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Numerical and experimental investigation of the near zone flow field in an array of confluent round jets
Linköping University, Department of Management and Engineering, Energy Systems. Linköping University, The Institute of Technology.ORCID iD: 0000-0002-4649-6449
Linköping University, Department of Management and Engineering, Energy Systems. Linköping University, The Institute of Technology.
Linköping University, Department of Management and Engineering, Energy Systems. Linköping University, The Institute of Technology.
Delft University of Technology, The Netherlands.
2014 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 46, 127-146 p.Article in journal (Refereed) Published
Abstract [en]

Numerical simulations, using three different turbulence models (i.e., standard kε, RNG kε and Reynolds Stress Model [RSM]) is performed in order to predict mean velocity field as well as turbulence characteristics in the near zone of a 6 × 6 in-line array of unconfined confluent round jets. The numerical results are compared with experimental data acquired by Particle Image Velocimetry (PIV).

All the turbulence models used are able to reproduce the mean velocity field and the development of turbulent kinetic energy of the confluent round jets, but in general, the standard kε and RSM model show better agreement with experimental data than the RNG model. In terms of mean velocity the second-order closure model (RSM) is not found to be superior to the less advanced standard kε model in spite of the mean flow curvature present in the flow field. The RSM model, however, provides information on individual Reynolds stresses. RSM show satisfactory agreement of streamwise normal Reynolds stress and shear stress, but generally underpredicts the normal Reynolds stress in the spanwise direction.

In comparison with plane twin jets, confluent round jets show a longer merging region. Within the merging region the maximum velocity of the confluent jets decay linearly. As the jets enter the combined region confluent jets have hardly any velocity decay, which leads to a higher maximum velocity for a combined confluent jet than a single round jet.

The jet’s position within the configuration has a substantial impact on the velocity decay, length of the potential core, and the lateral displacement of the confluent jets. Side jets show faster velocity decay, shorter potential core and higher turbulence level compared to central jets. Side jets are also deformed and has a kidney shaped cross-section in the merging region. Corner jets interact less with neighboring jets compared to side jets, thereby extending the potential core and reducing the velocity decay in the merging region compared to side jets.

Place, publisher, year, edition, pages
Elsevier, 2014. Vol. 46, 127-146 p.
Keyword [en]
Multiple jet array; Confluent jet; Reynolds Stress Model (RSM); Standard k–ε; RNG k–ε; Particle Image Velocimetry (PIV)
National Category
Fluid Mechanics and Acoustics
URN: urn:nbn:se:liu:diva-106378DOI: 10.1016/j.ijheatfluidflow.2014.01.004ISI: 000335103200010OAI: diva2:715789
Swedish Research Council, 2008-31145-61023-37
Available from: 2014-05-06 Created: 2014-05-06 Last updated: 2015-04-15Bibliographically approved
In thesis
1. Experimental and Numerical Investigations of Confluent Round Jets
Open this publication in new window or tab >>Experimental and Numerical Investigations of Confluent Round Jets
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Unconfined multiple interacting confluent round jets are interesting from a purely scientific point of view, as interaction between neighboring jets brings additional complexity to the flow field. Unconfined confluent round jets also exist in various engineering applications, such as ventilation supply devices, sewage disposal systems, combustion burners, chemical mixing or chimney stacks. Even so, little scientific attention has been paid to unconfined confluent round jets.

The present work uses both advanced measurement techniques and computational models to provide deeper understanding of the turbulent flow field development of unconfined confluent round jets. Both Laser Doppler Anemometry (LDA) and Particle Image Velocimetry (PIV) have been used to measure mean velocity and turbulence properties within two setups, consisting of a single row of 1×6 jets and a square array of 6×6 confluent jets.

Simulations using computational fluid dynamics (CFD) of the 6×6 setup were conducted using three different Reynolds Averaged Navier-Stokes (RANS) turbulence models: the standard k-ε, the RNG k-ε and the Reynolds Stress model (RSM). The results from the CFD simulations were compared with experimental data.

The employed RANS turbulence models were all capable of accurately predicting mean velocities and turbulent properties in the investigated confluent jet array. In general the RSM and k-ε std. models provided smaller deviations between numerical and experimental results than the RNG k-ε model. In terms of mean velocity the second-order closure model (RSM) was not found to be superior to the less complex standard k-ε model.

The validated CFD model was employed in a parametrical investigation, including five independent variables: inlet velocity, nozzle diameter, nozzle edge-to-edge spacing, nozzle height and the number of jets in the array. The parametrical investigations made use of statistical methods in the form of response surface methodology. The derived response surface models provided information on the principal influence and relative importance of the investigated parameters within the investigated design space.

The positions of the jets within the array strongly influence both mean velocity and turbulence. In all investigated setups the jets experience merging and combining. Square arrays also include considerable jet convergence, which was not present in the 1×6 jet array. Due to the jet convergence in square arrays the turbulent flow field, especially for jets far away from the array center, is affected by mean flow curvature.

Jets located along the sides of square jet arrays experience strong jet-to-jet interactions that result in considerable jet deformation, shorter potential core, higher turbulent kinetic energy and faster velocity decay compared to other jets. Jets located at the corners of the array do not interact as strongly with neighboring jets as do the jets along the sides. The locations of merging and combined points differ considerably between different jets and different jet configurations.

As the jets combine a zone with uniform stream-wise velocity and low turbulence intensity forms in the center of square jet arrays. This zone has been called Confluent Core Zone (CCZ) due to its similarities with the potential core zone of a single jet. Within the CCZ the appropriate scaling length changes from nozzle diameter to the effective source diameter.

The parametrical investigation showed that nozzle diameter and edge-to-edge nozzle spacing were the most important of the investigated parameters, reflecting a strong dependence on dimensionless jet spacing, S/d0. Higher S/d0 delays both merging and combining of the jets and leads to a CCZ with lower velocity and longer downstream extension. Increasing the array size leads to a reduced combined point distance, a stronger inwards displacement of jets in the outer part of the array, and reduced entrainment near the nozzles. A higher inlet velocity was found to increase the jet convergence in the investigated square confluent jet arrays. Nozzle height generally has minor impact on the investigated response variables.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2015. 110 p.
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1653
Confluent jets, Multiple jet array, Jet interactions, Confluent Core Zone (CCZ), Particle Image Velocimetry (PIV), Laser Doppler Velocimetry (LDA), Computational Fluid Dynamics (CFD), Response Surface Methodology
National Category
Energy Systems Fluid Mechanics and Acoustics
urn:nbn:se:liu:diva-117066 (URN)10.3384/diss.diva-117066 (DOI)978-91-7519-086-0 (print) (ISBN)
Public defence
2015-05-11, ACAS, hus A, Campus Valla, Linköping, 10:15 (Swedish)
Available from: 2015-04-15 Created: 2015-04-15 Last updated: 2015-04-15Bibliographically approved

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