Experimental Study Of Thermal-Hydraulic Characteristics During Refrigerant Condensing And Flow Boiling In Parallel Flow Microchannel Flat Tubes

Compared with traditional heat exchangers,parallel flow microchannel heat exchangers have the advantages of small size,light weight,low refrigerant charge and high heat transfer efficiency,and have received wide attention in recent years.As the basic component unit of parallel flow microchannel heat exchanger,the thermal characteristics of parallel flow microchannel flat tube directly affect the overall heat transfer performance of the equipment.However,the results of the heat transfer process of refrigerant two-phase flow in parallel flow microchannel flat tubes vary greatly among researchers,and a unified mechanism or conclusion has not been obtained to explain the differences in the results,which plays a crucial role in the prediction and optimization of heat transfer performance in different structural tubes.Therefore,in this dissertation,three parallel flow microchannel flat tubes with different channel structures are used to experimentally study the thermal characteristics of the two-phase flow of refrigerant in the tubes,analyze the mechanism of different factors on the heat transfer performance and resistance characteristics,and establish correlations for predicting the heat transfer characteristics based on the experimental results.This dissertation firstly summarizes the current research on parallel flow microchannel heat exchangers and parallel flow microchannel flat tubes,and then introduces the variable inclination angle test system and experimental method for twophase flow thermal characteristics in tested tubes,and verifies the accuracy,reliability and repeatability of the test system using single-phase flow experiments to ensure the correct conduct of experiments related to two-phase flow thermal characteristics.The heat transfer coefficient of the inner tooth tube is 1.18-1.42 times higher than that of the regular tube under the same working conditions,because the effective heat transfer area is significantly increased by the presence of the tooth structure,and the tooth structure enhances the fluid disturbance and strengthens the convective heat transfer process.In the three tested tubes the heat transfer performance and resistance characteristics are positively correlated with the mass flow rate,quality and heat flux.The increase of mass flow rate increases the difference of superficial velocity between the gas-liquid phase and enhances the intensity of the interphase heat transfer process;the increase of quality leads to the decrease of liquid phase and liquid film thickness near the wall surface,which reduces the thermal resistance.The newly proposed prediction correlation for heat transfer coefficient can predict the heat transfer characteristics of the three tested tubes more accurately,and the prediction deviation of all data points is within ±20%,the prediction deviation of 90.3% of the data points is within ±15%,and the prediction results are in high agreement with the change trend of experimental results.For the flow boiling heat transfer process in the parallel flow microchannel tube in the horizontal condition,the internal tooth tube has the relatively optimal heat transfer performance before dryout occurs,and the heat transfer coefficient of the internal tooth tube is 1.11-1.19 times higher than that of the regular tube under the same conditions.At the same time,the internal tooth tube has the highest friction pressure drop in all experimental conditions,which is directly related to the presence of tooth structure and the reduction in channel size.Changes in heat flux and quality can have a more pronounced effect on the heat transfer performance,while the increase in both heat flux and mass flow rate leads to earlier dryout in the annular flow region.This is because when the heat flux is increased,the evaporation rate of the liquid is accelerated,and the frequency of its evaporation is gradually higher than that of the rewetting of the wall by liquid,resulting in the continuous occurrence of the dryout phenomenon;when the mass flow rate is increased,the fracture probability of the liquid film is higher,and it is more difficult to rewet the wall.Based on the existing research results and experimental data,a critical quality prediction correlation formula applicable to the three tested tubes was established,and then a new correlation formula for heat transfer coefficient prediction was proposed.The validation results show that the prediction deviation of all data points can be controlled within ±15% using the new correlation formula,and the prediction deviations of 95.41% of the data points are within ±10%,while the average absolute deviation of the prediction values of the three tested tubes does not exceed 6%.For the two-phase flow heat transfer process in the parallel flow microchannel tube in the variable inclination angle condition,the effects of mass flow rate,quality and tube type changes on the thermal characteristics are not significantly different from those in the horizontal condition,but the effects of inclination angle changes on the heat transfer performance in the condensation and flow boiling processes are different.In the downward flowing condition,there is a certain degree of similarity in the variation pattern of the thermal characteristics of the condensing and flow boiling processes,both of which decrease with increasing inclination angle;in the upward flowing condition,the thermal characteristics of the condensing process are less different from those in the horizontal condition,while the thermal characteristics of the flow boiling process show a trend of increasing and then decreasing with increasing inclination angle.This phenomenon can be analyzed and explained by the distribution and interaction of the gas-liquid phase in the channel at different inclination angles.To determine whether the change of inclination angle under a certain working condition will significantly affect the thermal characteristics of the flow process,the discriminant of the change of inclination angle is established and the corresponding conditions of use are given.Based on this discriminant,new correlations for predicting heat transfer coefficients are proposed for condensation and flow boiling processes,respectively,under variable inclination angle conditions.In the applicable range,the new correlation for prediction of heat transfer coefficient for condensation with variable inclination angle can control the prediction deviation of all data points within ±10%,and 91.55% of the data points are within ±5%;the new correlation for prediction of heat transfer coefficient for flow boiling with variable inclination angle can control the prediction deviation of all data points within ±5%.The average absolute and average relative deviations of the two new heat transfer coefficient prediction correlations do not exceed 5%.