Balu Sekar

Reduced Chemical Kinetic Mechanisms for JP-8 Combustion

Abstract : Using CARM (Computer Aided Reduction Method), a computer program that automates the mechanism reduction process, six different reduced chemical kinetic mechanisms for JP-8 combustion have been generated. The reduced mechanisms have been compared to detailed chemistry calculations in simple homogeneous reactor calculations. Reduced mechanisms containing 15 and 20 species were found to give good agreement for both temperature and species concentrations (including NO) in adiabatic perfectly stirred reactor calculations for inlet temperatures from 300-1300 K, pressures from 10-40 atm, stoichiometric ratios from 0.5-2.0 and reactor residence times from 0.1 sec. to near blowout. Reduced mechanisms have also been created that compare well to available ignition delay measurements for JP-8.

Validation Of CFD++ Code Capability For Supersonic Combustor Flowfields

Numerical simulations of several turbulent supersonic flows related to scramjet combustors are carried out using a new unified-grid computational methodology. Five problems are considered: a 2-D ramp unit problem; a reattaching turbulent shear layer, the 3-D University of Virginia two-hole supersonic transverse Air-Air injector; and the NASA P2 and P8 supersonic inlets. The numerical simulations are conducted using the Reynoldsaveraged Navier-Stokes equations along with oneequation and three-equation pointwise turbulence models. Both turbulence models enable accurate prediction of the flowfields and numerical results compare favorably with experimental data in all cases.

A numerical study of the pulse detonation wave engine with hydrocarbon fuels

This paper explores some issues that arise in the analysis of pulse detonation wave engines with hydrocarbon fuels. One-dimensional and axisyrnmetric/two-dimensional simulations are employed along with reduced kinetic mechanisms to confirm the ability of the numerical approach to accurately compute relevant physical characteristics such as proper detonation wave speed, von-Neumann spike, aspiration, pressure time history and sequence of cycle events. It is shown that qualitatively and quantitatively reasonable results can be obtained with a careful treatment of the finite-rate-chemistry source terms. Some of the numerical difficulties that arise in dealing with unsteady detonation phenomena are discussed and improvements demonstrated. Onedimensional test cases with simplified H2-O2 and C3HgAir kinetics are used to verify correct detonation wave speed and testing boundary conditions. An axisymmetric case for the latter chemistry is studied with a generic inlet to illustrate the ability of the methodology to capture the relevant physics, namely, pressurization of thrust wall by the detonation wave and interaction of the reflected wave with rarefaction waves from the open end.

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.

LES-PDF Modeling of Flame Instability and Blow-out in Bluff-Body Stabilized Flames

Large Eddy Simulations (LES) were performed to predict the flame profile of bluff-body stabilized premixed flame at stable and blow-out conditions. Probability density function (PDF) based approach was used to solve the scalar transport by fully resolving the chemical source termwith 14 species, 44 step propane reduced chemical kinetic mechanism using

Experimental and Computational Studies of an Ultra-Compact Combustor

Reducing the weight and decreasing pressure losses of aviation gas turbine engines improves the thrust-to-weight ratio and improves efficiency. In ultra-compact combustors (UCCs), engine length is reduced and pressure losses are decreased by merging a combustor with adjacent components using a systems engineering approach. High-pressure turbine inlet vanes can be placed in a combustor to form a UCC. Eliminating the compressor outlet guide vanes (OGVs) and maintaining swirl through the diffuser can result in further reduction in engine length and weight. Cycle analysis indicates that a 2.4% improvement in engine weight and a 0.8% increase in thrust-specific fuel consumption are possible when a UCC is used. Experiments and analysis were performed in an effort to understand key physical and chemical processes within a trapped-vortex UCC. Experiments were performed using a combustor operating at pressures in the range of 520–1030 kPa (75–150 psi) and inlet temperature of 480–620 K (865–1120 °R). The primary reaction zone is in a single trapped-vortex cavity where the equivalence ratio was varied from 0.7 to 1.8. Combustion efficiencies and NOx emissions were measured and exit temperature profiles obtained, for various air loadings, cavity equivalence ratios, and configurations with and without turbine inlet vanes. A combined diffuser-flameholder (CDF) was used in configurations without vanes to study the interaction of cavity and core flows. Higher combustion efficiency was achieved when the forward-to-aft momentum ratios of the air jets in the cavity were near unity or higher. Discrete jets of air immediately above the cavity result in the highest combustion efficiency. The air jets reinforce the vortex structure within the cavity, as confirmed through coherent structure velocimetry of high-speed images. A more uniform temperature profile was observed at the combustor exit when a CDF is used instead of vanes. This is the result of increased mass transport along the face of the flame holder. Emission indices of NOx were between 3.5 and 6.5 g/kgfuel for all test conditions. Ultra-compact combustors (with a single cavity) can be run with higher air loadings than those employed in previous testing with a trapped-vortex combustor (two cavities) with similar combustion efficiencies being maintained. The results of this study suggest that the length of combustors and adjacent components can be reduced by employing a systems level approach.Copyright

Level-Set Flamelet/Large-Eddy Simulation of a Premixed Augmentor Flame Holder

The objective of the current study is to combine a high-fidelity large eddy simulation (LES) flow solver with a level-set flamelet algorithm for the prediction of premixed turbulent combustion. The same level of high accuracy is implemented for simulation at all speeds. The goal of this work is to accurately predict the unsteady turbulence-flame interaction for realistic industrial combustors with complex geometries. The numerical issues related to the numerical implementation of the LES equations, flamelet model and level-set algorithm are presented in detail. The accuracy of the numerical implementation is verified through comparisons with experimental data for an augmentor flame holder and a turbulent Bunsen burner flame.

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.

Comparison of three Navier-Stokes equation solvers for supersonic open cavity simulations

We compare the results obtained with an approximately factored lower-upper triangular solver, an explicit Runge-Kutta scheme, and a point implicit method for a variety of timesteps to assess solver error and timestep issues in computations of unsteady flows