Boiling Heat Transfer Characteristics Of Non-azeotropic Mixtures With Large Temperature Glide

Improving the energy efficiency of thermal system is an effective way to achieve the"double carbon"goal,which the research on working fluid plays an important role.For mechanical subcooling CO₂ air source heat pump,large temperature rise high temperature heat pump,organic rankine cycle and other positive and reverse cycle heat system,the use of large temperature glide non-azeotropic working fluid can significantly improve the system energy efficiency.This article in view of the large temperature glide of working fluid CO₂/R152a in horizontal tube diameter 2 mm small channel flow boiling heat transfer phenomenon,explore its component migration patterns,observation type gas-liquid two phase flow distribution characteristics,analysis the flow boiling heat transfer characteristics and pressure drop gradient,for large temperature glide non-azeotropic working fluid heat exchanger design optimization to provide theoretical and experimental support.In this paper,the effects of sliding speed ratio,evaporation pressure,heat exchange area in two-phase zone and system charge amount on component migration of large temperature glide fluid in horizontal tube are analyzed theoretically.The results show that with the increase of the speed ratio,the component migration in the horizontal tube is gradually weakened.At the same vapor quality,the degree of component migration increases with the decrease of pressure.The larger the volume of horizontal tube is,the more significant the component migration is.The degree of component migration can be reduced by increasing the charge rate.According to the flow pattern observation,the boiling two-phase flow pattern of large temperature glide non-azeotropic fluid CO₂/R152a in the tube can be divided into bubble flow,plug flow/slug flow,stratified flow,stratified-annular flow,annular flow and mist flow within the range of experimental test conditions.Compared with the corresponding flow patterns of conventional channels,the gas-liquid distribution of bubble flow and plug flow/slug flow is more regular and the gas-liquid interface is more obvious.With the increase of heat flux,the vapor quality of annular flow and the transition from dry state to mist flow decreases.Through digital image processing,it can be found that the distribution law of gray value of flow pattern image changing with time can be used to assist flow pattern division.By comparing the experimental observation flow pattern with the classical flow pattern prediction model,it is found that the prediction effect of Revellin model is better.The flow boiling heat transfer coefficient and pressure drop gradient of different components of large temperature glide non-azeotropic working fluid CO₂/R152a were measured,the flow boiling heat transfer law was obtained,and a prediction model for boiling heat transfer of large temperature glide non-azeotropic working fluid was proposed.The experimental results show that the convective heat transfer coefficient of large temperature glide working fluid increases slowly at first,then decreases rapidly and tends to be stable with the increase of vapor quality.At the same time,there is a critical drying point.The heat transfer effect with the mass fraction of CO₂ of 0.474 is lower than that with the mass fraction of 0.221 and 0.653.At the same time,the effects of different parameters on heat transfer coefficient,pressure drop gradient and flow pattern of large temperature glide working fluid were studied experimentally.The results show that with the increase of mass flow rate,heat flux and saturation temperature,the heat transfer coefficient of large temperature glide working fluid increases.The pressure drop gradient increases with the increase of mass flow rate and decreases with the increase of saturation temperature.The heat transfer coefficient of large temperature glide non-azeotropic working fluid CO₂/R152a is significantly lower than that of pure CO₂ and higher than that of R152a.Combined with the experimental data and considering the sensible thermal resistance and mass transfer resistance,a new correlation model for predicting heat transfer of non-azeotropic working fluid with large temperature glide is proposed,and its absolute deviation is 22.05%.