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 sub limation technique. The tests on the vane were performed at the nomina l flow angle, whereas for the turbine blade an off-design angle was ch osen to study the influence of a separation bubble on the heat transfe r. The exit Mach number was varied from M(2) = 0.2 to 0.4 and the exit Reynolds number ranged from Re-2 = 300,000 to 700,000. Comparisons wi th numerical codes have been conducted. The measurements were performe d in a linear test facility containing five airfoils. Two tailboards a nd two bypass vanes allowed us to achieve a good periodicity of the fl ow. The aerodynamic flow conditions were measured using pressure taps and Laser-Two-Focus (L2F) anemometry. About 40 static pressure taps ga ve a precise Mach number distribution over the suction and the pressur e side of the airfoil. L2F measurements were used to determine the dow nstream flow angles, The heat transfer coefficient was measured using the naphthalene sublimation technique. This method is based on the hea t and mass transfer analogy for incompressible flow, A 0.5 mm thin nap hthalene layer was applied to the middle airfoil and exposed to the fl ow for about 45 minutes. The sublimation was then measured in over 500 points on the airfoil, which allowed a high resolution of the heat tr ansfer coefficient. Due to its high resolution, the sublimation techni que shows the presence of and the precise location of the laminar-to-t urbulent transition point and the separation bubble. The measurements on the vane were compared with two separate two-dimensional boundary l ayer programs, which were TEXSTAN (Texas University) and TEN (Sussex U niversity), The programs incorporate the k-epsilon turbulence model wi th several different formulations. The laminar-turbulent transition wa s predicted quite well with TEN, which slightly damps out the producti on 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.