A misalignment between the combustor exit and the nozzle guide vane (NGV) platform commonly exists due to manufacturing tolerances and thermal transience. This study experimentally investigated the effect of the combustor-turbine misalignment on the heat transfer for an axisymmetric converging endwall with a jet purge cooling scheme at transonic conditions. Tests were conducted at engine-representative freestream exit Mach number of 0.85, inlet turbulence intensity of 16%, film cooling blowing ratio of 2.5 (design condition) and 3.5, and density ratio of 1.95. Three different step misalignments, combustor exit 4.9% span higher than turbine inlet (backward-facing), no step (baseline), and combustor exit 4.9% span lower than turbine inlet (forward-facing), were tested to demonstrate the misalignment effect on endwall Nusselt number, adiabatic film cooling effectiveness, and net heat flux reduction. A supportive numerical simulation was conducted to provide insight into the flow field.
Experimental and numerical results indicated a blowing ratio of 2.5 amid a backward-facing step leads to more turbulent mixing between the coolant and mainstream due to the horseshoe vortex and step-induced cavity vortex, reducing coolant coverage and adversely affecting coolant performance. For the same blowing ratio, a forward-facing step induced more lift-off and shifted the coverage downstream and toward the pressure side, ultimately slightly enhancing the cooling performance. Nevertheless, the forward-facing step leads to a large cooling effectiveness gradient in the pitch-wise direction. At a blowing ratio of 3.5, the baseline no-step case provided superior coolant performance, however, the effect of the step was much less significant as compared to the lower blowing case. The forward-facing step amid a blowing ratio of 3.5 results in severe jet lift-off, diminishing the returns traditionally associated with increasing coolant mass flow. Based on this study, a backward-facing step should be avoided to prevent endwall burnout in the nozzle guide. Additionally, a forward-facing step should be avoided, especially amid high blowing ratios.