The Effect of Area Ratio Change Via Increased Hole Length for Shaped Film Cooling Holes With Constant Expansion Angles

Abstract

Shaped film cooling holes are used as a cooling technology in gas turbines to reduce metal temperatures and improve durability, and they generally consist of a small metering section connected to a diffuser that expands in one or more directions. The area ratio (AR) of these holes is defined as the area at the exit of the diffuser, divided by the area at the metering section. A larger AR increases the diffusion of the coolant momentum, leading to lower average momentum of the coolant jet at the exit of the hole and generally better cooling performance. Cooling holes with larger ARs are also more tolerant of high blowing ratio conditions, and the increased coolant diffusion typically better prevents jet lift-off from occurring. Higher ARs have traditionally been accomplished by increasing the expansion angle of the diffuser while keeping the overall length of the hole constant. The present study maintains the diffuser expansion angles and instead increases the length of the diffuser, which results in longer holes. Various ARs have been examined for two shaped holes: one with forward and lateral expansion angles of 7 deg (7-7-7 hole) and one with forward and lateral expansion angles of 12 deg (12-12-12 hole). Each hole shape was tested at numerous blowing ratios to capture trends across various flow rates. Adiabatic effectiveness measurements indicate that for the baseline 7-7-7 hole, a larger AR provides higher effectiveness, especially at higher blowing ratios. Measurements also indicate that for the 12-12-12 hole, a larger AR performs better at high blowing ratios but the hole experiences ingestion at low blowing ratios. Steady Reynolds-averaged Navier–Stokes simulations did not accurately predict the levels of adiabatic effectiveness, but did predict the trend of improving effectiveness with increasing AR for both hole shapes. Flowfield measurements with particle image velocimetry (PIV) were also performed at one downstream plane for a low and high AR case, and the results indicate an expected decrease in jet velocity due to a larger diffuser.

Uncertainty Quantification of the Effects of Small Manufacturing Deviations on Film Cooling: A Fan-Shaped Hole

“…With larger hole length-to-diameter ratio values, the hole exit area will be larger. According to a study by Haydt et al [21] on the effects of area ratio on the distributions of η for the baseline 7-7-7 fan-shaped hole, η increases with the increase of the ratio of the hole exit area value to hole entrance area value. The lateral diffusion of the cool air for the case with L/D = 6 was considerably stronger than that of the case with L/D = 3.5, which made the lateral coverage area larger, but the streamwise coverage length shorter for the case with L/D = 6 than that for the case with L/D = 3.5.…”

“…With larger hole length-to-diameter ratio values, the hole exit area will be larger. According to a study by Haydt et al [21] on the effects of area ratio on the distributions of η for the baseline 7-7-7 fan-shaped hole, η increases with the increase of the ratio of the hole exit area value to hole entrance area value. Therefore, it can be a reason why the hole with L/D = 6 features better cooling performance than that of the hole with L/D = 3.5 even with conical angle adaption.…”

Effect of Mainstream Velocity on the Optimization of a Fan-Shaped Film-Cooling Hole on a Flat Plate

“…Energies 2021, 14, 3573 2 of 15 Anderson et al [10] experimentally studied the effects that the mainstream condition (e.g., Reynolds number, Mach number, and boundary layer thickness) had on the FCE of the fan-shaped hole. Haydt et al [11] investigated the effects that the area ratio had on the FCE of the fan-shaped hole by changing the hole length with a constant forward expansion angle. Zhang et al [12] installed a vortex generator at the exit of the fan-shaped hole to enhance the FCE, and they showed that the area-averaged FCE was enhanced by 25.5% at a blowing ratio of 2.0 compared to the fan-shaped hole without the vortex generator.…”

Research on the Hole Length Ratio of Fan-Shaped Holes in Flat Plate Film Cooling

“…Gritsch et al, 2005 conducted experiments on the film cooling efficiency of three fan-shaped holes with L/D 7.5, 9.5, and 11.5, and the results showed that the average cooling efficiency of the three cases was not much different. Haydt et al, 2018 used computational fluid dynamics prediction and particle image velocimetry measurement to study the adiabatic film efficiency of 7-7-7 holes with different hole lengths. In this study, the expansion mode was maintained unchanged, and different hole lengths led to different area ratios.…”