Effect of combustion mode on heat transfer inside a hybrid solar receiver combustor

A 3-D CFD model of a hybrid solar receiver combustor (HSRC) was developed to carry out a systematic assessment of multiple performance criteria (thermal efficiency and heat flux distribution) with the aim of identifying how the introduction of concentrated solar radiation inside the HSRC influences the relative performance of MILD and conventional combustion. The effect of changing the dominant receiver geometric features (i.e. the length-to-diameter cavity ratio) on the thermal performance of the device is also assessed. The model employs a Monte-Carlo ray tracing approach to calculate the radiation flux distribution on heat transfer fluid (HTF) pipes and thermal efficiency of the solar receiver under solar-only mode of operation. For the combustion-only mode of operation, two different burner designs were considered as representative cases for non-premixed swirl-stabilised flames and MILD combustion, respectively. It was found that through optimisation of the HSRC design it is possible to achieve similar thermal efficiency in both solar-only and combustion-only modes. Also, the thermal efficiency of the device increases with an increase in the length-to-diameter cavity ratio and solar concentration ratio. MILD and conventional combustion showed a similar thermal efficiency, although the total heat transfer rate was larger (up to 5%) under MILD conditions, implying a better utilisation of the energy source and/or fuel savings. On the other hand, the heat flux distribution on the HTF pipes for solar-only mode was closer to the combustion-only mode operating with conventional flames than MILD conditions, suggesting that the materials selection will be influenced by the combustion regime.