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.

/pdf/thermochemical-biofuel-production-in-hydrothermal-media-a-57itjyvhnt.pdf

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

Dynamics and stability of lean-premixed swirl-stabilized combustion

Fluid dynamics of coal combustion: A review

Results from an experimental investigation of the mechanisms governing the ash aerosol size segregated composition resulting from the combustion of pulverized coal in a laboratory scale down-flow combustor are described. The results of modeling activities used to interpret the results of the experiments conducted under his subtask are also described in this section. Although results from the entire program are included, Phase II studies which emphasized: (1) alkali behavior, including a study of the interrelationship between potassium vaporization and sodium vaporization; and (2) iron behavior, including an examination of the extent of iron-aluminosilicate interactions, are highlighted. Idealized combustion determination of ash particle formation and surface stickiness are also described.

/pdf/transformations-of-inorganic-coal-constituents-in-combustion-3qtvdp3kj3.pdf

Transformations of inorganic coal constituents in combustion systems

A Zone Model has been developed for the prediction of NO reduction by “reburning” in turbulent diffusion flames. The computations of fluid dynamics are decoupled from those of detailed combustion chemistry including those of the nitrogen species. The calculations begin with the computations of the stream function, heat release, temperature, and major species concentration distributions using a commercial CFD code and simple, one or two step chemistry. The modeled space is then subdivided into volume elements (zones) bounded by streamlines and some axial coordinate. The sizes of these zones are much larger than those of a computational grid so that it is practicable to use detailed chemistry for the calculation of progress of reaction within such a zone. In the experimental investigation natural gas with and without additive NH3 was used as “reburn” fuel. It was injected axially into the NO bearing combustion products of a 335 kW natural gas-air flame to obtain a parabolic flow which can be modele...

Modeling of NOx reburning in a pilot scale furnace using detailed reaction kinetics

Injection of additive such as ammonia or calcium compounds along with a small amount of hydrocarbon, preferably methane or natural gas, in a relatively high temperature region of the effluent stream for effectively reducing pollutants such as NO x and SO x . Preferably, injection is achieved by atomization of a liquid-form additive or additive solution with a small amount of gaseous hydrocarbon.

János M. Beér

Papers

Fluid dynamics of coal combustion: A review

Transformations of inorganic coal constituents in combustion systems

Modeling of NOx reburning in a pilot scale furnace using detailed reaction kinetics

Reduction of combustion effluent pollutants

Coal utilization in industrial boilers in China —a prospect for mitigating CO2 emissions