| It is urgent to carry out the work of energy conservation and emission reduction in today's world with increasingly prominent energy and environmental problems.The refrigeration heating ventilation and air conditioning is a basic industry related to people's work and daily life,of which the energy consumptions can not be ignored.Therefore,study on the energy saving technology of the industry has become an important task in the work of energy conservation and emission reduction.The conventional compression refrigeration cycle(CCRC)plays the major role in the refrigeration heating ventilation and air conditioning system.Throttling losses of the expansion valve in the CCRC system is significant.The two-phase ejector used as an expansion device in the refrigeration system is a kind of effective measure to recover the throttling losses.Therefore,studies on the two-phase ejector and the compression/ejection refrigeration system have drawn more and more attentions.The two-phase constant-area mixing ejector model is modified and used to investigate performances of the bi-evaporator compression/ejection refrigeration system(BCERC)with R134a as the refrigerant.On the basis of the theoretical analysis,the two-phase ejector is designed and manufactured.Furthermore,the experimental platform of the bi-evaporator compression/ejection refrigeration system is set up and used to study influences of operating parameters on performance of the refrigeration system and the two-phase ejector.Moreover,impacts of different geometrical dimensions of the two-phase ejector on performance of the refrigeration system and the two-phase ejector are studied using numerical simulations and experimental methods.The main contents and conclusions are as follows:(1)Based on the modified two-phase ejector model containing the real physical parameters of refrigerant and the friction losses in constant-area mixing chamber,thermodynamic analysis of the BCERC is carried out with R134a as the refrigerant.Influences of the operating parameters and ejector area ratio(AR)are obtained.Results show that the condensing temperature,evaporation temperature,subcooling degree and cooling capacity ratios have significant impacts on performance of the BCERC.However,influences of the superheat degree of entrained flow on the system performance are relatively small.Energy-saving effects of the bi-evaporator compression/ej ection refrigeration system are more obvious in the operating conditions of high condensing temperature,low evaporation temperature,low subcooling degree and high cooling capacity ratios.Compared to performance of the CCRC system,the maximum COP improvement of the BCERC can be 30.77%and the maximum qv improvement can be 43.77%.Furthermore,there exists an optimal ejector area ratio(ARopt)corresponding to the best performance of the BCERC system.The optimal ejector area ratio changes with different operating parameters.The relationship between the optimal ejector area ratio and the operating parameters is obtained to be used as a reference for the design and applications of the two-phase ejector,which is as follows:ARopt=-1.35 exp(4.06?-0.55Tc-0.002Tle-0.48?Tc+1.18?le)-3.17?-0.14Tc+0.06Tle+0.17?Tc+15.57(2)The two-phase ejector is designed and manufactured based on theoretical analysis.Moreover,effects of operating parameters on performance of the bi-evaporator compression/ej ection refrigeration system are investigated based on the R134a BCERC experimental platform.Performance comparisons of the BCERC system and the CCRC system are carried out under the same cooling capacities and operating parameters.Results show that the total exergy destructions in the BCERC are lower than that in the CCRC.The COP and the exergy efficiency of the BCERC are higher than that of the conventional system.The COP and exergy efficiency improvements become more obvious as the condenser water temperature increases,the high-temperature evaporator water temperature decreases and the rotational speed of compressor increases.In the BCERC with a constant rotational speed of compressor,the use of an ejector to recover the expansion work would improve the COP and the exergy efficiency by 16.94?30.59%,7.57?28.29%,respectively.The total irreversibility could be reduced by 8.38?26.15%.Compared with the performance of the CCRC,COP and exergy efficiency of the BCERC with a variable rotational speed of compressor could be increased by around 32.64%and 23.32%,respectively.As the increase of the entrained fluid mass flow,the exergy efficiency of the BCERC system decreases.COP of the BCERC increases firstly and then decreases slowly with the increase of the entrained fluid mass flow.The maximum COP is obtained with the entrained fluid mass flow of 45.5kg/h.Under the experimental operating conditions,impacts of the inlet water temperature at the low-temperature evaporator are relatively small.(3)For a fixed geometrical dimension of two-phase ejector,influences of the operating parameters on the ejector performance are experimentally studied based on the experimental platform of BCERC system.Results show that the ejector entrainment ratio decreases with the increase of the rotational speed of compressor,condenser inlet water temperature and high-temperature evaporator inlet water temperature.With the increase of the inlet water temperature at the low-temperature evaporator and mass flow rates of the entrained flow,the ejector entrainment ratio increases accordingly.The ejector pressure lift ratio rises with the increase of the rotational speed of compressor,inlet water temperature at the condenser and the high-temperature evaporator.However,the ejector pressure lift ratio decreases with the increase of the inlet water temperature at the low-temperature evaporator and the mass flow rates of the entrained flow.With the increase of the rotational speed of compressor,the ejector efficiency decreases firstly to a lowest value and then increases.The ejector efficiency increases with the increase of the inlet water temperature at the condenser and decreases with increasing high-temperature evaporator inlet water temperature and decreasing low-temperature evaporator inlet water temperature.With the increase of mass flow rates of the entrained flow,the ejector entrainment ratio increases firstly to a highest value and then decreases.(4)Numerical simulations of different ejector geometrical dimensions are carried out based on the Ansys Fluent.The user-defined functions(udf)with the gas-liquid phase change model and the boundary conditions with the experimental parameters are used in the calculation model.The calculation results show that the critical mass fluxes of the primary flow increases with the rise of inlet pressure and subcooling degree.The choking factor Cf for refrigerant R134a is obtained under the condition of inlet pressure 0.1?2MPa and dimensionless subcooling degree ?T*sub 0?1,namely:0??T*sub<0.15:Cf=0.96,0.15??T*ub?1:Cf=1.The choking factor of R134a is higher than that of water for 0??T*sub<0.15.Therefore,the metastable effects of R134a are weaker than that of water.Under the experimental operating conditions and ejector geometrical dimensions,the ejector performance decreases significantly with small primary nozzle throat diameter and large primary nozzle outlet diameter.The ejector pressure lift ratio is less than 1 with long mixing chamber lengths and small mixing chamber diameters.The performance of two-phase ejectors is better for the ratio of mixing chamber length to mixing chamber diameter of 10.The diffuser angles should not be larger than 9�.(5)Influences of different ejector geometrical dimensions on performance of the BCERC system and the two-phase ejector are experimentally studied.Reulsts show that the system COP decreases with the increase of the primary nozzle throat diameter.The system COP reaches the highest value with the mixing chamber diameter of 10mm,which is corresponding to the optimal ejector area ratio of 4.73. |
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