Flow patterns visualization during condensation of R134a inside horizontal microfin and smooth tubes

During a condensation process, microfin tubes exhibit a heat transfer enhancement, as compared to smooth tubes under equivalent geometrical and operating conditions: this enhancement depends on mass velocities and on two phase flow patterns. The aim of this work is to link the heat transfer enhancement for the microfin tube with the flow pattern during the process. Experimental observations of displayed flow patterns, carried out for a horizontal smooth and a microfin tube in a transparent test section with refrigerant 134a, are presented. Experimental results are plotted on a flow pattern map and compared with previous heat transfer data in order to link the transition between different flow regimes to a unique parameter. In particular, transition from stratified to annular flow regime is analysed. Relevance about an optimal value of the specific mass flux that maximises the heat transfer enhancement is given by the present preliminary results. Further investigation with different fluids and different geometries will certainly provide a deeper knowledge about this phenomenon.