| Among the many replacements for R22,R290,as a natural process,has obvious environmental affinity advantages compared to synthetic processes,but its flammable and explosive hazardous nature limits its scope of application significantly.R134a,despite its superior thermophysical properties,is gradually being discarded because of its high global warming potential,and it has been proposed to mix R290 with R134a to form a new hybrid process,which could be an ideal replacement for R22.As a typical representative of enhanced heat transfer pipes,micro-ribbed tubes have an extremely wide range of use in this industry and play an irreplaceable role in improving energy efficiency and reducing the size of heat exchangers.In order to explore the flow heat transfer performance of R290/R134a mixed working fluids and the flow boiling characteristics in common reinforced heat transfer pipes,this paper studies,the flow boiling characteristics of a mixed R290/R134a mass ratio of 4/6 in a micro-ribbed tube at different tube diameter levels are investigated by means of numerical simulations.Smooth tubes with diameters of 2 mm,5 mm,7 mm and 9.5 mm and micro-ribbed tubes with diameters of 7 mm and 9.5 mm were modelled.After validation of the numerical model,the flow heat transfer of the mixed mass R290/R134a（4/6）in different models was simulated separately using the mixture model.In the operating conditions range of 100 to 250 kg/（m2·s）mass flow rate,5 to 25 k W/m2 heat flow density,0 to 0.65 inlet dryness and 7.2°C saturation temperature.The flow pattern characteristics,gas phase distribution in the outlet section,gas phase volume fraction in the section and average temperature distribution of the mixed mass in the micro-ribbed tube were analyzed,and the effects of tube diameter,tube structure,heat flow density,mass flow rate and dryness on the boiling characteristics of the mixed mass R290/R134a（4/6）flow were investigated.which provided theoretical support for the system development and design of the mixed working fluid.And as a hybrid working fluid of R134a,it is intended to provide certain reference significance for the development and selection of R134a hybrid working fluid.The main findings of this paper are as follows:（1）The hybrid R290/R134a（4/6）has a superior heat transfer performance.Compared to a single process,the mixed process has a better compression ratio,lower exhaust gas temperatures and higher refrigeration efficiency;lower pressure losses further enhance heat transfer efficiency;and a larger refrigeration volume than a single process.（2）Under the same conditions,the influence of tube diameter on the change of flow pattern is more obvious,the larger the diameter of the tube,the wider the distribution of internal flow pattern,in the mass flow rate of 100 kg/（m~2·s）,heat flow density of 15 k W/m~2,inlet dryness of 0 conditions,2 mm tube diameter of the micro-fine tube flow pattern is mainly block flow and plug flow,while the conventional tube of 5 mm or more,the flow pattern distribution is more comprehensive.The flow pattern in the smooth tube is clearly stratified,and the smaller the tube diameter the more obvious the stratification,while in the micro-ribbed tube,due to the interference of the ribs,the stratification of the flow pattern has a greater interference effect,and from the exit section of the gas phase cloud diagram can be clearly seen,in the 9.5 mm micro-ribbed tube formed a secondary flow phenomenon.（3）The volume fraction of the gas phase in the cross-section of the smooth tube and the micro-ribbed tube and the average temperature of the cross-section both increase gradually in the direction of the tube length,and the tube diameter is the same as 7 mm,under the working conditions of mass flow rate of 100 kg/（m~2·s）,heat flow density of 15 k W/m~2 and inlet dryness of 0,the gas phase fraction in the micro-ribbed tube is 0.224,which is slightly higher than that of the smooth tube of 0.194,but the temperature in the smooth tube is The temperature in the smooth tube was slightly higher than that in the micro-ribbed tube,21.1°C for the smooth tube and 18.1°C for the micro-ribbed tube.（4）The influence of tube size and tube structure on the boiling heat transfer coefficient is relatively obvious,under the same working conditions,as the tube diameter decreases,the boiling heat transfer coefficient of the heat transfer tube increases significantly,the boiling heat transfer coefficient of the micro-fine tube with 2 mm diameter is the highest,and the boiling heat transfer coefficient of the smooth tube with 9.5 mm diameter is the lowest.The boiling heat transfer coefficient of the micro-ribbed tube is about 1.3 times higher than that of the smooth tube,and the boiling heat transfer coefficient of the micro-ribbed tube is increased by more than 23.7%compared with the smooth tube of the same diameter,which is a significant effect of the enhanced heat transfer of the micro-ribbed tube.（5）The boiling heat transfer coefficient increases and then decreases with increasing dryness.Among the effects of mass flow rate and heat flow density on the boiling heat transfer coefficient,the effect of heat flow density is more obvious.In the low dryness area,the increase in heat flow density leads to a significant increase in the boiling heat transfer coefficient,and when the heat flow density increases from 5 k W/m~2 to 15 k W/m~2,the boiling heat transfer coefficient of 9.5mm micro-ribbed tube increases by At the same time,the increase in heat flow density leads to an earlier critical dryness,and after the critical dryness,the boiling heat transfer coefficient decreases in a precipitous manner,and in smooth pipes,the boiling heat transfer coefficient decreases more rapidly.The effect of mass flow density on the heat transfer properties of the pipe is concentrated in the high dryness zone,and the increase in mass flow rate in the low dryness zone does not lead to an obvious increase in the heat transfer coefficient,and the mass The increase in mass flow rate from 100 kg/（m~2·s）to 250 kg/（m~2·s）increases the maximum boiling heat transfer coefficient by only 4.8%,except for the 9.5 mm smooth tube,but the critical dryness shifts back with the increase in mass flow rate,and the rate of decrease in heat transfer coefficient slows down significantly after the critical dryness.Figure 28 table 4 reference 90... |
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