Experimental investigations on flow boiling heat transfer in plate heat exchanger at low mass flux condition

• Flow boiling heat transfer at the low mass flux condition was investigated for PHX.
• Heat transfer was mainly dominated by nucleate boiling heat transfer.
• Excess temperature was discussed, which relates closely to nucleate boiling regime.
• This paper essentially extends the operating conditions of PHX.

In this paper, flow boiling heat transfer in a plate heat exchanger at the low mass flux condition was investigated for low temperature lift heat pump applications. The effects of vapor quality, heat flux, evaporation pressure, and mass flux were studied. The current study shows that the influence of convective boiling heat transfer is suppressed under given test conditions, and the effect of boiling heat transfer was dominant. This is evident from the insignificant effect of vapor quality on flow boiling heat transfer coefficient. It showed a dominant effect of nucleate boiling heat transfer. When heat flux was increased, the flow boiling heat transfer coefficient increased which is an indication of nucleate boiling heat transfer. Excess temperature was also studied, which closely relates to nucleate boiling regime. As evaporation pressure was decreased, it became easier for vapor bubbles to generate, which can be explained with a lower excess temperature. Furthermore, a more rigorous bubble movement at a low excess temperature range involved in this study enhanced heat transfer across the surface, so that flow boiling heat transfer coefficient increased. For the effect of mass flux, the flow boiling heat transfer coefficient increased slightly as the mass flux was increased. This means that convective boiling heat transfer is present, but it just played a minor role in the evaporation heat transfer because of a low fluid mass flux.