A theoretical and numerical study of natural convection in two-dimensional laminar incompressible flow in a trapezoidal enclosurein the presence of thermal radiation is conducted, motivated by energy systems applications. Heat flow visualization via the method of energy flux vectors (EFVs) is also included. The trapezoidal cavity has an inclined top wall which in addition to the bottom wall is maintained at constant temperature, whereas the remaining (vertical side) walls are adiabatic. The governing partial differential conservation equations are transformed using a vorticity-stream function formulation and non-dimensional variables and the resulting nonlinear boundary value problem is solved using a finite difference method with incremental time steps. EFVs provide abundant details of the heat flow at the core of the enclosure. The larger energy flux vectors indicate high temperature gradient zones and the sparse EFVs correspond to low temperature gradient zone. Heat flow distribution in the trapezoidal enclosure can be clearly elaborated via energy flux vectors and provides a deeper insight into thermal characteristics. A comprehensive parametric study is performed to evaluate the impact of Rayleigh number (buoyancy parameter) and radiation parameter on transport phenomena. The computations indicate that local Nusselt number and velocity are increasing functions of the Rayleigh number and radiation parameter. Significant changes in streamlines, temperature contours and energy streamlines for high Rayleigh number are observed. The energy flux vectors show that a large eddy is formed within the enclosure which migrates towards the cold wall. Greater thermal buoyancy force accelerates the primary flow whereas it decelerates the secondary flow. The simulations are relevant to solar collector systems, enclosure fire dynamics, electronic cooling and fuel cell systems. Furthermore, the computations furnish a good benchmark for more general computational fluid dynamics (CFD) analysis with commercial software e.g. ANSYS FLUENT. © 2019 Elsevier Ltd