Film Cooling Effectiveness and Heat Transfer Coefficient Distributions Around Diffusion Shaped Holes

Abstract

Presented in this paper is an experimental study focusing on the effects of diffusion hole-geometry on overall film cooling performance. The study consists of three different but closely related hole shapes: (1) Shape A: straight circular hole with a 30 deg inclined injection, (2) Shape B: same as Shape A but with a 10 deg forward diffusion, and (3) Shape C: same as Shape B with an additional 10 deg lateral diffusion. The blowing ratios tested are 0.5 and 1.0. The density ratio is nominally equal to one. Measurements of the experiment use a transient liquid crystal technique that reveals local distributions of both film effectiveness (η) and heat transfer coefficient (h). The data obtained indicate that Shape C with a combined forward and lateral diffusion produces a significant increase in η and decrease in h as compared to Shape A, the baseline case. These improvements combined yield an about 20 percent to 30 percent reduction in heat transfer or thermal load on the film protected surface. Shape B, with forward diffusion only, shows a much less significant change in both film effectiveness and overall heat transfer reduction than Shape C. However, it has the lowest heat transfer coefficient in the vicinity of the injection hole among all the three hole-shapes studied. A flow visualization study using pulsed laser sheet-light reveals that Shape A and Shape B inherit quite similar flow structures. The coolant injected out of Shape C flows much closer to the protected wall than that of Shape A and Shape B.

Experimental research on the thermal performance of converging slot holes with different divergence angles

“…The study of Goldstein et al [2], which is recognized as the first to demonstrate and qualify the film cooling effects due to shaped holes, reported a significant increase in film cooling effectiveness in the near hole region as well as greater lateral coolant coverage for fan-shaped holes compared with standard cylindrical holes. The improvement in film cooling effectiveness brought by the expansion shaped holes has also been reported by Schmidt et al [3], Gritsch et al [4] and Yu et al [5]. All these studies attributed the improvement primarily to the reduction in momentum flux ratio at the exit of the hole causing a decreased penetration of the jet into the mainstream.…”

“…In the former work including Vedula and Metzger [21], Drost et al [22], Yu et al [5] and Chambers et al [23], the initial wall temperature was always considered as constant. In the present measurement, the initial wall temperature is measured and approximated by power series of 3rd order:…”

“…According to Thole et al [6], expansion shaped holes produce less jet penetration, reduced velocity gradients, and lower turbulence production relative to cylindrical holes. Heat transfer coefficient measurements of expansion shaped holes were carried out by Makki and Jakubowski [7], Sen et al [8], Gritsch et al [9] and Yu et al [5]. They found that expansion shaped holes generally produce lower spatially averaged heat transfer coefficient ratios than cylindrical holes.…”

Film cooling on a gas turbine blade pressure side or suction side with axial shaped holes

“…As compared to the cylindrical hole, the shaped holes present significantly improved thermal protection on the surface downstream of the ejection location, particularly at high blowing ratios. Along similar lines, Yu et al [4] studied the film cooling effectiveness and heat transfer distributions on a flat plate with cylindrical holes with 10°forward diffusion, and an additional 10°lateral diffusion. In each case, the holes were inclined 30°to the surface.…”

Flow characteristics in an annular burner with fully film cooling

“…The cooling air is injected through small discrete holes into the tube internal boundary layer, forming a protective film on the surface. Different kinds of film-cooling technologies have been widely employed in modern gas turbine combustors and were extensively studied in the past 30 years [8,[12][13][14][15][16]. However, the most published investigations only modeled the flow within the combustor liner or just concentrated on flat or curve plates with film injection through slots or rows of cylindrical holes.…”