Due to the higher cooling requirements of novel combustor liners a comprehensive understanding of the phenomena concerning the interaction of hot gases with different coolant flows plays a major role in the definition of a well performing liner. An experimental analysis of a real engine cooling scheme was performed on a test article replicating a slot injection and an effusion array with a central large dilution hole. Test section consists of a rectangular cross-section duct and a flat perforated plate with 272 holes arranged in 29 staggered rows (d = 1.65 mm, Sx/d = 7.6, Sy/d = 6, L/d = 5.5, α = 30 deg); a dilution hole (D = 18.75 mm) is located at the 14th row. Both effusion and dilution holes are fed by a channel replicating combustor annulus, that allows to control cold gas side cross-flow parameters. Upstream the first effusion row, a 6.0 mm high slot ensure the protection of the very first region of the liner. Final aim was the measurement of adiabatic effectiveness of the cooling scheme by means of a steady-state Thermochromic Liquid Crystals (TLC) technique, considering the combined effects of slot, effusion and dilution holes. Experiments were carried out imposing three different effusion velocity ratios typical of modern engine working conditions (VReff = 3, 5, 7) and keeping constant slot flow parameters (VRsl = 1.1). CFD RANS calculations were also performed with the aim of better understanding interactions between coolant exiting from the slot and injected by effusion cooling rows. Numerical analysis revealed a large dependency on effusion velocity ratio. An in-house one-dimensional fluid network solver was finally used to compare experimental and numerical results with the ones predicted by correlations and then quantify the possibility of giving predictions. Both CFD and experimental results reveal that slot protection is reduced in the first rows by coolant injected with such high velocity ratios; nevertheless effusion, though in penetration regime, guarantees a significant effectiveness level in the more downstream region. Dilution hole alters the effectiveness growth rate, moreover leading to local protection lowering just after its injection.
The turbine blade tip clearances control in large aero-engines is currently performed by means of impinging fan air on the outer case flanges. The aim of the present study is to evaluate both the heat transfer coefficient and the adiabatic thermal effectiveness characteristics of an enginelike ACC system, and in particular, to comprehend the effects of the undercowl flow on the impingement jets. The considered geometry replicates the impingement tubes and the by-pass duct used in active control clearance systems. The tube's internal diameter is D = 12 mm, the cooling hole's diameter is d = 1 mm, and the span-wise pitch is Sy/d=12. In order to simulate the undercowl flow, the impingement arrays are inserted inside a tunnel that replicates the typical shape of a real engine by-pass duct. Tests were conducted varying both the mainstream Reynolds number and the jets Reynolds number in a range typical of real-engine operative conditions (Rej=2000-10000, β=1.05-1.15). Numerical calculations are finally proposed to point out if CFD is able to confidently reproduce the experimental evidences.
An experimental analysis of a real engine cooling scheme was performed on a test article replicating a slot injection and an effusion array with a central large dilution hole. Test section consists of a rectangular cross-section duct with a flat plate comprised of 270 holes arranged in 29 staggered rows (D = 1.65mm, Sx/D = 7.6, Sy/D = 6, L/D = 5.5, α = 30deg) and a dilution hole (D = 18.75mm) located at the 14th row. Both effusion and dilution holes are fed by a channel replicating combustor annulus, that allows to control cold gas side cross-flow parameters, especially in terms of Reynolds number of both annulus and effusion holes. Upstream the first row, a 6mm high slot, ensure the protection of the very first region of the liner. Final aim was the measurement of both heat transfer coefficient and Net Heat Flux Reduction of the cooling scheme, by means of a steady-state Thermochromic Liquid Crystals (TLC) technique with a thin Inconel heating foil. A data reduction procedure based on a Finite Element approach has been developed to take into account the non uniform heat generation and conduction due to the large amount of holes. Experiments were carried out considering the combined effects of slot, effusion and dilution holes. Three different effusion blowing ratios (BR = 3–5–7) are investigated, keeping constant the slot flow parameters (BR = 1.3). Results highlight a large influence of effusion blowing ratio on heat transfer coefficient. A steep increase was found in the first rows, while the large dilution hole does not influences significantly the heat transfer behaviour in the downstream area.
Due to expected increases in gas turbine performance, strictly related to firing temperature, heat transfer is a major issue in design processes. To keep components temperature levels below design requirements, cooling systems are commonly used. Nowadays, nozzle and blade cooling systems have reached a high degree of complexity. In a preliminary design stage, both experimental and 3D numerical analyses are usually not very suitable to define geometry, coolant mass flow rate or cooling system typology. This is mainly due to the uncertainty on several parameters, i.e. pressure distributions and materials properties, and their undefined interaction. This work presents a simulation tool useful to provide system cooling development with qualitative and quantitative information about metal temperature, coolant mass flow rate, heat transfer and much more. This tool couples energy, momentum and mass flow conservation equations together with experimental correlations for heat transfer and pressure losses. Metal conduction is solved by two dimensional calculations for several blade to blade sections. This methodology allows to investigate several cooling system configurations and compare them in a relatively short time. Main features of this simulation tool are shown comparing obtained results with experimental data.
Due to the higher cooling requirements of novel combustor liners a comprehensive understanding of the phenomena concerning the interaction of hot gases with different coolant flows plays a major role in the definition of a well performing liner. A numerical study of a real engine cooling scheme was performed on a test article replicating a slot injection and an effusion array with a central large dilution hole. Geometry consists of a rectangular cross-section duct with a flat plate comprised 272 holes arranged in 29 staggered rows (d = 1.65 mm, Sx/d = 7.6, Sy/d = 6, L/d = 5.5, α = 30 deg); a dilution hole (D = 18.75 mm) is located at the 14th row. A detailed experimental survey has been performed on this test article making possible to compare both predicted adiabatic effectiveness and heat transfer coefficient. The study has a twofold objective. On one hand it aims to assess the accuracy of standard industrial CFD analysis in the prediction of heat transfer on the hot side of realistic effusion cooled plates, and, on the other hand, it allows to better understand the structure of flow field, not investigated with experiments. Steady state RANS calculations have been performed on 3D computational domain with a full explicit discretization of effusion holes, with a sensitivity to standard two-equation turbulence models. Numerical results have pointed out a large dependence on effusion velocity ratio and, despite the well known deficiency of eddy viscosity models in the prediction of film effectiveness, CFD results have shown an excellent agreement with experiments in the prediction of hot side heat transfer coefficient. The entity of local heat transfer augmentation due to gas-jets interaction and its dependence on jets velocity ratio were predicted with very satisfactory agreement. The increase of heat transfer is usually located very close to jet exits and it is mainly due to local flow acceleration and vortices whose calculation is not affected by the inaccurate jet mixing prediction of first order turbulence models. Besides the comparison with experimental data of the companion paper, an additional numerical investigation was performed to assess the effect of a variable density ratio. Obtained results point out the opportunity to scale the increase in heat transfer coefficient with effusion jets velocity ratio.
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