Direct-contact condensation heat transfer with noncondensable gases and interfacial shear for co-current stratified wavy flow in nearly-horizontal channels

Abstract

The calibrating method for an electrochemical probe, neglecting the effect of the normal velocity on the mass transport, can cause large errors when applied to the measurement of wall shear rates in thin wavy flow with large amplitude waves. An extended calibrating method is developed to consider the contributions of the normal velocity. The turbulence-induced normal velocity component is included into the 2-D mass transport equation by means of its root mean square value multiplied by a random function. The wave-induced normal velocity component is postulated to be proportional to the normal interfacial velocity in thin wavy flow. The inclusion of the turbulence-induced normal velocity term is found to have a negligible effect on the mass transfer coefficient. The contribution of the wave-induced normal velocity can be classified on the dimensionless parameter, V. If V is above a critical value of V, $V_{crit}$, the effects of the wave-induced normal velocity become larger with an increase in V. While its effects negligible for $V<V_{crit}$. The unknown shear rate is numerically determined by solving the 2-D mass transport equation inversely. The present inverse method can predict the unknown shear rate more accurately in thin wavy flow with large amplitude waves than the previous method. If the normal velocity is neglected, the error that the predicted shear rate is lower than the real value is obtained. The interfacial shear stress is experimentally investigated for co-current air-water stratified flow in inclined rectangular channels having a length of 1854mm, width of 120mm and height of 40mm at almost atmospheric pressure. Experiments are carried out in several inclinations from 0˚ up to 10˚. The local film thickness and the wave height are measured at three locations, i.e., L/H=8, 23, and 40. According to the inclination angle, the experimental data are categorized into two groups; nearly horizontal data group (0˚≤θ≤0.7˚), and inclined channel data gro...