4.1. Experimental results

The heat flux distribution for experimental runs corresponding to the nominal system pressure, Pn, of 2, 3, and 4 bars, and including pure vapor and different mixture of air and vapor, are presented in Fig. 2, 3, and 4 respectively [4,6].

An increase in system pressure increases local heat flux and this can be attributed to the increase in wall subcooling degree that enhances the thermal driving force for heat transfer. Moreover, higher system pressure associated with the higher inlet temperature leads to a greater number of molecular collisions helping in the diffusive transport of energy. However, in our experimental investigation, the dependency of the wall subcooling degree, either measured (Tc-Tw) or predicted from Gibbs-Dalton Law (Ts-Tw), on system pressure is such that the wall subcooling degree remains nearly the same for the same inlet air mass fraction and for the different system pressure. This implies that the vapor mass flow rate may dominate over system pressure, concerning the effect on local heat flux, for cases with air/vapor mixture (Fig. 5). The situation is rather different in pure vapor runs, that is increase in system pressure has a strong effect on enhancement of predicted, and even measured, wall subcooling degree and hence on increase of local heat flux (Fig. 6).