Study On Flow Boiling Heat Transfer Characteristics Of CO₂/Propane In Pipe

The refrigerants currently used in heat pump products are mostly synthetic refrigerants,which have a serious i MPact on the environment and refrigerant replacement is imminent.The natural refrigerants carbon dioxide and propane（R290）have good thermodynamic properties,but also have disadvantages such as high critical pressure and high flammability respectively.The mixture of the two can improve the existing shortcomings,reduce flammability and system high pressure,the mixture has a good replacement.The study of the in-tube boiling heat transfer characteristics of CO₂/propane blends is of practical importance for refrigerant screening and replacement.The heat transfer characteristics of a binary CO₂/propane mixture in a horizontal circular tube with different composition ratios are investigated in the paper.A new heat transfer correlation equation was fitted and co MPared with experimental data.The following shows the main work content and conclusion of this study:（1）At present,there are few experimental research conditions for the heat transfer of CO₂/propane mixed masses,and there is a lack of qualitative analysis to summarize the trend of heat transfer coefficients with parameters.Therefore,Therefore,this paper has reviewed a large amount of literature,2292 sets of experimental data on heat transfer in a horizontal circular tube were collected to establish a database on boiling heat transfer in CO₂/propane tubes,and the influence of basic parameters such as saturation temperature,heat flow density,mass flow rate and tube diameter on the heat transfer coefficient was analysed.Under the condition of constant flow rate and heat flux,increasing the saturation temperature can increase the heat transfer coefficient.At a constant temperature and heat flow density and a dryness of about 0.67,The heat transfer coefficient of the mixed mass increases and decreases simultaneously with the mass flow velocity,that is to say,the flow velocity enhances the heat transfer.In the region of high mass flow velocity,there is a positive correlation between the heat transfer coefficient and the heat flux,but the trend slows down above 30k W/m².When the tube diameter is greater than 2mm,the heat transfer coefficient decreases with increasing tube diameter.（2）A combination of experiments and simulations was used to study the heat transfer and drying characteristics of the CO₂/propane mixture in the tube.A numerical model of the boiling heat transfer in the tube flow of CO₂/propane mixed mass was established to study the trend of the heat transfer coefficient with the change of parameters such as saturation temperature,mass flow rate,heat flow density and tube diameter.The heat transfer coefficient of boiling CO₂/propane mixture is positively correlated with the heat flow density;increasing the mass flow rate G also increases the heat transfer coefficient,and this trend is more obvious in the high dryness region;increasing the heat transfer coefficient is also acco MPanied by increasing the saturation temperature,but the effect of increasing the saturation temperature on the heat transfer coefficient is limited co MPared with the first two parameters;using a large tube diameter reduces the heat transfer coefficient co MPared with a small tube diameter The smaller tube diameter is chosen to enhance the heat transfer in the tube.A CO₂ binary mixture horizontal circular tube boiling heat transfer experiment was built to investigate the heat transfer in the tube of a CO₂/propane mixture.The results show that the boiling heat transfer coefficient increases with the increase of heat flow density,mass flow rate and saturation temperature,and that the appearance of the drying point lags with the increase of the propane component ratio,and the evaporation dryness phenomenon occurs mostly in the high dryness zone.The simulation results were verified on the basis of experiments,and the average deviation was 10.53%,with good model accuracy.（3）The prediction errors of several commonly used correlations were calculated and the Docoloubier heat transfer prediction model was found to be most accurate.Based on the experimental data in the database,the new prediction correlation was modified to obtain a new prediction correlation.The MAD（Mean Absolute Error）of the new correlation formula was reduced to 27.01%as verified by the remaining database data,while the MAD of the new correlation formula was 21.43%as verified by the experimental data in this paper,with 89.3%of the data points falling within the 30%error band.This also proves that the fitted formula is more accurate and more widely applicable.