Blockage ratio effects on heat transfer in 1:4 AR channels with developing flow is experimentally determined. The blockage ratios (e/D-h) were 0.078 and 0.156. Reynolds numbers up to 40 K and rotational speeds up to 400 rpm were considered. The rotation number (Ro) and local buoyancy parameter (Bo(x)) were extended to 0.65 and 1.5, respectively. The entrance dominates over rotation on the trailing surface first pass. The leading surface is dominated by rotation. Rotation effects are reduced by the ribs and heat transfer is similar for both walls in the second pass. Rotation effects were similar for both blockage ratios. The correlations show that the buoyancy parameter is useful to predict heat transfer in the extended range. (C) 2009 Elsevier Ltd. All rights reserved.