摘要:

A comprehensive theoretical analysis has been developed to study vaporization of liquid oxygen (LOX) droplets in hydrogen over a wide range of pressure. The formulation accommodates complete sets of conservation equations for both gas and liquid phases, and accounts for variable properties, thermodynamic non-idealities, and vapor-liquid phase equilibria. The model is capable of treating the entire history of a vaporizing LOX droplet, including the thermodynamic phase transition through the critical mixing point. Various distinct high-pressure effects on the droplet behavior were investigated in depth. In particular, a parametric study of the droplet lifetime as a function of the ambient pressure, temperature, and initial droplet diameter has been conducted. The droplet lifetime exhibits a strong pressure dependence and can be correlated well with the square of the initial diameter.

ISSN:0010-2202    

DOI:10.1080/00102209408935380

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The burning process of a fuel droplet adjacent to an oxidizer droplet under an oxidizing convective Rowis studied numerically by a quasi-steady body-fitted computation. Hypergolic propellants such as monomethyl hydrazine and nitrogen tetroxide served as the fuel oxidizer sources, respectively. The computation took into account the variable properties of bipropellant species and products, with a singlestep global finite-rate chemical reaction being assumed for the gas-phase combustion. The results obtained from the present numerical analysis show that multiple flame-configurations and vaporization-rates characteristic of temperature-sensitive, high-activation-energy Arrhenius kinetics, occurring under certain flow conditions for n-octane droplet burning in air flow(Jiang et al.,1993), are not exhibited by the present hypergolic propellants, which are characterized by a low-activation-energy of reaction. The oxidizer droplet adjacent to the fuel droplet substantially influences both flame configuration and the vaporizationrates of the fuel droplet by providing extra oxidizer vapor as well as changing the surrounding thermal-Row structures. At low Reynolds numbers (> 20), the fuel droplet vaporization rate is increased due to either a leading, or a trailing oxidizer droplet. Under these conditions, the fuel droplet vaporization rate is further increased by either a larger oxidizer droplet or smaller droplet center distance; results similar to those for biopropellant droplet burning in a stagnant environment. At high Reynolds numbers (< 20),the fuel droplet vaporization rate is not as significantly influenced by the accompanying oxidizer droplet as that at low Reynolds numbers. An excessively large leading oxidizer droplet, at Reynolds numbers higher than 60, results in a substantially lower fuel droplet vaporization rate than that exhibited by single droplet burning.

ISSN:0010-2202    

DOI:10.1080/00102209408935381

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Soot formation in laminar ethylene inverse diffusion flames has been investigated experimentally and modeled. Soot volume fraction and temperature measurements have been made and compared to numerical predictions using a soot formation model previously applied to normal diffusion flames. The inverse flame configuration serves as a good test of the applicability of the model and is relevant to practical combustor designs. Comparisons ofthe predictions and data show good agreement. The model reasonably predicts several features of the inverse flame, including flame length. the locations of soot-bearing regions of the flow, and the evolution of the soot volume fraction profile.

ISSN:0010-2202    

DOI:10.1080/00102209408935382

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Logging slash on a 486 hectare site in Ontario was burned as part of a Forestry Canadaforest management program.A 100hectareportion of this site was instrumentedbyseveralgroups interested in large scale fires. NIST utilized Forestry Canada data on mass loading before and after the fire, total burningarea as a functionof timeand burningduration to estimate the spatialand temporalpatternof heat release during the burning of the instrumented section of the fire. The heat release rate was estimated to reach 2-4 x 107kW during the time of interest, This information was utilized in the context ofa flow model due to Baum and McCaffrey to calculate the near-ground flow field induced by this heat release pattern; the results compared moderately well with point measurements made in the field.

ISSN:0010-2202    

DOI:10.1080/00102209408935383

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

A new concept of direct gasification of pulverized coal in a stream reactor has been studied by mathematical simulation of the process. The new method presumes that gasification of pulverized coal will occur in the annular region of a cylindrical reactor during the co-current flow of coal-oxidant mixture in the peripheral zone and hot combustion products-serving as a heat soure-in the core region of the reactor.The paper presents a model of the process which was treated as a reactive, multi-component, two-phase mixture flow with simultaneous combustion in the core and coal gasification in the peripheral annular region ofthe reactor, with radiative heat exchange between the two co-current streams. The gas phase was described in the Eulerian frame and the discrete phase in the Lagrangian frame with the account of the mutual mass and heat transfer between the two phases. The closure of the transport equations for the gas phase was achieved by applying a version ofthe k-6turbulence model modified, to account for the mass and momentum exchange between the phases due to chemical reactions. Heterogeneous chemical reactions of coal were treated in the kinetic-diffusion regime and the radiation heat exchange was modelled by a six-flux model. The model,verified in an example of pulverized coal combustion, showed that the concept is feasible and yielded basic information for the design of an experimental rig for a final verification of the concept.

ISSN:0010-2202    

DOI:10.1080/00102209408935384

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The paper presents some results of the mathematical simulation of a new concept of direct gasification of pulverized coal in a stream reactor. The new' method presumes that gasification of pulverized coal will occur in the annular region of a cylindrical reactor during the co-current flow of coal-oxidant mixture in the peripheral zone and hot combustion products-serving as a heat source-in the core region of the reactor. Part I. ofthe paper outlined the rationale ofthe gasification concept and described the model of hydrodynamics and of radiative heal transfer in the particle-laden gas flow. The process was modelled as a reactive, multi-component, two-phase mixture flow with simultaneous combustion in the core and coal gasification in the peripheral annular region of the reactor, with radiative heat exchange between the two co-current streams. The gas phase was described in the Eulerian frame and the discrete phase in the Lagrangian frame with an account for mutual mass and heat exchange between the two phases. Radiation heat exchange was modelled by the six-flux model. Part II deals with coal chemical reactions relevant to the considered gasification concept and presents some results of computation by the complete model of the process. Heterogeneous chemical reactions of coal were treated in a kinetic-diffusion regime. The model, verified in an example of pulverized coal combustion, showed that the concept is feasible and yielded basic information for the design of an experimental rig for a final verification of the new gasification technique.

ISSN:0010-2202    

DOI:10.1080/00102209408935385

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Following the recent computational result that the ignition of counterflowing hydrogen versus heated air possesses three distinct ignition limits similar to those of the homogeneous explosion limits of hydrogen/air mixtures, a large activation energy asymptotic analysis with reduced mechanisms has been conducted to provide an analytical description of this phenomenon. Starting with an appropriate eight-step mechanism for hydrogen oxidation, for each limit the relevant reduced mechanism is derived and the asymptotic structure analyzed. Results show that while the first and third limits are strongly affected by transport, the second limit is dominated by chemistry. In fact, the controlling chemistry for the second ignition limit is identical to that of the second explosion limit of the homogeneous mixture. The predicted ignition limits agree reasonably well with the numerical results obtained with full mechanism.

ISSN:0010-2202    

DOI:10.1080/00102209408935386

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Measurements of probability density functions (PDFs) and power spectral densities (PSDs) are needed to characterize minor species concentrations in turbulent flames. Current techniques can be used to measure PDFs, but they have limited ability to measure PSDs. We report here on a new strategy. picosecond time-resolved laser-induced fluorescence (PITLIF)), which can be used to simultaneously measure concentration PDFsand PSDsin turbulent flames. Knowledge of the local quenching environment, which is necessary to obtain accurate number densities, is obtained using a high-bandwidth channel capable of resolving individual temporal decays of the species of interest. In this initial investigation, we demonstrate the capabilities of the PITLIF instrument by studying fluctuating concentrations of sodium seeded into a laminar H2/O2/Ar diffusion flame. We show that PDFs and valid PSDs can be obtained for concentration fluctuations in the frequency range of interest for many studies of turbulent combustion.

ISSN:0010-2202    

DOI:10.1080/00102209408935387

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

In the framework of a nonlinear model which incorporales the Landau-Darrieus instability mechanism and curvature effects, we study thin premixed flames subjected to incoming velocity fluctuations of known r.m.s, intensity (∼ µ') and length-scale ( ∼ Lint). Specifically, we numerically integrate a forcedevolution equation of Michelson-Sivashinsky type, with periodic boundary conditions and focus attention on the mean spacing 'erul(t)between front crests. The forcing mimics the one-component, near Lorentzian spectrum of moderate, isotropic turbulence; the phases and turnover times are randomly sampled. Processing the results of many runs suggests that: i) in the long-time limit /(:resl fluctuates around a value Ll;lest which, once ensemble-averaged to give the quantity 1/' only feebly depends on the lateral flame extent (if the latter is large enough); instead, AcreI, mainly scales like the marginally-stable wavelength Aneutral of the linearized. unforced evolution equation.ii) oActnJou' 0 and, if u'/SL is small enough, an approximately logarithmic increase of Acrt.JAncutr' with decreasing u'ISL is obtained.iii) At fixed U'/SL' Acrc.JAneulf.1 only weakly depends on L inl_ apparently in a nonmonotonic way and with a shallow minimum when Lint and Ancu,ul approximately coincide.iv) When u'lSL is large enough, a new regime appears, where the front gets more and more often covered with traveling waves.The results i)-iii) are tentatively interpreted in terms of the influence of wrinkle-stretching on hydrodynamic instability. We propose a scaling law for A cm, in the low u'/SLregime; besides Aneutral' Lint and u', it explicitly includes the density ratio and the shape of the forcing energy-spectrum, and it is compatible with all our numerical findings. even for not-so-small intensities of forcing.

ISSN:0010-2202    

DOI:10.1080/00102209408935388

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

In this study a refined theoretical model was devised to estimate the ignition delay of a suspended coal-water slurry (CWS) droplet exposed to hot gas stream. A very detailed temporal temperature variation inside the droplet is taken into account. The effect of the surface radiation was also included in the model. The governing equations are numerically solved and the results are corroborated in comparison with the experimental ones. The outer porous layer could be pyrolyzed as the evaporation front kept going inward to the center of the droplet. Even before the binary mixture in inner region of the droplet was thoroughly evaporated, the volatile gases generated could be ignited in the vicinity of droplet surface. The ignition delay time was primarily comprised of both the heating time and the mixing time. But the heating time, which is required for a droplet to be vaporized and then pyrolyzed, was found to be controlling one. Experimental results were used to validate the numerical results. A reasonable agreement was observed in ignition delays of droplets, only when the surface radiation was incorporated in the model.

ISSN:0010-2202    

DOI:10.1080/00102209408935389

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor