Numerical simulation of laminar reacting flows with complex chemistry

2011

Recent computational studies have simulated a mode of distributed premixed combustion where turbulent mixing plays a significant role in the transport of mass and heat near the reaction zone. Under these conditions, molecular transport processes play a correspondingly smaller role. A consequence of burning in this regime is that a change in the gas mixture composition can occur within the flame zone, which modifies the burning rate. The composition depends on the Lewis number (ratio of molecular heat to mass diffusivity), and so the response to the transition to distributed burning will be different for fuels with different Lewis numbers. In this paper, we examine the role of Lewis number on flames in the distributed burning regime. We use high-resolution three-dimensional flame simulations with detailed transport models to explore the turbulent combustion of a range of fuels, specifically lean premixed hydrogen, methane and propane mixtures. The response of the burning rate is found to be more pronounced in hydrogen than in the other fuels.

Section: Computational Methodologymentioning

confidence: 99%

Lewis number effects in distributed flames

Aspden

2011

“…The transport coefficients, thermodynamic relationships and hydrogen kinetics (chemical source terms) are obtained from the the GRIMech 2.11 model [14] with the relevant carbon species removed. The reader is referred to [15] for details of the low Mach number model and its numerical implementation, and to [16] for previous applications of this methodology to the simulation of premixed turbulent flames.…”

Section: Computational Methodologymentioning

confidence: 99%

Characterization of low Lewis number flames

Aspden

2011

A recent numerical study of turbulence-flame interactions in lean premixed hydrogen, where the Lewis number was approximately 0.36, observed that flames at different equivalence ratios presented significantly different behavior despite having the same Karlovitz and Damköhler numbers. This differing behavior is due to the thermodiffusively-unstable nature of low Lewis number flames. In more than one dimension, differential diffusion focuses fuel into hot regions increasing the local burning rate, which was found to affect the leaner hydrogen flames more significantly. Ultimately, this difference between idealized flat flames and freely-propagating flames undermines the characterization of turbulent flames through Karlovitz and Damköhler numbers based on flat laminar quantities. This paper considers refining the definitions of these dimensionless numbers by replacing the flat laminar flame values with freely-propagating values. In particular, we perform three-dimensional simulations of freely-propagating flames over a range of equivalence ratios, and use data from those simulations to define modified Karlovitz and Damkölher numbers. We then perform a series of turbulent flame simulations that show that defining Karlovitz and Damkölher numbers based on the freely-propagating flames effectively eliminates dependence on equivalence ratio.

“…The complete integration algorithm is second-order accurate in space and time, and discretely conserves species mass and enthalpy. The reader is referred to [13] for details of the low Mach number model and its numerical implementation and to [12] for previous applications of this methodology to the simulation of premixed turbulent flames.…”

Section: Low Mach Number Modelmentioning

confidence: 99%

Numerical simulation of Lewis number effects on lean premixed turbulent flames

Cheng

et al. 2007

122

A dominant factor in determining the burning rate of a premixed turbulent flame is the degree to which the flame front is wrinkled by turbulence. Higher turbulent intensities lead to greater wrinkling of the flame front and an increase in the turbulent burning rate. This picture of turbulent flame dynamics must be modified, however, to accommodate the affects of variations in the local propagation speed of the flame front. Classical flame analysis characterizes these local variations in propagation speed by the Markstein number which represents the response of the flame front to curvature and strain. In this paper, we consider lean premixed flames for three different fuels having widely varying fuel Lewis numbers corresponding to widely varying Markstein numbers. In particular, we present numerical simulations of premixed turbulent flames for lean hydrogen, propane and methane mixtures in two dimensions. Each simulation is performed at turbulence conditions similar to those found in laboratory-scale experiments and is performed using detailed chemical kinetics and transport properties. We discuss the effect of Lewis number on the overall flame morphology and explore the dependence of local flame propagation speed on flame curvature. We also explore the relationship between local flame speed and experimentally accessible variables such as OH concentration. Finally, we focus on the low Lewis number case, hydrogen, in which the flame front is broken indicating local extinction.