The research team controlled coolant pulsation by adjusting the opening and closing times of two synchronized pulsed valves, installed in the coolant supply line, to create a combination of PF and DC. The investigation comprised respective DCs of 50%, 75%, and 100% (continuous coolant) with PFs of 5 and 10 Hz. DC is the ratio of a valve’s open time to its total time in the open-close cycle, expressed as a percentage value. Therefore, for a PF of 10 Hz and a DC of 75%, for example, the pulsed valve is open for 0.075 sec and closed for 0.025 sec for each cycle of the 10 Hz pulse.

To conduct the study, the team used hot mainstream air and injected the cooler, coolant air through a cylindrical hole in the test model. They released both flows onto the ambient-temperature surface of the model simultaneously in a transient mode. The researchers performed all tests at Reynolds number (Re) 60,000, based on the model’s leading edge diameter (D). They executed the test condition matrix by setting the continuous coolant at one of four blowing ratios (M): 0.75, 1.00, 1.50, or 2.00—where M is the mass flow rate per unit area for the jet, divided by the mass flow rate per unit area of the freestream—and subsequently generated the pulsed cases by modifying the DC and PF without changing the coolant flow rate setting.

The researchers employed the infrared imaging system to capture the response of the model’s surface temperature. Using both the measured surface temperatures and the mainstream and coolant temperature histories, they determined the local heat transfer coefficient and film effectiveness at every point of the imaging field (320 x 240 pixels). Next, the researchers calculated a dimensionless Frossling number (hD/k/Re0.5, where k is the thermal conductivity of the fluid studied) to represent the heat transfer coefficient. Film effectiveness is the temperature difference between the mainstream flow and the model wall, divided by the temperature difference between the mainstream and the cooling stream. The goal of film cooling is to obtain a higher film effectiveness (to better protect the turbine blade/vane) and a lower heat transfer coefficient (to prevent the hot-gas stream from transferring heat to the turbine blade/vane).