Mike Fox

Separate Effects of Mach Number and Reynolds Number on Jet Array Impingement Heat Transfer

Limited available data suggest a substantial impact of Mach number on the heat transfer from an array of jets impinging on a surface at fixed Reynolds number. Many jet array heat transfer correlations currently in use are based on tests in which the jet Reynolds number was varied by varying the jet Mach number. Hence, this data may be inaccurate for high Mach numbers. Results from the present study are new and innovative because they separate the effects of jet Reynolds number and jet Mach number for the purposes of validating and improving correlations that are currently in use. The present study provides new data on the separate effects of Reynolds number and Mach number for an array of impinging jets in the form of discharge coefficients, local and spatially averaged Nusselt numbers, and local and spatially averaged recovery factors. The data are unique because data are given for impingement jet Mach numbers as high as 0.60 and impingement jet Reynolds numbers as high as 60,000, and because the effects of Reynolds number and Mach number are separated by providing data at constant Reynolds number because the Mach number is varied, and data at constant Mach number because the Reynolds number is varied. As such, the present data are given for experimental conditions not previously examined, which are outside the range of applicability of current correlations.

Effect of Temperature Ratio on Jet Array Impingement Heat Transfer

This paper consider the effects of temperature ratio on the heat transfer from an array of jets impinging on a flat plate. At a constant Reynolds number of 18,000 and a constant Mach number of 0.2, different ratios of target plate temperature to jet temperature are employed. The spacing between holes in the streamwise direction X is 8D, and the spanwise spacing between holes in a given streamwise row Y is also 8D. The target plate is located 3D away from the impingement hole exits. Experimental results show that local, line-averaged, and spatially averaged Nusselt numbers decrease as the T wa /T j temperature ratio increases. This is believed to be due to the effects of temperature-dependent fluid properties, as they affect local and global turbulent transport in the flow field created by the array of impinging jets. The effect of temperature ratio on crossflow-to-jet mass velocity ratio and discharge coefficients is also examined.

Mach Number, Reynolds Number, Jet Spacing Variations: Full Array of Impinging Jets

Presented are data that illustrate the effects of Mach number, Reynolds number, and hole spacing on surface Nusselt numbers produced by an array of jets impinging on a flat plate. Considered are Reynolds numbers ranging from 17,300 to 60,000 and Mach numbers from 0.1 to 0.45. Impingement hole spacings are 5D, 8D, and 12D in the streamwise and spanwise directions. Local spatially resolved and spatially averaged Nusselt numbers, measured using infrared thermography and energy balance techniques, show strong dependence on the impingement jet Reynolds number for each situation as the jet Mach number is maintained constant. Nusselt numbers show negligible variations between Ma = 0.1 and 0.2; however, data taken at Mach numbers greater than 0.2 (as the Reynolds number is held constant) show that Mach number has a significant impact on local and spatially averaged Nusselt numbers. This Mach number dependence changes with hole spacing, with greater Nusselt number increases with the less dense impingement arrays. These variations are described using new correlations, which account for the effects of Mach number for all three impingement hole spacings.

Effect of Hole Spacing on Jet Array Impingement Heat Transfer

Data which illustrate the effects of hole spacing on the heat transfer from an array of jets impinging on a flat plate are presented. Considered are Reynolds numbers ranging from 8200, to 30500, and Mach numbers from 0.1 to 0.2. The spacing of the holes used to produce the impinging jets is either 8D or 12D in both the streamwise and spanwise directions. Local and spatially-averaged Nusselt numbers show strong dependence on the impingement jet Reynolds number for both situations. Experimental results show that local Nusselt numbers show some dependence on the Mach number for the smaller jet hole spacing, with negligible dependence for the larger jet hole spacing. This is partially a result of the accumulating cross-flows produced by the jets, as well as the interactions of the vortex structures which initially form around the jets, and then impact and interact as they advect away from stagnation points along the impingement target surface. Spatially-averaged Nusselt numbers generally decrease as x/D increases when hole spacing is 8D, whereas Nusselt numbers are generally about constant as x/D increases when hole spacing is 12D. This is partially due to cross-flow effects, as well as behavior of each jet in the array, which is similar to that of a single, isolated jet for the larger hole spacing. Spatially-averaged Nusselt numbers for 8D jet hole spacing are also often higher than values for the 12D jet hole spacing when compared at the same x/D location.Copyright

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

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. The test surface utilizes varying blowing ratio along the length of the contraction passage which contains the cooling hole arrangement. For the different experimental conditions examined, film cooling holes are sharp-edged and streamwise inclined either at 20° or 30° with respect to the liner surface. 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, 3, 4, and 5, blowing ratios (at the test section entrance) of 2.0, 5.0, and 10.0, coolant Reynolds numbers Refc of 10,000 to 12,000, freestream temperatures from 75°C to 115°C, a film hole diameter of 7 mm, and density ratios from 1.15 to 1.25. Non-dimensional streamwise and spanwise film cooling hole spacings, X/D and Y/D, are 6, and 5, respectively. When the streamwise hole inclination angle is 20°, spatially-averaged and line-averaged adiabatic effectiveness values at each x/D location are about the same as the contraction ratio varies between 1, 3, and 4, with slightly higher values at each x/D location when the contraction ratio Cr is 5. For each contraction ratio, there is a slight increase in effectiveness when the blowing ratio is increased from 2.0 to 5.0 but there is no further substantial improvement when the blowing ratio is increased to 10.0. Overall, line-averaged and spatially-averaged adiabatic film effectiveness data, and spatially-averaged heat transfer coefficient data are described as they are affected by contraction ratio, blowing ratio, hole angle α, and streamwise location x/D. For example, when α = 20°, the detrimental effects of mainstream acceleration are apparent since heat transfer coefficients for contraction ratios Cr of 3 and 5 are often higher than values for Cr = 1, especially for x/D > 100.Copyright