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Lewis Bryan J., Cimbala John M. Unsteady Computational Fluid Dynamic Analysis of the Behavior of Guide Vane Trailing‐Edge Injection and Its Effects on Downstream Rotor Performance in a Francis Hydroturbine J. Turbomach. 137(8), 081001 (2015) (9 pages);   Paper No: TURBO-14-1157;   doi:10.1115/1.4029427

A unique guide vane design, which includes trailing-edge jets, is presented for a mixed-flow Francis hydroturbine. The water injection causes a change in bulk flow direction at the inlet of the rotor. When properly tuned, altering the flow angle results in a significant improvement in turbine efficiency during off-design operation. Unsteady CFD simulations show nearly 1% improvement in overall turbine efficiency with the use of injection. This revolutionary concept also has the ability to reduce the intensity of the rotor–stator interactions (RSI) by compensating for the momentum deficit of the wicket gate wakes. This technology may be equally applied to other turbomachinery devices with problematic rotor–stator flow misalignments.

Commentary by Dr. Valentin Fuster
Vahdati Mehdi, Smith Nigel, Zhao Fanzhou. Influence of Intake on Fan Blade Flutter J. Turbomach. 137(8), 081002 (2015) (10 pages);   Paper No: TURBO-14-1276;   doi:10.1115/1.4029240

The main aim of this paper is to study the influence of upstream reflections on flutter of a fan blade. To achieve this goal, flutter analysis of a complete fan assembly with an intake duct and the downstream outlet guide vanes (OGVs) (whole low pressure (LP) domain) is undertaken using a validated computational fluid dynamics (CFD) model. The computed results show good correlation with measured data. Due to space constraints, only upstream (intake) reflections are analyzed in this paper. It will be shown that the correct prediction of flutter boundary for a fan blade requires modeling of the intake and different intakes would produce different flutter boundaries for the same fan blade. However, the “blade only” and intake damping are independent and the total damping can be obtained from the sum of the two contributions. In order to gain further insight into the physics of the problem, the pressure waves created by blade vibration are split into an upstream and a downstream traveling wave in the intake. The splitting of the pressure wave allows one to establish a relationship between the phase and amplitude of the reflected waves and flutter stability of the blade. By using this approach, a simple reflection model can be used to model the intake effects.

Commentary by Dr. Valentin Fuster
Pohl Stephanie, Frank Gabriele, Pfitzner Michael. Heat Transfer in Reacting Cooling Films: Influence and Validation of Combustion Modeling in Numerical Simulations J. Turbomach. 137(8), 081003 (2015) (10 pages);   Paper No: TURBO-14-1284;   doi:10.1115/1.4029350

The demand for increased performance and lower weight of gas turbines gives rise to higher fuel-to-air ratios and a more compact design of the combustion chamber, thereby increasing the potential of fuel escaping unburnt from the combustor. Chemical reactions are likely to occur when the coolant air, used to protect the turbine blades, interacts with the unreacted fuel. Within this work, Reynolds-averaged Navier–Stokes (RANS) simulations of reacting cooling films exposed to high temperature fuel-rich exhaust gases are performed using the commercial computational fluid dynamics (CFD) code ansys fluent and validated against experimental results obtained at the Air Force Research Laboratory in Ohio. The results underline that the choice of the turbulence model has a significant impact on the evolution of the flow field and the mixing effectiveness. The flamelet as well as the equilibrium combustion model is able to predict an adequate distance of the reaction zone normal to the wall. Its thickness, however, is still much smaller and its onset too far upstream as compared to the experimental results. According to the present analysis, the flamelet combustion model applied along with k–ω shear stress transport (SST) or k–ε turbulence model turned out to be an appropriate choice in order to model near wall reacting flows with reasonable prospect of success.

Commentary by Dr. Valentin Fuster
El-Gabry Lamyaa A., Saha Ranjan, Fridh Jens, et al. Measurements of Hub Flow Interaction on Film Cooled Nozzle Guide Vane in Transonic Annular Cascade J. Turbomach. 137(8), 081004 (2015) (9 pages);   Paper No: TURBO-14-1287;   doi:10.1115/1.4029242

An experimental study has been performed in a transonic annular sector cascade of nozzle guide vanes (NGVs) to investigate the aerodynamic performance and the interaction between hub film cooling and mainstream flow. The focus of the study is on the endwalls, specifically the interaction between the hub film cooling and the mainstream. Carbon dioxide (CO2) has been supplied to the coolant holes to serve as tracer gas. Measurements of CO2 concentration downstream of the vane trailing edge (TE) can be used to visualize the mixing of the coolant flow with the mainstream. Flow field measurements are performed in the downstream plane with a five-hole probe to characterize the aerodynamics in the vane. Results are presented for the fully cooled and partially cooled vane (only hub cooling) configurations. Data presented at the downstream plane include concentration contour, axial vorticity, velocity vectors, and yaw and pitch angles. From these investigations, secondary flow structures such as the horseshoe vortex, passage vortex, can be identified and show the cooling flow significantly impacts the secondary flow and downstream flow field. The results suggest that there is a region on the pressure side (PS) of the vane TE where the coolant concentrations are very low suggesting that the cooling air introduced at the platform upstream of the leading edge (LE) does not reach the PS endwall, potentially creating a local hotspot.

Commentary by Dr. Valentin Fuster
Tyacke James C., Tucker Paul G. Future Use of Large Eddy Simulation in Aero‐engines J. Turbomach. 137(8), 081005 (2015) (16 pages);   Paper No: TURBO-14-1308;   doi:10.1115/1.4029363

Computational fluid dynamics (CFD) has become a critical tool in the design of aero-engines. Increasing demand for higher efficiency, performance, and reduced emissions of noise and pollutants has focused attention on secondary flows, small scale internal flows, and flow interactions. In conjunction with low order correlations and experimental data, RANS (Reynolds-averaged Navier–Stokes) modeling has been used effectively for some time, particularly at high Reynolds numbers and at design conditions. However, the range of flows throughout an engine is vast, with most, in reality being inherently unsteady. There are many cases where RANS can perform poorly, particularly in zones characterized by strong streamline curvature, separation, transition, relaminarization, and heat transfer. The reliable use of RANS has also been limited by its strong dependence on turbulence model choice and related ad-hoc corrections. For complex flows, large-eddy simulation (LES) methods provide reliable solutions, largely independent of turbulence model choice, and at a relatively low cost for particular flows. LES can now be used to provide in depth knowledge of flow physics, for example, in areas such as transition and real wall roughness effects. This can be used to inform RANS and lower order modeling (LOM). For some flows, LES can now even be used for design. Existing literature is used to show the potential of LES for a range of flows in different zones of the engine. Based on flow taxonomy, best practices including RANS/LES zonalization, meshing requirements, and turbulent inflow conditions are introduced, leading to the proposal of a tentative expert system for industrial use. In this way, LES becomes a well controlled tool, suitable for design use and reduces the burden on the end user. The problem sizes tackled however have lagged behind potential computing power, hence future LES use at scale requires substantial progress in several key areas. Current and future solver technologies are thus examined and the potential current and future use of LES is considered.

Commentary by Dr. Valentin Fuster
Walther Benjamin, Nadarajah Siva. Optimum Shape Design for Multirow Turbomachinery Configurations Using a Discrete Adjoint Approach and an Efficient Radial Basis Function Deformation Scheme for Complex Multiblock Grids J. Turbomach. 137(8), 081006 (2015) (20 pages);   Paper No: TURBO-14-1314;   doi:10.1115/1.4029550

This paper proposes a framework for fully automatic gradient-based constrained aerodynamic shape optimization in a multirow turbomachinery environment. The concept of adjoint-based gradient calculation is discussed and the development of the discrete adjoint equations for a turbomachinery Reynolds-averaged Navier–Stokes (RANS) solver, particularly the derivation of flow-consistent adjoint boundary conditions as well as the implementation of a discrete adjoint mixing-plane formulation, are described in detail. A parallelized, automatic grid perturbation scheme utilizing radial basis functions (RBFs), which is accurate and robust as well as able to handle highly resolved complex multiblock turbomachinery grid configurations, is developed and employed to calculate the gradient from the adjoint solution. The adjoint solver is validated by comparing its sensitivities with finite-difference gradients obtained from the flow solver. A sequential quadratic programming (SQP) algorithm is then utilized to determine an improved blade shape based on the gradient information from the objective functional and the constraints. The developed optimization method is used to redesign a single-stage transonic flow compressor in both inviscid and viscous flow. The design objective is to maximize the isentropic efficiency while constraining the mass flow rate and the total pressure ratio.

Commentary by Dr. Valentin Fuster

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