摘要:

The switching of inhibition to catalysis at low pressures is analysed. It occurs at a certain concentration of hydrogen bromide, which depends on temperature and pressure. Catalysis is caused by very fine balancing of the processes H + HBr ⇌2H2+ Br. Other elementary processes can cause either inhibition or catalysis depending on the kinetic scheme in which they are involved.

ISSN:0010-2202    

DOI:10.1080/00102208008952375

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

In order to examine the relative contribution of surface and gas phase reactions to the exothermic conversion of a fuel-air gas mixture flowing over a catalytic surface, we have carried out a theoretical and experimental study of catalytic combustion under stagnation point flow conditions. In the presence of exothermic surface reaction the theoretical model provides an analytical solution; in the presence of both reaction modes, homogeneous and heterogeneous, computer solutions were obtained for the total heat flux at the surface and the distribution of temperature, reactant, and product concentrations in the stagnation point boundary layer. Having available kinetic data on the platinum-catalyzed oxidation of propane and on the gas phase reaction between propane and air, we selected this chemical fuel system for experimental and theoretical study. Experimentally the flow of the propane-air mixture (1 vol percent C3Hs) was directed at a quartz plate whose surface was coated with thin strips of vacuum-deposited platinum, that served both as catalyst and resistance thermometer. During an experiment the temperature of each strip was maintained constant (± 5 K.) by adjusting the electrical heat input as monitored by resistance measurements. By this procedure the heat released by exothermic reaction of the fuel-air mixture could be determined from the difference in electrical power required to keep each Pt strip at its original temperature. With the aid of the theoretical analysis we were able to compute the fractional contributions of catalytic and gas phase combustion to the total heat flux conducted to the catalytic surface. The relative contribution of each combustion mode' depended on catalyst activity, volumetric flow rate, and fuel-air ratio. Under our experimental conditions with transport limited surface reaction an increase in reactant flow rate enhances the surface-catalyzed contribution to the total heat release rate. The results obtained clearly indicate the conditions under which, either combustion mode predominates.

ISSN:0010-2202    

DOI:10.1080/00102208008952376

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

Measurements have been made in the turbulent premixed flame in an LPG/ air mixture issuing from a 50 mm diameter pipe. The flame was of the classical bunsen type with a half cone angle of 16° Measurements were made along a line at an angle of about 50° to the normal to the flame front. Concentration, or the progress variable c, was measured by the light scatter technique and velocity u by laser Doppler velocimetry. The correlation between u and c is found to be positive through much of the flame indicating a counter-gradient flux. The joint probability density function (pdf) ofuandcis well modelled by essentially burnt and unburnt gas with only a small contribution of the partially reacted gas. The conditioned pdf' s for velocity in the unburnt and burnt gas show a considerable increase in both mean and variance for the burnt gas.

ISSN:0010-2202    

DOI:10.1080/00102208008952377

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

Liquid fuel diffusion flames burning in a low velocity gas stream under ambient pressure were studied experimentally. Flames were established around fuel-wetted porous spheres of 1 to 6 mm diameter, porous cylinders of 3.6 to 6.7 mm diameter, and moving threads of 0.22 to 0.46 mm diameter. Burning rates and oxides of nitrogen emission indices were obtained for various geometries and conditions. The emission index was defined as the total oxides of nitrogen molar formation rate divided by the mass burning rate of the fuel. The approach stream conditions varied were velocity (0-80 cm/ s) temperature (25-650 ° ) and oxygen concentration (18.5-21 percent). Methanol, ethanol, w-pentane, andn-heptane were used as fuels. Fuel temperature was varied from 25° C to the boiling point of the fuel. The data showed that fuel temperature, air velocity, and liquid surface geometry had little effect on the emission index as compared to the much larger effects of fuel composition, approach stream temperature, and approach stream composition. Burning rates were relatively insensitive to approach stream temperature and velocity, but were sensitive to fuel temperature. Comparison of the data to calculated emission indices in the literature for spherically symmetric flames around droplets showed that such models are not accurate for flames such as the ones studied. The neglect of convection effects and detailed kinetics in the models is the probable source of disagreement.

ISSN:0010-2202    

DOI:10.1080/00102208008952378

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The one-dimensional stability of an isobaric burner-stabilized premixed flame is investigated for arbitrary Lewis number and stoichiometry in the asymptotic limit of large activation energy. Assuming a one-step irreversible chemical reaction in which fuel and oxidizer react to form a product, a linear stability analysis is Used to calculate the neutral stability boundary in Lewis number-activation energy space as a function of incoming flow velocity (or equivalently, the burned temperature) The major result is that although a steady-state adiabatic flame is likely to be stable for typical parameter values, a value of the incoming flow velocity sufficiently less than the adiabatic flame speed is destabilizing to the extent that the unstable region becomes feasible for many flames. Consequently, if all other parameters are fixed, there exists for such flames a critical value of the incoming flow velocity at which the time-asymptotic solution to the time-dependent problem bifurcates from the nontrivial steady state solution. Time-oscillating burner-stabilized flames are observable experimentally, and realistic numerical calculations of a fuel-rich H2IO2 premixed flame are presented which further verify the existence of such time-periodic solutions for sufficiently small incoming flow velocities. In addition, the numerical results indicate the existence of a secondary instability phenomenon in which a singly periodic pulsating solution abruptly becomes doubly periodic.

ISSN:0010-2202    

DOI:10.1080/00102208008952379

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

Atmospheric, premixed, flat H2-NO flames have been stabilized on a 3.5 cm diameter, Meker type burner in the temperature range of 2800-3100° K. Temperature profiles and concentration profiles of NO, OH and O were determined in the reaction and post-reaction zones using exclusively spectrophotometric techniques. These data in conjunction with the OH emission profiles were used to analyse the reaction mechanisms occurring in the flame. OH radiation was found to be of chemiluminescent origin according to a thermo-molecular recombination reaction: possibly H + OH + OH→ H2O+ OH* or O + Ha + OH * HaO + OH* NO decomposition appears to be rate controlled by two reactions: H + NO → N + OH (1) and O 4- NO → N + O2 (2r. Reaction (1) is more important in the early stage of combustion due to diffusion of H atoms from the reaction zone. Later the concentration of O atoms builds up and reaction (2) becomes significant. The rate constant for reaction (1) was determined using the flame data and was found to be: k1= 2.3 × 1010cm3/ mole-sec at 3000° K.

ISSN:0010-2202    

DOI:10.1080/00102208008952380

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

ISSN:0010-2202    

DOI:10.1080/00102208008952381

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

ISSN:0010-2202    

DOI:10.1080/00102208008952382

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor