Numerical three-dimensional studies of the natural convection and radiative heat loss from cavity receiver of different shapes with and without mouth-blockage have been investigated under isothermal wall condition. Convective heat loss is found to decrease for cavities having mouth blockage created by reducing aperture area (case I) whereas it enhances when mouth blockages are introduced by increasing the cavity dimensions and keeping the same aperture area (case II). Convective loss is

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... rized by using the convective zone area (A cb ). Conical cavity yields the lowest convective loss whereas hetro-conical cavity gives the highest convective loss among different shapes investigated. Radiative loss is independent of cavity inclination and is found to be nearly constant for all cavity shapes and cavity configurations (with or without mouth blockage) so long as the aperture area remains the same; it is proportional to the aperture area. However, investigations on decrease in heat loss of mouth-blocked cavities needed to be coupled with the estimation of concentrated flux.