Air curtains for reduction of natural convection heat loss from a heated plate: A numerical investigation

Abstract

Concentrating 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 reduction of convection losses by using a planar jet that disrupts the buoyant flow arising from the heated surface of an external CSP receiver. An isothermal flat plate with a height of 1.8 m was used to model the receiver, and a planar jet air curtain with a nozzle thickness of 3 mm was introduced near the upper edge of the wall. A computational fluid dynamics model was first validated and subsequently implemented to conduct a parametric study on the heat transfer from the isothermal plate with an air curtain varying four parameters: jet speed, jet angle, plate temperature and plate inclination. The results showed that the air curtain generates a stagnation zone adjacent to the wall which successfully reduces local convective heat losses. The effectiveness of an air curtain is defined here as the relative reduction in the local heat loss due to the air curtain, compared to the case of natural convection alone. A local maximum of 31.2% effectiveness is achieved in the stagnation zone below the jet outlet for a vertical wall with a jet speed of 2.5 ms−1 and jet angle of 45∘. The air curtain effectiveness at the stagnation region was found to decrease with increasing jet speeds, whereas the effectiveness increased near the laminar-to-turbulent transition region with increasing jet speed. Smaller air curtain angles relative to the wall resulted in lower effectiveness. A 45∘ air curtain on a vertical wall can offer performance benefits that are 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. Therefore, an air curtain can be used to reduce convective heat losses locally from a heated flat surface, including potentially when applied to CSP receivers.