Combustion and heat transfer processes were studied in a specially designed, rapid heating experimental furnace, equipped with a multijet combustion chamber for natural gas/air firing. The system has no special combustion tunnels or flame holders and results were obtained for high velocities (up to 400 m s(-1)) and firing rates. Maximum convective heat fluxes of up to 260 kW m(-2) were obtained with relatively 'cold' refractory wall temperatures (<1300 K). The combustion gas temperatures ranged from 1400 to 1700 K. The mean flame jet flow characteristics and temperature profiles together with rates of heat transfer were measured. A simple two-dimensional numerical model was developed to predict combustion and radiative as well as convective heat transfer to the load and refractories in order to obtain an improved understanding of the processes.