Experimental And Engineering Model Investigation Of Evaporation In Micro-fin Tubes Of Different Geometries

Heat transfer enhancement has been an important factor in obtaining energy efficiency improvements in refrigeration and air-conditioning applications. Currently, micro-fin tubes, i.e., tubes with numerous, very small integral fins on the inner surface, are routinely used because they can substantially enhance heat transfer coefficients with a small pressure drop penalty. Small diameter tubes can not only maintain a high working pressure and reduce the liquid-filled quality, but also can reduce the volume of the heat exchanger and reduce the pressure drop of the air outside the tube. But the complicated flow process and large amount of impact factors lead to difficulty to derive the analytic solution of two-phase flow. The experimental data is hard to predicted within a relatively low error band level using the existing correlations for two-phase flow. So it is hoped that a generalization of data getting from the experiments will help to optimize the existing correlations.Three parts of work are done in this paper. At first a literature review of two-phase flow in tubes was completed. Secondly, an experimental facility for in-tube evaporation was designed and built. Finally the experimental data analysis and the optimization of some correlations were carried out.An experimental investigation was performed with R22for single-phase flow and evaporation inside four micro-fin tubes with the same outer diameter5mm and helix angle18°. Data are for mass fluxes ranging from about100530kg/m2s to530kg/m2s. The nominal saturation temperature is279K, with inlet and outlet qualities of0.1and0.8, respectively. The results suggest that Tube2has the best thermal performance for evaporation when mass velocity is less than400kg/m2s, while Tube4has the best heat transfer performance at higher mass velocities before partial dry-out, with relatively low pressure drop penalties in the both tubes. The Beattie and Whalley correlation can predict the evaporation pressure drop data accurately the new asymptote model modified from the Cavallini et al.’s method can predict evaporation heat transfer coefficients, which can predict85%of the data points within a±30%error band.