Heat Transfer in Two-Pass Turbulated Channels Connected by Holes

Abstract

This is part of a continuing study of a new internal channel cooling design for modern gas-turbine blades. In previous studies, the enhanced cooling in the second pass of a serpentine channel was achieved by a combination of impingement and cross ow-induced swirl. A holed or slotted divider wall replaced the 180-deg U turn connecting the two legs of the serpentine channel. Flow from one coolant pass to the adjoining coolant pass was achieved through a series of straight and angled holes and a two-dimensional slot placed along the dividing wall. The focus is to enhance the heat transfer in the rst pass of the two-pass channel using traditional rib turbulators. The effect of ribs in the rst pass on the overall second pass heat transfer enhancement is compared to channels with no rib turbulators. Heat transfer distributions are compared for three channel ow Reynolds numbers ranging between 1:0 £ £ 10 4 and 5:0 £ £ 10 4 . Three different rib con gurations, 90-deg ribs, 60-deg angled forward facing toward the divider wall, and 60-deg angled backward facing away from divider wall, are studied for all Reynolds numbers and divider wall geometries. The presence of ribs in the rst pass does not only decrease the enhanced heat transfer in the second pass, but also provides higher heat transfer enhancement in the rst pass, resulting in an increase in overall heat transfer enhancement for the entire two-pass channel. Nomenclatureb = divider wall thickness D = square channel width or height D h = channel hydraulic diameter d = hole diameter f = Darcy friction factor, 21P.D h =L/=.½ N V 2 / f 0 = Darcy friction factor in all-smooth wall channel, 0:046Re ¡0:2 f N N = total channel averaged friction factor h = convective heat transfer coef cient k = thermal conductivity of test surface material k a = thermal conductivity of air L = length of each pass P m = mass-ow rate Nu = Nusselt number, hD h =k a Nu 0 = fully developed ow Nusselt number, 0:023Re 0:8 Pr 0:4 Nu = span averaged Nusselt number N N N u = area averaged Nusselt number P = pressure Pr = Prandtl number p = hole spacing Re = channel Reynolds number, ½ N V D=¹ T = temperature t = time of color change N V = average ow velocity X = axial distance from middle of turn ® = thermal diffusivity of test surface material