Experimental results and analysis of heat transfer from convex and concave walls (surfaces) and average heat transfer from both walls to hydrodynamically and thermally stabilized air flow in rectangular helical channels with two-sided heating over a wide range of flow hydrodynamic (Re = 10(3)-2 x 10(5)) and geometric parameters (relative curvature D/h = 5-90 and relative width b/h = 2-20) are presented. It is established, that in case of two-sided heating the heat transfer (Nu numbers) from convex and concave walls increases up to 20% in comparison with one-sided heating. Average heat transfer in helical channels increases up to 50% for laminar-vortex flow regime, and up to 20% for turbulent flow regime in comparison with heat transfer in straight flat channel. Correlations are proposed for stabilized heat transfer for different flow regimes. (C) 2010 Elsevier Ltd. All rights reserved.