Combined experimental and theoretical investigations have been carried out to study heat/mass transfer and combustion in the direct flame impingement (DFI) furnace for rapid heating of metals in materials processing. A large-size industrial DFI furnace, equipped with a multiflame combustion system, has been instrumented for performing detailed fluid dynamics and heat transfer measurements. The mean and local pressure, fuel mass fractions, temperatures and convective/radiative heat fluxes have been measured and are reported for high jet velocities (up to 230 m/s) and firing rates. In the case of natural gas-air firing, the convective heat fluxes as high as 500 kW/m(2) were recorded with relatively 'cold' refractory wall temperatures (<1400 K). The combustion gas temperature varied between 1500 and 1800 K. A simplified two-dimensional theoretical model was developed to analyze gas flow, flame jet combustion and heat/mass transfer in the DFI furnace. The model developed has been Validated against the experimental data and was used to obtain a fundamental understanding of the physical processes taking place in the furnace. In addition, the model has been used as a tool to optimize design and operation of the DFI furnace. <(c)> 2001 Elsevier Science Ltd. All rights reserved.