Comparison of system performance in a hybrid solar receiver combustor operating with MILD and conventional combustion. Part II: Effect of the combustion mode

Abstract

This paper reports on the effects of the influence of the MILD combustion mode relative to conventional combustion on the overall performance of a hybrid solar receiver combustor (HSRC) operating under the combustion-only mode of operation. The influence of the dominant reactor geometrical parameters and level of dilution of the combustion air stream on the thermal efficiency, heat transfer mechanisms and heat flux distribution within the cavity were investigated using a three-dimensional computational fluid dynamics (CFD) model of the device. It was found that the use of MILD combustion leads to an increase of the thermal performance of the device in comparison with a conventional flame, owing to a larger rate of convective heat transfer. Also, the level of dilution of the combustion air influences significantly the rate of radiative heat transfer and the ratio of radiative to convective heat transfer rate. The thermal efficiency of the device was also found to increase with the length-to-diameter cavity ratio (L/D) and/or a reduction of the jet inclination angle. In addition, operation in the MILD regime can achieve a similar thermal performance to the solar mode of operation given suitable configurations, such as the use of either a cavity of sufficiently length (L/D>5) or an appropriate arrangement of the HTF pipes within the cavity. However, the calculated heat flux distributions on the receiver pipes were found to be significantly different under the two modes of operation, implying that the difference in heat flux distribution is a key parameter that needs to be considered in the design strategies and materials selection. The present investigation also identified the configurations for which the analytical model of the device (operating in the MILD regime), developed previously, yields reasonable accuracy for the thermal efficiency of the device, notably for the case of long cavities (L/D>5).