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Near-field mixing of jets issuing from an array of round nozzles
University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering, Energy system. Linköping University.
University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering, Energy system.ORCID iD: 0000-0003-3472-4210
2014 (English)In: International Journal of Heat and Fluid Flow, ISSN 0142-727X, E-ISSN 1879-2278, Vol. 47, p. 84-100Article in journal (Refereed) Published
##### Abstract [en]

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

##### Place, publisher, year, edition, pages
2014. Vol. 47, p. 84-100
##### Keywords [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
Engineering and Technology
##### Identifiers
ISI: 000336773700008Scopus ID: 2-s2.0-84898077957OAI: oai:DiVA.org:hig-18353DiVA, id: diva2:770013
Available from: 2014-12-09 Created: 2014-12-09 Last updated: 2018-03-13Bibliographically 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 $\kappa$$\epsilon$ and Reynolds stress model) are validated with hot-wire anemometry (HWA) near different nozzles discharge in the array. A single round jet is then studied to find the appropriate turbulence models for the prediction of the three-dimensional flow field and to gain an understanding of the effect of the boundary conditions predicted at the nozzle inlet. In comparison with HWA measurements, the turbulence models with low Reynolds correction ($\kappa$ − $\epsilon$ and shear stress transport [SST] $\kappa$ − $\omega$) give reasonable flow predictions for the single round jet with the prescribed inlet boundary conditions, while the transition models ($\kappa$$\kappa$$\iota$ − $\omega$ and transition SST $\kappa$$\omega$) are unable to predict the flow in the turbulent region. The results of numerical prediction (low Reynolds SST $\kappa$$\omega$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.

##### Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1639
##### Keywords
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 −, Low Reynolds −, Response Surface Method
##### National Category
Fluid Mechanics and Acoustics
##### Identifiers
urn:nbn:se:hig:diva-18837 (URN)978-91-7519-161-4 (ISBN)
##### Public defence
2015-02-06, C3, C-huset, Campus Valla, Linköping, 10:15 (English)
##### Supervisors
Available from: 2015-01-23 Created: 2015-01-23 Last updated: 2018-03-13Bibliographically approved

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Ghahremanian, ShahriarMoshfegh, Bahram

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International Journal of Heat and Fluid Flow
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Cite
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