Numerical simulation of heat transfer between forced moist air and a flat plate with water condensation

The problem of heat transfer between forced moist air and a flat plate with water condensation is studied numerically by means of Fluent. The problem is modelled as a flow of a mixture of air and water vapour over a cooled vertical surface, with water condensation on the surface. The two-phase flow is modelled with a VOF model, with the gas flow flowing within a rectangular channel under a turbulent regime and exchanging heat with a cooled vertical surface. The water film or droplets creation on the surface is modelled by user defined functions (UDF) built textit{ad hoc}. For the phase change model it is assumed that: begin{itemize} item the condensation is homogeneous (i.e., no impurities are present to form nuclei); item the droplet growth is based on average representative mean radii; item the droplet is assumed to be spherical; item the heat capacity of the fine droplet is negligible compared with the latent heat released in condensation. end{itemize} The mass generation rate is modelled with two contributions for the droplets mass increase, the nucleation (the formation of critically sized droplets) and the growth/demise of these droplets. The expansion process is very rapid. Therefore, when the state path crosses the saturated-vapor line, the process will depart from equilibrium, and the supersaturation ratio will assume values greater than one. The condensation process involves two mechanisms, the transfer of mass from the vapor to the droplets and the transfer of heat from the droplets to the vapour in the form of latent heat. The formation of a liquid-phase in the form of droplets from a supersaturated phase is modelled by assuming the absence of impurities or foreign particles. The nucleation rate is modelled with source terms, added directly into momentum, energy, and scalar equations, by means of defined functions (UDF) for all the corresponding equations.\ The numerical solution is validated by a comparison with experiments performed by Bozzoli textit{et al.} [6]. \ The experimental apparatus consists of a closed-loop wind tunnel, with a flow of air and water vapour up to the saturation point by the steam produced by a variable-power boiler. In order to regulate both the water vapour content and the temperature, the humid air stream is forced through a finned tube heat exchanger connected to a heat bath. It enters then into a settling chamber and then into a channel where the test section is arranged. On one vertical wall an aluminium plate is placed, with no geometrical discontinuities on the channel’s wall on which the aluminium plate is fitted. The test section is located downstream the channel inlet section, in order to assure that a fully developed boundary layer is achieved for the whole Reynolds number range under investigation. The temperature along three sides of the metallic fin is controlled by means of direct contact with a copper tube where a fluid coming from a circulator bath flows. The external surface of the metallic plate has been coated by a thin film of opaque paint with uniform and known emissivity. The surface temperature distribution has been acquired by means of an infrared thermographic system. When the infrared thermographic technique is adopted, like in the present investigation, some problems arise, due to the reduced detector’s sensitivity at low temperatures shown by both cooled and uncooled sensors. The estimation procedure under question, based on the solution of the inverse heat conduction problem in the wall, has been here successfully adopted to estimate the local heat transfer coefficient for a humid air turbulent flow maintained at the saturation point, by considering three different Reynolds number values, ranging from 21000 to 35000. The heat transfer coefficient in wet condition is measured by means of an estimation technique of the local heat transfer coefficient, well established in single-phase conditions ([1]-[5]) and successfully applied to the two-phase...