An experimental investigation of subcooled pool boiling on downward-facing surfaces with microchannels

Abstract

Upward-facing pooling boiling enhancement employing micro-structure has been extensively studied, yet it is rarely applied in the cooling system in microgravity or In-Vessel Retention (IVR) in nuclear industry. The main challenge lies in effective removal of the vapor columns and replenishment of the fresh liquid without the aid of the buoyancy force. To solve the issue, additional pressure induced by microchannel is introduced to yield spontaneous bubble removal and liquid replenishment in the present work. Downward-facing pool boiling of subcooled FC-72 are conducted on Plain, Straight- and Expanding-Channeled Surfaces (PS, SCS and ECS) with visualization experiments. The superheat at the onset of nucleate boiling (ONB) on channeled surfaces was decreased about 3 K compared to that of the PS, and the maximum heat transfer coefficient (HTC) of the SCS and ECS were improved by 149 % and 217 %, respectively. Compared to PS, the channeled surfaces are more independent from saturated pressure, displaying high and relatively consistent heat transfer coefficients under various pressure conditions. Additionally, with the increasing subcooling degree, channeled surfaces showed greater enhancement in heat transfer performance. Proved by the bubble dynamics, the ECS enabled a directional movement and effective removal of vapor bubbles even though the buoyancy force cannot be utilized under downward-facing condition. The proposed enhancement mechanism attributed to separated vapor–liquid flow paths is independent on gravity and achieves a satisfied pool boiling performance, thus is expected to provide a promising solution for thermal management systems in aerospace and nuclear industry.