Performance Comparison of Aero-Engine Thrust Augmentors Stabilized by Bluff Body Flame Holders and a Flame-Holder-Less Concept

2011 ◽  
Author(s):  
Armin Tammer ◽  
John T. Cutright ◽  
Yedidia Neumeier ◽  
Ben T. Zinn
Keyword(s):  
Thermal Efficiency ◽  
Bluff Body ◽  
Weight Ratio ◽  
Performance Comparison ◽  
Flame Stabilization ◽  
Pressure Losses ◽  
Stable Combustion ◽  
Flame Holder ◽  
Engine Thrust ◽  
Flight Range

This study investigates the performance benefits of a flame-holder-less flame stabilization concept for thrust augmentors compared to the common flame holder design. The concept proposes to burn a small portion of the augmentor fuel in a rich mixture with air bled from the compressor to produce a highly reactive partially oxidized fuel-air mixture (POx). The POx mixture is injected into the turbine exit flow to enhance combustion kinetics in order to achieve stable combustion in the augmentor. Thermal efficiency during wet and dry operation is compared, taking into account both the pressure losses due to the flame holders and the reduction of core air for the flame-holder-less concept. Furthermore, the thrust-to-weight ratio and the corresponding flight range have been investigated with respect to the system weight and the induced losses. It was found that the thermal efficiency during dry operation is significantly increased when the pressure losses of the flame holders are eliminated. During wet operation, it was calculated that a flame holder system with only 2% total pressure loss of the flow would operate at the same thermal efficiency as the flame-holder-less concept when 3% air is bled from the compressor. If, for an engine operating at these conditions, the flame-holder-less system could maintain stable combustion using less than 3% bleed air, it would increase the thermal efficiency of that engine during wet operation. The results also suggest that a flame-holder-less system is lighter weight and has the potential to increase engine thrust-to-weight ratio and extend flight range when compared to a flame holder system designed to operate at the same overall engine thermal efficiency.

Dynamics of Non-Premixed Bluff Body-Stabilized Flames in Heated Air Flow

2010 ◽  
Author(s):  
Caleb Cross ◽  
Aimee Fricker ◽  
Dmitriy Shcherbik ◽  
Eugene Lubarsky ◽  
Ben T. Zinn ◽  
...  
Keyword(s):  
Heat Release ◽  
Vortex Shedding ◽  
High Speed ◽  
Bluff Body ◽  
Combustion Process ◽  
Flame Stabilization ◽  
Inlet Temperature ◽  
Fuel Injectors ◽  
Flame Holder ◽  
Process Heat

This paper describes a study of the fundamental flame dynamic processes that control bluff body-stabilized combustion of liquid fuel with low dilatation. Specifically, flame oscillations due to asymmetric vortex shedding downstream of a bluff body (i.e., the Be´nard/von-Ka´rma´n vortex street) were characterized in an effort to identify the fundamental processes that most affect the intensity of these oscillations. For this purpose, the spatial and temporal distributions of the combustion process heat release were characterized over a range of inlet velocities, temperatures, and overall fuel-air ratios in a single flame holder combustion channel with full optical access to the flame. A stream of hot preheated air was supplied to the bluff body using a preburner, and Jet-A fuel was injected across the heated gas stream from discrete fuel injectors integrated within the bluff body. The relative amplitudes, frequencies, and phase of the sinusoidal flame oscillations were characterized by Fourier analysis of high-speed movies of the flame. The amplitudes of the flame oscillations were generally found to increase with global equivalence ratio, reaching a maximum just before rich blowout. Comparison of the flame dynamics to the time-averaged spatial heat release distribution revealed that the intensity of the vortex shedding decreased as a larger fraction of the combustion process heat release occurred in the shear layers surrounding the recirculation zone of the bluff body. Furthermore, a complete transition of the vortex shedding and consequent flame stabilization from asymmetric to symmetric modes was clearly observed when the inlet temperature was reduced from 850°C to 400°C (and hence, significantly increasing the flame dilatation ratio from Tb/Tu ∼ 2.3 to 3.7).

Variations of Anchoring Pattern of a Bluff-Body Stabilized Laminar Premixed Flame as a Function of the Wall Temperature

2016 ◽  
Author(s):  
Sandrine Berger ◽  
Stéphane Richard ◽  
Florent Duchaine ◽  
Laurent Gicquel
Keyword(s):  
Flow Field ◽  
Boundary Conditions ◽  
Combustion Chamber ◽  
Wall Temperature ◽  
Bluff Body ◽  
Flame Stabilization ◽  
Premixed Flame ◽  
Strong Impact ◽  
Flame Holder ◽  
Thermal Boundary Conditions

Aircraft engine components are subject to hostile thermal environments. The solid parts in the hot stages encounter very high temperature levels and gradients that are critical for the engine lifespan. Combustion chamber walls in particular exhibit very heterogeneous thermal fields. The prediction of this specific thermal field is a very complex task as it results from complex interactions between fresh gas injections, cooling flow distributions, combustion, flame stabilization and thermal transfers to the solids. All these phenomena are tightly coupled and do not evolve linearly. Today, the design phase of a combustion chamber is strongly enhanced by the use of high fidelity computations such as Large Eddy Simulations (LES). However, thermal boundary conditions are rarely well known and are thus treated either as adiabatic or as approximated isothermal conditions. Such approximations on thermal boundary conditions can lead to several errors and inaccurate predictions of the combustion chamber flow field. With this in mind and to foresee the potential difficulties of LES based Conjugate Heat Transfer (CHT) predictions, the effect of the wall temperature on a laminar premixed flame stabilization is numerically investigated in this paper for an academic configuration. The considered case consists of a squared cylinder flame holder at a low Reynolds number for which several wall-resolved Direct Numerical Simulations (DNS) are performed varying the bluff-body wall thermal condition. In such a set-up, the reactive flow and the flame holder interact in a complex way with an underlying strong impact of the wall temperature. For a baseline configuration where the flame holder wall temperature is fixed at 700K, the flow field is steady with a flame stabilized thanks to the recirculation zone of the flame holder. As the wall temperature is decreased, the position of the stabilized flame moves further downstream. The flame remains steady until a threshold cold temperature is reached below which an instability appears. For solid temperatures above 700 K, the flame is seen to move further and further upstream. For very hot conditions, the flame even stabilizes ahead of the bluff-body. The various flow solution bifurcations as the flame stabilization evolves are detailed in this paper. Heat flux distribution along the bluff-body walls are observed to be dictated by the flame stabilization process illustrating different mechanisms while integration of these fluxes on the whole flame holder surface confirms that various theoretical equilibrium states may exist for this configuration. This suggests that computation of more realistic cases including thermal conduction in the bluff-body solid part could lead to different converged results depending on the initial thermal state.

Experimental and Computational Studies of an Ultra-Compact Combustor

2013 ◽  
Author(s):  
David Blunck ◽  
Dale Shouse ◽  
Craig Neuroth ◽  
Ryan Battelle ◽  
Amy Lynch ◽  
...  
Keyword(s):  
Systems Engineering ◽  
High Speed ◽  
Weight Ratio ◽  
Combustion Efficiency ◽  
Inlet Temperature ◽  
Pressure Losses ◽  
Flame Holder ◽  
Air Jets ◽  
Turbine Inlet ◽  
Trapped Vortex

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.

Unsteady Non-Reacting and Reacting Flow Simulations of a Triangular Bluff-Body Flameholder

2019 ◽  
Author(s):  
Manoj Mannari ◽  
A. T. Sriram ◽  
Gursharanjit Singh ◽  
S. Ganesan
Keyword(s):  
Bluff Body ◽  
Stokes Equations ◽  
Velocity Profiles ◽  
Flame Stabilization ◽  
Navier Stokes ◽  
Reacting Flow ◽  
Turbine Engine ◽  
Flame Holder ◽  
Eddy Simulation ◽  
Flow Features

Abstract Triangular bluff-body flame-holders, commonly called v-gutters, are used in gas turbine engine afterburners for flame stabilization. Unsteady flow dynamics in the v-gutter wake plays an important role in phenomenon such as combustion instability. The objective of the present work is to simulate the flow features for flow past v-gutter with Unsteady Reynolds-Averaged Navier–Stokes equations (URANS) and Large Eddy Simulation (LES) approach using Eddy Dissipation Model (EDM) for turbulent combustion. Volvo’s bluff body flame-holder rig configuration has been used for the current study. Both non-reacting and reacting simulations have been carried out in commercial Computational Fluid Dynamics (CFD) code FLUENT. In the case of non-reacting flow, the structure of instantaneous velocity field of LES differs from URANS approach. The predicted velocity profiles with LES have shown good agreement with the experiment in most of the regions than URANS for the case of non-reacting flow. In the case of reacting flow, LES simulations have shown unsteady flame with eddy structures. In contrast, URANS simulations have shown more like a steady flame. Overall, the temperature and velocity profiles were better predicted by LES than by URANS approach. In addition, simulations were carried out with circular flame-holder in an annular duct. The oscillation characteristics were found to be more or less similar for both straight and circular flame-holder.

Computational and Experimental Analysis of a Compact Combustor Integrated into a JetCat P90 RXi

2021 ◽  
Author(s):  
Daniel Holobeny ◽  
Brian T. Bohan ◽  
Marc D. Polanka
Keyword(s):  
Bluff Body ◽  
Flame Stabilization ◽  
Inlet Temperature ◽  
Gas Temperature ◽  
Turbine Engine ◽  
Pressure Losses ◽  
Primary Zone ◽  
Sustained Operation ◽  
Mass Flow Rates ◽  
Stable Flame

Abstract Ultra Compact Combustors (UCC) look to reduce the overall combustor length and weight in modern gas turbine engines. Previously, a UCC achieved self-sustained operation at sub-idle speeds in a JetCat P90 RXi turbine engine with a length savings of 33% relative to the stock combustor. However, that combustor experienced flameout as reactions were pushed out of the primary zone before achieving mass flow rates at the engine's idle condition. A new combustor that utilized a bluff body flame stabilization with a larger combustor volume looked to keep reactions in the primary zone within the same axial dimensions. This design was investigated computationally for generalized flow patterns, pressure losses, exit temperature profiles, and reaction distributions at three engine power conditions. The computational results showed the validity of this new Ultra Compact Combustor, with a turbine inlet temperature of 1080 K and a pattern factor of 0.67 at the cruise condition. The combustor was then built and tested in the JetCat P90 RXi with rotating turbomachinery and gaseous propane fuel. The combustor maintained a stable flame from ignition through the 36,000 RPM idle condition. The engine ran self-sustained from 25,000 to 36,000 RPM with an average exit gas temperature of 980 K, which is comparable to the stock engine.

Determination of Equivalence Ratio and Oscillatory Heat Release Distributions in Non-Premixed Bluff Body-Stabilized Flames Using Chemiluminescence Imaging

2011 ◽  
Author(s):  
Caleb Cross ◽  
Eugene Lubarsky ◽  
Dmitriy Shcherbik ◽  
Keary Bonner ◽  
Alex Klusmeyer ◽  
...  
Keyword(s):  
Heat Release ◽  
Vortex Shedding ◽  
Equivalence Ratio ◽  
Bluff Body ◽  
Fuel Injection ◽  
Flame Stabilization ◽  
Operating Conditions ◽  
Flame Holder ◽  
Air Stream ◽  
Local Equivalence

In an effort to elucidate the fundamental processes controlling bluff body flame stabilization, the dependence of the spatial distribution of the local equivalence ratio and the heat release dynamics upon the mode of fuel injection was studied. Experiments were performed in a single flame holder combustion channel which was supplied with a high-temperature air stream. Jet-A fuel was injected across the incoming air stream from one of two locations: a cylindrical fuel bar installed 0.25 m upstream of the bluff body, or from fuel injectors integrated within the bluff body 2.5 cm upstream of the trailing edge (i.e., close-coupled injection). The time-averaged spatial distributions of the combustion heat release were characterized by CH* and C2* chemiluminescence imaging of the flame, and ratios of the C2* to CH* light emission were used to characterize the local equivalence ratio. The spatial average of the C2*/CH* value in the flame was found to increase linearly with increasing global equivalence ratio for fuel injection upstream of the bluff body, whereas this value was relatively constant for close-coupled injection. This constant value equaled the same average C2*/CH* value obtained for upstream fuel injection at globally stoichiometric conditions, suggesting that combustion resulting from close-coupled fuel injection took place, on average, in stoichiometric flamelets throughout the combustor. The heat release dynamics due to asymmetric (von Ka´rma´n) vortex shedding were also investigated for each operating condition by recording high-speed movies of the flame at 24 kHz. Upon processing of these movies, the amplitudes of heat release fluctuations due to von Ka´rma´n vortex shedding were found to be significantly higher for close-coupled injection than for injection well upstream of the flame holder for all operating conditions. This is attributed to an increase in span-wise fuel-air mixing and near-wake heat release for upstream fuel injection, resulting in a hotter recirculation zone which suppressed the von Ka´rma´n instability more than the close-coupled case.

Performance Comparison and Parametric Analysis of sCO2 Power Cycles Configurations

2018 ◽  
Author(s):  
Ali S. Alsagri ◽  
Andrew Chiasson ◽  
Ahmad Aljabr
Keyword(s):  
Power Output ◽  
Thermal Efficiency ◽  
Performance Comparison ◽  
Specific Power ◽  
Brayton Cycle ◽  
Computationally Efficient ◽  
Power Cycles ◽  
Optimization Study ◽  
Specific Power Output ◽  
Improve Calculation

A thermodynamic analysis and optimization of four supercritical CO2 Brayton cycles were conducted in this study in order to improve calculation accuracy; the feasibility of the cycles; and compare the cycles’ design points. In particular, the overall thermal efficiency and the power output are the main targets in the optimization study. With respect to improving the accuracy of the analytical model, a computationally efficient technique using constant conductance (UA) to represent heat exchanger performances is executed. Four Brayton cycles involved in this compression analysis, simple recaptured, recompression, pre-compression, and split expansion. The four cycle configurations were thermodynamically modeled and optimized based on a genetic algorithm (GA) using an Engineering Equation Solver (EES) software. Results show that at any operating condition under 600 °C inlet turbine temperature, the recompression sCO2 Brayton cycle achieves the highest thermal efficiency. Also, the findings show that the simple recuperated cycle has the highest specific power output in spite of its simplicity.

Review and Analysis of Bluff Body Flame Stabilization Data

2008 ◽  
Author(s):  
Sajjad A. Husain ◽  
Ganesh Nair ◽  
Santosh Shanbhogue ◽  
Tim C. Lieuwen
Keyword(s):  
Activation Energy ◽  
Kinetic Modeling ◽  
First Principles ◽  
Bluff Body ◽  
Large Range ◽  
Flame Stabilization ◽  
Local Extinction ◽  
Data Set ◽  
Flame Sheet ◽  
Semi Empirical

This paper compiles and analyzes bluff body stabilized flame blowoff data from the literature. Many of these studies contain semi-empirical blowoff correlations that are, in essence, Damko¨hler number correlations of their data. This paper re-analyzes these data, utilizing various Damko¨hler number correlations based upon detailed kinetic modeling for determining chemical time scales. While the results from this compilation are similar to that deduced from many earlier studies, it demonstrates that a rather comprehensive data set taken over a large range of conditions can be correlated from “first-principles” based calculations that do not rely on empirical fits or adjustable constants (e.g., global activation energy or pressure exponents). The paper then discusses the implications of these results on understanding of blowoff. Near blowoff flames experience local extinction of the flame sheet, manifested as “holes” that form and convect downstream. However, local extinction is distinct from blowoff — in fact, under certain conditions the flame can apparently persist indefinitely with certain levels of local extinction. We hypothesize that simple Damko¨hler number correlations contain the essential physics describing this first stage of blowoff; i.e., they are correlations for the conditions where local extinction on the flame begins, but do not fundamentally describe the ultimate blowoff condition itself. However, such correlations are reasonably successful in correlating blowoff limits because the ultimate blowoff event appears to be correlated to some extent to the onset of this first stage.

Simulation of Two-Dimensional Turbulent Recirculating Flow Field Behind a V-Shaped Bluff Body

1994 ◽  
Author(s):  
Z. Gu ◽  
M. A. R. Sharif
Keyword(s):  
Flow Field ◽  
Bluff Body ◽  
Flame Stabilization ◽  
Bluff Bodies ◽  
Two Dimensional ◽  
Combustion Chambers ◽  
Recirculating Flow ◽  
Conservative Form ◽  
Curvilinear Grid ◽  
Predictor Corrector

Abstract The two-dimensional turbulent recirculating flow fields behind a V-shaped bluff body have been investigated numerically. Similar bluff bodies are used in combustion chambers for flame stabilization. The governing transport equations in conservative form are solved by a pressure based predictor-corrector method. The standard k-ϵ turbulence closure model and a boundary fitted multi-block curvilinear grid system are used in the computation. The code is validated against turbulent flow over a backward facing step problem. The predicted flow field behind the bluff body is also compared with experiment. It is found that while the qualitative features of the flow are well predicted, there is quantitative disagreement between the measurement and prediction. This disagreement can be partially attributed to the k-ϵ turbulence model which is known to be inadequate for recirculating flows. Parametric investigation of the flow field by varying the shape and size of the bluff body is also performed and the results are reported.

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