As part of an investigation into combustion behaviour at Aberthaw Power Station, experiments have been conducted using Innogy's 0.5 MW Combustion Test Facility (CTF) at Didcot A. Most tests involved combustion, but a. few were for isothermal conditions and included velocity measurements. Both isothermal and combusting flows have been calculated using a commercial computational fluid dynamics (CFD) program called Star-CD. The isothermal solutions have been validated against a simpler analytical solution and against rig measurements. The solutions with combustion and radiation have been validated qualitatively against the behaviour observed during rig tests and also quantitatively against incident heat flux measurements. The basis of the CFD for both the isothermal and combusting cases is described, together with the validation that has been achieved. (C) 2003 Elsevier Ltd. All rights reserved.