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An experimental investigation on performance of bubble pump with lunate channel for absorption refrigeration system
International Journal of Refrigeration 29 (2006) 815–822 www.elsevier.com/locate/ijrefrig
An experimental investigation on performance of bubble pump with lunate channel for absorption refrigeration system Lianying Zhanga,*, Yuyuan Wua, Hongfei Zhengb, Jingang Guoa, Dongsheng Chena b
a School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China School of Vehicle and Transport Engineering, Beijing Institute of Technology, Beijing, China
Received 19 May 2004; received in revised form 10 November 2005; accepted 16 November 2005 Available online 24 February 2006
Abstract An experimental research on the performance of the bubble pump for absorption refrigeration units was made. The bubble pump provides the drive for the absorption cycle and is a decisive component of the absorption refrigeration unit. The bubble pump’s property determines the efficiency of the absorption refrigeration system. A continuous experimental system with different size of bubbles pumps were designed, constructed and successfully worked. The experiments were performed by changing some of the parameters affecting the bubble pump performance. The experimental results shows that the performance of the bubble pump depends mainly on the driving temperature, the solution head and the combining tube diameters. With the suitable size of section area of the pump tubes the net elevating height of solution is 2.5 times as high as the solution submergence. The lunate channel has several outstanding characteristics, such as low starting temperature (minimum 68 8C), wide operating temperature range and lower requirement for vacuum condition (under 10 kPa). Then the elevating capability of the bubble pump with lunate channel is much better than others currently. It would provide well foundation for practical applications. q 2005 Elsevier Ltd and IIR. All rights reserved. Keywords: Absorption system; Experiment; Pump; Bubble; Geometry; Calculation
Syste`me frigorifique a` absorption: e´tude expe´rimentale sur la performance d’une pompe a` bulles munie d’un canal sous forme de croissant Mots cle´s : Syste`me a` absorption ; Expe´rimentation ; Pompe ; Bulle ; Ge´ome´trie ; Calcul
1. Introduction
* Corresponding author. E-mail address: [email protected] (L. Zhang).
0140-7007/$35.00 q 2005 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2005.11.006
For refrigeration purpose, absorption refrigeration systems are preferred when enough exhaust heat or lowcost alternated energy source is available. In this respect, the LiBr–H2O absorption refrigeration system is widely used,
816
L. Zhang et al. / International Journal of Refrigeration 29 (2006) 815–822
Nomenclature F h k m_ Q q T v_ x0
the wall area of the lunate channel (m2) motive head (m) heat transfer coefficient (W mK2 KK1) mass flow rate (g sK1) heat input (W) heat flux (W m-2) temperature (8C) volume flow rate (ml sK1) vapor mass fraction, dimensionless
Greek Symbols r density (kg mK3)
especially in air-conditioning. But a conventional airconditioning system is driven by higher grade energy source like mechanical/electrical energy to circulate the refrigerant–absorbent solution. Hence, miniaturization of the absorption refrigerators and reduction of electric power consumption requires the replacement of this mechanical pump by a pumping system driven by heat, e.g. a bubble pump. To obtain a higher coefficient of performance and to make it compatible with solar heat (low generator temperature), a two-fluid bubble pump is suggested. Chen et al. [1] study on improvement of the current cycle performance of a diffusion–absorption refrigerator. They designed and fabricated a new generator with heat exchanger to increase the COP of the cycle by 50%. Their generator make up of heating elements, a bubble pump and a coaxial heat exchanger. This configuration reduced heat losses so as to increase the efficiency of the heating process and to increase the COP. Studies on bubble pump both analytically and experimentally were developed by Pfaff et al. [2]. A bubble pump was modeled for intermittent slug flow of solution and vapor mixture. A test rig was built in glass to evaluate the performance of the bubble pump, to visualize the flow regime and to validate the analytical model. In order to study the effect of pipe diameter on the performance of the bubble pump. Pfaff et al. [2] built the test rig with several bubble pumps that were connected in parallel. The bubble pump of Pfaff et al. [2] operated at slug flow regime in cyclic intervals. They found that the pumping ratio (the ratio between the strong solution flow rate and the refrigerant flow rate) is independent of the bubble pump heat input within the operating ranges studied but increase with higher driving head, lower pump lift and smaller tube diameter. It was found that the frequency of the pumping action increases with rising the heat input to the bubble pump, increases in driving head, and decreases in pump lift and tube diameter. Pfaff et al. [2] found that a bubble pump with a tube diameter of 10 mm and heat input of 40 W is suitable for a refrigerator of about 100 W cooling capacity.
Subscript cw condensate water hw hot water hwi inlet of hot water hwo outlet of hot water ps poor solution rs rich solution si inlet of solution so outlet of solution w water
A number of parallel bubble pumps were required for increasing the capacity of the refrigerator. Koyfman et al. [3] presented an experimental investigation to study the performance of the bubble pump for diffusion absorption refrigeration units. A continuous experimental system was designed, built and successfully operated. In the experiments, some of the parameters affecting the bubble pump performance were changed. During the experimental investigation, photographs were taken. It can be seen that the bubble pump operates at slug flow regime with a churn flow regime at the entrance of the bubble pump tube. The prediction of rates of heat transfer and thermally induced flow (circulation rate) is the primary requirement for the design of a bubble pump. The experimental measurement of these rates was carried out earlier by Piret and Isbin [4,5] on an electrically heated pipe. Johnson [5] measured the circulation rate in a steam-heated vertical thermosyphon elevating tube of standard design. The effects of heat flux, submergence and physical properties were studied experimentally by many workers [6–9]. In this paper, the bubble pump is a full-length heating bubble pump with lunate channel, the working fluid is water-lithium bromide solution. The present experiment will study the effect of driving temperature, motive head and design parameters on performance of the bubble pump.
2. Theoretical foundation of boiling heat transfer enhancement for lunate channel Narrow slot thermosiphon boiling heat transfer has two notable characteristics [10]: both in remarkable heat transfer enhancement, especially at high heat flux, and in small temperature difference for boiling heat transfer. The lunate channel is formed by locating a tube extreme eccentrically inside a bigger tube so as to make the outer wall of smaller tube to contact the inner wall of the bigger tube at one straight line theoretically, which is shown in
L. Zhang et al. / International Journal of Refrigeration 29 (2006) 815–822
A
A
817
Convective heat Annular transfer through flow liquid film
Slug flow
Bubbly flow
Cross-section A-A Fig. 1. The construction of the lunate channel.
Fig. 1. Both inner and outer tubes are soldered together at both ends of the contacting line. This configuration have several advantages as follows [11] (1) it is convenience to machining; (2) the pointed corners of the lunate channel cross section can provide artificial nucleation sites, the bubbles will create easily at it; (3) the wider part of the lunate channel operate as liquid filling, which can prevent the wall being dried. Because the pointed triangles of lunate channel operate as boiling nuclei, bubble would be easily formed, especially with the inner and the outer wall heating fluids simultaneously. The hot boundary layer is thinner, so the superheat degree for forming bubble is decreased greatly. The flow regimes along the heated length of the test sample have been shown in Fig. 2 [13]. The subcooled liquid entering the tube gets heated by single phase convection and moves upwards. In the liquid ascending process, the liquid undergoes single-phase flow, bubbly flow and slug flow in turn. Monde [12] showed that slug flow is in favor of elevating solution much more. Because water–LiBr solution is a viscous fluids, in order to obtain slug flow inside the channel, it is important to choose the cross section size of the channel. If the channel is too narrow, the flow resistance of two-phase will be increased, which will weaken elevating solution. If the channel is too wider, the boiling required heat flux is increased. The circulation of fluid flow will be unstable. The choice of present experimental channel has considered all aspects.
Single Phase flow
Saturated nucleate boiling
Subcooled boiling
Convective heat transfer
Fig. 2. Flow patterns. Boiling regimes along a bubble pump.
3. Experimental apparatus and procedures The whole experimental system mainly consists of five portions: a test section (11), condenser (3), vapor–liquid separator (9), liquid container (6), heat source, pressure balanced tube (10) and connecting tube, etc. The system is made of stainless steel except heat-exchange element. The bubble pump served as the test section (11). The outer and inner tubes were made of purple copper. The outer tube was of 32 mm i.d. and 1660 mm long. The inner tubes were, respectively, of 16, 19, 22 mm i.d. and 1450 mm long. The elevation tube connected with the liquid container (6) and joined with a vapor–liquid separator (9) and total condenser vessels (3) forming a thermosiphon loop as schematically shown in Fig. 3. The hot water provided by heater and pumped by a centrifugal hot water pump entered the inner and the outer tube from the bottom, and then went out from the top. Thus, the vertical lunate channel was heated by hot water in the inner tube and outside the outer tube simultaneously. The weak solution of known concentration was boiled in the lunate channel, and was driven upward due to density difference between the liquid in the container (6) and boiling two-phase fluid in test section (11)
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L. Zhang et al. / International Journal of Refrigeration 29 (2006) 815–822
1 P
2 Inlet of cooling water
16
25
3
9
18
10 Outlet of cooling water
m Outlet of hot water
22
21
4 19 5
lift
24 15
m
m 14
8
12
11 23 7 6
13 m
Motive head Inlet of hot water
20 17 Fig. 3. Schematic diagram of the experimental set-up 1, vacuum watch; 2, vacuum valve; 3, condenser; 4, cooling water meter; 5, distilled water reflux tube; 6, liquid container; 7, hot water valve; 8, sleeve muff of heating solution; 9, vapor–liquid separator; 10, pressure balanced tube; 11, test section; 12, solution reflux tube; 13, hot water meter; 14, rich solution corrosion preventive rotameter; 15, distilled water rotameter; 16, cooling water valve; 17, drain valve; 18, inlet of cooling water thermocouple; 19, outlet of cooling water thermocouple; 20, inlet test section of poor solution thermocouple; 21, outlet test section of solution thermocouple; 22, outlet vapor–liquid of rich solution thermocouple; 23, inlet of hot water thermocouple; 24, outlet of hot water thermocouple; 25, partition.
and then entered into the separator (9). The rich solution was drained down the bottom of the separator (9) while vapors were led to a condenser vessel (3) through partition (25). The rich solution moved downwards and entered the liquid container through solution reflux tube (12). Distilled water out of the condenser (3) also entered the liquid container (6) through distilled water reflux tube (5). They were mixed in the liquid container (6) forming poor solution that re-entered the test section (11) through a joint. The distilled water volume flow rate was measured by a LZB-10 rotameter (15) and the rich solution volume flow rates were measured by a LZB-10F corrosion preventive flowmeters (14). The stabilized hot water was supplied through an electrical heater. The hot water flow rates measured by a hot water meter (13). The cooling water flow rate was maintained by means of a centrifugal water pump and was measured by an ordinary water meter (4). A TX-HB-FF copper-constantan thermocouple (20) was applied to measure the inlet liquid temperature. The temperature of the boiling liquid before entry to the vapor–liquid separator was measured by another thermocouple (21). TX-HB-FF copper-constantan thermocouples were also placed to measure the temperatures in and
around the condenser and other strategic locations in the reboiler loop to ensure a reliable computation of circulation rates through the heat balance. The electric potential signal offering by thermocouples are directly read through digital microvolt meter. Drain valve (17) was provided at the inlet of test section for feeding and draining solution. The entire set up was thoroughly lagged with asbestos rope and glass wool and finally covered with a thin aluminium sheet to reduce the heat losses. After steady state was reached as indicated by the thermocouples readings, the experimental data were recorded.
4. Data processing In the experimental system, the water mass flow rates were obtained by m_ cw Z v_cw rw
(1)
The poor solution volume flow rates were calculated by making a mass balance on the test section: v_ps z v_cw C v_rs
(2)
L. Zhang et al. / International Journal of Refrigeration 29 (2006) 815–822
Then, the poor solution mass flow rates were computed by (3)
The vapor mass fraction of the two-phase mixture at the exit of test section is: m_ x Z cw m_ ps 0
(4)
The total heat inputted to the solution by the hot water is: Q Z V_ hw rw ðThwi KThwo Þ
(5)
The heat flux is: qZ
Q F
(6)
18.0 Rich solution volume flow rate (mls–1)
m_ ps Z v_ps rps
Q F Thw KTs
d1/d2=19/32 h=68.2cm h=57.5cm h=47.5cm
16.5 15.0 13.5 12.0 10.5 9.0 7.5 6.0 4.5 70
The heat transfer coefficient is: kZ
(7)
819
72
74 76 78 80 82 Driving temperature T(°C)
84
86
Fig. 4. Variation of rich solution volume flow rate with driving temperature for different motive head.
where: 1 Z ðThwi C Thwo Þ Thw 2
(8)
1 Ts Z ðTsi C Tso Þ 2
(9)
5. Results and discussions The operating parameters were driving temperature, motive head and combining tube diameters. All the runs were conducted under vacuum condition (about 10 kPa). Here, the driving temperature specially refers to hot water temperature and the motive head is defined as the height of the poor solution level in the channel as is illustrated at Fig. 3. The influences of the major operational and design parameters on distilled water yield, elevating solution volume and the vapor mass fraction of the two-phase mixture at the exit of test section will be discussed as follows.
temperature is 75 8C, the water mass flow rate attained around 2.0 g sK1. If the water can vaporize completely in evaporator, the refrigerating capacity of the unit may reach more than 4.8 kW, which shows that the lunate channel has strong heat transfer at small temperature difference in compare to the annular or round channel [13], and the designed bubble pump has strong elevating ability. The volume flow rate elevated rich solution and water mass flow rate with difference motive head for 19/32 combining tube diameters are plotted versus the driving temperature in Figs. 4 and 5. The figures clearly illustrate that the flow rates of elevated rich solution and water basically increase with driving temperature after the minimum driving temperature is reached. When the driving temperature is above 85 8C, the flow rates increase slowly. Because the slug flow may change to the annular flow at the outlet of the channel due to the temperature increase. However, the void fraction inside the channel is higher at the higher heat flux, the shortage of 3.75
5.1. Influence of driving temperature It was found that a certain minimum driving temperature Tmin is required for the pump to operate. Before Tmin was reached, the driving force was not sufficient for pumping action. The minimum driving temperature depends on the operating conditions and the dimensions of the pump (combining tube diameters) and the system itself (motive head). The minimum driving temperature shows an upward tendency for lower driving head and for unsuitable combining tube diameters. For the 19/32 combining tubes pumping action started around 68 8C, whereas for the 16/32 combining tubes a minimum driving temperature close to 76 8C was required. If the hot water temperature is at 75–85 8C, the system runs well. When the driving
Water mass flow rate (gs–1)
3.50 3.25 3.00
d1/d2=19/32 : h=68.2cm : h=57.5cm : h=47.5cm
2.75 2.50 2.25 2.00 1.75 1.50 1.25 70
72
74
76
78
80
82
84
86
Driving temperature T(°C) Fig. 5. Variation of water mass flow rate with driving temperature for different motive head.
L. Zhang et al. / International Journal of Refrigeration 29 (2006) 815–822
0.23
d1/d2=19/32 h=68.2cm h=57.5cm h=47.5cm
vapor mass fraction
0.21 0.19 0.17 0.15 0.13 0.11 0.09 0.07 70
72
74
76
78
80
82
84
86
Driving temperature T(°C) Fig. 6. Variation of vapor mass fraction at exit of the pump with driving temperature for different motive head.
solution upside the channel aggravates the heat transfer, thus, the elevated solution decreases. Fig. 6 shows the variations of vapor mass fraction at the exit of the test channel with the driving temperature for different motive head and for 19/32 combining tubes. It indicates that the trends of the variation of the vapor mass fraction are somewhat similar to those of the water mass flow rate, whereas the increase of water mass flow rates is faster than that of the vapor mass fraction at the exit of the test at any motive head. For example the motive head is 57.5 cm, the water mass flow rate increases by 109% with the driving temperature increasing by 10 8C, whereas the vapor mass fraction increases by 15%. As the higher driving temperature, the more heat input to the fluids and the more water is vaporized, correspondingly which means a higher driving force developed by the bubble pump, thus more solution is pumped, however the vapor mass fraction increases with increase in driving temperature with less gradient. 5.2. Influence of motive head The motive head is a very important operational parameter for the bubble pump. Because the height (1660 mm) of exit port of the rich solution is fixed, the variation of the motive head is achieved only by increasing or decreasing the amount of poor solution in the liquid container. It can be observed from the Figs. 4–6 that a higher motive head leads to a higher solution volume rate and a lower vapor mass fraction at any driving temperature, and vice versa. But water mass flow rate is almost invariable with the variation of the motive head. The increase in the solution volume flow rate is caused by the increased driving force due to a increased motive head and by decreased flowing resistance of the two-phase mixture resulting from a lower pump lift. At the same time, the convective heat transfer regime and the subcooled boiling regime in flow channel are longer and correspondingly the vapor mass
fraction in the channel and at the exit of the channel are smaller. The amount of the water is equal to the amount of lifted solution times the vapor mass fraction at the exit of the channel, so the water mass flow rate depends on the vapor mass fraction and the amount of lifted solution. Contrarily, if the driving force decreases due to a lower motive head and the resistance of the mixture increases by a higher pump lift at the same time, only a little solution is elevated into the gas–liquid separator and the poor solution remains in the pump for a longer time. Thus a larger amount of the heat input into the bubble pump is utilized to heat the solution. But less heat input is needed for heating the incoming solution and so the vapor mass fraction increases. As the motive head is 475 mm, the elevating height is 1185 mm, which is about 2.5 times as high as the motive head. This clearly illustrates that the elevating capability of the bubble pump is very strong. If the operating temperature is higher or the system pressure is lower, its elevating capability will be further increased. This provides well foundation for the practical application of the bubble pump with lunate channel. 5.3. Influence of combining pipe diameters Due to increasing contacting area between solution and wall at the corner of lunate channel, the heat flux increases greatly in the corner. At the same time, the corner of the lunate cross section provides artificial nucleation sites. It enhances the boiling heat transfer. When the bubbles departed from the wall and moved to wider passage, it brings about the second flow in the channel. This greatly promotes boiling of solution inside the TEST. Though lunate channel increases frictional resistance, it is favorable to elevating solution due to enhanced heat transfer. Therefore, it is very important to choose reasonable passage area. It is observed from the experimental results that the 21 Rich solution volume flow rate(mls–1)
820
19 17
h=68.2cm d1/d2=16/32 d1/d2=19/32
15 13 11 9 7 70
72
74
76
78
80
82
84
86
88
Driving temperature T(°C) Fig. 7. Influence of the combing pipe diameters on the rich solution volume flow rate.
L. Zhang et al. / International Journal of Refrigeration 29 (2006) 815–822
2100
Water mass flow rate(gs–1)
3.20
heat transfer coefficient k(W.m–2.K)
3.70 3.45
h=68.2cm d1/d2=16/32 d1/d2=19/32
2.95 2.70 2.45 2.20 1.95 1.70 1.45 70
72
74 76 78 80 82 Driving temperature T(°C)
84
86
elevating efficiency and the minimum driving temperature to start circulation is strongly dependent on the combining tubes diameters. For example, the elevating effect of d1/d2Z 19/32 is the best and its required minimum driving temperature is about 68 8C. Then the elevating effect of d1/d2Z16/32 is better and its required minimum driving temperature is closely 76 8C. Finally that of d1/d2Z22/32 is the poorest, it is difficult to sustain continuous flow through the system and the experimental date are difficult to be recorded. The Figs. 7–9 illustrated the effect of d1/d2Z 16/32 and d1/d2Z19/32. The smaller area of flowing channel results in an increasing heated area but meanwhile results in an increasing friction greatly, consequently the pump elevating capability decreased. The larger area of flowing channel results in an decreasing heated area per unit solution, the vapor mass fraction inside the channel decreases at the same heat input, the thermosyphon driving 0.13 h=68.2cm d1/d2=16/32 d1/d2=19/32
0.12 Vapor mass fraction
2000 1900 1800
0.11 0.10 0.09 0.08
72
74
76
78
80
82
84
86
Driving temperature T(°C) Fig. 9. Influence of the combining pipe diameters on the vapor mass fraction at the exit of the bubble pump.
d1/d2=19/32 h=68.2cm h=57.5cm h=47.5cm
1700 1600 1500 1400 1300 1200 1100 25000
Fig. 8. Influence of the combing pipe diameters on the water mass flow rate.
0.07 70
821
29000
33000
37000
41000 45000
heat flux q(W.m–2) Fig. 10. Heat transfer coefficient vs heat flux for various motive head.
force decreased, which can weaken heat transfer and elevating capability. Our experiments show that the 19/32 combining tubes is reasonable for solution elevation purpose. 5.4. Characteristics of boiling heat transfer in lunate channel The characteristic of narrow channel heat transfer enhancement is correlative with the narrow channel boiling heat transfer mechanism. The nucleate boiling heat transfer mechanism in the lunate channel mainly includes the thin film beneath deformed bubbles evaporation mechanism, the bubbles disturbing mechanism and natural convective mechanism. The heat transfer coefficient has been plotted against the heat flux in Fig. 10. The figure clearly demonstrates that heat transfer coefficient increases with increase in heat flux, which may be attributed to the change heat transfer mechanism above-mentioned contribution to heat transfer due to bubbles behaviors in the channel, vapor mass fraction and two-phase flow regime change with the heat flux changing. The flow regime is mainly bubbly flow in the channel at lower heat flux. The most of bubbles are isolated sphericity or slightly flattened. The thin film evaporation mechanism to the heat transfer is retarded. The enhancement of heat transfer is weakened. The flow behavior is mainly slug flow in the channel with the heat flux increasing. The bubbles diameter increase, the bigger bubbles are formed due to aggregating one another. The bubble-groups are badly deformed due to the wall flattening. The thin film evaporation mechanism exerts adequately. At the same time, the bubbles disturbing mechanism are also enhanced. So the narrow channel heat transfer enhancement is intensified.
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L. Zhang et al. / International Journal of Refrigeration 29 (2006) 815–822
Because of the complexity of boiling heat transfer, there is not a theory that can fully interpret the mechanism of boiling heat transfer. Furthermore, the mechanism of boiling heat transfer in the narrow channel is more complex, it is very difficult to obtain a theoretic result. Thus, using the experimental data of 19/32 combining tube, a regression analysis of the data resulted in the following correlation for the average heat transfer coefficient: K0:65 0:58
k Z 128h
q
Acknowledgements The authors are grateful to the State National Science Foundation of China for the financial support to the project of 50176036 and 50276048.
References
(10)
6. Summary and conclusions An experimental test rig has been built to test the performance of the bubble pump with lunate channels. The LiBr–H2O solution was heated by the inner and the outer wall simultaneously. The average flow rate of the refrigerant (water) for driving temperature of 75 8C was about 2.0 g s-1.The latent heat of vaporization of water is 2481 kJ kgK1, thus a theoretical cooling capacity of about 4.8 kW can be obtained. The driving temperature is one of the most dominant parameters influencing the bubble pump performance. Changing the driving temperature by 10 8C will result in about 109% change in the water mass flow rates and 15% change in the vapor mass fraction at the exit of the pump tube. It was included that a higher driving temperature is recommended to achieve higher refrigerant flow rates and to obtain higher cooling capacity within the operating ranges. During this study, the elevating effect of this pump bubble is very high. For the reasonable combining tube diameters, the height of solution elevated is 2.5 times as high as motive head, the minimum driving temperature was about 68 8C and the maximum average driving temperature was below 90 8C. Hence, it is conclude that heat from lower potential sources, such as solar energy could be utilized in an absorption refrigerating system with LiBr–H2O solution may utilize. Finally, the boiling heat transfer in lunate channel was analyzed. A heat transfer coefficient on heat flux and motive head for 19/32 combining tubes is obtained.
[1] J. Chen, K.J. Kin, K.E. Herold, Performance enhancement of a diffusion–absorption refrigerator, Int J Refrigeration 19 (3) (1996) 208–218. [2] M. Pfaff, R. Saravanan, M.P. Maiya, M. Srinivasa, Studies on bubble pump for a water–lithium bromide vapor absorption refrigeration, Int J Refrigeration 21 (6) (1998) 452–462. [3] A. Koyfman, M. Jelinek, A. Levy, I. Borde, An experimental investigation of bubble pump performance for diffusion absorption refrigeration system with organic working fluids, Appl Therm Eng 23 (2003) 1881–1894. [4] E.J. Piret, H.S. Isbin, Two-phase heat-transfer in natural circulation evaporator, Chem Eng 50 (1954) 305–311. [5] A.I. Johnson, Circulation rates and overall temperature driving forces in a vertical thermosiphon reboiler, Chem Eng Prog Symp Ser 52 (1956) 37–43. [6] H. Ali, S.S. Alam, Circulation rates in thermosiphon reboiler, Int J Heat Fluid Flow 13 (1) (1992) 86–92. [7] M. Kamil, S.S. Alam, Experimental study of heat transfer to boiling binaries in a reboiler tube, Int J Heat Transfer 39 (7) (1996) 1489–1501. [8] M. Kamil, H. Ali, S.S. Alam, Heat transfer to boiling liquid in single vertical tube thermosiphon reboiler, Exp Therm Fluid Sci 10 (1995) 44–53. [9] S. Arneth, J. Stichlmair, Characteristics of thermosiphon reboiler, Int J Therm Sci 40 (2001) 385–391. [10] S. Aoki, A. Inoue, M. Aritomi, Experimental study on the boiling phenomena within a narrow gap, Int J Heat Mass Transfer 25 (7) (1982) 985–990. [11] Y.Y. Wu, Yu. Lu, L.F. Chen, Boiling heat transfer enhancement of two-phase flow in lunate channel, J Cryog 30 (1990) 287–291. [12] M. Monde, Measurement of liquid film thickness during passage of bubbles in a vertical rectangular channel, J Heat Transfer ASME 112 (2) (1990) 255–258. [13] S. Li, Study on the two-phase flow and boiling heat transfer enhancement of cryogenic fluid in narrow channel, PhD Thesis, Xi’an Jiaotong University, Xi’an, China, 1999.
An experimental investigation on performance of bubble pump with lunate channel for absorption refrigeration system Lianying Zhanga,*, Yuyuan Wua, Hongfei Zhengb, Jingang Guoa, Dongsheng Chena b
a School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China School of Vehicle and Transport Engineering, Beijing Institute of Technology, Beijing, China
Received 19 May 2004; received in revised form 10 November 2005; accepted 16 November 2005 Available online 24 February 2006
Abstract An experimental research on the performance of the bubble pump for absorption refrigeration units was made. The bubble pump provides the drive for the absorption cycle and is a decisive component of the absorption refrigeration unit. The bubble pump’s property determines the efficiency of the absorption refrigeration system. A continuous experimental system with different size of bubbles pumps were designed, constructed and successfully worked. The experiments were performed by changing some of the parameters affecting the bubble pump performance. The experimental results shows that the performance of the bubble pump depends mainly on the driving temperature, the solution head and the combining tube diameters. With the suitable size of section area of the pump tubes the net elevating height of solution is 2.5 times as high as the solution submergence. The lunate channel has several outstanding characteristics, such as low starting temperature (minimum 68 8C), wide operating temperature range and lower requirement for vacuum condition (under 10 kPa). Then the elevating capability of the bubble pump with lunate channel is much better than others currently. It would provide well foundation for practical applications. q 2005 Elsevier Ltd and IIR. All rights reserved. Keywords: Absorption system; Experiment; Pump; Bubble; Geometry; Calculation
Syste`me frigorifique a` absorption: e´tude expe´rimentale sur la performance d’une pompe a` bulles munie d’un canal sous forme de croissant Mots cle´s : Syste`me a` absorption ; Expe´rimentation ; Pompe ; Bulle ; Ge´ome´trie ; Calcul
1. Introduction
* Corresponding author. E-mail address: [email protected] (L. Zhang).
0140-7007/$35.00 q 2005 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2005.11.006
For refrigeration purpose, absorption refrigeration systems are preferred when enough exhaust heat or lowcost alternated energy source is available. In this respect, the LiBr–H2O absorption refrigeration system is widely used,
816
L. Zhang et al. / International Journal of Refrigeration 29 (2006) 815–822
Nomenclature F h k m_ Q q T v_ x0
the wall area of the lunate channel (m2) motive head (m) heat transfer coefficient (W mK2 KK1) mass flow rate (g sK1) heat input (W) heat flux (W m-2) temperature (8C) volume flow rate (ml sK1) vapor mass fraction, dimensionless
Greek Symbols r density (kg mK3)
especially in air-conditioning. But a conventional airconditioning system is driven by higher grade energy source like mechanical/electrical energy to circulate the refrigerant–absorbent solution. Hence, miniaturization of the absorption refrigerators and reduction of electric power consumption requires the replacement of this mechanical pump by a pumping system driven by heat, e.g. a bubble pump. To obtain a higher coefficient of performance and to make it compatible with solar heat (low generator temperature), a two-fluid bubble pump is suggested. Chen et al. [1] study on improvement of the current cycle performance of a diffusion–absorption refrigerator. They designed and fabricated a new generator with heat exchanger to increase the COP of the cycle by 50%. Their generator make up of heating elements, a bubble pump and a coaxial heat exchanger. This configuration reduced heat losses so as to increase the efficiency of the heating process and to increase the COP. Studies on bubble pump both analytically and experimentally were developed by Pfaff et al. [2]. A bubble pump was modeled for intermittent slug flow of solution and vapor mixture. A test rig was built in glass to evaluate the performance of the bubble pump, to visualize the flow regime and to validate the analytical model. In order to study the effect of pipe diameter on the performance of the bubble pump. Pfaff et al. [2] built the test rig with several bubble pumps that were connected in parallel. The bubble pump of Pfaff et al. [2] operated at slug flow regime in cyclic intervals. They found that the pumping ratio (the ratio between the strong solution flow rate and the refrigerant flow rate) is independent of the bubble pump heat input within the operating ranges studied but increase with higher driving head, lower pump lift and smaller tube diameter. It was found that the frequency of the pumping action increases with rising the heat input to the bubble pump, increases in driving head, and decreases in pump lift and tube diameter. Pfaff et al. [2] found that a bubble pump with a tube diameter of 10 mm and heat input of 40 W is suitable for a refrigerator of about 100 W cooling capacity.
Subscript cw condensate water hw hot water hwi inlet of hot water hwo outlet of hot water ps poor solution rs rich solution si inlet of solution so outlet of solution w water
A number of parallel bubble pumps were required for increasing the capacity of the refrigerator. Koyfman et al. [3] presented an experimental investigation to study the performance of the bubble pump for diffusion absorption refrigeration units. A continuous experimental system was designed, built and successfully operated. In the experiments, some of the parameters affecting the bubble pump performance were changed. During the experimental investigation, photographs were taken. It can be seen that the bubble pump operates at slug flow regime with a churn flow regime at the entrance of the bubble pump tube. The prediction of rates of heat transfer and thermally induced flow (circulation rate) is the primary requirement for the design of a bubble pump. The experimental measurement of these rates was carried out earlier by Piret and Isbin [4,5] on an electrically heated pipe. Johnson [5] measured the circulation rate in a steam-heated vertical thermosyphon elevating tube of standard design. The effects of heat flux, submergence and physical properties were studied experimentally by many workers [6–9]. In this paper, the bubble pump is a full-length heating bubble pump with lunate channel, the working fluid is water-lithium bromide solution. The present experiment will study the effect of driving temperature, motive head and design parameters on performance of the bubble pump.
2. Theoretical foundation of boiling heat transfer enhancement for lunate channel Narrow slot thermosiphon boiling heat transfer has two notable characteristics [10]: both in remarkable heat transfer enhancement, especially at high heat flux, and in small temperature difference for boiling heat transfer. The lunate channel is formed by locating a tube extreme eccentrically inside a bigger tube so as to make the outer wall of smaller tube to contact the inner wall of the bigger tube at one straight line theoretically, which is shown in
L. Zhang et al. / International Journal of Refrigeration 29 (2006) 815–822
A
A
817
Convective heat Annular transfer through flow liquid film
Slug flow
Bubbly flow
Cross-section A-A Fig. 1. The construction of the lunate channel.
Fig. 1. Both inner and outer tubes are soldered together at both ends of the contacting line. This configuration have several advantages as follows [11] (1) it is convenience to machining; (2) the pointed corners of the lunate channel cross section can provide artificial nucleation sites, the bubbles will create easily at it; (3) the wider part of the lunate channel operate as liquid filling, which can prevent the wall being dried. Because the pointed triangles of lunate channel operate as boiling nuclei, bubble would be easily formed, especially with the inner and the outer wall heating fluids simultaneously. The hot boundary layer is thinner, so the superheat degree for forming bubble is decreased greatly. The flow regimes along the heated length of the test sample have been shown in Fig. 2 [13]. The subcooled liquid entering the tube gets heated by single phase convection and moves upwards. In the liquid ascending process, the liquid undergoes single-phase flow, bubbly flow and slug flow in turn. Monde [12] showed that slug flow is in favor of elevating solution much more. Because water–LiBr solution is a viscous fluids, in order to obtain slug flow inside the channel, it is important to choose the cross section size of the channel. If the channel is too narrow, the flow resistance of two-phase will be increased, which will weaken elevating solution. If the channel is too wider, the boiling required heat flux is increased. The circulation of fluid flow will be unstable. The choice of present experimental channel has considered all aspects.
Single Phase flow
Saturated nucleate boiling
Subcooled boiling
Convective heat transfer
Fig. 2. Flow patterns. Boiling regimes along a bubble pump.
3. Experimental apparatus and procedures The whole experimental system mainly consists of five portions: a test section (11), condenser (3), vapor–liquid separator (9), liquid container (6), heat source, pressure balanced tube (10) and connecting tube, etc. The system is made of stainless steel except heat-exchange element. The bubble pump served as the test section (11). The outer and inner tubes were made of purple copper. The outer tube was of 32 mm i.d. and 1660 mm long. The inner tubes were, respectively, of 16, 19, 22 mm i.d. and 1450 mm long. The elevation tube connected with the liquid container (6) and joined with a vapor–liquid separator (9) and total condenser vessels (3) forming a thermosiphon loop as schematically shown in Fig. 3. The hot water provided by heater and pumped by a centrifugal hot water pump entered the inner and the outer tube from the bottom, and then went out from the top. Thus, the vertical lunate channel was heated by hot water in the inner tube and outside the outer tube simultaneously. The weak solution of known concentration was boiled in the lunate channel, and was driven upward due to density difference between the liquid in the container (6) and boiling two-phase fluid in test section (11)
818
L. Zhang et al. / International Journal of Refrigeration 29 (2006) 815–822
1 P
2 Inlet of cooling water
16
25
3
9
18
10 Outlet of cooling water
m Outlet of hot water
22
21
4 19 5
lift
24 15
m
m 14
8
12
11 23 7 6
13 m
Motive head Inlet of hot water
20 17 Fig. 3. Schematic diagram of the experimental set-up 1, vacuum watch; 2, vacuum valve; 3, condenser; 4, cooling water meter; 5, distilled water reflux tube; 6, liquid container; 7, hot water valve; 8, sleeve muff of heating solution; 9, vapor–liquid separator; 10, pressure balanced tube; 11, test section; 12, solution reflux tube; 13, hot water meter; 14, rich solution corrosion preventive rotameter; 15, distilled water rotameter; 16, cooling water valve; 17, drain valve; 18, inlet of cooling water thermocouple; 19, outlet of cooling water thermocouple; 20, inlet test section of poor solution thermocouple; 21, outlet test section of solution thermocouple; 22, outlet vapor–liquid of rich solution thermocouple; 23, inlet of hot water thermocouple; 24, outlet of hot water thermocouple; 25, partition.
and then entered into the separator (9). The rich solution was drained down the bottom of the separator (9) while vapors were led to a condenser vessel (3) through partition (25). The rich solution moved downwards and entered the liquid container through solution reflux tube (12). Distilled water out of the condenser (3) also entered the liquid container (6) through distilled water reflux tube (5). They were mixed in the liquid container (6) forming poor solution that re-entered the test section (11) through a joint. The distilled water volume flow rate was measured by a LZB-10 rotameter (15) and the rich solution volume flow rates were measured by a LZB-10F corrosion preventive flowmeters (14). The stabilized hot water was supplied through an electrical heater. The hot water flow rates measured by a hot water meter (13). The cooling water flow rate was maintained by means of a centrifugal water pump and was measured by an ordinary water meter (4). A TX-HB-FF copper-constantan thermocouple (20) was applied to measure the inlet liquid temperature. The temperature of the boiling liquid before entry to the vapor–liquid separator was measured by another thermocouple (21). TX-HB-FF copper-constantan thermocouples were also placed to measure the temperatures in and
around the condenser and other strategic locations in the reboiler loop to ensure a reliable computation of circulation rates through the heat balance. The electric potential signal offering by thermocouples are directly read through digital microvolt meter. Drain valve (17) was provided at the inlet of test section for feeding and draining solution. The entire set up was thoroughly lagged with asbestos rope and glass wool and finally covered with a thin aluminium sheet to reduce the heat losses. After steady state was reached as indicated by the thermocouples readings, the experimental data were recorded.
4. Data processing In the experimental system, the water mass flow rates were obtained by m_ cw Z v_cw rw
(1)
The poor solution volume flow rates were calculated by making a mass balance on the test section: v_ps z v_cw C v_rs
(2)
L. Zhang et al. / International Journal of Refrigeration 29 (2006) 815–822
Then, the poor solution mass flow rates were computed by (3)
The vapor mass fraction of the two-phase mixture at the exit of test section is: m_ x Z cw m_ ps 0
(4)
The total heat inputted to the solution by the hot water is: Q Z V_ hw rw ðThwi KThwo Þ
(5)
The heat flux is: qZ
Q F
(6)
18.0 Rich solution volume flow rate (mls–1)
m_ ps Z v_ps rps
Q F Thw KTs
d1/d2=19/32 h=68.2cm h=57.5cm h=47.5cm
16.5 15.0 13.5 12.0 10.5 9.0 7.5 6.0 4.5 70
The heat transfer coefficient is: kZ
(7)
819
72
74 76 78 80 82 Driving temperature T(°C)
84
86
Fig. 4. Variation of rich solution volume flow rate with driving temperature for different motive head.
where: 1 Z ðThwi C Thwo Þ Thw 2
(8)
1 Ts Z ðTsi C Tso Þ 2
(9)
5. Results and discussions The operating parameters were driving temperature, motive head and combining tube diameters. All the runs were conducted under vacuum condition (about 10 kPa). Here, the driving temperature specially refers to hot water temperature and the motive head is defined as the height of the poor solution level in the channel as is illustrated at Fig. 3. The influences of the major operational and design parameters on distilled water yield, elevating solution volume and the vapor mass fraction of the two-phase mixture at the exit of test section will be discussed as follows.
temperature is 75 8C, the water mass flow rate attained around 2.0 g sK1. If the water can vaporize completely in evaporator, the refrigerating capacity of the unit may reach more than 4.8 kW, which shows that the lunate channel has strong heat transfer at small temperature difference in compare to the annular or round channel [13], and the designed bubble pump has strong elevating ability. The volume flow rate elevated rich solution and water mass flow rate with difference motive head for 19/32 combining tube diameters are plotted versus the driving temperature in Figs. 4 and 5. The figures clearly illustrate that the flow rates of elevated rich solution and water basically increase with driving temperature after the minimum driving temperature is reached. When the driving temperature is above 85 8C, the flow rates increase slowly. Because the slug flow may change to the annular flow at the outlet of the channel due to the temperature increase. However, the void fraction inside the channel is higher at the higher heat flux, the shortage of 3.75
5.1. Influence of driving temperature It was found that a certain minimum driving temperature Tmin is required for the pump to operate. Before Tmin was reached, the driving force was not sufficient for pumping action. The minimum driving temperature depends on the operating conditions and the dimensions of the pump (combining tube diameters) and the system itself (motive head). The minimum driving temperature shows an upward tendency for lower driving head and for unsuitable combining tube diameters. For the 19/32 combining tubes pumping action started around 68 8C, whereas for the 16/32 combining tubes a minimum driving temperature close to 76 8C was required. If the hot water temperature is at 75–85 8C, the system runs well. When the driving
Water mass flow rate (gs–1)
3.50 3.25 3.00
d1/d2=19/32 : h=68.2cm : h=57.5cm : h=47.5cm
2.75 2.50 2.25 2.00 1.75 1.50 1.25 70
72
74
76
78
80
82
84
86
Driving temperature T(°C) Fig. 5. Variation of water mass flow rate with driving temperature for different motive head.
L. Zhang et al. / International Journal of Refrigeration 29 (2006) 815–822
0.23
d1/d2=19/32 h=68.2cm h=57.5cm h=47.5cm
vapor mass fraction
0.21 0.19 0.17 0.15 0.13 0.11 0.09 0.07 70
72
74
76
78
80
82
84
86
Driving temperature T(°C) Fig. 6. Variation of vapor mass fraction at exit of the pump with driving temperature for different motive head.
solution upside the channel aggravates the heat transfer, thus, the elevated solution decreases. Fig. 6 shows the variations of vapor mass fraction at the exit of the test channel with the driving temperature for different motive head and for 19/32 combining tubes. It indicates that the trends of the variation of the vapor mass fraction are somewhat similar to those of the water mass flow rate, whereas the increase of water mass flow rates is faster than that of the vapor mass fraction at the exit of the test at any motive head. For example the motive head is 57.5 cm, the water mass flow rate increases by 109% with the driving temperature increasing by 10 8C, whereas the vapor mass fraction increases by 15%. As the higher driving temperature, the more heat input to the fluids and the more water is vaporized, correspondingly which means a higher driving force developed by the bubble pump, thus more solution is pumped, however the vapor mass fraction increases with increase in driving temperature with less gradient. 5.2. Influence of motive head The motive head is a very important operational parameter for the bubble pump. Because the height (1660 mm) of exit port of the rich solution is fixed, the variation of the motive head is achieved only by increasing or decreasing the amount of poor solution in the liquid container. It can be observed from the Figs. 4–6 that a higher motive head leads to a higher solution volume rate and a lower vapor mass fraction at any driving temperature, and vice versa. But water mass flow rate is almost invariable with the variation of the motive head. The increase in the solution volume flow rate is caused by the increased driving force due to a increased motive head and by decreased flowing resistance of the two-phase mixture resulting from a lower pump lift. At the same time, the convective heat transfer regime and the subcooled boiling regime in flow channel are longer and correspondingly the vapor mass
fraction in the channel and at the exit of the channel are smaller. The amount of the water is equal to the amount of lifted solution times the vapor mass fraction at the exit of the channel, so the water mass flow rate depends on the vapor mass fraction and the amount of lifted solution. Contrarily, if the driving force decreases due to a lower motive head and the resistance of the mixture increases by a higher pump lift at the same time, only a little solution is elevated into the gas–liquid separator and the poor solution remains in the pump for a longer time. Thus a larger amount of the heat input into the bubble pump is utilized to heat the solution. But less heat input is needed for heating the incoming solution and so the vapor mass fraction increases. As the motive head is 475 mm, the elevating height is 1185 mm, which is about 2.5 times as high as the motive head. This clearly illustrates that the elevating capability of the bubble pump is very strong. If the operating temperature is higher or the system pressure is lower, its elevating capability will be further increased. This provides well foundation for the practical application of the bubble pump with lunate channel. 5.3. Influence of combining pipe diameters Due to increasing contacting area between solution and wall at the corner of lunate channel, the heat flux increases greatly in the corner. At the same time, the corner of the lunate cross section provides artificial nucleation sites. It enhances the boiling heat transfer. When the bubbles departed from the wall and moved to wider passage, it brings about the second flow in the channel. This greatly promotes boiling of solution inside the TEST. Though lunate channel increases frictional resistance, it is favorable to elevating solution due to enhanced heat transfer. Therefore, it is very important to choose reasonable passage area. It is observed from the experimental results that the 21 Rich solution volume flow rate(mls–1)
820
19 17
h=68.2cm d1/d2=16/32 d1/d2=19/32
15 13 11 9 7 70
72
74
76
78
80
82
84
86
88
Driving temperature T(°C) Fig. 7. Influence of the combing pipe diameters on the rich solution volume flow rate.
L. Zhang et al. / International Journal of Refrigeration 29 (2006) 815–822
2100
Water mass flow rate(gs–1)
3.20
heat transfer coefficient k(W.m–2.K)
3.70 3.45
h=68.2cm d1/d2=16/32 d1/d2=19/32
2.95 2.70 2.45 2.20 1.95 1.70 1.45 70
72
74 76 78 80 82 Driving temperature T(°C)
84
86
elevating efficiency and the minimum driving temperature to start circulation is strongly dependent on the combining tubes diameters. For example, the elevating effect of d1/d2Z 19/32 is the best and its required minimum driving temperature is about 68 8C. Then the elevating effect of d1/d2Z16/32 is better and its required minimum driving temperature is closely 76 8C. Finally that of d1/d2Z22/32 is the poorest, it is difficult to sustain continuous flow through the system and the experimental date are difficult to be recorded. The Figs. 7–9 illustrated the effect of d1/d2Z 16/32 and d1/d2Z19/32. The smaller area of flowing channel results in an increasing heated area but meanwhile results in an increasing friction greatly, consequently the pump elevating capability decreased. The larger area of flowing channel results in an decreasing heated area per unit solution, the vapor mass fraction inside the channel decreases at the same heat input, the thermosyphon driving 0.13 h=68.2cm d1/d2=16/32 d1/d2=19/32
0.12 Vapor mass fraction
2000 1900 1800
0.11 0.10 0.09 0.08
72
74
76
78
80
82
84
86
Driving temperature T(°C) Fig. 9. Influence of the combining pipe diameters on the vapor mass fraction at the exit of the bubble pump.
d1/d2=19/32 h=68.2cm h=57.5cm h=47.5cm
1700 1600 1500 1400 1300 1200 1100 25000
Fig. 8. Influence of the combing pipe diameters on the water mass flow rate.
0.07 70
821
29000
33000
37000
41000 45000
heat flux q(W.m–2) Fig. 10. Heat transfer coefficient vs heat flux for various motive head.
force decreased, which can weaken heat transfer and elevating capability. Our experiments show that the 19/32 combining tubes is reasonable for solution elevation purpose. 5.4. Characteristics of boiling heat transfer in lunate channel The characteristic of narrow channel heat transfer enhancement is correlative with the narrow channel boiling heat transfer mechanism. The nucleate boiling heat transfer mechanism in the lunate channel mainly includes the thin film beneath deformed bubbles evaporation mechanism, the bubbles disturbing mechanism and natural convective mechanism. The heat transfer coefficient has been plotted against the heat flux in Fig. 10. The figure clearly demonstrates that heat transfer coefficient increases with increase in heat flux, which may be attributed to the change heat transfer mechanism above-mentioned contribution to heat transfer due to bubbles behaviors in the channel, vapor mass fraction and two-phase flow regime change with the heat flux changing. The flow regime is mainly bubbly flow in the channel at lower heat flux. The most of bubbles are isolated sphericity or slightly flattened. The thin film evaporation mechanism to the heat transfer is retarded. The enhancement of heat transfer is weakened. The flow behavior is mainly slug flow in the channel with the heat flux increasing. The bubbles diameter increase, the bigger bubbles are formed due to aggregating one another. The bubble-groups are badly deformed due to the wall flattening. The thin film evaporation mechanism exerts adequately. At the same time, the bubbles disturbing mechanism are also enhanced. So the narrow channel heat transfer enhancement is intensified.
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L. Zhang et al. / International Journal of Refrigeration 29 (2006) 815–822
Because of the complexity of boiling heat transfer, there is not a theory that can fully interpret the mechanism of boiling heat transfer. Furthermore, the mechanism of boiling heat transfer in the narrow channel is more complex, it is very difficult to obtain a theoretic result. Thus, using the experimental data of 19/32 combining tube, a regression analysis of the data resulted in the following correlation for the average heat transfer coefficient: K0:65 0:58
k Z 128h
q
Acknowledgements The authors are grateful to the State National Science Foundation of China for the financial support to the project of 50176036 and 50276048.
References
(10)
6. Summary and conclusions An experimental test rig has been built to test the performance of the bubble pump with lunate channels. The LiBr–H2O solution was heated by the inner and the outer wall simultaneously. The average flow rate of the refrigerant (water) for driving temperature of 75 8C was about 2.0 g s-1.The latent heat of vaporization of water is 2481 kJ kgK1, thus a theoretical cooling capacity of about 4.8 kW can be obtained. The driving temperature is one of the most dominant parameters influencing the bubble pump performance. Changing the driving temperature by 10 8C will result in about 109% change in the water mass flow rates and 15% change in the vapor mass fraction at the exit of the pump tube. It was included that a higher driving temperature is recommended to achieve higher refrigerant flow rates and to obtain higher cooling capacity within the operating ranges. During this study, the elevating effect of this pump bubble is very high. For the reasonable combining tube diameters, the height of solution elevated is 2.5 times as high as motive head, the minimum driving temperature was about 68 8C and the maximum average driving temperature was below 90 8C. Hence, it is conclude that heat from lower potential sources, such as solar energy could be utilized in an absorption refrigerating system with LiBr–H2O solution may utilize. Finally, the boiling heat transfer in lunate channel was analyzed. A heat transfer coefficient on heat flux and motive head for 19/32 combining tubes is obtained.
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