Injection Hole Diameter Research Articles

The Influence of Curvature on Film Cooling Performance

The effects of injection rate and strength of curvature on film cooling performance of gas injected through a row of holes on a convex surface are studied. Comparisons are made to film cooling of concave and flat surfaces. Three different relative strengths of curvature (ratio of radius of curvature to radius of injection hole), two density ratios (0.95 and 2.0), and a wide range of blowing rates (0.3 to 2.7) are considered. A foreign gas injection technique (mass transfer analogy) is used. The strength of curvature was controlled by varying the injection hole diameter. At low blowing rates, film cooling is more effective on the convex surface than on a flat or a concave surface. The cross-stream pressure gradient present in curved flows tends to push the jet into the convex wall. As the injection rate is increased, normal and tangential jet momentum promote liftoff from the convex surface, thereby lowering performance. In contrast, previous studies show that on a concave surface, tangential jet momentum, flow instabilities, and blockage improve performance on a concave surface as blowing rate is increased.

Effects of Vortices With Different Circulations on Heat Transfer and Injectant Downstream of a Row of Film-Cooling Holes in a Turbulent Boundary Layer

Results are presented that illustrate the effects of single embedded longitudinal vortices on heat transfer and injectant downstream of a row of film-cooling holes in a turbulent boundary layer. Attention is focused on the changes resulting as circulation magnitudes of the vortices are varied from 0.0 to 0.15 m2/s. Mean temperature results are presented that show how injectant is distorted and redistributed by vortices, along with heat transfer measurements and mean velocity surveys. Injection hole diameter is 0.952 cm to give a ratio of vortex core diameter to hole diameter of about 1.5–1.6. The free-stream velocity is maintained at 10 m/s, and the blowing ratio is approximately 0.5. Film-cooling holes are oriented 30 deg with respect to the test surface. Stanton numbers are measured on a constant heat flux surface with a nondimensional temperature parameter of about 1.5. Two different situations are studied: one where the middle injection hole is beneath the vortex downwash, and one where the middle injection hole is beneath the vortex upwash. For both cases, vortex centers pass within 2.9–3.4 vortex core diameters of the centerline of the middle injection hole. To quantify the influences of the vortices on the injectant, two new parameters are introduced. S is defined as the ratio of vortex circulation to injection hole diameter times mean injection velocity. S1 is similarly defined except vortex core diameter replaces injection hole diameter. The perturbation to film injectant and local heat transfer is determined by the magnitudes of S and S1. When S is greater than 1–1.5 or when S1 is greater than 0.7–1.0, injectant is swept into the vortex upwash and above the vortex core by secondary flows, and Stanton number data show evidence of injectant beneath the vortex core and downwash near the wall for x/d only up to about 17.5. For larger x/d, local hot spots are present, and the vortices cause local Stanton numbers to be augmented by as much as 25 percent in the film-cooled boundary layers. When S and S1 are less than these values, some injectant remains near the wall beneath the vortex core and downwash where it continues to provide some thermal protection. In some cases, the protection provided by film cooling is augmented because of vortex secondary flows, which cause extra injectant to accumulate near upwash regions.

Effects of an Embedded Vortex on Injectant From a Single Film-Cooling Hole in a Turbulent Boundary Layer

Effects of embedded longitudinal vortices on heat transfer in turbulent boundary layers with injection from a single film-cooling hole are described. These results were obtained at a free-stream velocity of 10 m/s, with a film-cooling hole inclined 30 deg to the horizontal and a blowing ratio of about 0.50. The ratio of vortex core diameter to injection hole diameter was 1.58, and the ratio of circulation to injection velocity time hole diameter was about 3.16. Coolant distributions and spatially resolved heat transfer measurements indicate that injection hole centerlines must be at least 2.9–3.4 vortex core diameters away from the vortex center in the lateral direction to avoid significant alterations to wall heat transfer and distributions of film coolant. Under these circumstances, protection from film cooling is evident at least up to 55 hole diameters downstream of injection. When the injection hole is closer to the vortex center, secondary flows convect most injectant into the vortex upwash and thermal protection from film cooling is destroyed for streamwise locations from the injection hole greater than 17.5 hole diameters.

The Two-Dimensional Behavior of Film Cooling Jets on Concave Surfaces

Local impermeable wall effectiveness is measured along a concave surface downstream of a row of film cooling jets. Three different injection hole diameters, two density ratios (0.95 and 2.0), and a wide range of blowing rates (0.3 to 2.7) are considered. Except close to injection, where normal momentum is strong, an increase in blowing rate results in improved film cooling performance. This is attributed to an increase in thermal mass (capacity) of coolant, tangential momentum of the jet, and blockage. Far downstream of injection, the normal and tangential momentum weaken, but blockage and large thermal mass at high injection rates still keep effectiveness high. Lateral mixing of the jets caused by the unstable concave mainflow results in film-cooling performance, which correlates well with the same parameter, which is influential in slot injection on a flat surface. Deviation from this two-dimensional behavior is found only at very low blowing rates and weak wall curvature.