1 Copyright © 2004 by ASME Proceedings of ASME Turbo Expo 2004 Power for Land, Sea, and Air June 14-17, 2004, Vienna, Austria GT2004-53250 MEASURED ADIABATIC EFFECTIVENESS AND HEAT TRANSFER FOR BLOWING FROM THE TIP OF A TURBINE BLADE J. R. Christophel, E. Couch, and K. A. Thole Mechanical Engineering Virginia Polytechnic Institute and State University Blacksburg, VA 24061 F. J. Cunha Pratt & Whitney United Technologies Corporation East Hartford, CT 06108 ABSTRACT The clearance gap between the tip of a turbine blade and the shroud has an inherent leakage flow from the pressure side to the suction side of the blade. This leakage flow of combustion gas and air mixtures leads to severe heat transfer rates on the blade tip of the high pressure turbine. As the thermal load to the blade increases, blade alloy oxidation and erosion rates increase thereby adversely affecting component life. The subject of this paper is the cooling effectiveness levels and heat transfer coefficients that result from blowing through two holes placed in the forward region of a blade tip. These holes are referred to as dirt purge holes and are generally required for manufacturing purposes and expelling dirt from the coolant flow when operating in sandy environments. Experiments were performed in a linear blade cascade for two tip gap heights over a range of blowing ratios. Results indicated that the cooling effectiveness was highly dependent upon the tip gap clearance with better cooling achieved at smaller clearances. Also, heat transfer was found to increase with blowing. In considering an overall benefit of cooling from the dirt purge blowing, a large benefit was realized for a smaller tip gap as compared with a larger tip gap. INTRODUCTION The performance of a turbine engine is a strong function of the maximum gas temperature at the rotor inlet. Because turbine airfoils are exposed to hot gas exiting the combustion chamber(s), the materials and cooling methods are of critical importance. Turbine blade designers concentrate heavily on finding better cooling schemes to increase the overall operation life of all turbine airfoils, namely the high pressure turbine blades. The clearance between the blade tip and the associated shroud, also known as the blade outer air seal, provides a flow path across the tip that leads to aerodynamic losses and high heat transfer rates along the blade tip. The flow within this clearance gap is driven by a pressure differential between the pressure and suction side of the blade, but is also affected by the viscous forces as the fluid passes through the gap. From an operational point-of-view, engine removals from service are primarily dictated by the spent exhaust gas temperature (EGT) margin caused by deterioration of the high pressure turbine components. Increased clearance gaps accelerate effects of low cycle thermal-mechanical fatigue, oxidation, and erosion as a result of increased temperatures in the turbine and decreased EGT margin. In general, tip clearances for large commercial engines are of the order of 0.25 mm, which can reduce the specific fuel consumption by 1% and EGT by 10°C (Lattime and Steinetz, [1]). Improving the blade tip durability can, therefore, produce fuel and maintenance savings over hundreds of millions of dollars per year [1]. The work presented in this paper is on a realistic design for a turbine blade tip consisting of a flat tip with the exception of a small cavity in which two dirt holes are placed. The location of these holes is a direct consequence of the internal cooling passages within the blade. The purge hole cavity extends only over a small area in the front portion of the blade tip. The function of the dirt purge holes includes the following: (1) purge holes allow centrifugal forces to expel any dirt ingested by the compressor into the turbine rather than clogging the smaller diameter film cooling holes; and (2) purge holes provide a way to support the ceramic core during the lost-wax investment casting of the blade manufacturing process. The dirt purge cavity is present to insure that the purge holes remain open during eventual blade rubbing. This paper details the film-cooling and heat transfer associated with blowing from the dirt purge holes along the tip of a turbine blade. Measurements of adiabatic effectiveness and heat transfer coefficients are studied while varying the tip clearance and mass flux (blowing) ratios. RELEVANT PAST STUDIES The work presented in this paper is concerned with the effects of injecting coolant from the tip of a turbine blade whereby the experiments were completed for a stationary, linear cascade. As such, it is important to consider the relevance of past studies to evaluate the effects of the relative