Enhancement of heat transfer and hydraulic drag in gas-cooled helical channels with artificial roughness on convex wall

Results on experimental investigation of heat transfer enhancement in helical channels of rectangular cross-section in the hydrodynamically and thermally stabilized air flow are presented. The investigations have been carried out in the rough, with artificial three-dimensional pyramidal roughness (k/de ≈ 0.03) on the convex wall (surface), and in the smooth helical channels (relative curvature D/h ≈ 20 and relative width b/h ≈ 6) for a wide range of flow hydrodynamic parameter Re (103–2 × 105). The research has revealed that in the region of turbulent flow, heat transfer from convex surface of the rough channel more than twice exceeds heat transfer from convex surface of the smooth channel and reaches level of heat transfer from concave surface. Heat transfer from concave surfaces of the both rough and smooth helical channels is similar and is about 50% higher than of a straight flat channel. Heat transfer efficiency parameter St3/ξ for the rough channel is higher than for the smooth channel almost in the whole range of investigated Re, and the efficiency parameter η ((St/Stsm)/(ξ/ξsm)) is about 1.3. Correlations are proposed for calculation of stabilized heat transfer from convex and concave surfaces and average heat transfer and hydraulic drag in rough helical channels.