Experimental and CFD Studies of NGV Endwall Cooling

For engines operating at high turbine entry temperatures it is increasingly important to cool the high pressure nozzle guide vane (HP NGV) endwalls. This is particularly so for low NOx combustors operating with flatter outlet temperature distributions. Double-row arrangements of film/ballistic cooling holes upstream of the NGV passage have been employed in production engines. Optimisation of such systems is non-trivial, however, due to the complex nature of the flow in the endwall region. Previous studies have reported that strong cross passage pressure gradients lead to migration of coolant flow and boundary layer flow within the passage. In addition the vane potential field effects lead to non-uniform blowing ratios for holes upstream of the vanes. It has also been reported that inlet total pressure and turbulence profiles have a significant effect on the development of the film cooling layer. In this study, endwall film cooling flows are studied experimentally in a large-scale low-speed cascade tunnel with engine-realistic combustor geometry and turbulence profiles. At very low blowing ratios mild cross-passage migration effects are observed. At higher blowing ratios more realistic of the engine situation no cross-passage migration is observed. This finding is somewhat contrary to the classical view of endwall secondary flow, which is presented as significant at the scale of the vane passage by several authors. The difference arises in part because of the thinning of the boundary layer due to strong acceleration in the vane inlet contraction. The findings are further supported by CFD simulations. Methods of improving conventional double-row systems to offer improved cooling of the endwall are also discussed.