Hugh Thornburg

Solution adaptive grid strategies based on point redistribution

Solution adaptive grid strategies based on the redistribution of a fixed number of points are described in this paper. The redistribution is performed using weight functions that vary based on significant flow features. The weight functions are evaluated using an equidistribution principle. In this paper, emphasis is placed on the development of weight functions applicable to compressible flows exhibiting large scale separated vortical flows, vortex‐vortex and vortex‐surface interactions, separated shear layers and multiple shocks of diAerent intensities. Algebraic, elliptic and parabolic methods of grid generation have been utilized for structured grid redistribution. Additionally, a point movement scheme is presented for generalized (structured/unstructured/hybrid) grid adaptation. Computer Aided Geometry Design (CAGD) techniques are combined with redistribution schemes to maintain the fidelity of solid boundaries. In particular, solid boundaries are represented using Non-Uniform Rational B-Splines (NURBS). A grid generation software system ‐ Parallel Multiblock Adaptive Grid generation (PMAG) ‐ using an elliptic redistribution scheme is also described with emphasis placed on the parallel implementation for multiblock structured grids with unstructured blocking topologies and on interpolation issues. Computational examples demonstrating the influence of diAerent weight functions and grid redistribution strategies are presented. ” 2000 Elsevier Science S.A. All rights reserved.

Effect of Trapped Vortex Combustion with Radial Vane Cavity Arrangements on Predicted Inter-Turbine Burner Performance

The complex combustion processes, including chemical reactions, turbulence, unsteady, multiphase flow, evaporation and heat and mass transfer pose great challenges in modern propulsion system design and development. Ultra-short compact, high performance combustion systems are desirable for advanced propulsion systems from the standpoint of lower fuel consumption and increased material durability. AFRL has proposed placing an Ultra-Compact Combustor (UCC) between a high pressure turbine stage and low pressure turbine stage to create an innovative Inter-Turbine Burner (ITB) concept. This paper focuses on ITB combustor technologies that can enable the development of compact, highperformance combustion systems. Compact combustors weigh less and take up less volume in space-limited turbine engine for aero applications. The earlier designs conceived and developed at AFRL/RZTC is based on the idea that the flame speed under turbulent conditions is directly proportional to the square root of gravity and high-g flames offer increased flame speeds, which would aid in the design of shorter combustion systems. This idea led to an ITB with a circumferential cavity in which fuel and air injected at selected points led to rich combustion in the circumferential cavity. This was further followed by lean combustion and flame stabilization with the aid of a radial vane with notch. Even though this concept exhibited good merits through several rig tests and numerical studies carried out over the years at AFRL/RZTC, it does not allow scaling of the geometry and configuration for higher mass flow rates, larger size and increased thrust requirements. This paper presents an alternative concept for the UCC that uses a Trapped Vortex Cavity (TVC) to replace the high swirling circumferential cavity combustion to enhance mixing rates via a double vortex system in the TVC, followed by further mixing of the free stream air through the vane with a notch. Flow field predictions utilizing FLUENT are presented for concept evaluation in a systematic way to understand the flow development and physics, leading to the incremental combustion enhancement, total pressure loss, the entrainment and the calculated exit temperature profile. The analysis supplements the understanding of the design space required for future engine designs that may use this novel, compact combustion systems.

Effects of Main Swirl Direction on High -g Combustion

The use of high -g combustors promises to greatly decrease burner size and improve cost, weight, efficiency and durability of turbine engines . The latest design of the Ultra -Compact Combustor (UCC) is one such technology that utilizes swir l enhanced combustion at up to 10, 000 g’s . Recessed cavi ties create areas of flame stabilization and make possible the very low residence times experienced by this combustor. The use of turning vane passages simulates combustion in a turbine stator stage. Tests conducted at the Air Force Research Laboratory’s (AFRL) Atmospheric Pressure Combustion Research Center (APCRC) have studied the effect of the main swirl direction in the UCC using both standard and coal derived jet fuels. This research gives insight into the physics of the burner ’s operation.

PERFORMANCE OF AN INTER-TURBINE BURNER (ITB) CONCEPT WITH THREE-DIFFERENT VANE CAVITY SHAPES

Three-dimensional CFD simulations were conducted in this study to determine the performance of an UltraCompact-Combustor (UCC) for use as an Inter-Turbine Burning (ITB) in aero gas turbine engines. The study considered the AFRL novel UCC/ITB concept and its performance associated with three different Radial Vane Cavity (RVC) shapes, namely rectangular (or angled), backward and forward facing step cavities, was numerically obtained. The CFD results demonstrated that the performance of UCC/ITB is profoundly dependent upon the shape of the RVC utilized for radial transport of combustion products and continual lean burning. In particular, the radial transport of the combustion products from the Circumferential Cavity (CC) into the main airflow was predicted to be substantially lower for the backward facing step RVC than that for the rectangular (angled) and forward facing step cavities. Intense burning in the ITB circumferential cavity was predicted by the CFD simulations, suggesting strong flame stability characteristics, improved lean blowout performance, and high combustion efficiency of the AFRL UCC/ITB concept. The results further showed that the shape of the RVC plays an important role in determining the migration mechanism and shedding rate of the combustion products from the high g-loaded swirling circumferential cavity into the main airflow and as a result variant burning patterns were obtained downstream of the trailing edges of the cavities. These burning patterns, however, were found to produce somewhat unconventional radial temperature and fuel-air ratio profiles at the ITB exit plane and these radial profiles were attributed to the inadequate mixing of the combustion products and main airflow. This study, therefore, warrants further design, CFD, and experimental efforts to improve main air stream and combustion products mixing performance.

Effect of Vane Notch and Ramp Design on the Performance of a Rectangular Inter-Turbine Burner

This research is motivated by the need to improve and optimize the performance of AFRL’s Ultra-Compact Combustor (UCC) in terms of greater combustion efficiency, reduced pressure losses and exit temperature profile requirements. The UCC operates as an Inter-turbine Burner (ITB) situated in between the high and low pressure turbine stages. The detailed understanding of the effect of the vane cavities, that are essential for the transport and mixing of the combustion products and incoming air stream in a threedimensional ITB model would be very difficult to optimize and could be experimentally and computationally prohibited. Therefore, a simple representation of somewhat similar burner is used here for optimization of the vane cavities, for improved mixing and reduced losses, using modeling and simulation. The ITB generally contains vanes to redirect the flow direction and to assist the mixing, but in this investigation we model the Trapped Vortex Combustor (TVC) ITB with a single vane with various notch designs and with ramps typically found in high speed combustion applications. In addition, the full configuration is simplified with only two fuel injection sites to get a faster turn around and to get a better understanding of the local flow development. A total of five vane configurations are studied: (1) Vane (V), (2) Vane + Notch (VN), (3) Vane + Altered Notch (VAN), (4) Vane + Extended Altered Notch (VEAN), and (5) Vane + Double Notch (VDN). Two ramp configurations are tested as well: (1) Ramp (R) and (2) Reverse Ramp (RR). FLUENT is used for modeling the three-dimensional ITB using a global eddy dissipation mechanism for C12H23-air combustion with detailed thermodynamic and transport properties. Calculations are performed using the Realizable k-e RANS turbulence model. The combustor efficiency for all ITB configurations is above 99%. Results indicate the major contributor to total drag for all ITB vane and ramp configurations investigated is the pressure drag. The side walls of the combustor do not contribute to drag. The top wall of the ITB is primarily exposed to viscous drag, whereas the bottom wall, which includes the TVC, is primarily exposed to pressure drag with small contributions from viscous drag. The vanes and ramps mainly contribute to drag due to pressure drag. The vanes contribute the most to the overall combustor drag (or pressure loss). The total drag in the combustor decreases with the addition of vane notches (or cavities). Drag decreases in descending order from V to VN, VAN, VEAN, and VDN. Whereas VN and VAN decrease pressure drag (or pressure losses) by only ~3%, VEAN and

Heat Release in Turbine Cooling II: Numerical Details of Secondary Combustion Surrounding Shaped Holes

Film cooling plays a critical role in providing effective thermal protection to components in modern gas turbine engines. Most of the previous studies on film cooling were conducted using either cylindrical or shaped coolant holes with nonreactive pure gases in the cross-stream flow. In this paper, the chemically reactive film cooling over a surface with shaped coolant hole is investigated by a Reynolds-averaged Navier–Stokes approach with a shear-stress transport k-!model to simulate the turbulentflow.To take into account the secondary combustion resulting from the unburned fuels in the crossflow, a two-step reaction scheme was used for the combustion of propane. An eddydissipation concept approach was used to account for the turbulence–chemistry interaction. The three-dimensional simulation was performed on an unstructured hybrid grid. The characteristics of reactive thermal flows, jet– crossflow interactions, species transport, and fuel consumption were investigated at different equivalence ratios and blowing ratios. Numerical results provide insight into where reactions take place and how fuel is consumed.

Numerical Simulation of Inter-Turbine Burner (ITB) Flows With the Inclusion of V-Gutter Flame Holders

In this paper, a three-dimensional numerical simulation has been performed to study the complex reactive flows during the combustion in an inter-turbine burner (ITB) with the inclusion of V-gutter flame holders. The ITB configuration, which has straight radial vanes (SRV), was based on the innovative, high efficiency, high-g Ultra-Compact Combustor (UCC) concept developed at the Air Force Research Laboratory (AFRL). The V-gutter’s angle of attack was varied from −10 to 10 degrees at fixed JET-A fuel and air injections. The turbulent flow in the ITB was modeled with a RANS-based realizable k-e turbulence model, while the spray combustion is modeled with an eddy-dissipation model on an unstructured grid. Numerical results indicate that the V-gutter not only generates vortices behind itself, but also alters the turbulent flow feature and mixing behavior between main air flow and the circumferential and SRV cavity flows within the ITB. The exit temperature profile of the ITB can be modified substantially by the inclusion of the V-gutters at different angle of attack. The additional pressure drop incurred by the addition of the V-gutter was found to be less that 1%. Details of the vane cavity dynamics and increased entrainment physics are also discussed in the paper.Copyright

NUMERICAL STUDY OF AN INTER-TURBINE BURNER (ITB) CONCEPT WITH CURVED RADIAL VANE

The demand for significantly higher performance gas turbine engines has led to the exploration and identification of “Out of the Box” innovative engine design concepts. These demands include increased thrust-to-weight ratio goals that can primarily be met by substantial engine performance increases such as specific thrust, engine weight and size reductions, and repackaging of engine components to create compact engines. Concepts of an Ultra-Compact-Combustor (UCC) for use as a main combustor, or as an Inter-Turbine Burner (ITB) to boost engine work output, reduce pollutant emissions and engine weight are being explored. The available experimental results and observations indicate that UCC/ITB can operate at 95-99% combustion efficiency over a wide range of operating conditions and short flame lengths can result which are 50% shorter than those of conventional combustors. In the present study the radial curved vane ITB design concept has been modeled using three-dimensional CFD. The objectives are to predict flow field and combustion characteristics of the ITB, guide the ITB experiments, identify the key design parameters for best performance, and optimize the ITB design configurations. The CFD results demonstrated that intense burning in a high-g loaded cavity occurred which resulted in high combustion efficiency. The obtained results for the radial vane cavity are compared with the earlier results available for a straight vane cavity. This study indicates improved exit profile characteristics when compared to the earlier baseline.

Enhanced Mixing in Trapped Vortex Combustor with Protuberances Part 1: Single-Phase Nonreacting Flow

This research is motivated by the need to improve and optimize the performance of the Ultra-Compact Combustor (UCC) in terms of reduced drag and pressure losses, while enhancing mixing. Various geometries are now tested for understanding the effects of adding several vortex producing concepts to enhance mixing. This study deals with ramps and convex dimples, placed on the vane and in the Trapped Vortex Combustor (TVC) cavity to create additional vorticity to transport cavity air flow into the air main stream so that the improved understanding will guide when designing the fuel injected TVC for improved performance. The steady three-dimensional governing equations of continuity, momentum, and turbulence are solved using the coupled pressure-based solver of FLUENT. Turbulence is modeled by using the Realizable k- RANS model. Because the flow is weakly compressible, density is allowed to vary with pressure. The turbulence model includes production of turbulent kinetic energy by mean velocity gradients and consumption accounts for dilation dissipation. The upstream flow impinges on the leading edge of the vane and separation occurs at the leading edge of the notch. As seen by an observer looking into the TVC cavity, there is clockwise re-circulation flow inside the notch. Downstream the notch the flow separates again with a clockwise vorticity field further downstream. On the high pressure region of the vane, the flow is characterized by a counterclockwise vorticity field. The trailing edge of the vane also exhibits a counterclockwise vorticity flow field, indicating the location of a shear layer. The drag increases nearly linearly with increasing pressure losses, while mixing efficiency increases with increasing pressure losses. For all configurations, the total pressure losses do not exceed 1%. However, the configuration with no vane, no notch, and no protuberance exhibited the lowest pressure losses. The major effects of the vane with notch are to enhance mixing by creating a low pressure region in the transverse direction and to substantially increase the spanwise velocity magnitude downstream the TVC cavity. Adding ramps or dimples in the TVC’s cavity downstream wall decreases mixing efficiency in comparison to the TVC with vane and notch alone. Mixing efficiency can, however, be enhanced by adding dimples to the leading edge of the latter configuration. By also adding dimples at the trailing edge of this configuration the mixing efficiency is not enhanced, but drag and pressure losses are increased. By offsetting the TVC’s cavity downstream wall while maintaining vane leading edge dimples, the mixing efficiency remains constant, but the total pressure losses and drag decrease. This is found to be the best configuration tested in terms of drag, total pressure losses, and mixing. For the design of passive mixing devices the most important parameter is mixing, which inversely correlates with drag and pressure losses.

Parametric Mesh Deformation and Sensitivity Analysis for Design of a Joined-Wing Aircraft

*† ‡ # + § New processes that enable a state-of-the-art MDO framework for future air vehicles were explored in this paper. First, a unified technique for defining a high-quality geometry and associated Computational Fluid Dynamics (CFD) mesh for a candidate configuration was developed. This method was designed to augment an existing framework-oriented tool for high-fidelity analysis of air vehicle configurations developed at the Air Force Research Laboratory that accounts for system nonlinearities and vehicle flexibility. Second, the mesh generation technique was generalized to account for geometries that deform under the influence of either static or dynamic airloads. Fidelity of the deformation was found to be high, and found to be sufficient for CFD computations. Third, the concept of parametric shape functions was proposed to represent characteristic structural deformations in a fashion consistent with the geometry/mesh definition. As demonstrated with example problems, these functions analytically related the primary structural variables (defined in a generalized manner) to the aerodynamic mesh. The relation produced the grid sensitivity expression resulting from fluid/structure coupling. The significance of this quantity for enforcing conservation of energy during an aeroelastic interaction was discussed.