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Title: Extinction of counterflow diffusion flames with radiative heat loss and nonunity Lewis numbers

Abstract

The structure and extinction characteristics of counterflow diffusion flames with flame radiation and nonunity Lewis numbers of the fuel and oxidant are examined using multiscale asymptotic theory, and a model expressed in terms of the jump relations and reactant leakages with the proper consideration of the excess enthalpy overlooked in previous analyses is developed. The existence of the dual extinction limits in the presence of radiative heat loss, namely the kinetic limit at small Damkoehler number (high stretch rate) and the radiative limit at large Damkoehler number (low stretch rate), are identified. It is found that the former is minimally affected by radiative loss, while a substantial amount of heat loss is associated with the radiative limit. Reactant leakage, however, is the root cause for both limits. The influence of radiative loss on the extinction Damkoehler numbers is found to be through its effects on the flame temperature, the excess enthalpy, and the reduced extinction Damkoehler number. At both extinction limits, the contribution from the flame temperature is always important and dominant. The contributions from the other two, however, could be important in some special cases. At small Le{sub F}, the contribution from the reduced extinction Damkoehler number is largemore » and even dominant under small radiative loss. The contribution from the excess enthalpy is important for small Le{sub O} and it may be comparable to the contribution from the flame temperature when radiative loss is small. Thus, overlooking the excess enthalpy in previous analyses may have resulted in rather large error in the predicted extinction Damkoehler numbers, especially the kinetic one. (author)« less

Authors:
;  [1];  [2]
  1. Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544-5263 (United States)
  2. Department of Marine Engineering, National Taiwan Ocean University, Keelung (Taiwan)
Publication Date:
OSTI Identifier:
20864953
Resource Type:
Journal Article
Resource Relation:
Journal Name: Combustion and Flame; Journal Volume: 148; Journal Issue: 3; Other Information: Elsevier Ltd. All rights reserved
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ENTHALPY; HEAT LOSSES; FLAMES; DIFFUSION; COMBUSTION PROPERTIES; LEAKS; ERRORS; FLAME EXTINCTION; RADIATIVE COOLING; LIMITING VALUES

Citation Formats

Wang, H.Y., Law, C.K., and Chen, W.H. Extinction of counterflow diffusion flames with radiative heat loss and nonunity Lewis numbers. United States: N. p., 2007. Web. doi:10.1016/J.COMBUSTFLAME.2006.10.005.
Wang, H.Y., Law, C.K., & Chen, W.H. Extinction of counterflow diffusion flames with radiative heat loss and nonunity Lewis numbers. United States. doi:10.1016/J.COMBUSTFLAME.2006.10.005.
Wang, H.Y., Law, C.K., and Chen, W.H. Thu . "Extinction of counterflow diffusion flames with radiative heat loss and nonunity Lewis numbers". United States. doi:10.1016/J.COMBUSTFLAME.2006.10.005.
@article{osti_20864953,
title = {Extinction of counterflow diffusion flames with radiative heat loss and nonunity Lewis numbers},
author = {Wang, H.Y. and Law, C.K. and Chen, W.H.},
abstractNote = {The structure and extinction characteristics of counterflow diffusion flames with flame radiation and nonunity Lewis numbers of the fuel and oxidant are examined using multiscale asymptotic theory, and a model expressed in terms of the jump relations and reactant leakages with the proper consideration of the excess enthalpy overlooked in previous analyses is developed. The existence of the dual extinction limits in the presence of radiative heat loss, namely the kinetic limit at small Damkoehler number (high stretch rate) and the radiative limit at large Damkoehler number (low stretch rate), are identified. It is found that the former is minimally affected by radiative loss, while a substantial amount of heat loss is associated with the radiative limit. Reactant leakage, however, is the root cause for both limits. The influence of radiative loss on the extinction Damkoehler numbers is found to be through its effects on the flame temperature, the excess enthalpy, and the reduced extinction Damkoehler number. At both extinction limits, the contribution from the flame temperature is always important and dominant. The contributions from the other two, however, could be important in some special cases. At small Le{sub F}, the contribution from the reduced extinction Damkoehler number is large and even dominant under small radiative loss. The contribution from the excess enthalpy is important for small Le{sub O} and it may be comparable to the contribution from the flame temperature when radiative loss is small. Thus, overlooking the excess enthalpy in previous analyses may have resulted in rather large error in the predicted extinction Damkoehler numbers, especially the kinetic one. (author)},
doi = {10.1016/J.COMBUSTFLAME.2006.10.005},
journal = {Combustion and Flame},
number = 3,
volume = 148,
place = {United States},
year = {Thu Feb 15 00:00:00 EST 2007},
month = {Thu Feb 15 00:00:00 EST 2007}
}
  • The asymptotic structure of counterflow and stagnant diffusion flames are analyzed in the limit for large values of the overall, nondimensional activation energy, T/sub a/, characterizing the rate of the reaction, and results are given for small values of the stoichiometric fuel to oxygen mass ratio. The chemical reaction between the fuel and the oxidizer is represented by a one-step, irreversible process. A new approach is developed to characterize the influence of the Lewis number of the fuel, L/sub F/, and the Lewis number of the oxidizer, L/sub O/, on the outer and inner structure of near equilibrium diffusion flames.more » Explicit algebraic formulas to predict the critical conditions of flame extinction are also given. The results developed here are used to obtain overall chemical kinetic rate parameters characterizing the gas phase oxidation of methane using previously measured values of the critical conditions of flame extinction.« less
  • A principal effect of turbulence on premixed flames in the flamelet regime is to wrinkle the flame fronts. For nonunity Lewis numbers, Le [ne] 1, the local flame structure is altered in curved regions. This effect is examined using direct numerical simulations of three-dimensional isotropic turbulence with constant density, single-step Arrhenius kinetics chemistry. Simulations of Lewis numbers 0.8, 1.0, and 1.2 are compared. At the local level, curvature effects dominated changes to the flame structure while strain effects were insignificant. A strong Lewis-number-dependent correlation was found between surface curvature and the local flame speed. The correlation was positive for Lemore » < 1 and negative for Le > 1. At the global level, strain-related effects were more significant than curvature effects. The turbulent flame speed changed significantly with Lewis number, increasing as Le decreased. This was found to be due to strain effect that have a nonzero mean over the flame surface, rather than to curvature effects that have a nearly zero mean. The mean product temperature was also found to vary with Lewis number, being higher for Le > 1 and lower for Le < 1.« less
  • A theoretical analysis is given of the effect of nonluminous thermal radiation on the properties of counterflow diffusion flames at high pressure. The self-consistent analysis includes an expression for gas band radiant dissipation in the energy equation of a counterflow flame solver. NO[sub x] formation rates and other properties are studied as a function of strain rate for adiabatic, optically thin, and optical thickness-corrected flames. For adiabatic flames, NO[sub x] concentration and flame temperature increase continuously with decreasing strain rate. For radiating flames, a temperature level off is exhibited due to the competing effects of heat loss and extent ofmore » reaction as strain rate (inverse residence time) decreases. For the very lowest strain rates, up to 5% of the flame enthalpy is converted to radiation, and optical thickness corrections start to become important. Certain factors that need to be taken into account in relating the isolated flamelet results to practical gas turbine combustors are also discussed.« less
  • Extensive results from axisymmetric convergent-nozzle and straight-tube opposed jet burners (OJBs) characterized strain-induced extinction of unanchored (free-floating), laminar H{sub 2}/N{sub 2}-air flames. Parameters included (a) plug-flow and parabolic input velocity profiles, (b) jet exit diameters ranging 2.7 to 7.2 mm for nozzles and 1.8 to 10 mm for tubes, (c) various relative jet gaps, and (d) 14 to 100% H{sub 2} in the fuel jet. Extinction, a sudden rupture (blowoff) of the mostly-airside disk flame, occurred as fuel and air flows were slowly increased and a critical radial strain rate was exceeded. Focusing schlieren, thermocouple, and airside LDV (and PIV)more » data confirmed the (1-D) character of nozzle-OJB flow fields; axial widths of velocity- and thermal-layers varied as (input strain rate){sup {minus}1/2} for both nozzles and tubes. The global approximation of a 1-D applied stress rate (ASR), using average air jet velocity divided by exit diameter, enabled high quality correlations of extinction data with varied H{sub 2} concentrations for both nozzles and tubes. Pre-extinction ASRs for nozzles agreed closely with LDV-measured centerline input strain rates; for tubes, however, an empirical factor of 3 produced close agreement. For extinction of 100% H{sub 2}-air, an ASR of 5,670 1/s compared with 7,350, 8,140, and 8,060 from independent 1-D numerical evaluations using potential-flow inputs; for 50 to 14% H{sub 2} inputs, agreement was much closer. The nozzle-ASR/tube-ASR ratio for extinction was {ge} 3 for < 50% H{sub 2} inputs, 2.74 {+-} 0.03 for 50 to 100% H{sub 2} inputs.« less
  • In this paper the combined influence of the fuel and the oxidizer concentrations and Lewis numbers on the extinction of finite-jet counterflow diffusion flames is experimentally and theoretically investigated. In the experiments, either methane or butane mixed in nitrogen is the fuel jet, while mixtures of oxygen and nitrogen are used as the oxidizer jet. The critical minimum values of the fuel (oxidizer) concentrations at extinction are determined at different values of the oxidizer (fuel) concentration in the opposing jet and various fixed values of the jet velocity. Thus, the domain of 'flammability' of the diffusion flames within the completemore » range of the fuel/oxidizer molar concentration (X{sub F}, X{sub O}) is obtained at various fixed values of the mean nozzle velocities. The concept of the 'external' or global versus the internal or local equivalence ratio is introduced to describe the simultaneous impact of the fuel and the oxidizer concentrations on the burning characteristics of diffusion flames.« less