Turbulent free moist convection over horizontal surfaces

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MTech (Res); 2022; Mechanical engineering

The heat and moisture loss from water surfaces are important in many natural and industrial systems. For example, the natural water cycle where water evaporates from the lakes, rivers, and seas, rises in the atmosphere, forms a cloud, and returns to the earth's surface in the form of rain. In addition, evaporation is a significant source of atmospheric water. In the above cases, heat and mass transport are coupled with each other, making the problem more complex. Even though few studies have been conducted to understand the flow dynamics and scalar transport near the surface, our understanding of this problem is still insufficient. The current thesis deals with turbulent free moist convection over the horizontal surface. The study is carried out in two separate parts. The first part proposes a model to describe the near-wall dynamics in turbulent moist convection over the horizontal surface. In the second part, experiments are conducted to verify the model and study heat transfer and evaporation rates. The experimental set-up consists of an open tank of heated water kept in still ambient air. Diagnostics include temperature and humidity measurements near the water surface and visualization of the flow field, which typically consists of small water droplets from condensed vapour. Our experiments of turbulent moist convection over the heated water surface indicate that the mechanics of heat and mass transfer near the surface is line plumes. Based on this evidence, our model consists of a 2-D periodic array of line plumes and associated boundary layers. The model provides the value for the average plume spacing. The predicted value of the plume spacing is in reasonable agreement with the experiments. Moreover, it predicts the distribution of mean temperature, vapour density and supersaturation, and rms of fluctuation of the above quantities. To understand the effects of the condensation, we also solve the model using a numerical simulation where we include the condensation in the plume and the boundary layer. The distribution of various quantities mentioned above changed slightly, while the critical plume spacing remained constant. The simulation gives the liquid water concentration field and its mean and rms fluctuation. The experiments are carried out over the heated water surface to understand the planform structure and the heat & mass transfer for different input conditions. Visualization done using a laser sheet oriented vertically or horizontally just above the water surface confirms the existence of the line plumes close to the surface. The plumes are oriented randomly and move in the lateral direction and merge due to an entrainment field created by the upward moving plume and an incoming flow in the downward direction. At a higher Grashof number (surface temperature), we observed the mushroom-type structures in the side-view and spiral structures in the top-view. The origin and elongation of the plumes are similar to that observed in single component natural convection. Plume length and plume spacing are found manually using software from the top view images of the flow field. A continuous laser sheet and High-speed camera are used to visualize the particle motion, and Particle Image Velocimetry (PIV) is used to get the rms of horizontal and vertical velocity fluctuation. Here, the particles are the droplets formed due to condensation in the plume and the boundary layer. Temperature, vapour density, and relative humidity profiles are obtained with the help of thermocouples and humidity sensors. The distribution of mean temperature, vapour density, supersaturation, rms of temperature, and the plume spacing are in good agreement with the results from the model. A siphon system is used to get the evaporation rate. A power-law correlation is developed between the Sherwood number and Rayleigh number (Sh = 0.373 Sc1/3 Raea0.291). It is difficult to get the exact values of sensible heat flux directly from the experiments, so we used the model that relates the sensible heat Nusselt number, NuT and Raea. The values obtained from these equations are in good agreement with the experiments in the literature. The heat and mass transfer are also represented in non-dimensional form (GrδT−1/3and GrδM−1/3), where Gr is Grashof number that includes density contributions due to temperature and water vapour content, and δT and δM are conduction layer thickness based on temperature and vapour density. The experimental values of both the parameters lie between 0.13 and 0.15. These values are nearly independent of the Grashof number.