condensation, condensing superheated region, heat transfer
Heat transfer of R134a condensing in a horizontal smooth round tube with 6.1 mm inner diameter is invested in this study. Experimental results on the heat transfer coefficient (HTC) with mass flux from 50 to 200 kg m-2 s-1 and heat flux from 5 to 15 kW m-2 are provided. Heat transfer behavior is compared between condensing superheated (CSH) region and two-phase (TP) region. Experimental result shows that in TP region, higher mass flux results in higher HTC while heat flux does not affect HTC. In CSH region, however, mass flux does not affect HTC while HTC increases when heat flux increases. Based on the flow characterization, the heat transfer behavior in CSH region is contrary to that suggested by liquid film geometry, where higher mass flux means thinner film whose HTC is higher and heat flux does not alter HTC a lot. In addition, the consistent peak of HTC at quality one cannot be physically explained while the reason for the counterintuitive heat transfer behavior actually lies in the conventional definition of HTC. Therefore, a new definition of heat transfer coefficient, described as â€œfilm heat transfer coefficientâ€ is proposed to better represent the physics and serve as a preparation for the new heat transfer model.