With focus on the large combustion difference between the zones near the front and rear walls and serious slagging on the front and rear walls in down-fired pulverized-coal boilers, we investigated experimentally the aerodynamic field at different secondary-air angles of -10 degrees, -5 degrees, 0 degrees. 5 degrees, 10 degrees and 20 degrees within a cold 1:15-scaled model for a down-fired pulverized-coal 300 MW, utility boiler with secondary air being supplied through arches. At settings of 10 degrees and 5 degrees, a deflected flow field appeared in the lower furnace and the downward airflow near the front wall is directed upward earlier than that near the rear wall. Concurrently, downward airflow washed over both the front and rear walls. For settings of 0 degrees and 5 inverted perpendicular, flow field deflection disappeared in the furnace. At angles of 10 degrees and 20 degrees, another flow-field deflection appeared with deflection characteristics differing from that at 10 degrees and -5 degrees. The downward airflow near the front wall reverses direction far later than for near the rear wall and airflow sweeping disappears. To establish a symmetric flow field and an appropriate airflow penetration, an optimal angle setting for secondary air was found at 5 inverted perpendicular, particularly if slagging is to be avoided on the front and rear walls and in the dry bottom hopper. (C) 2011 Elsevier B.V. All rights reserved.