Study Of Heat Transfer And Dry-out Characteristics Of Carbon Dioxide During Flow Boiling In Tube

The natural working fluid CO2 has good thermodynamic properties with low surface tension.The ODP of CO2 is 0 and GWP is 1.However,compared with traditional refrigerants,CO2 has very different flow boiling heat transfer characteristics,and the process is prone to dry-out,which greatly reduces its heat transfer capacity.In this paper,the theoretical analysis and experimental study of CO2 flow boiling heat transfer dry-out are carried out:(1)At present,there are many researches on CO2 flow boiling heat transfer in tube,but their working conditions are limited.Therefore,a database of flow boiling heat transfer in CO2 tube with 4986 data points is established,and the influence trend of mass flow rate,heat flux,saturation temperature(or pressure)and pipe diameter on CO2 heat transfer coefficient is analyzed.When the dry-out quality is greater than 0.4,the heat transfer coefficient increases with the increase of mass flow rate;in most conditions,the heat transfer coefficient increases with the increase of heat flux,but in the conditions of small dry-out quality,high mass flow rate,high dry-out quality and high saturation temperature,the heat transfer coefficient decreases with the increase of heat flux;when the dry-out quality is less than 0.6,the saturation temperature is less than 0℃,with the increase of saturation temperature,the heat transfer coefficient increases,and the dry-out quality decreases.When the dry-out quality is less than 0.6,the heat transfer coefficient of most conventional channels decreases with the increase of pipe diameter,while the heat transfer coefficient of microchannel increases with the increase of pipe diameter,and the heat transfer coefficient of microchannel is lower than that of conventional channel;when the dry-out quality is more than 0.6,the heat transfer coefficient is larger when the pipe diameter is larger.(2)It is found that when the dry-out quality is less than 0.6,the dry-out quality decreases with the increase of mass flow rate,while when the dry-out quality is greater than 0.6,the dry-out quality increases with the increase of mass flow rate;in most cases,the dry-out quality decreases with the increase of heat flux,but in some cases when the dry-out quality is less than 0.6,the dry-out quality increases with the increase of heat flux and with the increase of saturation temperature.Compared with large diameter,the dry-out quality of small diameter is smaller,while for large diameter,the dry-out quality decreases with the increase of diameter.(3)In order to further verify the reliability of the conclusions and expand the working range of the database,an experimental platform for CO2 flow boiling heat transfer was set up,which can be used to analyze and study the characteristics of CO2 flow boiling heat transfer and drying under variable working conditions.The experimental conditions are as follows.The diameter of pipe is4mm.The heat flux is between 3.61 k W/m~2 to 5.89 k W/m~2.The mass flow rate is between 800kg/(m~2·s)to 2600 kg/(m~2·s).The evaporation temperature is between 20℃to 30℃.The effects of mass flow rate,heat flux and saturation temperature(pressure)on the dry-out characteristics of CO2 heat transfer were investigated.The results show that:in the experimental conditions,with the increase of mass flow rate,the heat transfer coefficient has no obvious change rule,and the dry-out quality degree decreases;with the increase of heat flux,the heat transfer coefficient increases,and the dry-out quality degree decreases,and no obvious dry-out phenomenon is observed in the high mass flow rate conditions.With the increase of saturation temperature,the heat transfer coefficient increases,and the dry-out occurs in advance.(4)On the basis of database analysis and experimental research,a new correlation of dry-out quality prediction is established.Through the comparison between the database data and several commonly used correlations of CO2 heat transfer,it is found that the deviation is large.Combined with the optimization algorithm,based on the existing heat transfer correlation,through a large number of calculation and analysis,a correlation which can accurately predict the CO2 flow boiling dry-out quality in a larger range of operating conditions is obtained.After verification,89%of the data points in the database are less than 15%and 87%of the experimental data points are less than20%,which is of great significance to prevent the deterioration of heat transfer.