Good combustion is required to maximize steam generation and avoid emissions in the wood waste fired boilers found frequently in the pulp and paper industry. The combustion process is assisted if velocities above the grate are minimized, if gases in the combustion zone are mixed intimately, and if gaseous combustion is concentrated above the grate. A 1/15 scale replica of a power boiler was constructed for isothermal flow modelling of the flow field above the grate, the overfire air jets and the furnace up to the generating banks. The flow was made visible with smoke, and velocities were measured with a pulsed wire anemometer. It was found that a very non uniform velocity profile occurred above the fuel grate because the under grate plenum did not adequately diffuse the under grate air flow. Also, non perforated areas of the fuel grate near the furnace walls caused recirculation zones to occur above the grate near the front wall. The recirculation zones of the overfire air jets caused high velocities to occur above the grate as well. A large space between the front wall and the front overfire air nozzles on the side walls, caused vertical stratification of the flow, in which gases rising from the grate, near the front wall did not mix with the overfire air. At part load, reduced velocity in the overfire air jets caused vertical stratification of the flow as well, in which mixture of gases rising from the grate and overfire air occurred well above the fuel grate. At high nozzle velocities, gases were well mixed, throughout the cross section, large recirculations occurred and mixing was concentrated near the fuel grate. Established models for jets for free turbulent jets do not accurately represent opposing banks of jets. Centerline velocity is overpredicted by a factor of two or more, and deflection of the jet are greatly underpredicted by the models. Throw and penetration, calculated with the models do not give reasonable predictions, indicating the need for more sophisticated models. At high overfire air flow rates, oscillation of the overfire air jets was observed, with a period in the order of one second. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate