A Novel Technique for Assessing Turbine Cooling System Performance

Abstract

A new experimental technique for the accurate measurement of steady-state metal temperature surface distributions of modern heavily film-cooled turbine vanes has been developed and is described in this paper. The technique is analogous to the thermal paint test but has been designed for fundamental research. The experimental facility consists of an annular sector cascade of high pressure (HP) turbine vanes from a current production engine. Flow conditioning is achieved by using an annular sector of deswirl vanes downstream of the test section, being both connected by a three-dimensionally contoured duct. As a result, a transonic and periodic flow, highly representative of the engine aerodynamic field, is established: Inlet turbulence levels, mainstream Mach and Reynolds numbers, and coolant-to-mainstream total pressure ratio are matched. Since the fully three-dimensional nozzle guide vane (NGV) geometry is used, the correct radial pressure gradient and secondary flow development are simulated and the cooling flow redistribution is engine-realistic. To allow heat transfer measurements to be performed, a mainstream-to-coolant temperature difference (up to 33.5°C) is generated by using two steel-wire mesh heaters, operated in series. NGV surface metal temperatures are measured (between 20°C and 40°C) by wide-band thermochromic liquid crystals. These are calibrated in situ and on a per-pixel basis against vane surface thermocouples, in a heating process that spans the entire color play and during which the turbine vanes can be assumed to slowly follow a succession of isothermal states. Experimental surface distributions of overall cooling effectiveness are presented in this paper. By employing resin vanes of the same geometry and cooling configuration (to implement adiabatic wall thermal boundary conditions) and the transient liquid crystal technique, surface distributions of external heat transfer coefficient and film cooling effectiveness can be acquired. By combining these measurements with those from the metal vanes, the results can be scaled to engine conditions with a good level of accuracy.

Methods to Extract Underlying Boundary Conditions from Transient Metal Effectiveness Measurements

“…R ESEARCH investment in metal effectiveness (or overall cooling effectiveness) measurement techniques has been driven by the desire to accurately assess the overall thermal performance of nozzle guide vanes or turbine blades at engine-realistic conditions [1][2][3][4][5][6]. The traditional approach of predicting overall thermal performance of such parts from a thermal model with boundary conditions obtained from separate experiments [7][8][9][10] has the disadvantage that errors in underlying measurements can accumulate in the final result.…”

“…For given boundary conditions on each side of the wall, the transient thermal response of the wall is solved using a finite difference approach to approximate the temporal and spatial derivatives of the one-dimensional heat equation [Eq. (2)]. The solving methodology was the one proposed by Bergman et al [11].…”

“…3. The Annular Sector Heat Transfer facility [2] and the Engine Component Aerothermal facility [5] (facilities performing metal effectiveness measurements at the University of Oxford) can achieve external flow temperature rises of time-constant ratios (τ rise ∕τ w ) of 0.25 and 15, respectively.…”

Calibrated Low-Order Transient Thermal and Flow Models for Robust Test Facility Design

“…The ECAT facility warm-core was developed from earlier facilities already at a high level of maturity, e.g. (Luque and Povey, 2011;Povey et al, 2011;. Unusually for an ambitious facility build, both the ECAT facility test-bed, the warm-core, and a number of associated research programmes were delivered on time and within budget.…”

Experimental Aerothermal Performance of Turbofan Bypass Flow Heat Exchangers

“…Vogt and Fransson [41] applied also annular sector cascade testing to aeromechanical research by employing semi-flexible inlet and outlet ducting sidewalls which allowed some controlled deformation in order to maintain the periodicity and radial pressure gradients established in the test section. A different approach to establish the back pressure boundary conditions in the test section on a transonic annular cascade was employed by Povey et al [42], for assessing turbine cooling system performances [43]. They developed an annular sector cascade of high pressure turbine vanes, where flow conditioning was achieved by an annular sector of deswirl vanes downstream of the test section.…”