Heat transfer coefficient calculated using a linear pressure gradient assumption and measurement for flow boiling in microchannels

To obtain local flow boiling heat transfer coefficients, some studies used a linear pressure gradient method. In this study, this method is compared with a pressure gradient measurement method. The measurement was carried out for flow boiling of de-ionized water in single horizontal microchannels. Three single microchannels with widths of 0.5 mm, 1 mm and 1.71 mm and a depth of 0.39 mm, giving hydraulic diameters of 0.438 mm, 0.561 mm and 0.635 mm, were tested. The channel length was the same, 62 mm. The nominal mass flux used was 800 kg/m2 s and heat fluxes ranging from 222 to 685 kW/m2 were applied. An inlet fluid temperature of 98 °C and an inlet pressure of 125 kPa (abs) corresponding to an inlet subcooling of 7 K, were maintained at the channel entrance. The trends of heat transfer coefficient decrease with quality at a fixed heat flux and mass flux along the channel. However, the heat transfer mechanism was dominated by nucleate boiling for the 0.561 mm and 0.635 mm channels indicated by the increased local heat transfer coefficient with heat flux, while for the 0.438 mm channel, it was dominated by convective boiling because there was no effect of heat flux on the local heat transfer coefficient. Local heat transfer coefficients determined using the two methods were found to be significantly different, especially at high heat fluxes. When the Φ is less than 9 W/m2 Pa for the 0.438 mm channel, less than 11 W/m2 Pa for the 0.561 mm channel and less than 16 W/m2 Pa for the 0.635 mm channel, the linear pressure distribution may be used with caution.