Experimental Heat Transfer of Rib Roughened Square, Rectangular and Trapezoidal Cooling Channels

This paper explores the heat transfer benefit of using turbulators within turbine airfoil cooling passages. The local and average heat transfer coefficients were measured in scaled models with trapezoidal, square and rectangular cross-section channels with aspect ratios of 0.15, 1, and 3 respectively. The friction factors through the square and rectangular channels were also calculated. To simulate a heated wall boundary condition, heaters were placed along the test section. Liquid crystals were mounted over the heaters to measure the temperature. For the square and rectangular test sections, the walls are rib-roughened on two opposite sides with and without wrap-arounds extending on the adjacent walls. For the trapezoidal test section, the walls have staggered turbulators mounted on two opposite sides with film bleed holes on one side. Comparisons are made for turbulator configurations where the height-to-hydraulic-diameter ratio (e/Dh) is 0.09 and 0.14; the turbulator height-to-width ratios (e/w) are 0.45, 0.63, and 0.73. Upon conclusion of this study, it was found that: (a) Based on the results from the square and rectangular channel studies: (1) Having wrap-arounds result in a heat transfer benefit to all of the channel walls and increases the friction factor. (2) The Nusselt Number increases as the flow travels from area one downstream towards area five. (b) Based on the results from the trapezoidal channel studies: (1) Nusselt Numbers are larger for test configurations with higher rib height to hydraulic diameter (e/Dh) ratios. (2) Nusselt Numbers are lower for test configurations with smaller rib height to width (e/w) ratios. (3) At the range of local Reynolds Numbers from 20,000 to 30,000, the Nusselt Numbers for the orientation where the free end of turbulators are pointed towards the airfoil root are higher than the Nusselt Numbers of the orientation where the free end of turbulators pointed towards the airfoil tip. (c) To support the experimental results and trends, a CFD analysis modelling the test geometry can be completed.