摘要:

Various similarity hypothesis for axisymmetric turbulent buoyant plumes with large density differences (strong plumes) are discussed in this paper, In contrast to buoyant flows having small density variations (Boussinesqu limit), controversies remain in modeling strong buoyant flows for which (▵e/e0=(e0-e)/e0=O(1)), We review: 1) previous attempts to model such flows. 2) experimental evidence that the flow properties for strong plumes have similar profiles. and 3) a consistent derivation of the similarity relations. A significant conclusion from this analysis is that the similarity profiles for velocity and density defect (or temperature rise) should be expressed in terms of an average stream function or equivalently the Howarth variable (e.g.,n=∫0r(e/e0rdr for axisymmetric plumes) which includes variable density effects. Others have suggested, instead, that the plume property profiles might be expressed by using a laboratory radial coordinate normalized by a local characteristic flow width, in analogy to weak buoyant plumes. We demonstrate that these latter similarity assumptions are inconsistent with the conservation equations for both overall momentum flow and overall mass flow. In addition, these latter similarity arguments encounter difficulties with generally accepted entrainment laws for strong plumes. These problems are overcome by using the somewhat more cumbersome stream function similarity. However, we point out that final resolution of the controversy may have to await definitive experimental examination of the radial similarity in strong plumes.

ISSN:0010-2202    

DOI:10.1080/00102208808923987

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

—The structure of a strained premixed laminar propane-air flame is examined. The flame is formed in the neighborhood of the stagnation point produced by the counterflow of fresh mixture and hot combustion products. The governing conservation equations are solved numerically by employing adaptive continuation techniques. The model includes detailed transport and complex kinetics using a reaction scheme due to Warnatz. Flame structures, ignition regimes and extinction regimes are exhibited. The results are in excellent qualitative agreement with the asymptotic theory of Libby and Williams and can be used to develop a flame let description of turbulent premixed propane-air combustion.

ISSN:0010-2202    

DOI:10.1080/00102208808923988

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

—A turbulent combustion model for a CO/H2/N2-air turbulent jet diffusion flame has been developed by combining a velocity-composition joint pdf turbulence closure, a Monte Carlo solution algorithm, and a two-scalar stretched laminar f1amelet chemistry approach. In the flamelet model, it is assumed that the structure of the turbulent diffusion flame is locally that of a laminar diffusion flame at the same instantaneous value of the mixture fraction (ξ) and scalar dissipation (X). A library of stretched laminar flamelets (density, temperature, and species concentrations vs, ξ and X) is generated for a laminar opposed-flow configuration including 15 chemical species, 32 reactions, and differential diffusion. A modeled transport equation for the joint probability density function of the velocities, ξ, and X is then solved numerically by a Monte Carlo method. Calculated profiles of mean mixture fraction, temperature, and species concentrations are compared both with experimental data and with earlier modeling studies based on different turbulence closures and different chemistry schemes. Overall, the level of agreement between flamelet model and experimentis comparable to that obtained using a two-scalar partial equilibrium model. The flamelet approach shows some advantage in the fuel-rich regions of the flow, but yields an overly rapid approach to chemical equilibrium downstream in the jet. The flamelet model could not be rigorously justified in this turbulent diffusion flame because calculated laminar flame thicknesses are larger than the estimated smallest turbulence mixing scales. Still, we believe that a flamelet model is of value because it is computationally tractable and because it is one of the few approaches that directly includes the important coupling between chemical reaction and molecular diffusion.

ISSN:0010-2202    

DOI:10.1080/00102208808923989

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

A Jet Stirred Reactor for Combustion Studies: Design and CharacterizationAbstract–A stainless steel jet stirred reactor (JSFR) has been designed and constructed, capable of operating under pressures up to 20 atm and temperatures up to 1000 K and residence times as short as 0.015 s.Complete mixedness has been experimentally checked by means of mass-spectrometric analysis of argon pulses in a nitrogen steady flow.A Nusselt type correlation has been developed to evaluate the heat transfer coefficient for this kind of reactors.JSF reactor technique has been applied to study the dynamic behaviour of air-stoichiometric mixtures of n.heptane and i.octane. Runs have been performed both at constant and variable operation pressure. Heat release rates (HRR) of combustion processes have also been systematically evaluated.

ISSN:0010-2202    

DOI:10.1080/00102208808923990

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The dependence of the critical parameter for the ignition of a combustible solid on the physical quantities of interest is explored. This analysis shows that there exists a minimum initial temperature and a minimum reactant concentration for the ignition of weakly exothermic systems. Application to combustion synthesis of ceramics is discussed.

ISSN:0010-2202    

DOI:10.1080/00102208808923991

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

In this paper, the joint droplet size and velocity distribution is derived by applying information theory to the atomization process, along with the normalization of the probability distribution function and the physical conservation laws of mass, momentum and energy. The obtained distribution contains the Weber number as a variable, and agrees with experimental observations. An equation for the Sauter mean diameter (D32) is obtained which agrees with several of the expressions that have been obtained from correlations of experimental data. When the Weber number exceeds 4000, the results given by Li and Tankin (1987) are appropriate.

ISSN:0010-2202    

DOI:10.1080/00102208808923992

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

Bubble Characterization in Pressurized Fluidized-Bed Combustors with Bed InternalsAbstract–Bubble characteristics within a tube bundle of a Pressurized Fluidized Bed Coal Combustor (PFBC) were studied. A model was developed to compute emulsion phase gas velocity, bubble throughflow velocity, and the visible bubble flow coefficient. Experimental data obtained from a PFBC unit were compared with the model prediction and a good agreement was observed.

ISSN:0010-2202    

DOI:10.1080/00102208808923993

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

An experimental study of combustion characteristics and structure of interacting 2, 3, and 5 propane jets in a cross flow is presented. The separation distance between the burners was set at 8, 12, and 16 exit diameters. The ratio of jet momentum flux to cross flow momentum flux was varied over a range of 44-390. Flame length, blowout stability, temperature, radiation emission, and concentrations of CO, NO, and O2are measured. The effects of number of jets, their separation distance, and their geometrical arrangement relative to cross flow, on flame length, radiation and stability characteristics are examined. The changes in the radial profiles of temperature, and concentrations of CO, O2, and NO are studied as functions of separation distance and ratio of jet momentum flux to cross flow momentum flux of 3-jet flames. The range between the upper and lower stability limits of the jet velocity of lifted flames in cross flow is higher for multiple-jet flames than for single-jet flames. A third limit, which is newly identified, however is independent of the number of jets or their separation distance. Carbon monoxide concentration and flame radiation are maximum at moderate values of separation distance and NO concentration decreases monotonically with it.

ISSN:0010-2202    

DOI:10.1080/00102208808923994

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

Transport and mixing of diffusive scalars in turbulent flows are simulated computationally based on a novel representation of the temporal evolution along a transverse line moving with the mean fluid velocity. The scalar field along this line evolves by Fickian diffusion, representing molecular processes, and by randomly occurring events called block inversions. Block inversion, representing the effect of turbulent convection, consists of the random selection of an interval (Y0− 1/2, Y0+1/2) of the line, where the interval size l may he either fixed or randomly selected, and replacement of the scalar field θ(y) within that interval by θ(2y0For fixed l, the model requires a single input parameter, the Peclet number. To demonstrate the performance of the model, this formulation is used to compute the spatial development of diffusive scalar fields downstream of several source configurations in homogeneous turbulence. Generalization to inhomogeneous turbulence is discussed, as well as a formulation which incorporates dependences on Reynolds number and Schmidt number. Finite rate chemical reactions can also be incorporated.

ISSN:0010-2202    

DOI:10.1080/00102208808923995

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

An infrared radiometer, which incorporated three optical paths and operated between 3.5-4.0 μm, was used to obtain transient surface temperatures in a fired engine. An integrated form of the equation of radiative transfer was applied to each optical path allowing gas emission, gas absorption and deposit surface radiation within the combustion-chamber to be characterized during a single cycle. Measurements of a simulated deposit were made at various temperatures and spark advance with the engine fired on methane and again on gasoline. Radiative characteristics of the combustion gases such as absorption and emission were investigated at several different operating conditions. A maximum error of 6% was found for temperature measurements between 660 and 900 Kelvins. Using a fast surface thermocouple with its junction located at the inner surface of the combustion-chamber wall, two combustion-chamber deposits were accumulated for different lengths of time. Deposit surface and interface temperatures were investigated at several different operating conditions for each deposit. Utilizing the experimentally obtained deposit surface and interface temperatures, deposit thermal conductivity (k) and heat capacity (Qc) were determined via a computational model which treated k as a function of temperature. Values obtained for k and Qc were well within the range of those reported for various forms of graphite.

ISSN:0010-2202    

DOI:10.1080/00102208808923996

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor