Influence of duct aspect ratio on heat/mass transfer in coolant passages with rotation

The combined effects of duct aspect ratio and rotation on heat/mass transfer characteristics are investigated. Mass transfer experiments are conducted to obtain detailed local heat/mass transfer coefficients on the leading and trailing surfaces in rotating two-pass ducts with a pair of opposite rib-roughened walls. The ducts of three different aspect ratios (W/H = 0.5, 1.0, and 2.0) are employed with a fixed hydraulic diameter (Dh) of 26.7 mm. In all duct cases, the rib height-to-hydraulic diameter ratio (e/Dh) is 0.056 and the rib pitch-to-rib height (p/e) is 10. The rotation number ranges from 0.0 to 0.20 while the Reynolds number is fixed at 10,000. To verify the heat/mass transfer augmentation, internal flow structures are calculated for a smooth two-pass square duct using numerical simulations. The results show that Sherwood number ratios are approximately 2.5 times higher than the fully developed value in a stationary smooth pipe due to the flow reattachment near ribbed surfaces. The overall heat/mass transfer coefficient increases as the duct aspect ratio increases. It is because the core flow is highly disturbed and accelerated in the midsections of the ribs, as the rib height-to-duct height ratio (e/H) increases. Dean vortices generated due to 180°-turn augment heat/mass transfer in the turning region and in the upstream region of the second pass. The rotation of duct produces heat/mass transfer discrepancy between leading and trailing surfaces, having higher Sherwood number on the trailing surface in the first pass and on the leading surface in the second pass. However, the effects of duct turning curvature and rotation on heat/mass transfer become less significant for the higher aspect ratio.