EFFECTS OF UPSTREAM STEP GEOMETRY ON AXISYMMETRIC CONVERGING VANE ENDWALL HEAT TRANSFER AND FILM COOLING AT TRANSONIC CONDITIONS

In real gas turbine engines, a gap/step interface commonly exits between upstream of the inlet guide vane endwall and combustor, called upstream endwall misalignment, due to the errors of assembly and the thermal expansion. This endwall misalignment, commonly being presented as the gap/step geometry with different heights, has a significant effect on the endwall heat transfer and film cooling coverage distributions. This paper presents a detailed experimental and numerical study on the effects of upstream endwall misalignment (step geometry) on the vane endwall heat transfer and film cooling in a transonic linear turbine vane passage. The experiment measurements were performed in a blowdown wind tunnel at simulated realistic gas turbine operating conditions (high inlet freestream turbulence level of 16 %, exit Mach number of 0.85 and exit Reynolds number of 1.7x10(6). Three types of upstream step geometry were tested at design blowing ratio (BR=2.5) for the same vane profile: I) baseline geometry with zero-step height of Delta H = 0 mm; II) forward-facing step geometry with negative step height of Delta H = -5 mm; III) backward-facing step geometry with positive step height of Delta H = 5 mm. The endwall thermal load and film cooling coverage distributions were measured using transient infrared thermography, being presented as endwall Nusselt number Nu and adiabatic film cooling effectiveness eta, respectively. Detailed comparisons of experiment measurements with numerical predictions were also presented and discussed for three types of upstream step configurations with with Delta H = -5, 0, 5 mm, respectively. The numerical simulations were performed by solving the steady-state Reynolds Averaged Navier Stokes (RANS) with Realizable k-epsilon turbulence model, based on the commercial CFD solver ANSYS Fluent v.15. The effects of upstream step geometry were numerically studied, at the same design blowing ratio BR = 2.5, by solving the endwall Nusselt number, film cooling effectiveness and secondary flow field for various upstream step heights: three forward-facing step heights (from -8 mm to -3 mm), a baseline step height (0 mm), and four backward-facing step heights (from 3 mm to 10 mm). The results show the upstream forward-facing step geometry is beneficial for the endwall thermal load and film cooling, though the improvement is weak for all step heights (less than 10% decrease in endwall heat transfer and less than 10% increase in endwall film cooling). However, the upstream backward-facing step geometry is pernicious for the endwall heat transfer and film cooling, and the influence increases with the increasing upstream backward-facing step height. The backward-facing step geometry obviously alters near endwall flow field, leading to an enhancement (up to 20%) in endwall heat transfer and significant reduction (up to 60%) in endwall film cooling effectiveness.