Condensation And Boiling Heat Transfer Investigation Of Alternative Refrigerant Outside The Enhanced Tubes

According to the Montreal protocol, the traditional refrigerant R22will be definitelyprohibited due to the harm to the Ozone layer. At present, R134a, R410a and R407c arewidely used as substitute refrigerants in the refrigeration and air-conditioning industry.Besides environmental protection problem, energy saving is an important issue that must besolved, too. Condenser and evaporator are the crucial heat exchanger equipments in therefrigeration unit, their heat transfer performances will directly affect the refrigeration unitefficiency and energy consumption. The significant demand for energy and material savingwill continue to promote the development of heat transfer enhancement technique and theapplication of high efficient condenser and evaporator.Past several decades, many enhanced heat transfer tubes were developed and used in therefrigeration and air-conditioning industry by some companies in Germany, American andJapan. Turbo C and B tubes were representative condensation and boiling enhanced tubes forheat transfer outside tubes, respectively. Novel condensation heat transfer enhancement tubessuch as petal-shaped finned tube (PF tube), helical groove petal-shaped finned Tube (HGPFtube) and stainless steel micro-fin tube (MF tube) and mechanically fabricated porous surfacetube (MFPS tube) for boiling heat transfer enhancement were autonomously developed by ourgroup. In present work, the experimental and theoretical researches on condensation andboiling heat transfer for refrigerant R22and its substitute refrigerants R134a, R410a andR407c outside novel enhanced tubes were conducted. The main works are summarized asfollowing:（1）At saturation temperature of39°C, the condensation heat transfer performance forsingle component refrigerants R22and R134a and azeotropic refrigerant mixture R410a wereconducted outside single smooth, PF and HGPF tubes. The condensation heat transfercoefficients（HTCs） decrease with increasing wall subcooling. HGPF tube is an inside andoutside enhanced tube, it can enhance both condensation heat transfer outside tube andconvective heat transfer for water flowing inside tube，and the condensation heat transferperformance is superior to Turbo C tube reported in the literature. The overall HTCs of R22,R134a and R410a are5.52~6.04,5.25~6.21and5.15~5.89times HGPF tube outside as largeas those of smooth tube, respectively. By inputting the surface tension coefficient, thecorrelation of condensation HTC HGPF tube was built, it could calculate condensation HTC well.（2）At saturation temperature of39°C, the condensation heat transfer performance forsingle component refrigerants R22and R134a and azeotropic refrigerant mixture R410a wereconducted outside smooth, PF and HGPF tubes. Both condensation HTC on each row andaverage tube bundle decreased with increasing wall subcooling. At the same wall subcooling,the average condensation HTC was the biggest for R410a, and R134a was the smallest. Thetube row effect was obvious for HGPF tube. Based on Nusselt model, the correlation of tuberow effect was produced. At the same wall subcooling, the average condensation HTC atsaturation temperature of35°C was higher than that of39°C.（3）At saturation temperature of39°C，the condensation heat transfer performances onnonazeotropic refrigerant mixture R407c were studied outside single PF tube, three row PFtubes and helical baffle condenser. With increasing the wall subcoolinbg, the condensationHTC increased, this phenomenon was adverse with R134a and R410a, the reason is that themass transfer in vapour phase affected the condensation heat transfer process. The resultshowed that the enhancement factor for single PF tube is4.6~5.35. The tube row effect wasobvious for the PF tube, too. At the same flux, the condensation HTCs of helically baffledcondenser with PF tubes were about1.56times as large as that of helically baffled condenserwith low finned tubes (LF tubes) at the same heat flux. Correlations have been suggested forboth the shell-side condensation HTCs for the two condensers with different tube types andgive very good agreement with experimental results.（4）Aimed at the application of sea water as the coolant in refrigeration unit, the novelstainless steel MF tube was developed. At saturation temperature of39°C，the condensationheat transfer performance of R22、R134a and R410a were researched. The condensation HTCdecreased with increasing wall subcooling. The enhancement factors of R134a、R22andR410a were2.23~2.81、2.4~3.55and2.65~3.55, respectively. They were lower than those PFtube.（5）At saturation or vaour temperature of39°C，the pool boiling HTCs of R22, R410aand R407c were studied on MFPS tube bundle. It is found that the pool boiling HTC increaseswith increasing the heat flux. In three tube rows, the boiling HTC on the second row was thehighest for R22、R134a and R410a, and it was the lowest for the third row. But for R407c, theboiling HTC on the first row was the lowest. For the evaporator with the MFPS tube bundle,it was found that boiling HTCs for R410a are1.25–1.81times and6.33–7.02times higherthan that for R22and R407c, respectively. The experimental correlations for the pool boiling HTCs of R22, R407c, and R410a on the present enhanced tubes bundle were developed. Thethermal resistance analysis reveals that the thermal resistance of the water side was acontrolling factor for the evaporator for R22and R410a. However, for R407c, the thermalresistance of the refrigerant side was slightly higher than that of the water side. The heattransfer performance of the flooded evaporator with helical groove MFPS tubes was tested.