Study of flow transport mechanisms in rectangular channels with different heat transfer augmentation devices.

摘要

The present study examines flow transport mechanisms in rectangular channels with different heat transfer augmentation devices: (i) crossed- and parallel-rib turbulators, (ii) pin fins, and (iii) various dimpled surface geometries. All of these devices act to increase secondary flows and turbulence transport, and to form coherent fluid motions in the form of streamwise oriented vortices and vortex pairs. Such vortices and secondary flows act to increase not only secondary advection of heat away from surfaces, but also three-dimensional turbulence production by increasing shear and creating gradients of velocity over significant flow volumes. The overall purpose of such devices in internal passages is enhancement of turbulence transport and surface heat transfer rates over larger portions of the flow area with minimal streamwise pressure losses.; Two different rib arrangements with perpendicular and parallel orientations on two opposite surfaces are investigated. Comparisons show important local Nusselt number differences for the two rib arrangements, especially just upstream of the ribs, which are due to significant differences in global and local primary- and secondary-flow characteristics. Flow structure consists of one single, large cell of fluid motion for the crossed ribs, whereas two cells of large-scale motion are present for the parallel-ribs.; Flow structural characteristics are also presented for a stationary channel with an aspect ratio of 8 and a staggered array of pin fins between two of the surfaces. Local Nusselt numbers, measured on one endwall, are highest beneath primary and secondary horseshoe vortices located just upstream of individual pins, beneath pin wakes, and downstream of the pins.; Flow structural characteristics over the surfaces of channels with spherical dimples on the wall are experimentally and numerically studied. Data are presented for nondimensional dimple depths (ratio of depth to dimple print diameter) of 0.1, 0.2 and 0.3, respectively. As the dimple depth becomes larger, larger deficits of total pressure and streamwise velocity are present, along with higher magnitudes of streamwise vorticity, vortex circulation, and Reynolds normal stress. This implies bigger and stronger vortices and increasing turbulence transport due to advection of reattaching and recirculating flow from locations within the dimple cavities.