A DNS study of the ignition of lean PRF/air mixtures with temperature inhomogeneities under high pressure and intermediate temperature

Combustion Science and Technology

Wooldridge

et al. 2015

Section: The Regime Diagram and Discussionmentioning

confidence: 99%

A Regime Diagram for Autoignition of Homogeneous Reactant Mixtures with Turbulent Velocity and Temperature Fluctuations

Combustion Science and Technology

Wooldridge

et al. 2015

“…Ihme and co-workers [22,23] demonstrated using a reduced order modeling approach that turbulent fluctuations could result in significant advancement of overall ignition. A number of DNS studies [24][25][26][27][28] have revealed that thermal stratification and turbulence can influence auto-ignition phenomena in LTC engines.…”

Section: Introductionmentioning

confidence: 99%

Computational characterization of ignition regimes in a syngas/air mixture with temperature fluctuations

Proceedings of the Combustion Institute

Valorani

Arias

et al. 2017

Auto-ignition characteristics of compositionally homogeneous reactant mixtures in the presence of thermal non-uniformities and turbulent velocity fluctuations were computationally investigated. The main objectives were to quantify the observed ignition characteristics and numerically validate the theory of the turbulent ignition regime diagram recently proposed by Im et al. [H.G. Im, P. Pal, M.S. Wooldridge, A.B. Mansfield, Combustion Science and Technology, 2015] that provides a framework to predict ignition behavior a priori based on the thermo-chemical properties of the reactant mixture and initial flow and scalar field conditions. Ignition regimes were classified into three categories: weak (where deflagration is the dominant mode of fuel consumption), reaction-dominant strong, and mixing-dominant strong (where volumetric ignition is the dominant mode of fuel consumption). Two-dimensional (2D) direct numerical simulations (DNS) of auto-ignition in a lean syngas/air mixture with uniform mixture composition at highpressure, low-temperature conditions were performed in a fixed volume. The initial conditions considered two-dimensional isotropic velocity spectrums, temperature fluctuations and localized thermal hot spots. A number of parametric test cases, by varying the characteristic turbulent Damköhler and Reynolds numbers, were investigated. The evolution of the auto-ignition phenomena, pressure rise, and heat release rate were analyzed. In addition, combustion mode analysis based on front propagation speed and computational singular perturbation (CSP) was applied to characterize the auto-ignition phenomena. All results supported that the observed ignition behaviors were consistent with the expected ignition regimes predicted by the theory of the regime diagram. This work provides new high-fidelity data on syngas ignition characteristics over a broad range of conditions and demonstrates that the regime diagram serves as a predictive guidance in the understanding of various physical and chemical mechanisms controlling auto-ignition in thermally inhomogeneous and compositionally homogeneous turbulent reacting flows.

“…and τ ig,10% represents the shortest ignition timescale of the initial mixture [34] and is defined as the homogeneous ignition delay time at the temperature T min + 0.9(T max − T min ), where T min and T max are the minimum and maximum initial temperatures in the domain; α 0 denotes the thermal diffusivity at the initial mean thermodynamic conditions. When the Da mix becomes less than O(1), the passive scalar dissipation effects are expected to become important.…”

Section: Role Of Passive Scalar Mixingmentioning

confidence: 99%

A computational study of syngas auto-ignition characteristics at high-pressure and low-temperature conditions with thermal inhomogeneities

Combustion Theory and Modelling

Mansfield

Arias

et al. 2015

A computational study was conducted to investigate the characteristics of auto-ignition in a syngas mixture at high-pressure and low-temperature conditions in the presence of thermal inhomogeneities. Highly resolved one-dimensional numerical simulations incorporating detailed chemistry and transport were performed. The temperature inhomogeneities were represented by a global sinusoidal temperature profile and a local Gaussian temperature spike (hot spot). Reaction front speed and front Damköhler number analyses were employed to characterise the propagating ignition front. In the presence of a global temperature gradient, the ignition behaviour shifted from spontaneous propagation (strong) to deflagrative (weak), as the initial mean temperature of the reactant mixture was lowered. A predictive Zel'dovich-Sankaran criterion to determine the transition from strong to weak ignition was validated for different parametric sets. At sufficiently low temperatures, the strong ignition regime was recovered due to faster passive scalar dissipation of the imposed thermal fluctuations relative to the reaction timescale, which was quantified by the mixing Damköhler number. In the presence of local hot spots, only deflagrative fronts were observed. However, the fraction of the reactant mixture consumed by the propagating front was found to increase as the initial mean temperature was lowered, thereby leading to more enhanced compression-heating of the end-gas. Passive scalar mixing was not found to be important for the hot spot cases considered. The parametric study confirmed that the relative magnitude of the Sankaran number translates accurately to the quantitative strength of the deflagration front in the overall ignition advancement.