Heat Transfer Enhancement Using a Convex-Patterned Surface

The heat transfer rate from a smooth wall in an internal cooling passage can be significantly enhanced by using a convex patterned surface on the opposite wall of the passage. This design is particularly effective for a design that requires the heat transfer surface to be free of any augmenting features (smooth). Heat transfer coefficients on the smooth wall in a rectangular channel, which had convexities on the opposite wall were experimentally investigated. Friction factors were also measured to assess the thermal performance. Relative clearances (δ/d) between the convexities and the smooth wall of 0, 0.024, and 0.055 were investigated in a Reynolds number (Re_HD) range from 15,000 to 35,000. The heat transfer coefficients were measured in the thermally developed region using a transient thermochromic liquid crystal technique. The clearance gap between the convexities and the smooth wall adversely affected the heat transfer enhancement (Nu_HD). The friction factors (f), measured in the aerodynamically developed region, were largest for the cases of no clearance (δ/d = 0). The average heat transfer enhancement (Nu_HD) was also largest for the cases of no clearance (δ/d = 0), as high as 3.08 times at a Reynolds number of 11,456 in relative to that (Nu_o) of an entirely smooth channel. The normalized Nusselt numbers (Nu_HD/Nu_o), as well as the normalized friction factors (f/f_o), for all three cases, decreased with Reynolds numbers. However, the decay rate of the friction factor ratios (f/f_o) with Reynolds numbers was lower than that of the normalized Nusselt numbers. For all three cases investigated, the thermal performance ((Nu_HD/Nu_o) /(f/f_o)^1/3) values were within 5% to each other. The heat transfer enhancement using a convex patterned surface was thermally more effective at a relatively low Reynolds numbers (less than 20,000 for δ/d = 0) than that of a smooth channel.