Reduction of convective heat loss from an external solar receiver using an air curtain
Creators: name: Mondal, Razon
Publisher: name: The Australian National University
Year: 2023

description: Concentrated Solar Power (CSP) plants encounter inefficiencies at all stages of electricity generation. Convection from the solar thermal receiver is a significant mode of heat loss in CSP systems and is challenging to mitigate. This study investigates the mitigation of convective heat loss from an external heated surface by introducing an air curtain that disrupts the buoyant flow rising along the heated surface. The external CSP receiver surface is an important setting where this mode of heat transfer frequently occurs. An isothermal flat plate with a height of 1.8 m is used to model the receiver, and an obliquely introduced planar jet is used to form the air curtain. This thesis aims to understand the potential benefits of air curtains for CSP receiver heat loss reduction via a series of numerical case studies at the smaller 1.8 scale. The computational fluid dynamics (CFD) model is first validated for heated wall and air curtain cases. The subsequently validated models are then implemented to conduct a parametric study on the heat transfer from the isothermal plate with an air curtain, varying different parameters. The results showed that the air curtain successfully generates a stagnation zone adjacent to the wall and does reduce convective heat losses locally. The air curtain effectiveness, defined as the relative reduction in local heat loss when the air curtain is added, reaches a local value of 31.2 percent in the stagnation zone for a vertical wall when the air curtain had a jet speed and angle of 2.5 meter/second and 45 degree, respectively. Smaller air curtain angles relative to the wall resulted in lower effectiveness. A 45 degree air curtain on a vertical wall can offer performance benefits similar in magnitude to inclining a wall from the vertical. A higher wall temperature was accompanied by better effectiveness near the jet outlet, particularly in the stagnation region, while lower wall temperatures produced higher effectiveness further from the jet. Following that, different air curtain parameters, including jet speed, angle, thickness, and gap, are optimised to increase the mitigation of convection loss from a vertical external heated surface, focusing on global air curtain effectiveness. More than 18 percent global effectiveness was achieved for an optimum jet with a speed of 5.5 meter/second, an angle of 45 degree, a thickness of 30 mm, and a gap of 30 mm. The jet thickness found to be the key parameter for significant improvement of air curtain effectiveness. As an option for using waste heat streams in a CSP plant, the potential benefit of heated jets was considered; the effectiveness is gradually decreased with increasing the temperature of the air curtain. Finally, the comparison between 2D and 3D simulations was performed, where no significant changes were revealed. A validation has also been conducted between the 3D CFD modelling result against the experimental data, conducted by Abbasi Shavazi (2019). Both results show a similar trend and confirm that an overall beneficial air curtain configuration was found to reduce convective heat losses from the heated surface. Overall, this thesis thoroughly investigates the performance of air curtains to minimise convection loss from an external heated surface. A significant improvement of more than 18\% global effectiveness was achieved by using an air curtain. This thesis also contributes to a rigorous validation of numerical modelling with experimental data. Therefore, using this concept in the CSP plant may provide significant economic value to mitigate convection losses from the receiver surface.