Investigation On Liquid-vapor Separation Evaporator And Its Application In Air Conditioning System

As the proportion of air conditioning system’s energy consumption keeps increasing,air conditioning system has become the largest energy consumption terminal in both residential and non-residential buildings.Improving the energy efficiency of the system and reducing energy consumption is the key to energy conservation and emission reduction.Heat exchangers have a significant effect on the performance of air conditioning system.By optimizing the behaviours of heat exchangers,the performance of air conditioning system can be effectively improved.One of heat transfer enhancement methods called liquid-vapor separation heat transfer enhancement technology(LVST),which has been applied to condenser can improve heat transfer coefficient and reduce pressure drop,obviously promoting the performance of air conditioning system.Applying the LVST to evaporator can theoretically improve its behaviours by redistributing the vapor quality and mass flow rate of refrigerant.However,at present,the behaviours of liquid-vapor separation evaporator(LVSE),and the influence of LVSE on the air conditioning system still need to be further studied.Firstly,the paper developed a numerical model of LVSE based on distributed parameter method.And then,the average heat transfer coefficient and pressure drop of conventional parallel flow evaporator and LVSE under different operating conditions were obtained by experiments,and the numerical model was verified and modified.The results show that the root-mean-square error of average heat transfer coefficient of the LVSE model is 33.7 % and the root-mean-square error of pressure drop is 28.3 %.Secondly,based on the numerical model,the influence of evaporator refrigerant circuitry,liquid-vapor separation efficiency,inlet vapor quality and mass flow rate on LVSE were investigated though Single controlled variable analysis.The results discover that the average heat transfer coefficient of LVSE decreases,the pressure drop has no obvious change,and the comprehensive performance of evaporator deteriorates with the increase of liquid-vapor separation efficiency during the inlet vapor quality is relatively low.When the inlet vapor quality is high,the average heat transfer coefficient is less affected by the liquidvapor separation efficiency,the pressure drop first increases and then decreases,and the comprehensive performance first deteriorates and then increases significantly.Therefore,the LVST has negative influence on the evaporator behaviours when the inlet vapor quality is low,and only when the inlet vapor quality is high can the effect of enhancing the evaporator behaviours be realized.Besides,the results also discover that,when the inlet mass flow rate is low,the average heat transfer coefficient does not change significantly with the liquid-vapor separation efficiency,the pressure drop increases first and then decreases,and the comprehensive performance deteriorates first and then increases,but the changes are insignificant.When the inlet mass flow rate is high,the average heat transfer coefficient increases firstly and then decreases.The pressure drop increases first and then decreases significantly,and the comprehensive performance of the evaporator firstly becomes worse and then significantly improved.Therefore,the LVST cannot give full play to the enhancement effect when the inlet mass flow rate is low,but it can effectively improve the comprehensive performance of evaporator under high mass flow rate.Furthermore,the optimal refrigerant circuitry and liquid-vapor separation efficiency were designed based on the numerical model of LVSE under a given working condition.The optimization results show that the optimal refrigerant circuitry of the three-pass LVSE is9-6-9 and the optimal liquid-vapor separation efficiency is 0.64.Compared with the conventional parallel flow evaporator with the optimal refrigerant circuitry,the average heat transfer coefficient of the LVSE decreased by 0.7 %,the pressure drop decreased by 44.37 %,and the comprehensive performance improved by 14.14 %.Finally,a liquid-vapor separation heat exchanger was applied to the outdoor unit of air conditioning system.At cooling mode,the outdoor heat exchanger where LVST located operates as the liquid-vapor separation condenser,and at heating mode,the outdoor heat exchanger is transformed into the LVSE.Based on the system experiment results,the advanced exergy analysis find that at cooling mode,the advanced exergy efficiency of liquid-vapor separation air conditioning system(LVSA)with the same refrigerant charge and capillary tube length with original system is 19.2 % higher than that of the original air conditioning system(OA),and the advanced exergy efficiency of optimal LVSA after matching is 20.8 % higher than that of OA.At heating mode,the advanced exergy efficiency of optimal LVSA is 9.6 % higher than that of OA.Besides,the effects of liquid-vapor separation heat exchanger on components and systems were quantified and analysed by the advanced exergy analysis.The analysis results show that at cooling mode,the avoidable exergy destruction of compressor of optimal LVSA reduced by 45.4 %,and that of outdoor exchanger where the LVST located reduced by 19.0 %,comparing to that of OA.At heating mode,the avoidable exergy destruction of compressor of optimal LVSA is 33.9 % smaller and that of outdoor heat exchanger is 23.5 % smaller than that of OA.The results discover that the LVST can effectively reduce the avoidable exergy destruction of the compressor and the heat exchanger where it located,significantly improving the performance of air conditioning system.By modelling and experimental studying of LVSE behaviour characteristics and influence factors,and experiment and exergy analysis to the effect of liquid-vapor separation heat exchanger on air conditioning system,the advanced behaviour of LSVE as well as the great application potential of air conditioning system with liquid-vapor separation heat exchanger were proved.