Gas/particle two-phase flow experiments at different over-fire air ratios were conducted using a 1:20 small-scale model of a down-fired 350 MWe superficial utility boiler designed with multiple injections and multiple staging combustion technology. The mean velocity, fluctuating velocity and particle volume flux were measured using particle dynamics anemometer measurements. None of four conditions of pulverized coal short-circuit occurred. When the over-fire air percentage was 1.9% and after the arch air flow reached the tertiary air, the position of maximum gas flow velocity of the arch secondary air and the maximum particle volume flux in the fuel-rich flow was close to and eroded the furnace hopper side wall. When the over-fire air percentage was increased to 9.6%, the position of maximum gas flow velocity of the arch secondary air and the maximum particle volume flux in the fuel-rich flow started to deviate from the hopper side wall but did not erode the furnace side wall. However, when the over-fire air percentage was 9.6%, the gas velocity of the arch air flow entering the furnace hopper was still high and the arch air flow downstream depth was too deep. Therefore, use of an over-fire air percentage greater than 19.1% was recommended. Published by Elsevier Ltd.