperature ratio = 3.18
-2
0.85
Friction factor x 10
Hot water Reynolds number = 4400 Driving temperature ratio = 3.02
Mass transfer effectiveness
Heat transfer effectiveness
0.8
+25%
350 -25%
300 250 200 150 100
13000
18000
23000
28000
Two phase Reynolds number
50 75
125
175
225
275
325
375
Experimental Nusselt number Fig. 12. Effect of solution Reynolds number on friction factor at different driving temperature ratios.
Fig. 14. Comparison of predicted against experimental Nusselt number.
P. Balamurugan, A. Mani / International Journal of Thermal Sciences 61 (2012) 118e128
127
7. Correlation for Sherwood number
8. Conclusions
Based on the above experiments, a correlation for Sherwood number has been developed as a function of two phase Reynolds number, Schmidt number, concentration gradient and heat flux.
The influence of solution two phase Reynolds number, driving temperature ratio, driving pressure ratio and initial R134a concentration on the performance of tubular generator is studied experimentally with R134a-DMF as working fluid. Correlations for Nusselt number and Sherwood number have been proposed from the present experimental studies with a maximum deviation of 25% and 15% respectively. The results showed that desorption ratio, Sherwood number, Nusselt number increase with driving temperature ratio, R134a initial concentration, and two phase Reynolds number but decrease with increase in driving pressure ratio. Driving temperature ratio varied from 2.5 to 4.2, whereas driving pressure ratio varied from 1.7 to 3.1. However the increase in the heat and mass transfer rates are not infinite with respect to the temperature and pressure ratios. Heat and mass transfer effectiveness of generator decrease two to three times faster with increase in solution Reynolds number than the rate of increase with increase in driving temperature ratio. Heat transfer effectiveness increases with increase in solution initial concentration. However mass transfer effectiveness decreases with increase in solution concentration. The two phase friction factor varied from 0.02 to 0.035 and it decreases with decrease in driving pressure ratio and with increase in the Reynolds number and driving temperature ratio.
e Sh ¼ D Retp ðScl Þf ðXsw Þg ðqÞj
(19)
where
Sh ¼ convective mass transfer=diffusive transfer ¼
MD2i rl Dc;l (20)
Retp ¼ Inertial force/Viscous force ¼ Rel
Rel ¼
4ml
pDi ml
and F ¼ 2:35
1
ctt
F 1:25 ,
where
0:736 þ 0:213
(21)
Scl ¼ Momentum diffusivity=Mass diffusivity ¼
ml rl Dc;l
(22)
Xsw ¼ Concentration difference between solutions at entry and exit ¼ Xss Xws
(23) References
Generic form of the correlation includes convective mass transfer, diffusive mass transfer, inertial force, viscous force, momentum diffusivity, mass diffusivity, desorption potential and heat source. Fig. 15 shows the comparison of predicted Sherwood number with the experimental Sherwood number from the present study. The regression technique using Generalized Autoregressive Conditional Heteroskedasticity (GARCH) tool box in MATLAB is employed to determine the coefficients in the generic form of the correlations. The following correlation has been developed with R134a-DMF mixture with 15% error band.
1:079 Sh ¼ 4:819 106 Retp ðScL Þ0:242 ðXsw Þ0:884 ðqÞ1:586 (24) The above correlation is valid in the range for Retp ¼ 8160 to 27,810; ScL ¼ 30 to 42; Xsw ¼ 0.07e0.4; q ¼ 2e20.
Predicted Sherwood number
12000 +15%
Sh = 4.819 x 10 6 (Retp) -1.079 (ScL) -0.242 (Xsw)
-0.884
(q)
1.586
-15%
8000
4000
0 0
4000
8000
12000
Experimental Sherwood number Fig. 15. Comparison of predicted against experimental Sherwood number.
[1] M. Fatouh, S. Srinivasa Murthy, Performance of an HCFC22 based vapour absorption refrigeration system, Int. J. Refrig. 18 (1995) 465e476. [2] M. Fatouh, S. Srinivasa Murthy, Comparison of R22-absorbent pairs for absorption cooling based on P-T-X data, J. Renew. Energy 3 (1993b) 31e37. [3] M. Suresh, A. Mani, Heat and mass transfer studies on R134a/DMF solution based on phenomenological theory, Int. J. Heat Mass Transf. 53 (2010) 2813e2825. [4] M. Fatouh, Studies on HCFC based vapour absorption refrigeration systems suitable for low potential heat sources, PhD thesis, Indian Institute of Technology Madras, 1994. [5] G. Karthikeyan, A. Mani, S. Srinivasa Murthy, Performance of different working fluids in transfer-tank operated vapour absorption refrigeration systems, Renew. Energy 6 (1995) 835e842. [6] K.S. Sujatha, A. Mani, S. Srinivasa Murthy, Analysis of a bubble absorber working with R22 and five organic absorbents, Heat Mass Transf. 32 (1997) 255e259. [7] Y. Nezu, N. Hisada, T. Ishiyama, K. Watanabe, Thermodynamic properties of working-fluid pairs with R-134a for absorption refrigeration system, in: Natural Working-Fluids, IIR Gustav Lorentzen Conf. 5th, China, Sept. 17e20, 2002, pp. 446e453. [8] A. Yokozeki, Theoretical performances of various refrigeranteabsorbent pairs in a vapour absorption refrigeration cycle by the use of equation of state, Appl. Energy 80 (2005) 383e399. [9] A. Mani, Studies on Compact Bubble Absorber of the Vapour Absorption Refrigeration System, a Report Submitted Department of Science and Technology, Government of India, 2009. [10] D.L. Bennett, J.C. Chen, Forced convective boiling in vertical tubes for saturated pure components and binary mixtures, AIChE J. 26 (1980) 454e461. [11] W.W.S. Charters, V.R. Megler, W.D. Chen, Y.F. Wang, Atmospheric and subatmospheric boiling of H2O and LiBreH2O solutions, Int. J. Refrig. 5 (1982) 107e114. [12] A. Matsuda, K.H. Choi, K. Hada, T. Kawamura, Effect of pressure and concentration on performance of a vertical falling-film type of absorber and generator using lithium bromide aqueous solutions, Int. J. Refrig. 17 (1994) 538e542. [13] H.K. Varma, R.K. Mehrotra, K.N. Agrawal, Heat transfer during pool boiling of LiBr-water solutions at sub atmospheric pressures, Int. Commun. Heat Mass Transf. 21 (1994) 539e548. [14] W. Rivera, R. Best, Boiling heat transfer coefficients inside a vertical smooth tube for watereammonia and ammoniaelithium nitrate mixtures, Int. J. Heat Mass Transf. 42 (1999) 905e921. [15] W. Rivera, V. Velez, A. Xicale, Heat transfer coefficients in two-phase flow for mixtures used in solar absorption refrigeration systems, Sol. Energy Mater. & Sol. Cells 63 (2000) 401e411. [16] W. Rivera, A. Xicale, Heat transfer coefficients in two phase flow for the waterelithium bromide mixture used in solar absorption refrigeration systems, Sol. Energy Mater. & Sol. Cells 70 (2001) 309e320.
128
P. Balamurugan, A. Mani / International Journal of Thermal Sciences 61 (2012) 118e128
[17] T. Inoue, M. Monde, Y. Teruya, Pool boiling heat transfer in binary mixtures of ammoniaewater, Int. J. Heat Mass Transf. 45 (2002) 4409e4415. [18] H. Arima, M. Monde, Y. Mitsutake, Heat transfer in pool boiling of ammoniaewater mixture, Heat Mass Transf. 39 (2003) 535e543. [19] L. Roriz, A. Mortal, L.F. Mendes, in: Study of a Plate Heat Exchanger Desorber With a Spray Column for a Small Solar Powered Absorption Machine, 3rd Int. Conf. Heat Powered Cycles, Cyprus, 2004. [20] T. Khir, R.K. Rahim, N. Ghaffour, A. Ben Brahim, Experimental study on forced convective boiling of ammoniaewater mixtures in a vertical smooth tube, Arab. J. Sci. Eng. 30 (2005) 47e63. [21] M.P. Mishra, H.K. Varma, C.P. Sharma, Heat transfer coefficients in forced convective evaporation of refrigerant mixtures, Lett. Heat Mass Transf. 8 (1981) 127e136. [22] F. Taboas, M. Valles, M. Bouruois, A. Coronas, Pool boiling of ammoniaewater and its pure components: comparison of experimental data in the literature with predictions of standard correlations, Int. J. Refrig. 30 (2007) 778e788.
[23] F. Taboas, M. Valles, M. Bouruois, A. Coronas, Flow boiling heat transfer of ammoniaewater mixture in a plate heat exchanger, Int. J. Refrig. 33 (2010) 695e705. [24] P. Balamurugan, A. Mani, Heat and Mass Transfer Studies on Vertical Tubular Generator in R134a-DMF Absorption Refrigeration System, Int. Symp. Innov. Mater. Process. Energy Syst., Singapore, Nov 30eDec 1, 2010, 335e342. [25] P. Balamurugan, A. Mani, Numerical studies on vertical tubular generator in vapour absorption refrigeration system, Int. J. Air Cond. Refrig. 19 (2011) 121e129. [26] R.C. Reid, J.M. Prausnitz, B.E. Poling, The Properties of Gases and Liquids, fourth ed., McGraw-Hill Book Company, New York, 1989. [27] Y.Y. Yan, T.F. Lin, Evaporation heat transfer and pressure drop of refrigerant R134a in a plate heat exchanger, J. Heat Transf. 121 (1999) 118e127. [28] F.P. Incropera, D.P. DeWitt, T.L. Bergman, A.S. Lavine, Fundamentals of Heat and Mass Transfer, sixth ed., John Wiley & Sons, USA, 2007. [29] S.J. Kline, F.A. McClintock, Describing uncertainties in single sample experiments, J. Mech. Eng. 75 (1953) 3e12.
Contents lists available at SciVerse ScienceDirect
International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts
Experimental studies on heat and mass transfer in tubular generator for R134a-DMF absorption refrigeration system P. Balamurugan, A. Mani* Refrigeration & Airconditioning Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600 036, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 September 2011 Received in revised form 8 March 2012 Accepted 5 June 2012 Available online 20 July 2012
Experimental investigations have been carried out to study heat and mass transfer during desorption of Tetrafluoro ethane (R134a) from R134a and Dimethyl formamide (DMF) solution in a tubular generator. Absorption refrigeration system has been built with brazed plate heat exchangers as condenser, absorber, evaporator and solution heat exchanger and with stainless steel concentric tubes as the generator. Effects of operational parameters viz., solution two phase Reynolds number, driving temperature ratio, driving pressure ratio, solution initial concentration on generator performance are analyzed. Desorption ratio, Sherwood number and Nusselt number increase as the solution Reynolds number, solution initial concentration, driving temperature ratio increase whereas these parameters decrease as the driving pressure ratio increases. Finally a correlation for Nusselt number and Sherwood number are proposed based on the experimental studies. Ó 2012 Elsevier Masson SAS. All rights reserved.
Keywords: Desorption R134a DMF Tubular generator Heat transfer Mass transfer
1. Introduction In the vapour absorption refrigeration system, even though they are traditionally used, ammonia/water suffers from toxicity, corrosion and rectification requirements. Water/lithium bromide encounters limitation of evaporator temperature, vacuum pressure in the components and possibility of crystallization. From the literature [1e3], it is observed that the above disadvantages associated with the traditional working pairs have prompted researchers in the past, to search for alternative working fluids. Though R22-organic solvent based absorption refrigeration systems have been extensively studied by Fatouh [4], Karthikeyan et al. [5] and Sujatha et al. [6], HCFCs along with CFCs, are being phased out by Montreal and other International Protocols. Hence attention was shifted towards HFCs as an alternate to CFCs and HCFCs. They are not destructive to the ozone layer though they have a slight effect on global warming. R134a is the commonly used HFC refrigerant. The Kyoto protocol during 1997 has put R134a as one of the green house gases. However there is no phase out date for this refrigerant and it is expected to be highly used in refrigeration industries. Nezu et al. [7] examined the possibility of testing R134a as a refrigerant in VARS with various organic solvents and showed
* Corresponding author. Tel.: þ91 44 22574666; fax: þ91 44 22570509. E-mail address: [email protected] (A. Mani). 1290-0729/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.ijthermalsci.2012.06.001
that the R134a-DMA and the R134a-DMF systems are considered attractive as the working-fluid pairs for the absorption refrigeration system than other R-134a-absorbent combinations. Yokozeki [8] compared the performances of various refrigeranteabsorbent pairs in a VARS cycle, by the use of equations of state and concluded that R134a-DMF and R134a-DMA systems exhibited better performance, compared to other R134a-absorbent combinations. Also the circulation ratio is less and COP is more for R134aDMF system compared to R134a-DMA system. Mani [9] carried out experimental studies on R134a-DMF based compact vapour absorption refrigeration system with plate heat exchangers and reported that this system could be very competitive for applications ranging from 10 C to 10 C, with heat source temperature in the range of 80 Ce90 C and with cooling water as coolant for absorber and condenser in the range of 20 Ce35 C. Generator, one of the crucial components of the vapour absorption refrigeration system, influences the system performance significantly. In the generator, desorption of refrigerant vapour takes place from refrigeranteabsorbent solution by simultaneous heat and mass transfer phenomena. A detailed heat and mass transfer study of desorption in a generator will contribute to the improvement of absorption refrigeration technologies. Bennett and Chen [10] studied forced convective boiling with aqueous ethylene glycol solutions and developed a correlation to predict the heat transfer coefficient. This correlation improved the understanding of flow boiling of binary mixtures. Charters et al. [11]
P. Balamurugan, A. Mani / International Journal of Thermal Sciences 61 (2012) 118e128
Nomenclature A Cp Di Dc F G H Hfg h L m M Nu P Re Sh Sc q Q T x X
heat transfer area, m2 specific heat, J kg1 K1 inner diameter of inner tube, m diffusion coefficient, m2 s1 friction factor mass flux, kg m2 s1 enthalpy, J kg1 latent heat of vaporization, J kg1 heat transfer coefficient, W m2 K1 tube length, m mass flow rate, kg s1 mass transfer coefficient, kg m3 s1 Nusselt number pressure, Pa Reynolds number Sherwood number Schmidt number heat flux, W m2 heat transfer rate, W temperature, C vapour quality liquid concentration
measured heat transfer coefficients experimentally for H2OeLiBr boiling on an electrically heated vertical copper tube. A correlation was proposed to predict the experimental data for pool boiling heat transfer on surfaces of platinum, copper and brass. Matsuda et al. [12] carried out experiments on generator in a H2OeLiBr absorption refrigerating machine using a vertical falling-film type of stainless steel column. The measured evaporation rate decreased with reducing pressure and increasing concentration of LiBr in the falling liquid. Varma et al. [13] measured heat transfer coefficients during pool boiling of H2OeLiBr solutions over horizontal stainless steel tubes. Heat transfer coefficient of lithium bromideewater solution was found to be slightly smaller than that of pure water. Also average heat transfer coefficient decreased as the concentration of LiBr increased. Rivera et al. [14] studied heat transfer in forced convective boiling experimentally for NH3eH2O and NH3eLiNO3 mixtures in a vertical tube. Correlations were proposed to predict the experimental local heat transfer coefficients. Rivera et al. [15] analyzed NH3eH2O, NH3eLiNO3 and H2OeLiBr in saturated upward flow nucleate boiling in a uniformly heated vertical tube. It was observed that the highest values of average heat transfer coefficients were obtained for NH3eH2O mixture. Local and average heat transfer coefficients in saturated nucleate boiling were measured by Rivera et al. [16], for H2OeLiBr mixture in a uniformly heated vertical tube, which was the generator of a solar absorption refrigeration system. It was observed that the average heat transfer coefficients increased for the mixture with an increase in the heat flux and with decrease in solution concentration and temperature difference between tube wall and fluid. Inoue et al. [17] conducted experimental study for heat transfer during nucleate pool boiling on the horizontal heated wire in mixtures of NH3eH2O. The heat transfer coefficients in the mixtures were less than those in single component substances. An applicability of existing correlations to the experimental data was discussed. Arima et al. [18] measured boiling heat transfer coefficient of NH3eH2O mixture on a horizontal heated surface. Heat transfer coefficients were compared with existing correlations and a revised correlation was proposed to predict them. Roriz et al. [19]
119
Subscripts abs absorber av average d desorption eq equilibrium gen generator hw hot water i inlet l liquid o outlet ph preheater s solution ss strong solution tp two phase v vapour wallin inner wall ws weak solution Greek symbols 3 effectiveness h mass transfer efficiency q non dimensional temperature m dynamic viscosity, Pa-s r density, kg m3
compared the overall performance of the absorption system and the heat transferred in the generator with that of a system with falling film desorber and thus proved the feasibility of a compact generator. Khir et al. [20] performed an experiment on forced convective boiling heat transfer of NH3eH2O mixtures inside a vertical tube. Experimental data were compared with correlations of Bennett and Chen [10], Mishra et al. [21], etc. Taboas et al. [22] compiled experimental data available in the open literature on nucleate boiling of NH3, H2O and NH3eH2O mixture. The experimental data were compared with three correlations available in the literature for pool boiling of mixtures. A new correlation was proposed which satisfactorily predicted the experimental data. Francisco Taboas et al. [23] carried out experiments to measure heat transfer coefficient and pressure drop for NH3eH2O mixtures under flow boiling conditions in a vertical brazed plate heat exchanger, at different operating conditions. Balamurugan and Mani [24] developed a numerical model on heat and mass characteristics of R134a-DMF based tubular generator based on ColburneDrew formulation and validated by comparing with the results available in literature. Numerical analysis of forced convective boiling of refrigeranteabsorbent solution in a stainless steel tube in tube generator has been carried out by Balamurugan and Mani [25]. Using NH3eH2O, the model has been validated and compared with the experimental results in the literature. In the present experimental work, heat and mass transfer parameters are measured during desorption of R134a from R134aDMF solution in the tubular generator. Vapour absorption refrigeration setup has been built using brazed plate heat exchangers for other major heat and mass transfer components. The performance of tubular generator is evaluated using dimensionless parameters. The influence of solution two phase Reynolds number, driving temperature ratio, driving pressure ratio and solution initial concentration on desorption is presented and examined. 2. Experimental setup Fig. 1 shows the schematic diagram of experimental setup. This consists of a stainless steel tubular generator, condenser,
120
P. Balamurugan, A. Mani / International Journal of Thermal Sciences 61 (2012) 118e128
T22 T5
T6
NV1
T8
Condenser
R134a/DMF solution line
P4
L2
BV5
Refrigerant line Water line
T21
BV6
BV8 Evaporator
MS S1
P2
T18
T7
Liquid Refrigerant Receiver
T4
P3
T9
L1
S5 BV7
BV4 T15
BV9 Chilled water pump
Gas separator
Vapour Generator
T20 S3
BV12
S4
T14 T12
T13 P6
BV2
BV15
BV16 T2 Online denstiy meter BV13 Preheater Hot water simulator
T17
Hot water T16 pump
BV3
Absorber
Cooling load simulator
P7 NV2
Solution heat exchanger
S4
T19
BV10 T10 T1
S L
Ball valve Needle valve BV1 Flow meter Level gauge
T
Temperature gauge
P
Pressure gauge
SG
Absorber Tank
Sight glass
L3
Cold water pump
P5
T11
BV11
Cooling water simulator
T3 P1 BV14
Solution Pump
Fig. 1. Schematic diagram of tubular generator experimental setup.
evaporator, absorber and solution heat exchanger, solution pump, cooling water simulator, hot water simulator, cooling load simulator, instrumentation and valves. Table 1 shows the specifications of major components used in the setup. The system consists of refrigerant circuit and solution circuit. In refrigerant circuit, R134a vapour coming from gas separator is condensed in the condenser and accumulated in the receiver. Heat of condensation is removed by the cooling water. Liquid R134a from the receiver is expanded through throttle valve and evaporated in the evaporator. Chilled water from the simulator acts as the heat load. In solution circuit, R134a vapour from the evaporator is absorbed by the weak DMF solution in the absorber. Integral enthalpy of solution is removed by water from cooling water simulator. Strong solution collected in the absorber tank is pumped through solution heat exchanger and solution preheater to the bottom of inner tube of generator. Tubular generator which is used for the present study consists of two concentric tubes as shown in Fig. 2. Hot water is supplied as heat source through generator annulus in counter flow direction. R134a vapour boiled off in the generator is separated in the gas separator. Weak solution remaining in the gas separator is sent through solution heat exchanger and pressure reducing valve to absorber for absorption. A picture of the present experimental setup is shown in Fig. 3. Hot water simulator consists of a hot water tank insulated with glass wool, electric heaters, pump, flow meter, PT100 sensor, PID temperature controller, contactor, piping and valves. It supplies hot water to the generator. Cooling water simulator consists of R22 based vapour compression refrigeration (VCR) circuit, a cooling water tank insulated with expanded polyethylene (EPE) sheets,
electric heaters, pump, flow meter, PT100 sensor, PID temperature controller, contactor, piping and valves. VCR circuit consists of a hermetically sealed reciprocating compressor, an air cooled condenser, a thermostatic expansion valve and cooling coil. Cooling water simulator supplies cooling water to absorber and condenser. Cooling load simulator consists of a chilled water tank insulated with expanded polyethylene sheets, electric heaters, pump, flow meter, PT100 sensor, PID temperature controller, contactor, piping and valves. It supplies water as heat load to evaporator and maintains a constant desired value of chilled water temperature at evaporator inlet. This system also includes various measuring devices such as temperature sensors, pressure sensors, flow meters and online density meter fitted at suitable locations as shown in Fig. 1. All these measuring instruments are calibrated. Thirty two numbers of calibrated coppereconstantan thermocouples are used as temperature sensors with a measurement uncertainty up to 0.5 C. Ten thermocouples have been fixed to the tube as shown in Fig. 2, to measure the wall temperature and the bulk solution temperature along the generator. Seven numbers of piezoeelectric type pressure transducers are used as pressure sensors with a measurement uncertainty up to 4.6%. Metal tube rotameters are used to measure the flow of solution, liquid refrigerant and hot water with a measurement uncertainty up to 5% and glass rotameters are used to measure the flow of cooling water and cooling load with a measurement uncertainty up to 2.5%. An online density meter is used to measure the density of strong and weak solutions with a measurement uncertainty of 0.3%. Concentrations of strong and weak solutions are evaluated from the measured density values
P. Balamurugan, A. Mani / International Journal of Thermal Sciences 61 (2012) 118e128 Table 1 Specifications of major components. Tubular generator Type Fluid circuit Material Inside tube Outside tube Tube length Evaporator Type Fluid circuit Fluid direction Heat transfer area, m2 Length, m Width, m Height, m No. of plates No. of effective plates No. of passes Condenser Type Fluid circuit Fluid direction Heat transfer area, m2 Length, m Width, m Height, m No. of plates No. of effective plates No. of passes Absorber Type Fluid circuit Fluid direction Heat transfer area, m2 Length, m Width, m Height, m No. of plates No. of effective plates No. of passes Solution heat exchanger Type Fluid circuit Fluid direction Heat transfer area, m2 Length, m Width, m Height, m No. of plates No. of effective plates No. of passes Solution pump Type Capacity Discharge pressure Plunger diameter SPM Online density transmitter Tube inner diameter Tube length (Inlet to outlet) Measuring range Measuring repeatability Accuracy in the adjusted range Temperature range Pressure range Flow rate (water) Pressure transducers Type Measuring range Temperature Input Output Accuracy Response time
Tube in tube Tube side: solution Annulus: hot water SS 316 ID: 20 mm, OD: 23 mm ID: 30 mm, OD: 33 mm 1000 mm Brazed plate heat exchanger Cold side: R134a, hot side: water Countercurrent 0.16 0.064 0.077 0.207 14 12 1
121
Table 1 (continued ) Material Flow meters Type Flow rate Operating pressure End fitting material Float material Tube material Tube taper length Accuracy Type Flow rate Operating pressure End fitting material Float material Tube material Tube taper length Accuracy Type Flow rate
SS316L Glass rotameter Minimum: 0.025 m3 h1, maximum: 0.25 m3 h1 2 105 Pa (abs.) SS316 SS316 Borosilicate glass 250 mm þ/1% on F.S.D Glass rotameter Minimum: 0.018 m3 h1, maximum: 0.18 m3 h1 2 105 Pa (abs.) SS316 SS316 Borosilicate glass 250 mm þ/1% on F.S.D Metal tube rotameter Minimum: 0.010 m3 h1, maximum: 0.104 m3 h
Operating pressure End fitting material Float material Tube material Tube taper length Accuracy Type Flow rate Operating pressure End fitting material Float material Tube material Tube taper length Accuracy Type Flow rate Operating pressure End fitting material Float material Tube material Tube taper length Accuracy
11 105 Pa (abs.) SS316 SS316 SS316 102 mm þ/1% on F.S.D Metal tube rotameter Minimum: 0.002 m3 h1, maximum: 0.02 m3 h1 11 105 Pa (abs.) SS316 SS316 SS316 102 mm þ/1% on F.S.D Metal tube rotameter Minimum: 0.033 m3 h1, maximum: 0.33 m3 h1 2 105 Pa (abs.) SS316 SS316 SS316 102 mm þ/1% on F.S.D
1
Brazed plate heat exchanger Cold side: water, hot side: R134a Countercurrent 0.16 0.064 0.077 0.207 14 12 1 Brazed plate heat exchanger Cold side: water, hot side: R134a þ DMF Countercurrent 0.1 0.055 0.077 0.207 10 8 1 Brazed plate heat exchanger Cold side: R134a þ DMF, hot side: R134a þ DMF Countercurrent 0.37 0.059 0.094 0.324 18 16 1 Metring pump 0.050 m3 h1 40 105 Pa 25 mm 100 6.6 mm 500 mm 0 to 3000 kgm3 5 102 kg m3 2 101 kgm3, 0.1% 0e100 C 0e50 105 Pa 0.1e0.5 m3 h1 Piezoresistive 0e16 105 Pa absolute 15e100 C 8e30 V DC 0e5 V DC (3 wires) 1% of full scale Maximum: 250 m Sec
using HBT (HankinsoneBrobsteThomson) equation used by Reid et al. [26]. Readings from all these instruments and sensors are recorded continuously by connecting them to a data acquisition system and a computer. 3. Experimental procedure Initially the refrigerant and solution circuit are separated by closing the valve between (i) gas separator and condenser and (ii) evaporator and absorber. Hot water simulator and cooling water simulator are started. Hot water is circulated through the annulus of the generator at a temperature higher than that to be maintained in the generator. Cooling water is circulated through absorber and condenser at a temperature lower than that to be maintained in the respective components. Cooling load simulator is operated by circulating water through evaporator. Water temperature in the chilled water tank is maintained constant by operating the heaters equivalent to cooling capacity of system. Solution pump is started to circulate strong solution through the inner tube of generator. Level of weak solution collected in the gas separator, level of strong solution in the absorber storage tank and pressure in each component of solution circuit are monitored continuously. When pressure in the gas separator becomes higher than that in the condenser, the valve between them is opened to allow refrigerant vapour to get condensed in condenser. Level of liquid refrigerant
122
P. Balamurugan, A. Mani / International Journal of Thermal Sciences 61 (2012) 118e128
Weak solution out
225
Hot water in
1000
225
Thermocouple
225
225
Thermocouple
Hot water out Strong solution in φ 20 φ 23 φ 30 φ 33 All dimensions in mm Fig. 2. Schematic diagram of tubular generator.
accumulated in the liquid refrigerant receiver is monitored. After collecting sufficient amount of liquid refrigerant, it is allowed through the throttle valve to enter the evaporator. Flow rate of weak solution and liquid refrigerant are adjusted to maintain steady flow in the system. When all the readings of pressure transducers, thermocouples, flow meters and level gauges remain constant over a period of time, steady state readings are recorded in the computer. Water flow rates in the hot water simulator, cooling water simulator and cooling load simulator are maintained at the desired constant values. Experimental runs are repeated for different operating conditions. While shutting down the system after the end of the experimental test, solution circuit and refrigerant circuit are again isolated by closing the valves. 4. Data reduction Experimental tests are conducted in tubular generator by varying the operational parameters within the range as given in Table 2. The range of generator temperature for the present study has been chosen to suit the hot water temperature that could be obtained by harnessing solar energy. Experimentation with varying parameters has been planned based on a preliminary thermodynamic analysis carried out on the vapour absorption refrigeration system using R134a-DMF pair. Experimental runs are repeated for different operating conditions. In every run of the experiment, the system is allowed to run for a couple of hours to attain steady state
Fig. 3. Tubular generator experimental setup.
conditions. Calculations are carried out using the recorded values at these conditions. The following assumptions are considered for the calculating the heat and mass transfer parameters: (i) As the temperature of hot water varies nearly linearly along the generator, heat flux imposed on the test section is assumed to be uniform over the entire length of generator; (ii) Bulk solution temperature is considered to be constant in a given cross section of generator; (iii) Generator is assumed to have uniform thermal conductivity across its wall; (iv) Thermodynamic equilibrium exists in the test section; (v) As the distance between preheater and the generator is very short, the exit quality of preheater is assumed as the entry quality for the generator. From the measured values of pressures, temperatures and compositions of the solution, enthalpies of solution at the inlet and outlet of generator are found using thermodynamic equations for mixtures. The dimensionless operating parameters, which are used for evaluating performance of the generator, driving pressure ratio (Pgen/Pabs) and driving temperature ratio (Tgen/Tabs), besides desorption ratio, Nusselt number and Sherwood number, vapour quality, heat and mass transfer effectiveness, friction factor are
Table 2 Operating range of parameters. Solution flow rate Generator pressure Solution initial concentration Generator temperature Condenser temperature Evaporator temperature Hot water flow rate Hot water temperature
0.02e0.05 m3 hr1 650e1000 kPa 0.58e0.76 kgkg1 78e90 C 15e30 C 5e13 C 0.08e0.33 m3 hr1 82e98 C
P. Balamurugan, A. Mani / International Journal of Thermal Sciences 61 (2012) 118e128
found using the derived quantities from the experimental data. Thermodynamic and physical properties of R134a-DMF used are obtained from experimental correlations from Reid et al. [26], Nezu et al. [7] and Yokozeki [8]. Wall temperature and solution temperature at core are nondimensionalized using Eq. (1).
q ¼
ðT Tice Þ ðTsteam Tice Þ
(1)
Heat energy supplied is obtained from energy balance on hot water side of generator as in Eq. (2).
Qhw ¼ mhw Cphw Thw;i Thw;o
immediately to the tubular generator for desorption, the exit condition of preheater is assumed to be the same as the entry condition of generator.
xph;o ¼
xgen;o ¼ xgen;i þ
(2)
(3)
Volumetric mass transfer coefficient is obtained from the log mean concentration difference (LMCD) and the mass of refrigerant vapour desorbed as in Eq. (5).
md M ¼ p 2 Di L LMCD 4
(5)
Desorption ratio (md/mss) is defined as the ratio of mass of refrigerant vapour desorbed to the total mass flow rate of solution flowing through the generator. Local heat transfer coefficient is calculated from the wall and bulk solution temperatures and the heat flux:
Q ¼ hðTwallin Ts Þ q ¼ A
(6)
Inner wall temperature is found from the measured outside wall temperature by radial heat conduction. Average heat transfer coefficient is found from the local heat transfer coefficients at five thermocouple stations.
hav ¼
h1 þ h2 þ h3 þ h4 þ h5 5
(7)
Preheater is used in the experimentation to evaluate (Yan et al. [27]) the solution vapour quality at the entry of the generator. Heat transferred from the preheater is calculated from the voltage supplied and the resistance of the electric heater. In the preheater, heat is transferred to the solution both to rise its temperature (sensible heat transfer) and for evaporation of refrigerant in the solution (latent heat transfer). Knowing the amount of heat transferred in the preheater, vapour quality at the exit of preheater could be evaluated from the energy balance across the preheater as given in Eqs. (8)e(11).
Qph ¼ Qph;sensible þ Qph;latent
(8)
Qph;sensible ¼ ms Cps Ts;ph;o Ts;ph;i
(9)
Qph;latent ¼ ms Hfg xph;o
(10)
Above equations are rearranged to get the vapour quality at the exit of preheater. As the solution from the preheater is sent
Qgen ms Hfg
! (12)
Xs;i Xs;o ¼ Xs;i Xeq;s;o
(13)
Friction factor associated with the flow boiling of R134a-FMF solution is found (Incropera and Dewitt [28],) from the correlation
f ¼ (4)
(11)
Mass transfer effectiveness is calculated from 3
Xeq;s;i Xs;i Xs;o Xeq;s;o LMCD ¼ Xeq;s;i Xs;i ln Xs;o Xeq;s;o
! 1 Qph Qph;sensible ¼ xgen;i ms Hfg
Quality of refrigerant vapour at the exit of generator is found from the energy balance on the solution side in the generator as given in Eq. (12).
While heat gained by solution across the generator is determined from the enthalpy difference and solution mass flow rate as
Qs ¼ mss ðHi Ho Þ:
123
1
! (14)
2 0:79lnRetp 1:64
The uncertainties in the measured quantities namely temperature, pressure, flow rates, and density are determined from the minimum value of the measured output and accuracy of the instruments. The uncertainties in derived quantities namely desorption ratio, concentration, Sherwood number, Nusselt number, heat transfer effectiveness, mass transfer effectiveness, quality and friction factor have been determined using the formula proposed by Kline and McClintock [29]. Table 3 gives the results of the uncertainty analysis. 5. Results and discussion Using the equations given in previous section, performance of R134a-DMF tubular generator is deduced, for the range of operating conditions. Effect of the solution Reynolds number, driving temperature ratio, driving pressure ratio and solution initial concentration on desorption ratio, Nusselt number, Sherwood number, heat and mass transfer effectiveness, vapour quality, friction factor, etc, are plotted and discussed. Fig. 4 depicts the effect of driving temperature ratio on non-dimensional wall temperature, solution temperature and local Nusselt number along the generator. With the increase in driving temperature ratio, which is realized by the increase in temperature of simulated hot water
Table 3 Estimated uncertainties. Measured quantities Solution flow rate Pressure Temperature Density Hot water flow rate Derived quantities Desorption ratio Concentration Sherwood number Nusselt number Mass transfer effectiveness Heat transfer effectiveness Quality Friction factor
5.2% 4.6% 0.5 C 0.3% 2.8% 7.9% 2.7% 7.0% 4.7% 4.6% 7.5% 8.0% 13.5%
P. Balamurugan, A. Mani / International Journal of Thermal Sciences 61 (2012) 118e128
Driving temp.ratio = 4.05 Driving temp.ratio = 3.70
1
0.9
600 Twall ----- Ts Nu
400
0.8
200 Driving Temp.ratio = 3.60 0
0.7 0
0.2 0.4 0.6 0.8 Non-dimensional length of generator
1
Fig. 4. Variation of non-dimensional wall temperature, solution temperature and local Nusselt number for different driving temperature ratios.
supplied to the generator, heat transfer rate to the wall and subsequently to the solution increases, resulting in higher wall and solution temperatures and the Nusselt number. For the uniform heat flux imposed on the generator, difference between the wall temperature and solution temperature along the generator decreases, causing the local Nusselt number to increase. However, as the driving temperature ratio increases, reduction in difference between wall and solution temperatures is more pronounced. Hence, increase in Nusselt number is steep at latter half of the generator which may be due to the increased utilization of quantum of heat to boil refrigerant from the solution. Fig. 5 shows the variation of average Nusselt number, Sherwood number and desorption ratio with solution Reynolds number for various driving temperature ratios. Heat transfer rate increases due to increased solution flow rates, as more heat need to be supplied at higher desorption rates. As the Reynolds number increases, quantity of solution available for desorption is more, resulting in higher mass generation of R134a vapour. Heat transfer rate also increases as the driving temperature ratio increases resulting in higher generation of refrigerant vapour. Hence, increase in heat transfer rate and mass transfer rate results in the increase in Nusselt number and Sherwood number respectively, with solution Reynolds number and driving temperature ratio. However, as the Reynolds number increases, even though the mass generation of R134a vapour increases, increase in Nusselt number with Reynolds number slows down, as solution approaches the maximum limit of
300 Nusselt number Sherwood number ---- Desorption ratio
150
5000
Sherwood number
Nusselt number Desorption ratio x 2000
Driving pressure ratio = 1.91 Hot water Reynolds number = 4960 Initial concentration = 0.67 10000
0 8000
0.36
15000
600
450
proportional desorption for a given operating condition. This effect is more pronounced at higher driving temperature ratios, revealing that the increase in Nusselt number is not infinite. As the Nusselt number and Sherwood number increase, for the reasons explained above, desorption ratio increases with the increase in Reynolds number and driving temperature ratio. However, as the increase in Nusselt number with Reynolds number is not significant, desorption ratio also flattens at higher values of Reynolds number and driving temperature ratios. Fig. 6 shows the effect of solution Reynolds number and driving temperature ratio on exit vapour quality and concentration difference across the generator during desorption of R134a in the tubular generator. At higher driving temperature ratios and Reynolds number, the solution undergoing desorption is subjected to increased heat transfer rate, probably because of higher temperatures, resulting in the generation of higher quantity of R134a vapour. This increases the refrigerant vapour quality at the exit of the generator. However at higher Reynolds number, though the heat transfer rate and desorption rate increase, effective desorption decreases which results in a higher value of weak solution concentration at the exit of generator. Thus the difference in concentration is less for higher solution Reynolds number. At higher driving temperature ratio, for a given solution flow rate, both heat transfer and desorption rates are higher. Weak solution concentration at the exit is lower and hence difference in concentration increases. Thus the concentration difference between the entry and exit of generator increases with driving temperature ratios and decreases with Reynolds number. Variation of heat transfer effectiveness and mass transfer effectiveness with solution Reynolds number for different driving temperature ratios is depicted in Fig. 7. Heat transfer effectiveness increases with the increase in driving temperature ratio due to the increase in heat transfer rate. However with the increase in Reynolds number, though heat transfer rate is high, heat transfer effectiveness decreases because the ratio, of heat transferred to solution flow rate, and the utilization of heat decreases as the Reynolds number increases. Hence effective heat transfer decreases at higher Reynolds number. Also for the reason explained above for Fig. 6, at higher Reynolds number, though the heat transfer rate and mass generation rate are higher, effective desorption decreases which results in a higher value of weak solution concentration at the exit of generator. This causes an increase in the gap between the actual weak concentration and the equilibrium weak concentration at the exit of generator. Hence, the tendency of weak solution at the exit of generator to get closer to equilibrium concentration decreases as
Driving temperature ratio = 4.05 Driving temperature ratio = 3.70 Driving temperature ratio = 3.60 0 13000
18000
23000
28000
Two phase Reynolds number Fig. 5. Effect of two phase Reynolds number on Nusselt number, Sherwood number and desorption ratio for different driving temperature ratios.
Hot water Reynolds number = 4960 Driving pressure ratio = 1.91 Initial concentration = 0.67
0.6
0.27 0.4 Concentration difference ---- Exit vapour quality 0.18
0.2
Exit vapour quality
Retp = 10700 Driving pressure.ratio = 1.9 800 Re = 3140 hw Xsi = 0.7
Concentration difference across generator
Local Nusselt number
1000
Non-dimensional temperature
124
Driving temperature ratio = 4.05 Driving temperature ratio = 3.70 Driving temperature ratio = 3.60 0.09 8000
0 13000
18000
23000
28000
Two phase Reynolds number Fig. 6. Effect of two phase Reynolds number on concentration difference across generator and exit vapour quality for different driving temperature ratios.
P. Balamurugan, A. Mani / International Journal of Thermal Sciences 61 (2012) 118e128
0.85 ---- Mass transfer effectiveness Heat tranfser effectiveness
0.6
0.8 Driving temperature ratio = 4.05 Driving temperature ratio = 3.70 Driving temperature ratio = 3.60
0.5 8000
0.75 13000
18000
23000
0.8 Hot water Reynolds number = 4700 Driving temperature ratio = 3.18
Exit vapour quality
0.7
0.8
0.6
0.6 ---- Exit quality Concentratrion difference
0.4
0.2
0.2 Driving pressure ratio = 1.72 Driving pressure ratio = 2.03
0 8000
28000
0.4
Concentration difference across generator
0.9
Hot water Reynolds number = 4960 Driving pressure ratio = 1.91 Initial concentration = 0.67
Mass tranfser effectiveness
Heat transfer effectiveness
0.8
125
0 13000
18000
23000
Two phase Reynolds number
Two phase Reynolds number Fig. 7. Effect of two phase Reynolds number on heat and mass transfer effectiveness for different driving temperature ratios.
Fig. 9. Effect of solution Reynolds number on exit vapour quality and concentration difference across generator for different driving pressure ratios.
the Reynolds number increases whereas it increases as the driving temperature ratio increases, for a given solution flow rate. Hence mass transfer effectiveness decreases as the Reynolds number increases and increases as driving temperature ratio increases. Fig. 8 depicts the variation of Nusselt number, Sherwood number and desorption ratio with Reynolds number at different driving pressure ratios. At lower driving pressure ratios, the solution enters the generator at a lower temperature resulting in a higher temperature gradient for a given driving temperature ratio and solution flow rate. This increases the heat transfer rate which results in increase in the mass generation rate of R134a vapour, with decrease in pressure ratio. Thus increased heat and mass transfer rates results in increased Nusselt number and Sherwood number, respectively, with decrease in pressure ratio. As explained earlier, as the Reynolds number increases both Nusselt number and Sherwood number increase respectively. However the rate of increase slows down at lower values of driving pressure ratios due to the solution approaching the saturation limit of effective desorption, as explained earlier with reference to Fig. 5. As the Nusselt number and Sherwood number are higher, desorption ratio increases at lower pressure ratio and at higher Reynolds number respectively. However, the rate of increase in desorption ratio with Reynolds number is not eternal, as it decreases with decrease in pressure ratio, due to the similar behaviour of Nusselt number and Sherwood number with Reynolds number at lower pressure ratios. Fig. 9 shows the effect of driving pressure ratio on exit vapour quality and concentration difference across the generator. The
vapour quality and the difference between strong and weak solution concentration increase as the driving pressure ratio decreases due to increase in heat transfer rate for a given solution flow rate. Fig. 10 depicts the effect of solution initial concentration on desorption ratio, exit vapour quality and concentration difference between the strong and weak solution of the generator. As the solution concentration increases, equilibrium inlet temperature of solution decreases resulting in increasing the temperature potential for higher heat transfer to solution. This causes both Nusselt number and Sherwood number to increase, at higher concentration. Thus desorption ratio increases as solution initial concentration increases. As explained earlier, the rate of increase in desorption ratio is limited at higher values of Reynolds number, as the effective desorption decreases. For the reason explained above, the vapour quality and the concentration difference increase with the increase in solution initial concentration due to the increase in heat transfer rate to the solution in the generator. Fig. 11 shows the effect of initial concentration of solution on the heat and mass transfer effectiveness during flow boiling of R134aDMF solution in the tubular generator. Heat transfer effectiveness increases with the increase in solution initial concentration due to the increase in heat transfer rate. However, as the solution concentration increases, equilibrium inlet temperature and pressure of solution decrease, resulting in lower weak solution equilibrium concentration at the exit of the generator. Thus at higher initial concentration, though difference between strong and weak solution actual concentration is higher, there exists a gap between
Hot water Reynolds number = 4700 Driving temp. ratio = 3.18
12000
---- Desorption ratio Nusselt number Sherwood number 140
4000
Driving pressure ratio = 2.03 0 13000
18000
0.3
23000
Two phase Reynolds number Fig. 8. Effect of solution Reynolds number on Nusselt number, Sherwood number and desorption ratio for different driving pressure ratios.
Initial concentration = 0.70 Initial concentration = 0.58
0.4
0.3 0.2 0.1
Driving pressure ratio = 1.72
50 8000
Exit vapour quality Desorption ratio
8000
230
0.4 Sherwood number
Nusselt number Desorption ratio x 1000
320
Hot water Reynolds number = 4400 Driving temperature ratio = 3.02
Exit quality Concentration difference
---- Desorption ratio 0 8000 13000 18000 Two phase Reynolds number
0.2
Concentration difference
0.5
0.5
0.1 23000
Fig. 10. Effect of solution Reynolds number on desorption ratio, exit vapour quality and concentration difference across generator for different solution initial concentration.
126
P. Balamurugan, A. Mani / International Journal of Thermal Sciences 61 (2012) 118e128
3.6
Heat transfer effectiveness ---- Mass transfer effectiveness
0.7
0.83
0.6
0.81 Initial concentration = 0.70 Initial concentration = 0.58
0.5 8000
Fig. 11. Effect of solution Reynolds number on generator heat and mass transfer effectiveness for different solution initial concentration.
the exit weak solution actual concentration and equilibrium concentration, as the gradient between the strong solution inlet concentration and the exit weak solution equilibrium concentration is still higher reducing the mass transfer effectiveness. Hence as solution concentration increases, mass transfer effectiveness decreases. Fig. 12 shows the effect of solution Reynolds number and driving temperature ratio on two phase friction factor associated with flow boiling of R134a-DMF in the tubular generator. At higher Reynolds number, mass velocity of solution undergoing desorption is higher, thus causing a decrease in friction factor. As driving temperature ratio decreases, mass generation of vapour decreases resulting in lower mean vapour quality. This increases average two phase density between the entry and the exit of the generator causing an increase in friction factor. Thus friction factor decreases with the increase in Reynolds number and the driving temperature ratio. The effect of driving pressure ratio on friction factor is depicted in Fig. 13. At lower pressure ratios, mass generation of R134a vapour is more, contributing for decreased two phase density. This causes a decrease in friction factor at lower pressure ratios as shown in Fig. 13. 6. Correlation for Nusselt number A correlation for Nusselt number has been proposed as a function of two phase Reynolds number, Martinelli parameter and Boiling number.
2.4 8000
2.3 8000
Driving pressure ratio = 1.91 Hot water Reynolds number = 4960 Initial concentration = 0.67
18000
23000
Fig. 13. Effect of solution Reynolds number on friction factor at different driving pressure ratios.
b Nu ¼ a Retp ð1=Xtt Þc ðBoÞd
(15)
where
1=Xtt ¼ Lockhart Martinelli parameter 0:5 0:1 rl mv x 0:9 ¼ rv ml 1x Bo ¼ Boiling number ¼
(16)
q GHfg
(17)
In the present work, Eq. (15) is used as the generic form of correlation to correlate the experimental heat transfer coefficients for R134a-DMF mixture. Based on the present experimental data, values of the constant a and indices b, c and d are obtained. In Fig. 14, experimental Nusselt numbers are compared with the Nusselt numbers predicted using the proposed correlation. The proposed correlation is able to predict more than 93% of the data within 25% deviation. The correlation obtained for R134a-DMF boiling with 25% error band is given by:
0:188 Nu ¼ 3:848 104 Retp ð1=Xtt Þ0:116 ðBoÞ0:469
(18)
The above correlation is valid in the range of Retp ¼ 8100 to 27,800; 1=ctt ¼ 0.95 to 14.54; Bo ¼ 4.05 104 to 3.4 103. 450 Nu = 3.848 x 104 (Retp) -0.188 (1/Xtt)-0.116(Bo)0.469
400 Predicted Nusselt number
-2
Friction factor x 10
2.7
13000
Two phase Reynolds number
Driving temperature ratio = 3.60 Driving temperature ratio = 3.70 Driving temperature ratio = 4.05
2.9
2.5
2.8
Driving pressure ratio = 1.72
0.79 23000
18000
Two phase Reynolds number
3.1
3.2
Driving pressure ratio = 2.03
13000
3.3
Hot water Reynolds number = 4700 Driving temperature ratio = 3.18
-2
0.85
Friction factor x 10
Hot water Reynolds number = 4400 Driving temperature ratio = 3.02
Mass transfer effectiveness
Heat transfer effectiveness
0.8
+25%
350 -25%
300 250 200 150 100
13000
18000
23000
28000
Two phase Reynolds number
50 75
125
175
225
275
325
375
Experimental Nusselt number Fig. 12. Effect of solution Reynolds number on friction factor at different driving temperature ratios.
Fig. 14. Comparison of predicted against experimental Nusselt number.
P. Balamurugan, A. Mani / International Journal of Thermal Sciences 61 (2012) 118e128
127
7. Correlation for Sherwood number
8. Conclusions
Based on the above experiments, a correlation for Sherwood number has been developed as a function of two phase Reynolds number, Schmidt number, concentration gradient and heat flux.
The influence of solution two phase Reynolds number, driving temperature ratio, driving pressure ratio and initial R134a concentration on the performance of tubular generator is studied experimentally with R134a-DMF as working fluid. Correlations for Nusselt number and Sherwood number have been proposed from the present experimental studies with a maximum deviation of 25% and 15% respectively. The results showed that desorption ratio, Sherwood number, Nusselt number increase with driving temperature ratio, R134a initial concentration, and two phase Reynolds number but decrease with increase in driving pressure ratio. Driving temperature ratio varied from 2.5 to 4.2, whereas driving pressure ratio varied from 1.7 to 3.1. However the increase in the heat and mass transfer rates are not infinite with respect to the temperature and pressure ratios. Heat and mass transfer effectiveness of generator decrease two to three times faster with increase in solution Reynolds number than the rate of increase with increase in driving temperature ratio. Heat transfer effectiveness increases with increase in solution initial concentration. However mass transfer effectiveness decreases with increase in solution concentration. The two phase friction factor varied from 0.02 to 0.035 and it decreases with decrease in driving pressure ratio and with increase in the Reynolds number and driving temperature ratio.
e Sh ¼ D Retp ðScl Þf ðXsw Þg ðqÞj
(19)
where
Sh ¼ convective mass transfer=diffusive transfer ¼
MD2i rl Dc;l (20)
Retp ¼ Inertial force/Viscous force ¼ Rel
Rel ¼
4ml
pDi ml
and F ¼ 2:35
1
ctt
F 1:25 ,
where
0:736 þ 0:213
(21)
Scl ¼ Momentum diffusivity=Mass diffusivity ¼
ml rl Dc;l
(22)
Xsw ¼ Concentration difference between solutions at entry and exit ¼ Xss Xws
(23) References
Generic form of the correlation includes convective mass transfer, diffusive mass transfer, inertial force, viscous force, momentum diffusivity, mass diffusivity, desorption potential and heat source. Fig. 15 shows the comparison of predicted Sherwood number with the experimental Sherwood number from the present study. The regression technique using Generalized Autoregressive Conditional Heteroskedasticity (GARCH) tool box in MATLAB is employed to determine the coefficients in the generic form of the correlations. The following correlation has been developed with R134a-DMF mixture with 15% error band.
1:079 Sh ¼ 4:819 106 Retp ðScL Þ0:242 ðXsw Þ0:884 ðqÞ1:586 (24) The above correlation is valid in the range for Retp ¼ 8160 to 27,810; ScL ¼ 30 to 42; Xsw ¼ 0.07e0.4; q ¼ 2e20.
Predicted Sherwood number
12000 +15%
Sh = 4.819 x 10 6 (Retp) -1.079 (ScL) -0.242 (Xsw)
-0.884
(q)
1.586
-15%
8000
4000
0 0
4000
8000
12000
Experimental Sherwood number Fig. 15. Comparison of predicted against experimental Sherwood number.
[1] M. Fatouh, S. Srinivasa Murthy, Performance of an HCFC22 based vapour absorption refrigeration system, Int. J. Refrig. 18 (1995) 465e476. [2] M. Fatouh, S. Srinivasa Murthy, Comparison of R22-absorbent pairs for absorption cooling based on P-T-X data, J. Renew. Energy 3 (1993b) 31e37. [3] M. Suresh, A. Mani, Heat and mass transfer studies on R134a/DMF solution based on phenomenological theory, Int. J. Heat Mass Transf. 53 (2010) 2813e2825. [4] M. Fatouh, Studies on HCFC based vapour absorption refrigeration systems suitable for low potential heat sources, PhD thesis, Indian Institute of Technology Madras, 1994. [5] G. Karthikeyan, A. Mani, S. Srinivasa Murthy, Performance of different working fluids in transfer-tank operated vapour absorption refrigeration systems, Renew. Energy 6 (1995) 835e842. [6] K.S. Sujatha, A. Mani, S. Srinivasa Murthy, Analysis of a bubble absorber working with R22 and five organic absorbents, Heat Mass Transf. 32 (1997) 255e259. [7] Y. Nezu, N. Hisada, T. Ishiyama, K. Watanabe, Thermodynamic properties of working-fluid pairs with R-134a for absorption refrigeration system, in: Natural Working-Fluids, IIR Gustav Lorentzen Conf. 5th, China, Sept. 17e20, 2002, pp. 446e453. [8] A. Yokozeki, Theoretical performances of various refrigeranteabsorbent pairs in a vapour absorption refrigeration cycle by the use of equation of state, Appl. Energy 80 (2005) 383e399. [9] A. Mani, Studies on Compact Bubble Absorber of the Vapour Absorption Refrigeration System, a Report Submitted Department of Science and Technology, Government of India, 2009. [10] D.L. Bennett, J.C. Chen, Forced convective boiling in vertical tubes for saturated pure components and binary mixtures, AIChE J. 26 (1980) 454e461. [11] W.W.S. Charters, V.R. Megler, W.D. Chen, Y.F. Wang, Atmospheric and subatmospheric boiling of H2O and LiBreH2O solutions, Int. J. Refrig. 5 (1982) 107e114. [12] A. Matsuda, K.H. Choi, K. Hada, T. Kawamura, Effect of pressure and concentration on performance of a vertical falling-film type of absorber and generator using lithium bromide aqueous solutions, Int. J. Refrig. 17 (1994) 538e542. [13] H.K. Varma, R.K. Mehrotra, K.N. Agrawal, Heat transfer during pool boiling of LiBr-water solutions at sub atmospheric pressures, Int. Commun. Heat Mass Transf. 21 (1994) 539e548. [14] W. Rivera, R. Best, Boiling heat transfer coefficients inside a vertical smooth tube for watereammonia and ammoniaelithium nitrate mixtures, Int. J. Heat Mass Transf. 42 (1999) 905e921. [15] W. Rivera, V. Velez, A. Xicale, Heat transfer coefficients in two-phase flow for mixtures used in solar absorption refrigeration systems, Sol. Energy Mater. & Sol. Cells 63 (2000) 401e411. [16] W. Rivera, A. Xicale, Heat transfer coefficients in two phase flow for the waterelithium bromide mixture used in solar absorption refrigeration systems, Sol. Energy Mater. & Sol. Cells 70 (2001) 309e320.
128
P. Balamurugan, A. Mani / International Journal of Thermal Sciences 61 (2012) 118e128
[17] T. Inoue, M. Monde, Y. Teruya, Pool boiling heat transfer in binary mixtures of ammoniaewater, Int. J. Heat Mass Transf. 45 (2002) 4409e4415. [18] H. Arima, M. Monde, Y. Mitsutake, Heat transfer in pool boiling of ammoniaewater mixture, Heat Mass Transf. 39 (2003) 535e543. [19] L. Roriz, A. Mortal, L.F. Mendes, in: Study of a Plate Heat Exchanger Desorber With a Spray Column for a Small Solar Powered Absorption Machine, 3rd Int. Conf. Heat Powered Cycles, Cyprus, 2004. [20] T. Khir, R.K. Rahim, N. Ghaffour, A. Ben Brahim, Experimental study on forced convective boiling of ammoniaewater mixtures in a vertical smooth tube, Arab. J. Sci. Eng. 30 (2005) 47e63. [21] M.P. Mishra, H.K. Varma, C.P. Sharma, Heat transfer coefficients in forced convective evaporation of refrigerant mixtures, Lett. Heat Mass Transf. 8 (1981) 127e136. [22] F. Taboas, M. Valles, M. Bouruois, A. Coronas, Pool boiling of ammoniaewater and its pure components: comparison of experimental data in the literature with predictions of standard correlations, Int. J. Refrig. 30 (2007) 778e788.
[23] F. Taboas, M. Valles, M. Bouruois, A. Coronas, Flow boiling heat transfer of ammoniaewater mixture in a plate heat exchanger, Int. J. Refrig. 33 (2010) 695e705. [24] P. Balamurugan, A. Mani, Heat and Mass Transfer Studies on Vertical Tubular Generator in R134a-DMF Absorption Refrigeration System, Int. Symp. Innov. Mater. Process. Energy Syst., Singapore, Nov 30eDec 1, 2010, 335e342. [25] P. Balamurugan, A. Mani, Numerical studies on vertical tubular generator in vapour absorption refrigeration system, Int. J. Air Cond. Refrig. 19 (2011) 121e129. [26] R.C. Reid, J.M. Prausnitz, B.E. Poling, The Properties of Gases and Liquids, fourth ed., McGraw-Hill Book Company, New York, 1989. [27] Y.Y. Yan, T.F. Lin, Evaporation heat transfer and pressure drop of refrigerant R134a in a plate heat exchanger, J. Heat Transf. 121 (1999) 118e127. [28] F.P. Incropera, D.P. DeWitt, T.L. Bergman, A.S. Lavine, Fundamentals of Heat and Mass Transfer, sixth ed., John Wiley & Sons, USA, 2007. [29] S.J. Kline, F.A. McClintock, Describing uncertainties in single sample experiments, J. Mech. Eng. 75 (1953) 3e12.