Visualisation of the heat transfer enhancement during condensation in a microfin tube.

Microfins tubes are largely used in refrigeration industry for in-tube refrigerant condensation, because of the heat transfer enhancement when compared to equivalent smooth tubes under the same operating conditions. But not much evidence about the effect of microfins on the condensation flow patterns is available in the open literature. There is agreement in the open literature that the mechanisms of heat transfer are intimately linked with the prevailing two-phase flow regime. The present authors have recently measured the heat transfer coefficient during condensation of R410A in a microfin tube. The heat transfer enhancement in this tube can be experimentally evaluated by comparing those coefficients to the ones measured by Cavallini et al. (2001) in a plain tube, at the same operating conditions. The same operative conditions (saturation temperature, vapor quality and mass flux), occurring during the heat transfer measurements, were reproduced in a different section for visualization of flow patterns during condensation of R410A. The flow visualization has been carried out both in the plain tube and in the microfin tube. The objective of the present paper is to present the heat transfer enhancement during condensation of R410A and to show the flow visualized at the same operating condition for both the smooth and the microfin tube, aiming to link the heat transfer enhancement to the flow pattern variation.