Experimental and CFD Studies of NGV Endwall Cooling

Abstract

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.

A novel scalar tracking method for optimising film cooling systems

“…Thomas et al, 1 describe experimental validation of the CFD methods presented in this study: experimental maps of endwall eff obtained at a number of coolant to mainstream mass flux ratios, showing good agreement with CFD predictions. Implementation in FLUENT and validation.…”

“…A two-passage CFD domain was used to simulate flow through a test cascade. The geometry of the cascade is that used in a large-scale platform cooling test facility in Oxford University, 1 which includes a linearised NGV geometry from a recent commercial engine. Geometric details of the cascade are provided in Table 1.…”

“…The simulations described in this study were performed at test facility representative flow conditions. 1 The boundary conditions used are given in Table 3. Adiabatic conditions were prescribed on all wall surfaces.…”

“…Thomas et al 1 studied a double-row endwall cooling system with holes upstream of the vanes. Experiments and CFD were used to show that coolant injection altered near-wall flow patterns.…”

Influence of different film cooling arrangements on endwall cooling

“…Takeishi et al [8] studied the influence of secondary flows in the passage on endwall film cooling and found that the endwall film cooling jets were deflected from the pressure side to the suction side. Thomas et al [9] conducted experimental and numerical investigation of 1st stage vane endwall film cooling and found that due to the complex vortex and effect of pressure gradient, the areas close to leading edge and pressure side as well as the region around trailing edge were difficult to be cooled. Wright et al [10] showed that the vortices formed upstream of the blade cascade would greatly reduce the endwall film cooling effectiveness.…”