Heat transfer characteristic of R-600a during flow boiling inside horizontal plain tube

In this paper, heat transfer characteristics and flow visualization of the hydrocarbon refrigerant R-600a during flow boiling inside horizontal smooth tube are investigated. For this purpose the well instrumented experimental apparatus including pump, flow meter, pre-heaters, test evaporator, by-pass and condenser was designed, fabricated and installed in order to measure the experimental data on flow boiling heat transfer and flow pattern of hydrocarbon refrigerant. The test section was a plain copper tube with an inside diameter of 8.7 mm and a wall thickness of 0.41 mm. The experimental tests were carried out varying: (i) the refrigerant mass fluxes within the range of 130-380 kg/m2 s; (ii) the vapor qualities up to 0.70; and (iii) the heat fluxes from 10 to 27 kW/m2. The results indicate that, intermittent and annular flow patterns are the dominant flow patterns in the ranges of operating parameters of the present study. Also, the flow boiling heat transfer coefficient increased as the mass velocity increased in the fixed vapor quality. Moreover, heat transfer coefficient increased by the increase of vapor quality as the mass velocity was fixed. Most of the flow boiling heat transfer and flow pattern prediction methods have been developed for HFC and HCFC and CFC refrigerants and there are no general methods which can be applicable for all fluids. Therefore the present study experimental data for flow boiling heat transfer coefficient and flow pattern were compared with some existing models in order to find out which method could predict the convective boiling heat transfer and flow pattern characteristics of R-600a as a hydrocarbon refrigerant more accurately. The results indicate that Kattan-Thome-Favrat [23] flow map was closer to the flow patterns observed in this study. In addition, Gungor-Winterton [8] correlation had the best consistency with the present study experimental heat transfer data.