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Numerical studies of turbulent wall-jets for mixing and combustion applications
KTH, School of Engineering Sciences (SCI), Mechanics.
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Direct numerical simulation is used to study turbulent plane wall-jets. The investigation is aimed at studying dynamics, mixing and reactions in wall bounded flows. The produced mixing statistics can be used to evaluate and develop models for mixing and combustion. An aim has also been to develop a simulation method that can be extended to simulate realistic combustion including significant heat release. The numerical code used in the simulations employs a high order compact finite difference scheme for spatial integration, and a low-storage Runge-Kutta method for the temporal integration. In the simulations the inlet based Reynolds and Mach numbers of the wall-jet are Re = 2000 and M=0.5 respectively, and above the jet a constant coflow of 10% of the inlet jet velocity is applied. The development of an isothermal wall-jet including passive scalar mixing is studied and the characteristics of the wall-jet are compared to observations of other canonical shear flows. In the near-wall region the jet resembles a zero pressure gradient boundary layer, while in the outer layer it resembles a plane jet. The scalar fluxes in the streamwise and wall-normal direction are of comparable magnitude. In order to study effects of density differences, two non-isothermal wall-jets are simulated and compared to the isothermal jet results. In the non-isothermal cases the jet is either warm and propagating in a cold surrounding or vice versa. The turbulence structures and the range of scales are affected by the density variation. The warm jet contains the largest range of scales and the cold the smallest. The differences can be explained by the varying friction Reynolds number. Conventional wall scaling fails due to the varying density. An improved collapse in the inner layer can be achieved by applying a semi-local scaling. The turbulent Schmidt and Prandtl number vary significantly only in the near-wall layer and in a small region below the jet center. A wall-jet including a single reaction between a fuel and an oxidizer is also simulated. The reactants are injected separately at the inlet and the reaction time scale is of the same order as the convection time scale and independent of the temperature. The reaction occurs in thin reaction zones convoluted by high intensity velocity fluctuations.

Place, publisher, year, edition, pages
Stockholm: Mekanik , 2007.
Series
Trita-MEK, ISSN 0348-467X ; 2007/08
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-4564OAI: oai:DiVA.org:kth-4564DiVA: diva2:12872
Public defence
2007-12-14, D3, Huvudbyggnaden, Lindstedtsvägen 5, Stockholm, 10:15
Opponent
Supervisors
Note
QC 20100621Available from: 2007-12-05 Created: 2007-12-05 Last updated: 2010-06-21Bibliographically approved
List of papers
1. Direct numerical simulation of a plane turbulent wall-jet including scalar mixing
Open this publication in new window or tab >>Direct numerical simulation of a plane turbulent wall-jet including scalar mixing
2007 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 19, no 6, 065102- p.Article in journal (Refereed) Published
Abstract [en]

Direct numerical simulation is used to study a turbulent plane wall-jet including the mixing of a passive scalar. The Reynolds and Mach numbers at the inlet are Re=2000 and M=0.5, respectively, and a constant coflow of 10% of the inlet jet velocity is used. The passive scalar is added at the inlet enabling an investigation of the wall-jet mixing. The self-similarity of the inner and outer shear layers is studied by applying inner and outer scaling. The characteristics of the wall-jet are compared to what is reported for other canonical shear flows. In the inner part, the wall-jet is found to closely resemble a zero pressure gradient boundary layer, and the outer layer is found to resemble a free plane jet. The downstream growth rate of the scalar is approximately equal to that of the streamwise velocity in terms of the growth rate of the half-widths. The scalar fluxes in the streamwise and wall-normal direction are found to be of comparable magnitude. The scalar mixing situation is further studied by evaluating the scalar dissipation rate and the mechanical to mixing time scale ratio.

Keyword
Turbulent plane wall-jet, Wall-jet mixing
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-7748 (URN)10.1063/1.2732460 (DOI)000247625900015 ()2-s2.0-34447313400 (Scopus ID)
Note
QC 20100621Available from: 2007-12-05 Created: 2007-12-05 Last updated: 2010-11-08Bibliographically approved
2. Direct numerical simulation of non-isothermal turbulent wall-jets
Open this publication in new window or tab >>Direct numerical simulation of non-isothermal turbulent wall-jets
2009 (English)In: Physics of fluids, ISSN 1070-6631, E-ISSN 1089-7666, Vol. 21, no 3Article in journal (Refereed) Published
Abstract [en]

Direct numerical simulations of plane turbulent nonisothermal wall jets are performed and compared to the isothermal case. This study concerns a cold jet in a warm coflow with an ambient to jet density ratio of ρa/ρj = 0.4, and a warm jet in a cold coflow with a density ratio of ρa/ρj = 1.7. The coflow and wall temperature are equal and a temperature dependent viscosity according to Sutherland’s law is used. The inlet Reynolds and Mach numbers are equal in all these cases. The influence of the varying temperature on the development and jet growth is studied as well as turbulence and scalar statistics. The varying density affects the turbulence structures of the jets. Smaller turbulence scales are present in the warm jet than in the isothermal and cold jet and consequently the scale separation between the inner and outer shear layer is larger. In addition, a cold jet in a warm coflow at a higher inlet Reynolds number was also simulated. Although the domain length is somewhat limited, the growth rate and the turbulence statistics indicate approximate self-similarity in the fully turbulent region. The use of van Driest scaling leads to a collapse of all mean velocity profiles in the near-wall region. Taking into account the varying density by using semilocal scaling of turbulent stresses and fluctuations does not completely eliminate differences, indicating the influence of mean density variations on normalized turbulence statistics. Temperature and passive scalar dissipation rates and time scales have been computed since these are important for combustion models. Except for very near the wall, the dissipation time scales are rather similar in all cases and fairly constant in the outer region.

Keyword
flow simulation, jets, shear turbulence
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-7749 (URN)10.1063/1.3081554 (DOI)000264782100032 ()2-s2.0-64249085661 (Scopus ID)
Note
QC 20100617Available from: 2007-12-05 Created: 2007-12-05 Last updated: 2012-01-18Bibliographically approved
3. Direct numerical simulation of a reacting turbulent wall-jet
Open this publication in new window or tab >>Direct numerical simulation of a reacting turbulent wall-jet
2007 (English)Report (Other academic)
Place, publisher, year, edition, pages
Stockholm: KTH, 2007
Series
Linne Flow Centre, Dept. of Mechanics
Identifiers
urn:nbn:se:kth:diva-7750 (URN)
Note
QC 20100621Available from: 2007-12-05 Created: 2007-12-05 Last updated: 2010-06-21Bibliographically approved
4. A numerical method for simulation of turbulence and mixing in a compressible wall-jet
Open this publication in new window or tab >>A numerical method for simulation of turbulence and mixing in a compressible wall-jet
2007 (English)Report (Other academic)
Series
Technical Report, Linne Flow Centre, Dept. of Mechanics
Identifiers
urn:nbn:se:kth:diva-7751 (URN)
Note
QC 20100621Available from: 2007-12-05 Created: 2007-12-05 Last updated: 2010-06-21Bibliographically approved

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