In this paper, a numerical study of combined natural convection and surface radiation heat transfer in a solar trapezoidal cavity absorber for Compact Linear Fresnel Reflector (CLFR) is presented. The CFD package, FLUENT 6.3 is used to develop the 2-D, non-Boussinesq, steady state, laminar, combined natural convection and surface radiation heat transfer model for a trapezoidal cavity absorber. The validation of the present non-Boussinesq numerical procedure is compared with other closed cavity model. Based on the validated non-Boussinesq model, the combined heat loss coefficients are predicted for various parameters such as Grashof number, absorber angles, surface emissivity, aspect ratio, temperature ratio and radiation-conduction number. The numerical simulation results are presented in terms of Nusselt number correlation to show the effect of these parameters on combined natural convection and surface radiation heat loss. (C) 2011 Elsevier Ltd. All rights reserved.