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Near-field mixing of jets issuing from an array of round nozzles
Linköping University, Department of Management and Engineering, Energy Systems. Linköping University, The Institute of Technology. University of Gävle, Sweden.
Linköping University, Department of Management and Engineering, Energy Systems. Linköping University, The Institute of Technology.ORCID iD: 0000-0002-4649-6449
Delft University of Technology, The Netherlands.
Linköping University, Department of Management and Engineering, Energy Systems. Linköping University, The Institute of Technology. University of Gävle, Sweden.
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. Vol. 47, 84-100 p.
Keyword [en]
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: urn:nbn:se:liu:diva-106380DOI: 10.1016/j.ijheatfluidflow.2014.01.007ISI: 000336773700008OAI: oai:DiVA.org:liu-106380DiVA: diva2:715796
Funder
Swedish Research Council, 2008-31145-61023-37
Available from: 2014-05-06 Created: 2014-05-06 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Near-Field Study of Multiple Interacting Jets: Confluent Jets
Open this publication in new window or tab >>Near-Field Study of Multiple Interacting Jets: Confluent Jets
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis deals with the near-field of confluent jets, which can be of interest in many engineering applications such as design of a ventilation supply device. The physical effect of interaction between multiple closely spaced jets is studied using experimental and numerical methods. The primary aim of this study is to explore a better understanding of flow and turbulence behavior of multiple interacting jets. The main goal is to gain an insight into the confluence of jets occurring in the near-field of multiple interacting jets.

The array of multiple interacting jets is studied when they are placed on a flat and a curved surface. To obtain the boundary conditions at the nozzle exits of the confluent jets on a curved surface, the results of numerical prediction of a cylindrical air supply device using two turbulence models (realizable 𝑘 − 𝜖 and Reynolds stress model) are validated with hot-wire anemometry (HWA) near different nozzles discharge in the array. A single round jet is then studied to find the appropriate turbulence models for the prediction of the three-dimensional flow field and to gain an understanding of the effect of the boundary conditions predicted at the nozzle inlet. In comparison with HWA measurements, the turbulence models with low Reynolds correction (𝑘 − 𝜖 and shear stress transport [SST] 𝑘 − 𝜔) give reasonable flow predictions for the single round jet with the prescribed inlet boundary conditions, while the transition models (𝑘 − 𝑘l − 𝜔𝜔 and transition SST 𝑘 − 𝜔) are unable to predict the flow in the turbulent region. The results of numerical prediction (low Reynolds SST 𝑘 − 𝜔 model) using the prescribed inlet boundary conditions agree well with the HWA measurement in the nearfield of confluent jets on a curved surface, except in the merging region.

Instantaneous velocity measurements are performed by laser Doppler anemometry (LDA) and particle image velocimetry (PIV) in two different configurations, a single row of parallel coplanar jets and an inline array of jets on a flat surface. The results of LDA and PIV are compared, which exhibit good agreement except near the nozzle exits.

The streamwise velocity profile of the jets in the initial region shows a saddle back shape with attenuated turbulence in the core region and two off-centered narrow peaks. When confluent jets issue from an array of closely spaced nozzles, they may converge, merge, and combine after a certain distance downstream of the nozzle edge. The deflection plays a salient role for the multiple interacting jets (except in the single row configuration), where all the jets are converged towards the center of the array. The jet position, such as central, side and corner jets, significantly influences the development features of the jets, such as velocity decay and lateral displacement. The flow field of confluent jets exhibits asymmetrical distributions of Reynolds stresses around the axis of the jets and highly anisotropic turbulence. The velocity decays slower in the combined regio  of confluent jets than a single jet. Using the response surface methodology, the correlations between characteristic points (merging and combined points) and the statistically significant terms of the three design factors (inlet velocity, spacing between the nozzles and diameter of the nozzles) are determined for the single row of coplanar parallel jets. The computational parametric study of the single row configuration shows that spacing has the greatest impact on the near-field characteristics.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2015. 125 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1639
Keyword
Multiple interacting jets, confluent jets, axisymmetric/round jet, Low Reynolds number jet, Particle Image Velocimetry (PIV), Laser Doppler Anemometry (LDA), Hot-Wire anemometry (HWA), RANS turbulence models, SST 𝑘 − 𝜔, Low Reynolds 𝑘 − 𝜖, Response Surface Method
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:liu:diva-113259 (URN)10.3384/diss.diva-113259 (DOI)978-91-7519-161-4 (ISBN)
Public defence
2015-02-06, C3, C-huset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Supervisors
Available from: 2015-01-13 Created: 2015-01-13 Last updated: 2015-01-13Bibliographically approved
2. 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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Ghahremanian, ShahriarSvensson, KlasMoshfegh, Bahram

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