Effect of thermodynamic parameters on the performance of refrigerator-ejector

Abstract

A 3D calculation model of a refrigerator-ejector was built and simulated in a compression/injection hybrid refrigeration cycle system by using the FLUENT software of CFD. The effect of thermodynamic parameters (the pressure of primary fluid and secondary fluid) on the performance of the refrigerator-ejector was studied. The boundary conditions were set according to the actual operating condition and the parameters of refrigerator experimental sample. The numerical calculation results show that there is one optimal pressure of primary fluid, i.e., p_popt = 0.06612 MPa, corresponding to the maximum entrainment ratio, i.e., u = 0.568; and there is one optimal pressure of secondary fluid, i.e., p_hopt = 0.04837 MPa, corresponding to the maximum entrainment ratio, i.e., u = 0.564.

Keywords

refrigerator-ejector / FLUENT / thermodynamic parameters / entrainment ratio performance

Introduction

The refrigerator-ejector, a product of the integration of the steam ejector device and the refrigeration science, is designed according to the actual operation condition and the internal pipeline system of the refrigerator and applied to a two-temperature refrigerator system.

A new type of system called compression/injection hybrid refrigeration cycle system is built if a refrigerator-ejector is integrated into the refrigeration system. A series of studies on such compression/injection hybrid refrigeration cycle system have been performed theoretically and experimentally in Refs. [1-4]. The results show that this hybrid system has obvious advantages and is energy-saving efficient. It can significantly reduce the excessive heat transfer temperature difference in traditional two-temperature refrigerator systems, thereby improving the coefficient of performance of the refrigerator system. Theoretical calculations show that the coefficient of performance of the compression/injection hybrid refrigeration cycle system can be 8% to 12% higher than that of the traditional one, and the refrigeration capacity can be increased by 10% to 18%.

The performance of the refrigerator system and the numerical simulation methods about the steam ejector have been investigated in Refs. [5-7]. The effect of the location of the nozzle, the size of the throat, the computational model and the thermodynamic model of the entrainment ratio, and the structure of the nozzle, etc., on the performance of the steam ejector have been studied [8-15], and some useful conclusions have been reached. The internal flow field and the system performance of the ejector have been numerically simulated in Ref. [16].

The 3D calculation model of a refrigerator-ejector in a compression/injection hybrid refrigeration cycle system is simulated in this paper using the FLUENT software of CFD. Moreover, the effect of thermodynamic parameters (the pressure of primary fluid and secondary fluid) on the performance of the refrigerator-ejector is also studied.

Calculation model of refrigerator-ejector

Calculation grid of refrigerator-ejector

Figure 1 shows the schematic diagram of the refrigerator-ejector. It can be seen that the left end is primary fluid inlet, to be connected with the outlet of the refrigerator evaporator; the bottom is secondary fluid inlet, to be connected with the outlet of the freezer evaporator; and the right end is the hybrid fluid outlet, to be connected with the suction inlet of the compressor.

Figure 2 shows the 3D calculation grid of the refrigerator-ejector. In order to study the flow of primary fluid and secondary fluid in the refrigerator-ejector conveniently and to reduce the computation time, the structured and unstructured meshes are used.

Fluid materials and boundary conditions

As is known, the flowing fluid in the refrigerator-ejector of a refrigerator system is called refrigerant, so primary fluid and secondary fluid are both R600a, whose density, specific heat at constant pressure, thermal conductivity, and viscosity are initialized manually in the boundary condition setting. The pressure-inlet boundary is adopted at both primary fluid inlet and secondary fluid inlet. The initial pressure, temperature, and appropriate turbulence condition are given. The pressure-outlet boundary is also adopted at the hybrid fluid outlet. The outlet pressure and appropriate back-flow condition are given too. No slip stationary wall and heat insulation boundary are used.

The entire flow field in the refrigerator-ejector is numerically simulated using the second-order precision finite volume discrete control equation, the turbulence model of standard κ-ϵ equation and the method of amended wall function near the wall studied. In the process of numerical simulation, the pressure, temperature, and velocity of primary fluid are 0.08851 MPa, 251 K, and 0.05 m/s, respectively; the pressure, temperature, and velocity of secondary fluid are 0.05793 MPa, 248 K, and 0.03 m/s, respectively; and the pressure and temperature of hybrid fluid is 0.06853 MPa and 252 K, respectively.

Results and analysis of numerical simulation

Effect of the pressure of primary fluid

When other thermodynamic parameters remain unchanged, four different pressure conditions of primary fluid are selected to simulate the refrigerator-ejector. Figure 3 shows the fluid flow pressure distribution under four different pressures of primary fluid. It can be seen that different pressure of primary fluid corresponds to different pressure drop from the outlet of the nozzle. Therefore, after primary fluid and secondary fluid are adequately mixed, the hybrid fluid would have diverse initial pressure. After being further compressed in the mixing chamber, the hybrid fluid will have different pressure at the throat. After diverse diffusion, the hybrid fluid with different kinds of pressure will flow out of the refrigerator-ejector. It can be seen from the results that the fourth condition (when the pressure of primary fluid is 0.06605 MPa) has a higher pressure at the outlet.

Figure 4 shows two kinds of primary pressure (0.08851 MPa and 0.07825 MPa) distribution curves along the central axis of symmetry. It can be seen that the pressure along the central axis of symmetry decreases at the outlet of the nozzle. In this process, the pressure is translated into velocity. The pressure of the hybrid fluid increases after these two fluids are adequately mixed. In the mixing chamber, it decreases gradually when the hybrid fluid is compressed. At the throat, it decreases again. However, in the first half of the throat, it counterpoises; in the last half, it ascends a little. It then slowly rises and gradually, completing the processing of velocity being translated into pressure in the diffuser. At last, the hybrid fluid flows out of the refrigerator-ejector with a higher pressure and enters the compressor. Under two different primary pressure conditions, the pressure along the central axis of symmetry is more or less uniform. When the pressure of the primary fluid is lower, there is less pressure difference at the outlet of the nozzle, but there is greater pressure difference at the throat. Finally, the pressure at the outlet is commensurable under these two conditions.

Besides, the effect of pressure on entrainment ratio is studied by just changing the pressure of primary fluid (11 different pressures of primary fluid are selected) when other thermodynamic parameters remain unchanged. Figure 5 shows the changes of entrainment ratio under different pressures of primary fluid.

From Fig. 5, it can be seen that the entrainment ratio changes in the range of 0.435 to 0.568 under different pressures of primary fluid and that there is one optimal pressure, i.e., p_popt = 0.06612 MPa, corresponding to the maximum entrainment ratio, i.e., u = 0.568. When p_p <p_popt, the entrainment ratio increases with the rise of the pressure. When p_p>p_popt, the entrainment ratio decreases with the drop of the pressure. When the pressure of primary fluid is higher than 0.08156 MPa, the entrainment ratio basically remains unchanged. This shows that the increase in the pressure of primary fluid does not necessarily improve the performance of the refrigerator-ejector, because the increase of the pressure of primary fluid can hike up the evaporation temperature of the refrigerator evaporator, and accordingly increasing the running time of the refrigerator, which is, naturally, disadvantageous to the performance of the refrigerator.

Effect of the pressure of secondary fluid

When other thermodynamic parameters remain unchanged, four different pressure conditions of secondary fluid are selected to simulate the refrigerator-ejector. Figure 6 shows the fluid flow pressure distribution in the refrigerator-ejector under four different pressures of secondary fluid. It can be seen that different pressures of secondary fluid correspond to different flow status. Under the same pressure of primary fluid, the pressure drop at the outlet of the nozzle is identical. However, due to the different pressures of secondary fluid, the hybrid fluid has diverse initial pressures. From Fig. 6, a conclusion can be made that, after being further compressed in the mixing chamber, the pace of change in the pressure distribution of the hybrid fluid determines the pressure at the outlet. It can be seen from the result that the third condition (when the pressure of secondary fluid is 0.04616 MPa) has a higher pressure at the outlet.

By the same token, the effect of pressure on the entrainment ratio is studied by selecting 11 pressures of secondary fluid. Figure 7 shows changes of the entrainment ratio under different pressures of secondary fluid.

From Fig. 7, it can be seen that the entrainment ratio changes in the range of 0.328 to 0.564 under different pressures of secondary fluid and that there is one optimal pressure of secondary fluid, i.e., p_hopt = 0.04837 MPa, corresponding to the maximum entrainment ratio, i.e., u = 0.564. When p_h<p_hopt, the entrainment ratio increases slowly with the rise of the pressure. When p_h>p_hopt, the entrainment ratio decreases significantly with the drop of the pressure. This shows that, in the pressure range of secondary fluid, increasing the pressure of secondary fluid can improve the performance of refrigerator-ejector, but once the critical pressure is exceeded, the entrainment ratio reduces dramatically and the performance of the refrigerator-ejector deteriorates. Besides, it is known from the actual operation conditions of the refrigerator that excessive pressure of secondary fluid can also hike up the evaporation temperature of the freezer evaporator, which is inconsistent with the equitable capacity allocation requirements of the refrigerator and freezer.

Conclusions

1) The refrigerant flow in the refrigerator-ejector is very complicated. The 3D numerical simulation of the refrigerator-ejector in this paper can provide an in-depth comprehension of the internal flow mechanism of the refrigerator-ejector.

2) The performance of the refrigerator-ejector under four different pressure conditions of primary fluid (or secondary fluid) is compared, and the refrigerant flow mechanism in the refrigerator-ejector is obtained. The simulation results further prove the working principle of the refrigerator-ejector and at the same time prove the correctness of the model itself, which can effectively predict the flow of the refrigerant in the refrigerator-ejector.

3) For different pressures of primary fluid, there is one optimal pressure of primary fluid, i.e., p_popt = 0.06612 MPa, corresponding to the maximum entrainment ratio, i.e., u = 0.568. When p_p<p_popt, the entrainment ratio increases with the rise of the pressure. When p_p>p_popt, the entrainment ratio decreases with the drop of the pressure. When the pressure of primary fluid is higher than 0.08156 MPa, the entrainment ratio basically remains unchanged.

4) For different pressures of primary fluid, there is one optimal pressure of secondary fluid, i.e., p_hopt = 0.04837 MPa, corresponding to the maximum entrainment ratio, i.e., u = 0.564. When p_h<p_hopt, the entrainment ratio increases slowly with the rise of the pressure. When p_h>p_hopt, the entrainment ratio decreases significantly with the drop of the pressure.

References

[1] Liu Yicai, Cao Lihong, Yang Zhihui, Liu Zhenli, Huang Qian, Xin Tianlong. Study on a new-typed compression/injection hybrid refrigeration cycle for household refrigerator. Cryogenics and Superconductivity, 2008, 36(1): 42-45 (in Chinese)

[2] Cao Lihong, Liu Yicai, Yin Zheng, Liu Zhenli, Huang Qian, Zhang Mingyan. Experiment research and analysis on compression/injection hybrid refrigeration cycle for household refrigerators. Chinese Association of Refrigeration 2007 Air-conditioning Academic Convention, Hangzhou, China, 2007, 641-645 (in Chinese)

[3] Cao Lihong, Liu Yicai, Yang Zhihui, Liu Zhenli, Huang Qian, Yin Zheng. Study on a new kind of compression/injection hybrid refrigeration cycle. The 10th Academic Convention of Engineering Thermophysics, Chongqing, China, 2006, 446-452

[4] Liu Yicai, Cao Lihong, Yang Zhihui, Liu Zhenli, Huang Qian. Analysis of energy saving technique of refrigerators. China Appliance Technology, 2006, (5): 42-44

[5] Aphornratana S, Eames I W. A small capacity steam-ejector refrigerator—experimental investigation of a system using ejector with movable primary nozzle. International Journal of Refrigeration, 1997, 20(5): 352-358

[6] Chaiwongsa P, Wongwises S. Effect of throat diameters of the ejector on the performance of the refrigeration cycle using a two-phase ejector as an expansion device. International Journal of Refrigeration, 2007, 30(4): 601-608

[7] Xu Haitao. The theories and numerical research of steam-jet ejector. Dissertation for the Doctoral Degree. Nanjing: School of Mechanical and Power Engineering, Nanjing University of Technology, 2003, 69-85

[8] Xu Haitao, Sang Zhifu. Thermodynamic models for calculating entrainment ratio of steam-jet ejector. Journal of Chemical Industry and Engineering, 2004, 55(5): 704-710

[9] Liu Yicai, Cao Lihong, Yang Zhihui, Liu Zhenli, Huang Qian. Theoretical research on gas-liquid two-phase flow at outlet of refrigerator capillary. Journal of Central South University (Science and Technology), 2007, 38(3): 450-453 (in Chinese)

[10] Liu Yicai, Cao Lihong, Yang Zhihui, Liu Zhenli, Huang Qian. Numerical modeling and experimental investigation of non-adiabatic capillary tube in the refrigerator. ICCR2008, Shanghai, 2008

[11] Wang Quan, Xiang Xiongbiao. A study of method for calculating jet coefficient of steam jet compressor. Acta Energiae Solaris Sinica, 1997, 18(3): 314-321

[12] Liao Guojin, Ba Dechun, Wang Xiaodong. Study on supersonic flow field in steam ejector by 3D simulation. The Academic Convention of Vacuum Metallurgy and Superficial Engineering, 2005, 263-266

[13] Li Haijun, Shen Shengqiang, Zhang Bo, Li Sufen. Numerical analysis on flow parameters and performance of steam ejector. Journal of Thermal Science and Technology, 2005, 4(1): 52-57

[14] Yang Yanqin, An Zhiqiang, Jing Shudong. Study on numerical simulation of ejector flow field. Journal of Southwest University for Nationalities Natural Science Edition, 2006, 32(2): 316-324

[15] Rusly E, Aye L, Charters W W S, Ooi A. CFD analysis of ejector in a combined ejector cooling system. International Journal of Refrigeration, 2005, 28(7): 1092-1101

[16] Bartosiewicz Y, Aidoun Z, Desevaux P, Mercadier Y. Numerical and experimental investigations on supersonic ejectors. International Journal of Heat and Fluid Flow, 2005, 26(1): 56-70

RIGHTS & PERMISSIONS

Higher Education Press and Springer-Verlag Berlin Heidelberg