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On the influence of array size and jet spacing on jet interactions and confluence in round jet arrays
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.
Linköping University, Department of Management and Engineering, Energy Systems. Linköping University, The Institute of Technology.
2016 (English)In: Journal of Fluids Engineering - Trancactions of The ASME, ISSN 0098-2202, E-ISSN 1528-901X, Vol. 138, no 8, 081206Article in journal (Refereed) Published
Abstract [en]

Arrays of unconfined confluent round jets exist in a number of engineering applications, including ventilation supply devices, sewage disposal systems, combustion burners, chemical mixing, and chimney stacks. Interacting confluent round jets are also interesting from a purely scientific point of view, as jet interactions and confluence bring additional complexity to the flow field. Yet little scientific attention has been paid to unconfined confluent round jets and detailed scientific investigations are scarce.

The present work uses computational models to study the effects of confluence and jet-to-jet interactions in four different confluent jet arrangements, reporting on the influence of jet array size and dimensionless jet spacing, 𝑆⁄𝑑0. The results show that both jet spacing and jet array size largely influence the jet-to-jet interactions and flow field development in confluent jet arrays. The jet interactions in the investigated setups result in regions of negative static pressure between jets, jet deformation, high spanwise velocity and jet displacement. Generally smaller jet spacing and larger array size results in stronger influence of jet interactions.

After the jets have combined the confluent jets form a zone with constant maximum streamwise velocity and decay of turbulence intensity, called a Confluent Core Zone (CCZ). During the CCZ the combined jet will have asymmetric spreading rates leading to axisswitching. The entrainment rate of the CCZ is constant, but the volumetric flow of the combined jet is substantially affected by the degree of entrainment before the jets have combined.

Place, publisher, year, edition, pages
2016. Vol. 138, no 8, 081206
Keyword [en]
Computational Fluid Dynamics (CFD), Confluent jets, Confluent Core Zone (CCZ), Jet interactions, Axis-switching.
National Category
Energy Systems
Identifiers
URN: urn:nbn:se:liu:diva-117079DOI: 10.1115/1.4033024ISI: 000379589700012OAI: oai:DiVA.org:liu-117079DiVA: diva2:805419
Note

Vid tiden för disputation förelåg publikationen som manuskript

Funding agencies:The authors gratefully acknowledge the financial support received from the Swedish Research Council (Grant No. 2008-31145-61023-37) and Linkoping University (Sweden). The National Supercomputer Centre (NSC) is acknowledged for providing computational resources. The authors thank Dr. Mark Tummers, Delft University of Technology, and Ph.D. student Shahriar Ghahremanian, Linkoping University, for the fruitful cooperation when conducting the experimental work used for validation.

Available from: 2015-04-15 Created: 2015-04-15 Last updated: 2017-12-04Bibliographically 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.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1653
Keyword
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
Identifiers
urn:nbn:se:liu:diva-117066 (URN)10.3384/diss.diva-117066 (DOI)978-91-7519-086-0 (ISBN)
Public defence
2015-05-11, ACAS, hus A, Campus Valla, Linköping, 10:15 (Swedish)
Opponent
Supervisors
Available from: 2015-04-15 Created: 2015-04-15 Last updated: 2015-04-15Bibliographically approved

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