A model for predicting the pressure gradient along a heated channel during flow boiling.

Abstract

A model has been derived to predict the frictional pressure gradient along a heated channel. It focuses mainly on flow boiling at high-pressure conditions (10 MPa). Based on a force-momentum balance, the overall pressure gradient is divided into three components: friction, acceleration and gravity. This model analyzes the frictional pressure drop for all heat-transfer modes encountered inside a heated channel: single-phase heat transfer, subcooled boiling, saturated (bulk) boiling, forced convective evaporation and film boiling. The heating effect is introduced through a change in flow structure in two-phase flow (i.e., the phase distribution), particularly the significant differences in entrained liquid fraction between adiabatic and boiling two-phase flow (two separate correlations have been derived in this study). One of the major components in this model is the relation between shear stress and velocity gradient. Based primarily on the theory in single-phase flow, this relation is extended to two-phase flow boiling through the assumption of a homogeneous mixture having the characteristics of a single-phase flow. In the forced-convective evaporation region (mainly wispy-annular flow), a four-layer structure is introduced to analyze the liquid film and two-phase core. A number of assumptions and modifications to the expressions for single-phase flow are introduced to account for the presence of two phases. Empirical corrections, however, are used for the effects of surface roughness and viscosity difference between near-wall fluid and bulk fluid on two-phase frictional pressure drop. Experimental data for validating the present model were obtained in a vertical tubular test section, installed in a high-pressure steam-water facility at the Chalk River Laboratories of AECL Research. A strong effect of surface heating on frictional pressure drop was observed. With increasing heat flux, the frictional pressure drop decreases in single-phase flow, increases in nucleate boiling, and decreases in forced convective evaporation. The effect of heat flux in the film-boiling region (post-dryout conditions) appears to be small. However, the frictional pressure gradient is much smaller in the film-boiling region than in the pre-CHF region for the same flow conditions (i.e., pressure, mass flux and quality). For most flow conditions, a maximum in two-phase frictional pressure drop is encountered at a quality lower than the value corresponding to the occurrence of dryout. In addition to heating, other effects on frictional pressure drop (such as pressure, mass flux, etc.) have also been examined. (Abstract shortened by UMI.)