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Experimental studies on heat and mass transfer characteristics for R134a–DMF bubble absorber
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Experimental studies on heat and mass transfer characteristics for R134aeDMF bubble absorber M. Suresh, A. Mani* Refrigeration & Airconditioning Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai, Tamilnadu 600036, India
article info
abstract
Article history:
Experimental investigations have been carried out to study heat and mass transfer char-
Received 5 January 2011
acteristics of Tetrafluoro ethane (R134a) in Dimethyl formamide (DMF) in a glass absorber.
Received in revised form
Effects of operational parameters viz., gas flow rate, solution flow rate, solution initial
1 December 2011
concentration, solution pressure, solution temperature and cooling water flow rate on
Accepted 7 January 2012
absorber performance are analyzed. Absorption rate and heat transfer rate increase as the
Available online 24 January 2012
gas flow rate, solution flow rate, cooling water flow rate and solution pressure increase whereas they decrease as the solution initial concentration and solution temperature
Keywords:
increase. Heat and mass transfer rates determined from the experiments are compared
Absorption
with numerical model and it is found that agreement is good. A correlation for mass
R134a
transfer coefficient is presented from the experimental studies within 20% error band. ª 2012 Elsevier Ltd and IIR. All rights reserved.
DMF Bubble Mass transfer Heat transfer
Etudes expe´rimentales sur les caracte´ristiques de transfert de chaleur et de masse d’un absorbeur a` bulles au R134a / DMF Mots cle´s : Absorption ; R134a ; DMF ; Bulle ; Transfert de masse ; Transfert de chaleur
1.
Introduction
Many environment friendly fluid combinations have been suggested by number of investigators in order to overcome some of the limitations of well known working pairs viz., ammoniaewater and lithium bromideewater for the vapor absorption refrigeration systems (VARS). Though R22-organic solvent based absorption refrigeration systems have been
extensively studied by Fatouh (1994), Karthikeyan (1995) and Sujatha (1997), HCFCs along with CFCs, are also covered by Montreal and other International Protocols and are being phased out. So environment friendly R134a based VARS are being investigated. Nezu et al. (2002) examined the possibility of testing R134a as a refrigerant in VARS with various organic solvents and showed that the R134aeDMA and the R134aeDMF systems are considered attractive as the working-
* Corresponding author. Tel.: þ91 44 22574666; fax: þ91 44 22570509. E-mail address: [email protected] (A. Mani). 0140-7007/$ e see front matter ª 2012 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2012.01.011
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 1 0 4 e1 1 1 4
Nomenclature A D Dc h m M p p0 Q Re Sc Sh T Uo V X
2
absorber cross-section area, m absorber diameter, m diffusion coefficient, m2 s1 solution enthalpy, kJ kg1 mass flow rate, kg s1 mass transfer coefficient, kg m3 s1 solution pressure, bar atmospheric pressure, bar heat transfer rate, W Reynolds number Schmidt number Sherwood number temperature, C overall heat transfer coefficient, W m2 K1 volumetric flow rate, m3 s1 liquid mass fraction, kg kg1
fluid pairs for the absorption refrigeration system than other R134aeabsorbent systems. Yokozeki (2005) studied theoretical performance of various refrigeranteabsorbent pairs in a VARS cycle by the use of equations of state. Of these, R134aeDMF and R134aeDMA systems exhibit better performance, compared to other R134aeabsorbent systems. Also the circulation ratio is less and COP is more for R134aeDMF system compared to R134aeDMA system. Mani (2009) carried out experimental studies on R134a/DMF based compact vapor 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 80e90 C and with cooling water as coolant for absorber and condenser in the temperature range of 20e35 C. Absorber is considered as one of the crucial components in the vapor absorption refrigeration system. Kang et al. (2000) carried out analytical investigation of falling film and bubble type absorbers and found that absorption rate of the bubble type absorber was found to be always higher than that of the falling film mode. Bubble type absorber provides better heat and mass transfer coefficients, also good wettability and mixing between the liquid and the vapor (Kang et al., 2000). Absorption process is characterized by simultaneous heat and mass transfer phenomena. These mechanisms, though complicated, influence the system performance significantly. Elperin and Fominykh (2003) studied combined heat and mass transfer mechanisms at all stages of bubble growth and rise in a bubble absorber, which can be useful in design calculations of gaseliquid absorbers. Lee et al. (2003) performed both the numerical and experimental analyses in the absorption process of a bubble absorber. Numerical model in these studies can be used for the optimum design of absorber. Merrill and Perez-Blanco (1997) developed an analytical model to predict bubble dynamics in binary sub-cooled solutions. This model improves the understanding of bubble absorption dynamics.
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Subscripts a absorption eq equilibrium g gas ht heat transfer i inlet l liquid mt mass transfer o outlet s solution v volumetric w water 1 inner 2 outer Greek symbols m dynamic viscosity, Pa s g kinematic viscosity, m2 s1 r density, kg m3 h mass transfer efficiency
Sujatha et al. (1997a,b) carried out numerical analysis in a vertical tubular bubble absorber working with R22 as refrigerant and five organic fluids namely DMF, DMA, DMETEG, DMEDEG and NMP as absorbents. This model is validated by comparing with the results available in the literature. Based on these results, a correlation for mass transfer coefficient has been suggested for the vertical tubular bubble absorber. Sujatha et al. (1999) have also carried out experimental studies on a vertical tubular bubble absorber working with R22-DMF. Experimental pressure drop, heat transfer coefficient and mass transfer coefficient are compared with the results obtained from the numerical model. Kang et al. (1998) developed a model for a bubble absorber with a plate type heat exchanger by considering the combined heat and mass transfer analysis in both liquid and vapor regions. All geometric variables such as distance between two plates, number of plates, and width of the plates could be selected optimally for given thermal conditions by the developed design model for ammoniaewater combination. Staicovici (2000a,b,c) used non-equilibrium phenomenological theory to evaluate gaseliquid interaction. Design of bubble absorber, based on non-equilibrium thermodynamics could be suited to a modern compact plate type construction and offers better absorption efficiency and minimum pressure loss on the gas side. Suresh and Mani (2010) developed a numerical model on bubble dynamics, heat and mass characteristics of R134a/DMF based bubble absorber using phenomenological theory and validated by comparing with the results available in literature. Suresh and Mani also carried out experimental studies on bubble characteristics on a vertical glass bubble absorber and presented a correlation for bubble diameter during detachment. Kang et al. (2002a) developed a correlation for initial bubble diameter, which can be used to calculate the interfacial area in the design of ammoniaewater bubble absorber. Kang et al. (2002b) also developed an experimental correlation of mass transfer coefficient for ammoniaewater bubble absorption.
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Cerezo et al. (2009) carried out experimental studies using a plate heat exchanger as absorber and ammoniaewater as working fluid. They concluded that increase in pressure, solution and cooling flow rates positively affects the absorber performance and increase in the concentration, cooling and solution temperature negatively affects the absorber performance. Present experimental work is carried out to study heat and mass transfer characteristics and effects of operational parameters viz., gas flow rate, solution flow rate, solution initial concentration, solution pressure, solution temperature and cooling water flow rate on the performance of R134aeDMF bubble absorber.
2.
the bottom of inner tube by a solution pump. R134a gas is supplied from a storage cylinder through a mass flow controller unit and injected through a nozzle installed at the bottom of inner tube. Strong DMF solution is collected in the strong solution tank at the top of the absorber. Cooling water is supplied, by cooling water simulator, through the absorber annulus in counter flow direction to the solution and gas. Cooling water simulator consists of a R22 based vapour compression refrigeration (VCR) circuit of 3.4 TR capacity, 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 a cooling coil. This system also includes various measuring devices such as temperature sensors, pressure sensors, flow meters, mass flow controller and online density meter fitted at suitable locations as shown in Fig. 1. All these measuring instruments are pre-calibrated. Twelve numbers of calibrated coppereconstantan thermocouples are used as temperature sensors with a measurement uncertainty up to 0.78%. Three numbers of piezo-electric type pressure transducers are used as pressure sensors with a measurement uncertainty up to
Experimental setup
Fig. 1 shows the schematic diagram of experimental setup. This consists of a glass bubble absorber, strong and weak solution tanks, solution pump, cooling water simulator, instrumentation and valves. Tables 1 and 2 show the specifications of main components and instruments in the setup, respectively. Glass absorber consists of two concentric tubes. DMF solution is pumped from weak solution tank, through R134a gas collection tank
NRV1
HSV4
Ice box
P3 T4
Strong solution tank
L
HSV10
T3
HSV11
Glass absorber
T5 T6
Online density meter
NV1
Solution charging line
L
HSV6
T7
Video camera recorder
T8
HSV5
GV2
GV1
T9
HSV9
HSV8
Weak solution tank
High speed Water camera pump
T10
P2
T11
T2
S2
HSV2 S1
T12
HSV3
Gas charging line
HSV7
Solution pump
HSV1 M
Computer with Data Acquisition system
P1
T1
Pressure regulating valve
R134a gas cylinder
Hand shut-off valve Non-Return valve
R134a-DMF Solution line
Gate valve
R134a refrigerant line Water line
Cooling water thermostat
Needle valve S Flow meter Mass flow controller unit L Level gauge
M
T P
Temperature gauge Pressure gauge
Fig. 1 e Schematic diagram of glass bubble absorber experimental setup.
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Table 1 e Specifications of main components.
Table 2 e Specifications of main instruments.
Glass absorber Type
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)
Tube in tube
Fluid circuit
Tube side: solution Annulus: cooling water
Material Inside tube Outside tube Tube length
Glass ID: 33 mm, OD: 37 mm ID: 46 mm, OD: 50 mm 1000 mm
Nozzle Type Material Inner diameter Outer diameter
Tube Copper 2.2 mm 4.0 mm
Solution pump Type Capacity Discharge pressure Plunger diameter SPM
Metering pump 50 lph 40 bar 25 mm 100
4.55%. Glass rotameters are used to measure the flow of solution and cooling water with a measurement uncertainty up to 4.8%. Mass flow controller unit is used to measure the volume flow rate of gas with a measurement uncertainty of 1.25%. 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 using HBT (HankinsoneBrobsteThomson) equation used by Reid et al. (1989). Readings from all these instruments and sensors are monitored continuously by connecting them to a data acquisition system and a computer. An uncertainty analysis associated with various measured and derived quantities has been discussed in Appendix B.
3.
Mass flow controller Flow rate Response time Accuracy Linearity Repeatability Control valve type Input power
6.6 mm 500 mm 0e3 g m3 5 105 g m3 2 104 g m3, 0.1% 0e100 C 0e50 bar 100e500 l/h
Power consumption Input command signal Output indication
10 ccpm to 2500 ccpm <2 s within 2% of set point Within 0.05% of full scale Within 0.5% of full scale Within 0.2% of full scale Normally closed solenoid þ15 V DC at 25 mA to 15 V DC at 180 mA 31 W (Max.) 0e5 V DC 0e5 V DC
Pressure transducers Type Measuring range Temperature Input Output Accuracy Response time Material
Piezoresistive 0e10 bar absolute 20 to 80 C 8e28 V DC 0e5 V DC (3 wires) 1% of full scale Maximum: 250 ms SS316L
Flow meters Type Flow rate Operating pressure End fitting material Float material Tube material Tube taper length Accuracy
Glass rotameter Minimum: 0.2 lpm, Maximum: 2.0 lpm 4 bar (abs.) SS316 SS316 Borosilicate glass 250 mm 1% on F.S.D
Experimental procedure
Initially DMF solution is charged into the weak solution tank. Then it is pumped through inner tube of glass absorber by solution pump. Solution is collected in the strong solution tank at the top of the absorber. Solution is returned to the weak solution tank through a needle valve after each set of experimentation. Initial concentration of the solution at the absorber inlet is measured by online density meter. Solution inlet pressure and temperature are monitored continuously and kept constant. Cooling water is allowed at a constant flow rate, through outer tube of the glass absorber, counter flow to the solution. Then R134a gas is injected from a storage cylinder through the nozzle at the bottom of the absorber. Gas flow rate is measured by mass flow controller unit. Gas temperature is maintained constant. During this process, by keeping solution flow rate constant, gas flow rate is varied and measured using DC power supply unit connected to mass flow controller. All the parameters viz., solution inlet and outlet pressure, temperature and concentration, solution flow
rate, gas flow rate, gas pressure and temperature, cooling water flow rate, inlet and outlet temperature are monitored and recorded in the computer using data acquisition unit. These experiments are repeated for various solution flow rates. In the next run of experiment, by keeping solution, gas and water flow rates constant, the solution initial concentration is increased by injecting R134a gas through a charging line in weak solution tank and monitored by online density meter. All readings are monitored and noted. These experiments are repeated for various solution pressures and solution initial concentrations. In another run, solution inlet temperature is varied by varying cooling water temperature and flow rate whereas all other parameters are kept constant. All readings are monitored and noted. The entire test facility (especially solution tanks) could not be maintained at steady state conditions since it is a once through a system. However, absorption process in the glass
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absorber was maintained at steady state by the following steps:
80 Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution flow rate = 0.025 m h Solution inlet pressure = 120 kPa Solution inlet temperature = 30 ºC Solution inlet concentration = 0.01 kgkg Cooling water flow rate = 0.05 m h
4.
Results and discussion
Experimentation was conducted in glass absorber by varying the operational parameters viz., gas flow rate from 0.03 m3 h1 to 0.15 m3 h1, solution flow rate from 0.025 m3 h1 to 0.05 m3 h1, solution pressure from 1.2 bar to 4 bar, solution initial concentration from 0.01 kg kg1 to 0.2 kg kg1, solution temperature from 20 C to 30 C and cooling water flow rate from 0.05 m3 h1 to 0.075 m3 h1. Experimental results are compared with the numerical model for the bubble absorber developed earlier by the authors (Suresh and Mani, 2010) to study heat and mass transfer characteristics using phenomenological theory. Experimental values of absorption rate, heat transfer rate, volumetric mass transfer coefficient and overall heat transfer coefficient are calculated using the equations given in Appendix A. Fig. 2 shows the variation of solution temperature along the height of absorber for various gas flow rates. The temperature increases to a maximum in the nozzle region, then drops along the height. This indicates that absorption is more in the nozzle region. As the gas flow rate increases, the solution temperature increases along the absorber height due to increase in absorption rate. Fig. 3 shows the variation of solution temperature along the height of absorber for various solution flow rates. As the solution flow rate increases, the solution temperature decreases along the absorber height due to the availability of more amount of solution for absorption. Fig. 4 shows the variation of solution temperature along the height of absorber for various solution initial concentrations. As the solution initial concentration increases, the solution temperature decreases along the absorber height due to decrease in absorption rate. Fig. 5 compares the experimental absorption rate with that of numerical model for various gas flow rates, solution flow rates, pressures, temperatures and initial concentrations. The agreement is good within a maximum deviation of 10%.
60
50
40
30 Gas flow rate = 0.03 m h
20
Gas flow rate = 0.09 m h Gas flow rate = 0.15 m h
10 0
0.2
0.4
0.6
0.8
1
1.2
Absorber height, m
Fig. 2 e Effect of gas flow rate on solution temperature profile along absorber height.
Fig. 6 compares the experimental heat transfer rate with that of numerical model for various gas flow rates, solution flow rates, pressures, temperatures and initial concentrations. The agreement is fair within a maximum deviation of 20%. Fig. 7 depicts variation of absorption rate with gas flow rate for various solution and cooling water flow rates. As gas flow rate increases, amount of gas absorbed in the solution is more, resulting in high absorption rates. Also, at high solution flow rates, quantity of solution available for gas absorption is more, resulting in increase in absorption rate. At high cooling water flow rates, heat rejected from solution is more, resulting in high absorption rates. Fig. 8 depicts variation of heat transfer rate with gas flow rate for various solution and cooling water flow rates. As gas and solution flow rates increase, heat transfer rate also increases due to high absorption rates. At high cooling water flow rates, heat rejected from solution is more, resulting in high heat transfer rates. Fig. 9 shows the variation of solution outlet concentration with gas flow rate
80 Gas flow rate = 0.09 m h Gas inlet pressure = 650 kPa Gas inlet temperature = 32°C Solution inlet pressure = 120 kPa Solution inlet temperature = 30 °C Solution inlet concentration = 0.01 kgkg Cooling water flow rate = 0.05 m3h
70
Solution temperature, °C
i. Refrigerant gas from a large gas cylinder at high pressure was injected into the inner tube of glass absorber through a nozzle. Flow rate of gas is metered through a mass flow controller unit. So the operating parameters viz. gas flow rate, pressure and temperature were kept constant at absorber inlet. ii. Solution (Dimethyl formamide) was pumped through the absorber inner tube using a diaphragm pump by maintaining constant flow rate, pressure and temperature at absorber inlet. iii. Cooling water was circulated through the outer tube of absorber inlet at constant flow rate and at constant pressure and temperature. iv. Gas injection was continued and flow rate, pressure and temperature of solution and gas at absorber inlet were monitored continuously by connecting respective instruments and sensors to a data acquisition system and a computer. When all these readings remain constant during a time period, they were recorded in the computer.
Solution temperature, °C
70
60
50
40
30 Solution flow rate = 0.025 m h
20
Solution flow rate = 0.0375 m h Solution flow rate = 0.05 m h
10 0
0.2
0.4
0.6
0.8
1
1.2
Absorber height, m
Fig. 3 e Effect of solution flow rate on solution temperature profile along absorber height.
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300
80 Gas inlet pressure = 650 kPa Gas inlet temperature = 32 °C Solution flow rate = 0.05 m3h-1 Solution inlet pressure = 200 kPa Solution inlet temperature = 30 °C Cooling water flow rate = 0.05 m3h-1
70
Solution temperature, °C
Experimental heat transfer rate, W
Gas flow rate = 0.12 m3h-1
60
50
40
30 Solution inlet concentration = 0.01 kgkg
20
250 +20 % 200
150 -20 % 100
50
0
Solution inlet concentration = 0.1 kgkg
0
50
Solution inlet concentration = 0.2 kgkg
100
150
200
250
300
Heat transfer rate - Numerical model, W
10 0
0.2
0.4
0.6
0.8
1
1.2
Absorber height, m
Fig. 4 e Effect of solution initial concentration on solution temperature profile along absorber height.
for various solution flow rates. Solution concentration increases as gas flow rate increases due to higher absorption rates. However at higher solution flow rates, the concentration decreases due to dilution. Fig. 10 shows that as the gas and solution flow rates increase, solution pressure drop across the absorber increases due to increase in flow velocity and friction. Fig. 11 shows that mass transfer efficiency decreases as the gas flow rate increases and increases as the solution flow rate increases. At lower gas flow rates and higher solution flow rates, the solution outlet concentration approaches closer to equilibrium concentration resulting in higher mass transfer efficiency whereas at higher gas flow rates and lower solution flow rates, the solution outlet concentration is far away from equilibrium concentration resulting in lower mass transfer efficiency. Fig. 12 depicts variation of volumetric mass transfer coefficient with gas flow rate at various solution and cooling water flow rates, and overall heat transfer coefficient with gas flow rate at various solution flow rates. It increases as the gas, solution and cooling water flow rates increase due to increase in absorption rates. Fig. 13 depicts variation of overall heat transfer coefficient with gas flow rate
Fig. 6 e Variation of heat transfer rate based on experiments with heat transfer rate based on numerical model.
at various solution and cooling water flow rates. It increases marginally as gas and solution flow rates increase due to marginal increase in heat transfer rate. At high gas flow rates, heat and mass transfer efficiencies become lower due to small volume of mixing zone in test section. Due to this, the rate of increase in absorption and heat transfer slows down. So overall heat transfer coefficient tends to flatten at higher gas flow rates. It increases as cooling water flow rate increases. At high cooling water flow rates, water side heat transfer coefficient increases due to increase in flow velocity. This results in high overall heat transfer coefficient. Fig. 14 depicts variation of absorption rate with solution pressure for various solution inlet concentrations. Absorption rate increases marginally with solution pressure due to marginal increase in bubble life at high pressures. Fig. 15 depicts variation of heat transfer rate with solution pressure for various solution inlet concentrations. Heat transfer rate does not vary much with respect to solution pressure due to marginal increase in absorption rate. Figs. 16 and 17 depict respectively the variation of volumetric mass transfer coefficient and overall heat transfer coefficient with respect to
Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution inlet pressure = 120 kPa Solution inlet temperature = 30 ºC Solution inlet concentration = 0.01 kgkg
1
Absorption rate, x 10 -3 kgs-1
Experimental absorption rate, x 10-3 kgs-1
1.2
1.5
1.25 +10 % 1
0.75 -10 % 0.5
0.25
0.8
0.6
0.4
---- Cooling water flow rate = 0.05 m h Cooling water flow rate = 0.075 m h
0.2
Solution flow rate = 0.025 m h Solution flow rate = 0.0375 m h Solution flow rate = 0.05 m h
0
0
0
0.25
0.5
0.75
1
1.25 -3
1.5
-1
Absorption rate - Numerical model, x 10 kgs
Fig. 5 e Variation of absorption rate based on experiments with absorption rate based on numerical model.
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
3 -1
Gas flow rate, m h
Fig. 7 e Effect of gas flow rate on absorption rate for different solution and cooling water flow rates.
0.16
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100
250 Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution inlet pressure = 120 kPa Solution inlet temperature = 30 ºC Solution inlet concentration = 0.01 kgkg
90
Mass transfer efficiency, %
Heat transfer rate, W
200
150
100 ---- Cooling water flow rate = 0.05 m h Cooling water flow rate = 0.075 m h
50
Solution flow rate = 0.025 m h
80
70 Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution inlet pressure = 120 kPa Solution inlet temperature = 30 ºC Solution inlet concentration = 0.01 kgkg Cooling water flow rate = 0.05 m h
60
50
Solution flow rate = 0.0375 m h
Solution flow rate = 0.025 m h Solution flow rate = 0.0375 m h Solution flow rate = 0.05 m h
Solution flow rate = 0.05 m h
40
0 0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0
0.16
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
3 -1
Gas flow rate, m h
3 -1
Gas flow rate, m h
Fig. 11 e Effect of gas flow rate on mass transfer efficiency for different solution flow rates.
Fig. 8 e Effect of gas flow rate on heat transfer rate for different solution and cooling water flow rates.
Volumetric mass transfer coefficient, kgm-3s-1
12
Solution outlet concentration, kgkg -1
0.14 Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution inlet pressure = 120 kPa Solution inlet temperature = 30 ºC Solution inlet concentration = 0.01 kgkg Cooling water flow rate = 0.05 m h
0.12 0.1 0.08 0.06 0.04
Solution flow rate = 0.025 m h
0.02
Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution inlet pressure = 120 kPa Solution inlet temperature = 30 ºC Solution inlet concentration = 0.01 kgkg
10
8 ---- Cooling water flow rate = 0.05 m h Cooling water flow rate = 0.075 m h
6
4
Solution flow rate = 0.025 m h
2
Solution flow rate = 0.0375 m h Solution flow rate = 0.05 m h
0
Solution flow rate = 0.0375 m h
0
Solution flow rate = 0.05 m h
0.02
0.04
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.06
0.08
0.1
0.12
0.14
0.16
3 -1
Gas flow rate, m h
0 0.16
3 -1
Gas flow rate, m h
Fig. 9 e Effect of gas flow rate on solution outlet concentration for different solution flow rates.
Fig. 12 e Effect of gas flow rate on volumetric mass transfer coefficient for different solution and cooling water flow rates.
Overall heat transfer coefficient, Wm -2K-1
225
Solution pressure drop across absorber, Pa
25 Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution inlet pressure = 120 kPa Solution inlet temperature = 30 ºC Solution inlet concentration = 0.01 kgkg Cooling water flow rate = 0.05 m h
20
15
10
5
Solution flow rate = 0.025 m h
---- Cooling water flow rate = 0.05 m h Cooling water flow rate = 0.075 m h
200
175
Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution inlet pressure = 120 kPa Solution inlet temperature = 30 ºC Solution inlet concentration = 0.01 kgkg
150
Solution flow rate = 0.025 m h Solution flow rate = 0.0375 m h Solution flow rate = 0.05 m h
Solution flow rate = 0.0375 m h
125
Solution flow rate = 0.05 m h
0
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
3 -1
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
Gas flow rate, m h
3 -1
Gas flow rate, m h
Fig. 10 e Effect of gas flow rate on solution pressure drop across absorber for different solution flow rates.
Fig. 13 e Effect of gas flow rate on overall heat transfer coefficient for different solution and cooling water flow rates.
0.16
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165.5
0.95
Gas flow rate = 0.12 m h Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution flow rate = 0.05 m h Solution inlet temperature = 30 ºC Cooling water flow rate = 0.05 m h
Overall heat transfer coefficient, Wm -2K-1
Absorption rate, x 10 -3 kgs-1
1
0.9
0.85
Solution inlet concentration = 0.01 kgkg Solution inlet concentration = 0.1 kgkg Solution inlet concentration = 0.2 kgkg
200
250
300
350
400
163.5 Solution inlet concentration = 0.01 kgkg Solution inlet concentration = 0.1 kgkg Solution inlet concentration = 0.2 kgkg
162.5 150
0.8 150
164.5
450
Gas flow rate = 0.12 m h Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution flow rate = 0.05 m h Solution inlet temperature = 30 ºC Cooling water flow rate = 0.05 m h
200
Solution pressure, kPa
190
Heat transfer rate, W
170
Gas flow rate = 0.12 m h Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution flow rate = 0.05 m h Solution inlet temperature = 30 ºC Cooling water flow rate = 0.05 m h
160 150 140 Solution inlet concentration = 0.01 kgkg Solution inlet concentration = 0.1 kgkg
130
Solution inlet concentration = 0.2 kgkg
120 150
200
250
300
350
400
450
Solution pressure, kPa
Fig. 15 e Effect of solution pressure on heat transfer rate for different solution inlet concentrations.
Gas flow rate = 0.12 m h Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution flow rate = 0.05 m h Solution inlet temperature = 30 ºC Cooling water flow rate = 0.05 m h
12
Solution inlet concentration = 0.2 kgkg
450
solution pressure for various solution inlet concentrations. Mass transfer coefficient decreases as solution pressure increases and increases as solution initial concentration increases. Though absorption rate is almost constant with respect to solution pressure and solution initial concentration, the log mean concentration difference (LMCD) increases as solution pressure increases resulting in lower mass transfer coefficients and LMCD decreases as solution inlet concentration increases resulting in higher mass transfer coefficients. Heat transfer coefficient does not vary much with respect to solution pressure and solution initial concentration. Fig. 18 shows variation of solution outlet concentration, mass transfer efficiency and heat transfer efficiency with solution inlet temperature. At high solution inlet temperatures, bubble life is shorter because bubble interface reaches the equilibrium temperature faster. This results in lower absorption rates. Due to this, solution outlet concentration decreases. This also leads to decrease in mass transfer efficiency. Due to reduction in absorption rate, heat transfer rate also decreases resulting in reduction in heat transfer efficiency. Fig. 19 depicts the variation of volumetric mass transfer coefficient and overall heat transfer coefficient with respect to solution inlet temperature. Mass transfer 100
9
6
3
0 150
400
Fig. 17 e Effect of solution pressure on overall heat transfer coefficient for different solution inlet concentrations.
Heat and mass transfer efficiency, %
Solution inlet concentration = 0.1 kgkg
350
0.1
90 0.08 80 0.06 Gas flow rate = 0.12 m h Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution flow rate = 0.05 m h Solution inlet pressure = 120 kPa Solution inlet concentration = 0.01 kgkg Cooling water flow rate = 0.05 m h
70
60
0.04 Heat transfer efficiency Mass transfer efficiency
Solution outlet concentration, kgkg -1
Volumetric mass transfer coefficient, kgm -3s-1
15 Solution inlet concentration = 0.01 kgkg
300
Solution pressure, kPa
Fig. 14 e Effect of solution pressure on absorption rate for different solution inlet concentrations.
180
250
Solution outlet concentration
200
250
300
350
400
450
Solution pressure, kPa
50
0.02 15
20
25
30
35
o
Fig. 16 e Effect of solution pressure on volumetric mass transfer coefficient for different solution inlet concentrations.
Solution inlet temperature, C
Fig. 18 e Effect of solution inlet temperature on heat and mass transfer efficiency and solution outlet concentration.
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165
8
164
6
163
Gas flow rate = 0.12 m h Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution flow rate = 0.05 m h Solution inlet pressure = 120 kPa Solution inlet concentration = 0.01 kgkg Cooling water flow rate = 0.05 m h
4
2
162
161 Volumetric mass transfer coefficient Overall heat transfer coefficient
0 15
20
25
2500
Experimental Sherwood number
10
Overall heat transfer coefficient, Wm -2K-1
-3 -1 Volumetric mass transfer coefficient, kgm s
1112
1500 -20%
1000
500 Sh = 1.7 × 10 (Re
)
(Sc )
(xgl)
( )
0 0
160 35
30
+20%
2000
500
1000
1500
2000
2500
Correlation - Sherwood number
o
Solution inlet temperature, C
Fig. 19 e Effect of solution inlet temperature on volumetric mass transfer coefficient and overall heat transfer coefficient.
coefficient increases as solution inlet temperature increases. Though absorption rate decreases with respect to solution inlet temperature, the log mean concentration difference (LMCD) decreases as solution inlet temperature increases resulting in higher mass transfer coefficients. Heat transfer coefficient does not vary much with respect to solution inlet temperature.
Fig. 20 e Variation of Sherwood number based on experiments with Sherwood number based on empirical correlation.
Generalized Autoregressive Conditional Heteroskedasticity (GARCH) tool box in MATLAB is employed to find out the coefficients in the generic form of the correlation in Eq. (1). The following correlation has been developed with R134aeDMF mixture with 20% error band. 1:422 2:128 ðScl Þ0:932 ðXglÞ ðhÞ1:413 Sh ¼ 1:7 104 Relþg
(7)
The above correlation is valid in the range for Relþg ¼ 700e2800; Scl ¼ 115e135; Xgl ¼ 0.79e0.98; h ¼ 0.6e0.9.
5.
Correlation for mass transfer coefficient
Based on the above experiments, a correlation for mass transfer coefficient has been developed as a function of Sherwood number, two phase Reynolds number, Schmidt number, concentration gradient and mass transfer efficiency. c3 c4 c5 Sh ¼ c1 Rec2 lþg Scl Xgl h
(1)
where Sh ¼ convective mass transfer=diffusive transfer ¼
Ml;v D2 rl Dc;l
(2)
Relþg ¼ Inertial force=Viscous force ¼
rl Ql þ Qg D
p where A ¼ D2 4
Aml (3)
Scl ¼ Momentum diffusivity=Mass diffusivity ¼ Xgl ¼ Xg Xl h ¼ mass transfer efficiency ¼
nl Dc;l
(4)
(5) Xl;o Xl;i Xl;o;eq Xl;i
(6)
Generic form of the correlation includes convective mass transfer, diffusive mass transfer, inertial force, viscous force, momentum diffusivity, mass diffusivity and absorption potential. Fig. 20 shows the experimental correlation of the Sherwood number from the present study. A regression technique using
6.
Conclusions
Experimental investigations have been carried out on a glass bubble absorber to study heat and mass transfer characteristics of R134a in DMF. Absorption rate and heat transfer rate determined from the experiments are compared with numerical model for various gas flow rates, solution flow rates, pressures, temperatures and inlet concentrations. The agreement is generally fair within a maximum deviation of 20%. Absorption rate and heat transfer rate increase as the gas flow rate, solution flow rate and cooling water flow rate increase. Absorption rate increases marginally with solution pressure and does not vary much with respect to solution initial concentration. Heat transfer rate also does not vary much with respect to solution pressure and initial concentrations. Absorption rate and heat transfer rate decrease as the solution inlet temperature increases. Volumetric mass transfer coefficient and overall heat transfer coefficients increase as gas flow rate, solution flow rate and cooling water flow rate increase. Volumetric mass transfer coefficient decreases as solution pressure increases and increases as solution initial concentration increases. Overall heat transfer coefficient does not vary much with solution pressure and solution initial concentration. Volumetric mass transfer coefficient increases as solution inlet temperature increases. Overall heat transfer coefficient does not vary much with solution inlet temperature. Solution outlet concentration increases as gas flow rate increases due to higher absorption rates. However at higher solution flow
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rates, the concentration decreases due to dilution. Solution pressure drop across the absorber increases as the gas and solution flow rates increase. Mass transfer efficiency decreases as the gas flow rate increases and increases as the solution flow rate increases. Mass transfer and heat transfer efficiencies decrease as solution inlet temperature increases. Efficient removal of heat of mixing and large interfacial area between the vapour and the weak solution will result in high heat and mass transfer efficiencies respectively and improve absorber performance.
Appendix A
1113
transfer coefficient, mass transfer efficiency and heat transfer efficiency have been calculated using the method proposed by Kline and McClintock (1953).
Uncertainty in measured quantities For example glass rotameter is used to measure solution flow rate. Accuracy of flow meter is 1% of full scale. Full scale value is 2 lpm (120 lph). So accuracy is 1.2 lph. Minimum value of the measured solution flow rate is 25 lph. So the maximum uncertainty in the measurement of solution flow rate is (1.2/25) 100 ¼ 4.8%. Table I shows the uncertainty in various measured quantities.
Absorption rate (ma) ms;i Xs;i þ ma Xg ¼ ms;i þ ma Xs;o
(A.1)
Measured quantities
Heat rejected from solution (Qs) Qs ¼ ms hs;i hs;o
(A.2)
Solution inlet and outlet enthalpies (hs,i, hs,o) are calculated from pressure and temperature at solution inlet and outlet respectively. Heat rejected to cooling water (Qw) Qw ¼ mw Tw;o Tw;i
Table I
Pressure, p Temperature, T Solution volume flow rate, Vl Gas volume flow rate, Vg Density, r
Uncertainty 4.55% 0.78% 4.8% 1.25% 0.3%
(A.3)
Uncertainty in derived quantities
(A.4)
For example Heat transfer rate is calculated from water flow rate, inlet and outlet temperatures. Uncertainty in the calculation of heat transfer rate is:
Heat transfer efficiency (hht) hht
Qw ¼ Qs
Mass transfer efficiency (hmt) hmt ¼
ma mg
(A.5)
Ts;i Tw;o Ts;o Tw;i ! Ts;i Tw;o ln Ts;o Tw;i
"
dVl Vl
2
2 #1=2 h i1=2 dT þ2 ¼ ð0:048Þ2 þ2ð0:0078Þ2 ¼ 4:93% T
Table II shows the uncertainty in various derived quantities.
Overall heat transfer coefficient (Uo) LMTD ¼
dQ ¼ Q
(A.6) Table II Derived quantities
Uo ¼
Qw ðpD2 LÞLMTD
(A.7)
Volumetric mass transfer coefficient (Ml,v) LMCD ¼
Xeq;s;i Xs;i Xeq;s;o Xs;o Xeq;s;i Xs;i ln Xeq;s;o Xs;o
ma Ml;v ¼ p D21 L LMCD 4
(A.8)
(A.9)
Appendix B The maximum possible uncertainty in various measured quantities namely temperature, pressure, flow rate, density and voltage is estimated from the minimum value of the measured output and accuracy of the instrument. Uncertainties in derived quantities namely concentration, absorption rate, heat transfer rate, mass transfer coefficient, heat
Concentration Absorption rate Heat transfer rate Overall heat transfer coefficient Volumetric mass transfer coefficient Heat transfer efficiency Mass transfer efficiency
Uncertainty 2.67% 5.15% 4.93% 5.24% 6.39% 7.24% 5.3%
references
Cerezo, J., Bourouis, M., Valles, M., Coronas, A., Best, R., 2009. Experimental study of an ammoniaewater bubble absorber using a plate heat exchanger for absorption refrigeration machines. Appl. Therm. Eng. 29 (5e6), 1005e1011. Elperin, T., Fominykh, A., 2003. Four stages of the simultaneous mass and heat transfer during bubble formation and rise in a bubbly absorber. Chem. Eng. Sci. 58, 3555e3564. Fatouh, M., 1994. Studies on HCFC based vapour absorption refrigeration systems suitable for low potential heat sources. PhD thesis, Indian Institute of Technology Madras.
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Kang, Y.T., Christensen, R.N., Kashiwagi, T., 1998. Ammoniaewater bubble absorber with a plate heat exchanger. ASHRAE Trans. 104 (1), 1e11. Kang, Y.T., Akisawa, A., Kashiwagi, T., 2000. Analytical investigation of two different absorption modes: falling film and Bubble types. Int. J. Refrigeration 23 (6), 430e443. Kang, Y.T., Nagano, T., Kashiwagi, T., 2002a. Visualization of bubble behavior and bubble diameter correlation for NH3eH2O bubble absorption. Int. J. Refrigeration 25 (1), 127e135. Kang, Y.T., Nagano, T., Kashiwagi, T., 2002b. Mass transfer correlation of NH3eH2O bubble absorption. Int. J. Refrigeration 25 (7), 878e886. Karthikeyan, G., 1995. Studies on transfer tank operated vapour absorption refrigeration system. PhD thesis, Indian Institute of Technology Madras. Kline, S.J., McClintock, F.A., 1953. Describing uncertainties in single sample experiments. J. Mech. Eng. 75, 1e3. Lee, J.C., Lee, K.B., Chun, B.H., Lee, C.H., Ha, J.J., Kim, S.H., 2003. A study on numerical simulations and experiments for mass transfer in bubble mode absorber of ammonia and water. Int. J. Refrigeration 26 (5), 551e558. Mani, A., 2009. Studies on compact bubble absorber of the vapour absorption refrigeration system. A report submitted to Department of Science and Technology, Government of India. Merrill, T.L., Perez-Blanco, H., 1997. Combined heat and mass transfer during bubble absorption in binary solutions. Int. J. Heat Mass Transfer 40 (3), 589e603. Nezu, Y., Hisada, N., Ishiyama, T., Watanabe, K., 2002. Thermodynamic properties of working-fluid pairs with R-134a for absorption refrigeration system. In: Natural WorkingFluids 2002, IIR Gustav Lorentzen Conf. 5th., China, Sept. 17e20, pp. 446e453. Reid, R.C., Prausnitz, J.M., Poling, B.E., 1989. The Properties of Gases and Liquids, fourth ed. McGraw-Hill Book Company.
Staicovici, M.D., 2000a. A phenemenological theory of polycomponent interactions in non-ideal mixtures. Application to NH3/H2O and other working pairs. Int. J. Refrigeration 23 (2), 153e167. Staicovici, M.D., 2000b. A non-equilibrium phenomenological theory of the mass and heat transfer in physical and chemical interactions. Part IdTheory and its application to NH3/H2O and other working systems. Int. J. Heat Mass Transfer 43 (22), 4153e4173. Staicovici, M.D., 2000c. A non-equilibrium phenomenological theory of the mass and heat transfer in physical and chemical interactions. Part IIdModeling of the NH3/H2O bubble absorption, analytical study of absorption and experiments. Int. J. Heat Mass Transfer 43 (22), 4175e4188. Sujatha, K.S., 1997. Studies on absorber and transfer tank of the vapour absorption refrigeration system. PhD thesis, Indian Institute of Technology Madras. Sujatha, K.S., Mani, A., Srinivasa Murthy, S., 1997a. Finite element analysis of a bubble absorber. Int. J. Numer. Meth. Heat Fluid Flow 7 (7), 737e750. Sujatha, K.S., Mani, A., Srinivasa Murthy, S., 1997b. Analysis of a bubble absorber working with R22 and five organic absorbents. Heat Mass Transfer 32, 255e259. Sujatha, K.S., Mani, A., Srinivasa Murthy, S., 1999. Experiments on a bubble absorber. Int. Commun. Heat Mass Transfer 26 (7), 975e984. Suresh, M., Mani, A., 2010. Heat and mass transfer studies on R134a bubble absorber in R134a/DMF based on phenomenological theory. Int. J. Heat Mass Transfer 53 (13e14), 2813e2825. Yokozeki, A., 2005. Theoretical performances of various refrigeranteabsorbent pairs in a vapour absorption refrigeration cycle by the use of equation of state. Appl. Energy 80, 383e399.
Available online at www.sciencedirect.com
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journal homepage: www.elsevier.com/locate/ijrefrig
Experimental studies on heat and mass transfer characteristics for R134aeDMF bubble absorber M. Suresh, A. Mani* Refrigeration & Airconditioning Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai, Tamilnadu 600036, India
article info
abstract
Article history:
Experimental investigations have been carried out to study heat and mass transfer char-
Received 5 January 2011
acteristics of Tetrafluoro ethane (R134a) in Dimethyl formamide (DMF) in a glass absorber.
Received in revised form
Effects of operational parameters viz., gas flow rate, solution flow rate, solution initial
1 December 2011
concentration, solution pressure, solution temperature and cooling water flow rate on
Accepted 7 January 2012
absorber performance are analyzed. Absorption rate and heat transfer rate increase as the
Available online 24 January 2012
gas flow rate, solution flow rate, cooling water flow rate and solution pressure increase whereas they decrease as the solution initial concentration and solution temperature
Keywords:
increase. Heat and mass transfer rates determined from the experiments are compared
Absorption
with numerical model and it is found that agreement is good. A correlation for mass
R134a
transfer coefficient is presented from the experimental studies within 20% error band. ª 2012 Elsevier Ltd and IIR. All rights reserved.
DMF Bubble Mass transfer Heat transfer
Etudes expe´rimentales sur les caracte´ristiques de transfert de chaleur et de masse d’un absorbeur a` bulles au R134a / DMF Mots cle´s : Absorption ; R134a ; DMF ; Bulle ; Transfert de masse ; Transfert de chaleur
1.
Introduction
Many environment friendly fluid combinations have been suggested by number of investigators in order to overcome some of the limitations of well known working pairs viz., ammoniaewater and lithium bromideewater for the vapor absorption refrigeration systems (VARS). Though R22-organic solvent based absorption refrigeration systems have been
extensively studied by Fatouh (1994), Karthikeyan (1995) and Sujatha (1997), HCFCs along with CFCs, are also covered by Montreal and other International Protocols and are being phased out. So environment friendly R134a based VARS are being investigated. Nezu et al. (2002) examined the possibility of testing R134a as a refrigerant in VARS with various organic solvents and showed that the R134aeDMA and the R134aeDMF systems are considered attractive as the working-
* Corresponding author. Tel.: þ91 44 22574666; fax: þ91 44 22570509. E-mail address: [email protected] (A. Mani). 0140-7007/$ e see front matter ª 2012 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2012.01.011
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Nomenclature A D Dc h m M p p0 Q Re Sc Sh T Uo V X
2
absorber cross-section area, m absorber diameter, m diffusion coefficient, m2 s1 solution enthalpy, kJ kg1 mass flow rate, kg s1 mass transfer coefficient, kg m3 s1 solution pressure, bar atmospheric pressure, bar heat transfer rate, W Reynolds number Schmidt number Sherwood number temperature, C overall heat transfer coefficient, W m2 K1 volumetric flow rate, m3 s1 liquid mass fraction, kg kg1
fluid pairs for the absorption refrigeration system than other R134aeabsorbent systems. Yokozeki (2005) studied theoretical performance of various refrigeranteabsorbent pairs in a VARS cycle by the use of equations of state. Of these, R134aeDMF and R134aeDMA systems exhibit better performance, compared to other R134aeabsorbent systems. Also the circulation ratio is less and COP is more for R134aeDMF system compared to R134aeDMA system. Mani (2009) carried out experimental studies on R134a/DMF based compact vapor 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 80e90 C and with cooling water as coolant for absorber and condenser in the temperature range of 20e35 C. Absorber is considered as one of the crucial components in the vapor absorption refrigeration system. Kang et al. (2000) carried out analytical investigation of falling film and bubble type absorbers and found that absorption rate of the bubble type absorber was found to be always higher than that of the falling film mode. Bubble type absorber provides better heat and mass transfer coefficients, also good wettability and mixing between the liquid and the vapor (Kang et al., 2000). Absorption process is characterized by simultaneous heat and mass transfer phenomena. These mechanisms, though complicated, influence the system performance significantly. Elperin and Fominykh (2003) studied combined heat and mass transfer mechanisms at all stages of bubble growth and rise in a bubble absorber, which can be useful in design calculations of gaseliquid absorbers. Lee et al. (2003) performed both the numerical and experimental analyses in the absorption process of a bubble absorber. Numerical model in these studies can be used for the optimum design of absorber. Merrill and Perez-Blanco (1997) developed an analytical model to predict bubble dynamics in binary sub-cooled solutions. This model improves the understanding of bubble absorption dynamics.
1105
Subscripts a absorption eq equilibrium g gas ht heat transfer i inlet l liquid mt mass transfer o outlet s solution v volumetric w water 1 inner 2 outer Greek symbols m dynamic viscosity, Pa s g kinematic viscosity, m2 s1 r density, kg m3 h mass transfer efficiency
Sujatha et al. (1997a,b) carried out numerical analysis in a vertical tubular bubble absorber working with R22 as refrigerant and five organic fluids namely DMF, DMA, DMETEG, DMEDEG and NMP as absorbents. This model is validated by comparing with the results available in the literature. Based on these results, a correlation for mass transfer coefficient has been suggested for the vertical tubular bubble absorber. Sujatha et al. (1999) have also carried out experimental studies on a vertical tubular bubble absorber working with R22-DMF. Experimental pressure drop, heat transfer coefficient and mass transfer coefficient are compared with the results obtained from the numerical model. Kang et al. (1998) developed a model for a bubble absorber with a plate type heat exchanger by considering the combined heat and mass transfer analysis in both liquid and vapor regions. All geometric variables such as distance between two plates, number of plates, and width of the plates could be selected optimally for given thermal conditions by the developed design model for ammoniaewater combination. Staicovici (2000a,b,c) used non-equilibrium phenomenological theory to evaluate gaseliquid interaction. Design of bubble absorber, based on non-equilibrium thermodynamics could be suited to a modern compact plate type construction and offers better absorption efficiency and minimum pressure loss on the gas side. Suresh and Mani (2010) developed a numerical model on bubble dynamics, heat and mass characteristics of R134a/DMF based bubble absorber using phenomenological theory and validated by comparing with the results available in literature. Suresh and Mani also carried out experimental studies on bubble characteristics on a vertical glass bubble absorber and presented a correlation for bubble diameter during detachment. Kang et al. (2002a) developed a correlation for initial bubble diameter, which can be used to calculate the interfacial area in the design of ammoniaewater bubble absorber. Kang et al. (2002b) also developed an experimental correlation of mass transfer coefficient for ammoniaewater bubble absorption.
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Cerezo et al. (2009) carried out experimental studies using a plate heat exchanger as absorber and ammoniaewater as working fluid. They concluded that increase in pressure, solution and cooling flow rates positively affects the absorber performance and increase in the concentration, cooling and solution temperature negatively affects the absorber performance. Present experimental work is carried out to study heat and mass transfer characteristics and effects of operational parameters viz., gas flow rate, solution flow rate, solution initial concentration, solution pressure, solution temperature and cooling water flow rate on the performance of R134aeDMF bubble absorber.
2.
the bottom of inner tube by a solution pump. R134a gas is supplied from a storage cylinder through a mass flow controller unit and injected through a nozzle installed at the bottom of inner tube. Strong DMF solution is collected in the strong solution tank at the top of the absorber. Cooling water is supplied, by cooling water simulator, through the absorber annulus in counter flow direction to the solution and gas. Cooling water simulator consists of a R22 based vapour compression refrigeration (VCR) circuit of 3.4 TR capacity, 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 a cooling coil. This system also includes various measuring devices such as temperature sensors, pressure sensors, flow meters, mass flow controller and online density meter fitted at suitable locations as shown in Fig. 1. All these measuring instruments are pre-calibrated. Twelve numbers of calibrated coppereconstantan thermocouples are used as temperature sensors with a measurement uncertainty up to 0.78%. Three numbers of piezo-electric type pressure transducers are used as pressure sensors with a measurement uncertainty up to
Experimental setup
Fig. 1 shows the schematic diagram of experimental setup. This consists of a glass bubble absorber, strong and weak solution tanks, solution pump, cooling water simulator, instrumentation and valves. Tables 1 and 2 show the specifications of main components and instruments in the setup, respectively. Glass absorber consists of two concentric tubes. DMF solution is pumped from weak solution tank, through R134a gas collection tank
NRV1
HSV4
Ice box
P3 T4
Strong solution tank
L
HSV10
T3
HSV11
Glass absorber
T5 T6
Online density meter
NV1
Solution charging line
L
HSV6
T7
Video camera recorder
T8
HSV5
GV2
GV1
T9
HSV9
HSV8
Weak solution tank
High speed Water camera pump
T10
P2
T11
T2
S2
HSV2 S1
T12
HSV3
Gas charging line
HSV7
Solution pump
HSV1 M
Computer with Data Acquisition system
P1
T1
Pressure regulating valve
R134a gas cylinder
Hand shut-off valve Non-Return valve
R134a-DMF Solution line
Gate valve
R134a refrigerant line Water line
Cooling water thermostat
Needle valve S Flow meter Mass flow controller unit L Level gauge
M
T P
Temperature gauge Pressure gauge
Fig. 1 e Schematic diagram of glass bubble absorber experimental setup.
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Table 1 e Specifications of main components.
Table 2 e Specifications of main instruments.
Glass absorber Type
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)
Tube in tube
Fluid circuit
Tube side: solution Annulus: cooling water
Material Inside tube Outside tube Tube length
Glass ID: 33 mm, OD: 37 mm ID: 46 mm, OD: 50 mm 1000 mm
Nozzle Type Material Inner diameter Outer diameter
Tube Copper 2.2 mm 4.0 mm
Solution pump Type Capacity Discharge pressure Plunger diameter SPM
Metering pump 50 lph 40 bar 25 mm 100
4.55%. Glass rotameters are used to measure the flow of solution and cooling water with a measurement uncertainty up to 4.8%. Mass flow controller unit is used to measure the volume flow rate of gas with a measurement uncertainty of 1.25%. 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 using HBT (HankinsoneBrobsteThomson) equation used by Reid et al. (1989). Readings from all these instruments and sensors are monitored continuously by connecting them to a data acquisition system and a computer. An uncertainty analysis associated with various measured and derived quantities has been discussed in Appendix B.
3.
Mass flow controller Flow rate Response time Accuracy Linearity Repeatability Control valve type Input power
6.6 mm 500 mm 0e3 g m3 5 105 g m3 2 104 g m3, 0.1% 0e100 C 0e50 bar 100e500 l/h
Power consumption Input command signal Output indication
10 ccpm to 2500 ccpm <2 s within 2% of set point Within 0.05% of full scale Within 0.5% of full scale Within 0.2% of full scale Normally closed solenoid þ15 V DC at 25 mA to 15 V DC at 180 mA 31 W (Max.) 0e5 V DC 0e5 V DC
Pressure transducers Type Measuring range Temperature Input Output Accuracy Response time Material
Piezoresistive 0e10 bar absolute 20 to 80 C 8e28 V DC 0e5 V DC (3 wires) 1% of full scale Maximum: 250 ms SS316L
Flow meters Type Flow rate Operating pressure End fitting material Float material Tube material Tube taper length Accuracy
Glass rotameter Minimum: 0.2 lpm, Maximum: 2.0 lpm 4 bar (abs.) SS316 SS316 Borosilicate glass 250 mm 1% on F.S.D
Experimental procedure
Initially DMF solution is charged into the weak solution tank. Then it is pumped through inner tube of glass absorber by solution pump. Solution is collected in the strong solution tank at the top of the absorber. Solution is returned to the weak solution tank through a needle valve after each set of experimentation. Initial concentration of the solution at the absorber inlet is measured by online density meter. Solution inlet pressure and temperature are monitored continuously and kept constant. Cooling water is allowed at a constant flow rate, through outer tube of the glass absorber, counter flow to the solution. Then R134a gas is injected from a storage cylinder through the nozzle at the bottom of the absorber. Gas flow rate is measured by mass flow controller unit. Gas temperature is maintained constant. During this process, by keeping solution flow rate constant, gas flow rate is varied and measured using DC power supply unit connected to mass flow controller. All the parameters viz., solution inlet and outlet pressure, temperature and concentration, solution flow
rate, gas flow rate, gas pressure and temperature, cooling water flow rate, inlet and outlet temperature are monitored and recorded in the computer using data acquisition unit. These experiments are repeated for various solution flow rates. In the next run of experiment, by keeping solution, gas and water flow rates constant, the solution initial concentration is increased by injecting R134a gas through a charging line in weak solution tank and monitored by online density meter. All readings are monitored and noted. These experiments are repeated for various solution pressures and solution initial concentrations. In another run, solution inlet temperature is varied by varying cooling water temperature and flow rate whereas all other parameters are kept constant. All readings are monitored and noted. The entire test facility (especially solution tanks) could not be maintained at steady state conditions since it is a once through a system. However, absorption process in the glass
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absorber was maintained at steady state by the following steps:
80 Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution flow rate = 0.025 m h Solution inlet pressure = 120 kPa Solution inlet temperature = 30 ºC Solution inlet concentration = 0.01 kgkg Cooling water flow rate = 0.05 m h
4.
Results and discussion
Experimentation was conducted in glass absorber by varying the operational parameters viz., gas flow rate from 0.03 m3 h1 to 0.15 m3 h1, solution flow rate from 0.025 m3 h1 to 0.05 m3 h1, solution pressure from 1.2 bar to 4 bar, solution initial concentration from 0.01 kg kg1 to 0.2 kg kg1, solution temperature from 20 C to 30 C and cooling water flow rate from 0.05 m3 h1 to 0.075 m3 h1. Experimental results are compared with the numerical model for the bubble absorber developed earlier by the authors (Suresh and Mani, 2010) to study heat and mass transfer characteristics using phenomenological theory. Experimental values of absorption rate, heat transfer rate, volumetric mass transfer coefficient and overall heat transfer coefficient are calculated using the equations given in Appendix A. Fig. 2 shows the variation of solution temperature along the height of absorber for various gas flow rates. The temperature increases to a maximum in the nozzle region, then drops along the height. This indicates that absorption is more in the nozzle region. As the gas flow rate increases, the solution temperature increases along the absorber height due to increase in absorption rate. Fig. 3 shows the variation of solution temperature along the height of absorber for various solution flow rates. As the solution flow rate increases, the solution temperature decreases along the absorber height due to the availability of more amount of solution for absorption. Fig. 4 shows the variation of solution temperature along the height of absorber for various solution initial concentrations. As the solution initial concentration increases, the solution temperature decreases along the absorber height due to decrease in absorption rate. Fig. 5 compares the experimental absorption rate with that of numerical model for various gas flow rates, solution flow rates, pressures, temperatures and initial concentrations. The agreement is good within a maximum deviation of 10%.
60
50
40
30 Gas flow rate = 0.03 m h
20
Gas flow rate = 0.09 m h Gas flow rate = 0.15 m h
10 0
0.2
0.4
0.6
0.8
1
1.2
Absorber height, m
Fig. 2 e Effect of gas flow rate on solution temperature profile along absorber height.
Fig. 6 compares the experimental heat transfer rate with that of numerical model for various gas flow rates, solution flow rates, pressures, temperatures and initial concentrations. The agreement is fair within a maximum deviation of 20%. Fig. 7 depicts variation of absorption rate with gas flow rate for various solution and cooling water flow rates. As gas flow rate increases, amount of gas absorbed in the solution is more, resulting in high absorption rates. Also, at high solution flow rates, quantity of solution available for gas absorption is more, resulting in increase in absorption rate. At high cooling water flow rates, heat rejected from solution is more, resulting in high absorption rates. Fig. 8 depicts variation of heat transfer rate with gas flow rate for various solution and cooling water flow rates. As gas and solution flow rates increase, heat transfer rate also increases due to high absorption rates. At high cooling water flow rates, heat rejected from solution is more, resulting in high heat transfer rates. Fig. 9 shows the variation of solution outlet concentration with gas flow rate
80 Gas flow rate = 0.09 m h Gas inlet pressure = 650 kPa Gas inlet temperature = 32°C Solution inlet pressure = 120 kPa Solution inlet temperature = 30 °C Solution inlet concentration = 0.01 kgkg Cooling water flow rate = 0.05 m3h
70
Solution temperature, °C
i. Refrigerant gas from a large gas cylinder at high pressure was injected into the inner tube of glass absorber through a nozzle. Flow rate of gas is metered through a mass flow controller unit. So the operating parameters viz. gas flow rate, pressure and temperature were kept constant at absorber inlet. ii. Solution (Dimethyl formamide) was pumped through the absorber inner tube using a diaphragm pump by maintaining constant flow rate, pressure and temperature at absorber inlet. iii. Cooling water was circulated through the outer tube of absorber inlet at constant flow rate and at constant pressure and temperature. iv. Gas injection was continued and flow rate, pressure and temperature of solution and gas at absorber inlet were monitored continuously by connecting respective instruments and sensors to a data acquisition system and a computer. When all these readings remain constant during a time period, they were recorded in the computer.
Solution temperature, °C
70
60
50
40
30 Solution flow rate = 0.025 m h
20
Solution flow rate = 0.0375 m h Solution flow rate = 0.05 m h
10 0
0.2
0.4
0.6
0.8
1
1.2
Absorber height, m
Fig. 3 e Effect of solution flow rate on solution temperature profile along absorber height.
1109
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 1 0 4 e1 1 1 4
300
80 Gas inlet pressure = 650 kPa Gas inlet temperature = 32 °C Solution flow rate = 0.05 m3h-1 Solution inlet pressure = 200 kPa Solution inlet temperature = 30 °C Cooling water flow rate = 0.05 m3h-1
70
Solution temperature, °C
Experimental heat transfer rate, W
Gas flow rate = 0.12 m3h-1
60
50
40
30 Solution inlet concentration = 0.01 kgkg
20
250 +20 % 200
150 -20 % 100
50
0
Solution inlet concentration = 0.1 kgkg
0
50
Solution inlet concentration = 0.2 kgkg
100
150
200
250
300
Heat transfer rate - Numerical model, W
10 0
0.2
0.4
0.6
0.8
1
1.2
Absorber height, m
Fig. 4 e Effect of solution initial concentration on solution temperature profile along absorber height.
for various solution flow rates. Solution concentration increases as gas flow rate increases due to higher absorption rates. However at higher solution flow rates, the concentration decreases due to dilution. Fig. 10 shows that as the gas and solution flow rates increase, solution pressure drop across the absorber increases due to increase in flow velocity and friction. Fig. 11 shows that mass transfer efficiency decreases as the gas flow rate increases and increases as the solution flow rate increases. At lower gas flow rates and higher solution flow rates, the solution outlet concentration approaches closer to equilibrium concentration resulting in higher mass transfer efficiency whereas at higher gas flow rates and lower solution flow rates, the solution outlet concentration is far away from equilibrium concentration resulting in lower mass transfer efficiency. Fig. 12 depicts variation of volumetric mass transfer coefficient with gas flow rate at various solution and cooling water flow rates, and overall heat transfer coefficient with gas flow rate at various solution flow rates. It increases as the gas, solution and cooling water flow rates increase due to increase in absorption rates. Fig. 13 depicts variation of overall heat transfer coefficient with gas flow rate
Fig. 6 e Variation of heat transfer rate based on experiments with heat transfer rate based on numerical model.
at various solution and cooling water flow rates. It increases marginally as gas and solution flow rates increase due to marginal increase in heat transfer rate. At high gas flow rates, heat and mass transfer efficiencies become lower due to small volume of mixing zone in test section. Due to this, the rate of increase in absorption and heat transfer slows down. So overall heat transfer coefficient tends to flatten at higher gas flow rates. It increases as cooling water flow rate increases. At high cooling water flow rates, water side heat transfer coefficient increases due to increase in flow velocity. This results in high overall heat transfer coefficient. Fig. 14 depicts variation of absorption rate with solution pressure for various solution inlet concentrations. Absorption rate increases marginally with solution pressure due to marginal increase in bubble life at high pressures. Fig. 15 depicts variation of heat transfer rate with solution pressure for various solution inlet concentrations. Heat transfer rate does not vary much with respect to solution pressure due to marginal increase in absorption rate. Figs. 16 and 17 depict respectively the variation of volumetric mass transfer coefficient and overall heat transfer coefficient with respect to
Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution inlet pressure = 120 kPa Solution inlet temperature = 30 ºC Solution inlet concentration = 0.01 kgkg
1
Absorption rate, x 10 -3 kgs-1
Experimental absorption rate, x 10-3 kgs-1
1.2
1.5
1.25 +10 % 1
0.75 -10 % 0.5
0.25
0.8
0.6
0.4
---- Cooling water flow rate = 0.05 m h Cooling water flow rate = 0.075 m h
0.2
Solution flow rate = 0.025 m h Solution flow rate = 0.0375 m h Solution flow rate = 0.05 m h
0
0
0
0.25
0.5
0.75
1
1.25 -3
1.5
-1
Absorption rate - Numerical model, x 10 kgs
Fig. 5 e Variation of absorption rate based on experiments with absorption rate based on numerical model.
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
3 -1
Gas flow rate, m h
Fig. 7 e Effect of gas flow rate on absorption rate for different solution and cooling water flow rates.
0.16
1110
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 1 0 4 e1 1 1 4
100
250 Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution inlet pressure = 120 kPa Solution inlet temperature = 30 ºC Solution inlet concentration = 0.01 kgkg
90
Mass transfer efficiency, %
Heat transfer rate, W
200
150
100 ---- Cooling water flow rate = 0.05 m h Cooling water flow rate = 0.075 m h
50
Solution flow rate = 0.025 m h
80
70 Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution inlet pressure = 120 kPa Solution inlet temperature = 30 ºC Solution inlet concentration = 0.01 kgkg Cooling water flow rate = 0.05 m h
60
50
Solution flow rate = 0.0375 m h
Solution flow rate = 0.025 m h Solution flow rate = 0.0375 m h Solution flow rate = 0.05 m h
Solution flow rate = 0.05 m h
40
0 0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0
0.16
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
3 -1
Gas flow rate, m h
3 -1
Gas flow rate, m h
Fig. 11 e Effect of gas flow rate on mass transfer efficiency for different solution flow rates.
Fig. 8 e Effect of gas flow rate on heat transfer rate for different solution and cooling water flow rates.
Volumetric mass transfer coefficient, kgm-3s-1
12
Solution outlet concentration, kgkg -1
0.14 Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution inlet pressure = 120 kPa Solution inlet temperature = 30 ºC Solution inlet concentration = 0.01 kgkg Cooling water flow rate = 0.05 m h
0.12 0.1 0.08 0.06 0.04
Solution flow rate = 0.025 m h
0.02
Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution inlet pressure = 120 kPa Solution inlet temperature = 30 ºC Solution inlet concentration = 0.01 kgkg
10
8 ---- Cooling water flow rate = 0.05 m h Cooling water flow rate = 0.075 m h
6
4
Solution flow rate = 0.025 m h
2
Solution flow rate = 0.0375 m h Solution flow rate = 0.05 m h
0
Solution flow rate = 0.0375 m h
0
Solution flow rate = 0.05 m h
0.02
0.04
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.06
0.08
0.1
0.12
0.14
0.16
3 -1
Gas flow rate, m h
0 0.16
3 -1
Gas flow rate, m h
Fig. 9 e Effect of gas flow rate on solution outlet concentration for different solution flow rates.
Fig. 12 e Effect of gas flow rate on volumetric mass transfer coefficient for different solution and cooling water flow rates.
Overall heat transfer coefficient, Wm -2K-1
225
Solution pressure drop across absorber, Pa
25 Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution inlet pressure = 120 kPa Solution inlet temperature = 30 ºC Solution inlet concentration = 0.01 kgkg Cooling water flow rate = 0.05 m h
20
15
10
5
Solution flow rate = 0.025 m h
---- Cooling water flow rate = 0.05 m h Cooling water flow rate = 0.075 m h
200
175
Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution inlet pressure = 120 kPa Solution inlet temperature = 30 ºC Solution inlet concentration = 0.01 kgkg
150
Solution flow rate = 0.025 m h Solution flow rate = 0.0375 m h Solution flow rate = 0.05 m h
Solution flow rate = 0.0375 m h
125
Solution flow rate = 0.05 m h
0
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
3 -1
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
Gas flow rate, m h
3 -1
Gas flow rate, m h
Fig. 10 e Effect of gas flow rate on solution pressure drop across absorber for different solution flow rates.
Fig. 13 e Effect of gas flow rate on overall heat transfer coefficient for different solution and cooling water flow rates.
0.16
1111
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 1 0 4 e1 1 1 4
165.5
0.95
Gas flow rate = 0.12 m h Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution flow rate = 0.05 m h Solution inlet temperature = 30 ºC Cooling water flow rate = 0.05 m h
Overall heat transfer coefficient, Wm -2K-1
Absorption rate, x 10 -3 kgs-1
1
0.9
0.85
Solution inlet concentration = 0.01 kgkg Solution inlet concentration = 0.1 kgkg Solution inlet concentration = 0.2 kgkg
200
250
300
350
400
163.5 Solution inlet concentration = 0.01 kgkg Solution inlet concentration = 0.1 kgkg Solution inlet concentration = 0.2 kgkg
162.5 150
0.8 150
164.5
450
Gas flow rate = 0.12 m h Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution flow rate = 0.05 m h Solution inlet temperature = 30 ºC Cooling water flow rate = 0.05 m h
200
Solution pressure, kPa
190
Heat transfer rate, W
170
Gas flow rate = 0.12 m h Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution flow rate = 0.05 m h Solution inlet temperature = 30 ºC Cooling water flow rate = 0.05 m h
160 150 140 Solution inlet concentration = 0.01 kgkg Solution inlet concentration = 0.1 kgkg
130
Solution inlet concentration = 0.2 kgkg
120 150
200
250
300
350
400
450
Solution pressure, kPa
Fig. 15 e Effect of solution pressure on heat transfer rate for different solution inlet concentrations.
Gas flow rate = 0.12 m h Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution flow rate = 0.05 m h Solution inlet temperature = 30 ºC Cooling water flow rate = 0.05 m h
12
Solution inlet concentration = 0.2 kgkg
450
solution pressure for various solution inlet concentrations. Mass transfer coefficient decreases as solution pressure increases and increases as solution initial concentration increases. Though absorption rate is almost constant with respect to solution pressure and solution initial concentration, the log mean concentration difference (LMCD) increases as solution pressure increases resulting in lower mass transfer coefficients and LMCD decreases as solution inlet concentration increases resulting in higher mass transfer coefficients. Heat transfer coefficient does not vary much with respect to solution pressure and solution initial concentration. Fig. 18 shows variation of solution outlet concentration, mass transfer efficiency and heat transfer efficiency with solution inlet temperature. At high solution inlet temperatures, bubble life is shorter because bubble interface reaches the equilibrium temperature faster. This results in lower absorption rates. Due to this, solution outlet concentration decreases. This also leads to decrease in mass transfer efficiency. Due to reduction in absorption rate, heat transfer rate also decreases resulting in reduction in heat transfer efficiency. Fig. 19 depicts the variation of volumetric mass transfer coefficient and overall heat transfer coefficient with respect to solution inlet temperature. Mass transfer 100
9
6
3
0 150
400
Fig. 17 e Effect of solution pressure on overall heat transfer coefficient for different solution inlet concentrations.
Heat and mass transfer efficiency, %
Solution inlet concentration = 0.1 kgkg
350
0.1
90 0.08 80 0.06 Gas flow rate = 0.12 m h Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution flow rate = 0.05 m h Solution inlet pressure = 120 kPa Solution inlet concentration = 0.01 kgkg Cooling water flow rate = 0.05 m h
70
60
0.04 Heat transfer efficiency Mass transfer efficiency
Solution outlet concentration, kgkg -1
Volumetric mass transfer coefficient, kgm -3s-1
15 Solution inlet concentration = 0.01 kgkg
300
Solution pressure, kPa
Fig. 14 e Effect of solution pressure on absorption rate for different solution inlet concentrations.
180
250
Solution outlet concentration
200
250
300
350
400
450
Solution pressure, kPa
50
0.02 15
20
25
30
35
o
Fig. 16 e Effect of solution pressure on volumetric mass transfer coefficient for different solution inlet concentrations.
Solution inlet temperature, C
Fig. 18 e Effect of solution inlet temperature on heat and mass transfer efficiency and solution outlet concentration.
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 1 0 4 e1 1 1 4
165
8
164
6
163
Gas flow rate = 0.12 m h Gas inlet pressure = 650 kPa Gas inlet temperature = 32 ºC Solution flow rate = 0.05 m h Solution inlet pressure = 120 kPa Solution inlet concentration = 0.01 kgkg Cooling water flow rate = 0.05 m h
4
2
162
161 Volumetric mass transfer coefficient Overall heat transfer coefficient
0 15
20
25
2500
Experimental Sherwood number
10
Overall heat transfer coefficient, Wm -2K-1
-3 -1 Volumetric mass transfer coefficient, kgm s
1112
1500 -20%
1000
500 Sh = 1.7 × 10 (Re
)
(Sc )
(xgl)
( )
0 0
160 35
30
+20%
2000
500
1000
1500
2000
2500
Correlation - Sherwood number
o
Solution inlet temperature, C
Fig. 19 e Effect of solution inlet temperature on volumetric mass transfer coefficient and overall heat transfer coefficient.
coefficient increases as solution inlet temperature increases. Though absorption rate decreases with respect to solution inlet temperature, the log mean concentration difference (LMCD) decreases as solution inlet temperature increases resulting in higher mass transfer coefficients. Heat transfer coefficient does not vary much with respect to solution inlet temperature.
Fig. 20 e Variation of Sherwood number based on experiments with Sherwood number based on empirical correlation.
Generalized Autoregressive Conditional Heteroskedasticity (GARCH) tool box in MATLAB is employed to find out the coefficients in the generic form of the correlation in Eq. (1). The following correlation has been developed with R134aeDMF mixture with 20% error band. 1:422 2:128 ðScl Þ0:932 ðXglÞ ðhÞ1:413 Sh ¼ 1:7 104 Relþg
(7)
The above correlation is valid in the range for Relþg ¼ 700e2800; Scl ¼ 115e135; Xgl ¼ 0.79e0.98; h ¼ 0.6e0.9.
5.
Correlation for mass transfer coefficient
Based on the above experiments, a correlation for mass transfer coefficient has been developed as a function of Sherwood number, two phase Reynolds number, Schmidt number, concentration gradient and mass transfer efficiency. c3 c4 c5 Sh ¼ c1 Rec2 lþg Scl Xgl h
(1)
where Sh ¼ convective mass transfer=diffusive transfer ¼
Ml;v D2 rl Dc;l
(2)
Relþg ¼ Inertial force=Viscous force ¼
rl Ql þ Qg D
p where A ¼ D2 4
Aml (3)
Scl ¼ Momentum diffusivity=Mass diffusivity ¼ Xgl ¼ Xg Xl h ¼ mass transfer efficiency ¼
nl Dc;l
(4)
(5) Xl;o Xl;i Xl;o;eq Xl;i
(6)
Generic form of the correlation includes convective mass transfer, diffusive mass transfer, inertial force, viscous force, momentum diffusivity, mass diffusivity and absorption potential. Fig. 20 shows the experimental correlation of the Sherwood number from the present study. A regression technique using
6.
Conclusions
Experimental investigations have been carried out on a glass bubble absorber to study heat and mass transfer characteristics of R134a in DMF. Absorption rate and heat transfer rate determined from the experiments are compared with numerical model for various gas flow rates, solution flow rates, pressures, temperatures and inlet concentrations. The agreement is generally fair within a maximum deviation of 20%. Absorption rate and heat transfer rate increase as the gas flow rate, solution flow rate and cooling water flow rate increase. Absorption rate increases marginally with solution pressure and does not vary much with respect to solution initial concentration. Heat transfer rate also does not vary much with respect to solution pressure and initial concentrations. Absorption rate and heat transfer rate decrease as the solution inlet temperature increases. Volumetric mass transfer coefficient and overall heat transfer coefficients increase as gas flow rate, solution flow rate and cooling water flow rate increase. Volumetric mass transfer coefficient decreases as solution pressure increases and increases as solution initial concentration increases. Overall heat transfer coefficient does not vary much with solution pressure and solution initial concentration. Volumetric mass transfer coefficient increases as solution inlet temperature increases. Overall heat transfer coefficient does not vary much with solution inlet temperature. Solution outlet concentration increases as gas flow rate increases due to higher absorption rates. However at higher solution flow
i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 5 ( 2 0 1 2 ) 1 1 0 4 e1 1 1 4
rates, the concentration decreases due to dilution. Solution pressure drop across the absorber increases as the gas and solution flow rates increase. Mass transfer efficiency decreases as the gas flow rate increases and increases as the solution flow rate increases. Mass transfer and heat transfer efficiencies decrease as solution inlet temperature increases. Efficient removal of heat of mixing and large interfacial area between the vapour and the weak solution will result in high heat and mass transfer efficiencies respectively and improve absorber performance.
Appendix A
1113
transfer coefficient, mass transfer efficiency and heat transfer efficiency have been calculated using the method proposed by Kline and McClintock (1953).
Uncertainty in measured quantities For example glass rotameter is used to measure solution flow rate. Accuracy of flow meter is 1% of full scale. Full scale value is 2 lpm (120 lph). So accuracy is 1.2 lph. Minimum value of the measured solution flow rate is 25 lph. So the maximum uncertainty in the measurement of solution flow rate is (1.2/25) 100 ¼ 4.8%. Table I shows the uncertainty in various measured quantities.
Absorption rate (ma) ms;i Xs;i þ ma Xg ¼ ms;i þ ma Xs;o
(A.1)
Measured quantities
Heat rejected from solution (Qs) Qs ¼ ms hs;i hs;o
(A.2)
Solution inlet and outlet enthalpies (hs,i, hs,o) are calculated from pressure and temperature at solution inlet and outlet respectively. Heat rejected to cooling water (Qw) Qw ¼ mw Tw;o Tw;i
Table I
Pressure, p Temperature, T Solution volume flow rate, Vl Gas volume flow rate, Vg Density, r
Uncertainty 4.55% 0.78% 4.8% 1.25% 0.3%
(A.3)
Uncertainty in derived quantities
(A.4)
For example Heat transfer rate is calculated from water flow rate, inlet and outlet temperatures. Uncertainty in the calculation of heat transfer rate is:
Heat transfer efficiency (hht) hht
Qw ¼ Qs
Mass transfer efficiency (hmt) hmt ¼
ma mg
(A.5)
Ts;i Tw;o Ts;o Tw;i ! Ts;i Tw;o ln Ts;o Tw;i
"
dVl Vl
2
2 #1=2 h i1=2 dT þ2 ¼ ð0:048Þ2 þ2ð0:0078Þ2 ¼ 4:93% T
Table II shows the uncertainty in various derived quantities.
Overall heat transfer coefficient (Uo) LMTD ¼
dQ ¼ Q
(A.6) Table II Derived quantities
Uo ¼
Qw ðpD2 LÞLMTD
(A.7)
Volumetric mass transfer coefficient (Ml,v) LMCD ¼
Xeq;s;i Xs;i Xeq;s;o Xs;o Xeq;s;i Xs;i ln Xeq;s;o Xs;o
ma Ml;v ¼ p D21 L LMCD 4
(A.8)
(A.9)
Appendix B The maximum possible uncertainty in various measured quantities namely temperature, pressure, flow rate, density and voltage is estimated from the minimum value of the measured output and accuracy of the instrument. Uncertainties in derived quantities namely concentration, absorption rate, heat transfer rate, mass transfer coefficient, heat
Concentration Absorption rate Heat transfer rate Overall heat transfer coefficient Volumetric mass transfer coefficient Heat transfer efficiency Mass transfer efficiency
Uncertainty 2.67% 5.15% 4.93% 5.24% 6.39% 7.24% 5.3%
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