Heat Transfer Measurements on Turbine Airfoils Using the Naphthalene Sublimation Technique

Results of heat transfer measurements on a typical turbine blade and a vane in a linear cascade have been obtained using the naphthalene sublimation technique. The tests on the vane were performed at the nominal flow angle, whereas for the turbine blade an off-design angle was chosen to study the influence of a separation bubble on the heat transfer. The exit Mach number was varied from M₂ = 0.2 to 0.4 and the exit Reynolds number ranged from Re₂ = 300,000 to 700,000. Comparisons with numerical codes have been conducted. The measurements were performed in a linear test facility containing five airfoils. Two tailboards and two bypass vanes allowed us to achieve a good periodicity of the flow. The aerodynamic flow conditions were measured using pressure taps and Laser-Two-Focus (L2F) anemometry. About 40 static pressure taps gave a precise Mach number distribution over the suction and the pressure side of the airfoil. L2F measurements were used to determine the downstream flow angles. The heat transfer coefficient was measured using the naphthalene sublimation technique. This method is based on the heat and mass transfer analogy for incompressible flow. A 0.5 mm thin naphthalene layer was applied to the middle airfoil and exposed to the flow for about 45 minutes. The sublimation was then measured in over 500 points on the airfoil, which allowed a high resolution of the heat transfer coefficient. Due to its high resolution, the sublimation technique shows the presence of and the precise location of the laminar-to-turbulent transition point and the separation bubble. The measurements on the vane were compared with two separate two-dimensional boundary layer programs, which were TEXSTAN (Texas University) and TEN (Sussex University). The programs incorporate the k–epsilon turbulence model with several different formulations. The laminar–turbulent transition was predicted quite well with TEN, which slightly damps out the production of turbulent kinetic energy in order to ensure a smooth transition zone. In the case of the blade, the naphthalene sublimation technique was able to predict the size and the location of the separation bubble as well as the reattachment with a very high precision.