Full-Coverage Film Cooling: Film Effectiveness and Heat Transfer Coefficients for Dense and Sparse Hole Arrays at Different Blowing Ratios

Abstract

Experimental results are presented for a full coverage film cooling arrangement which simulates a portion of a gas turbine engine, with appropriate streamwise static pressure gradient and varying blowing ratio along the length of the contraction passage which contains the cooling hole arrangement. Film cooling holes are sharp-edged, streamwise inclined at 20 deg with respect to the liner surface, and are arranged with a length to diameter ratio of 8.35. The film cooling holes in adjacent streamwise rows are staggered with respect to each other. Data are provided for turbulent film cooling, contraction ratios of 1 and 4, blowing ratios (at the test section entrance) of 2.0, 5.0, and 10.0, coolant Reynolds numbers Refc from 10,000 to 12,000 (for a blowing ratio of 5.0), freestream temperatures from 75 °C to 115 °C, a film hole diameter of 7 mm, and density ratios from 1.15 to 1.25. Changes to X/D and Y/D, nondimensional streamwise and spanwise film cooling hole spacings, with Y/D of 3, 5, and 7, and with X/D of 6 and 18, are considered. For all X/D=6 hole spacings, only a slight increase in effectiveness (local, line-averaged, and spatially-averaged) values are present as the blowing ratio increases from 2.0 to 5.0, with no significant differences when the blowing ratio increases from 5.0 to 10.0. This lack of dependence on blowing ratio indicates a condition where excess coolant is injected into the mainstream flow, a situation not evidenced by data obtained with the X/D=18 hole spacing arrangement. With this sparse array configuration, local and spatially-averaged effectiveness generally increase continually as the blowing ratio becomes larger. Line-averaged and spatially-averaged heat transfer coefficients are generally higher at each streamwise location, also with larger variations with streamwise development, with the X/D=6 hole array, compared to the X/D=18 array.