Experimental study of flow boiling heat transfer on a downward-facing heated wall

An experimental study was conducted to investigate the effects of wall inclination and void fraction on the flow boiling heat transfer coefficient (HTC) from a downward-facing heated wall. Experiments with water at atmospheric condition were performed with varying void fraction in the range from 0.1 to 0.6 considering wall inclinations of 0°, 30° and 60°. Temperature distribution of the boiling surface was measured using a high-speed infrared camera and the HTC was calculated using the applied heat flux and the measured average surface temperature. As the horizontal inclination of the downward-facing wall increased, the HTC was degraded due to the poor heat transfer underneath the elongated slug bubbles, which resulted from the severe coalescence of nucleating boiling bubbles. However, the void fraction had an insignificant influence in the range of the present study. From the experimental data, a suppression factor to predict the degradation of flow boiling HTC on a downward-facing wall was proposed as a function of wall inclination angle.