摘要:

A direct stiffness method of analyzing the elastic ftexural-torsional buckling of rigid-jointed plane frames composed of l-section members and subjected to in-plane loads is presented. The in-plane stiffness matrix and the fixed-end resultants are obtained from the member stiffness matrices derived from the in-plane differential equations. These member stiffness matrices are assembled and solved, and their solutions are used to linearize the flexural-torsional buckling equations. The out-of-plane member stiffness matrices are then obtained numerically from the buckling equations by the method of finite integrals. The out-of-plane frame -stiffness matrix is assembled, and the critical loads are obtained when its determinant is zero. A computer program is developed which carries out either a first- or second-order in-plane analysis, and then determines the flexural-torsional buckling loads. The effects of in-plane deformations prior to buckling can be included. Very good agreement is obtained between the results computed by this program and known solutions, and its ability to analyze large complex frames is demonstrated.

ISSN:0010-2202    

DOI:10.1080/00102209408935443

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

We demonstrate a picosecond pump/probe absorption instrument for measuring absolute concentrations in a rapidly fluctuating flame environment at atmospheric pressure. The instrument is based on a regeneratively mode-locked Ti:Sapphire laser. Atomic potassium is seeded into an atmospheric, premixed CH4/air flame. A detection limit for atomic potassium of 1.1 x 1011cm-3is obtained for an integration time of 1 second without the need for calibration. We predict a detection limit for CH of 1.0 x 1013cm-3if simple modifications are made to the pump/probe instrument, including a reduction in laser spectral bandwidth by a factor of 50.The spatial resolution for our measurements is 0.137cm. We have also demonstrated the temporal resolution that the picosecond pump/probe instrument offers. Here, 50-Hz fluctuations were induced in an otherwise laminar flame, and the resulting fluctuations in the potassium concentration were resolved on power spectral density (psd) plots. A detection limit or 1.5 x 1011cm-3was obtained for the psd system. Weestimate that a corresponding detection limit or 1.5 x 1013cm-3for CH is possible. The temporal resolution of our system is presently limited to that of the borrowed detection electronics, and simple modifications will allow resolution beyond 1 MHz.

ISSN:0010-2202    

DOI:10.1080/00102209408935444

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

ABSTRACT—analyze the structure of a steady laminar isobaric planar deflagration propagating through a uniform mixture of pressed powders. Uniformity here refers to spatial invariance in the stoichiometric ratio for a volume with dimension large relative to the characteristic particle size. The powders reacts rapidly to generate copious gas, as well as solid (and possibly molten) products of combustion. Specifically, we seek the rate of flame propagation through a mixture of sodium azide (NaN3) and hematite–derived iron oxide (Fe2O3) for practically interesting stoichiometries, for which the oxide is present in excess in the pristine pyrotechnic. For a uniform grain, the hot–boundary products are nitrogen gas (N2) and the solid-phase species iron (Fe) and sodium oxide (Na2O), We obtain the flame–propagation rate as a function of stoichoirnetry and other mixture properties for a two–step model: (1) a roughly energy-neutral decomposition of the sodium azide occurs at a relatively low temperature, and (2) a relatively exothermic, heterogeneous. kinetically–limited reaction between the resulting sodium vapor and the solid–phase iron oxide occurs at relatively high temperature. We account for the departure of the sodium vapor from saturation near the “hot boundary” for azide–deficient mixtures, such that all the sodium is “fixed” in the burned mixture. The role of the oxide is both to provide the heat that sustains the propagation, and to counter the presence of sodium vapor in the hot products. We discuss briefly extensions to account for nonuniformity in the mixture, and to excess–azide mixtures.

ISSN:0010-2202    

DOI:10.1080/00102209408935445

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

This paper describes an experimental exploration of droplet–droplet and droplet–flame interactions in a self-sustained rectilinear monodispersed droplet stream flame. Fuel droplets in the stream have an initial diameter of approximately 1OOµm and travel at an average velocity of 1Om/s (Reynolds number approximately 100) vertically upward. The initial center to center droplet spacing is between 2 and 10 droplet diameters. The droplet stream ignites as it passes through an electrically heated coil of bare nickel–chromium wire. Vaporization rates are computed from changes in droplet size that are measured from photographs. The results indicate that, as the spacings examined, droplet vaporization rates in the stream flame differ from isolated droplet vaporization rates. In this spacing regime, the decrease in the vaporization rate is almost linear with decreasing droplet spacing. At a constant mass flow rate in the droplet stream, the total mass vaporization increases as the initial droplet size and spacing decreases, even though the individual droplet vaporization rates decreases. The increased total surface area with decreased droplet size counteracts the droplet interaction effects. The trends of the vaporization results are independent offuel type, but the structure of the stream flame, and particularly the behavior of the sooting region, is very sensitive to the fuel used. A thin blue non–sooting flame escapes the top of the soot plume when pure hydrocarbon fuels burn. indicating that shoot formation ends before the liquid fuel completely burns out.

ISSN:0010-2202    

DOI:10.1080/00102209408935446

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

—A linear stability analysis has been performed to identify the role of the hydrodynamic instability when combustion occurs in a closed tube. The model treats the flame as a surface of discontinuity in an otherwise inviscid, isentropic mixture. The burning rate is obtained from an analysis of the internal flame structure. It is shown that flame acceleration, a consequence of pressure buildup, has a damping influence on the long wavelength disturbances. Disturbances of shorter wavelengths grow but their growth rate exceeds the propagation speed, which can only then be identified as an instability. The growth rate is found to be always lower than the corresponding growth rate for a freely propagating flame.

ISSN:0010-2202    

DOI:10.1080/00102209408935447

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

An assessment is sought of the stretched laminar flamelet approach, in particular as far as its capability to account for finite-rate chemistry effects in nonpremixed turbulent combustion is concerned. It is used here with the k - e - 9 turbulence model, in order to obtain a computational model which can easily be implemented on current codes. A particularly convenient form is adopted, which limits the computational overhead over nonreacting computations to a minimum. An appropriate plane is identified to check the combustion regime in the different regions of the flame. Test cases are reported which involve quantities most directly affected by finite-rate chemistry, namely mean and variance of OH, CO and NO concentration in different flames. Numerical predictions worked out with the present model are compared to experimental results and predictions by other authors. Results confirm the potentialities of the proposed approach, which in addition can beextended to include insofar neglected effects. The pros and cons of the different approaches proposed for finite-rate chemistry are also discussed.

ISSN:0010-2202    

DOI:10.1080/00102209408935448

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The characteristic modes and structure of the nonpremixed circular-disc stabilized flamesare experimentally studied. Using direct and schlieren photography techniques, the flames in the stability domain are systematically classified into seven characteristic modes:recirculated,transition,unsteady detached,laminar ring,developing,split flashing, andlifted. The flame behaviorand the combusting and cold flow patterns in each characteristic mode are comprehensively discussed. Two stability limits which are closely related to the flow structure are found. The flame and lift-off heights are correlated to the central-to-annular jet momentum ratio. The structure and transport mechanism in the recirculation bubble dominate the flame behavior for central-to-annular jet momentum ratio lower than 7.5. For the central-to annular jet monentum ratio higher than 7.5, the combusting flow behaves like a turbulent diffusion flame.The temperature profiles are probed by a fine-wire thermocouple. The thermal structure in the recirculation area is found 10 be complicatedly dominated by the vortex structure in the bubble.

ISSN:0010-2202    

DOI:10.1080/00102209408935449

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

A model for early flame kernel development that allows for individual stochastic realizations is proposed. The following concepts are used in the model: the flamelet concept, the flame broadening concept, the concepts of flame quenching by turbulence and early flame kernel extinction due to heat loss into the spark electrodes. The model simulates both cycle-by-cycle variation and lean misfire limit in an 51 engine.Comparison between numerical results and measurements of spark-ignited kernels under a variety of conditions verify the validity and universality of the model.

ISSN:0010-2202    

DOI:10.1080/00102209408935450

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

ABSTRACT–Fuel pyrolysis is an important step in gas-phase flame spread over condensed fuels. First generation models treat pyrolysis as a vaporization phenomenon and assign a constant vaporization temperature, an experimentally determined unknown, to the pyrolyzing fuel. In second generation models,and Arrhenius expression is used to evaluate the surface mass flux. The kinetic parameters, i.e., the pre-exponential factor and the activation energy, replace the vaporization temperature as unknown constants. The values used for these parameters for a particular fuel vary over a wide range in the literature.Here a parametric study is presented to evaluate the importance ofthese two kinetic parameters in relation to flame spread over a thin cellulosic fuel. An increase in the pre-exponential factor or a decrease in the activation energy causes the vaporization temperature to decrease, which in turn affects the flame structure and the spread rate. However, the two kinetic parameters do not act independently, and sets of kinetic parameters are found to beequivalent for a given environmental condition. A dimensionless parameter, comprising the kinetic constants as well as several environmental variables, that is related to the vaporization temperature through a simple formula has been identified. This formula predicts the vaporization temperature for a given pair of kinetic constants and given environmental conditions. The effect of the pyrolysis kinetics on flame spread can be reproduced by a constant-vaporization-temperature model by using the developed correlation between the vaporization temperature and the kinetic constants.

ISSN:0010-2202    

DOI:10.1080/00102209408935451

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

ABSTRACT–Numerical modeling of propagating fronts in non-uniform two-dimensional flow fields is performed in orderto simulatetheeffect of such flowson premixed flamefronts. In particular, the influence of the flow disturbance intensity (uύ) on the mean front propagation rate (ST) is examined.A second-order numerical technique is employed that combines the level set (G-equation) formulation to describe the self-propagation of the front and a multidimensional upwind technique to describe the convection of the front by the flow field. In this way the effect of the non-dimensional disturbance intensity (uύISL) on the non-dimensional propagation rate (STISL) at values of uύ/SL>> I is computed. The dependence of the laminar propagation speed (SJ on the flame stretch (including both the front curvature and the velocity strain effects) is incorporated in this formulation. We focus on front propagation in simulated Taylor-Couette flows in the “Taylor vortex#rdquo; regime and the results are found to compare favorably with recent experiments on the propagation or isothermal chemical fronts in this flow. Theformation of “islands” of reactants is observed and its relation to front propagation rates is discussed.

ISSN:0010-2202    

DOI:10.1080/00102209408935452

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor