摘要:

We study the propagation of combustion waves through porous samples in which two reactions occur. The first is a gastess solid-solid reaction between two solid species in the porous solid matrix which react to synthesize a solid product. The second is a solid-gas reaction in which gas, delivered to the reaction site through the pores of the sample, reacts with one of the solid species to form gaseous products. We consider the case of counter-flow filtration, in which the direction of gas flow is opposite to the direction of propagation of the reaction wave. Our purpose in considering such systemsis that the solid-solid reaction is sometimes only weakly exothermic or even endothermic, so that propagation, and lherefore synthesis, in the absence of enhancement of the reaction rate would not be possible. Enhancement is achieved by arranging for counterflow gas filtration, which leads to the solid-gas reaction. Heat released in the solid-gas reaction raises the temperature in the combustion wave, thus increasing the solid-solid reaction rate. Using the large activation energy approximation, which describes narrow reaction zones, we determine the structure and characteristics of the combustion wave.

ISSN:0010-2202    

DOI:10.1080/00102209408907685

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Idealized, two dimensional combustion is considered in terms of irrotational, premixed gases upstream of a discontinuous flame which propagates with a constant specified velocity and results in a specified decrease in density. The flame is anchored by a small flame-holder which is located on the lower wall and which plays an essential role in determining the flow. With heat release the flow downstream of the flame is rotational and involves a stagnation region adjacent to the upper wall. Numerical solutions are obtained for a range of propagation speeds down lo roughly 0.70 of the velocity of the approaching stream and for several density jumps including the zero heat release case. In this latter case the assumed influence of the flame holder results in flow nonuniformity.

ISSN:0010-2202    

DOI:10.1080/00102209408907686

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Approximately 1/3 of the useful energy of the fuel is destroyed during the combustion process used in electrical power generation. This study is an attempt to clarify and categorize the reasons for the exergy destruction taking place in combustion processes. The entropy production is separated into three subprocesses: (1) combined diffusion/fuel oxidation, (2) “internal thermal energy exchange” (heat transfer), and (3) the product constituent mixing process. Four plausible process paths are proposed and analyzed. The analyses are performed for two fuels: hydrogen and methane. The results disclose that the majority (about 3/4) of the exergy destruction occurs during the internal thermal energy exchange. The fuel oxidation, by itself, is relatively efficient, having an exergetic efficiency of typically 94% to 97%.

ISSN:0010-2202    

DOI:10.1080/00102209408907687

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

This is a study on the combustion characteristics of poly (styrene) (PS) particles either plain, or with various degrees of crosslinking in the range of 5-25%. Effects of crosslinking were examined by producing and burningindividual particles in the size range of 47—63 /an, while effects of particle size were examined by burning particles of the same degree of crosslinking (8%) in the size range of 47-350 jira. Measurements during combustion of styrene monomer droplets were also conducted. Combustion took place in a laminar, drop tube furnace at thermochemical conditions pertinent to municipal waste incinerators, i.e., in air at high heating rates and gas temperatures ranging from 1050 to 1400 K. Combustion of single particles/drops was monitored by three-color pyrometry and high-speed cinematography, to obtain temporal information on the radiant intensity, size, temperature and soot content of the flame, as well as the total burnout time of the particles. Plain (uncrosslinked) PS particles, in the size range of 47-63 urn, formed constant diameter envelope flames, somewhat larger than the maximum fame diameter of similar size crosslinked particles. As the degree of crosslinking increased, the total burnout time also increased and the instantaneous flame diameter decreased throughout the combustion period of these particles. Heterogeneous (char) combustion was observed in crosslinked particles only, upon extinction of the volatile flames. At high degrees of crosslinking, indications of a second wave of pyrolysis, pronounced in the plain PS, ceased to be present. Comparisons between low heating rate TGA experiments and high heating rate combustion experiments for plain PS indicated that the pyrolysis (decomposition) mechanisms depend on the heating rate. The maximum flame diameters were 10 to 25 times larger than the initial particle diameters and the flame/char temperatures ranged from 1800 to 2200 K. Average burning rates were estimated to be in the range of 0.01 to 0.14 mg/s. For particles in the range of 150- 340 μm the average mass burning rate as well as the maximum flame diameter were found to be proportional to the initial particle diameter, which suggests that the combustion was diffusion controlled. Smaller particles (47-63μm) were found to burn faster than what the above trend would predict, probably due to enhanced diffusion of oxygen to the flame. The monomer droplets burned with elongated and, overall, larger and hotter flames than those of similar size polymer particles. The furnace wall temperature was found to affect the burnout time and the ignition delay of the particles, but not the flame temperature, which was, instead, influenced by the particle size. Instantaneous soot volume fractions in the flame were found to change throughout combustion, averaging in the range of 4 x 10−4to 4 x 10−5.

ISSN:0010-2202    

DOI:10.1080/00102209408907688

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

We study the large scale effective flame front equations for premixed turbulent combustion with separated scale turbulent velocity fields recently formulated and derived by Majda and Souganidis. For the particular case of a steady incompressible velocity field consisting of a mean flow plus a small scale periodic shear we use analytical expressions and numerical quadrature to study the effective turbulent flame front velocity, its dependence on the turbulence intensity, and the role played by the mean flow. In general the dependence of the turbulent flame speed on the turbulence intensity is nonlinear, and the local logarithmic rate of growth can take a continuum of values, depending on the magnitudes of the mean and turbulent intensities. In the weak turbulence limit this dependence can be either linear or quadratic, depending on the magnitude and direction of the mean flow relative to the shear. In the strong turbulence limit the dependence is always linear. This behavior is documented for various forms of the small scale shear flow.

ISSN:0010-2202    

DOI:10.1080/00102209408907689

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Toluene was used as the fuel in a heterogeneous spouted bed combustor to investigate the effect of bed material on pollutant emissions. Experimental measurements were conducted in the annulus region (preflame) and include Destruction and Removal Efficiency (DRE) of the fuel, and concentrations of three products of Incomplete combustion (PICs) of toluene; CO, benzene and benzaldehyde. Two types of bed material were used: Sand and quartz. Higher conversion of the fuel (higher DRE) and higher concentration of CO, benzene and benzaldehyde (up to one order of magnitude) were observed with sand particles. Predictions of a detailed homogeneous gas phase mechanism for the oxidation of toluene (Emdee et al., 1992), were compared to the experimental data to reveal possible influences of surface reactions on the conversion of toluene and formation of CO, benzene and benzaldehyde.

ISSN:0010-2202    

DOI:10.1080/00102209408907690

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The kinetics of the oxidation of natural gas blends (CH4/C3H8and CH4/C2H6/C3H8) has been studied in a jet stirred reactor (900 < T/K < 1230, 1 < P/atm < 10, 0.1 < equivalence ratio < 1.5). Mole fractions of reactants, intermediates and products have been measured in a JSR as a function of temperature and mean residence time. These results have been used to validate a detailed kinetic reaction mechanism. Literature ignition delay times of CH4/C3H8/O2mixtures measured in shock tube have also been modeled. A general good agreement between the data and the model is found. The same mechanism has also been used to sucessfully represent the oxidation of methane, ethyne, ethene, ethane, propene, and propane in various conditions including JSR, shock tube and flame. The present study clearly shows the importance of traces of ethane and propane on the oxidation of methane. The computations indicate that the oxidation of methane is initiated by its reaction with O2and by thermal dissociation when no other hydrocarbon is present. In CH4/C3H8/O2and/or C2H6mixtures, ethane and propane react first leading to the formation of OH, H and O radicals which initiate methane oxidation.

ISSN:0010-2202    

DOI:10.1080/00102209408907691

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

This paper is aimed at validating a detailed reaction mechanism for the formation of the first aromatic rings (benzene and phenyl radical) in a rich premixed acetylene-oxygen-argon flame. Validation has been carried out by confrontation of computed mole fraction profiles of stable and labile species against experimental results. With only specific changes in kinetic data of a few reactions a good agreement was obtained for the mole fraction profiles of C4species. Distinction between singlet and triplet methylene radical was a more profound change brought to the mechanism and needed to obtain a good modelling of C3H3. These changes result in a marked improvement in the modelling of C4H2, C4H3, C4H4, C4H5, C3H3, C3H4and C6H6, accompanied by a better coherence of the mechanism with recent kinetic data measurements. Pathways analyses of bemiene formation show that acetylene addition to C4H5takes place close to the burner and is rapidly superseded by addition of propargyl radicals to allene.

ISSN:0010-2202    

DOI:10.1080/00102209408907692

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Experiments were conducted in which air was injected radially into simple, nonpremixed, jet flames at axial locations of either 0.25, 0.50, or 0.75 times the visible flame length (Lo). One objective of these experiments was to determine if NOxemissions could be lowered by enhanced mixing of air into the broadened reaction zones that lie near the flame tip. Also of interest were the emissions of carbon monoxide and NO2-to-NOxt ratios. Experiments were conducted with fuel jet Reynolds numbers of 8, 100 and 34,800 for methane and propane fuels, respectively. Radial air injection dramatically altered the flame shape from a relatively slender jet to a ball flame. Air injection reduced global flame residence times only for injection at x/Lo= 0.25. NOxemissions generally increased, rather than decreased, with air injection, although a decreasing trend was observed after an initial increase in some cases. The combined effects of residence times and radiant heat losses are thought to govern these trends. For large air injection rates at the x/Lo= 0.25 location, rapid entrainment and mixing increased combustion intensity and resulted in high CO emissions and high NO2-to-NOxratios, consistent with the formation of HO2radicals, which oxidize NO to NO2and the destruction of OH radicals, which oxidize CO to CO2. The results obtained in this study are qualitatively similar to those found by others for coaxial air injection in hydrocarbon jet flames.

ISSN:0010-2202    

DOI:10.1080/00102209408907693

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

We examine an expanding spherical premixed flame propagating in a mixture containing inert dust. The presence of the dust generates a significant radiative energy flux that influences the flame speed. With the adoption of a hydrodynamic description in which stretch effects are accounted for using an empirical formula, and with the use of the Eddington approximation to describe the radiation, a simple numerical problem is formulated valid for that initial time interval in which the flame-temperature changes by O(e) amounts where c is the small inverse activation energy. Solutions are constructed using parameter values appropriate to lean CH^/air mixtures, and for modest particle loadings. We describe the competition between the Zeldouich-Spalding effect in which radiative losses on the diffusive scale tend to quench the flame, and the Joulin effect in which radiative preheating on the scale of the Planck length tends to strengthen the flame.

ISSN:0010-2202    

DOI:10.1080/00102209408907694

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor