Film cooling is widely used to protect modern gas turbine blades and vanes from the ever increasing inlet temperatures. Film cooling involves a very complex turbulent flow-field, the characterization of which is necessary for reliable and economical design. Several experimental studies have focused on gas turbine blade, vane and end-wall film cooling over the past few decades. Measurements of heat transfer coefficients, film cooling effectiveness values and heat flux ratios using several different experimental methods have been reported. The emphasis of this current review is on the Pressure Sensitive Paint (PSP) mass transfer analogy to determine the film cooling effectiveness. The theoretical basis of the method is presented in detail. Important results in the open literature obtained using the PSP method are presented, discussing parametric effects of blowing ratio, momentum ratio, density ratio, hole shape, surface geometry, free-stream turbulence on flat plates, turbine blades, vanes and end-walls. The PSP method provides very high resolution contours of film cooling effectiveness, without being subject to the conduction error in high thermal gradient regions near the hole.

Adiabatic film-cooling effectiveness is examined on a high-pressure turbine blade by varying three critical engine parameters, viz., coolant blowing ratio, coolant-to-mainstream density ratio, and freestream turbulence intensity. Three average coolant blowing ratios (BR=1.2, 1.7, and 2.2 on the pressure side and BR=1.1, 1.4, and 1.8 on the suction side), three average coolant density ratios (DR=1.0, 1.5, and 2.5), and two average freestream turbulence intensities (Tu=4.2% and 10.5%) are considered. Conduction-free pressure sensitive paint (PSP) technique is adopted to measure film-cooling effectiveness. Three foreign gases—N2 for low density, CO2 for medium density, and a mixture of SF6 and argon for high density are selected to study the effect of coolant density. The test blade features two rows of cylindrical film-cooling holes on the suction side (45 deg compound), 4 rows on the pressure side (45 deg compound) and 3 around the leading edge (30 deg radial). The inlet and the exit Mach numbers are 0.24 and 0.44, respectively. The Reynolds number of the mainstream flow is 7.5×105 based on the exit velocity and blade chord length. Results suggest that the PSP is a powerful technique capable of producing clear and detailed film-effectiveness contours with diverse foreign gases. Large improvement on the pressure side and moderate improvement on the suction side effectiveness is witnessed when blowing ratio is raised from 1.2 to 1.7 and 1.1 to 1.4, respectively. No major improvement is seen thereafter with the downstream half of the suction side showing drop in effectiveness. The effect of increasing coolant density is to increase effectiveness everywhere on the pressure surface and suction surface except for the small region on the suction side, xss/Cx<0.2. Higher freestream turbulence causes effectiveness to drop everywhere except in the region downstream of the suction side where significant improvement in effectiveness is seen.

Experiments to determine heat transfer coefficients and friction factors are conducted on a stationary 45 deg parallel rib-roughened square channel, which simulates a turbine blade internal coolant passage. Copper plates fitted with silicone heaters and thermocouples are used to measure regionally averaged heat transfer coefficients. Reynolds numbers studied range from 30,000 to 400,000. The ribs studied have rounded (filleted) edges to account for manufacturing limitations of actual engine blades. The rib height (e) to hydraulic diameter (D) ratio (e/D) ranges from 0.1 to 0.2, while spacing (p) to height ratio (p/e) ranges from 5 to 10. Results indicate an increase in the heat transfer due to the ribs at the cost of a higher friction factor, especially at higher Reynolds numbers. Round-edged ribs experience a similar heat transfer coefficient and a lower friction factor compared with sharp-edged ribs, especially at higher values of the rib height. Correlations predicting Nu and f as a function of e/D, p/e, and Re are presented. Also presented are correlations for the heat transfer and friction roughness parameters (G and R, respectively).

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The effect of a step positioned upstream of a row of film cooling holes on the film cooling effectiveness is studied systematically using the steady state Pressure Sensitive Paint (PSP) technique. The upstream step effect is studied on four separate hole geometries: simple angled (axial angle 30°) and compound angled (axial angle 30°, compound angle 45°) cylindrical and fan-shaped film cooling holes. Each plate considered has 7 holes, each hole 4mm in diameter. The plates with cylindrical holes have a spacing of 3 diameters (12mm) between the centers of two consecutive holes, while the fan-shaped holes have a spacing of 3.75d (15mm). Three different step heights (12.5%d, 25%d and 37.5%d) are studied. Also studied is the effect of the width of the step; the distance of the step upstream of the hole and the positioning of the step downstream of the film-cooling hole. Four separate blowing ratios are reported for all tests: M = 0.3, M = 0.6, M = 1.0 and M = 1.5. All studies have been conducted with a mainstream of 25m/s velocity at an ambient temperature of 22C. Results indicate an increase in film-cooling effectiveness in the region near the hole due to the upstream step for all the plates considered. This increase due to the step is found to be most significant in the case of compound angled cylindrical holes and least significant in the case of simple angled fan-shaped holes.