Influence of Film Cooling Unsteadiness on Turbine Blade Leading Edge Heat Flux

Abstract

Film cooling in the hot gas path of a gas turbine engine can protect components from the high temperature main flow, but it generally increases the heat transfer coefficient h partially offsetting the benefits in reduced adiabatic wall temperature. We are thus interested in adiabatic effectiveness η and h which are combined in a formulation called net heat flux reduction (NHFR). Unsteadiness in coolant flow may arise due to inherent unsteadiness in the external flow or be intentionally introduced for flow control. In previous work it has been suggested that pulsed cooling flow may, in fact, offer benefits over steady blowing in either improving NHFR or reducing the mass flow requirements for matched NHFR. In this paper we examine this hypothesis for a range of steady and pulsed blowing conditions. We use a new experimental technique to analyze unsteady film cooling on a semicircular cylinder simulating the leading edge of a turbine blade. The average NHFR with pulsed and steady film cooling is measured and compared for a single coolant hole located 21.5° downstream from the leading edge stagnation line, angled 20° to the surface and 90° to the streamwise direction. We show that for moderate blowing ratios at blade passing frequencies, steady film flow yields better NHFR. At higher coolant flow rates beyond the optimum steady blowing ratio, however, pulsed film cooling can be advantageous. We present and demonstrate a prediction technique for unsteady blowing at frequencies similar to the blade passing frequency that only requires the knowledge of steady flow behavior. With this important result, it is possible to predict when pulsing would be beneficial or detrimental.

CFD Evaluations of Film Cooling Flow Scaling Between Engine and Experimental Conditions

“…The coolant hole inlet boundary condition was modeled to have a turbulence intensity of Tu = 1% and a length scale of Λ / d = 0.42. These turbulence characteristics are similar to the turbulence levels of the wind tunnel on which the computational model was based [10].…”

“…6 shows the  results spanwise averaged over the region -3.93 ≤ y/d ≤ 3.93 (similar to a case with a pitch spacing of 7.86 d between the holes) compared to the corresponding cases using the experimental methodology on an identical geometry and described in Ref. 10. The spanwise averaged CFD data shown in Fig.…”

“…The leading edge to coolant hole diameter ratio was D/d = 18.7 and the coolant hole length to diameter ratio was L/d = 11.69, and was located in the middle of the computational model. One computational model had a leading edge diameter of D = 8.89 cm, matching that of experimental studies [10] that were designed to be approximately 10x scale. This computational model was scaled down 10x to create a second model to be used for engine condition simulations.…”

Flow structures and thermal field with modulated jet near the semi-circular leading edge

“…In contrary, Ekkad et al 13 reported through measurements that pulsed jet enhanced by lowering heat transfer coefficient as compared to steady jet, while considering a leading-edge model with a single film cooling hole. Heat flux measurements by Rutledge et al 14 also elucidated that the net heat flux was normally increased by pulsed film cooling. However, it could be advantageous at higher blowing ratio beyond the optimum value of its steady operation.…”

Numerical investigations of pulsed film cooling on an entire turbine vane

“…Rutledge and King [10] conducted a computational study to determine how the coolant pulse affects film cooling performance in LE region, and they thought that the poor performance of pulsed cooling occurs because the jet necessarily spends some time operating at an inefficient blowing rate. Then Rutledge et al [11] experimentally investigated net heat flux reduction (NHFR) on semicircular cylinder simulating the LE of a turbine blade, and found that for moderate BRs, a steady film flow yields better NHFR, while for higher coolant flow rates beyond the optimum BR, pulsed film cooling is better.…”