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Numerical Investigation on Three-fluid Heat Exchanger for Hybrid Energy Source Heat Pumps
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 105 (2017) 1692 – 1699
The 8th International Conference on Applied Energy – ICAE2016
Numerical Investigation on three-fluid heat exchanger for hybrid Energy source heat pumps Weijia Zhang1,2,3 Shuangquan Shao1,2 Hainan Zhang1,2 Changqing Tian1,2 1. Key Laboratory of Cryogenics, TIPC, CAS, Beijing 100190, China; 2. Beijing Key Laboratory of Thermal Science and Technology, TIPC, CAS, Beijing 100190, China, 3. University of Chinese Academy of Sciences
Abstract Hybrid energy source heat pump system is one of the best solutions to achieve efficient and stable utilization of renewable energies such as solar energy, geothermal energy, etc. The Three-fluid heat exchanger can realize heat transfer from the refrigerant to the outside air and the inside water simultaneously or independently, which will keep the refrigerant well distributed under difference working modes. The simulation model is built and validated for performance analysis of the three fluid heat exchanger based on Matlab and REFPROP. Three heat exchangers with different flow path designs are investigated for the heat transfer and pressure loss. The results show that the heat exchanger can give better performance by optimization of heat transfer coefficient and the pressure loss. Moreover, the influence on the heat exchanger from the variation of the air flow speed, air temperature, water flow speed and water temperature are also been analyzed. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy.
Key words:hybrid energy source heat pump, three-fluid heat exchanger, simulation
1.
Introduction
From 1854, the concept of the heat pump has been firstly proposed. The research of the heat pump technique has not been in rapid development until the emergence of the energy problem and environmental problem. By the energy source of the systems, we can divide the heat pumps into different kinds such as ground source heat pumps, air source heat pumps, hybrid energy source heat pumps, etc. Before 1970s, ground source heat pumps market has been slow-growing for the expensive initial investment. This situation has not been changed until the energy crisis while people have found the energy conservation potential of this system[1]. After that, the ground source heat pumps have been in a rapid development especially in the 21th century for their stable performance. However, the construction of the ground source heat pumps is easily affected by the local environment, and the unreasonable construction also influence the local soil even the soil ecosystem. 澔 * Corresponding author. Tel.:+86-10-82543433; fax: +86-10-82543433. E-mail address: [email protected] (S. Q. Shao).
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.551
Weijia Zhang et al. / Energy Procedia 105 (2017) 1692 – 1699
Compared to ground source heat pumps, air source heat pumps have been applied more widely for their low initial investment and ubiquitous energy source. However, the coefficient of performance (COP) and the heating capacity of air source heat pumps can be easily influenced by the local environment. Especially in the low temperature environment, COP and the heating capacity of air source heat pumps become very low, furthermore, they also suffering from the frost problem which influences the efficiency of the heat exchanger. To sum up, it is feasible to combine the ground source heat pumps with the air source heat pumps to build the hybrid energy source heat pumps which could run stably, economically and efficiently. Ooka et al. built a hybrid energy source heat pumps which could simultaneously or separately use solar energy, ground energy and air energy as energy source, and the results of the simulation and analysis show that this system could reduce electric power outages by 44% in summer and 39% in winter[2]. Currently, the traditional way of using energy source in hybrid energy source heat pumps could be divided into parallel mode and series mode. However, the distribution of refrigerant in heat exchanger is difficult to control in both ways of using energy source and the hidden trouble of switching solenoid valve cannot avoid in the traditional way of using energy source. These two problems can be avoided by using three-fluid heat exchanger, because the three-fluid heat exchanger can achieve heat exchange between the refrigerant and the other two fluid media in only one heat exchanger[3,4]. This paper established a mathematical model of the three-fluid heat exchanger applied to the hybrid energy source heat based on Matlab software and REFPROP software, and used the mathematical model to analyse the performance of the three-fluid heat exchanger in order to establish research basis of evaluating the performance of threefluid heat exchanger and the hybrid energy source heat pumps. 2.
Modeling and Simulation
The structure of the three-fluid heat exchanger applies to hybrid energy source heat pumps is shown in Figure 1. The three-fluid heat exchanger consists of double-pipe heat exchanger with fins. The hot water flows in the inner pipe, the refrigerant flows in the outer pipe and the air flows across the fins. In this heat exchanger, the refrigerant can simultaneously exchange heat with hot water and air or with either of both independently. This paper employs the distributed parameter methods to establish mathematical model applied to the three-fluid heat exchanger and divided the three-fluid heat exchanger into cells along the pipe like Figure 2. The mathematical model successively calculates the heat exchange capacity between the hot water and the refrigerant in every infinitesimal along the direction of refrigerant flow, and calculates the heat exchange capacity between the air and the refrigerant in every cell along the direction of refrigerant flow. By this model, we can calculate the heat exchange capacity of the three-fluid heat exchanger after inputting the parameters of inlet refrigerant, inlet hot water and inlet air.
. Figure 1. The structure of the heat exchanger
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Figure 2. The structure of the infinitesimal
To simplify the model and improve the computing speed of the model, it is necessary to make following assumptions[5]. (1) The refrigerant flow is supposed to be one-dimensional flow along the pipe. (2) Refrigerant in the connection parts which connected one pipe and another pipe, is calculated the pressure change, regardless of the refrigerant enthalpy change. (3) Gravitational pressure drop is only considered in connection parts which connected one pipe and another pipe. (4) The pressure and enthalpy of the refrigerant do not change in section orientation in one pipe. (5) The pressure drop of the refrigerant in the superheated zone could be ignored. The calculation flow chart of the mathematical model is described in Figure 3. The models of pressure and enthalpy applied to refrigerant, hot water and air are described in following part. 濧濨濕濦濨 澽濢濤濩濨澔濝濢濠濙濨澔濤濕濦濕濡濙濨濙濦濧澔濕濢濘澔濡濣濘濙濠澔濧濝濮濙 澵濧濧濩濡濝濢濛澔濨濜濙澔濣濩濨濠濙濨澔濤濕濦濕濡濙濨濙濦濧 澷濕濠濗濩濠濕濨濙澔濨濜濙澔濜濙濕濨澔濕濖濧濣濦濤濨濝濣濢澔濣濚澔濨濜濙澔濦濙濚濦濝濛濙濦濕濢濨
澷濕濠濗濩濠濕濨濙澔濨濜濙澔濦濙濚濦濝濛濙濦濕濢濨澔濤濦濙濧濧濩濦濙澔濘濦濣濤澔濣濚澔濨濜濙澔濗濙濠濠 澷濣濡濤濩濨濝濢濛澔濨濜濙澔濢濙濬濨澔濝濢濚濝濢濝濨濙濧濝濡濕濠 濡
澵濘濞濩濧濨澔濨濜濙澔 濜濭濤濣濨濜濙濧濝濧澔 濤濕濦濕濡濙濨濙濦濧
濋濜濙濨濜濙濦澔濨濣澔 濦濙濕濗濜澔濨濜濙澔 濣濩濨濠濙濨 濬 澷濣濡濤濕濦濝濧濣濢澔 濣濚澔濨濜濙澔濦濙濧濩濠濨濧澔 濕濗濗濩濦濕濗濭 濬 濃濩濨濤濩濨澔濨濜濙澔濦濙濧濩濠濨 澹濢濘澔 Figure 3. Algorithm flow chart
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2.1. The model of pressure drop and heat transfer coefficient The pressure drop of the refrigerant can be divided into gravitational pressure drop, acceleration pressure drop, frictional pressure drop and partial drop of pressure. The pipe was arranged horizontally, so the gravitational pressure drop of the refrigerant in pipe was not considered[6,7]. The equations used in the model are listed in the table 1. Table 1 the equations used in the model
dp Pressure drop of the refrigerant
Xb C
§ C 1 · dp l ¨¨1 2 ¸¸ © Xb Xb ¹ § U" · ¨¨ U' ¸¸ © ¹
0.5
0.5 ª § U' U" · º ¸¸ » «1 C2 1 ¨¨ «¬ © U' ¹ »¼
G' G"
ª§ U' ·0.5 § U" ·0.5 º «¨¨ ¸¸ ¨¨ ¸¸ » «¬© U" ¹ © U' ¹ »¼
C2=3.2
澳
Convection heat transfer coefficient of the air outside the finned tube[8]
;
Convective heat transfer coefficient of the hot water[9] Convective heat transfer coefficient of the refrigerant in the superheated zone[9] Convective heat transfer coefficient of the refrigerant in the two phase state[10]
澳
The simulation experiments have showed the effective of the mathematical model. The error range of f6%~10%[11] could accurately simulate the three-fluid heat exchanger[12]. 3. Performance analysis The mathematical model is used to analyse the performance of the three-fluid heat exchanger applied to the hybrid energy source heat pumps. The structure of the heat exchanger is shown in the Figure 2. The heat exchanger was consisted of 24 1-meter-tubes arranged in two rows. Three different flow routes are compared by the simulation analysis. And the three different flow paths are described in the Figure 4, and the three types were named “A” (consisted of 8 single loops, and each loops had 6 tubes), “B” (consisted of 6 single loops, and each loops has 8 tubes), “C” (consisted of 4 single loops, and each loops has 12 tubes) in turn. The operating parameters are listed in the Table 2. Air flow speed Wet-bulb temperature of the air Water flow speed The enthalpy of the refrigerant in the outlet
Table 2. The operating parameters 2.0m/s Dry-bulb temperature of the air 6.0oC Water temperature 1.0m/s The enthalpy of the refrigerant in the inlet 407kJ/kg The pressure of the refrigerant in the inlet
7.0 oC 10 oC 270kJ/kg 500kPa
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Figure 4. Three different flow paths
Under the condition of the operation, the heat transfer rate and the pressure drop of the three different flow routes are described in the Figure 5and Figure 6.
Figure 5. Heat transfer rate
Figure 6. Pressure drop of refrigerant
From Figure 5 and Figure 6, the B type has best performance in heat transfer, and the C type has better performance in heat transfer compared to the A type. The A type has the best performance in pressure drop, and the B type has the better performance compared to the C type. The reason why A type has the best performance in pressure drop is that A type had the shortest length of the single loop. The changing rule of the heat-transfer coefficient and the temperature of the refrigerant along the tube are described in the Figure 7, Figure 8 and Figure 9. With the decreasing of the number of the flow paths, the heat-transfer coefficient of the refrigerant-hot water and the heat-transfer coefficient of the refrigerantair are increasing, and the reason is that the flow speed of the refrigerant and the flow speed of the hot water are increasing with the decreasing of the number of the flow paths. With the decreasing of the number of the flow paths, the pressure drop of the refrigerant is increasing which results in the increasing of the evaporating temperature of the refrigerant described in Figure 9, and the decreasing of the mean temperature difference. Synthesizes both influences, the B type has best performance in the heat transfer rate.
Weijia Zhang et al. / Energy Procedia 105 (2017) 1692 – 1699
Figure 7. Variation of the refrigerant-air heat transfer coefficient Figure 8.Variation of the refrigerant-water heat transfer coefficient
Figure 9. The temperature of the refrigerant along the tube
The effect of the variation of the speed of the inlet air, the temperature of the inlet air, the speed of the inlet hot water and the speed of the inlet hot water on the three-fluid heat exchanger have been analysed in this paper by the simulation model. The inlet refrigerant parameters and the outlet refrigerant parameters have retained unchanged while the external factors have been changed. And the final results are represented in the Figure10, Figure11, Figure 12 and Figure 13. It is necessary to point out that the “T” means total heat transfer rate, “W” means the heat transfer rate from water to refrigerant and “A” means heat transfer rate from air to refrigerant in the flowing figures.
Figure 10. The influence of the air speed
Figure 11. The influence of the air temperature
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Figure 12. The influence of the water speed
Figure 13. The influence of the water temperature
Results 1: While the speed of the inlet air increase, the quantity of exchanged heat from air to the refrigerant and the total exchanged heat are increasing. Because the parameters of the inlet refrigerant and the outlet refrigerant need to maintain unchanged, the flow rate of the refrigerant increase which increase the coefficient of heat transfer, the quantity of exchanged heat from water to the refrigerant also increase. Results 2: The features of the simulated results which only changed the temperature of the inlet air kind of like the reluts1. Results 3: While the speed of the inlet water increase, the quantity of exchanged heat from water to the refrigerant and the total exchanged heat are increasing. Because the parameters of the inlet refrigerant and outlet refrigerant need to maintain unchanged, the flow rate of the refrigerant increase which increase the coefficient of heat transfer. However, the length of the overheated zone increase from the simulated results which increase the mean temperature difference. In Figure 12, the quantity of exchanged heat from air to the refrigerant is initially increasing until the speed arrive to the 1.1m/s. And from that, the quantity of exchanged heat from air to the refrigerant is decreasing. Results 4: The features of the simulated results which only changed the temperature of the inlet water kind of like the results 3. It is obvious that the quantity of the total exchanged heat increase while the temperature of the inlet heating media or the speed of the inlet heating media increase. However, while the speed or the temperature of the inlet water increase, the quantity of the exchanged heat from air to the refrigerant is initially increasing and decreasing finally. And from the simulation results, the variation of the heating medium temperature affects more obviously on the quantity of the exchanged heat than the variation of the heating medium speed. 4. Conclusion This paper establishes the simulation model with the distributed-parameter method to analyse the performance of the three-fluid heat exchanger applied to hybrid energy source heat pumps. Moreover, the accuracy of the simulation model is verified experimentally. The different flow path designs of the threefluid heat exchanger are analysed by the simulation model. The results show that with the decrease of the number of flow paths, the length of each flow path increases, and the mean heat transfer coefficient increases, while the pressure loss of the refrigerant increases, which results in the decrease of the mean temperature difference for heat transfer. So synthesizing both influences of the heat transfer coefficient and the pressure loss is required in designing the three-fluid heat exchanger, and the type B of the threefluid heat exchanger reaches the best heat transfer capacity. Moreover, the influence on the heat exchanger from the variation of the air flow speed, air temperature, water flow speed and water temperature are analysed in this paper. And the results show the while the temperature or the flow speed of the heating medium increase, the heat transfer rate of the heat exchanger is increasing. The simulation
Weijia Zhang et al. / Energy Procedia 105 (2017) 1692 – 1699
model and the results in this paper laid the foundation to the simulation modelling of the hybrid energy source heat pumps system and the optimization of the system. Acknowledgements The authors gratefully acknowledge the financial support from Key International Program of Chinese Academy of Sciences (CAS-DOE, 1A1111KYSB20150014). References [1] Zhang S. Shallow discussion on ground source heat pump for new HVAC technology [J]. SHANXI ARCHITECT URE, 2010, 16: 165. [2] Okka R, Hino T, Sato H, et al. Development of multi-source and multi-use heat pump system[C].10th IEA Heat Pump Conference. Japan, 2011 [3] Zhang HN, Shao SQ, et al. Integrated system of mechanical refrigeration and thermosyphon for free cooling of data centers. Applied Thermal Engineering, 2015, 75: 185-192. [4] Zhou GH, Zhang C, Liu Y, et al. Experimental Study on Heating Performance of a Solar-Air Multisource Heat Pump. Journal of Hunan University(Natural Sciences), 2009, 36(12): 19-21. [5] Han L. Research on the Integrated Air Conditioner with Thermosyphon and its Application [D]. Tsinghua University, 2014 [6] Imura H, Saito Y, Katsumata Y. Flow and heat transfer characteristics in a two-phase loop thermosiphon[J]. Trans. of the JSRAE, 1988, 5(1): 66-73. [7] Yan C. Nuclear Science And Echnology. Harbin Engineering University Press, 2007: 107~172. [8] Yan Q, Shi W, Tian C. Refrigeration Technology for Air Conditioning. China Architecture & Building Press, 2004: 87-88 [9] Yang S, Tao W. Heat Transfer. Higher Education Press, 2006: 246-247 [10] Kandlikar SG. A General Correlation for Saturated Two-Phase Flow Boiling Heat Transfer Inside Horizontal and Vertical Tubes[J].Journal of Heat Transfer ,1990,112; 219-228 [11] Zhang HN, Shao SQ, et al. Numerical investigation on fin-tube three-fluid heat exchanger for hybrid source HVAC&R systems[J]. Applied Thermal Engineering, 2016, 95; 157-164 [12] Zhang HN, Shao SQ, et al. Numerical investigation on integrated system of mechanical refrigeration and thermosyphon for free cooling of data centers[J]. International journal of refrigeration, 2015, 60; 9-18 Biography Shuangquan Shao was born in 1975. He received his Ph.D. in 2005 from Tsinghua University. He is now an Associate Professor of the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences. Prof. Shao’s research interests include heat pump, thermal management, building energy saving, et al.
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ScienceDirect Energy Procedia 105 (2017) 1692 – 1699
The 8th International Conference on Applied Energy – ICAE2016
Numerical Investigation on three-fluid heat exchanger for hybrid Energy source heat pumps Weijia Zhang1,2,3 Shuangquan Shao1,2 Hainan Zhang1,2 Changqing Tian1,2 1. Key Laboratory of Cryogenics, TIPC, CAS, Beijing 100190, China; 2. Beijing Key Laboratory of Thermal Science and Technology, TIPC, CAS, Beijing 100190, China, 3. University of Chinese Academy of Sciences
Abstract Hybrid energy source heat pump system is one of the best solutions to achieve efficient and stable utilization of renewable energies such as solar energy, geothermal energy, etc. The Three-fluid heat exchanger can realize heat transfer from the refrigerant to the outside air and the inside water simultaneously or independently, which will keep the refrigerant well distributed under difference working modes. The simulation model is built and validated for performance analysis of the three fluid heat exchanger based on Matlab and REFPROP. Three heat exchangers with different flow path designs are investigated for the heat transfer and pressure loss. The results show that the heat exchanger can give better performance by optimization of heat transfer coefficient and the pressure loss. Moreover, the influence on the heat exchanger from the variation of the air flow speed, air temperature, water flow speed and water temperature are also been analyzed. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy.
Key words:hybrid energy source heat pump, three-fluid heat exchanger, simulation
1.
Introduction
From 1854, the concept of the heat pump has been firstly proposed. The research of the heat pump technique has not been in rapid development until the emergence of the energy problem and environmental problem. By the energy source of the systems, we can divide the heat pumps into different kinds such as ground source heat pumps, air source heat pumps, hybrid energy source heat pumps, etc. Before 1970s, ground source heat pumps market has been slow-growing for the expensive initial investment. This situation has not been changed until the energy crisis while people have found the energy conservation potential of this system[1]. After that, the ground source heat pumps have been in a rapid development especially in the 21th century for their stable performance. However, the construction of the ground source heat pumps is easily affected by the local environment, and the unreasonable construction also influence the local soil even the soil ecosystem. 澔 * Corresponding author. Tel.:+86-10-82543433; fax: +86-10-82543433. E-mail address: [email protected] (S. Q. Shao).
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.551
Weijia Zhang et al. / Energy Procedia 105 (2017) 1692 – 1699
Compared to ground source heat pumps, air source heat pumps have been applied more widely for their low initial investment and ubiquitous energy source. However, the coefficient of performance (COP) and the heating capacity of air source heat pumps can be easily influenced by the local environment. Especially in the low temperature environment, COP and the heating capacity of air source heat pumps become very low, furthermore, they also suffering from the frost problem which influences the efficiency of the heat exchanger. To sum up, it is feasible to combine the ground source heat pumps with the air source heat pumps to build the hybrid energy source heat pumps which could run stably, economically and efficiently. Ooka et al. built a hybrid energy source heat pumps which could simultaneously or separately use solar energy, ground energy and air energy as energy source, and the results of the simulation and analysis show that this system could reduce electric power outages by 44% in summer and 39% in winter[2]. Currently, the traditional way of using energy source in hybrid energy source heat pumps could be divided into parallel mode and series mode. However, the distribution of refrigerant in heat exchanger is difficult to control in both ways of using energy source and the hidden trouble of switching solenoid valve cannot avoid in the traditional way of using energy source. These two problems can be avoided by using three-fluid heat exchanger, because the three-fluid heat exchanger can achieve heat exchange between the refrigerant and the other two fluid media in only one heat exchanger[3,4]. This paper established a mathematical model of the three-fluid heat exchanger applied to the hybrid energy source heat based on Matlab software and REFPROP software, and used the mathematical model to analyse the performance of the three-fluid heat exchanger in order to establish research basis of evaluating the performance of threefluid heat exchanger and the hybrid energy source heat pumps. 2.
Modeling and Simulation
The structure of the three-fluid heat exchanger applies to hybrid energy source heat pumps is shown in Figure 1. The three-fluid heat exchanger consists of double-pipe heat exchanger with fins. The hot water flows in the inner pipe, the refrigerant flows in the outer pipe and the air flows across the fins. In this heat exchanger, the refrigerant can simultaneously exchange heat with hot water and air or with either of both independently. This paper employs the distributed parameter methods to establish mathematical model applied to the three-fluid heat exchanger and divided the three-fluid heat exchanger into cells along the pipe like Figure 2. The mathematical model successively calculates the heat exchange capacity between the hot water and the refrigerant in every infinitesimal along the direction of refrigerant flow, and calculates the heat exchange capacity between the air and the refrigerant in every cell along the direction of refrigerant flow. By this model, we can calculate the heat exchange capacity of the three-fluid heat exchanger after inputting the parameters of inlet refrigerant, inlet hot water and inlet air.
. Figure 1. The structure of the heat exchanger
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Figure 2. The structure of the infinitesimal
To simplify the model and improve the computing speed of the model, it is necessary to make following assumptions[5]. (1) The refrigerant flow is supposed to be one-dimensional flow along the pipe. (2) Refrigerant in the connection parts which connected one pipe and another pipe, is calculated the pressure change, regardless of the refrigerant enthalpy change. (3) Gravitational pressure drop is only considered in connection parts which connected one pipe and another pipe. (4) The pressure and enthalpy of the refrigerant do not change in section orientation in one pipe. (5) The pressure drop of the refrigerant in the superheated zone could be ignored. The calculation flow chart of the mathematical model is described in Figure 3. The models of pressure and enthalpy applied to refrigerant, hot water and air are described in following part. 濧濨濕濦濨 澽濢濤濩濨澔濝濢濠濙濨澔濤濕濦濕濡濙濨濙濦濧澔濕濢濘澔濡濣濘濙濠澔濧濝濮濙 澵濧濧濩濡濝濢濛澔濨濜濙澔濣濩濨濠濙濨澔濤濕濦濕濡濙濨濙濦濧 澷濕濠濗濩濠濕濨濙澔濨濜濙澔濜濙濕濨澔濕濖濧濣濦濤濨濝濣濢澔濣濚澔濨濜濙澔濦濙濚濦濝濛濙濦濕濢濨
澷濕濠濗濩濠濕濨濙澔濨濜濙澔濦濙濚濦濝濛濙濦濕濢濨澔濤濦濙濧濧濩濦濙澔濘濦濣濤澔濣濚澔濨濜濙澔濗濙濠濠 澷濣濡濤濩濨濝濢濛澔濨濜濙澔濢濙濬濨澔濝濢濚濝濢濝濨濙濧濝濡濕濠 濡
澵濘濞濩濧濨澔濨濜濙澔 濜濭濤濣濨濜濙濧濝濧澔 濤濕濦濕濡濙濨濙濦濧
濋濜濙濨濜濙濦澔濨濣澔 濦濙濕濗濜澔濨濜濙澔 濣濩濨濠濙濨 濬 澷濣濡濤濕濦濝濧濣濢澔 濣濚澔濨濜濙澔濦濙濧濩濠濨濧澔 濕濗濗濩濦濕濗濭 濬 濃濩濨濤濩濨澔濨濜濙澔濦濙濧濩濠濨 澹濢濘澔 Figure 3. Algorithm flow chart
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2.1. The model of pressure drop and heat transfer coefficient The pressure drop of the refrigerant can be divided into gravitational pressure drop, acceleration pressure drop, frictional pressure drop and partial drop of pressure. The pipe was arranged horizontally, so the gravitational pressure drop of the refrigerant in pipe was not considered[6,7]. The equations used in the model are listed in the table 1. Table 1 the equations used in the model
dp Pressure drop of the refrigerant
Xb C
§ C 1 · dp l ¨¨1 2 ¸¸ © Xb Xb ¹ § U" · ¨¨ U' ¸¸ © ¹
0.5
0.5 ª § U' U" · º ¸¸ » «1 C2 1 ¨¨ «¬ © U' ¹ »¼
G' G"
ª§ U' ·0.5 § U" ·0.5 º «¨¨ ¸¸ ¨¨ ¸¸ » «¬© U" ¹ © U' ¹ »¼
C2=3.2
澳
Convection heat transfer coefficient of the air outside the finned tube[8]
;
Convective heat transfer coefficient of the hot water[9] Convective heat transfer coefficient of the refrigerant in the superheated zone[9] Convective heat transfer coefficient of the refrigerant in the two phase state[10]
澳
The simulation experiments have showed the effective of the mathematical model. The error range of f6%~10%[11] could accurately simulate the three-fluid heat exchanger[12]. 3. Performance analysis The mathematical model is used to analyse the performance of the three-fluid heat exchanger applied to the hybrid energy source heat pumps. The structure of the heat exchanger is shown in the Figure 2. The heat exchanger was consisted of 24 1-meter-tubes arranged in two rows. Three different flow routes are compared by the simulation analysis. And the three different flow paths are described in the Figure 4, and the three types were named “A” (consisted of 8 single loops, and each loops had 6 tubes), “B” (consisted of 6 single loops, and each loops has 8 tubes), “C” (consisted of 4 single loops, and each loops has 12 tubes) in turn. The operating parameters are listed in the Table 2. Air flow speed Wet-bulb temperature of the air Water flow speed The enthalpy of the refrigerant in the outlet
Table 2. The operating parameters 2.0m/s Dry-bulb temperature of the air 6.0oC Water temperature 1.0m/s The enthalpy of the refrigerant in the inlet 407kJ/kg The pressure of the refrigerant in the inlet
7.0 oC 10 oC 270kJ/kg 500kPa
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Figure 4. Three different flow paths
Under the condition of the operation, the heat transfer rate and the pressure drop of the three different flow routes are described in the Figure 5and Figure 6.
Figure 5. Heat transfer rate
Figure 6. Pressure drop of refrigerant
From Figure 5 and Figure 6, the B type has best performance in heat transfer, and the C type has better performance in heat transfer compared to the A type. The A type has the best performance in pressure drop, and the B type has the better performance compared to the C type. The reason why A type has the best performance in pressure drop is that A type had the shortest length of the single loop. The changing rule of the heat-transfer coefficient and the temperature of the refrigerant along the tube are described in the Figure 7, Figure 8 and Figure 9. With the decreasing of the number of the flow paths, the heat-transfer coefficient of the refrigerant-hot water and the heat-transfer coefficient of the refrigerantair are increasing, and the reason is that the flow speed of the refrigerant and the flow speed of the hot water are increasing with the decreasing of the number of the flow paths. With the decreasing of the number of the flow paths, the pressure drop of the refrigerant is increasing which results in the increasing of the evaporating temperature of the refrigerant described in Figure 9, and the decreasing of the mean temperature difference. Synthesizes both influences, the B type has best performance in the heat transfer rate.
Weijia Zhang et al. / Energy Procedia 105 (2017) 1692 – 1699
Figure 7. Variation of the refrigerant-air heat transfer coefficient Figure 8.Variation of the refrigerant-water heat transfer coefficient
Figure 9. The temperature of the refrigerant along the tube
The effect of the variation of the speed of the inlet air, the temperature of the inlet air, the speed of the inlet hot water and the speed of the inlet hot water on the three-fluid heat exchanger have been analysed in this paper by the simulation model. The inlet refrigerant parameters and the outlet refrigerant parameters have retained unchanged while the external factors have been changed. And the final results are represented in the Figure10, Figure11, Figure 12 and Figure 13. It is necessary to point out that the “T” means total heat transfer rate, “W” means the heat transfer rate from water to refrigerant and “A” means heat transfer rate from air to refrigerant in the flowing figures.
Figure 10. The influence of the air speed
Figure 11. The influence of the air temperature
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Figure 12. The influence of the water speed
Figure 13. The influence of the water temperature
Results 1: While the speed of the inlet air increase, the quantity of exchanged heat from air to the refrigerant and the total exchanged heat are increasing. Because the parameters of the inlet refrigerant and the outlet refrigerant need to maintain unchanged, the flow rate of the refrigerant increase which increase the coefficient of heat transfer, the quantity of exchanged heat from water to the refrigerant also increase. Results 2: The features of the simulated results which only changed the temperature of the inlet air kind of like the reluts1. Results 3: While the speed of the inlet water increase, the quantity of exchanged heat from water to the refrigerant and the total exchanged heat are increasing. Because the parameters of the inlet refrigerant and outlet refrigerant need to maintain unchanged, the flow rate of the refrigerant increase which increase the coefficient of heat transfer. However, the length of the overheated zone increase from the simulated results which increase the mean temperature difference. In Figure 12, the quantity of exchanged heat from air to the refrigerant is initially increasing until the speed arrive to the 1.1m/s. And from that, the quantity of exchanged heat from air to the refrigerant is decreasing. Results 4: The features of the simulated results which only changed the temperature of the inlet water kind of like the results 3. It is obvious that the quantity of the total exchanged heat increase while the temperature of the inlet heating media or the speed of the inlet heating media increase. However, while the speed or the temperature of the inlet water increase, the quantity of the exchanged heat from air to the refrigerant is initially increasing and decreasing finally. And from the simulation results, the variation of the heating medium temperature affects more obviously on the quantity of the exchanged heat than the variation of the heating medium speed. 4. Conclusion This paper establishes the simulation model with the distributed-parameter method to analyse the performance of the three-fluid heat exchanger applied to hybrid energy source heat pumps. Moreover, the accuracy of the simulation model is verified experimentally. The different flow path designs of the threefluid heat exchanger are analysed by the simulation model. The results show that with the decrease of the number of flow paths, the length of each flow path increases, and the mean heat transfer coefficient increases, while the pressure loss of the refrigerant increases, which results in the decrease of the mean temperature difference for heat transfer. So synthesizing both influences of the heat transfer coefficient and the pressure loss is required in designing the three-fluid heat exchanger, and the type B of the threefluid heat exchanger reaches the best heat transfer capacity. Moreover, the influence on the heat exchanger from the variation of the air flow speed, air temperature, water flow speed and water temperature are analysed in this paper. And the results show the while the temperature or the flow speed of the heating medium increase, the heat transfer rate of the heat exchanger is increasing. The simulation
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model and the results in this paper laid the foundation to the simulation modelling of the hybrid energy source heat pumps system and the optimization of the system. Acknowledgements The authors gratefully acknowledge the financial support from Key International Program of Chinese Academy of Sciences (CAS-DOE, 1A1111KYSB20150014). References [1] Zhang S. Shallow discussion on ground source heat pump for new HVAC technology [J]. SHANXI ARCHITECT URE, 2010, 16: 165. [2] Okka R, Hino T, Sato H, et al. Development of multi-source and multi-use heat pump system[C].10th IEA Heat Pump Conference. Japan, 2011 [3] Zhang HN, Shao SQ, et al. Integrated system of mechanical refrigeration and thermosyphon for free cooling of data centers. Applied Thermal Engineering, 2015, 75: 185-192. [4] Zhou GH, Zhang C, Liu Y, et al. Experimental Study on Heating Performance of a Solar-Air Multisource Heat Pump. Journal of Hunan University(Natural Sciences), 2009, 36(12): 19-21. [5] Han L. Research on the Integrated Air Conditioner with Thermosyphon and its Application [D]. Tsinghua University, 2014 [6] Imura H, Saito Y, Katsumata Y. Flow and heat transfer characteristics in a two-phase loop thermosiphon[J]. Trans. of the JSRAE, 1988, 5(1): 66-73. [7] Yan C. Nuclear Science And Echnology. Harbin Engineering University Press, 2007: 107~172. [8] Yan Q, Shi W, Tian C. Refrigeration Technology for Air Conditioning. China Architecture & Building Press, 2004: 87-88 [9] Yang S, Tao W. Heat Transfer. Higher Education Press, 2006: 246-247 [10] Kandlikar SG. A General Correlation for Saturated Two-Phase Flow Boiling Heat Transfer Inside Horizontal and Vertical Tubes[J].Journal of Heat Transfer ,1990,112; 219-228 [11] Zhang HN, Shao SQ, et al. Numerical investigation on fin-tube three-fluid heat exchanger for hybrid source HVAC&R systems[J]. Applied Thermal Engineering, 2016, 95; 157-164 [12] Zhang HN, Shao SQ, et al. Numerical investigation on integrated system of mechanical refrigeration and thermosyphon for free cooling of data centers[J]. International journal of refrigeration, 2015, 60; 9-18 Biography Shuangquan Shao was born in 1975. He received his Ph.D. in 2005 from Tsinghua University. He is now an Associate Professor of the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences. Prof. Shao’s research interests include heat pump, thermal management, building energy saving, et al.
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