Experimental Study Of The Heattransfer And Fluid Flowcharacteristics Of R290,R600a And Their Mixtures During Inboiling In Microchannels

Hydrocarbon refrigerants(HCs)such as R290 and R600 a have zero ozone depletion potential(ODP),low greenhouse effect efficiency(GWP),good thermodynamic properties,and a wide range of sources,and are the primary choice of alternative refrigerants.The hybrid refrigerant R290/R600 a,also has the advantages of low GWP,good thermodynamic properties,and high refrigeration capacity per unit volume,etc.As a non-azeotropic mixture,its temperature slip characteristics can reduce irreversible losses in the heat transfer process,and it has a good prospect of applying the hybrid refrigerant to refrigeration equipment.Studying microchannels,with the reduction of channel scale,on the one hand,can strengthen the heat exchange in the tube and make the heat exchange equipment compact;on the other hand,it is beneficial to reduce the tank charge of HCs flammable and explosive refrigerants.The study of the flow boiling heat exchange and pressure drop characteristics of R290,R600 a,and their mixtures in microchannels is important for the replacement of refrigerants and the development of compact and efficient heat exchangers.In this paper,we designed an experimental system for flow boiling heat transfer in horizontal microchannels,under the experimental conditions of heat flow density of20～30 k W/m2,a mass flow rate of 200～400 kg/(m2·s),saturation temperature of22 ℃,24 ℃,26 ℃,and dryness of 0～1,and conducted experimental studies on pure R290,R600 a,mixed R290/R600a(mass ratio of 0.7/0.3,0.5/0.5,and 0.3/0.7)for flow boiling heat transfer and adiabatic two-phase frictional pressure drop experiments.The central studies include:1.The results of flow boiling experiments of pure refrigerant R290,R600 a,and mixed refrigerant R290/R600 a were obtained.The experimental results show that:with the increasing dryness,the mass flow rate promotes the boiling heat transfer coefficient more obviously,and the heat transfer coefficient increases first and then decreases;The effect of saturation temperature on pure and mixed working fluids is not significant at low heat flux,and the heat transfer coefficient of pure and mixed working fluids decreases with the increase of heat flux,the change of heat flux has little effect on the heat transfer coefficient of pure working medium R600 a.The heat transfer coefficient of mixed refrigerant R290/R600 a is found to be between that of pure refrigerant R290 and R600 a when the dryness is less than 0.1,and the heat transfer coefficient of mixed refrigerant R290/R600 a is found to be between that of pure refrigerant R290 and R600 a when the dryness is less than 0.1,the heat transfer coefficient of the mixed refrigerant increases slowly with the increase of dryness.2.The experimental results of pure working substances R290 and R600 a were compared with the empirical correlations.The results showed that the Bertsch correlations were more accurate in predicting R290 and R600 a,with average absolute relative deviations of 14.22% and 11.02%,respectively.The experimental data of mixed refrigerant R290/R600 a were compared with the Jung correlation,Zou correlation,and Choi correlation.It was found that the Zou correlation could predict the heat transfer coefficient of mixed refrigerant most accurately,the mean absolute relative deviation was 14.12%,the mean relative deviation was-7.67% and 90.3% of the experimental results were within ± 30%.3.The flow boiling pressure drop data of pure refrigerant R290,R600 a,and mixed refrigerant R290/R600 a were obtained.The results show that the adiabatic twophase friction pressure drop of pure refrigerant R290,R600 A,and mixed refrigerant R290/R600 a increases with mass flow rate.The effect of mass flow rate on friction pressure drop increases with dryness.The increase in dryness decreases the friction pressure drop of pure refrigerant R290,R600 a,and mixed refrigerant R290/R600 a from large to small.The effect of saturation temperature is that the friction pressure drop decreases with the increase of saturation temperature.The friction pressure drop of the mixture will increase with the increase of the mass ratio of R600 a,and the increase of the friction pressure drop will also increase with the increase of the dryness.4.The experimental data of friction pressure drop of pure R290,R600 A,and mixed R290/R600 A were compared with three empirical correlations,among which the müller-steinhagen correlation had the best prediction accuracy for pure R290 and R600A;the average absolute relative deviation was 38.56% and 34.05% respectively.The accuracy of the Müller-steinhagen correlation and Kim correlation for predicting the friction pressure drop of R290/R600 A is not high,with 52.1% and 58.3% of the experimental results within ± 30%,respectively.In summary,the experimental study on the flow boiling heat transfer and adiabatic two-phase friction pressure drop of pure R290,R600 a,and their mixtures in microchannels was conducted to analyze the influence of each influencing factor on the boiling heat transfer coefficient and two-phase friction pressure drop.The experimental data of flow boiling heat transfer and two-phase friction pressure drop are compared with the prediction results of the empirical correlation formula to investigate the accuracy of the correlation formula in predicting the experimental results in this paper,which is essential for subsequent research.