摘要:

Fluidized beds often burn fuels containing aromatic hydrocarbons and chlorine. This work uses a bench-scale fluidized bed and some detailed calculations to investigate the inhibition of benzene oxidation by hydrogen chloride. Carbon monoxide and carbon dioxide were measured in the freeboard of the fluidized bed reactor over a range of HCI and oxygen concentrations and bed temperatures. For fuel-lean conditions, the presence of HCI causes the CO emissions to increase significantly. HCI addition also increases the unburned hydrocarbon levels. For fuel-rich conditions, the inhibitory effect of HCI is reduced. The experimental measurements indicate, in spite of the large surface areas available to quench radicals, most of the oxidation occurs within the fluidized bed and not in the freeboard. Model predictions agree qualitatively with experimental measurements made in the bench-scale fluidized bed; the trends with HCI concentration, equivalence ratio and temperature are correct. However, the model tends to under-predict the HCI inhibition of hydrocarbon and carbon monoxide oxidation. The model suggests that the inhibition of CO oxidation by HCI is the result of reduced radical levels due to reduced chain branching. The model shows less HCI inhibition of hydrocarbon oxidation because of CI radical reactions with benzene and intermediate hydrocarbons.

ISSN:0010-2202    

DOI:10.1080/00102200008947308

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

We consider the combustion, in a gravitational field, of a heterogeneous powder mixture compressed into a solid sample, in which the high temperature ahead of the reaction zone destroys the solid, due, e.g. to melting of some of components of the mixture. Thus, a suspension is formed, consisting of a liquid bath containing solid or liquid particles. Processes such as heat and mass transfer as well as chemical reactions in the suspension determine the structure of the combustion wave and its propagation velocity. Under the influence of gravitational forces there is the possibility of relative motion of the liquid and solid. Previous theoretical analyses considered the rate of heat transfer between the solid and liquid phases to be sufficiently large that their two distinct temperatures rapidly equilibrated to a single temperature. In addition to this case, we also consider the case when the rate of heat transfer is not so large and the model involves the separate temperatures of the solid and liquid phases. We find that multiplicity of traveling wave structures is possible. In particular, in addition to a low velocity structure, which is essentially the same as that obtained from the one temperature description, we find a high velocity structure, which does not exist in the one temperature description, but rather depends on the fact that the solid and fluid temperatures differ from each other. Both structures can exist for the same parameter values in a given range. We describe the dependence of the combustion characteristics of the two structures on gravitational forces and other factors. In particular, we compare the characteristics in gravity and microgravity environments.

ISSN:0010-2202    

DOI:10.1080/00102200008947309

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

This paper presents the results of an experimental parametric study of water mist suppression of large-scale liquid pool fires. The experiments were conducted with 50cm diameter pan heptane and JP8 pool fires. Mist was injected into the fire from the base at 90°and 45°and from the top at 90°. The results show that base injection of droplets enhanced their suppression effectiveness by as much as two times. Secondly, optimum suppression effectiveness is obtained with small droplets injected at the base of the fire. This is because the droplets evaporated quickly within the lower region of the fire where a greater effect of oxygen dilution and water vapor higher heat capacity is fully realized. Finally, a comparison of the results with the two fuels show that water mist is more effective in suppressing the JP8 fires than the heptane fires. It is concluded that the additional effects of direct surface cooling contributed significantly to the observed difference.

ISSN:0010-2202    

DOI:10.1080/00102200008947310

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

We consider the SHS (self-propagating high-temperature synthesis) process for synthesizing materials. In this process a powder mixture of reactants is cold pressed into a sample, which is ignited at one end. A high temperature combustion wave then propagates through the sample converting reactants to the desired product material. In this process, melting of some or all the components is often observed. Therefore, we study combustion waves propagating through a high caloricity inorganic powder mixture whose combustion temperature exceeds the melting temperatures of many components. The solid matrix is thus destroyed by the propagating combustion wave due to melting ahead of the reaclion zone, and a liquid bath is formed which contains gaseous bubbles. The waves propagate in the presence of a gravitational field. Due to the effect of gravity, there is relative motion between the rising bubbles and the descending bath, which affects the composition of the medium, its thermophysical properties, the “liquid flame” structure, and the propagation velocity. To enhance our understanding of phenomena associated with the interaction of the relative motion with the propagating combustion wave we formulate and analyze a relatively simple mathematical model of liquid flames in a gravitational field. We describe the wave structure and combustion characteristics including the combustion velocity. We compare our results to existing experimental observations and suggest new experiments to be performed. We consider the effects of gravity and, in particular, examine both microgravity and large gravity conditions.

ISSN:0010-2202    

DOI:10.1080/00102200008947311

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

This paper investigates the accuracy of the typical experimental practices that estimate the unsteady pressure in the flame region of an unstable combustor from pressure measurements along the combustor walls. This measurement, along with that of the unsteady heat release, is often used to determine information about the flame's driving characteristics. These wall pressure measurements are only meaningful, however, if the unsteady pressure is nearly one-dimensional in the combustor; that is, if the transverse gradient of the pressure in the flame region is small. This paper presents computational results of the interior acoustic field in a combuslor undergoing longitudinal oscillations in order to quantify the difference between the acoustic pressures at the wall (where they are typically measured) and the flame. These results show that the duct pressure at the flame and the wall typically differ in magnitude by five to twenty five percent, and in phase by ten to twenty degrees. It is concluded from these results that wall pressure measurements provide qualitative information about the fluctuating pressure at the flame and can, in some cases, be used in conjunction with heat release measurements to characterize the flame driving.

ISSN:0010-2202    

DOI:10.1080/00102200008947312

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Laser-induced fluorescence (LIF) has been used to detect AIO and TiO in low pressure hydrogen/oxygen flames with added metalorganic precursors. Such flames are used for synthesis of alumina and titania nanopowders. Knowledge of the monoxide profiles in these flames is useful in developing chemical models for the complex pyrolysis of metalorganics in flames. Challenges to detection of AIO and TiO include strong scattering background from the particle-laden flame, as well as interferences from broadband fluorescence. We present strategies that allow for sensitive detection of these species in the presence of such interferences, with minimum temperature correction required.

ISSN:0010-2202    

DOI:10.1080/00102200008947313

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

In a previous study, we proposed a two-dimensional equation to model fire spread. This study was in keeping with our long-term goal to create a forest fire simulator. The model was tested for laboratory fire experiments conducted on pine needle litter and allowed us to exhibit the main features of these fires. The study was carried out for a single fuel load however. The aim of the present paper is to further validate this model. First the influence of fuel load on the model predictions for horizontal fire spread is examined. These results which are in agreement with the experimental data, were then compared to predictions from other physical models. In addition, comparisons are made between numerical and experimental results under slope conditions for line-ignition fires. In this last case, the influence of the fuel load is also considered.

ISSN:0010-2202    

DOI:10.1080/00102200008947314

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

This paper reports the experimental research on combustion and pollution properties of several typical Chinese individual coals and coal blends and the methods determined optimum blending ratios. Thermogravimetry analysis technique and a drop-tube furnace were used to investigate the ignition and burnout behavior and NOx emission characteristics. Ash chemical composition and fusion temperature was also measured. The effects of the blending ratios on ignition, burnout, slagging and NOx emissions have been analyzed. The fuzzy logic approach has been used to determine the optimum blending ratios based on experimental data which provide a scientific basis for design and operation of power station blending coal. Calculation results were comparable with that of results obtained from combustion test carried out in a boiler simulator furnace.

ISSN:0010-2202    

DOI:10.1080/00102200008947315

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Partially-premixed flames (PPF) can contain multiple reaction zones, e.g., one or two with a premixed-Iike structure and one being a nonpremixed reaction zone. An intrinsic feature of partially premixed flames pertains to the synergistic interactions between these two types of reaction zones that are characterized by heat and mass transfer between them. Since these interactions are strongly dependent on the distribution of the radical and stable species concentrations, an accurate representation of the flame chemistry involving these species is critical for simulating their behavior. The role of C2-chemistry in determining the structure of partially premixed methane-air flames is investigated herein by employing two relatively detailed chemical mechanisms. The first involves only C1-con-taining species and consists of 52 reactions involving 17 species, while the second mechanism represents both C1- and C2-chemistry and consists of 81 reactions that involve 24 species. A planar two-dimensional partially premixed flame established on a rectangular slot burner is simulated. The simulation is based on the numerical solution of the time-dependent conservation equations for mass continuity, momentum, species, and energy. The computations are validated by comparison with the experimentally-obtained chemiluminescent emission from excited-C2free radical species, as well as with velocity measurements using particle image velocimetry. A numerical study is then conducted to examine the applicability of C1and C2mechanisms for predicting the structure of partially premixed flames for different levels of partial premixing and reactant velocity. Results indicate that both the mechanisms reproduce the global structure of PPF over a wide range of reactant velocity and stoichiometry. Since the C1mechanism is known to be inadequate for fuel-rich premixed flames, its relatively good performance can be attributed to the interactions between the two reaction rones that characterize the PPF structure. There are, however, important quantitative differences between the predictions of the two mechanisms. The C2mechanism is overall superior compared to the C, mechanism in that its predictions are in closer agreement with our experimental results. The rich premixed reaction zone height obtained with the C2mechanism is more sensitive to variations in the equivalence ratio as compared with predictions that are obtained using the C1-mechanism. In addition, for high levels of partial premixing, the methane consumption in the inner reaction zone is significantly increased when the C2-mechanism is employed, compared to when the C1-mechanism is used. Consequently, the amount of methane that leaks from the rich premixed to nonpremixed reaction zone is significantly lower when the C2mechanism is used. The interactions between the inner and outer reaction zones arestronger when the C2mechanism is employed. Finally, the maximum temperature predicted by the C2-mechanism is slightly lower as compared to that obtained using the C1-chemistry alone. These differences are attributed to the presence of the C2-chain in the 81-step mechanism, which strongly affects the inner premixed reaction zone.

ISSN:0010-2202    

DOI:10.1080/00102200008947316

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Liquid-phase models for vaporization and combustion of emulsified water-hydrocarbon droplets have been developed that assume diffusive transport in the liquid phase and take into account the transient heating period. These models have been coupled to vaporization and combustion gas-phase models through a new interface model which has been developed lo account for vaporization with finite rate supply of each component to the droplet surface. The inclusion of liquid phase mass transfer allows the simulation of droplet vaporization and combuslion in the diffusion and distillation limits. Experimental data of water-heptane droplet combustion in convection-free environment were reasonably predicted by the model. Simulations in the diffusion and distillation limits were used to interpret experimental data on water-hydrocarbon emulsion droplet microexplosion in near stagnant conditions. Comparison of estimates of droplet temperature at microexplosion to homogeneous nucleation theory predictions showed good agreement only for combustion in the distillation limit, implying the existence of microdroplet dispersion for the analyzed experimental data.

ISSN:0010-2202    

DOI:10.1080/00102200008947317

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor