Computational fluid dynamics (CFD) is used to isolate the flow physics responsible for hot crossflow ingestion, a phenomenon that can cause failure of a film cooled gas turbine component. In the gas turbine industry, new compound-angle shaped hole (CASH) geometries are currently being developed to decrease the heat transfer coefficient and increase the adiabatic effectiveness on film cooled surfaces. These new CASH geometries can have unexpected flow patterns that result in hot crossflow ingestion at the film hole. This investigation examines a 15 deg forward-diffused film hole injected streamwise at 35 deg with a compound angle of 60 deg (FDIFF60) and with a length-to-diameter ratio (L/D) of 4.0. Qualitative and quantitative aspects of computed results agreed well with measurements, thus lending credibility to predictions. The FDIFF60 configuration is a good representative of a typical CASH geometry, and produces flow mechanisms that are characteristic of CASH film cooling. FDIFF60 has been shown to have impressive downstream film cooling performance, while simultaneously having undesirable ingestion at the film hole. In addition to identifying the physical mechanisms driving ingestion, this paper documents the effects on ingestion of the blowing ratio, the density ratio, and the film hole Reynolds number over realistic gas turbine ranges of 0.5 to 1.88, 1.6 to 2.0, and 17,350 to 70,000, respectively. The results of this study show that hot crossflow ingestion is caused by a combination of coolant blockage at the film hole exit plane and of crossflow boundary layer vorticity that has been re-oriented streamwise by the presence of jetting coolant. Ingestion results when this re-oriented vorticity passes over the blocked region of the film hole. The density ratio and the film hole Reynolds number do not have a significant effect on ingestion over the ranges studied, but the blowing ratio has a surprising nonlinear effect. Another important result of this study is that the blockage of coolant hampers convection and allows diffusion to transfer heat into the film hole even when ingestion is not present. This produces both an undesirable temperature gradient and high temperature level on the film hole wall itself. Lessons learned about the physics of ingestion are generalized to arbitrary CASH configurations. The systematic computational methodology currently used has been previously documented and has become a standard for ensuring accurate results. The methodology includes exact modeling of flow physics, proper modeling of the geometry including the crossflow, plenum, and film hole regions, a high quality mesh for grid independent results, second order discretization, and the two-equation k–ε turbulence model with generalized wall functions. The steady, Reynolds-averaged Navier–Stokes equations are solved using a fully elliptic and fully implicit pressure-correction solver with multiblock unstructured and adaptive grid capability and with multigrid convergence acceleration.

Numerical results are presented for a three-dimensional discrete-jet in crossflow problem typical of a realistic film-cooling application in gas turbines. Key aspects of the study include: (1) application of a systematic computational methodology that stresses accurate computational model of the physical problem, including simultaneous, fully elliptic solution of the crossflow, film-hole, and plenum regions; high-quality three-dimensional unstructured grid generation techniques, which have yet to be documented for this class of problems; the use of a high-order discretization scheme to reduce numerical errors significantly; and effective turbulence modeling; (2) a three-way comparison of results to both code validation quality experimental data and a previously documented structured grid simulation; and (3) identification of sources of discrepancy between predicted and measured results, as well as recommendations to alleviate these discrepancies. Solutions were obtained with a multiblock, unstructured/adaptive grid, fully explicit, time-marching, Reynolds-averaged Navier–Stokes code with multigrid, local time stepping, and residual smoothing type acceleration techniques. The computational methodology was applied to the validation test case of a row of discrete jets on a flat plate with a streamwise injection angle of 35 deg, and two film-hole length-to-diameter ratios of 3.5 and 1.75. The density ratio for all cases was 2.0, blowing ratio was varied from 0.5 to 2.0, and free-stream turbulence intensity was 2 percent. The results demonstrate that the prescribed computational methodology yields consistently more accurate solutions for this class of problems than previous attempts published in the open literature. Sources of disagreement between measured and computed results have been identified, and recommendations made for future prediction of film-cooling problems.

The flow physics of film cooling with compound-angle shaped holes is documented for realistic gas turbine parameters. For the first time in the open literature, the combined effects of compound-angle injection and hole shaping are isolated and the dominant mechanisms are examined. Results provide valuable insight into the valuable insight into the flowfield of this class of film-cooling jets. Computational and experimental results are presented for a row of holes injected at 35 deg on a flat plate with three distinct geometric configurations: (1) streamwise injected cylindrical holes (reference case); (2) 15 deg forward-diffused holes injected at a 60 deg compound angle; and (3) 12 deg laterally diffused holes injected a 45 deg compound angle. Detailed field and surface data, including adiabatic effectiveness (η) and heat transfer coefficient ( h ), of the two compound-angle shaped holes are provided and compared to: (i) the references streamwise cylindrical case; (ii) results from Part II detailing the compound-angle flowfield for cylindrical holes; (iii) results of Part III detailing the streamwise injected shaped-hole flowfield; and (iv) experimental data. The 60 deg compound-angle forward-diffused holes provided excellent lateral coolant distribution, but suffered from crossflow ingestion at the film-hole exit plane. The 45 deg compound-angle lateral-diffused hole and had much steeper lateral effectiveness variations. A previously documented and validated computational methodology was utilized. Computations were performed using a multiblock, unstructured-adaptive grid, fully implicity pressure-correction Navier–Stokes code with multigrid and underrelaxation type convergence accelerators. All simulations had fixed length-to-diameter ratio of 4.0, pitch-to-diameter ratio of 3.0, nominal density ratio of 1.55 and film-hole Reynolds number of 17,350, which allowed isolation of the combined effects of compound-angle injection and hole shaping for nominal blowing ratios of 1.25 and 1.88. The results demonstrate the ability of the prescribed computational methodology to predict accurately the complex flowfield associated with compound-angle shaped-hole film-cooling jets. [S0889-504X(00)01501-4]

A previously documented systematic computational methodology is implemented and applied to a jet-in-crossflow problem in order to document all of the pertinent flow physics associated with a film-cooling flowfield. Numerical results are compared to experimental data for the case of a row of three-dimensional, inclined jets with length-to-diameter ratios similar to a realistic film-cooling application. A novel vorticity-based approach is included in the analysis of the flow physics. Particular attention has been paid to the downstream coolant structures and to the source and influence of counterrotating vortices in the crossflow region. It is shown that the vorticity in the boundary layers within the film hole is primarily responsible for this secondary motion. Important aspects of the study include: (1) a systematic treatment of the key numerical issues, including accurate computational modeling of the physical problem, exact geometry and high-quality grid generation techniques, higher-order numerical discretization, and accurate evaluation of turbulence model performance; (2) vorticity-based analysis and documentation of the physical mechanisms of jet–crossflow interaction and their influence on film-cooling performance; (3) a comparison of computational results to experimental data; and (4) comparison of results using a two-layer model near-wall treatment versus generalized wall functions. Solution of the steady, time-averaged Navier–Stokes equations were obtained for all cases using an unstructured/adaptive grid, fully explicit, time-marching code with multigrid, local time stepping, and residual smoothing acceleration techniques. For the case using the two-layer model, the solution was obtained with an implicit, pressure-correction solver with multigrid. The three-dimensional test case was examined for two different film-hole length-to-diameter ratios of 1.75 and 3.5, and three different blowing ratios, from 0.5 to 2.0. All of the simulations had a density ratio of 2.0, and an injection angle of 35 deg. An improved understanding of the flow physics has provided insight into future advances to film-cooling configuration design. In addition, the advantages and disadvantages of the two-layer turbulence model are highlighted for this class of problems. [S0889-504X(00)01201-0]

The physics of the film cooling process for shaped, streamwise-injected, inclined jets is studied for blowing ratio (M=1.25,1.88), density ratio (DR=1.6), and length-to-diameter ratio (L/D=4) parameters typical of gas turbine operations. A previously documented computational methodology is applied for the study of five distinct film cooling configurations: (1) cylindrical film hole (reference case); (2) forward-diffused film hole; (3) laterally diffused film hole; (4) inlet shaped film hole, and (5) cusp-shaped film hole. The effect of various film hole geometries on both flow and thermal field characteristics is isolated, and the dominant mechanisms responsible for differences in these characteristics are documented. Special consideration is given to explaining crucial flow mechanisms from a vorticity point of view. It is found that vorticity analysis of the flow exiting the film hole can aid substantially in explaining the flow behavior downstream of the film hole. Results indicate that changes in the film hole shape can significantly alter the distribution of the exit-plane variables, therefore strongly affecting the downstream behavior of the film. Computational solutions of the steady, Reynolds-averaged Navier–Stokes equations are obtained using an unstructured/adaptive, fully implicit, pressure-correction solver. Turbulence closure is obtained via the high-Reynolds-number k–ε model with generalized wall functions. Detailed field results as well as surface phenomena involving adiabatic film effectiveness (η) and heat transfer coefficient ( h ) are presented. When possible, computational results are validated against corresponding experimental cases from data found in the open literature. Detailed comparisons are made between surface and field results of the film hole shapes investigated in this work; design criteria for optimizing downstream heat transfer characteristics are then suggested. [S0889-504X(00)01401-X]

Detailed analyses of computational simulations with comparisons to experimental data were performed to identify and explain the dominant flow mechanisms responsible for film cooling performance with compound angle injection, Φ, of 45, 60, and 90 deg. A novel vorticity and momentum based approach was implemented to document how the symmetric, counterrotating vortex structure typically found in the crossflow region in streamwise injection cases, becomes asymmetric with increasing Φ. This asymmetry eventually leads to a large, single vortex system at Φ=90 deg and fundamentally alters the interaction of the coolant jet and hot crossflow. The vortex structure dominates the film cooling performance in compound angle injection cases by enhancing the mixing of the coolant and crossflow in the near wall region, and also by enhancing the lateral spreading of the coolant. The simulations consist of fully elliptic and fully coupled solutions for field results in the supply plenum, film hole, and crossflow regions and includes surface results for adiabatic effectiveness η and heat transfer coefficient h . Realistic geometries with length-to-diameter ratio of 4.0 and pitch-to-diameter ratio of 3.0 allowed for accurate capturing of the strong three-way coupling of flow in this multiregion flowfield. The cooling configurations implemented in this study exactly matched experimental work used for validation purposes and were represented by high-quality computational grid meshes using a multiblock, unstructured grid topology. Blowing ratios of 1.25 and 1.88, and density ratio of 1.6 were used to simulate realistic operating conditions and to match the experiments used for validation. Predicted results for η and h show good agreement with experimental data. [S0889-504X(00)01301-5]

Large-scale computational analyses have been conducted and results compared with experiments to understand coolant jet and crossflow interaction in discrete-jet film cooling. Detailed three-dimensional elliptic Navier–Stokes solutions, with high-order turbuence modeling, are presented for film cooling using a new model enabling simultaneous solution of fully coupled flow in plenum, film-hole, and cross-stream regions. Computations are carried out for the following range of film cooling parameters typically found in gas turbine airfoil applications: single row of jets with a film-hole length-to-diameter ratio of 1.75 and 3.5; blowing ratio from 0.5 up to 2; coolant-to-crossflow density ratio of 2; streamwise injection angle of 35 deg; and pitch-to-diameter ratio of 3. Comparison of computational solutions with experimental data give good agreement. Moreover, the current results complement experiments and support previous interpretations of measured data and flow visualization. The results also explain important aspects of film cooling, such as the development of complex flow within the film-hole in addition to the well-known counterrotating vortex structure in the cross-stream.

Extensive numerical analyses and experiments have been conducted to understand mixing phenomena in multistage, axial-flow compressors. For the first time in the literature the following are documented: Detailed three-dimensional Navier–Stokes solutions, with high order turbulence modeling, are presented for flow through a compressor vane row at both design and off-design (increased) loading; comparison of these computations with detailed experimental data show excellent agreement at both loading levels; the results are then used to explain important aspects of mixing in compressors. The three-dimensional analyses show the development of spanwise (radial) and circumferential flows in the stator and the change in location and extent of separated flow regions as loading increases. The numerical solutions support previous interpretations of experimental data obtained on the same blading using the ethylene tracer-gas technique and hot-wire anemometry. These results, plus new tracer-gas data, show that both secondary flow and turbulent diffusion are mechanisms responsible for both spanwise and circumferential mixing in axial-flow compressors. The relative importance of the two mechanisms depends upon the configuration and loading levels. It appears that using the correct spanwise distributions of time-averaged inlet boundary conditions for three-dimensional Navier–Stokes computations enables one to explain much of the flow physics for this stator.