A hybrid approach portraying the dynamics of free convection condensation around a circular cylinder

Abstract

We introduce a hybrid framework to model the fluid dynamics of free convection condensation of saturated steam outside a circular cylinder. The liquid film is modeled analytically, and the adjacent vapor flow field is resolved numerically. The model incorporates temperature-dependent thermophysical properties of the condensate. We explore the subtle role of Jakob number (Ja) and Weber number (We) in the two-phase flow and establish the criteria for dynamic similarity of the flow field with two new dimensionless numbers, viz., free-fall Reynolds number and similarity number. For fixed base Prandtl number of the condensate, the increases in the subcooling rate with Ja and the cylinder's surface area to volume ratio with We cause film thickening. We develop a theoretical correlation to predict the average film thickness (δm). We identify entrainment and bypass zones in the vapor flow field demarcated by a separating streamline. The entrainment zone's streamlines converge to the interface, yielding a net condensate drainage. The bypass streamlines never reach the interface and reduce the condensation efficiency. The results show that the tangential velocity is dominant within the liquid film, the radial velocity is dominant within the vapor flow field, and they are of the same order at the interface. We locate the point of flow separation influenced by a surface tension-induced adverse pressure gradient. The location of the flow separation point shifts upstream, and the separating streamlines become steeper with the increase in We. Our investigation reveals the zone of tangential flow reversal near the interface, promoted with increasing We.