摘要:

We employ an accurate numerical algorithm to simulate two model flames—an unconfined lifted and a confined, coflowing, methane-air jet diffusion flame using detailed chemistry and complex transport models. The algorithm employs Newton's method to obtain the primitive variable solution of the large system of strongly coupled elliptic governing equations. The Newton equations are solved by a block-line tridiagonal method. We employ a global grid refinement technique which equidistributes meshes according to the gradients and curvatures of the solution obtained on the previous mesh and bounds the ratio of adjacent grid step size. The algorithm can be applied to problems ranging from non-reacting flows to reacting flows in two- or three-dimensional configurations. In the unconfined case, the lifted flame and the “triple flame” are both predicted in the numerical solution. The computed solutions agree well with the experimental results. The comparison of the present solutions with the previously reported stream function-vorticity computations shows that the primitive variable numerical approach is robust and yields a more accurate solution due to its implementation of the flow boundary conditions.

ISSN:0010-2202    

DOI:10.1080/00102209308907619

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A mathematical model of transport phenomena (heat, momentum and mass transfer) and chemical processes (primary and secondary reactions) of the thermal degradation of wood is presented. Implicit finite difference equations for energy, momentum and chemical species mass balances are formulated according to an operator-splitting technique and are numerically solved. The progress of the pyrolysis process along a wooden slab, radiatively heated on one side, is characterized by the following main processes: 1( a virgin wood region, crossed by a slow flow of pyrolysis products, where temperature and pressure values decrease as the non-irradiated boundary is approached; 2( a primary pyrolysis region where, due to the relatively low temperatures, secondary reactions are not active and 3( a char layer where volatile products of primary pyrolysis mainly flow and, temperature being higher, undergo secondary reactions. For low medium permeabilities, a peak in the gas overpressure is observed, separating the virgin wood and the pyrolysis region and two velocity distributions, directed towards the virgin wood and the char layer. Time and space evolution of main variables and reaction product distribution, as internal flow convection varies on dependence of wood and char permeabilities, are simulated. The effects of variations in the kinetic data and energetics of primary and secondary reactions, with special emphasis on the coupling between flow convection and extent of secondary reactions, are also analyzed.

ISSN:0010-2202    

DOI:10.1080/00102209308907620

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A high-speed spectral infrared imaging system has been developed for facilitating investigation of thermal processes, including those in combustion devices. The new system has several unique performance characteristics: variable and high speed frame rate; variable and short exposure period; independent control of imaging parameters; digital imaging in high dynamic resolution; spectral imaging, externally triggered individual images, and large number of frames per each data acquisition. Its design and performance verification was made by obtaining instantaneous thermal images that permit visualization of reaction fronts and subsequent mixing of combustion products of a Bunsen burner, a rectangular Poiseuille-flow burner, and a spark-ignition engine. Some results from this work are compared with those obtained using conventional methods reported in other literature. It is demonstrated that findings, otherwise difficult to obtain, are made possible by this newly developed tool.

ISSN:0010-2202    

DOI:10.1080/00102209308907621

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The oxidation of n-heptane, iso-octane and a toluene/n-heptane mixture was studied in a jet-stirred flow reactor operating under pressure at stoichiometric feed ratio in the low-temperature range. In particular, the transition from slow-combustion to a higher reactivity condition was examined by analyzing the composition of the reaction products as the residence time was increased.

ISSN:0010-2202    

DOI:10.1080/00102209308907622

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Convective combustion of mono-fuel aero-suspensions in an open-closed channel are treated numerically. The general approach to the problem of the simulation of the dispersed material behaviour to be exploited here is based upon incorporation of multiphase flow equations. The Lax-Wendroff two-step method is used for a solution of two-dimensional planar unsteady problems. The effect of acoustic oscillations caused by combustion upon combustion itself is studied. The different regimes (accelerated and decelerated– of convective combustion spreading are identified and the constitutive criterion of the realization of those regimes is suggested. The computational results to be obtained further are compared with available experimental data.

ISSN:0010-2202    

DOI:10.1080/00102209308907623

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

In a previous analysis, the stability of a premixed flame in a stagnation point How was discussed for a restricted class of disturbances that are self-similar to the basic undisturbed flow. Thus, flame fronts with corrugations only in the cross stream direction have been considered. In this paper, we consider a more general class of three-dimensional flame front perturbations, which permits corrugations in the streamwise direction as well. It is shown that, because of the stretch experienced by the flame, the hydrodynamic instability is limited only to disturbances of short wavelength. If, in addition, diffusion effects have a stabilizing influence, as would be the case for mixtures with Lewis number greater than one, a stretched flame could be absolutely stable. Instabilities occur when the Lewis number is below some critical value less than one. Neutral stability boundaries are presented in terms of the Lewis number, the strain rate and the appropriate wavenumbers. Beyond the stability threshold, the two-dimensional, self-similar modes always grow first. However, if disturbances of long wavelength are excluded, it is possible for the three-dimensional modes to be the least stable ones. Accordingly, the pattern that will be observed on the flame front, at the onset of the instability, will consist of either ridges in the direction of stretch, or the more common three-dimensional cellular structure.

ISSN:0010-2202    

DOI:10.1080/00102209308907624

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

An approximate simple semi-empirical model is developed to bound the extent of heterogeneously catalyzed chemical reaction occurring between two initially separated gaseous species in an axisymmelric compound jet, i.e., a jet with averaged nozzle-exit speed greater than that of a uniform enveloping coflow. The dynamic properties of the compound jet are adopted from measurements reported by Maezynski (1962). The chemical reaction between species B (initially present in the issuing jet only) and a species A (initially present in the coflow only) occurs only on the surface of particles (initially present in the issuing jet only). The particles participate purely as catalysts; an upper bound on the extent of reactant conversion is obtained by taking the (unknown) rate of surface catalysis to be mass-transfer-limited. The results furnish quantitative estimates for a chemical reacting shear flow without resort to experimentally discredited gradient-diffusion concepts to model the turbulent transport. An application, in the context of a solid-rocket motor with a conventional metallized grain (ammonium-perchlorale crystals and aluminum spheroids in a synthetic-rubber binder), associates species A with chlorine nitrate (C1ONO2), species B with hydrogen chloride (HO), and the particles with alumina (AI2O3), known to be heterogeneously catalytic in other contexts. The alumina might locally and transiently serve to generate the photochemicalty sensitive, potentially ozone-destructive species chlorine (O2) from the photochemically insensitive (“chlorine-reservoir”) species HC1 and CIONO2-The analysis suggests that even the upper bound on the amount of chlorine produced by alumina-particle catalysts (on the multihour time scale of a discernible-jet-exhaust flow) is negligible, owing to the diluteness of the ambient chlorine nitrate. Any longer-term, cumulative ozone destruction, arising because broadly dispersed alumina submicron-sized particles may fall out of the stratosphere only on a time scale of years, requires treatment of the general stratospheric circulation, and is not addressed by the analysis.

ISSN:0010-2202    

DOI:10.1080/00102209308907625

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

ISSN:0010-2202    

DOI:10.1080/00102209308907626

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor