The standard κ-ϵ equations and other turbulence models are evaluated with respect to their applicability in swirling, recirculating flows. The turbulence models are formulated on the basis of two separate viewpoints. The first perspective assumes that an isotropic eddy viscosity and the modified Boussinesq hypothesis adequately describe the stress distributions, and that the source of predictive error is a consequence of the modeled terms in the κ-ϵ equations. Both stabilizing and destabilizing Richardson number corrections are incorporated to investigate this line of reasoning. A second viewpoint proposes that the eddy viscosity approach is inherently inadequate and that a redistribution of the stress magnitudes is necessary. Investigation of higher-order closure is pursued on the level of an algebraic stress closure. Various turbulence model predictions are compared with experimental data from a variety of isothermal, confined studies. Supportive swirl comparisons are also performed for a laminar flow case, as well as reacting flow cases. Parallel predictions or contributions from other sources are also consulted where appropriate. Predictive accuracy was found to be a partial function of inlet boundary conditions and numerical diffusion. Despite prediction sensitivity to inlet conditions and numerics, the data comparisons delineate the relative advantages and disadvantages of the various modifications. Possible research avenues in the area of computational modeling of strongly swirling, recirculating flows are reviewed and discussed.

Bubbles, Drops, and Particles

Mechanism and modeling of nitrogen chemistry in combustion

Multi-criteria decision analysis (MCDA) methods have become increasingly popular in decision-making for sustainable energy because of the multi-dimensionality of the sustainability goal and the complexity of socio-economic and biophysical systems. This article reviewed the corresponding methods in different stages of multi-criteria decision-making for sustainable energy, i.e., criteria selection, criteria weighting, evaluation, and final aggregation. The criteria of energy supply systems are summarized from technical, economic, environmental and social aspects. The weighting methods of criteria are classified into three categories: subjective weighting, objective weighting and combination weighting methods. Several methods based on weighted sum, priority setting, outranking, fuzzy set methodology and their combinations are employed for energy decision-making. It is observed that the investment cost locates the first place in all evaluation criteria and CO2 emission follows closely because of more focuses on environment protection, equal criteria weights are still the most popular weighting method, analytical hierarchy process is the most popular comprehensive MCDA method, and the aggregation methods are helpful to get the rational result in sustainable energy decision-making.

Review on multi-criteria decision analysis aid in sustainable energy decision-making

Hydrothermal technologies are broadly defined as chemical and physical transformations in high-temperature (200–600 °C), high-pressure (5–40 MPa) liquid or supercritical water. This thermochemical means of reforming biomass may have energetic advantages, since, when water is heated at high pressures a phase change to steam is avoided which avoids large enthalpic energy penalties. Biological chemicals undergo a range of reactions, including dehydration and decarboxylation reactions, which are influenced by the temperature, pressure, concentration, and presence of homogeneous or heterogeneous catalysts. Several biomass hydrothermal conversion processes are in development or demonstration. Liquefaction processes are generally lower temperature (200–400 °C) reactions which produce liquid products, often called “bio-oil” or “bio-crude”. Gasification processes generally take place at higher temperatures (400–700 °C) and can produce methane or hydrogen gases in high yields.

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Thermochemical biofuel production in hydrothermal media: A review of sub- and supercritical water technologies

Dynamics and stability of lean-premixed swirl-stabilized combustion

Experimental measurements and modeling of the thermal ignition of chlorinated hydrocarbon droplet streams are presented. The auto-ignition temperatures of droplet streams of toluene, o-dichloro-benzene, ethylene dichloride and trichloroethylene were measured as a function of gas oxygen content in a drop-tube furnace. Effective ignition activation energies were estimated from the data. Values of global kinetic parameters at ignition were then derived from perfectly-stirred reactor computations employing detailed chemical mechanisms of simple chlorinated hydrocarbon oxidation. These rate parameters were found to be applicable over a wide range of temperatures. The computed ignition ordering of fuels agreed with published hydrocarbon auto-ignition temperature data. Inhibition of the ignition of fuels with high chlorine content, as well as promotion of ignition at low fuel chlorine content, were found, consistent with the experimental data.

Chemical Effects of Fuel Chlorine on the Envelope Flame Ignition of Droplet Streams

Samples of gaseous and solid species taken from the central axis of a 1 megawatt heat-input natural gas flame were tested in vitro for mutagenic activity and teratogenic potential. Mutagenicity was determined by a Salmonella typhimurium forward mutation assay. Potential teratogenicity was indicated by the ability of samples to interfere with the attachment of mammalian cells to a lectin coated surface. Both the mutagenic and anti-attachment activities were found to peak in samples originating from the flame regions where the total polyaromatic compound (PAC) species concentration reached a maximum, indicating a strong correlation between PAC presence in the samples and biological activity. Additional anti-attachment activity was found close to the injection nozzle. No biologically active material was detected beyond the luminous portion of the flame.

Generation of biologically active substances in a natural gas flame.

Summary Experimental studies have been made on a hollow cone water spray emerging from a pressure jet swirl atomizer into a uniform air stream. A double spark light source was used for high-speed photography of drops in flight. Measurements were made of drop size, velocity and angle of flight, from which spatial drop-size distributions, mean drop diameters and drop trajectories were calculated. Results are presented for water sprays at 100 lb/in2 injection pressure, 5·4 gal/hr and 80° nominal cone angle. Initial drop velocities and angles of drop trajectory were found to vary over wide limits within the break-up region. Segregation of drops occurs downstream from the break-up region with large drops moving with high velocity in the outer regions of the spray, while smaller drops with lower velocities are carried into the central regions of the spray under the influence of drag of the air stream. Trajectories and drop velocities have been computed using standard drag coefficients data for smooth spheres and based on drops emanating from a liquid sheet with a given angle and velocity. Comparisons of predictions with experiment show that the form of the distributions are similar but quantitative agreement is not obtained due to a large element of randomness in drop velocities and angle in the break-up region.

Hollow-cone liquid spray in uniform air stream

EVER since Nusselt1 introduced the boundary layer concept, about forty years ago, for calculating rates of heat and mass transfer to a spherical particle, it has been universally accepted (without proof) by subsequent theoretical work (see ref. 2) that the reaction-rate of particles in a dust flame is determined by the rate of supply of oxygen through a diffusional boundary layer surrounding the particles. That this might not in fact be so was first suggested3 about five years ago, the alternative proposition being that the rate control was chemical, not physical ; the source of the implied error is attributed to the mis-interpretation of reaction data in terms of diffusion instead of chemisorption which, according to Brunauer4, has been a common mistake in the past. Support for the alternative hypothesis, however, has now been provided by analysis of experimental results obtained recently from an anthracite flame.

Control of Reaction Rate in Dust Flames

Further experimental results are described from the multi-annular swirl burner (MASB). The burner comprises a plurality of concentric annular divergent nozzles with the inner end of each nozzle aligned substantially with the outer end of the adjacent nozzle. The MASB makes use of the principle that under turbulent conditions high volumetric heat release rates can be achieved by matching the concentrations and directions of flow of reactants in such a way that regions of high fuel concentration overlap regions of large shear stresses in the flow. Wider stability limits and better mixing of reactants are found in this MASB compared to the conventional type of single annular swirl burner. Laser anemometry system was used to obtain the three components of mean and r.m.s. velocity together with the higher moments of turbulence. Results were obtained under non-combustive and combustive conditions both with and without extension tube attached to the burner exit. The results indicate clearly an increase ...

János M. Beér

Papers

Chemical Effects of Fuel Chlorine on the Envelope Flame Ignition of Droplet Streams

Generation of biologically active substances in a natural gas flame.

Hollow-cone liquid spray in uniform air stream

Control of Reaction Rate in Dust Flames

On the Operational Characteristics of a Multi-annular Swirl Burner