Boiling heat transfer in small rectangular channels at low Reynolds number and mass flux

Abstract

Boiling heat transfer experiments were performed in a small rectangular channel (hydraulic diameter, De=3.0mm) with water and methanol. The channel size represents the tube configurations in plate-fin heat exchangers, which are commonly used for generating hydrogen from hydrogen-rich liquid fuels. The tested mass flux ranged from 6.59 to 21.97kgm−2s, heat flux from 13.3 to 30.1kWm−2 and inlet temperatures from ambient to 80°C. Tests were carried out at atmospheric pressure. Local heat transfer coefficients were determined experimentally as a function of vapor quantity along the length of the test section. Insights into heat transfer mechanisms in small channels are presented over a range of Reynolds number from 38 to 263. Experimental results and comparisons with state-of-the-art predictive correlations are presented. The Chen correlation was found to be most suitable for predictions of water boiling heat transfer under the recommended application range for minichannels. For methanol, the Shah correlation was more sound over the range with Re larger than 50, while the Gungor–Winterton equation was more accurate when Re was smaller than 50. In addition, flow boiling instability in minichannels was detected when the flow was dominated by subcooled boiling. However, the periodic oscillations of temperature and pressure disappeared when the heat transfer became dominated by saturated boiling.