Ignition in nonpremixed counterflowing hydrogen versus heated air: Computational study with detailed chemistry

doi:10.1016/0010-2180(95)00121-2

Combustion and Flame

Volume 104, Issues 1–2, January 1996, Pages 157-175

https://doi.org/10.1016/0010-2180(95)00121-2 Get rights and content

Abstract

Forced ignition in counterflowing jets of N₂-diluted H₂ versus heated air has been investigated over a wide range of temperature, pressure, and strain rate by numerical modeling with detailed chemistry and transport. Ignition temperatures calculated at constant strain rates are seen to exhibit a Z-shaped pressure dependence similar to that observed in explosion limits of homogeneous H₂/air mixtures. As with the corresponding explosion limits, the first and second ignition limits are governed by the competition for hydrogen radicals between chain branching (H + O₂ → O + OH) and termination (H + O₂ + M → HO₂ + M) pathways, and the third ignition limit involves additional propagation (2HO₂ → H₂O₂ + O₂ → 2OH + O₂) and branching (HO₂ + H₂ → H₂O₂ + H → 2OH + H) pathways that compete with chain termination. Ignition in this inhomogeneous, diffusive system is found to involve a spatially localized ignition “kernel,” identified as the region near the point of maximum temperature where the rate of hydrogen radical chain branching is maximized. Mass transport of radicals out of the ignition kernel affects the ignition process by competing with chemical reactions within the kernel, particularly in the first and third limits where the dominant ignition chemistry is relatively slow. Ignition temperatures in these limits are found to be much more sensitive to aerodynamic straining than in the second limit. By controlling the width of the ignition kernel and thus the characteristic residence time of key radicals within it, the strain rate is found to determine the dominant chemistry and the relevant ignition limit at any given pressure.

References (22)

U. Maas et al.
Combust. Flame
(1988)
D.G. Vlachos et al.
Combust. Flame
(1993)
T.G. Kreutz et al.
Combust. Flame
(1994)
A. Liñán
Acta Astronaut.
(1974)
U. Maas et al.
G. Goyal et al.
J. Sato et al.
C.K. Law
N. Darabiha et al.
Combust. Sci. Technol.
(1992)
G. Balakrishnan et al.

S.R. Lee et al.

Combust. Sci. Technol.

(1994)

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At elevated temperature, exceeding the auto-ignition temperature, lifted flames were also observed in jets in a hot co-flow [23] and in Direct Numerical Simulation [24]. However, it is unclear if the same trend would persist with increasing ambient pressure and temperature, as the ignition falls to a different ignition limit regime associated with different chemical kinetics and reaction paths [25,26]. In CI engine-relevant conditions, there are very few studies of H2 ignition and flame structure.
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Effects of hydrogen addition on non-premixed ignition of iso-octane by hot air in a diffusion layer
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They observed that the incipient ignition spot is always associated with the most reactive fraction and it is nearly independent of turbulence intensity. Similar observations were made in counterflow diffusion flames [16–18]. Besides, effects of different molecular diffusion models [9,10], initial gradient of fuel composition and strain rate [11,12,19, 20] on non-premixed ignition were also examined.
Hydrogen addition is widely used to improve the combustion performance of single-component fuel. In this study, the effects of hydrogen addition on non-premixed ignition of iso-octane by hot air in a diffusion layer were examined and interpreted numerically. Detailed chemistry and transport were considered in simulation. The non-premixed ignition delay times at different hydrogen blending levels were obtained and analyzed. It was found that hydrogen addition greatly reduces the ignition delay. This is mainly due to the fact that the preferential mass diffusion of hydrogen over iso-octane significantly increases the local hydrogen blending level at the ignition kernel. Besides, for the non-premixed ignition process, two modes of reaction front propagation were identified through the analysis based on Damköhler number and consumption speeds. One is the reaction-driven mode characterized by local or sequential homogeneous autoignition; and the other is the diffusion-driven mode, which depends on the balance of mass diffusion, heat transfer and chemical reaction. These two modes lead to different ignition behaviors. For pure iso-octane with low mass diffusivity, ignition is mainly caused by local homogeneous reaction occurring at the most reactive position. With the increase of diffusion layer thickness, the local temperature at the most reactive position increases and therefore the non-premixed ignition delay time of pure iso-octane decreases. However, when hydrogen with high mass diffusivity is added into iso-octane, the non-premixed ignition is controlled by fuel diffusion. With the increase of diffusion layer thickness, the concentration gradient becomes smaller and thereby less hydrogen diffuses into the ignition kernel. Consequently, unlike pure iso-octane, the non-premixed ignition delay time of hydrogen/iso-octane blends increases with the diffusion layer thickness.
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The non-monotonic ignition response of counterflowing premixed hydrogen/oxygen mixtures with nitrogen dilution versus heated nitrogen is studied numerically and theoretically. It is shown that the three ignition limits can be theoretically obtained by considering only the linear system involving at most only one radical in each reaction, while the influences of the nonlinear reactions, each involving two radicals, together with thermal feedback, introduce higher-order corrections, particularly for the third ignition limit. It is also demonstrated that the high diffusivity of H₂ promotes ignition at the third limit. On the other hand, the high diffusivity of the H atom suppresses ignition at the first limit, while the assumption of unity Lewis number for H yields remarkably good results for the other two limits. Furthermore, by solving the time evolution of the crucial H and HO₂ radicals, simplified formulations of the three individual limits and the two quadratic double limits are obtained analytically, in analogy with results for the homogeneous explosion problem.
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Auto-ignition of turbulent stratified mixing layer between hydrogen and hot air under elevated pressures p = 1–30 atm is studied using direct numerical simulations (DNS) in this work. Homogeneous isotropic turbulence is superimposed on the field. Detailed chemical mechanism and multicomponent diffusion model are employed. Other than turbulent mixing ignition (TMI), homogeneous mixing ignition (HMI) and laminar mixing ignition (LMI) are also investigated for comparison. For both laminar and turbulent cases, the onset of auto-ignition always happens at the same most reactive mixture fraction isosurfaces. Most reactive mixture fractions in diffusion auto-ignition are inconsistent with HMI calculations and shift to the rich side owing to diffusion for all pressures. At elevated pressures, auto-ignition chemistry is different from low pressures. The importance of H₂O₂ and HO₂ is highlighted as radical sinks during the ignition process, and can also be used as an indicator for locating the ignition spots. Moreover, OH radicals can be used as a marker variable for the transition of auto-ignition to flame propagation under high pressures. Two stages are involved in the diffusion ignition process: radical explosion and thermal runaway. According to our study, under elevated pressures, turbulence has little influence on the radical explosion stage. The role of turbulence is to accelerate the thermal runaway stage in the kernels to make the ignition delay time (IDT) shorter than laminar cases.
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^☆: This work was supported by the Army Research Office (DAAL03-90-G0-0220) under the technical monitoring of Dr. D. Mann.

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Ignition in nonpremixed counterflowing hydrogen versus heated air: Computational study with detailed chemistry☆

Abstract

Combust. Flame

Combust. Flame

Combust. Flame

Acta Astronaut.

Combust. Sci. Technol.

Combust. Sci. Technol.