Flow Boiling Study in Mini-Channels

Mini-channel heat exchangers improve thermal performance in comparison to conventional macro-channel heat exchangers, being highly efficient, compact and requiring low fluid mass. However, classical correlations for two-phase flow in macro-channels fail in predicting the heat transfer coefficient in mini-channels. Therefore, new studies are needed in order to provide better knowledge on flow boiling phenomena in confined spaces. The proposed paper presents an experimental study on two-phase vertical flow boiling in mini-channels. The aim of this work is to determine the heat transfer coefficient and to study the pressure drop in a mini-channel heat exchanger (hydraulic diameter of 840μm) in order to obtain better understanding of the flow boiling mechanisms. A vertical upward flow test section is connected to a primary HFE-7100 circuit. A preheater imposes a given sub-cooled fluid temperature or a given two-phase vapour quality at the inlet. Downstream in the test loop, the fluid is condensed and pumped again into the test section. The pressure drop and the heat transfer coefficient in the test section have been measured for a variety of conditions. Different heat flux, inlet vapour quality and mass flow rate values have been tested. For the heat transfer coefficient, a correlating model is proposed as a function of the superficial velocity. This parameter appears to be much more appropriate than the vapour quality or the mass flow rate for dry-out occurrence prediction. A single critical velocity value has been found.