CSAR Seminar

SPEAKER: Alex Briones, University of Illinois at Chicago

TITLE: A Numerical Investigation of Partially Premixed Flames

DATE: Wednesday, March 14, 2007
TIME: 12:00 Noon
PLACE: 2240 DCL
1304 W. Springfield Ave., Urbana, IL

ABSTRACT

The investigation of partially premixed flames (PPFs) is important from both fundamental and practical considerations. PPFs occur in numerous combustion applications, including spray combustion systems, such as gas turbines and diesel engines, in which the vaporization of smaller droplets and/or poor mixing lead to locally fuel vapor-rich regions. Generally, PPFs are often exposed to hydrodynamic, curvature-induced, and unsteady stretch effects, in addition to radiation effects, all of which strongly affect the flame structure and flame dynamics. Therefore, axisymmetric coflowing CH4-air flames are an appropriate choice for simulation in order to study the above effects. CH4 is selected as the fuel since it is the simplest model hydrocarbon fuel and thereby serves as a starting point for investigating higher hydrocarbon fuels. Uniform velocity profiles are used in both the inner and annular burner. The numerical model solves the time-dependent continuity, species, momentum, and energy equations in axisymmetric configuration. Viscous dissipation is neglected due to low speed flows. The chemistry is modeled using a detailed chemical mechanism that includes 48 species and 277 elementary reactions. The thermodynamic and transport properties appearing in the governing equations are temperature and species dependent. The thermal conductivity and viscosity of the individual species are based on Chapman-Enskog collision theory; those of the mixture are determined using the Wilke semi-empirical formulas. The Chapman-Enskog collision theory and the Lennard-Jones potentials are used to estimate the binary-diffusion coefficient between each species and nitrogen. A sink term based on an optically thin gas assumption is included in the energy equation to account for thermal radiation. An isothermal insert simulates the inner burner wall. The numerical model is validated in terms of flame topology, liftoff height, and flame propagation speed.

The investigation of flame structure and emissions, stabilization, and propagation of CH4-air PPFs over a wide range of conditions, such as various levels of partial premixing, CO2 dilution, and H2 enrichment, is thus a major extension into an area that has very sparse literature. First, the effects of partial premixing on flame structure and NOx emissions is investigated by simulating the complete partially premixed regime that extends from premixed flames to triple flames and then to double flames. This is done by suitably varying the fuel/air equivalence ratios in the inner jet and annular jet, while maintaining the global fuel/air equivalence ratio fixed. This specific study is relevant for understanding the NOx emissions occurring in combustors due to incomplete mixing of reactants. Second, the effect of fuel stream CO2 dilution on flame stabilization is investigated. Fuel-air mixture is introduced through the annular burner, whereas air is introduced through the annular burner. At a critical fuel stream CO2 dilution the flame lifts off. The understanding of this liftoff phenomenon in laminar flows might improve the fundamental understanding of diesel-spray flame liftoff which occurs in the presence of EGR. Last, the effect of H2 enrichment on propagating flames is investigated. These flames are established by igniting the jet-mixing layer. The ignition event is simulated by providing, in a rectangular cross-sectional area, a small high-temperature zone and small amounts of radicals. This high-temperature zone generates an ignition kernel that propagates upstream and rapidly develops into a triple flame, which then propagates upstream towards the burner rim and eventually stabilizes at the rim. A fundamental understanding of this phenomenon is relevant for flame propagation in direct-injection spark-ignited engines.