Effect of NO on extinction and re-ignition of vortex-perturbed hydrogen flames

doi:10.1016/j.combustflame.2009.10.014

Combustion and Flame

Volume 157, Issue 2, February 2010, Pages 217-229

https://doi.org/10.1016/j.combustflame.2009.10.014 Get rights and content

Abstract

The catalytic effect of nitric oxide (NO) on the dynamics of extinction and re-ignition of a vortex-perturbed non-premixed hydrogen–air flame is studied in a counterflow burner. A diffusion flame is established with counterflowing streams of nitrogen-diluted hydrogen at ambient temperature and air heated to a range of temperatures that brackets the auto-ignition temperature. Localized extinction is induced by impulsively driving a fuel-side toroidal vortex into the steady flame, and the recovery of the extinguished region is monitored by planar laser-induced fluorescence (PLIF) of the hydroxyl radical (OH). The dynamics of flame recovery depend on the air temperature and fuel concentration, and four different recovery modes are identified. These modes involve combinations of edge-flame propagation and the expansion of an auto-ignition kernel that forms within the extinguished region. The addition of a small amount of NO significantly alters the re-ignition process by shifting the balance between chain-termination and chain-propagation reactions to enhance auto-ignition. The ignition enhancement by this catalytic effect causes a shift in the conditions that govern the recovery modes. In addition, the effects of NO concentration and vortex strength on the flame recovery are examined. Direct numerical simulations of the flame–vortex interaction with and without NO doping show how the small amount of OH produced by NO-catalyzed reactions has a significant impact on the development of an auto-ignition kernel. This joint experimental and numerical study provides detailed insight into the interaction between transient flows and ignition processes.

Introduction

Flame extinction and re-ignition are important processes that affect the operating envelope of practical combustors. In turbulent non-premixed flames, fluctuations in the strain rate induce localized extinction, and the re-ignition of extinguished regions is dependent on the complex interactions between the turbulent flow and flame chemistry. Accurate modeling of extinction and re-ignition remains a challenge, especially for capturing the local structure of quenching in turbulent flames [1]. The extinguished regions can remain hot for a period of time following local extinction [2], [3]. The high temperature extinguished region may be re-ignited by a reduction in the local strain rate or the addition of heat from a hot boundary or neighboring reaction zone. Detailed investigations of the coupling between flame chemistry and transient flows are required to understand and control these phenomena.

The interactions between NO_x (NO and NO₂) and the oxidation of fuel has been referred to as mutual sensitization, and catalytic effects of NO on fuel ignition as well as the promoting action of hydrocarbons on the conversion of NO to NO₂ are representative effects [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. The catalytic effect of NO can lead to changes in the auto-ignition timing of reciprocating engines (i.e. engine knock and onset of low temperature combustion in HCCI engines), and more fundamentally, to changes in flammability limits and flame stabilization. Recently, the effects of NO on the oxidation of n-heptane, iso-octane, toluene [5], methanol [5], [7], [8], n-butane [6], ethane [9], di-methyl ether (DME) [10], methane [11], propane [12], and 1-pentene [13] have been reported. The influence of NO on the ignition phasing in an HCCI engine was studied [18], and nitrate additives, such as 2-ethylhexyl nitrate (2-EHN), were used to provide a source of NO₂ to reduce the ignition delay time [19]. In a supersonic flow, ignition enhancement with NO_x addition was investigated by using a plasma jet of a N₂/O₂ mixture [20]. For hydrogen, the impact of NO on ignition has been well established. NO is a catalyst that significantly alters the balance between the chain terminating steps related to HO₂ formation (H + O₂ + M $\Leftrightarrow$ HO₂ + M) and chain-propagation steps related to OH formation (NO + HO₂ $\Leftrightarrow$ NO₂ + OH, NO₂ + H $\Leftrightarrow$ NO + OH) [14], [15], [16], [17], [18], [21], [22], [23]. Previous studies were conducted in a homogeneous environment [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16] or in steady counterflow flames [17]. However, nearly all practical applications operate in an inhomogeneous environment, often involving a non-premixed, unsteady convective–diffusive environment where there may be significant overlap in chemical and diffusive timescales. If the local mixing environment encompasses recirculation of product and intermediate gases, there is the possibility of hot vitiated gases and NO impacting ignition delay times, potentially altering the highly dynamic process of re-ignition. Hence, it is necessary to understand the dynamics of coupling between transport and NO-catalyzed ignition chemistry in an unsteady strained flow.

Fundamental investigations of these transport-chemistry interactions are conducted in highly-repeatable isolated flow-flame interactions. The use of a canonical flow configuration and simple chemistry facilitates comparisons with detailed numerical simulations of modest computational expense. In the present study, a pulsed counterflow burner is used to produce highly repeatable flame–vortex interactions in a non-premixed hydrogen–air flame. Hydrogen combustion is used because of its simple chemistry, but it is also an important submechanism of hydrocarbon fuels. The flame–vortex interaction configuration has been used extensively to understand the transient response of the inner structure of non-premixed flames [24], [25], [26], [27], ignition in unsteady flow [27], [28], [29], and dynamics of edge-flame propagation [30], [31], [32], [33], [34]. In the present study, localized extinction is induced by a fuel-side vortex and the recovery process of the extinguished region is monitored with OH planar laser-induced fluorescence (PLIF). The heated air stream provides a steady source of enthalpy such that a re-ignition kernel is formed within the extinguished region, and different recovery modes are produced by interactions between the ignition kernel and annular edge-flame. The influence of NO on the dynamics of the re-ignition process is investigated for a range of fuel concentrations and air temperatures, while maintaining a constant vortex strength and initial strain rate. Addition of a small amount of NO significantly alters the re-ignition process and changes the recovery mechanism. To understand the catalytic effect of NO in a diffusive environment, direct numerical simulations (DNS) are conducted in the same counterflow configuration as the experiment. The simulations employ detailed chemistry that includes a recent NO reaction mechanism [22]. The coupling between experiment and simulation allows for the investigation of a broad parameter space and identification of the key elementary reactions that couple with unsteady transport to affect the re-ignition dynamics.

Section snippets

Experimental method

The experiments were conducted in a laminar axisymmetric counterflow burner, which consisted of opposed jets of heated air and N₂-diluted hydrogen at 298 K. Fig. 1 shows a diagram of the burner, which is based on the design described in [27], [28] with modifications to provide higher temperatures of heated air and a pulsed vortex generator. The top of the burner consists of concentric ceramic tubes with heated air flowing from the central tube. The flame is shielded from external disturbances by

Extinction and re-ignition without NO doping

We first consider the effects of the air stream temperature and the hydrogen content of the fuel stream on the dynamics of extinction and re-ignition without NO doping. The sequences of OH PLIF measurements in Fig. 4a and b show the temporal evolution of the flame–vortex interaction using fuel stream hydrogen mole fractions of X_H2 = 0.2 and X_H2 = 0.25, respectively, and an air stream at ambient temperature. The strength of the fuel-side vortex is the same for both cases, and the global strain rate

Direct numerical simulation

To better understand the extinction and re-ignition in the experiments and the effects of NO on these processes, we performed axisymmetric DNS calculations of the transient interaction of a flame with a fuel-side vortex impulsively driven into the flame. To facilitate comparison with the experiments, we consider two DNS cases; Case 1 has no NO doping, and for Case 2 160 ppm of NO are added to the heated air stream. The air temperature and fuel concentration are chosen to correspond with the

Conclusion

The catalytic effect of NO on extinction and re-ignition dynamics was studied in a vortex-perturbed hydrogen/heated air counterflow flame. Four different recovery mechanisms were observed. The dependence of the re-ignition mechanisms on air temperature and fuel concentration was measured, and the influence of the NO catalytic effect on each mode was investigated. When the air temperature was below the auto-ignition limit, the extinguished region of the flame recovered by edge-flame propagation.

Acknowledgments

We thank Prof. F.L. Dryer of Princeton University for providing the H₂/O₂/NO_x chemical mechanism, Dr. S.A. Kaiser of Sandia National Laboratories (SNL) for valuable discussions, and R.J. Sigurdsson of SNL for assistance in the laboratory. This research was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corporation, a Lockheed

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¹: Present address: KITECH, Cheonan, Chungnam 330-825, Republic of Korea.

²: Present address: School of Mechanical and Advanced Materials Engineering, Ulsan National Institute of Science and Technology, Ulsan 689-798, Republic of Korea.

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