Turbulent mixing and combustion are investigated in a gaseous shear layer formed between two streams: one containing a low concentration of hydrogen in nitrogen and the other containing a low concentration of fluorine in nitrogen. The resulting temperature field is measured simultaneously at eight points across the width of the layer using fast-response cold-wire thermometry. The results show the presence of large, hot structures separated by tongues of cool fluid that enter the layer from either side. The usual bell-shaped mean-temperature profiles therefore result from a duty cycle whereby a fixed probe sees alternating hot and cool fluid, which results in the local mean. The adiabatic flame temperature is not achieved in the mean, at any location across the layer. For fixed velocities, it is found that, in general, two different mean-temperature profiles result from a given pair of reactant compositions if the sides of the layer on which they are carried are exchanged (‘flipped’). This finding is a direct consequence of the asymmetric entrainment of fluid into the layer. Results are compared with the predictions of Konrad and discussed in the context of the Broadwell–Breidenthal model. By comparison with the liquid result of Breidenthal, the amount of product formed in the layer at high Reynolds number is found to be dependent upon the Schmidt number. Results for a helium–nitrogen layer are discussed briefly.

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Mixing and combustion with low heat release in a turbulent shear layer

To reveal the effects of particle characteristics, including particle thermal characteristics and size distributions, on flame propagation mechanisms during dust explosions clearly, the flame structures of dust clouds formed by different materials and particle size distributions were recorded using an approach combining high-speed photography and a band-pass filter. Two obviously different flame propagation mechanisms were observed in the experiments: kinetics-controlled regime and devolatilization-controlled regime. Kinetics-controlled regime was characterized by a regular shape and spatially continuous combustion zone structure, which was similar to the premixed gas explosions. On the contrary, devolatilization-controlled regime was characterized by a complicated structure that exhibited heterogeneous combustion characteristics, discrete blue luminous spots appeared surrounding the yellow luminous zone. It was also demonstrated experimentally that the flame propagation mechanisms transited from kinetics-controlled to devolatilization-controlled while decreasing the volatility of the materials or increasing the size of the particles. Damkohler number was defined as the ratio of the heating and devolatilization characteristic time to the combustion reaction characteristic time, to reflect the transition of flame propagation mechanisms in dust explosions. It was found that the kinetics-controlled regime and devolatilization-controlled regime can be categorized by whether Damkohler number was less than 1 or larger than 1.

Flame propagation mechanisms in dust explosions

The effect of Lewis and Damköhler numbers on the flame propagation through micro-organic dust particles

http://scinet.dost.gov.ph/union/Downloads/science_012_314515.pdf

Modeling of laminar flame propagation through organic dust cloud with thermal radiation effect

Dust flames are associated with two-phase combustion phenomena where flame characteristics depend on interactions between solid and gas phases. Since organic dust particles can be effectively utilized in energy production systems, investigation of this phenomenon is essential. In this study, an analytical model is presented to simulate the combustion process of moist organic dust. The flame structure is divided into three zones: preheat zone, reaction zone, and postflame zone. To determine the effects of moisture content and volatile evaporation, the preheat zone is also divided into four subzones: first heating subzone and drying subzone, second heating subzone, and volatile evaporation subzone. The results obtained from the presented model are in reasonable agreement with experimental data for lycopodium particles. An increase in moisture content causes a reduction in burning velocity owing to moisture evaporation resistance. Consequently, the effects of some important parameters, like volatilization temperature, volatilization Damkohler number and drying Damkohler number are investigated. In special cases, like high moisture content, low volatilization temperature, and high drying resistance, the second heating subzone is omitted.

/pdf/a-parametric-study-of-lycopodium-dust-flame-ywmdozwb70.pdf

A parametric study of lycopodium dust flame

In turbulent diffusion flames chemical reaction and mixing occur simultaneously in thin strained and distorted laminar mixing layers separating fuel from the oxidizer.
If the reaction can be modelled in terms of an irreversible reaction
F + n 02 —> Products
the relative importance of chemical production and transportation terms in the conservaron
equations is measured by the Damkohler number, or ratio of the character-istic mixing time tm and the characteristic chemical time tc. A typical mixing time is the inverse of the straining rate y, while tc~' is proportional to the frequency factor of the reaction times the Arrhenius exponential exp (-E/RT) involving the ratio of the activation energy of the reaction E to the thermal energy TR. Combustion
reactions are exothermic and with large values of the ratio E/RT, and, as a result we find in the flow-field regions of low temperature, where the Damkohler number is very small and, therefore, the chemical reaction is frozen. These regions coexist with regions of high temperature where because the reaction is so fast, one of the reactants is depleted, so that either the local concentration of the fuel or of the oxidizer must be zero.

/pdf/lewis-number-effects-on-the-structure-and-extinction-of-4fkkjaylkd.pdf

A. Liñan

Papers

Lewis number effects on the structure and extinction of diffusion flames due to strain