Flow and heat transfer investigation behind trapezoidal rib using PIV and LCT measurements

Abstract

The present work is an experimental investigation inside a rectangular duct for flow behind a trapezoidal type of rib with chamfering angle α (toward the direction of flow) at different Reynolds numbers. Chamfering angle α has been varied in between 0° and 20° with an increment of 5° and subsequently detailed fluid flow and heat transfer experiments have been performed at four different Reynolds numbers, that is, 9,400, 27,120, 44,600, and 61,480 (based on hydraulic diameter of the duct). In order to investigate the detailed fluid flow and heat transfer characteristics together, a distinct experimental setup has been designed while using 2-D particle image velocimetry and liquid crystal thermography, respectively. Flow investigations have been restricted within the streamwise location of x/e ≤ 11, while the region of interest for heat transfer measurement goes up to x/e ≤ 50. The emphasis is toward assessing and analyzing the potential impact of varying chamfering angle over the flow structures, and its subsequent effect on far downstream heat transfer enhancement, as well as its role in obviating the hot spots in the adjacent vicinity behind the chamfered rib turbulators. Transient heat transfer investigation has been performed for evaluating the surface heat transfer enhancement. Results are documented in terms of stream traces, mean and rms velocity fields, streamwise Reynolds stresses and vorticity distribution, and surface and spanwise averaged augmentation Nusselt numbers. The reattachment length has been identified for all of the configurations, and the turbulent characteristics have been discussed in reference to the reattaching shearing layer and its potential impact on the size of the recirculation bubble for different configurations and conditions. The result showed the successful impact of changing the trapezoidal angle α by manipulating the small-scale vortices at the leeward corner of the rib which helps in obviating the hot spots. Furthermore, the presence of large scale unsteady vortical structure within the shear layer has been confirmed, and it has been subsequently associated with heat transfer enhancement in the far downstream region.