Influence of the depth of single-row oval-trench dimples inclined to laminar air flow on heat transfer enhancement in a narrow micro-channel

Abstract

Enhancement of heat transfer in laminar air flow in narrow channels, whose walls are provided with regularly spaced single-row inclined oval-trench dimples, is associated with designing mini- and micro-channels of microelectronics cooling devices, compact miniature heat exchangers, air capacitors, radiators. Vortex heat transfer is considered using the computational domain of the narrow micro-channel of dimensionless height 1, width 6 and length 4 at periodic boundary conditions. An oval-trench dimple of dimensionless width 1 and length 4.5 is located in the center of the heated wall at an angle of 45° to incoming flow. To solve the Navier–Stokes equations and the energy equation, multiblock computational technique realized in the VP2/3 code on overlapping grids of different topology and with a different density of computational cells has found use. The Reynolds number is equal to 103. The dimple depth Δ is varied from 0 to 0.375. Dimples are classified: shallow dimples and dimples of moderate and large depth. As Δ is increased, substantial laminar separated flow augmentation on the entrance portion of the inclined dimple is revealed and explained. Maximum absolute value of relative friction at Δ = 0.375 is twice increased in comparison to this quantity at Δ = 0.1125–0.25. A maximum absolute value of the secondary flow velocity in the dimple reaches 0.72 of bulk velocity. In the case of single-row inclined oval-trench dimples with a depth of more than 0.25, in the narrow channel the phenomenon of laminar flow acceleration with a 1.5-fold growth of a maximum core velocity is discovered. It is established that a reason for laminar separated and secondary flow augmentation in the inclined oval-trench dimple lies in a sharp static pressure drop (maximum pressure value is 0.34 and minimum pressure value is −0.14 at Δ = 0.3125) at a very small distance between the centers of high and low pressure zones on the entrance portion of the dimple. By increasing Δ, local relative heat loads on the entrance portion of the dimple on the windward side grow and reach values of 16–17. Maximum thermal performance determined by relative total Nusselt numbers averaged over a streamlined wall section with an inclined dimple is 1.8 at Δ = 0.3125; maximum thermal-hydraulic performance is 1.3 at Δ = 0.25.