The transient cooling process in a sudden-expansion channel with the injection of cold air from the porous bottom wall is investigated experimentally. Four cooling patterns are categorized based on the structure of the corner eddy and the recirculation zone. Two mechanisms, porous-wall cooling and film cooling, are proposed to interpret the transition of the cooling patterns as the rate of wall injection is increased. At the early phase of wall injection, both the reattachment effect and the increased coolant injection induced by the low-pressure center are considered to produce the two maxima in spatial distributions of the Stanton number. (C) 2009 Elsevier Ltd. All rights reserved.