HEAT TRANSFER IN TRIANGULAR CHANNELS WITH ANGLED ROUGHNESS RIBS ON TWO WALLS

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Abstract
Leading edges of internally cooled gas turbine engine airfoils are often cooled convectively by flow through triangular cross-sectioned passages containing roughness ribs on two of their three sides. The roughness augments heat transfer coefficients and thereby increases the ability of the cooling air to cool the airfoil leading edge. Ribs are normally placed on the two sides of the passage that constitute forward sections of the airfoil pressure and suction surfaces, and the third side, a cooling passage divider or internal bulkhead, is usually left smooth. The present study is an experimental investigation of developing local convection heat transfer in triangular ducts with square ribs at various rib angles, orientations, and pitch-to-diameter ratios on two of the three channel walls in conjunction with a smooth third wall. Stream-wise development of spanwise-averaged Nusselt numbers is presented for both rough and smooth walls, along with typical spanwise variations. Results were obtained for combinations of three different rib angles and three different sets of rib orientations. Both rib angle and rib orientation have significant effects on the Nusselt number results. The angled ribs produce a steep streamwise rise in Nusselt numbers followed by oscillation in heat transfer. This behavior, together with strong spanwise heat transfer variations on the ribbed surfaces, is explained as the result of secondary flow generated by the angled ribs. Heat transfer augmentation for the case of angled ribs in triangular channels is higher on both smooth and rough walls than that measured on square channels in prior studies