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Statistical description of turbulent reacting media

By Kexin Liu

Abstract

Present work concerns the interaction of chemistry and turbulence in a turbulent reacting flow. Both self-ignition and flame propagation are studied.\ud \ud For self-ignition study, the combined effects of temperature/concentration inhomogeneities and turbulence were studied numerically. For this purpose two statistic\ud turbulent combustion models, namely Linear Eddy Model (LEM) and Reference Scalar Field (RSF), were applied for simulations of statistically homogeneous reacting media.\ud because these two models allow the calculation of the averaged reaction rates in fluctuating media from the first principles. Self-ignition delays, species concentration\ud and temperature evolutions were computed for three kinds of initial conditions, where temperature pdfs were given as Dirac's 8 peak pdf, rectangular and bimodal shapes. The\ud results obtained from the two models mentioned above were compared. The effect of heat loss on ignition delay was also studied with the RSF model.\ud \ud For the study of turbulent flame propagation, RSF model was applied to the problem of I-D flame propagation in a spherical fan-stirred bomb. This problem is selected because\ud of its simplest possible flow field, hence reduced computation cost and easy implementation. For turbulent convection different conditionally averaged velocity\ud models were introduced and evaluated. Pressure during gas explosion, and averaged mean values such as temperature and species concentration were calculated. The evolution of temperature pdf was also obtained from statistics of the reference scalar field. Flame radii and turbulent mass burning rates were determined from the calculated\ud pressure rise and the mass burning rates were compared with two existing correlations of Bradley et al. and Zimont as well as with measurements.\ud \ud Two types of reactive mixtures were studied, one was the methane/air flame and the other one was DTBPIN2 decomposition flame. Experiments with both mixtures were\ud carried out in a spherical fan-stirred bomb. In particular, pressure trace during explosion was recorded and this provided reference data for the modelling studies. Methane/air combustion was simulated with a reduced two-step chemical kinetics mechanism instead of the single-step kinetics commonly used for turbulent reacting flows\ud modelling. The two-step kinetics employed was developed according to the experimental observations of two-stage oxidation of hydrocarbons, in which the first\ud stage is related to the consumption of the fuel and the second stage represents the oxidation of CO and H2• Firstly, the kinetics was "calibrated" in the laminar situations to produce a reasonable agreement with measured flame speed. Then the kinetics was used with turbulent models to simulate the turbulent explosions. While DTBPIN2\ud decomposition flame is described by a single step kinetics and the reaction constants have been well-studied. So for simulation ofDTBPIN2 flame, any ambiguity resulting in\ud uncertainties in chemical mechanism is avoided

Publisher: School of Mechanical Engineering (Leeds)
Year: 2002
OAI identifier: oai:etheses.whiterose.ac.uk:1194

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