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Turbulent Mixing of Two Gas Streams with High Density Ratio

English title Turbulent Mixing of Two Gas Streams with High Density Ratio
Applicant Prasser Horst-Michael
Number 141025
Funding scheme Project funding (Div. I-III)
Research institution Institut für Energietechnik ETH Zürich
Institution of higher education ETH Zurich - ETHZ
Main discipline Fluid Dynamics
Start/End 01.09.2012 - 31.07.2016
Approved amount 256'539.00
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All Disciplines (2)

Discipline
Fluid Dynamics
Other disciplines of Physics

Keywords (6)

shear layer; isokinetic mixing; high density difference; stable stratification; mixing layer; turbulent mixing

Lay Summary (English)

Lead
Lay summary

The Ph.D. project addresses turbulent mixing in the presence of stable and unstable density stratifications with a particular focus on high density ratios. This class of mixing phenomena can be found in numerous applied problems, such as chemical processes, the dispersion of accidentally released liquefied gases, the simulation of fires and severe nuclear accidents. One example from the latter field is the accumulation, transport and dispersion of hydrogen in the containment of a LWR. Hydrogen is formed by chemical reactions between the fuel rod cladding material and vapour in case of cooling failure. A reliable prediction of hydrogen distributions in the containment is of crucial importance for the design of countermeasures to avoid detonations endangering containment structures. The latter provides the main motivation to the Laboratory of Thermal Hydraulics of PSI to performing research in this area. The project will provide experimental data for improving CFD models including coarse mesh containment codes for turbulent mixing of fluids with high density ratios.

Despite the technical relevance of the phenomenon, related literature is surprisingly scarce. There are only a few references outlining past shear layer experiments at high density differences (He-N2), and no examples of isokinetic experiments at high density differences were located. Past numerical simulations of the above mentioned mixing scenarios have often deviated from experimental results. This failure is due in part to the violation of the Boussinesq approximation of the first kind, i.e. the assumption that the density variation generates only a small correction to the inertia term of the momentum equation and can be thus neglected, only considering the density variation in conjunction with the acceleration of gravity.

For the measurement of the velocity and concentration fields in the HOMER test section, Particle Image Velocimetry (PIV) and Laser Induced Fluorescence (LIF) will be used. This will be complemented by Laser Doppler Anemometry (LDA) at selected positions in the test section. From the obtained series of instantaneous velocity and concentration fields, data needed for the model development and validation can be derived: (1) distributions of mean velocities, of the velocity fluctuations as well as of the Reynolds stresses, and (2) mean concentrations and concentration fluctuations.

On the basis of the mean and rms values of the velocity and concentration fields, appropriate experimental growth and entrainment laws will be derived and compared with values from the literature as much as possible. Some efforts will be undertaken to derive a universal growth law covering the different density stratifications. This approach will likely be based on a local gradient Richardson number.

The Optical Flow technique will be applied to some of the PIV recordings and will be further developed. It is expected that this new method will allow for an enhanced spatial resolution of the velocity field (compared with the classical cross-correlation approach) in order to resolve small scale coherent structures responsible for the details of the mixing process. This will also support the identification of the – most probably – Richardson number-dependent, and therefore stratification strength-dependent, dominant mixing mechanisms on which there is currently no consensus.

Direct link to Lay Summary Last update: 21.02.2013

Responsible applicant and co-applicants

Employees

Name Institute

Publications

Publication
Turbulent Gas Mixing in Strong Density Stratified Shear and Non-Shear Flows
Krohn Benedikt, Sharabi Medhat, Niceno Bojan, Prasser Horst-Michael, Bijleveld Henny, Shams Afaque, Roelofs Ferry (2015), Turbulent Gas Mixing in Strong Density Stratified Shear and Non-Shear Flows, in Proceedings of NURETH-16, Chicago IL USAAmerican Nuclear Society, Chicago IL USA.
A Novel Method to Measure Passive Scalar- and Velocity Fields Simultaneously by Means of PIV in a Density Stratified Planar Turbulent Mixing Layer
Krohn Benedikt, Wolff Nicolas Alexander, Prasser Horst-Michael, A Novel Method to Measure Passive Scalar- and Velocity Fields Simultaneously by Means of PIV in a Density Stratified Planar Turbulent Mixing Layer, in 27th International Symposium on Transport Phenomena (ISTP27), Honolulu, Hawaii, USAPacific Center of Thermal Fluids Engineering (PCTFE), Honolulu.

Collaboration

Group / person Country
Types of collaboration
Nuclear Research and consultancy Group (NRG) Netherlands (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Publication
TU Braunschweig Germany (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Exchange of personnel
Laboratory of Thermal Hydraulics, Paul Scherrer Institute Switzerland (Europe)
- in-depth/constructive exchanges on approaches, methods or results
- Publication
- Research Infrastructure

Associated projects

Number Title Start Funding scheme
132659 Temperature Fluctuations in Fluid and Pipe Walls induced by Turbulent Mixing 01.02.2011 Project funding (Div. I-III)

Abstract

The project addresses turbulent mixing in the presence of stable and unstable density stratifications. Despite its technical relevance in the fields of safety of nuclear reactors, chemical process engineering and the transport of liquefied gases, as well in the simulation of fires, related literature is surprisingly scarce; there are only a few references outlining past shear layer experiments at high density differences (He-N2), and no examples of isokinetic experiments at high density differences were located. Past numerical simulations of the above mentioned mixing scenarios have often deviated from experimental results. This failure is due in part to the violation of the Boussinesq approximation of the first kind, i.e. the assumption that the density variation generates only a small correction to the inertia term of the momentum equation and can be thus neglected, leaving the density variation considered only in conjunction with the acceleration of gravity. The current proposal requests funding for one PhD student as well as funding for an extension of the already available gas LIF system towards a combined PIV-LIF system. This system will be used to measure the turbulent concentration fluxes, which are of key importance for further turbulence modeling. The PhD student is dedicated to the HOMER (HOrizontal Mixing Experiment in a Rectangular channel) project to be conducted in the Laboratory for Thermal Hydraulics at the Paul Scherrer Institute in Switzerland. The aim of this project is to conduct experiments studying the development of the mixing layer formed along the density interface between two parallel co-flowing gas streams of differing densities. Shear and isokinetic mixing layers will be studied at a variety of velocities, and diverse density differences will be examined. The primary objectives of the project are as follows: (1) Measure 2D velocity and concentration fields for isokinetic and shear layer conditions including velocity and concentration fluctuations as well as the Reynolds stresses. (2) Deduce entrainment and growth laws with some emphasis put on a universal growth law covering the different density stratifications. (3) Highlight the effect of the grid generated turbulence on the developing mixing zone for the isokinetic mixing layer and the shear layer. (4) Demonstrate the capabilities of numerical codes. The measurement campaign will be complemented with numerical calculations using different RANS based turbulence models as well as LES calculations for selected cases.(5) Apply the new optical flow method to the PIV recordings to enhance the spatial resolution of the resulting velocity field in order to resolve small scale coherent structures responsible for the details of the mixing process.(6) Collect simultaneous PIV-LIF measurements to allow for the measurement of turbulent concentration fluxes in combination with the Optical Flow Method. This would push the spatial resolution of the calculated concentration fluxes to a level not reported in the literature.The knowledge gained from this project includes first hand experience on an important class of fundamental mixing processes relevant for the containment of a LWR. From a fundamental point of view, these mixing processes have nuclear safety relevance due to their possible effects on gas stratification, hydrogen distribution and associate risk of deflagration and detonation. Additionally, the study of non-Boussinesq effects is an important issue for safety relevant processes such as the accidental release of hazardous gases in the chemical industry, the storage and transport of liquefied gases or the simulation of fires. The better understanding of the detailed mechanisms of mixing in the presence of high density gradients requires well posed high quality experiments to extend our knowledge both theoretically and with respect to turbulence modeling issues to properly predict such flows.
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