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Title: Effects of substituting fuel spray for fuel gas on flame stability in lean premixtures

Abstract

We analyze flame propagation through a homogeneous three-component premixture composed of fuel gas, small fuel droplets, and air. This analytical study is carried out within the framework of a diffusional-thermal model with the simplifying assumption that both fuels - the fuel in the gaseous phase and the gaseous fuel evaporating from the droplets - have the same Lewis number. The parameter that expresses the degree of substitution of spray for gas is {delta}, the liquid loading, i.e., the ratio of liquid fuel mass fraction to overall fuel mass fraction in the fresh premixture. In this substitution of liquid fuel for gaseous fuel, the overall equivalence ratio is lean and is kept identical. We hence obtain a partially prevaporized spray, for which we analytically study the dynamics of the plane spray-flame front. The investigated model assumes the averaged distance between droplets to be small compared with the premixed flame thickness (i.e., small droplets and moderate pressure). Le, the Lewis number, Ze, the Zeldovich number, and {delta} are the main parameters of the study. Our stability analysis supplies the stability diagram in the plane {l_brace}Le,{delta}{r_brace} for various Ze values and shows that, for all Le, the plane front becomes unstable for highmore » liquid loading. At large or moderate Lewis number, we show that the presence of droplets substantially diminishes the onset threshold of the oscillatory instability, making the appearance of oscillatory propagation easier. Oscillations can even occur for Le<1 when sufficient spray substitution is operated. The pulsation frequency occurring in this regime is a tunable function of {delta}. At low Lewis number, substitution of spray for gas leads to a more complex situation for which two branches can coexist: the first one still corresponding to the pulsating regime, the other one being related to the diffusive-thermal cellular instability. (author)« less

Authors:
; ;  [1]
  1. Modelisation et Simulation Numerique en Mecanique et Genie des Procedes (MSNM-GP), UMR-CNRS n and cir 6181, Aix-Marseille Universites - ECM, I.M.T./La Jetee/L3M, 38, rue Frederic Joliot-Curie, 13451 Marseille Cedex 20 (France)
Publication Date:
OSTI Identifier:
20880665
Resource Type:
Journal Article
Resource Relation:
Journal Name: Combustion and Flame; Journal Volume: 149; 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; GAS FUELS; DROPLETS; LIQUID FUELS; SPRAYS; FLAMES; COMBUSTION; STABILITY; AIR; INSTABILITY; DIAGRAMS; FLAME PROPAGATION; PULSATIONS; FUNCTIONS; DISTANCE; OSCILLATIONS; DIESEL ENGINES; FURNACES

Citation Formats

Nicoli, C., Haldenwang, P., and Suard, S. Effects of substituting fuel spray for fuel gas on flame stability in lean premixtures. United States: N. p., 2007. Web. doi:10.1016/J.COMBUSTFLAME.2006.12.018.
Nicoli, C., Haldenwang, P., & Suard, S. Effects of substituting fuel spray for fuel gas on flame stability in lean premixtures. United States. doi:10.1016/J.COMBUSTFLAME.2006.12.018.
Nicoli, C., Haldenwang, P., and Suard, S. Tue . "Effects of substituting fuel spray for fuel gas on flame stability in lean premixtures". United States. doi:10.1016/J.COMBUSTFLAME.2006.12.018.
@article{osti_20880665,
title = {Effects of substituting fuel spray for fuel gas on flame stability in lean premixtures},
author = {Nicoli, C. and Haldenwang, P. and Suard, S.},
abstractNote = {We analyze flame propagation through a homogeneous three-component premixture composed of fuel gas, small fuel droplets, and air. This analytical study is carried out within the framework of a diffusional-thermal model with the simplifying assumption that both fuels - the fuel in the gaseous phase and the gaseous fuel evaporating from the droplets - have the same Lewis number. The parameter that expresses the degree of substitution of spray for gas is {delta}, the liquid loading, i.e., the ratio of liquid fuel mass fraction to overall fuel mass fraction in the fresh premixture. In this substitution of liquid fuel for gaseous fuel, the overall equivalence ratio is lean and is kept identical. We hence obtain a partially prevaporized spray, for which we analytically study the dynamics of the plane spray-flame front. The investigated model assumes the averaged distance between droplets to be small compared with the premixed flame thickness (i.e., small droplets and moderate pressure). Le, the Lewis number, Ze, the Zeldovich number, and {delta} are the main parameters of the study. Our stability analysis supplies the stability diagram in the plane {l_brace}Le,{delta}{r_brace} for various Ze values and shows that, for all Le, the plane front becomes unstable for high liquid loading. At large or moderate Lewis number, we show that the presence of droplets substantially diminishes the onset threshold of the oscillatory instability, making the appearance of oscillatory propagation easier. Oscillations can even occur for Le<1 when sufficient spray substitution is operated. The pulsation frequency occurring in this regime is a tunable function of {delta}. At low Lewis number, substitution of spray for gas leads to a more complex situation for which two branches can coexist: the first one still corresponding to the pulsating regime, the other one being related to the diffusive-thermal cellular instability. (author)},
doi = {10.1016/J.COMBUSTFLAME.2006.12.018},
journal = {Combustion and Flame},
number = 3,
volume = 149,
place = {United States},
year = {Tue May 15 00:00:00 EDT 2007},
month = {Tue May 15 00:00:00 EDT 2007}
}
  • Previously we show that fuels with higher laminar flame speed also have increased tolerance to EGR dilution. In this work, the effects of fuel laminar flame speed on both lean and EGR dilute spark ignition combustion stability were examined. Fuels blends of pure components (iso-octane, n-heptane, toluene, ethanol, and methanol) were derived at two levels of laminar flame speed. Each fuel blend was tested in a single-cylinder spark-ignition engine under both lean-out and EGR dilution sweeps until the coefficient of variance of indicated mean effective pressure increased above thresholds of 3% and 5%. The relative importance of fuel laminar flamemore » speed to changes to engine design parameters (spark ignition energy, tumble ratio, and port vs. direct injection) was also assessed. Our results showed that fuel laminar flame speed can have as big an effect on lean or EGR dilute engine operation as engine design parameters, with the largest effects seen during EGR dilute operation and when changes were made to cylinder charge motion.« less
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