Heat Transfer And Pressure Drop Of Refrigerant-oil Mixture Flow Boiling Inside Tubes Filled With Metal Foam

Metal foam is a kind of porous material with high porosity and largespecific surface area. It is expected to improve the efficiency of evaporatorwhen filled in heat exchange tubes. In the evaporator of practicalrefrigeration system, refrigerant is always mixed with lubricating oil. Theoil would have an effect on the heat transfer and pressure dropcharacteristics of refrigerant flow boiling inside circular tubes due to thegreat difference of property. So it`s necessary to investigate the heattransfer and pressure drop characteristics of refrigerant-oil flow boilinginside circular tubes filled with metal foam to make clear the applicabilityof metal foam in evaporator.In this paper, the heat transfer coefficient and pressure drop gradientof refrigerant-oil mixture flow boiling in metal foam filled tubes weremeasured. And the influence of oil on the heat transfer and pressure dropcharacteristics of refrigerant flow boiling inside metal foam filled tubeswas analyzed and summarized. Then correlations to predict the heattransfer coefficient and the pressure drop were developed based on theexperiment. The main results are summarized as following:1) Experimental study for the heat transfer characteristics of R410A flowboiling inside metal foam filled tubes was performed. The test resultsshow that the presence of metal foam enhanced the heat transfercoefficient. The enhancement was150%in13.8mm,10PPI metalfoam filled tube, and185%in23.4mm,10PPI filled tube. For13.8mm tubes, heat transfer coefficient decreases in low mass flux conditionswhile increases in high mass flux conditions with the increase ofquality. For23.4mm tubes filled with metal foam, heat transfercoefficient always decreases with the increase of quality.2) Experimental study for the pressure drop characteristics of R410Aflow boiling inside metal foam filled tubes was performed. The testresults show that presence of metal foam increase the pressure dropgradient. The pressure drop gradient can be56kPa/m when the massflux is90kg m^-2·s and the quality is0.8in tube filled with10PPImetal foam. The pressure drop in10PPI metal foam filled tube isabout1.2times of the one in5PPI metal foam filled tube in the sameworking conditions.3) Experimental study for the heat transfer characteristics of R410A-oilmixture flow boiling inside metal foam filled tubes was performed.The test results show that for13.8mm tubes filled with metal foam oilenhance the heat transfer performance in low quality conditions whiledeteriorate it in high quality conditions. For23.4mm tubes filled withmetal foam, oil always deteriorate the heat transfer performance，andthe deterioration can be as much as30%when oil concentration is5%.A new correlation to predict the heat transfer coefficient of R410A-oilmixture flow boiling inside metal foam filled tubes was developedbased on the experimental data, and it agrees with90%of theexperimental data within the deviation of±30%.4) Experimental study for the pressure drop characteristics of R410A-oilmixture flow boiling inside metal foam filled tubes was performed.The test results show that oil increase the pressure drop and the higheroil concentration the greater increase. The presence of oil can increasethe pressure drop by23%and50%respectively in5PPI and10PPImetal foam filled tubes when oil concentration is5%. A newcorrelation to predict the frictional pressure drop of R410A-oil mixtureflow boiling inside metal foam filled tubes was developed based on the experimental data, and it agrees with90%of the experimental datawithin the deviation of±15%.