The Research Of Heat Transfer Characteristics Of Carbon Dioxide In Microchannel Evaporator

With the improvement of people’s living standards,the awareness of environmental protection is gradually enhanced.The greenhouse gas refrigerants are in danger of being eliminated.While carbon dioxide has been regarded as the potential alternative refrigerant due to its high efficiency and environmental safety.As the microchannel has the advantages of high efficiency and saving energy,it has been widely concerned and has broad application prospects.In this paper,the carbon dioxide and microchannel technology were combined.The heat transfer and flow characteristics of carbon dioxide boiling in microchannel evaporator were studied.It provides a theoretical basis for carbon dioxide microchannel evaporator design and has important academic significance and practical value.Firstly,the existing carbon dioxide boiling heat transfer and flow correlations in microchannel were firstly analyzed in this paper.The use scope and characteristics of the correlation were fully considered.According to the actual situation of the experiment bench,the carbon dioxide microchannel evaporator boiling and flow correlation were obtained through comparison and analysis.When the carbon dioxide is in a boiling two-phase region in microchannel,Cheng correlations should be used,because the carbon dioxide flow boiling heat transfer correlation is based on the flow state.While in superheat region,as the Reynolds number Re ≥ 2300,the Gnielinski correlation should be applied to solve the convective heat transfer coefficient.However,when he Reynolds number Re < 2300,the Sieder-Tate correlation should be used to solve it.Several different forms of frictional pressure drop model were analyzed.If G≥ 43 kg · m-2 · s-1,the Jassim and Mewell correlation which is a probabilistic drop model should be used to work out the frictional pressure drop value in boiling two-phase region.If G < 43 kg · m-2 · s-1,the Friedel correlation which is a phase separation type drop mode was suggested to solve the frictional pressure drop value in boiling two-phase region.On this basis,based on finite element analysis,a steady state distributed parameter model for microchannel evaporator was established by using MATLAB combined with REFPROP technology and considering the dry and wet conditions on air side as well as the overheat condition on carbon dioxide side.The impacts of various parameters on carbon dioxide boiling heat transfer were studied.According to the simulation result,the carbon dioxide convective heat transfercoefficient reaches a maximum near the dryness point in two-phase region.After the dryness,the convective heat transfer coefficient decreased rapidly.The pressure drop value in superheat region is smaller than in boiling two-phase region.Then,the experiment bench of carbon dioxide boiling heat transfer in microchannel evaporator was build.According to the experiment result,the carbon dioxide and the air outlet temperature of each segment were gotten.Meanwhile,the convective heat transfer of carbon dioxide of each segment in microchannel evaporator was calculated.Comparisons of analytical and experimental data were made to verify and validate the model.The phenomenon of liquid separation inequality at the inlet of carbon dioxide evaporator was found by the infrared imaging technology.Because of this,the heat deterioration phenomenon occurred in microchannel.Several improvement measures were been put forward to alleviate this problem.Finally,the irreversible loss of heat transfer process of carbon dioxide in microchannel was represented by a dimensionless number — entropy generation number NS.The situation of wet and dry conditions on air side were fully considered.At the same time,the entropy generation model in boiling two-phase and superheat region were built separately.The distribution of each infinitesimal segment irreversible loss was analyzed.The results from the mathematic model showed that system entropy was mostly caused by temperature difference of heat transfer between carbon dioxide and air sides.The overheating segment irreversible loss is far less than the two-phase segment.The irreversible loss of the infinitesimal segment near dry-point reaches the maximum value.