摘要:

Previous coal combustion models using assumed-shape PDF's to treat turbulence-chemistry interactions have used only one progress variable to treat products from coal reactions. This assumes that the products of all coal reactions have the same composition. However, the composition of the combustion products of coal particles is known to vary with burnout, especially between devolatilization and char oxidation. In this work, two progress variables were implemented which distinguish between the products of devolatilization and those of char oxidation. This new approach requires as input the specified volatile content and elemental release during devolatilization. The values for these parameters were estimated based on elemental release data obtained in flat-flame burners. Predictions of the new and the old approaches for the major variables of the field were not appreciably different. However, NO pollutant predictions of the new method were, in general, better than those of the old method, particularly at downstream locations.

ISSN:0010-2202    

DOI:10.1080/00102200008952115

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The heterogeneous oxidation of methane-air mixture in a honeycomb catalytic reactor is investigated numerically in the present study. An improved multi-step surface reaction mechanism for methane oxidation on platinum is proposed so that surface ignition of lean methane-air mixtures is better modeled. First, this surface mechanism is used to determine the apparent activation energy of methane-air catalytic combustion. The predicted activation energies are found to agree well with the experimental data by Trimm and Lam (1980) and by Griffin and Pfefferle (1990). The chemical model indicates that, depending on the surface temperature, the surface reaction rate is dominated by either the oxygen desorption rate or by the methane adsorption rate.

ISSN:0010-2202    

DOI:10.1080/00102200008952116

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Results from reduced-gravity experiments on combustion of heptane/hexadecane mixture droplets and heptane droplets are presented. Initial droplet diameters were about 1 mm, and initial hexadecane mass fractions in the droplets were 0.6. The droplets were burned in air at 0.1 MPa. Attenuation of the beam from a diode laser was used to gain information on average soot volume fractions in the vicinities of the droplets. Results indicate that average soot volume fractions increased over the observation time for heptane droplets. For heptane/hexadecane mixture droplets, however, results indicate that average soot volume fractions increase after ignition, decrease when a flame contraction occurs, and then increase again after the flame contraction has been completed. Interpretation of the data indicates that peak soot volume fractions as high as about 50 ppm existed in these experiments. Simplified theory is developed to predict effects of droplet heating on quasisteady soot shell locations. The theory indicates that sufficiently-large droplet heating rates, which might occur during flame contractions, may cause soot to be deposited onto droplet surfaces as a result of thermophoretic forces.

ISSN:0010-2202    

DOI:10.1080/00102200008952117

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Post-rebuild profiles of velocity, species concentration (02, CO, and CO2), and gas temperature are reported in the portnecks of a regenerative, side-port, 550-ton/day, gas-fired, flat-glass furnace. These measurements are also compared to similar ones made before the same furnace was rebuilt. Measurements were also made below one of the regenerators in the tunnel leading to the furnace stack after the rebuild. Fewer variations were observed in the exhaust profiles of most measured variables after the rebuild. Flat inlet velocity profiles were measured with a magnitude of approximately 11 m/s before and after the rebuild. The temperature of the inlet preheat air was generally speaking higher and the furnace exhaust temperature lower before the rebuild. Locations of low O2concentration in the effluent are consistent with high CO concentrations before and after the furnace rebuild. CO2concentrations are nearly uniform across the portneck height, more so after the rebuild. The measurements in the tunnel after the rebuild indicate a stratification effect in the species concentration measurements. These measurements also indicate that the combustion reactions continue inside the regenerators resulting in overall complete combustion as indicated by the very low CO levels in the tunnel. A mass balance analysis for the overall combustion reaction based on the measurements of O2and CO2and fuel flow rate in each port showed that (1) before and after the furnace rebuild the predicted CO2formed in the glass is within 15% of the value estimated by Ford personnel; and (2) the overall stoichiometry was not much different before and after the rebuild (22.5% excess air before compared to 19.2% after). The total airflow rate calculated by this analysis after the rebuild is within 8% of the plant-measured value.

ISSN:0010-2202    

DOI:10.1080/00102200008952118

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The combustion zone propagating through an iron particle cloud and the combustion behavior of individual iron panicles have been examined by using high-speed photomicrographs. Propagation of the combustion zone of 4˜5 mm in width was observed as the movement of a luminous zone which consists of burning iron particles. In the region just behind the leading edge, burning particles of various diameters are examined. As the distance from the leading edge becomes larger, smaller particles are fading away, and then only large particles are observed to remain luminous in the region where the distance is larger than 2 mm. Each iron particle bums at the combustion zone without gas phase flame. The burn-out time (the duration of light emission) is proportional to the diameter of iron particle when the particle diameter is not so large. It agrees well with the result of a simple analysis. As the particle diameter becomes larger, the burn-out time becomes much larger than that predicted by the simple analysis.

ISSN:0010-2202    

DOI:10.1080/00102200008952119

出版商:Taylor & Francis Group    

年代:1990

数据来源: Taylor

摘要:

Colliding interaction of a vortex pair with a premixed flame is numerically studied with a two-step reaction model including chain-branching and chain-breaking reactions. The vortex pair has a maximum circumferential velocity ranging from 4.7 to 54.7 times the laminar burning velocity and has a core diameter ranging from 1.1 to 1.55 times the flame thickness. Besides well-known interacting behaviors such as the flame wrinkling by a weak vortex pair and the pocket formation by a moderate vortex pair, an elongation of the concave flame without entraining the burned gas appears in the interaction with a strong vortex pair. The different behaviors of interaction are attributed to the ratio of the moving velocity of the vortex pair to the burning velocity and the criterion is represented by the ratio of the maximum circumferential velocity to the burning velocity. For moderate and strong vortices, vorticity generation due to the baroclinic effect and reduction of distance between vortex cores result in an acceleration of the vortex pair. Flame propagation reduces the vorticity owing to dilatation of the burned gas and increase of the kinematic viscosity, but it accelerates the moving speed of the vortex pair by decreasing the distance between the vortex cores and increasing the induced velocity. The local stretch rate is dominated by the local strain rate for moderate and strong vortices except at the trailing edge of the mushroom head where the large positive curvature has an effect. The temporal evolution of the curvature, the strain rate and the stretch rate at the stagnation point does not depend on the Lewis number but depends on the vortex strength. However, the local burning velocity at the stagnation point becomes smaller for larger Lewis numbers and stronger vortices. For Le=0.6. there is a delay of response for the positive stretch that is known as the effect of unsteadiness. The increase of global burning rate is generally proportional to the increase of flame surface for Le = 1.0. On the other hand, the global burning rate is amplified by the increase of reaction rate as well as the increase of the flame surface for Le=0.6. and it is reduced by the decrease of reaction rate in spite of the increase of the flame surface for Le=1.6.

ISSN:0010-2202    

DOI:10.1080/00102200008952120

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

A basic experiment has been designed to improve the understanding of certain phenomena like droplet interaction occurring in dense sprays. The interaction effect in a stream of monosized ethanol droplets on the drag coefficient (Cd) for non evaporating and reacting conditions is investigated. Also the burning rate is studied. The spacing parameter C (ratio of interdroplet distance to droplet diameter) is the main parameter retained here to represent the interaction effects. Non intrusive optical techniques have been developed to measure, simultaneously, along the droplet path, the droplet size, velocity and mean temperature. The stream is first studied in non evaporating conditions. The data compiled for a large range of parameters (drag coefficient Cd. Reynolds number Re, spacing parameter C) permit to establish a new correlation for the drag coefficient taking into account the spacing parameter. The first experimental results are compared with those previously published. Then the stream is ignited by an electrically heated coil to investigate close coupled droplets (C<6) under reacting conditions. The previous optical methods are applied for those conditions. The CARS thermometry technique is used to determine the temperature fields around the droplets for different values of the spacing parameter. The detailed analysis of the two phases in the burning droplet stream allows to improve the knowledge of the fundamental mechanisms on droplet/droplet interactions. For example, the evolution of the drag coefficient with the spacing parameter is preponderant to compute the droplet trajectory in dense sprays. Particularly, the results show an important effect of droplet spacing on the burning rate and the drag coefficient Cd for this basic configuration

ISSN:0010-2202    

DOI:10.1080/00102200008952121

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Unlike the widely used Eulenan gas-Lagrangian particle models (particle trajectory models), two versions of two-fluid models—a pure two-fluid (FTF) model and a two-fluid-trajectory (continuum-trajectory, CT) model are proposed for simulating turbulent reacting gas-particle flows and coal combustion. Both of diem are based on Eulerian gas-phase equations, Eulenan particle-phase continuity and momentum equations, κ-ϵ-κ two-phase turbulence model, EBU-Arrhenius turbulent combustion model, six-flux radiation model, coal moisture evaporation, devolatilization and char combustion models with simultaneous three reactions. To account for the particle history effect, including the mass change due to moisture evaporation, devolatilization and char combustion and the particle temperature change due to the heat transfer between two phases, the FTF model uses Eulerian or partial differential conservation equations of particle mass, particle daf-coal mass, particle moisture and energy, while the CT model uses Lagrangian or ordinary differential equations and algebraic expressions to predict the particle mass and temperature change. Thermal NO and fuel NO formation models with a second-order moment closure to treat the turbulence-chemistry interaction are incorporated into the FTF model. Simulation of 3-D turbulent reacting gas-particle with coal combustion in sudden-expansion combustors with high-velocity jets shows that the predictions using both these two models give reasonable and qualitatively similar results. The results reveal the mechanism of fast ignition and good flame stabilization in the combustor. However, the PTF model can give more detailed information of particle flow field, for example, particle temperature and daf-coal mass field in 3-D space, than that given by the CT model. The FTF model has no difficulty in dealing with particles entering or leaving the recirculation zones, sometimes encountered in the CT model and pure particle trajectory model. The developed computer codes PERT-3 and LEAGAP-3 based on these models have already been applied in optimization design of coal burners for boiler furnaces and cement kilns

ISSN:0010-2202    

DOI:10.1080/00102200008952122

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The reduction of nitric oxide (NO) by ethane in simulated reburning conditions has been studied in a fused silica jet-stirred reactor operating at 1 atm, in the temperature range 900-1400 K, in diluted conditions. In the present experiments, the initial mole fraction of NO was 1000 ppm, that of ethane was 4400 ppm. The equivalence ratio has been varied from 0.75 to 2. It was demonstrated that the reduction of NO varies as the temperature and that, for a given temperature, a maximum NO reduction occurs slightly above stoichiometric conditions. Then, optimal NO-reburning conditions can be achieved for particular combinations of equivalence ratio and temperature. The present results generally show the same trends as observed in previous studies using simple hydrocarbons or natural gas (NG) as reburn fuel. A detailed chemical kinetic modeling of the present experiments was performed using an updated and improved kinetic scheme (877 reversible reactions and 122 species). An overall reasonable agreement between the present data and the modeling was obtained although improvements of the model are still necessary. The proposed kinetic mechanism, already successfully used to model the reduction of NO by ethylene, acetylene and HCN, and the low temperature interactions between NO and simple alkanes in a JSR, was also validated through the modeling of the reduction of NO by a NG blend. According to this study, the main route to NO-reduction by ethane involves kete-nyl radical. The model indicates that the reduction of NO proceeds through the reaction paths: HCCO + NO → HCNO + CO followed by HCNO + H → HCN + OH; HCN + O → NCO → HNCO → NH2; NHi+ NO → N2; NH + NO → N2O; N2O + H, O → N2

ISSN:0010-2202    

DOI:10.1080/00102200008952123

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor