Improvement of the k-epsilon Turbulence Model
for the case of Density Stratified Swirl Flows (such as in Swirling Flames)
for the case of Density Stratified Swirl Flows (such as in Swirling Flames)
References on Research
In reacting flows, where heat is produced by chemical reaction resulting spatial variations of density, the effect of turbulence damping (or generation) may considerably alter both the flow properties and chemical reaction rates. In a coaxial swirling flow, where the inner flow is of high temperature and the outer is cold, a radially stratified flow can develop. However, due to neglection of certain terms within their derivation, the standard turbulence models (k-ε or Reynolds Stress Model) cannot correctly describe turbulent flows in which such density stratification occurs. The density stratification is characteristic of the flame structure of many swirled gas-oil burners, which fact substantiates the modification of the turbulence model.
In this project the k-ε model of turbulence was modified to allow for the effect of damping of turbulence by positive radial density gradient in swirling flow. The inclusion of density fluctuations in the basic conservation equations yielded a modified set of equations for kinetic energy and dissipation of turbulence. The new equations were used to modify the FLUENT 4.51 source code. Results of CFD calculations made with and without the modified turbulence model were compared with experimental data previously obtained on a ducted turbulent methane diffusion flame in swirling annular air flow. Agreement between experiment and computations greatly improved when the modified turbulence model was used.
In this project the k-ε model of turbulence was modified to allow for the effect of damping of turbulence by positive radial density gradient in swirling flow. The inclusion of density fluctuations in the basic conservation equations yielded a modified set of equations for kinetic energy and dissipation of turbulence. The new equations were used to modify the FLUENT 4.51 source code. Results of CFD calculations made with and without the modified turbulence model were compared with experimental data previously obtained on a ducted turbulent methane diffusion flame in swirling annular air flow. Agreement between experiment and computations greatly improved when the modified turbulence model was used.
It must be highligted that the co-authors, PhD. László E. Barta and Prof. PhD. János M. Beér, deserve the merit for the whole concept, furthermore laying the mathematical and experimental foundation of this development, among other things. While Lajos L. Varga's work was confined to comprehension of the original FLUENT source code segments, their modification due to the new set of equations; doing the FLUENT modeling by means of the original and the modified FLUENT versions, testing and comparison of the results.
CFD-aided development of Domestic Fireplaces (2005, 2007)
The main objective is the reduction of CO emission of domestic fireplaces, while keeping the thermal efficiency high. This may seem easy at first glance, since these goals are well-known from problem-solving of heavy duty boilers, furthermore, the volume and the capacity of domestic stoves are smaller by several orders of magnitude than that of industrial boilers.
The fact of the matter is that making a proper domestic fireplace can be quite complicated because the designer has small degree of control over the burning cycle. The initial amount of fuel has been placed into the stove first, and you can only change the hydraulic loss of the air duct, through which the combustible air passively – due to instantaneous buoyancy force– enters into the equipment. There is neither fan, nor any other active controlling device. So the designers of domestic stoves must think in advance.
Therefore, simulations of distinct domestic fireplaces have been performed by means of FLUENT software, in order to gain a better understanding of the main processes that take place. Several geometry variations
The fact of the matter is that making a proper domestic fireplace can be quite complicated because the designer has small degree of control over the burning cycle. The initial amount of fuel has been placed into the stove first, and you can only change the hydraulic loss of the air duct, through which the combustible air passively – due to instantaneous buoyancy force– enters into the equipment. There is neither fan, nor any other active controlling device. So the designers of domestic stoves must think in advance.
Therefore, simulations of distinct domestic fireplaces have been performed by means of FLUENT software, in order to gain a better understanding of the main processes that take place. Several geometry variations
Fireplace Gyártó és Kereskedelmi Kft.
Budapest University of Technology and Economics
(BME, Hungary), Faculty of Mechanical Engineering,
Department of Fluid Mechanics
(BME, Hungary), Faculty of Mechanical Engineering,
Department of Fluid Mechanics
CALTECH Dep. Co.
The collaborating parties of this project were:
have been scrutinized under various boundary conditions, which led useful type-specific conclusions.
Since domestic stoves have passive combustion air supply, it is more than likely that in the second half of the burning cycle the fuel gets insufficient oxidizer because of either the decreased amount of overall combustion air or the limited molecular mixing in the furnace. Consequently, partial burnout takes place which produces certain amount of CO emission.
The idea – raised during the development– is based upon the modification of the admission of combustion air: the tertiary air has been introduced tangentially into the reaction zone in order to intensify the mixing process. More intensive mixing fosters the oxygen molecules to reach the fuel, so the burnout has a higher degree and, consequently, the stove produces less CO emission.
The benefits of tangential air admission have been proved by numerical modeling and by experimental measurements on the prototype as well.
Since domestic stoves have passive combustion air supply, it is more than likely that in the second half of the burning cycle the fuel gets insufficient oxidizer because of either the decreased amount of overall combustion air or the limited molecular mixing in the furnace. Consequently, partial burnout takes place which produces certain amount of CO emission.
The idea – raised during the development– is based upon the modification of the admission of combustion air: the tertiary air has been introduced tangentially into the reaction zone in order to intensify the mixing process. More intensive mixing fosters the oxygen molecules to reach the fuel, so the burnout has a higher degree and, consequently, the stove produces less CO emission.
The benefits of tangential air admission have been proved by numerical modeling and by experimental measurements on the prototype as well.
See more: Journal of the Institute of Energy,
December 2000. 73. pp 191-195.
December 2000. 73. pp 191-195.
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