Experimental correlation of natural convection losses from a scale-model solar cavity receiver with non-isothermal surface temperature distribution

Correlations for natural convection heat loss from solar cavity receivers are widely based on isothermal surface temperature assumptions, which do not occur in practice due to the local heat balance varying with position. An open question thus exists regarding the suitability of such correlations for non-isothermal conditions. This paper addresses this issue by presenting a new Nusselt correlation developed from an experimental investigation of natural convection heat loss from a non-isothermal scale-model cylindrical cavity receiver. Cavities that are considered in this work have length-to-diameter ratios of 1 and 2, are operated at peak temperatures ranging from 355 °C to 650 °C, and exhibit temperature differences along the cavity wall between 40 °C and 342 °C. Stagnation and convection zones, as well as view factor profiles, are observed to contribute to the wall temperature distribution as the cavity is inclined downwards. An energy balance undertaken for steady state provides insight into the effects of non-uniform surface temperature distribution and inclination-dependent surface areas on radiative and convective losses. Natural convection heat loss results from this work are compared with widely-used correlations from the literature that assume isothermal wall conditions, and systematic discrepancies are observed. The proposed Nusselt correlation which accounts for the temperature non-uniformity, cavity inclination and geometric aspect ratio is evaluated against experimental data from this and other studies. It is found to produce excellent predictions of Nusselt numbers for cylindrical cavity receivers in the Grashof number range of 2.6 × 10⁵ to 1.4 × 10⁷.