摘要:

Experimental data are presented for the extinction limits of pilot-stabilised turbulent nonpremixed flames of methane and propane fuels inhibited by Bromotrifluoromethane, CF3Br commonly known as halon-1301. Compared with thermal inhibitors like N2and At it is shown that CF3Br is far more effective because it acts chemically as well. The halon is mixed either with the fuel or with the coflowing air stream. Results indicate that as the turbulent mixing rates increase, the mass fraction of CF3Br required to extinguish the flame decreases linearly with jet velocity regardless of whether the inhibitor is added to the fuel or to the air stream.

ISSN:0010-2202    

DOI:10.1080/00102209408935355

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Velocity field imaging techniques were used to observe how a single toroidal vortex, which represents one eddy in a turbulent flow, exerts aerodynamic strain on a premised flame. By achieving dense seeding of the flow, the spatial derivatives of velocity were determined accurately, which allows the following to be measured as a function of space and time: the aerodynamic strain rate that is tangential to the flame, the rate of flame stretch, the vorticity field, the shear strain rate field and the flow pattern near the forward stagnation point. The vortex strength was sufficient to cause quenching of the flame. An unexpected result is that the maximum strain on the flame does not occur on the centerline near the forward stagnation point. Instead the strain rate distribution is significantly different from numerical simulations of Poinsot, et al., for which strain is maximum on centerline. The difference is due to the different vortex sizes considered, which indicates that small vortices exert a different strain rate distribution on a flame than larger vortices, and that the process cannot be modelled as being self-similar. During flame quenching, the maximum local strain rate is measured to be 35 sec-1, which is similar to the value of 42 sec-1that is required to quench a steady, planar counterflow flame of the same equivalence ratio

ISSN:0010-2202    

DOI:10.1080/00102209408935356

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The current work seeks to determine the feasibility of using radicals to improve the combustion of propulsion systems fueled by hydrogen. An ArF excimer laser (193 nm) with focused optics was used to photodissociate molecular oxygen in hydrogen/oxygen mixtures, producing ground-state atomic oxygen, O(3P). In these experiments, the laser penetrated more than 20 cm into the mixture, there producing a maximum atomic oxygen concentration which leads to pure photochemical ignition. The minimum laser pulse energy required for ignition has been determined over a range of equivalence ratios (0.2 to 2.0) and initial temperatures (400 to 520°C) at a baseline pressure of 0.5 atm. Based on these values, the atomic oxygen concentration produced in the focal volume has been calculated. For these conditions, both the minimum laser energy and atomic oxygen concentration decrease strikingly as the initial mixture temperature increases. Results from a simplified ignition model agree with the experimental data. Both show only a weak dependence of the atomic oxygen concentration required for ignition on the equivalence ratio of the mixture over the ranges studied

ISSN:0010-2202    

DOI:10.1080/00102209408935357

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Structure of hydrogen/air diffusion flames has been analyzed by adopting a stagnant diffusion layer as a model problem. Starting from a twelve-step hydrogen oxidation mechanism, a two-step reduced mechanism has been deduced by applying appropriate steady-state and partial equilibrium assumptions. Various flame structures have been identified depending on the strain rate which affects the relative rate of the recombination and branching reactions. In the small strain rates, all the radicals can be assumed in steady-state that a one-step reduced mechanism can describe the flame structure in which the branching and recombination reactions occur simultaneously in a thin reaction zone. Radical profiles and their relative ratios have been derived and the results agree well with the numerical calculations

ISSN:0010-2202    

DOI:10.1080/00102209408935358

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Goal of this article is to present a detailed kinetic scheme able to describe a large variety of macroscopic experimental data related to methane pyrolysis, partial oxidation and combustion. Due to the importance of C2 chemistry, detailed comparisons for pyrolysis and oxidation of ethane, ethylene and acetaldehyde are also reported. The main effort has been put in collecting and unifying these experimental knowledge into a single and comprehensive kinetic scheme. The result of this analysis is to enlarge the simulation capabilities of a single and general kinetic scheme; such a model, expensive in terms of species and reactions (more than 70 real and ‘;equivalent’ species involved in more than 1600 elementary reactions), maintains the typical advantages of mechanistic kinetic models. Therefore, once the submechanisms of the simpler species are validated and tested on the basis of a large set of experimental data, it becomes feasible to analyze the oxidation and decomposition of larger hydrocarbon fuels with only a very limited set of new primary elementary reactions

ISSN:0010-2202    

DOI:10.1080/00102209408935359

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

A simple and accurate method is presented to determine the flame temperature and the adiabatic burning velocity of laminar premixed flat flames, using a specially constructed flat flame burner. The heat loss of the flat flame is determined from measurement of the temperature profile of the burner plate. The adiabatic burning velocity is found when the burner plate temperature profile is uniform, implying that the net heat loss of the flame is zero. Several methods are presented to determine the thermal conductivity of the burner plate. The results for methane/air mixtures are compared with experimental data found in the literature and one-dimensional flame calculations, and close agreement is found. The method is particularly useful in providing an accurate reference for other temperature measurement techniques (e.g. spectroscopic laser diagnostics), and to determine adiabatic burning velocities over the entire range of flamm ability

ISSN:0010-2202    

DOI:10.1080/00102209408935360

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

A theoretical investigation into the effects of rotation on the burning characteristics of an isolated fuel droplet is presented. Rotation induces a secondary flow in the gas phase inward towards the droplet poles and outward from the equator. Overall, additional heat is transported from the flame to the droplet, enhancing the vaporization and burning processes and shortening the droplet lifetime. Explicit expressions are given for the burning rate and for the variations in the droplet diameter. The distorted flame shape and the modified flame stand-off distance are also determined. Although it enhances the burning, the swirling makes the flame more susceptible to extinction. It is found that when extinction occurs it first takes place locally either at the poles of the flame sheet or at the equator, depending on how far conditions are from stoichiometry

ISSN:0010-2202    

DOI:10.1080/00102209408935361

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The autoignition of gaseous mixtures representative of coal derived low-heating value fuels in a direct-injected diesel engine was theoretically investigated over a pressure and temperature ranges of 10 to 50 atm and 800 to 1000 K respectively- A computer model was validated with experimental data from an engine operated on direct injected synthetic coal gas. The computed results demonstrated the importance of the characteristic time associated with the chemical kinetics when compared to the fluid mixing. High gas temperatures resulting from high compression ratios, high boost, high inlet air temperatures or other ignition aids are needed to ignite these gases in a compression-ignition engine in a timely manner. The predicted sensitivity of the autoignition delay time to changes in the engine operating conditions was significant for temperature but almost negligible for pressures up to and slightly in excess of 30 atm

ISSN:0010-2202    

DOI:10.1080/00102209408935362

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

A model has been developed to simulate combustion and heat transfer in a porous matrix combustor-heater. The system consists of a packed bed in which a premixed natural gas-air mixture combusts. Radiative and convective heat transfer occurs from the flame zone to a fluid flowing through a bank of tubes embedded in the packed bed. Radiative heat transfer in the bed is modelled as a diffusion process, and the flow and temperature fields of the gas and solid matrix in the bed are calculated by solving the mass, momentum, energy and species conservation equations accounting for variations of thermophysical properties with temperature. Allowance is made for a finite heat transfer rate between the gas and the solid matrix, i.e., the temperatures of the gas and the solid matrix are not equal locally. The effects of particle diameter, excess air, and firing rate are examined and their influences on the thermal performance of the porous matrix combustor-heater are discussed. The sensitivity of the model to the volumetric heat transfer coefficient is also investigated and discussed. The numerically obtained results are compared with available experimental data for a similar system, and the predictions are found to agree with test data within the experimental uncertainty. The difference between the predictions of this model and a previous model based on the assumption of thermal equilibrium between the gas and solid matrix increases as the bed particle diameter increases

ISSN:0010-2202    

DOI:10.1080/00102209408935363

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

A new method is proposed to compare models for the premixed turbulent burning velocity on the basis of the model-predicted turbulent burning velocity diagrams in three coordinate planes (Re-Da, ηk/ηO-u′; /SLOand L/ηO-u′;/LO) which were usually used to show the phase-diagrams of flame structures in premixed turbulent combustion. As a turbulent burning velocity diagram can show in detail almost all the possible model predictions of turbulent burning velocities at various combustion conditions and the overall characteristics of the model on the same plane, it provides a thorough and direct exhibition of the model. Therefore, comparison of the models on the basis of their respective diagrams is more effective and more thorough than the conventional method which compares models by showing the model-predictions on a u’/SLO- ST/SLOplane for some specific mixtures at different turbulence intensities

ISSN:0010-2202    

DOI:10.1080/00102209408935364

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor