Heat Transfer Enhancement in a Dimpled Narrow Channel during the Transformation of Separated Turbulent Flow with Increasing Slope Angle of a Solitary Conical Dimple

A numerical study of turbulent (Re = 40 000) flow around a solitary conical dimple with a depth of 0.233 of the spot diameter on the wall of a narrow channel (with a cross section of 2.5 × 0.33) is carried out with change in slope angle θ. The transformation of the jet-vortex structure of the mean flow from a symmetric system of paired vortices to a monotornado flow leads to an intensification of the separated flow and heat transfer enhancement. The maximum velocities of the return and secondary flows increase by 25 and 40%, respectively, while the maximum turbulent energy drops abruptly by 30%. The minimum relative negative friction at the bottom of the dimple decreases by a factor of 2.5 (reaching –1.5). After θ = 45°, there is a fairly rapid decrease in the intensity of the separated flow in the dimple. The thermal efficiency of the control section with a dimple noticeably lags behind the growth of relative hydraulic losses; however, thermal-hydraulic efficiency THE = (Nu mm /Nu mm pl )/(ξ/ξ pl ) 1/3 is close to unity.