Heat transfer characteristics inside two-pass variable cross-section channels with ribs and bleed holes

Heat transfer characteristics were experimentally studied for two-pass channels with variable cross-section with and without ribs. Rectangular ribs were placed symmetrically on two opposite walls with attack angles of 90° and 60° to the main flow. The bleed holes were only distributed on the rib roughened surface along the second pass. The cross-section area of the channel varied in the way that it increased along the first pass and decreased along the second. The relative roughness heights were 0.092 and 0.123; the relative roughness pitches were 10 and 20. The experiments were carried out at Reynolds numbers from 7100 to 60000. Analysis of the experimental results may lead to the following conclusions: (1) The heat transfer was enhanced by the presence of variable cross section. (2) The 60° ribs was not necessarily the optimistic option as published in the literature, test showed that 90° ribs produced a hither heat transfer for the relative roughness height of 0.092. (3) For up to 30% of bleeding were outlet, the existence of bleed holes enhanced the heat transfer process.