摘要:

Differential diffusion between species often substantially effects the results of laboratory-scale experiments in turbulent combustion. The present work documents these effects for a simple jet flow by simultaneous 2D imaging of LIF from biacetyl and Lorenz-Mie scattering from submicron TiO2particles, at low to moderate Reynolds numbers. This novel technique, involving a frequency-tripled Nd:YAG laser and two slow-scan CCD cameras, captures directly the structural differences that arise when two species with widely different Schmidt numbers mix in a classical jet How where molecular diffusion makes a substantial contribution to overall mixing. Matched instantaneous image pairs of biacetyl and particle concentration show differences, with the particle images exhibiting sharper contours and a more convoluted structure with finer detail. The fluorescence images show a “superlayer” similar to that found in Rayleigh images in flows of this kind. Subtraction of one (normalized) image from the other in each pair reveals greater diffusive spreading of the biacetyl gas, a direct indication of its molecular contribution to total mixing. These findings support the argument that molecular diffusivity in general, and differential diffusion between species in particular, must be accounted for in the interpretation of laboratory flame results that are used to validate and test combustion models.

ISSN:0010-2202    

DOI:10.1080/00102209308907672

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The newly developed Finite Volume Method (FVM) was incorporated into a general pulverized fuel (PF) flame model to predict radiative heat transfer in furnaces. The code was applied to a benchmark test case where exact radiative heat transfer solutions were available and to a pilot scale front-firing tunnel furnace where flame measurements had been taken. The FVM predictions were found to approach the exact solutions in the benchmark case and compare well to the measured data in the pilot scale furnace. The results demonstrate the feasibility and potential of the FVM for radiation modelling in PF flames.

ISSN:0010-2202    

DOI:10.1080/00102209308907673

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

In this paper a laminar diffusion flame is predicted using a local grid refinement technique and non-staggered meshes. A single coarse grid covers the whole domain and local grid refinement is carried out in the regions of high gradients without changing the basic grid structure. The Navier-Stokes, the energy and the chemical species transport equations are solved and a finite rate chemical kinetics combustion model is employed. Transport equations for the species concentration are solved using a pseudo time splitting technique. The predictions are in good agreement with the experimental measurements for a confined laminar methane flame. The numerical method proved to be robust and accurate.

ISSN:0010-2202    

DOI:10.1080/00102209308907674

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A study was conducted to investigate the effects of jet size, jet number and jet orientation on the combustion rate in a jet-stirred constant volume combustion ‘bomb’. Qualitative information was obtained from flow visualization experiments using a schlieren photography system. A two-component LDV system was employed to measure velocities during and after charging of the chamber. Combustion pressure traces were recorded from piezoelectric pressure transducer output and were used to calculate mass fraction bumed curves.

ISSN:0010-2202    

DOI:10.1080/00102209308907678

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Planar laser induced fluorescence from the hydroxyl radical in spark-ignited, freely propagating turbulent flame kernels of methane/air mixtures has been used to study the early growth rate under varying turbulence conditions and mixture stoichiometry and to determine the inner cutoff of the fractal Hame surfaces. Methane/air mixtures were spark-ignited in a vertical wind tunnel and expanding turbulent Hame kernels were studied in a grid-generated decaying isotropic turbulent flow. The cold flow turbulence structure was determined by laser Doppler anemometry. An extensive use of the computational image processing and analysis techniques has been made to determine the kernel sizes and the fractal characteristics. Inner cutoff values of the self similarity of the flame surfaces representing the smallest scale of the flame wrinkling have been measured to be 20-30 times larger than the flame thickness and 13-15 times larger than the Kolmogorov length scale. Results suggest that the laminar flames are unaffected by the flow turbulence in a greater region than suggested by the criteria of turbulent Reynolds number < 1. The flame kernel growth for freely expanding methane/air flames in moderately turbulent flows (U1/UF= 1.62 – 2.73) is consistent with it being unaffected by the flow turbulence for kernel sizes smaller than the integral length scale of turbulence. For larger kernel sizes and the fully wrinkled flame geometries the kernel growth tends to become higher than the unstretched laminar values showing a weak dependence on the turbulence intensity.

ISSN:0010-2202    

DOI:10.1080/00102209308907675

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A numerical investigation is presented for multicomponent droplet evaporation under high-temperature laminar convective conditions. The study improves an existing model that did not account for the temperature dependence of liquid-phase densities and heat capacities. The new model, which accounts for variable gas and liquid-phase thermophysical properties, has also resolved previously reported numerical problems encountered during the early stages of the simulations. Detailed data on the temporal and spatial variation of the liquid-phase concentration fields throughout the lifetime of a bicomponent droplet provide new insight on the liquid mass-transport mechanisms within typical vaporizing hydrocarbon droplets. The current simulations indicate that the variation of liquid densities with temperature must be taken into account for an accurate representation of the transport processes in droplet convective vaporization. On the contrary, the effect of the variation of liquid heat capacities with temperature proves to be rather minor.

ISSN:0010-2202    

DOI:10.1080/00102209308907676

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The transient behaviors of three simplified methane mechanisms are numerically studied for two idealized turbulent nonpremixed combustion situations: the perfectly stirred reactor and the flamdet model. Two of the simplified mechanisms contain four global steps, and one of them has truncated source terms. The third mechanism is an improved version of the four-step mechanisms with the inclusion of an additional species CH3leading to a five-step mechanism. Comparisons of the steady-state flame results indicate that the three reduced reaction mechanisms predict the major species concentrations and the flame extinction limits in close agreement with those from a detailed mechanism. The performances of the reduced reaction mechanisms are further assessed by examining their transient responses to a simulated turbulent environment. The results reveal that truncation of global source terms leads to unsatisfactory results especially for lean flames and the performance of the five-step reduced mechanism is found to be superior to the four-step mechanisms only for flames under rich conditions. The flame response is found to be more sensitive to the fluctuation frequency than to the amplitude.

ISSN:0010-2202    

DOI:10.1080/00102209308907677

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The non-linear wave equation governing the progress of short length scale pressure perturbations across a conventional premixed flame is solved numerically. The particular acoustic disturbances considered have length scales of the same order as that of the flame and fractional amplitudes limited to O(l/θ), where θ is the dimensionless activation energy. These restrictions imply that the effects of the reaction and diffusion processes within the flame are negligible over the time scale of the passage of the pressure signals. The length scale of such a disturbance is of the order of a typical diffusion length, so the spatially varying temperature and density profile within the preheat and reaction zones must be considered (McIntosh 1989, McIntosh and Wilce 1991). However, in this work the exact steady temperature distribution, generated numerically, is used in the hyperbolic governing equations, enabling disturbances with length scales near to those of shock waves to be considered.

ISSN:0010-2202    

DOI:10.1080/00102209308907679

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

This paper discusses a number of proposed design modifications and changes in operational conditions which have a major influence on the overall performance of the Sheffield municipal solid waste incinerator plant. This 35 MW unit incorporates a heat recovery system for district heating. Four different secondary air injection systems and various primary air distribution patterns along the grate were investigated using computational fluid dynamics in an attempt to eliminate the existing slagging problems and at Ihe same time obtain optimum combustion conditions which would minimise potential emissions of toxic pollutants and reduce maintenance costs at the plant. The modelling work showed that the use of four tangential high speed secondary air jets together with optimising the primary air distribution along the grate, produces substantially longer residence times, improves the temperature profile at the exit, reduces the concentration of toxic pollutants and increases the combustion efficiency of the incinerator. The novel feature of the proposed secondary air injection system is the formation of a large size and significantly strong recirculation zone located above the burning refuse bed, in the middle of the furnace shaft. The presence of this strong recirculation zone improves the overall performance of the incinerator due to intensive mixing of the hot gaseous products (CO, volatile matter and hydrocarbons) evolving from the refuse bed with the combustion air supplied as the secondary air, and thereby greatly improves the gas phase combustion and helps to reduce emissions of chlorinated organic compounds; i.e. Dioxins.

ISSN:0010-2202    

DOI:10.1080/00102209308907680

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

First results are presented of a new flame code for modeling transport processes in two-dimensional flames using a one-step reaction model. The code is applied to a study on the flame shape of methane/air flames on a triple-slit burner. The results compare reasonably well with experimental data. We expect further improvement when more accurate measurements become available. These measurements are necessary to come to a more definite conclusion on the reliability of the model.

ISSN:0010-2202    

DOI:10.1080/00102209308907681

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor