R245fa flow boiling heat transfer in a sintering and electroplating modulated tube

Abstract

• R245fa flow boiling heat transfer is investigated in a modulated tube coated by using sintering and electroplating technology. • Flow patterns and dynamic liquid height analyses are proposed to explore the heat transfer enhancement mechanisms. • The micro-nano scale coating on the tube inner wall could modulate flow pattern to enhance heat transfer. • The maximum EF and PEC can reach 2.87 and 2.79. The flow boiling performance of R245fa was investigated in a bare stainless-steel tube (BT) and a sintering and electroplating modulated stainless-steel tube (SET). The test tubes had the same inner diameters of 10.02 mm and effective heat transfer lengths of 800 mm. The mass fluxes, inlet vapor mass qualities and heat fluxes were in ranges of 192.61-705.35 kg/(m 2 ·s), 0.01–0.9 and 4.99-74.97 kW/m 2 , respectively. Three flow patterns were identified in both BT and SET. The annular flow pattern conversion line of SET was earlier than that of the BT. With the increase in vapor quality, the boiling heat transfer coefficient increased and then decreased at q≤ 45 kW/m 2 , whereas it decreased at q ≥60 kW/m 2 . For the BT, the heat transfer coefficient was weakly affected by the mass flux, but it increased with the increase in mass flux for the SET. For the BT and SET, the frictional pressure drop increased and then decreased with the increase in mean vapor quality at a certain heat flux. Owing to the strong wettability of SET, the thick liquid film at the tube bottom in stratified flow was forced to the tube top and the liquid droplets in annular flow were captured by the surface coating. Thus, flow patterns were modulated to enhance heat transfer. The maximum of the heat transfer enhancement ratio and performance evaluation parameter can reach 2.87 and 2.79, respectively.