Two-phase ejector cycle
Vapor compression cooling cycles deviate from the Carnot refrigeration cycle in several ways, such as isenthalpic expansion of saturated liquid at the condenser outlet and desuperheating of refrigerant vapor at the compressor outlet. Therefore, COPs of vapor compression cooling cycles are always lower than those of a Carnot cycle under the same working conditions. Isenthalpic expansion imposes a two-fold penalty on cycle performance compared with isentropic expansion in the Carnot cycle: the cooling capacity is reduced and the compressor work is increased. Expansion work recovery devices such as ejectors which recover the kinetic energy released during the expansion instead of dissipating it in a throttling process are known to be beneficial to cycle performance. Figure 1 shows the layout and pressure-specific enthalpy diagram of a two-phase ejector cooling cycle first proposed by Gay [1].
In this cycle, high pressure motive flow leaving the condenser enters the ejector through the motive inlet. The motive flow is expanded in the motive nozzle and creates a low pressure zone at the nozzle outlet, which entrains the suction flow from the evaporator. The two streams are mixed in the mixing chamber and kinetic energy is transferred from the motive flow to the suction flow. The mixed fluids leave the ejector through the diffuser. The fluid velocity is reduced in the diffuser which results in recompression of the mixed fluids by converting velocity energy back into pressure energy. Therefore, the ejector diffuser outlet pressure is higher than the suction flow pressure (that is, the evaporator pressure). The two-phase flow then gets separated in the separator. Saturated vapor enters the compressor while saturated liquid gets throttled and is fed into the evaporator via a metering valve. That way, kinetic energy released during expansion is utilized to compress the fluid from the evaporator. As a result, some compressor work is saved while the cooling capacity is increased if the heat rejection capacity remains constant.
Disawas and Wongwises [2] proposed that, in addition to serving as an expansion device, the ejector can also act as a refrigerant pump for the low-pressure side of the system. The evaporator is therefore flooded with refrigerant and operates as in a liquid recirculation system. Their experimental results showed that the COP of the two-phase ejector refrigeration cycle using R134a was higher than that of the baseline cycle using expansion valve over the whole range of experimental conditions. The maximum improvement achieved was about 13% at low heat sink and heat source temperatures. Liquid recirculation can improve evaporator performance by sending more liquid to the evaporator than is actually evaporated so that dryout in the evaporator can be reduced. It can also improve refrigerant distribution for evaporators with inlet headers by feeding only single-phase liquid to the inlet headers instead of two-phase refrigerant which often results in nonhomogeneous distribution of two-phase flow into the parallel channels. Therefore, liquid recirculation can result in higher evaporation pressure, and higher system COP compared to a direct expansion cycle (Lawrence and Elbel [3]).
[1] Gay, N. H., “Refrigerating System,” U.S. Patent 1,836,318, 1931.
[2] Disawas, S. and Wongwises, S., “Experimental investigation on the performance of the refrigeration cycle using a two-phase ejector as an expansion device,” International Journal of Refrigeration, 2 7(6): 587-594, 2004.
[3] Lawrence, N., and Elbel, S., “Experimental and Numerical Study on the Performance of R410A Liquid Recirculation Cycles with and without Ejectors," 15th International Refrigeration and Air Conditioning Conference at Purdue, West Lafayette, IN, USA”, Paper 2187, 2014.
[4] Zhu, J. and Elbel, S., "A New Control Mechanism for Two-Phase Ejector in Vapor Compression Cycles for Automotive Applications Using Adjustable Motive Nozzle Inlet Swirl," SAE Int. J. Passeng. Cars - Mech. Syst. 9(1): 2016, doi:10.4271/2016-01-0243.
In this cycle, high pressure motive flow leaving the condenser enters the ejector through the motive inlet. The motive flow is expanded in the motive nozzle and creates a low pressure zone at the nozzle outlet, which entrains the suction flow from the evaporator. The two streams are mixed in the mixing chamber and kinetic energy is transferred from the motive flow to the suction flow. The mixed fluids leave the ejector through the diffuser. The fluid velocity is reduced in the diffuser which results in recompression of the mixed fluids by converting velocity energy back into pressure energy. Therefore, the ejector diffuser outlet pressure is higher than the suction flow pressure (that is, the evaporator pressure). The two-phase flow then gets separated in the separator. Saturated vapor enters the compressor while saturated liquid gets throttled and is fed into the evaporator via a metering valve. That way, kinetic energy released during expansion is utilized to compress the fluid from the evaporator. As a result, some compressor work is saved while the cooling capacity is increased if the heat rejection capacity remains constant.
Disawas and Wongwises [2] proposed that, in addition to serving as an expansion device, the ejector can also act as a refrigerant pump for the low-pressure side of the system. The evaporator is therefore flooded with refrigerant and operates as in a liquid recirculation system. Their experimental results showed that the COP of the two-phase ejector refrigeration cycle using R134a was higher than that of the baseline cycle using expansion valve over the whole range of experimental conditions. The maximum improvement achieved was about 13% at low heat sink and heat source temperatures. Liquid recirculation can improve evaporator performance by sending more liquid to the evaporator than is actually evaporated so that dryout in the evaporator can be reduced. It can also improve refrigerant distribution for evaporators with inlet headers by feeding only single-phase liquid to the inlet headers instead of two-phase refrigerant which often results in nonhomogeneous distribution of two-phase flow into the parallel channels. Therefore, liquid recirculation can result in higher evaporation pressure, and higher system COP compared to a direct expansion cycle (Lawrence and Elbel [3]).
[1] Gay, N. H., “Refrigerating System,” U.S. Patent 1,836,318, 1931.
[2] Disawas, S. and Wongwises, S., “Experimental investigation on the performance of the refrigeration cycle using a two-phase ejector as an expansion device,” International Journal of Refrigeration, 2 7(6): 587-594, 2004.
[3] Lawrence, N., and Elbel, S., “Experimental and Numerical Study on the Performance of R410A Liquid Recirculation Cycles with and without Ejectors," 15th International Refrigeration and Air Conditioning Conference at Purdue, West Lafayette, IN, USA”, Paper 2187, 2014.
[4] Zhu, J. and Elbel, S., "A New Control Mechanism for Two-Phase Ejector in Vapor Compression Cycles for Automotive Applications Using Adjustable Motive Nozzle Inlet Swirl," SAE Int. J. Passeng. Cars - Mech. Syst. 9(1): 2016, doi:10.4271/2016-01-0243.