Effect of gravity vector on flow boiling heat transfer, flow pattern map, and pressure drop of R245fa refrigerant in mini tubes

Abstract

Organic Rankine cycles have been proposed to address the on-going issue of global warming by recovering waste heat. R245fa refrigerant has high performance as a working fluid in such cycles. This study looks at the flow boiling and heat transfer mechanism of this refrigerant in a 3-mm stainless-steel tube. Horizontal flow and vertical flow are both considered. The flow pattern in mini tubes is visualized, and flow pattern maps are presented. Annular flow, slug flow, and bubbly flow are present in the horizontal flow. The vertical flow shows annular flow, churn flow, slug flow, and bubbly flow. Heat transfer coefficient data are also given for a mass flux range of 200–700kg/m2s, heat flux of 10–40kW/m2, and inlet quality of 0.1–0.7. The pressure drop and heat transfer mechanism are discussed. The horizontal flow data were compared with the vertical upward flow data. The experimental data for heat transfer coefficient and pressure drop was compared to the well-known correlations from the literature. The correlation by Liu & Winterton for heat transfer coefficient and the homogenous equation of Cicchitti et al. for pressure drop was found to wok the best for this range of experimental conditions. Also the flow patterns were well-predicted by using Ong and Thome correlation. The flow pattern base model of Cioncolini and Thome was able to capture most of the heat transfer coefficient and pressure drop data in annular flow. The results can be used for designing evaporators to consider the differences between vertical and horizontal flow regarding phase-change characteristics in heat exchangers orientation.