Heat Transfer Enhancement Using a Convex-Patterned Surface

Abstract

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 (ReHD) 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 (NuHD). 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 (NuHD) 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 (Nuo) of an entirely smooth channel. The normalized Nusselt numbers (NuHD/Nuo), as well as the normalized friction factors (f/fo), for all three cases, decreased with Reynolds numbers. However, the decay rate of the friction factor ratios (f/fo) with Reynolds numbers was lower than that of the normalized Nusselt numbers. For all three cases investigated, the thermal performance ((NuHD/Nuo)/(f/fo)1/3) values were within 5% to each other. The heat transfer enhancement using a convex patterned surface was thermally more effective at a relative low Reynolds numbers (less than 20,000 for δ/d=0) than that of a smooth channel.