Down-fired boilers currently suffer from a number of different issues including high levels of fly ash in the combustible matter, serious slagging and a high level of NOx emissions. In this study, the combustion system of a down-fired 350 MWe supercritical utility boiler was designed with multiple injections and multiple staging to experimentally investigate the impact of the primary air to vent air ratio on the aerodynamic field in the furnace. We constructed a cold 1:10 scale model of the down-fired boiler, and used this to compare the distribution characteristics of the furnace vertical velocity component along the horizontal direction, the decay curves of the downward airflow, and the effect of the secondary air supply ejector on the primary air. It was found that changing the ratio of primary air:vent air had very little effect on the downward flow. Taking the pulverized-coal concentration, the primary air velocity, and other factors into account, we found that the optimal ratio of primary air:vent air was 5:5. (C) 2012 Elsevier Ltd. All rights reserved.