Effect of mass flow rate on bubble size distribution in boiling flow in temperature-controlled annular test section

Abstract

Subcooled flow boiling experiments were performed in an annular horizontal channel with inner diameter of 12 mm and the gap of 2 mm. The annular channel is designed as a double-pipe heat exchanger, where the inner tube of the channel is heated by a water flow, providing a temperature-controlled boiling surface. The influence of mass flow rate of the working fluid R245fa on flow boiling patterns and heat transfer has been studied. Subcooled flow boiling patterns were recorded with a high-speed camera. The recorded images of the flow patterns were then further processed to determine the bubble size distributions, characterized as distributions of the void (vapour) volume given per bubble size. The observed bubble size distributions followed two distinct distribution functions: the Rayleigh distribution at larger and bimodal distribution at smaller mass flow rates. This indicates the existence of two distinct boiling flow regimes at different mass flow rates. The heat flux was estimated by local temperature measurements with thermocouples positioned along the boiling surface and in the heating water. The heat flux increased moderately with increasing mass flow rate, apparently without disruption at the observed change in the flow regime. On the contrary, the total void volume in the observed part of the test section is declining moderately with increased refrigerant mass flow rate, which, in this mode of operation of the test section, acts as a prevailing mechanism. The experimental setup, methods of experimental analysis and the results are presented and discussed.