Experimental investigation of external convection from scale-model solar thermal receivers

Abstract

Instrumental in increasing the thermal efficiency of a solar thermal receiver is the ability to accurately understand and model its various thermal losses. Convective heat losses from receivers are more difficult to quantify than radiative and reflective losses due to non-linearities in heat and mass transfer phenomena, complex receiver geometries, large length scales with complex flow regimes, and variable ambient conditions. This thesis reports on three experimental investigations for studying convection heat transfer from scale-model solar thermal receivers or receiver surfaces, aiming to increase the accuracy in predicting convection heat loss. Additionally, this thesis provides novel experimental proof of a strategy to reduce convection heat loss from a scale-model solar receiver geometry. Correlations for natural convection heat loss from solar cavity receivers are often based on isothermal surface temperature assumptions, which do not occur in practice. This thesis presents a new Nusselt number correlation developed on the basis of an experimental investigation of natural convection heat loss from a non-isothermal model cylindrical cavity receiver. Cavities with length-to-diameter ratios of one and two, operated at peak temperatures ranging from 355°C to 650°C, exhibit temperature differences along the cavity wall between 40°C and 342°C. The proposed correlation, accounting for temperature non-uniformity, cavity inclination, and geometric aspect ratio, predicts 92% of the experimental data within ±30% in the Grashof number range of 2.6 × 105 to 1.4 × 107. This correlation is simpler and as effective as the Clausing correlation, suggesting that temperature inhomogeneity significantly affects convective heat losses. The second experimental investigation studied a scale-model representation of the surface of an externally-irradiated CSP receiver in the form of a heated flat-plate setup. The design allowed investigation of turbulent natural convection for various inclined plate configurations. Comparison with relevant correlations showed good agreement, and datasets indicative of the flow development from laminar to turbulent regimes were obtained. The setup design included sidewalls to enable the investigation of the effect of flows at the plate edge on heat loss. Experiments with the sidewalls removed from the plate showed an increase in thermal loss, justified by the increased temperature gradient due to spanwise flow across the plate. The final experimental body of work investigated an air curtain system designed to mitigate convection losses from the heated flat plate. Parameters such as plate temperature and inclination, and air jet discharge angle and velocity were varied. For certain combinations of these parameters, the air curtain reduced convection heat loss compared to natural convection without the air curtain. The smallest jet discharge angle of 25° increased convection losses, while slightly tilted plate configurations resulted in larger reductions. A plate inclination of 15–30° and jet discharge angle of 35–45° produced the most favourable conditions for reducing convection. Lower jet velocities resulted in larger reductions in convection loss near the jet outlet, while higher velocities reduced convection loss further downstream. The presence of two regions of relatively higher effectiveness has been independently verified via computational fluid dynamics simulations (Mondal, 2023), increasing confidence in the contribution of this experimental work to understanding convection loss reduction from solar thermal receivers. Show more