condensation, condensing superheated region, flow regime, void fraction, film thickness
Flow characterization of R134a condensing in a horizontal smooth round tube with 6.1 mm inner diameter is invested in this study. The paper presents flow visualization and liquid film thickness measurement with mass flux from 50 to 200 kg m-2 s-1 and heat flux from 5 to 15 kW m-2, showing the effect of mass flux and heat flux on the onset of condensation, flow regime, film distribution and void fraction. The result of flow visualization reveals that the condensation always starts in bulk superheated (SH) region, as annular flow and the flow regime is strongly affected by mass flow, not heat flux. Based on the flow visualization in condensing superheated (CSH) region, a new diabetic flow map is proposed to better represent the physics and predict flow regimes in CSH region. A film thickness measurement technique for round tube refined from critical angle method is described and calibrated. The calibration and comparison between film thickness measurement result and flow visualization shows that the refined critical angle method is able to catch the film distribution inside the tube. The film thickness measurement demonstrates that void fraction drops below one in SH region, which is not taken into account for most conventional void fraction models. Besides, void fraction measurement illustrates that mass flux has greater effect on slip ratio than heat flux. Besides, having film distribution information provides an opportunity to more objectively determine the flow regime and develop a more mechanistic model by utilizing the film geometry inside of the tube.