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Experimental investigations on a R134a ejector applied in a refrigeration system
Applied Thermal Engineering 110 (2017) 1061–1065
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Experimental investigations on a R134a ejector applied in a refrigeration system Jiwei Yan, Guangming Chen, Chengyan Liu, Liming Tang, Qi Chen ⇑ Key Laboratory of Refrigeration and Cryogenic Technology of Zhejiang Province, Institute of Refrigeration and Cryogenics, Zhejiang University, Hangzhou 310027, Zhejiang Province, China
h i g h l i g h t s Back pressure plays an important role on performance of a R134a ejector. Primary fluid pressure has positive effects on critical back pressure. An optimal primary fluid pressure corresponds to a maximum entrainment ratio. A logarithmic relation to predict critical back pressure was presented.
a r t i c l e
i n f o
Article history: Received 14 June 2016 Revised 2 September 2016 Accepted 9 September 2016 Available online 10 September 2016 Keywords: R134a Ejector Back pressure Entrainment ratio Critical back pressure
a b s t r a c t An experimental investigation on the performance of an ejector applied in a refrigeration system is conducted. The system uses R134a as a working fluid. The experimental results indicate that back pressure plays an important role in the entrainment ratio of an ejector because this parameter determines the operation mode of an ejector under certain primary fluid pressure and secondary fluid pressure. Increasing in the primary fluid pressure can improve the critical back pressure while deteriorate the best performance of an ejector. Moreover, an optimal value of the primary fluid pressure exists corresponding to a maximum value of the entrainment ratio under a given condition. A logarithmic relation is developed to predict the critical back pressure and to determine operating conditions in a R134a ejector refrigeration system. The research findings will be meaningful for researchers to have a better understanding of the R134a ejector applied in a refrigeration system. Ó 2016 Published by Elsevier Ltd.
1. Introduction The first ejector refrigeration system was developed by Maurice Leblance about 1910 in Paris [1]. Being driven by heat instead of mechanical work or electric energy, an ejector system is appropriate for occasions where such resource shortages occur. After experiencing a brief season of prosperity, the ejector system was substituted for the more efficient vapor-compression system. However, in present times with growing concerns about the environment and energy, investigations and applications of ejector refrigeration systems have been increasingly conducted in recent years, as these systems can be non-polluting and be powered by low-grade heat energy or renewable energy [2,3]. The critical component of the system, the ejector, invented by Charles Parsons, originally for pumping air out of the condenser of a steam power ⇑ Corresponding author at: Institute of Refrigeration and Cryogenics, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China. E-mail address: [email protected] (Q. Chen). http://dx.doi.org/10.1016/j.applthermaleng.2016.09.046 1359-4311/Ó 2016 Published by Elsevier Ltd.
plant [1]. Afterwards, many theoretical and experimental studies have been carried out to investigate its fundamentals and operational behavior, as studies have demonstrated that the performance of an ejector system depends mostly on the operation and performance of an ejector [4]. Riffat et al. [5] and Besagni et al. [6] have presented comprehensive literature reviews on ejectors and ejector systems, including ejector models, ejector geometric optimization, performance and applications of ejectors, refrigerants and so on. Huang et al. [7] developed Joseph Henry Keenan et al.’s one dimensional model by assuming that the two fluids start to mix at the hypothetical throat with an uniform pressure. It was demonstrated experimentally that this model can accurately predict the performance of ejector within ±23% of error. Allouche et al. [8] presented a numerical study of a steam ejector using CFD to identify the suitable experimental conditions. It was found that, for a given ejector configuration, there exists an optimum primary vapor temperature at particular condenser and evaporator temperatures, which yields maximum entrainment ratio and COP. Selvaraju and Mani [9] investigated the performance of a vapor
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ejector refrigeration system and carried out a parametric study with six ejectors, having different geometrical dimensions. They concluded that the performance of an ejector and ejector system strongly relies on the ejector configuration. During the phase-out of CFCs and HFCs under the Montreal Protocol and the Kyoto Protocol, manufactures of refrigerating systems and heat pumps such as car air conditioners and household refrigerators have modified their products to use HFCs instead. The new F-gas Regulation has come into force on 1 January 2015 and strengthened measures on limiting the application of F-gases. However, America and Canada have disaffiliated from the Kyoto Protocol. Furthermore, there are no existed regulations or standards to definitely weed out F-gases in Africa. In other words, HFCs still have broad application prospect. Among these substitutes, R134a has been taken as a leading alternative refrigerant. Because of its preeminent safety performance, R134a has been classified into A1 group by the ISO 5149-1-2014 [10]. A lot of research has also demonstrated that R134a is suitable for lowgrade heat source [11–13]. R134a has being received a partiality in most markets at present, especially in car air conditioners [14,15]. Recently its application in ejector systems has attracted much attention [16–18]. Huang et al. [7] have demonstrated that the back pressure of an ejector is an important parameter as it has an immediate impact on the efficiency of an ejector. However, through literature researches, we know that experimental studies on the effect of back pressure on the performance of an ejector operating with R134a have not been conducted intensively. We believe that these works are indispensable for further applications and popularizations of ejector refrigeration systems with R134a. In the present paper, by utilizing R134a as the working fluid, experimental approaches are employed to specially investigate the variation of the performance of an ejector with back pressure. 2. Experimental ejector H62 brass was chosen as the material to fabricate the ejector investigated in this paper and a schematic diagram is presented in Fig. 1. As can be seen in the figure, this ejector consists of four parts: Laval nozzle, suction chamber, mixing chamber and diffuser. The main parameters of each part are listed in Table 1. The working process in an ejector can be described as the following: the primary fluid with high pressure and temperature flows into Laval nozzle and then undergoes an adiabatic expansion. At the exit of the nozzle, the primary fluid with supersonic velocity produces a vacuum region and the secondary fluid is entrained into suction chamber. These two fluids mix with each other in mixing chamber to complete mixing process. After that, the mixed fluid with subsonic velocity is produced and subsequently enters into the diffuser, where its velocity gradually decreases while the pressure escalates. Ultimately, the compressed fluid with higher
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Table 1 Main parameters of the tested ejector. Geometry parameters
Values/mm
Diameter of Laval nozzle inlet Diameter of Laval nozzle throat Diameter of Laval nozzle exit Diameter of mixing chamber Diameter of diffuser exit Laval nozzle exit position
7.00 1.08 1.41 2.88 14.00 2.65
pressure than that of the secondary fluid discharges out of the ejector. This working process is usually accompanied by supersonic flows, chockes, shock waves and other complex flow phenomena [19]. The widespread use of optical methods and Computational Fluid Dynamics (CFD) have given a big boost to the visualization research on ejectors [5,20]. To characterize the ejector performance, the entrainment ratio is utilized as an indicator, which is defined as:
l ¼ Gh =Gp
ð1Þ
where Gh is the secondary fluid mass flow rate, and Gp is the primary fluid mass flow rate.
3. Experimental apparatus and measuring system 3.1. Experimental rig To investigate the performance of the above ejector, an experimental apparatus was designed and constructed. As shown in Fig. 2, the apparatus was mainly composed of an ejector refrigeration system, a cooling water system, a chilled water system and a hot water system. R134a was utilized as the working fluid. The major components of the ejector refrigeration system include the ejector, a condenser, an evaporator, a generator, a metering pump and a throttle valve. A coaxial condenser (ET072SC), produced by the Extek Energy Equipment (Zhejiang) Co. Ltd., was used and cooled by the cooling water system. The temperatures of the inlet and outlet of this system was monitored. A reconstructive compression-type water chiller was prepared to guarantee the normal work of the cooling water system when the cooling water temperature got too high. The evaporator was also provided by the Extek Energy Equipment (Zhejiang) Co. Ltd. The chilled water system, heated by a U-electrical heating rod with a power of 1.5 kW, uninterruptedly supplied hot water to the shell and coil evaporator, resulting in the liquid R134a evaporates in the heat exchanger. That was how the secondary fluid produced. The heating power of this electrical heating rod was adjusted by a contacting voltage-regulator, so that the heat exchange capacity of the evaporator was controlled.
1.42
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Fig. 1. A schematic diagram of the tested ejector.
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Ejector
Evaporator
Condenser
Cooling water
Liquid-storage tank
Throttle valve
Chilled water Generator
Hot water Metering pump Fig. 2. A schematic view of the ejector refrigeration system.
The generator, fabricated from a stainless steel cylinder with a diameter of 426 mm and a length of 600 mm, was used to generate the primary fluid. The hot water system, powered by three electrical heating rods with a total power of 6.6 kW, served as a heat source for the generation of the primary fluid. A Hydro/3-025170 hydraulic diaphragm metering pump, produced by the ProMinent Fluid Controls, Inc., was installed between the generator and the liquid-storage tank. An adjustable flow could feed the generator because the stroke length of this metering pump could be adjusted between 0 and 100%.
3.2. Measuring system (1) Copper-constantan thermocouples and PT100 platinum resistances with an uncertainty of ±0.1 K were used to measure the temperatures. To improve the reliability of the measurement, all the measurement elements were affixed onto the pipe walls, and all the pipes were covered by thermal insulation materials. (2) All the pressures were monitored by high-precision pressure transducers with an accuracy of ±0.2% of their full scales. Their measuring ranges were 0–1.0 MPa for the inlet and outlet of the evaporator and inlet of the secondary fluid, 0–2.0 MPa for the outlet of the condenser, 0–2.5 MPa for the inlet of the condenser and outlet of the ejector, and 0–4.0 MPa for the inlet of the primary fluid, respectively. (3) Two Coriolis-type mass flow meters were mounted to monitor the mass flow rates of the primary fluid and the secondary fluid. The accuracy of these two meters is ±0.35% of measured values where their ranges were 0–0.04 kg/s and 0–0.01 kg/s, respectively. The output from all the measurement devices except two mass flow meters were collected and transferred to a desktop computer through the Agilent 34970A Data Acquisition/Switch Unit in realtime. Being directly connected to the computer with connectors, the data of these mass flow meters were recorded by the Data Acquisition and Communication Program.
According to the formula (1), the uncertainty of the entrainment ratio could be represented as [21]:
uðlÞ ¼ sqrtðð@ l=@Gh Þ2 uðGh Þ2 þ ð@ l=@Gp Þ2 uðGp Þ2 Þ
ð2Þ
Thus the relative uncertainty of the entrainment ratio could be estimated by:
uðlÞ=l ¼ sqrtðuðGh Þ2 =G2h þ uðGp Þ2 =G2p Þ
ð3Þ
Having integrated the experimental data and the accuracy of flow meters, the experimental uncertainty analysis has been conducted and demonstrates that the relative uncertainty of the entrainment ratio of the experimental ejector is about 0.50%. 4. Results and discussion The experimental investigations of the performance of the R134a ejector were performed under the conditions of the primary fluid and the secondary fluid pressures are 2.14–2.495 MPa and 0.31 MPa respectively, the back pressure varies from 0.44 MPa to 0.64 MPa. The variations of the entrainment ratio l with the back pressure Pc is drawn in Fig. 3. Additionally Fig. 4 depicts how the secondary fluid mass flow rate Gh varies with the back pressure Pc. According to Huang et al.’s definitions [7], Fig. 3 suggests that ejector operates with three modes under constant primary fluid pressure Pp and constant secondary fluid pressure Ph. The first one is double-choking mode: the primary fluid and the secondary fluid are both choked and the entrainment ratio l remains unchanged. The second one is single-chocking mode: only the primary fluid is choking and the entrainment ratio l lowers gradually with the back pressure Pc increasing. The third one is back-flow mode: neither the primary fluid nor the secondary fluid is choked but the secondary fluid is refluent, the entrainment ratio l is negative and ejector is out of work. From Fig. 3, it is also observed that a plateau of the entrainment ratio l, which is corresponding to the best operation of the ejector, keeps constant until the back pressure increases to the critical back pressure. As the back pressure progressively raises, the entrainment ratio l goes down gradually. All three curves in Fig. 3 indicate
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1.0
0.7 Pp=2.148MPa, Ph=0.31MPa
0.6
Pp=2.34MPa, Ph=0.31MPa
0.8
Pp=2.495MPa, Ph=0.31MPa
0.5 0.6
μ
μ
0.4 0.3
0.4
0.2
Pc=0.71 MPa, PH=0.46 MPa Pc=0.74 MPa, PH=0.46 MPa
0.2
0.1
Pc=0.86 MPa, PH=0.63 MPa
0.0
Pc=0.89 MPa, PH=0.63 MPa
0.0
0.44 0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60 0.62 0.64 0.66
2.2
Pc (MPa)
0.005 Pp=2.148MPa, Ph=0.31Mpa Pp=2.34MPa, Ph=0.31Mpa
Gh (kg/s)
2.6
2.8
3.0
3.2
Pp (MPa)
Fig. 3. Effect of back pressure on entrainment ratio.
Pp=2.495MPa, Ph=0.31Mpa
0.004
2.4
0.003
Fig. 5. Effect of primary fluid pressure on entrainment ratio.
On the basis of Huang et al.’s report [7], the Engineering Equation Solver was employed to predict the ejector performance at critical and sub-critical operational regimes. Then a logarithmic relation between the critical back pressure and the primary fluid pressure Pp, for the secondary fluid pressure is 0.31 MPa while the back pressure varying from 0.3 MPa to 0.66 MPa and the primary fluid pressure ranging from 1.5 MPa to 3.0 MPa, can be developed:
0.002
Pc ¼ 0:2151 ln Pp þ 0:3166
0.44 0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60 0.62 0.64 0.66
Pc (MPa) Fig. 4. Effect of back pressure on secondary fluid mass flow rate.
the same trend. Fig. 4 can be used to interpret the above phenomena. As seen in this figure, when the back pressure is lower than the critical back pressure, the ejector operates with the doublechoking mode that the secondary fluid is choked, the secondary fluid mass flow rate Gh is steady. As the back pressure Pc continues to increase, the ejector works with the single-choking mode and the secondary fluid is not choked, contributing to the secondary fluid mass flow rate Gh decreases constantly. However, the primary fluid mass flow rate Gp has been invariable under the certain conditions. Consequently, increasing the back pressure makes the entrainment ratio be a constant and then decrease. In addition, it is evident that in Fig. 3(a) for the same secondary fluid pressure, an increase in the primary fluid pressure Pp leads to an increase in the critical back pressure, and (b) for a given ejector, the greater the primary fluid pressure Pp, the lower the maximum entrainment ratio that the ejector can perform. That is because at a given secondary fluid pressure Ph, an increase in the primary fluid pressure Pp can increase the primary fluid mass flow rate Gp. When the back pressure Pc and the secondary fluid pressure Ph remain unchanged, variation of the entrainment ratio l versus the primary fluid pressure Pp is displayed in Fig. 5. It can be found that l increases as Pp increases because an increase in Pp causes an increase in Gp while Gh entering the ejector increases more relatively. However, after an optimal value of Pp, a further increase in Pp leads to a decrease in l because the degree of increase in Gh is lower.
0.65 Experimental values
0.60 0.55 0.50
*
0.000
ð4Þ
The correlation coefficient for the critical back pressure and the primary fluid pressure Pp is as high as 0.9952. Fig. 6 shows comparison of experimental values of the critical back pressure and the prediction curve. It can be found that the experimental values have a good agreement with the predictions. Although the critical back pressure has great effects on ejector performance, there is few research touching upon the relationship between this parameter and other ones in published literatures. Obviously, the relation proposed above has great significance for predicting the critical back pressure, the R134 ejector refrigeration system will run in the best possible way because the optimal operating conditions of this system can be determined. Furthermore, this relation can provide guidance for designing of R134a ejector.
Pc (MPa)
0.001
0.45 0.40 0.35 0.30
1.6
1.8
2.0
2.2
2.4
2.6
2.8
Pp (MPa) Fig. 6. Comparison of experimental values and the prediction curve.
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5. Conclusions In the present study, the performance of an existed ejector was investigated preliminarily with R134a based on an ejector refrigeration system. The experimental data have verified the effects of back pressure on the performance of an ejector. The following conclusions can be summarized:
[3]
[4] [5] [6]
(1) The back pressure exerts a tremendous influence on the entrainment ratio. When the back pressure is lower than the critical back pressure, ejector operates with the best performance. Otherwise, a deterioration of ejector performance occurs as the back pressure is greater and greater. This indicates that the ejector had better perform with the critical mode for a better performance. (2) An increase in the primary fluid pressure induces an increase in the critical back pressure and a decrease in the maximum entrainment ratio at a given secondary fluid pressure. (3) An optimal value of the primary fluid pressure exists, corresponding to a maximum value of the entrainment ratio under a certain condition. (4) A logarithmic relation was presented to predict the critical back pressure under certain conditions in a R134a ejector refrigeration system. This relation can provide important references for predicting the critical back pressure and guiding operation conditions design.
[7] [8]
[9] [10] [11]
[12] [13]
[14]
[15] [16]
[17]
Acknowledgements This research work is financially supported by the National Natural Science Foundation of China under Award No. 51376156. The assistance of Xuan Chen is greatly appreciated by the first author for polishing up this paper. References [1] W.B. Gosney, Principle of Refrigeration, Cambridge University Press, New York, 1982. [2] C. Li, Y.Z. Li, W.J. Cai, Y. Hu, H.R. Chen, J. Yan, Analysis on performance characteristics of ejector with variable area-ratio for multi-evaporation
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[21]
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Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Experimental investigations on a R134a ejector applied in a refrigeration system Jiwei Yan, Guangming Chen, Chengyan Liu, Liming Tang, Qi Chen ⇑ Key Laboratory of Refrigeration and Cryogenic Technology of Zhejiang Province, Institute of Refrigeration and Cryogenics, Zhejiang University, Hangzhou 310027, Zhejiang Province, China
h i g h l i g h t s Back pressure plays an important role on performance of a R134a ejector. Primary fluid pressure has positive effects on critical back pressure. An optimal primary fluid pressure corresponds to a maximum entrainment ratio. A logarithmic relation to predict critical back pressure was presented.
a r t i c l e
i n f o
Article history: Received 14 June 2016 Revised 2 September 2016 Accepted 9 September 2016 Available online 10 September 2016 Keywords: R134a Ejector Back pressure Entrainment ratio Critical back pressure
a b s t r a c t An experimental investigation on the performance of an ejector applied in a refrigeration system is conducted. The system uses R134a as a working fluid. The experimental results indicate that back pressure plays an important role in the entrainment ratio of an ejector because this parameter determines the operation mode of an ejector under certain primary fluid pressure and secondary fluid pressure. Increasing in the primary fluid pressure can improve the critical back pressure while deteriorate the best performance of an ejector. Moreover, an optimal value of the primary fluid pressure exists corresponding to a maximum value of the entrainment ratio under a given condition. A logarithmic relation is developed to predict the critical back pressure and to determine operating conditions in a R134a ejector refrigeration system. The research findings will be meaningful for researchers to have a better understanding of the R134a ejector applied in a refrigeration system. Ó 2016 Published by Elsevier Ltd.
1. Introduction The first ejector refrigeration system was developed by Maurice Leblance about 1910 in Paris [1]. Being driven by heat instead of mechanical work or electric energy, an ejector system is appropriate for occasions where such resource shortages occur. After experiencing a brief season of prosperity, the ejector system was substituted for the more efficient vapor-compression system. However, in present times with growing concerns about the environment and energy, investigations and applications of ejector refrigeration systems have been increasingly conducted in recent years, as these systems can be non-polluting and be powered by low-grade heat energy or renewable energy [2,3]. The critical component of the system, the ejector, invented by Charles Parsons, originally for pumping air out of the condenser of a steam power ⇑ Corresponding author at: Institute of Refrigeration and Cryogenics, Zhejiang University, 38 Zheda Road, Hangzhou 310027, China. E-mail address: [email protected] (Q. Chen). http://dx.doi.org/10.1016/j.applthermaleng.2016.09.046 1359-4311/Ó 2016 Published by Elsevier Ltd.
plant [1]. Afterwards, many theoretical and experimental studies have been carried out to investigate its fundamentals and operational behavior, as studies have demonstrated that the performance of an ejector system depends mostly on the operation and performance of an ejector [4]. Riffat et al. [5] and Besagni et al. [6] have presented comprehensive literature reviews on ejectors and ejector systems, including ejector models, ejector geometric optimization, performance and applications of ejectors, refrigerants and so on. Huang et al. [7] developed Joseph Henry Keenan et al.’s one dimensional model by assuming that the two fluids start to mix at the hypothetical throat with an uniform pressure. It was demonstrated experimentally that this model can accurately predict the performance of ejector within ±23% of error. Allouche et al. [8] presented a numerical study of a steam ejector using CFD to identify the suitable experimental conditions. It was found that, for a given ejector configuration, there exists an optimum primary vapor temperature at particular condenser and evaporator temperatures, which yields maximum entrainment ratio and COP. Selvaraju and Mani [9] investigated the performance of a vapor
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ejector refrigeration system and carried out a parametric study with six ejectors, having different geometrical dimensions. They concluded that the performance of an ejector and ejector system strongly relies on the ejector configuration. During the phase-out of CFCs and HFCs under the Montreal Protocol and the Kyoto Protocol, manufactures of refrigerating systems and heat pumps such as car air conditioners and household refrigerators have modified their products to use HFCs instead. The new F-gas Regulation has come into force on 1 January 2015 and strengthened measures on limiting the application of F-gases. However, America and Canada have disaffiliated from the Kyoto Protocol. Furthermore, there are no existed regulations or standards to definitely weed out F-gases in Africa. In other words, HFCs still have broad application prospect. Among these substitutes, R134a has been taken as a leading alternative refrigerant. Because of its preeminent safety performance, R134a has been classified into A1 group by the ISO 5149-1-2014 [10]. A lot of research has also demonstrated that R134a is suitable for lowgrade heat source [11–13]. R134a has being received a partiality in most markets at present, especially in car air conditioners [14,15]. Recently its application in ejector systems has attracted much attention [16–18]. Huang et al. [7] have demonstrated that the back pressure of an ejector is an important parameter as it has an immediate impact on the efficiency of an ejector. However, through literature researches, we know that experimental studies on the effect of back pressure on the performance of an ejector operating with R134a have not been conducted intensively. We believe that these works are indispensable for further applications and popularizations of ejector refrigeration systems with R134a. In the present paper, by utilizing R134a as the working fluid, experimental approaches are employed to specially investigate the variation of the performance of an ejector with back pressure. 2. Experimental ejector H62 brass was chosen as the material to fabricate the ejector investigated in this paper and a schematic diagram is presented in Fig. 1. As can be seen in the figure, this ejector consists of four parts: Laval nozzle, suction chamber, mixing chamber and diffuser. The main parameters of each part are listed in Table 1. The working process in an ejector can be described as the following: the primary fluid with high pressure and temperature flows into Laval nozzle and then undergoes an adiabatic expansion. At the exit of the nozzle, the primary fluid with supersonic velocity produces a vacuum region and the secondary fluid is entrained into suction chamber. These two fluids mix with each other in mixing chamber to complete mixing process. After that, the mixed fluid with subsonic velocity is produced and subsequently enters into the diffuser, where its velocity gradually decreases while the pressure escalates. Ultimately, the compressed fluid with higher
16.48
Table 1 Main parameters of the tested ejector. Geometry parameters
Values/mm
Diameter of Laval nozzle inlet Diameter of Laval nozzle throat Diameter of Laval nozzle exit Diameter of mixing chamber Diameter of diffuser exit Laval nozzle exit position
7.00 1.08 1.41 2.88 14.00 2.65
pressure than that of the secondary fluid discharges out of the ejector. This working process is usually accompanied by supersonic flows, chockes, shock waves and other complex flow phenomena [19]. The widespread use of optical methods and Computational Fluid Dynamics (CFD) have given a big boost to the visualization research on ejectors [5,20]. To characterize the ejector performance, the entrainment ratio is utilized as an indicator, which is defined as:
l ¼ Gh =Gp
ð1Þ
where Gh is the secondary fluid mass flow rate, and Gp is the primary fluid mass flow rate.
3. Experimental apparatus and measuring system 3.1. Experimental rig To investigate the performance of the above ejector, an experimental apparatus was designed and constructed. As shown in Fig. 2, the apparatus was mainly composed of an ejector refrigeration system, a cooling water system, a chilled water system and a hot water system. R134a was utilized as the working fluid. The major components of the ejector refrigeration system include the ejector, a condenser, an evaporator, a generator, a metering pump and a throttle valve. A coaxial condenser (ET072SC), produced by the Extek Energy Equipment (Zhejiang) Co. Ltd., was used and cooled by the cooling water system. The temperatures of the inlet and outlet of this system was monitored. A reconstructive compression-type water chiller was prepared to guarantee the normal work of the cooling water system when the cooling water temperature got too high. The evaporator was also provided by the Extek Energy Equipment (Zhejiang) Co. Ltd. The chilled water system, heated by a U-electrical heating rod with a power of 1.5 kW, uninterruptedly supplied hot water to the shell and coil evaporator, resulting in the liquid R134a evaporates in the heat exchanger. That was how the secondary fluid produced. The heating power of this electrical heating rod was adjusted by a contacting voltage-regulator, so that the heat exchange capacity of the evaporator was controlled.
1.42
19.55
78.48 109.64
Fig. 1. A schematic diagram of the tested ejector.
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Ejector
Evaporator
Condenser
Cooling water
Liquid-storage tank
Throttle valve
Chilled water Generator
Hot water Metering pump Fig. 2. A schematic view of the ejector refrigeration system.
The generator, fabricated from a stainless steel cylinder with a diameter of 426 mm and a length of 600 mm, was used to generate the primary fluid. The hot water system, powered by three electrical heating rods with a total power of 6.6 kW, served as a heat source for the generation of the primary fluid. A Hydro/3-025170 hydraulic diaphragm metering pump, produced by the ProMinent Fluid Controls, Inc., was installed between the generator and the liquid-storage tank. An adjustable flow could feed the generator because the stroke length of this metering pump could be adjusted between 0 and 100%.
3.2. Measuring system (1) Copper-constantan thermocouples and PT100 platinum resistances with an uncertainty of ±0.1 K were used to measure the temperatures. To improve the reliability of the measurement, all the measurement elements were affixed onto the pipe walls, and all the pipes were covered by thermal insulation materials. (2) All the pressures were monitored by high-precision pressure transducers with an accuracy of ±0.2% of their full scales. Their measuring ranges were 0–1.0 MPa for the inlet and outlet of the evaporator and inlet of the secondary fluid, 0–2.0 MPa for the outlet of the condenser, 0–2.5 MPa for the inlet of the condenser and outlet of the ejector, and 0–4.0 MPa for the inlet of the primary fluid, respectively. (3) Two Coriolis-type mass flow meters were mounted to monitor the mass flow rates of the primary fluid and the secondary fluid. The accuracy of these two meters is ±0.35% of measured values where their ranges were 0–0.04 kg/s and 0–0.01 kg/s, respectively. The output from all the measurement devices except two mass flow meters were collected and transferred to a desktop computer through the Agilent 34970A Data Acquisition/Switch Unit in realtime. Being directly connected to the computer with connectors, the data of these mass flow meters were recorded by the Data Acquisition and Communication Program.
According to the formula (1), the uncertainty of the entrainment ratio could be represented as [21]:
uðlÞ ¼ sqrtðð@ l=@Gh Þ2 uðGh Þ2 þ ð@ l=@Gp Þ2 uðGp Þ2 Þ
ð2Þ
Thus the relative uncertainty of the entrainment ratio could be estimated by:
uðlÞ=l ¼ sqrtðuðGh Þ2 =G2h þ uðGp Þ2 =G2p Þ
ð3Þ
Having integrated the experimental data and the accuracy of flow meters, the experimental uncertainty analysis has been conducted and demonstrates that the relative uncertainty of the entrainment ratio of the experimental ejector is about 0.50%. 4. Results and discussion The experimental investigations of the performance of the R134a ejector were performed under the conditions of the primary fluid and the secondary fluid pressures are 2.14–2.495 MPa and 0.31 MPa respectively, the back pressure varies from 0.44 MPa to 0.64 MPa. The variations of the entrainment ratio l with the back pressure Pc is drawn in Fig. 3. Additionally Fig. 4 depicts how the secondary fluid mass flow rate Gh varies with the back pressure Pc. According to Huang et al.’s definitions [7], Fig. 3 suggests that ejector operates with three modes under constant primary fluid pressure Pp and constant secondary fluid pressure Ph. The first one is double-choking mode: the primary fluid and the secondary fluid are both choked and the entrainment ratio l remains unchanged. The second one is single-chocking mode: only the primary fluid is choking and the entrainment ratio l lowers gradually with the back pressure Pc increasing. The third one is back-flow mode: neither the primary fluid nor the secondary fluid is choked but the secondary fluid is refluent, the entrainment ratio l is negative and ejector is out of work. From Fig. 3, it is also observed that a plateau of the entrainment ratio l, which is corresponding to the best operation of the ejector, keeps constant until the back pressure increases to the critical back pressure. As the back pressure progressively raises, the entrainment ratio l goes down gradually. All three curves in Fig. 3 indicate
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1.0
0.7 Pp=2.148MPa, Ph=0.31MPa
0.6
Pp=2.34MPa, Ph=0.31MPa
0.8
Pp=2.495MPa, Ph=0.31MPa
0.5 0.6
μ
μ
0.4 0.3
0.4
0.2
Pc=0.71 MPa, PH=0.46 MPa Pc=0.74 MPa, PH=0.46 MPa
0.2
0.1
Pc=0.86 MPa, PH=0.63 MPa
0.0
Pc=0.89 MPa, PH=0.63 MPa
0.0
0.44 0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60 0.62 0.64 0.66
2.2
Pc (MPa)
0.005 Pp=2.148MPa, Ph=0.31Mpa Pp=2.34MPa, Ph=0.31Mpa
Gh (kg/s)
2.6
2.8
3.0
3.2
Pp (MPa)
Fig. 3. Effect of back pressure on entrainment ratio.
Pp=2.495MPa, Ph=0.31Mpa
0.004
2.4
0.003
Fig. 5. Effect of primary fluid pressure on entrainment ratio.
On the basis of Huang et al.’s report [7], the Engineering Equation Solver was employed to predict the ejector performance at critical and sub-critical operational regimes. Then a logarithmic relation between the critical back pressure and the primary fluid pressure Pp, for the secondary fluid pressure is 0.31 MPa while the back pressure varying from 0.3 MPa to 0.66 MPa and the primary fluid pressure ranging from 1.5 MPa to 3.0 MPa, can be developed:
0.002
Pc ¼ 0:2151 ln Pp þ 0:3166
0.44 0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60 0.62 0.64 0.66
Pc (MPa) Fig. 4. Effect of back pressure on secondary fluid mass flow rate.
the same trend. Fig. 4 can be used to interpret the above phenomena. As seen in this figure, when the back pressure is lower than the critical back pressure, the ejector operates with the doublechoking mode that the secondary fluid is choked, the secondary fluid mass flow rate Gh is steady. As the back pressure Pc continues to increase, the ejector works with the single-choking mode and the secondary fluid is not choked, contributing to the secondary fluid mass flow rate Gh decreases constantly. However, the primary fluid mass flow rate Gp has been invariable under the certain conditions. Consequently, increasing the back pressure makes the entrainment ratio be a constant and then decrease. In addition, it is evident that in Fig. 3(a) for the same secondary fluid pressure, an increase in the primary fluid pressure Pp leads to an increase in the critical back pressure, and (b) for a given ejector, the greater the primary fluid pressure Pp, the lower the maximum entrainment ratio that the ejector can perform. That is because at a given secondary fluid pressure Ph, an increase in the primary fluid pressure Pp can increase the primary fluid mass flow rate Gp. When the back pressure Pc and the secondary fluid pressure Ph remain unchanged, variation of the entrainment ratio l versus the primary fluid pressure Pp is displayed in Fig. 5. It can be found that l increases as Pp increases because an increase in Pp causes an increase in Gp while Gh entering the ejector increases more relatively. However, after an optimal value of Pp, a further increase in Pp leads to a decrease in l because the degree of increase in Gh is lower.
0.65 Experimental values
0.60 0.55 0.50
*
0.000
ð4Þ
The correlation coefficient for the critical back pressure and the primary fluid pressure Pp is as high as 0.9952. Fig. 6 shows comparison of experimental values of the critical back pressure and the prediction curve. It can be found that the experimental values have a good agreement with the predictions. Although the critical back pressure has great effects on ejector performance, there is few research touching upon the relationship between this parameter and other ones in published literatures. Obviously, the relation proposed above has great significance for predicting the critical back pressure, the R134 ejector refrigeration system will run in the best possible way because the optimal operating conditions of this system can be determined. Furthermore, this relation can provide guidance for designing of R134a ejector.
Pc (MPa)
0.001
0.45 0.40 0.35 0.30
1.6
1.8
2.0
2.2
2.4
2.6
2.8
Pp (MPa) Fig. 6. Comparison of experimental values and the prediction curve.
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J. Yan et al. / Applied Thermal Engineering 110 (2017) 1061–1065
5. Conclusions In the present study, the performance of an existed ejector was investigated preliminarily with R134a based on an ejector refrigeration system. The experimental data have verified the effects of back pressure on the performance of an ejector. The following conclusions can be summarized:
[3]
[4] [5] [6]
(1) The back pressure exerts a tremendous influence on the entrainment ratio. When the back pressure is lower than the critical back pressure, ejector operates with the best performance. Otherwise, a deterioration of ejector performance occurs as the back pressure is greater and greater. This indicates that the ejector had better perform with the critical mode for a better performance. (2) An increase in the primary fluid pressure induces an increase in the critical back pressure and a decrease in the maximum entrainment ratio at a given secondary fluid pressure. (3) An optimal value of the primary fluid pressure exists, corresponding to a maximum value of the entrainment ratio under a certain condition. (4) A logarithmic relation was presented to predict the critical back pressure under certain conditions in a R134a ejector refrigeration system. This relation can provide important references for predicting the critical back pressure and guiding operation conditions design.
[7] [8]
[9] [10] [11]
[12] [13]
[14]
[15] [16]
[17]
Acknowledgements This research work is financially supported by the National Natural Science Foundation of China under Award No. 51376156. The assistance of Xuan Chen is greatly appreciated by the first author for polishing up this paper. References [1] W.B. Gosney, Principle of Refrigeration, Cambridge University Press, New York, 1982. [2] C. Li, Y.Z. Li, W.J. Cai, Y. Hu, H.R. Chen, J. Yan, Analysis on performance characteristics of ejector with variable area-ratio for multi-evaporation
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[21]
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