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 $\rm Re = 40\,000)$ 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 $2.5 \times 0.33)$ is carried out with change in slope angle $\theta$. 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 $\theta = 45^{\circ}$, 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 $THE = (\mathrm{Nu}_{mm}/\mathrm{Nu}_{mm\mathrm{pl}})/(\xi/\xi_{\mathrm{pl}})^{1/3}$ is close to unity.