Influences of Particulate Deposition on Vane Trailing-Edge Film Cooling: Adiabatic and Overall Cooling Effectiveness

Abstract

Thermal design for the trailing edge of a turbine vane is critically challenging because the thinnest trailing-edge region has a low-thermal capacity, resulting in a much quicker thermal response to the hot gas. Cooling air for this region is usually extracted from the internal cooling passages inside the vane main body and thus the post-cooling air has a relatively weaker cooling potential. The thermal mismatch in the metal can be further exacerbated by the deposition of fine contaminant particulates resulting from the intake air as well as the unburned hydrocarbon fuel. Changes of the surface conditions caused by deposition are able to affect film cooling and to enhance heat transfer substantially. However, the deposition layer has low-thermal conductivity (approximately 0.1 W · m−1 · K−1), which thermally insulates the wall from the hot gas. The presence of deposition thereby further complicates the heat convection and conduction issues in this area. The current study had the uniqueness of evaluating deposition effects on vane trailing-edge film cooling by conducting deposition simulation experiments, where a scaled-up vane model featured cutback slots with a thick lip on the pressure surface. The coupling effects of deposition and the slot discharge coolant were investigated by examining the dynamic evolution of deposition and flow mixing of the coolant with the mainstream flows for various slot discharge ratios. The resulting film cooling performance was further assessed in adiabatic and conjugate heat transfer conditions to document the deposition-induced changes of heat/mass transfer and conduction. The thick slot lip and higher discharge ratios were found to reduce deposition, but simultaneously, to generate stronger flow mixing. Deposition on the pressure-surface degraded adiabatic cooling effectiveness regardless of deposition thickness, with a maximum reduction of 35%, whereas thicker deposition on the cutback surface was favorable to film cooling at the highest discharge ratio. The insulation effects of the deposition layer were not able to erase out the reduction in adiabatic effectiveness and potentially increased heat transfer levels over the rough deposition surface, resulting in lower overall cooling effectiveness. The reduction in overall effectiveness, however, was alleviated by increasing the discharge ratio.