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Experimental and Numerical Investigations of Confluent Round Jets
Linköping University, Department of Management and Engineering, Energy Systems. Linköping University, The Institute of Technology.
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 [en]
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: urn:nbn:se:liu:diva-117066DOI: 10.3384/diss.diva-117066ISBN: 978-91-7519-086-0 (print)OAI: oai:DiVA.org:liu-117066DiVA: diva2:805327
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
List of papers
1. Near-field development of a row of round jets at low Reynolds numbers
Open this publication in new window or tab >>Near-field development of a row of round jets at low Reynolds numbers
2014 (English)In: Experiments in Fluids, ISSN 0723-4864, E-ISSN 1432-1114, Vol. 55, no 8, 1789- p.Article in journal (Refereed) Published
Abstract [en]

This article reports on an experimental investigation of the near-field behavior of interacting jets at low Reynolds numbers (Re = 2125, 3290 and 4555). Two measurement techniques, particle image velocimetry (PIV) and laser Doppler anemometry (LDA), were employed to measure mean velocity and turbulence statistics in the near field of a row of six parallel coplanar round jets with equidistant spacing. The overall results from PIV and LDA measurements show good agreement, although LDA enabled more accurate measurements in the thin shear layers very close to the nozzle exit. The evolution of all six coplanar jets showed initial, merging, and combined regions. While the length of the potential core and the maximum velocity in the merging region are Reynolds number-dependent, the location of the merging points and the minimum velocity between jets were found to be independent of Reynolds number. Side jets at the edges of the coplanar row showed a constant decay rate of maximum velocity after their core region, which is comparable to a single round jet. Jets closer to the center of the row showed reducing velocity decay in the merging region, which led to a higher maximum velocity compared to a single round jet. A comparison with the flow for an in-line array of 6 × 6 round jets showed that the inward bending of streamwise velocity, which exists in the near field of the 6 × 6 jet array, does not occur in the single row of coplanar jets, although both setups have identical nozzle shape, spacing, and Reynolds number.

Place, publisher, year, edition, pages
Springer Berlin/Heidelberg, 2014
National Category
Mechanical Engineering Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:liu:diva-109451 (URN)10.1007/s00348-014-1789-2 (DOI)000340838300014 ()
Available from: 2014-08-19 Created: 2014-08-19 Last updated: 2017-12-05Bibliographically approved
2. Near-field mixing of jets issuing from an array of round nozzles
Open this publication in new window or tab >>Near-field mixing of jets issuing from an array of round nozzles
2014 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 47, 84-100 p.Article in journal (Refereed) Published
Abstract [en]

This article presents results of an experimental study of the confluence of low Reynolds number jets inthe near field of a 6 6 in-line array of round nozzles. Particle Image Velocimetry (PIV) and Laser DopplerAnemometry (LDA) were employed to measure mean velocities and turbulence statistics. The comparisonof the results from PIV and LDA measurements along different cross-sectional profiles and geometricalcenterlines showed good agreement. However, LDA enabled more accurate results very close to the nozzleexits. The evolution of all the individual jets in the array into a single jet showed flow regions similarto twin jets (i.e., initial, converging including mixing transition, merging and combined regions). The lateraldisplacements play an important role for a confluent jet, where all jets to some degree are deflectedtowards the center of the nozzle plate. The jet development in terms of velocity decay, length of potentialcore and lateral displacement varies significantly with the position of the jet in the array. A comparisonwith single jet and twin jets flow showed considerable differences in velocity decay as well as locationand velocity in the combined point. The flow field of confluent jets showed asymmetrical distributionsof Reynolds stresses around the axis of the jets and highly anisotropic turbulence. Additionally, the lateraldisplacement as well as the turbulence development in the proximal region of the studied confluent jetwas shown to be dependent on Reynolds number.

Place, publisher, year, edition, pages
Elsevier, 2014
Keyword
Low Reynolds number round jet, Jet-to-jet interaction, Multiple jet array, Confluent jets, Particle Image Velocimetry (PIV), Laser Doppler Anemometry (LDA)
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:liu:diva-106380 (URN)10.1016/j.ijheatfluidflow.2014.01.007 (DOI)000336773700008 ()
Funder
Swedish Research Council, 2008-31145-61023-37
Available from: 2014-05-06 Created: 2014-05-06 Last updated: 2017-12-05Bibliographically approved
3. Numerical and experimental investigation of flow behavior in a confluent jet ventilation system for industrial premises
Open this publication in new window or tab >>Numerical and experimental investigation of flow behavior in a confluent jet ventilation system for industrial premises
2012 (English)Conference paper, Published paper (Other academic)
Abstract [en]

A conventional supply principle, such as rmxing or displacement ventilation, in industrial applications often results in low ventilation efficiency and high draught. A possible way to improve the ventilation efficiency in industrial premises is to implement a new type of supply system known as confluent jet ventilation. The confluent system can be described as a number of free jets issued in a plane, parallel to each other. In the proximity of the diffuser, the confluent jets behave as separate jets, but downstream the jets starts to merge with each other and eventually behave as a single jet. The main advantage of the confluent jet system is its ability to conserve momentum in an efficient way. The high level of momentum makes the ventilation system less sensitive to mechanical disturbances and buoyancy forces than displacement ventilation. This effect can be used to enhance the ventilation efficiency.

The purpose of this study is to investigate both numerically and experimentally the flow behavior of a confluent jet system in the region close to the diffuser.

In the present study, a diffusor consisting of 36 jets with an in-line anangement using equidistant spacing has been studied. The Reynolds number of the jet, based on the nozzle diameter, is Red= 3290. ARANS simulation using the Reynolds Stress Model (RSM) has been used to predict the mean velocity field and the tmbulence characteristics of the confluent jet configuration. The numerical simulations are compared with measurements using Particle Image Velocimetty (PIV), performed in a region extending out to a downsttream distance of 26 times the nozzle diameter. The flow behavior of the confluent jets showed good agreement with the experimental results.

Keyword
Confluent jet ventilation system, Near zone behavior, PIV measurements, CFD, Reynolds Stress Model
National Category
Energy Engineering
Identifiers
urn:nbn:se:liu:diva-89789 (URN)
Conference
The 10th International Conference on Industrial Ventilation, September 17-19, 2012, Paris, France
Projects
Ett nytt ventilationskoncept för industrilokaler
Funder
Swedish Research Council, 2008-31145-61023-37
Available from: 2013-03-13 Created: 2013-03-06 Last updated: 2015-04-15Bibliographically approved
4. Numerical and experimental investigation of the near zone flow field in an array of confluent round jets
Open this publication in new window or tab >>Numerical and experimental investigation of the near zone flow field in an array of confluent round jets
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
Keyword
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
Identifiers
urn:nbn:se:liu:diva-106378 (URN)10.1016/j.ijheatfluidflow.2014.01.004 (DOI)000335103200010 ()
Funder
Swedish Research Council, 2008-31145-61023-37
Available from: 2014-05-06 Created: 2014-05-06 Last updated: 2017-12-05Bibliographically approved
5. A computational parametric study on the development of confluent round jet arrays
Open this publication in new window or tab >>A computational parametric study on the development of confluent round jet arrays
2015 (English)In: European journal of mechanics. B, Fluids, ISSN 0997-7546, E-ISSN 1873-7390, Vol. 53, 129-147 p.Article in journal (Refereed) Published
Abstract [en]

In this study, Computational Fluid Dynamics (CFD) and response surface methodology is employed in a parametrical investigation of an in-line array of confluent round jets. Confluent round jet arrays are common within several fields of engineering, as detailed knowledge of the flow field development of confluent round jets is of great importance to design engineers working with, for example, chemical mixing, multiple jet burners, waste water disposal systems or ventilation supply devices. In this paper, five independent factors affecting flow field development are investigated with a multi-variable approach using a Box–Behnken design method.

The results include decay of maximum velocity, turbulence intensity, location of merging and combined points and development of volumetric flow rate. Dimensionless nozzle spacing, S/d0S/d0, is an important design parameter and has a large impact on several properties, such as merging and combined points, decay of maximum velocity, and development of turbulence intensity. Other factors, such as the number of jets per row and inlet velocity, are also of importance. The analysis of decay in maximum velocity led to the definition of a new zone of development, referred to as the Confluent Core Zone (CCZ), as its behaviour is reminiscent of the potential core of a single jet. The CCZ has uniform velocity, lacks considerable decay in streamwise velocity and has a rather low turbulence intensity. The CCZ has a characteristic footprint in confluent round jet arrays, and its properties are investigated in detail.

The development of volumetric flow can be divided into two regions. The initial region, close to the nozzles, features a high entrainment but decreasing entrainment rate. As the jets combine, the entrainment rate is lower, but rather constant. While S/d0S/d0 is generally an important design parameter, there is no direct correlation between S/d0S/d0 and entrainment rate of the combined jet.

Place, publisher, year, edition, pages
Elsevier, 2015
Keyword
Multiple jet array, Confluent jets, Computational Fluid Dynamics (CFD), Response Surface Methodology, Box-Behnken, Confluent Core Zone (CCZ)
National Category
Energy Systems
Identifiers
urn:nbn:se:liu:diva-117078 (URN)10.1016/j.euromechflu.2015.03.012 (DOI)000358968500012 ()
Available from: 2015-04-15 Created: 2015-04-15 Last updated: 2017-12-04Bibliographically approved
6. On the influence of array size and jet spacing on jet interactions and confluence in round jet arrays
Open this publication in new window or tab >>On the influence of array size and jet spacing on jet interactions and confluence in round jet arrays
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.

Keyword
Computational Fluid Dynamics (CFD), Confluent jets, Confluent Core Zone (CCZ), Jet interactions, Axis-switching.
National Category
Energy Systems
Identifiers
urn:nbn:se:liu:diva-117079 (URN)10.1115/1.4033024 (DOI)000379589700012 ()
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

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