Shichuan Ou

A Transient Infrared Thermography Method for Simultaneous Film Cooling Effectiveness and Heat Transfer Coefficient Measurements From a Single Test

In film cooling situations, there is a need to determine both local adiabatic wall temperature and heat transfer coefficient to fully assess the local heat flux into the surface. Typical film cooling situations are termed three temperature problems where the complex interaction between the jets and mainstream dictates the surface temperature. The coolant temperature is much cooler than the mainstream resulting in a mixed temperature in the film region downstream of injection. An infrared thermography technique using a transient surface temperature acquisition is described which determines both the heat transfer coefficient and film effectiveness (nondimensional adiabatic wall temperature) from a single test. Hot mainstream and cooler air injected through discrete holes are imposed suddenly on an ambient temperature surface and the wall temperature response is captured using infrared thermography. The wall temperature and the known mainstream and coolant temperatures are used to determine the two unknowns (the heat transfer coefficient and film effectiveness) at every point on the test surface. The advantage of this technique over existing techniques is the ability to obtain the information using a single transient test. Transient liquid crystal techniques have been one of the standard techniques for determining h and η for turbine film cooling for several years. Liquid crystal techniques do not account for nonuniform initial model temperatures while the transient IR technique measures the entire initial model distribution. The transient liquid crystal technique is very sensitive to the angle of illumination and view while the IR technique is not. The IR technique is more robust in being able to take measurements over a wider temperature range which improves the accuracy of h and η. The IR requires less intensive calibration than liquid crystal techniques. Results are presented for film cooling downstream of a single hole on a turbine blade leading edge model.

Effect of Jet Pulsation and Duty Cycle on Film Cooling From a Single Jet on a Leading Edge Model

The effect of jet pulsation and duty cycle on film effectiveness and heat transfer was investigated on a film hole located on the circular leading edge of a blunt body. A transient infrared technique was used to measure both heat transfer coefficients and film effectiveness from a single test. Detailed Frossling number and film effectiveness distributions were obtained for all flow conditions. Jet pulsing frequencies of 5 Hz, 10 Hz, and 20 Hz have been studied. The effect of duty cycle created by the valve opening and closing times was also set at different levels of 10%, 25%, 50%, and 75% of designated 100% fully open condition for different blowing ratios from 0.25 to 2.0. The combination of pulse frequency and duty cycle was investigated for different blowing ratios on a single leading edge hole located at 22 deg from geometric leading edge. Results indicate that higher effectiveness and lower heat transfer coefficients are obtained at the reduced blowing ratios, which result from reduced duty cycles. The effect of varying the pulsing frequency from 5 Hz to 20 Hz is not discernable beyond the level of experimental uncertainty. Effective blowing ratio due to lowering of the duty cycle at a given blowing ratio seems to play a more important role in combination with pulsing, which provides improved cooling effectiveness at lower heat transfer coefficients.

Shaped-Hole Film Cooling With Pulsed Secondary Flow

The effects of the coolant jet pulsing frequency (PF), duty cycle (DC), and hole shape geometry on heat transfer coefficient and film effectiveness were investigated with a film hole located on a semicircular leading edge test model with an afterbody. Cylindrical and diffusion-shaped holes located at 21.5° from the stagnation line were investigated. An infrared thermography technique with a single transient test was used to determine both the heat transfer coefficient and film effectiveness. Spanwise averaged heat transfer coefficient and film effectiveness were computed from the local values for all test conditions under the same Reynolds number (Re) of 60,000 and density ratio (DR) of 1.11. A dimensionless Frossling number (Fr) was used to represent the heat transfer coefficient. The effects of duty cycles of 50%, 75%, and 100% (continuous coolant) on film effectiveness and heat transfer coefficient were investigated at coolant jet pulsing frequencies of 5 Hertz (Hz) and 10 Hertz. The duty cycle and pulsing frequency were controlled by the opening and closing time settings of two synchronized pulsed valves. The blowing parameters investigated included continuous coolant at the blowing ratios (M) of 0.75, 1.00, 1.50 and 2.00. The subsequent pulsed cases for a combination of pulsing frequency and duty cycle were varied from the corresponding continuous case without changing the coolant flow rate (or blowing ratio) setting for a total of 40 cases for the shaped and cylindrical film holes. The shaped hole provides higher local film effectiveness values than the classical cylindrical hole when coolant flow is steady at M = 1.00. The higher local film effectiveness for the shaped hole was also observed for pulsed cases at M = 1.50 (Meff = 1.25) and M = 2.00 (Meff = 1.07) due to wider film spreading or coverage. The pulsed coolant cases provide higher spanwise averaged film effectiveness than the continuous coolant at M = 1.50 for both hole geometries. In contrast to the film effectiveness, the spanwise averaged Frossling numbers of pulsed coolant are lower compared to the continuous coolant for both hole shapes at the same blowing ratio. Combining the effects of heat transfer coefficient and film effectiveness, one can compute a relative heat load ratio to evaluate the performance of the film cooling. The pulsed coolant cases in general perform better than continuous coolant. The shaped hole geometry provides better film cooling performance than the cylindrical hole geometry for all blowing ratios including the continuous and the pulsed coolant cases studied.Copyright

Transient Liquid Crystal Measurement of Leading Edge Film Cooling Effectiveness and Heat Transfer With High Free Stream Turbulence

This paper studies the film effectiveness and heat transfer coefficients on a large scale symmetric circular leading edge with three rows of film holes. The film hole configuration focuses on a smaller injection angle of 20° and a larger hole pitch with respect to the hole diameter (P/d = 7.86). The study includes four blowing ratios (M = 1.0, 1.5, 2.0 and 2.5), two Reynolds numbers (Re = 30,000 and 60,000), and two free stream turbulence levels (approximately Tu = 1% and 20% depending on the Reynolds number). The method used to obtain the film cooling effectiveness and the heat transfer coefficient in the experiment is a transient liquid crystal technique. The distributions of film effectiveness and heat transfer coefficient are obtained with spatial resolutions of about 0.6 mm or 13% of the film cooling hole diameter. Results are presented for detailed and spanwise averaged values of film effectiveness and Frossling number. Blowing ratios investigated result in up to 2.8 times the lowest blowing ratio’s film effectiveness. Increasing the Reynolds number from 30,000 to 60,000 results in increasing the effectiveness by up to 55% at high turbulence. Turbulence intensity has up to a 60% attenuation on effectiveness between rows at Re = 30,000. The turbulence intensity has the same order of magnitude but opposite effect as Reynolds number, which also has the same order of magnitude effect as blowing ratio on the film effectiveness. A crossover from attenuation to improved film effectiveness after the second row of film holes is found for the high turbulence case as blowing ratio increases. The blowing ratio of two shows a spatial coupling of the stagnation row of film holes with the second row (21.5°) of film holes which results in the highest film effectiveness and also the highest Frossling numbers.Copyright