Effect of Forced Convection Heat Transfer on Vapor Quality in Subcooled Flow Boiling

A new theoretical model of vapor quality in subcooled flow boiling is proposed based on energy balance and well-known heat transfer correlations. This model takes into account the enhancement of forced convection heat transfer due to the presence of vapor. It is shown that the vapor quality predicted by our model is much less than that by a previous model for low pressure. This result demonstrates that the convective heat transfer coefficient (HTC) cannot be constant, and the effect of gas phase on forced convection heat transfer cannot be neglected even for subcooled flow boiling, particularly at low pressures. However, the difference between the present and previous models decreases as the pressure increases because (i) the increase of the convective heat transfer coefficient is weakened, and (ii) boiling heat transfer becomes dominant. The difference becomes large if the mass flux is increased or the wall heat flux is decreased, owing to the difference in the form of the convective heat flux. Furthermore, the present model has the capability of locating the point at which bulk boiling commences. In general, this saturation point moves downstream as the wall heat flux and pressure increase, and upstream as the mass flux and tube diameter increase. In addition, the present model can be simplified to a one-variable model, which is a good approximation of the original one especially for low pressures and wall heat flux and high mass flux.