Barry Kiel

A Detailed Investigation of Bluff Body Stabilized Flames

Abstract : Reduced Order Models (ROMs) and Computational Fluid Dynamics (CFD) codes are tools used to predict the extinction of flames behind bluff bodies. Accurate prediction of these models and codes is predicated on their validation with experimental data. This paper describes detailed experiments to obtain validation data for bluff body stabilized flames over a wide range of conditions. Included are non-reacting data from CFD and LDV, lean blowout and high speed images for three different flame holders. In our previous paper (Kiel 2006) it was asserted that the large vortices were a major driver of extinction. Those assertions are further supported here. It is concluded that the vortex dynamics and not geometry is the dominant mechanism for bluff body flame extinction. This conclusion is supported by the lean blowout data, by the high speed images and reference data from NACA.

LES Blowout Analysis of Premixed Flow Past V-gutter Flameholder

In this study, we have analyzed premixed propane/air reacting in a duct, with the flame being anchored by a V-gutter flameholder. 3D Large Eddy Simulation (LES) analyses were performed for flow conditions of one atmosphere, 700K, and inlet velocity of 52.1 m/s (Reynolds number of 29,000 based on flameholder height). Three types of analyses were performed: 1) nonreacting flow with nonvitiated air, 2) reacting flow with nonvitiated air at various equivalence ratios leading to blowout, and 3) reacting flow with vitiated air at various equivalence ratios leading to blowout. The predictions were qualitatively compared to experimental measurements of time-averaged mean and rms velocities, Strouhal number, flame images, and lean blowout. Unfortunately, quantitative comparisons could not be made because of differences between flow conditions and geometry (open versus closed V-gutter) of the experiments and calculations. The predictions were able to capture the basic flame structure near blowout of a Kelvin Helmholtz shear layer off the lip of the V-gutter, followed by von Karman structures further downstream. Blowout predictions for reacting flow with nonvitiated air showed reasonable agreement with experimental measurements, although improvements are needed. Analysis of reacting flow with vitiated air showed that blowout occurred at a fuelflow that was 24% more than the nonvitiated case, caused by lower reaction rates for the vitiated case. The basic flame structure was similar for the nonvitiated and vitiated blowout scenarios.

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

Influence of Turbulence-Chemistry Interaction in Blow-out Predictions of Bluff-Body Stabilized Flames

Large Eddy Simulations (LES) were performed to investigate the effect of turbulence –chemistry interaction on flame instability and flame-vortex interactions in bluff body stabilized premixed flames. A semi-global reduced kinetics mechanism and a skeletal mechanism were developed and implemented with a Laminar Chemistry (LC) model and an Eddy Dissipation Concept (EDC) model to simulate bluff-body stabilized propane-air flames using the experimental conditions of Kiel et al. (2007). Simulations were performed for reactive and non-reactive cases with coarse (0.65 million cells) and fine (2.4 million cells) grids. Simulations with fine grids were able to predict the recirculation zone thickness correctly as observed in the experiments. Simulation results also show that the near-field wake behind the bluff body was dominated by the Von-Karman vortex shedding for the non-reacting case as well as the reacting case with EDC models, while a shear layer generated vortex sheet was observed for reacting flow cases with the LC models. The simulation results demonstrate that turbulence-chemistry interactions play a major role in predicting the blow-out conditions. LES predictions with the EDC model show that the blow-out occurs at 0.6 equivalence ratio as observed experimentally at a DeZubay number of ~10.

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.

Identifying coherent structures in a 3-stream supersonic jet flow using time-resolved schlieren imaging

We analyze time-resolved schlieren images of the near-field of a 3-stream supersonic jet. The primary jet operates in the vicinity of Ma = 1.6, and the images are collected at the rate of 50 to 400 kfps. We analyze transverse-axial images by constructing time series from more than 400 points selected for their possible significance, based on a qualitative analysis of the schlieren images. The points are grouped along the various shear layers and in the near-field outside the jet. We examine in turn the power spectra and cross-correlations between points. Overall qualitative and quantitative trends in both spectra and correlation are noted, revealing a strong dependence of both on transverse and axial location in the flow field. Defining features in the spectra give insight into the frequency bands which will be more closely analyzed in future phases of this study. The results from this preliminary study point to the validity of using time-resolved schlieren imaging as a non-intrusive experimental method to generate time series, to which a range of analysis methods is applicable.

Effects of Vitiation and Pressure on Laminar Flame Speeds of n-Decane

There is currently a lack of experimental data required for kinetic model validation of the effect of oxidizer vitiation on laminar flame speeds of aviation fuels. This study examines the role of vitiation through the introduction of CO2 and H2O to the oxidizer stream at varying pressures (0.5 - 5.0 atm) at 450 K, conditions relevant to vitiated combustion devices, using n-decane as the model fuel. The experimental portion of this effort has acquired laminar flame speed data of n-decane in vitiated air using two separate techniques. A well-validated Bunsen Flame Technique was used to primarily examine the effect of total dilution and vitiation over a range of equivalence ratios and the Combustion Bomb Technique was used to investigate vitiation effects at various pressures and equivalence ratios. Overlap between measurement techniques has been performed as well as comparison to an analytical model to better understand the thermodynamic and chemical kinetic effects that vitiation has on hydrocarbon fuel combustion and flame structure. Experimental data shows that CO2 has the largest effect in reducing the flame speed over the range of equivalence ratios and pressures studied. Based on a kinetic analysis, chemical kinetic effects play a major role in reducing the flame speed when CO2 is present. The impact of chemical kinetic effects due to the diluent species on flame speed was found to have the following trend: CO2 > H2O > N2.

Improved Correlation for Blowout of Bluff Body Stabilized Flames

Abstract : With the advent of high-speed diagnostics and computers, new observations concerning the extinction process have been made, with the most general conclusion being that the extinction process is a wake phenomenon, where the flame is highly strained and dominated by large vortices. In the present paper a new correlation for lean extinction is derived using a linear least-squares fit and more than 800 data points from historical and current experiments. Fits of various dimensionless parameters are made, but the best fit is that of a Damkoehler number with ignition delay as the chemical time scale, verifying many previous conclusions. Finally, it is concluded that flame-holder size--not shape--is the driving parameter that represents the flame-holder geometry.

Investigation of the Effect of Nitric Oxide on the Autoignition of JP-8 at Low Pressure Vitiated Conditions

Currently there is very little data available for jet fuel oxidation at low pressure, vitiated conditions found in some aircraft combustion systems. Due to the lack of this information, current kinetics models do not have the necessary data for validation within these combustions regimes. A previous screening study [1] by the authors has shown that the amount of NO present in the vitiated oxidizer significantly influences the ignition of jet fuel in addition to temperature and oxygen levels. The current study examines the effect of NO on the ignition of JP-8 in detail at temperatures (700 K 900 K), pressures (0.5 atm and 1.0 atm) and oxygen levels (12% and 20%) relevant to vitiated combustion in aircrafts. Experimental results show that small amounts of NO (varied up to 1000 ppm) are capable of dramatically enhancing the oxidization of JP-8 with percent reduction in ignition delay time up to 80%. It is also found that significant coupling exists between NO and the other design variables (temperature, oxygen level and pressure) related to the effect of NO on ignition.

Characterization of Bluff-Body-Flame Vortex Shedding Using Proper Orthogonal Decomposition

Flame stabilization has been of interest for many decades. Bluff-body flame stabilization has been incorporated in gas turbine engines as a means of secondary combustion in high-speed flows. The current work is focused on understanding vortex shedding and its contribution to both blow off and flame stability. Two modes of shedding, Kelvin-Helmoltz and Von-Karman, have been observed to play a major role in the stability and blow off of these bluff-body flames. Typically researchers have observed these modes visually but have been unable to quantify the effective contribution under various flow conditions. The present work is focused on the implementation of Proper Orthogonal Decomposition (POD) as a means of characterizing the energy and nature of these shedding modes as flames transition to acoustic instabilities and blow off. POD provides a new method of assessing the shedding mode and complements the pure visualization and vorticity calculations performed to date. POD is implemented on high-speed images of bluff-body flames at multiple equivalence ratios in an experimental test section. During this equivalence-ratio scan, the flame transitions to an acoustic instability. By incorporation of POD, the symmetric and asymmetric energy contributions through instability and blow off can be described.