Numerical Investigation of Spark-ignition in a Laminar Methane-air Counterflow

Simulations of forced ignition in a non-premixed laminar counterflow are used to study the effect of the strain rate on ignition success. A one dimensional calculation is performed, using detailed methane chemical kinetics, and treating the ignition event as an instantaneous heat release. Ignition success is seen to depend on the mixture composition and spark location, resulting in lean and rich ignitability limits for a given spark that can be different from the nominal flammability limits. Ignition is also prohibited by excessive strain rates, in some cases at levels well below the extinction value. The structure of the evolving ignition region is examined in terms of its temperature, heat release rates and its composition. In the case of successful ignition, the high temperature reached due to the spark energy deposition causes local autoignition. Subsequently, intense burning rapidly consumes the reactants in the remaining region of flammable methane-air mixture. As this intense burning subsides a partially premixed and then a non-premixed diffusion flame are seen to survive. Introduction An improved understanding of mixture ignitability and the modeling of ignition events is needed for the satisfactory treatment of forced ignition in flows with wide variations in fuel concentration. Gas turbine combustors and atmospheric releases of flammable material provide important examples of such flows. Auto-ignition of non-premixed flow and the forced ignition of premixed flow are frequently studied, however there are aspects of the forced ignition of nonpremixed fluid which require further fundamental investigation. Birch et al. [1] demonstrated the randomness spark ignition in a turbulent non-premixed jet flames. Alvani and Fairweather [2] proposed a largely successful model for the ignition probability of inhomogeneous mixtures in the terms of the probability of finding flammable mixture at the spark location. Ahmed and Mastorakos’ measurements [3] of spark ignition in a turbulent jet show a number of fluid dynamic influences on the early kernel development in addition to the compositional effects. Simple chemistry DNS [4] shows the development of a tribrachial structure in a turbulent nonpremixed flame kernel. Rashkovsky [5] used the laminar non-premixed counterflow as a paradigm for the flow processes relevant to the ignition of a turbulent inhomogeneous fuel-air mixture, and was able to demonstrate that fluid dynamics, as characterized by the chemical Damköhler number, have a rôle in determining the success of ignition. The laminar counterflow configuration provides a well defined flow field in which to examine the effect of strain on flame structure, consequently it has provided significant insight to the study of turbulent non-premixed combustion [6]. The non-premixed configuration involving opposed jets of fuel and oxidizer has been used to investigate transient auto-ignition processes both numerically [7] and experimentally [8]. The configuration is also convenient for studies of some aspects of forced ignition processes. Forced ignition is distinguished from auto-ignition by the presence of a localized transient heat source. The heat source or igniter can take a variety of forms, including heated surfaces, electrical sparks and laser pulses. Even igniters of the same type can have very different characteristics in terms of the spatial and temporal evolution of the energy input. Laser Induced Spark Ignition (LISI) delivers a measurable quantity of energy to the fluid at the focus of the laser over a relatively short period of time, 5-500ns [9]. The LISI process lends itself to a simple numerical representation as an instantaneous heat release, and the absence of a solid igniter in the interesting region of the flow is also an advantage. A simulation of ignition in a laminar methane-air counterflow mixing layer, using LISI, has been devised in order to display the variety of ignition behaviour possible for a range of strain rates. Details of the flow and ignition formulation are given in the following section. The resulting ignitability limits are then discussed, and the transient flame structures are examined in order to discover the mechanisms which give rise to successful and failed ignition. The flame structures are presented in terms of mixture fraction, and neglecting topographical effects such as flame curvature, similar flame structures may be expected during the ignition of certain turbulent flows, for example a globally homogeneous turbulent flow with fuel-air segregation experiencing equivalent strain conditions. Formulation Flame Equations The computational configuration is a non-premixed counterflow arrangement with planar geometry. RUN1DL is used to solve a low Mach number formulation of the flame equations in terms of a distance variable [10], including detailed molecular transport. Variable molecular properties and the GRI-Mech 3.0 detailed chemical scheme are employed for reactions between methane and air [11]. The 53 species GRI-Mech 3.0 which has been optimized for combustion of natural gas using data in the range 1000-2500 K, also shows realistic extinction behaviour. The solution is evaluated by integrating the system on an adaptive grid with a modified Newton method [10], ensuring grid independence of the solution. The fuel and air inlets are at y=-6 mm and y=6 mm respectively, and the boundary temperature and pressures are 293 K and 100 kPa. The stagnation point is fixed at y=0 and the inlet velocities are determined according to a specified strain value a∞ [10]. The values of mixture fraction used to report data from the counter flow calculations are calculated using the nitrogen atom conservation. Forced Ignition The simulated LISI spark is considered to cause instantaneous heating of a volume with thickness ∆yspark =0.25mm, with a smooth temperature profile peaking at 4000K centred at yspark. The initial condition used for calculations in this investigation is a converged inert flow with the spark's temperature profile subsequently imposed. The low Mach number formulation described above does not capture the pressure waves that result from the sudden temperature change, nor is the chemical mechanism valid at such a high temperature. The very early evolution of the chemical composition and of the velocity field should hence not be given undue credence. Despite the doubt this casts over the early transport of the spark energy, the total spark energy is conserved. Dissipative processes cause a rapid decrease of the peak temperature and the subsequent predictions of the chemical and thermal evolution are better founded. We will be comparing the mixture fractions that ignite with the nominal static flammability limits of methane, which give ξlean=0.0284 and ξrich=0.089 [12]. Table 1. Details of Spark events A-C. a∞ (s) yspark (mm)