Effect of Permeable Ribs on Thermal-Flow Characteristics in an Internal Cooling Duct

This study aims to understand the effect of flow structures within the inter-rib regions of a novel permeable rib configuration in vertical and horizontal streamwise planes upon surface heat transfer parameters. In this investigation, the liquid crystal thermography (LCT) and particle image velocimetry (PIV) are used to extract the local heat transfer and flow-field information, respectively. The effect of slit-converging angle (ϕ = 0 deg, 5 deg, 10 deg, and 15 deg) are examined at a typical Reynolds number of 42,500 and relative rib pitch ratio of 10. Surface- and spanwise-average and overall augmentation Nusselt numbers are obtained along with the pressure drop measurements. Flow-field experiments are performed in both vertical and horizontal streamwise planes, and the results are expressed in terms of mean velocities, stream traces, turbulent statistics, coherent structures, and turbulent kinetic energy budgets. Critical points are also identified on the basis of critical point theory, which provides evidences of the different flow phenomena accountable for enhance mixing between the ribs. The secondary flow coming from the slit shows three-dimensionality in the flow resulting to higher turbulence intensity and rotational motion (say higher turbulent mixing), and thereby leading to high heat transfer just behind the permeable rib. The permeable ribs are also helpful in the reduction of friction factor by 32% with a typical ϕ value of 5 deg, compared to solid ribs, while the thermohydraulic performance increases with increasing ϕ from 0 deg to 15 deg up to 21%. The pentagonal ribs with convergent slit provide comparable or better performance among the permeable rib geometries used in the pertinent literature.