Caimao Luo

ALIGNMENT EFFECT ON THE STRUCTURE OF METHANE - AIR COUNTERFLOW DIFFUSION FLAMES

A three dimensional model was developed to describe the scalar structure which exists in a countercurrent turbulent diffusion flame. The model was solved numerically by combining CHEMKIN thermochemical and transport databases within the CFX4.3 software. The distance between the axis of the upper oxidiser duct and the lower fuel duct, termed eccentricity, was used to describe the severity of the torch misalignment. Due to the misalignment, the original horizontal flat flame becomes inclined and distorted. The flame structure along the centre line of the torch is not sensitive to eccentricity. However, the cross sectional contours of velocity, density and species mass fractions illustrate significant effect on eccentricity. Finally, the paper makes suggestions for experimental verification of the alignment of the counterflow burner in experiments and examines the validity of constant 1/r(∂p/∂r) assumption in a misaligned burner.

COMPUTATIONAL STUDY ON TOXIC GASES RELEASED FROM COMPARTMENT FIRES SUPPRESSED WITH HALOGENATED AGENTS

The evolution of toxic gases in the hot layer of an enclosure fire is simulated by a batch perfectly stirred reactor model using the SENKIN program from the CHEMKIN distribution package. The initial species composition was obtained from experimental data both for slightly underventilated and highly underventilated conditions. The formation rates of toxic gases were studied using a comprehensive kinetic mechanism, including the GRI 3.0 submechanism representing hydrocarbon oxidation, the National Institute of Standards and Technology (NIST) hydrofluorocarbon mechanism, and the NIST CBrF3 and CF3I inhibition mechanisms for the initial mole fractions of CBrF3 and CF3I ranging from 0.5 to 5.0%. An analysis of modeling results, including the prediction of the equilibrium concentration, provided important insight into the effects of adding CBrF3 and CF3I on the formation of toxic gases in the hot layer. Sensitivity analysis was performed under the highly underventilated conditions, leading to the identification of the chemical reactions that have the most significant influence on the formation of toxic products.

Three-dimensional numerical study on flames

A three-dimensional (3D) model of a methane-air counter-flow, non-premixed flame with a global reaction step for methane oxidation was developed using computational fluid dynamics (CFD) simulation. A specific computational domain, and relevant thermodynamic and transport data calculated by the chemical kinetic code CHEMKIN, were incorporated into the model. The model was validated by comparing predictions with the spontaneous Raman scattered profiles of major combustion species reported in the literature. The model was employed to carefully examine the self-similarity assumptions normally invoked in simulating counter-flow non-premixed flames. It was found that while most assumptions were strictly satisfied within the jet region for the case of plug flow boundary conditions (B.C.) along the central axis, for the quadratic boundary condition case (corresponding to uniform plug-flow), assumptions were only approximately valid within the jet region. Also, the influence of shroud gas was examined by setting the surrounding gas as air and increasing the shroud gas through widening the shroud gas gap while maintaining a constant shroud gas velocity. Calculations revealed that, the resulting flames for shroud gas gaps greater than half of the jet radius, were totally insulated from mixing with the ambient air. The effect of buoyancy on the flame structures was also studied by comparing contours of the combustion products, temperature and turbulent properties.