Hydrodynamic and thermal model for gravity-driven smooth laminar film flow undergoing flash evaporation cooling: Case study and correlation development

Abstract

Hydrodynamic and thermal analyses have been carried out for gravity-driven smooth laminar film flow, undergoing flash evaporation at the free surface. A classical one-dimensional semi-analytical approach has been adopted to address a unique problem where hydrodynamic and thermal boundary layers (TBLs) approach from opposite directions and eventually intersect each other. This occurs due to the rapid evaporation cooling at the film-free surface exposed to the low-pressure ambiance, leading to the growth of a TBL from the free surface. In contrast, the hydrodynamic boundary layer (HBL) grows from the solid wall over which the film flow occurs. The intersections between the TBL and HBL edges, HBL edge and the free surface, and TBL edge and the wall, in conjunction with the attainment of a fully developed hydrodynamic condition, result in the division of the overall film domain into three distinct hydrodynamic and five distinct thermal sub-zones requiring zone-specific formulations. The model is successfully validated for hydrodynamic formulations with the existing experimental data. However, the lack of available experimental studies limits the validation of the proposed thermal model. Correlations for relevant thermal and hydrodynamic parameters, such as local Nusselt number, local free surface temperature, local bulk mean temperature, and local film thickness, are developed based on the model predictions. The proposed model and the correlations derived from its predictions are anticipated to serve as crucial benchmarks for optimizing the design of thermal management and desalination systems that are fundamentally driven by the film evaporation process.