摘要:

A new asymptotics exploiting the disparity of the orders of magnitude of Damköhler numbers is uncovered in complex kinetics. In contrast to the conventional activation energy asymptotics where the non-dimensional activation energy 0 is algebraically large, the new asymptotics here only requires 0 to be logarithmically large, and is therefore a more appropriate approach for the analysis of problems where the activation energy 0 is not very large. The two-step Zeldovich-Liñán model with a chain-branching and a chain-breaking step is chosen to demonstrate the use of this new asymptotics.

ISSN:0010-2202    

DOI:10.1080/00102208808924015

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

A study on the differential diffusion caused by the difference in the molecular diffusivities in turbulent H2/N2nonpremixed flames in coaxial air jet are investigated. The approach is to use the conventional conserved scalar method with the perturbation theory. The turbulence model used is κ - ϵ -gmodel. The combustion model assumes that instantaneous species concentrations and temperature are in a chemical equilibrium state. Then the fluctuations of these quantities are considered as clipped Gaussian probability density function. Computational results considering the differential diffusion forRe= 4200 are better than those which do not consider it. The effect appears great in fuel rich region which is consistent with the experimental result of Drake. The same computational procedure are done for high Reynolds number ofRe= 11000 to know the dependence of differential diffusion on the Reynolds number. As the Reynolds number increases, turbulent diffusion becomes dominant over molecular diffusion. So no sensible differences are found between the results by the perturbation method and by the conventional method.

ISSN:0010-2202    

DOI:10.1080/00102208808924016

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

A numerical analysis is conducted of the initiation and evolution of a combustion reaction generated by fuel vapor absorption of radiation over an evaporating combustible surface in an oxidizer stagnation point flow. The combustible is initially evaporating due to an externally applied irradiance, and is assumed to be in equilibrium vaporization. The transient, stagnation point, gas conservation equations, including one-step Arrhenius type kinetics and fuel vapor absorption of radiation, are solved numerically for the case of PMMA as combustible evaporating in air. Detailed calculations are presented, for a specific case of irradiance and flow velocity, of the evolution of the velocity field, and temperature and species concentration distributions during the ignition of the mixture, and subsequent establishment of a diffusion flame over the combustible surface. Ignition is characterized by thermal run-away of the gas and it is considered to have occurred if, after discontinuing the external irradiance, a self-sustained reaction is present. It is predicted that ignition occurs in the fuel-rich side of the mixing layer formed around the dividing streamlines of the fuel and oxidizer opposing stagnation flows. After ignition a premixed reaction front moves toward the lean side separating the fuel from the oxidizer and leaving behind a diffusion flame which eventually reaches steady conditions. A parametric study is also conducted on the effect of the flow velocity on the minimum irradiance for ignition and on the ignition time. It is predicted that the minimum irradiance for ignition increases approximately linearly with the velocity and that for a given velocity the ignition time decreases approximately inversely with the irradiance.

ISSN:0010-2202    

DOI:10.1080/00102208808924017

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor