Research On Novel Two-stage Heat Pump With Vapor Expander And Its Key Thermal Processes Using Zeotropic Mixtures

The performance of the vapor compression heat pump system deteriorates greatly as the cycle temperature lift increases,which limits its application as an energy-saving heat supply technology in cold climate regions.The individual thermodynamic process should be optimized simultaneously in order to improve the cycle performance and to expand its application range.At present,most of the investigation focuses on the heat transfer and compression processes.The temperature glide matching and multi-stage compression have been recognized as two representative energy-saving techniques.However,little attention has been paid on the throttling process and it lacks effective method to reduce the throttling loss in the subcritical cycles.Hence,the present research attempts to introduce vapor expansion technology into subcritical vapor compression cycle to recover the expansion work,and a novel two-stage heat pump system is proposed.The first challenge for developing the proposed cycle is the selection of working fluid.Since the system comprises vapor expander and compressor simultaneously,from the point of view of avoiding liquid slugging in both machines,isentropic fluid with a vertical vapor boundary on temperature-entropy diagram should be as the first option for working fluid selection.However,very few refrigerants could be regarded as isentropic fluid.As a result,this study proposes to form isentropic fluid by mixing different pure refrigerant.Firstly,the temperature-entropy property of the refrigerant mixture was studied quantitatively,and the relation between characteristic parameter?and molecular structure was confirmed.Based on the given working condition with evaporating and condensing temperature being-15oC and 60oC respectively,a total of 30 isentropic mixtures were screened out.The influence of mixture composition and system pressure on the temperature-entropy property was investigated respectively.Then the effect of the fluid's temperature-entropy property on cycle performance was studied,and calculation results indicated that the exhaust temperature was reduced and the effective heat transfer area in condenser was increased when using isentropic mixtures.It was found that the isentropic mixture could be separated into two fluids with different temperature-entropy property under certain conditions,making the isentropic mixture be the desired working fluid for the proposed heat pump cycle.After determining the working fluid,the steady state thermodynamic model of the proposed cycle was established.Calculation was conducted based on mixture R290/R600a with three different circulating concentrations.The results showed that there existed an optimal vapor quality at the condenser outlet to obtain the maximum heating COP.Then the performance of the proposed cycle was compared with other cycles including single-stage cycle,two-phase expansion cycle and vapor injection cycle.Calculation results indicated that when the cycle temperature lift was increased to 72.5oC,the COP of the proposed cycle could be increased by 16.2%compared to single-stage cycle.So far as thermodynamic performance and payback period were concerned,however,the proposed cycle was not superior to the vapor injection cycle.The second key problem is the achievement of component separation of zeotropic mixture within the vapor compression cycle.Impacting T-junction was chosen as the partial phase separator,since it could lead to the compact design of system and cost reduction.An experimental system was designed and set up to investigate the separation characteristics of vapor-liquid two phase flow of refrigerants in T-junction.Based on R134a,the influences of inlet mass velocity,vapor quality and mass flow ratio of the downwards outlet on the liquid separation efficiency were investigated,and the flow pattern at T-junction inlet was recorded with a high-speed camera.The test results showed that the liquid separation efficiency could be over 90%under the condition where mass velocity and mass flow ratio being 200 kg·m^-2·s^-1 and 75%respectively.Based on the visualization experiments,a functional relationship between falling liquid ratio and vapor phase Froude number was derived,and in combination of the streaming line method,a model for predicting the phase distribution characteristic of annular and wave flow was proposed.Based on the same experimental system,the effects of inlet mass velocity,quality,mass flow ratio,circulating composition and outlet tube diameter on the distribution of constituents of binary zeotropic mixtures R134a/R245fa and R290/R600a in the impacting T-junction were investigated.In this study,the circulating concentration of the mixture was determined indirectly based on the relationship between the composition and the density of the liquid phase mixture under certain temperature and pressure condition.The density of the liquid mixture was measured with Coriolis mass flow meter.The experimental results indicated that,for T-junction with a 17.5mm outlet tube diameter,the mixture R134a/R245fa(x_R134a=0.7075)showed the best separation performance with difference in component separation efficiency being-14.6%,under the condition where inlet mass velocity,quality and mass flow ratio being 200 kg·m^-2·s^-1,0.49 and 0.5 respectively.The circulating concentration and outlet tube diameter affected the distribution of component via changing the pressure difference across the T-junction.Overall,a smaller pressure drop could lead to a better separation performance.Finally,the relationship between phase separation efficiency and component separation efficiency was discussed.