Study On Boiling Characteristics And Heat Transfer Mechanisms Of R134a/R245fa Zeotropic Mixtures

The heat dissipation problem encountered in the process of the development of new electronic equipments and mechanical components towards miniaturization has become the bottleneck of continuous progress in the field of mechanical and electronic control engineering,and the flow boiling heat transfer technology in microchannels may provide the effective cooling guarantee for this purpose.Given the reasonable operating temperature ranges of devices and the diversity of coolant evaporation temperature and pressure choices,refrigerants have become an important option,especially for zeotropic mixtures,which can not only modulate the properties of pure components,but also present the unique phase change heat transfer characteristics with great application prospects.Based on this background,the present paper systematically carried out the research on boiling characteristics and heat transfer mechanisms for zeotropic mixtures as well as their pure components,mainly including three aspects:nucleate pool boiling heat transfer,flow boiling heat transfer in a single microchannel and enhancement of flow boiling heat transfer in multi-microchannels by channel configuration.First of all,in order to understand the basic characteristics of boiling heat transfer process of zeotropic mixtures,the visualization experimental system of nucleate pool boiling was set up.R134a/R245fa zeotropic mixtures and their pure components were used as the test refrigerants,and the experimental study on nucleate pool boiling heat transfer characteristics were carried out followed by the theoretical analysis and exploration of heat transfer mechanisms.According to the visualization results for each test refrigerant,the corresponding nucleate pool boiling curves were subdivided into up to four zones,i.e.,natural convection region,isolated bubble region,bubble coalescence region and vapor column region.The slope of the boiling curves in bubble coalescence region was greater than that in isolated bubble region.The onset of nucleate boiling point(ONB point)of zeotropic mixtures obviously lagged behind that of pure components,namely,the higher wall superheat was required for the nucleation and boiling process.The variation trend of wall superheat at ONB point with the blending ratio was basically consistent with the temperature slip characteristic curve.The nucleate pool boiling heat transfer coefficient of zeotropic mixtures,although increasing with the increase of heat flux,was degraded compared to the corresponding ideal heat transfer coefficient,except for some specific cases with high heat flux.Based on the relationship between the actual heat transfer coefficient and the ideal heat transfer coefficient,the nucleate pool boiling heat transfer degradation factor of zeotropic mixtures was defined.Through in-depth analysis of the variation law of heat transfer degradation factors and the mechanism of additional mass transfer resistance,the three-stage model for nucleate pool boiling process of zeotropic mixtures was proposed,including pseudo fully developed nucleate boiling(P-FDNB)stage,pseudo double boiling(P-DB)stage and lagging boiling stage.Two inflection points of heat transfer degradation factors(P-FDNB point and P-DB point)were then determined.Different from the ONB point and the P-FDNB point,the wall superheat of zeotropic mixtures at P-DB point was stable near a certain value and did not vary with the blending ratio.In order to quantify and characterize the effect of mass transfer resistance for zeotropic mixtures at P-DB point,the mathematical analysis model of heat transfer degradation factor at P-DB point was established based on the introduced thermodynamic fluctuation theory.The calculation results showed good agreement with the experimental data.Then,based on the above research,in order to understand the basic phenomena and rules of the flow boiling process in a single microchannel using zeotropic mixtures as the coolants,the open circuit flow boiling visualization experimental system of single microchannel was set up.R134a/R245fa zeotropic mixtures and their pure components were also used as the test refrigerants,and the mechanism exploration of flow pattern and heat transfer characteristics of flow boiling in a single rectangular microchannel were experimentally investigated as well as the theoretical research of corresponding heat transfer prediction method.According to the visual observations for each test refrigerant,five typical flow patterns during the flow boiling process in the microchannel were identified,including bubbly flow,confined bubble flow,slug flow,churn-annular flow and annular flow.Different bubble coexistence phenomena(Type and Type ?)were observed for pure components and zeotropic mixtures,respectively.Especially,the nodes where the coexisted bubbles(Type ?)occurred coincided with the inflection points of heat transfer coefficient for zeotropic mixtures.The hysteresis of flow pattern transition for zeotropic mixtures was strongly affected by both of the temperature glide and blending ratio.Besides,through the comparison with pure components,the flow boiling heat transfer characteristics of zeotropic mixtures were analyzed and discussed in detail with the consideration of mixture effects as well as flow pattern features.Taking account of capillary effect and Marangoni effect in confined space as well as other impact factors,the newly proposed prediction method exhibited satisfactory accuracy in predicting the microchannel flow boiling heat transfer coefficient for zeotropic mixtures.Finally,combined with the above two aspects of the research content,the closed loop flow boiling experimental system for multi-microchannels was set up considering the practical application of microchannel flow boiling heat transfer technology.The flow boiling heat transfer research of R134a/R245fa zeotropic mixtures and their pure compartments in parallel multi-microchannels with different channel configurations were experimentally studied.The concept of segmented microchannels with interconnected areas was proposed and the impact of channel configuration on both of flow boiling heat transfer and pressure drop characteristics were emphatically discussed.According to the results of comparative study,the test refrigerant of R134a presented higher heat transfer coefficient than R245fa(contrary to the nucleate pool boiling heat transfer characteristics),but it was more prone to local dry out spots in advance.The small addition of volatile component of R 134a was found to be beneficial for improving the heat transfer performance of R245fa at higher heat flux as well as the flow boiling CHF.Compared with continuous microchannels,the heat transfer performance was enhanced a little for pure components but slightly suppressed for zeotropic mixtures in segmented microchannels,however,in which the interconnected area was helpful to delay the occurrence of the transfer deterioration and improve the flow boiling CHF regardless of pure or zeotropic refrigerants.Besides,the flow boiling pressure drop in segmented microchannels was obviously reduced for zeotropic mixtures and their pure components.The smaller the liquid-vapor density ratio of test refrigerants,the higher the reduction of pressure drop with the help of interconnected area.