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Title: Lifted methane-air jet flames in a vitiated coflow

Abstract

The present vitiated coflow flame consists of a lifted jet flame formed by a fuel jet issuing from a central nozzle into a large coaxial flow of hot combustion products from a lean premixed H{sub 2}/air flame. The fuel stream consists of CH{sub 4} mixed with air. Detailed multiscalar point measurements from combined Raman-Rayleigh-LIF experiments are obtained for a single base-case condition. The experimental data are presented and then compared to numerical results from probability density function (PDF) calculations incorporating various mixing models. The experimental results reveal broadened bimodal distributions of reactive scalars when the probe volume is in the flame stabilization region. The bimodal distribution is attributed to fluctuation of the instantaneous lifted flame position relative to the probe volume. The PDF calculation using the modified Curl mixing model predicts well several but not all features of the instantaneous temperature and composition distributions, time-averaged scalar profiles, and conditional statistics from the multiscalar experiments. A complementary series of parametric experiments is used to determine the sensitivity of flame liftoff height to jet velocity, coflow velocity, and coflow temperature. The liftoff height is found to be approximately linearly related to each parameter within the ranges tested, and it is most sensitivemore » to coflow temperature. The PDF model predictions for the corresponding conditions show that the sensitivity of flame liftoff height to jet velocity and coflow temperature is reasonably captured, while the sensitivity to coflow velocity is underpredicted.« less

Authors:
; ;  [1]; ;  [2]
  1. Mechanical Engineering Department, University of California, Berkeley, CA 94720 (United States)
  2. Combustion Research Facility, Sandia National Laboratories, Livermore, CA 94550 (United States)
Publication Date:
OSTI Identifier:
20681466
Resource Type:
Journal Article
Resource Relation:
Journal Name: Combustion and Flame; Journal Volume: 143; Journal Issue: 4; Other Information: Elsevier Ltd. All rights reserved
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; 03 NATURAL GAS; METHANE; AIR; FLAMES; MATHEMATICAL MODELS; JETS; TEMPERATURE DISTRIBUTION; VELOCITY; TEMPERATURE DEPENDENCE

Citation Formats

Cabra, R., Chen, J.-Y., Dibble, R.W., Karpetis, A.N., and Barlow, R.S.. Lifted methane-air jet flames in a vitiated coflow. United States: N. p., 2005. Web. doi:10.1016/j.combustflame.2005.08.019.
Cabra, R., Chen, J.-Y., Dibble, R.W., Karpetis, A.N., & Barlow, R.S.. Lifted methane-air jet flames in a vitiated coflow. United States. doi:10.1016/j.combustflame.2005.08.019.
Cabra, R., Chen, J.-Y., Dibble, R.W., Karpetis, A.N., and Barlow, R.S.. Thu . "Lifted methane-air jet flames in a vitiated coflow". United States. doi:10.1016/j.combustflame.2005.08.019.
@article{osti_20681466,
title = {Lifted methane-air jet flames in a vitiated coflow},
author = {Cabra, R. and Chen, J.-Y. and Dibble, R.W. and Karpetis, A.N. and Barlow, R.S.},
abstractNote = {The present vitiated coflow flame consists of a lifted jet flame formed by a fuel jet issuing from a central nozzle into a large coaxial flow of hot combustion products from a lean premixed H{sub 2}/air flame. The fuel stream consists of CH{sub 4} mixed with air. Detailed multiscalar point measurements from combined Raman-Rayleigh-LIF experiments are obtained for a single base-case condition. The experimental data are presented and then compared to numerical results from probability density function (PDF) calculations incorporating various mixing models. The experimental results reveal broadened bimodal distributions of reactive scalars when the probe volume is in the flame stabilization region. The bimodal distribution is attributed to fluctuation of the instantaneous lifted flame position relative to the probe volume. The PDF calculation using the modified Curl mixing model predicts well several but not all features of the instantaneous temperature and composition distributions, time-averaged scalar profiles, and conditional statistics from the multiscalar experiments. A complementary series of parametric experiments is used to determine the sensitivity of flame liftoff height to jet velocity, coflow velocity, and coflow temperature. The liftoff height is found to be approximately linearly related to each parameter within the ranges tested, and it is most sensitive to coflow temperature. The PDF model predictions for the corresponding conditions show that the sensitivity of flame liftoff height to jet velocity and coflow temperature is reasonably captured, while the sensitivity to coflow velocity is underpredicted.},
doi = {10.1016/j.combustflame.2005.08.019},
journal = {Combustion and Flame},
number = 4,
volume = 143,
place = {United States},
year = {Thu Dec 01 00:00:00 EST 2005},
month = {Thu Dec 01 00:00:00 EST 2005}
}
  • The joint velocity-turbulence frequency-composition PDF method is applied to a lifted turbulent jet flame with H{sub 2}/N{sub 2} fuel issuing into a wide coflow of lean combustion products, which are at a temperature of 1045 K. Model calculations with detailed chemistry are performed using three existing mixing models (IEM, MC, and EMST) and two chemistry mechanisms (the Mueller and Li mechanisms). Numerically accurate results are obtained and compared with the experimental data. Recent experiments have shown that the stabilization height of this lifted flame is very sensitive to the coflow temperature, much more than to the inlet velocity profile ormore » the initial temperature of the fuel. One percent (i.e., 10 K) change in the coflow temperature (which is well within the experimental uncertainty) can double the lift-off height. The joint PDF calculations capture this sensitivity very well and are in good agreement with the measurements for the velocity, mixture fraction, and species. The three mixing models give relatively similar results, implying that the cases studied here are mainly controlled by chemical kinetics. The Li mechanism results in earlier ignition than the Mueller mechanism and hence gives shorter lift-off heights over the whole test range. The joint PDF calculations generally give better agreement with the measurements than previous composition PDF calculations [A.R. Masri et al., Combust. Theory Modelling 8 (2004) 1-22]. A new parallel algorithm, involving domain partitioning of particles, has been implemented to facilitate these computations.« less
  • The autoignition characteristics of laminar lifted flames of methane, ethylene, ethane, and n-butane fuels have been investigated experimentally in coflow air with elevated temperature over 800 K. The lifted flames were categorized into three regimes depending on the initial temperature and fuel mole fraction: (1) non-autoignited lifted flame, (2) autoignited lifted flame with tribrachial (or triple) edge, and (3) autoignited lifted flame with mild combustion. For the non-autoignited lifted flames at relatively low temperature, the existence of lifted flame depended on the Schmidt number of fuel, such that only the fuels with Sc > 1 exhibited stationary lifted flames. Themore » balance mechanism between the propagation speed of tribrachial flame and local flow velocity stabilized the lifted flames. At relatively high initial temperatures, either autoignited lifted flames having tribrachial edge or autoignited lifted flames with mild combustion existed regardless of the Schmidt number of fuel. The adiabatic ignition delay time played a crucial role for the stabilization of autoignited flames. Especially, heat loss during the ignition process should be accounted for, such that the characteristic convection time, defined by the autoignition height divided by jet velocity was correlated well with the square of the adiabatic ignition delay time for the critical autoignition conditions. The liftoff height was also correlated well with the square of the adiabatic ignition delay time. (author)« less
  • Autoignition of hydrocarbon fuels is an outstanding research problem of significant practical relevance in engines and gas turbine applications. This paper presents a numerical study of the autoignition of methane, the simplest in the hydrocarbon family. The model burner used here produces a simple, yet representative lifted jet flame issuing in a vitiated surrounding. The calculations employ a composition probability density function (PDF) approach coupled to the commercial CFD package, FLUENT. The in situ adaptive tabulation (ISAT) method is used to implement detailed chemical kinetics. An analysis of species concentrations and transport budgets of convection, turbulent diffusion, and chemical reactionmore » terms is performed with respect to selected species at the base of the lifted turbulent flames. This analysis provides a clearer understanding of the mechanism and the dominant species that control autoignition. Calculations are also performed for test cases that clearly distinguish autoignition from premixed flame propagation, as these are the two most plausible mechanisms for flame stabilization for the turbulent lifted flames under investigation. It is revealed that a radical pool of precursors containing minor species such as CH{sub 3}, CH{sub 2}O, C{sub 2}H{sub 2}, C{sub 2}H{sub 4}, C{sub 2}H{sub 6}, HO{sub 2}, and H{sub 2}O{sub 2} builds up prior to autoignition. The transport budgets show a clear convective-reactive balance when autoignition occurs. This is in contrast to the reactive-diffusive balance that occurs in the reaction zone of premixed flames. The buildup of a pool of radical species and the convective-reactive balance of their transport budgets are deemed to be good indicators of the occurrence of autoignition. (author)« less
  • Abstract not provided.
  • This paper details a quantitative joint temperature, OH, and CH{sub 2}O imaging experiment designed to investigate the stabilization of lifted turbulent methane flames issuing into a high temperature vitiated coflow. Temperature is determined through Rayleigh imaging, and the data are used to quantify OH-LIF excited at 283.011 nm, and to enable to semi-quantification of CH{sub 2}O-LIF excited at 355 nm. A fuel with Rayleigh cross-section equal to that of the vitiated coflow was used to improve accuracy in the processing of the Rayleigh temperature. Results of the experiment have been presented, and compared to simulations of laminar transient autoignition flamelets.more » The images were classified in three main categories: (i) CH{sub 2}O only, (ii) ignition kernels, and (iii) liftoff flames. Images of type (i) and (ii) were dominant in the early part of the jet, while images of type (iii) were dominant after the mean stabilization height. By examining OH and CH{sub 2}O conditional on the size of the kernel, it was found that the sequence of conditional data was analogous to the evolution of autoignition, following the key stages of (1) build-up of a precursor pool, (2) initiation of reaction, and (3) formation of a steady flame. Viewed in such a sequence, CH{sub 2}O peaks prior to the autoignition and then decays after ignition, and OH is found to peak at ignition and these peaks are maintained into the established steady flames. This is in qualitative agreement with the laminar transient flamelet calculations. The data are consistent with the view that autoignition is the main stabilization mechanism in this lifted flame. (author)« less