The thermal management of aero gas turbine engine combustion systems commonly employs effusion-cooling in combination with various cold-side convective cooling schemes. The combustor liner incorporates many small holes which are usually set in staggered arrays and at a shallow angle to the cooled surface; relatively cold compressor delivery air is then allowed to flow through these holes to provide the full-coverage film-cooling effect. The efficient design of such systems requires robust correlations of film-cooling effectiveness and heat transfer coefficient at a range of aero-thermal conditions, and the use of appropriately validated computational models. However, the flow conditions within a combustor are characterised by particularly high turbulence levels and relatively large length scales. The experimental evidence for performance of effusion-cooling under such flow conditions is currently sparse. The work reported here is aimed at quantifying typical effusion-cooling performance at a range of combustor relevant free-stream conditions (high turbulence), and also to assess the importance of modeling the coolant to free-stream density ratio. Details of a new laboratory wind-tunnel facility for the investigation of film-cooling at high turbulence levels are reported. For a typical combustor effusion geometry that uses cylindrical holes, spatially resolved measurements of adiabatic effectiveness, heat transfer coefficient and net heat flux reduction are presented for a range of blowing ratios (0.48 to 2), free-stream turbulence conditions (4 and 22%) and density ratios (0.97 and 1.47). The measurements reveal that elevated free-stream turbulence impacts on both the adiabatic effectiveness and heat transfer coefficient, although this is dependent upon the blowing ratio being employed and particularly the extent to which the coolant jets detach from the surface. At low blowing ratios the presence of high turbulence levels causes increased lateral spreading of the coolant adjacent to the injection points, but more rapid degradation in the downstream direction. At high blowing ratios, high turbulence levels cause a modest increase in effectiveness due to turbulent transport of the detached coolant fluid. Additionally, the augmentation of heat transfer coefficient caused by the coolant injection is seen to be increased at high free-stream turbulence levels.
This paper investigates heat transfer in a rotating disk system using preswirled cooling air from nozzles at high and low radius. The experiments were conducted over a range of rotational speeds, flow rates, and preswirl ratios. Narrow-band thermochromic liquid crystal (TLC) was specifically calibrated for application to experiments on a disk, rotating at ∼5000 rpm and subsequently used to measure surface temperature in a transient experiment. The TLC was viewed through the transparent polycarbonate disk using a digital video camera and strobe light synchronized to the disk frequency. The convective heat transfer coefficient h was subsequently calculated from the one-dimensional solution of Fourier's conduction equation for a semi-infinite wall. The analysis was accounted for the exponential rise in the air temperature driving the heat transfer, and for the experimental uncertainties in the measured values of h. The experimental data was supported by “flow visualization,” determined from CFD. Two heat transfer regimes were revealed for the low-radius preswirl system: a viscous regime at relatively low coolant flow rates, and an inertial regime at higher flow rates. Both regimes featured regions of high heat transfer where thin, boundary layers replaced air exiting through receiver holes at high radius on the rotating disk. The heat transfer in the high-radius preswirl system was shown to be dominated by impingement under the flow conditions tested.
This paper investigates heat transfer in a rotating disc system using pre-swirled cooling air from nozzles at high and low radius. The experiments were conducted over a range of rotational speeds, flow rates and pre-swirl ratios. Narrow-band thermochromic liquid crystal (TLC) was specifically calibrated for application to experiments on a disc rotating at ∼ 5000 rpm and subsequently used to measure surface temperature in a transient experiment. The TLC was viewed through the transparent polycarbonate disc using a digital video camera and strobe light synchronised to the disc frequency. The convective heat transfer coefficient, h, was subsequently calculated from the one-dimensional solution of Fourier’s conduction equation for a semi-infinite wall. The analysis accounted for the exponential rise in the air temperature driving the heat transfer, and for experimental uncertainties in the measured values of h. The experimental data was supported by ‘flow visualisation’ determined from CFD. Two heat transfer regimes were revealed for the low-radius pre-swirl system: a viscous regime at relatively low coolant flow rates; and an inertial regime at higher flow rates. Both regimes featured regions of high heat transfer where thin, boundary layers replaced air exiting through receiver holes at high radius on the rotating disc. The heat transfer in the high radius pre-swirl system was shown to be dominated by impingement under the flow conditions tested.
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