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A study on proximal region of low reynolds confluent jets: Part 2: Numerical prediction of the flow field
University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering. Division of Energy Systems, Department of Management and Engineering, Linköping University.
University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering. Division of Energy Systems, Department of Management and Engineering, Linköping University.ORCID iD: 0000-0003-3472-4210
2014 (English)In: ASHRAE Transactions, 2014, no PART 1, 271-285 p.Conference paper (Refereed)
Abstract [en]

Conventional ventilation systems (mixing and displacement) produce low air quality in industrial premises. A new air supply system (confluent jet system) may improve both ventilation and energy efficiency. When round jets are issued from coplanar nozzles with enough spacing, they converge, merge, and combine at a certain downstream distance, which are called "confluent jets." In this study, the velocity field of the proximal region of confluent jets was recorded by traversing a hot-wire probe across the jets in one column at selected distances from the nozzles' exit in order to examine the performance of SST k - ω turbulence model. The experimental and numerical results from this work are summarized in a set of mapping fields of mean velocity for the confluent jet zones, which are presented in a generalized non-dimensional form. The existence of an initial, a converging, a merging, and a combined region in the confluent jets has been found for three low Reynolds numbers. Three different confluent jets can be seen in the array of jets studied placed six by six symmetrically on the long side of a cylindrical supply device. The streamwise velocity of the geometrical centerline of side jets and corner jets decays faster than that for the fully confluent jets, due to deflection towards their adjacent neighboring jets. Side jets and corner jets deflect to their adjacent jets and finally merge and combine with them, while fully confluent jets normally spread and amalgamate with each other. Low local pressure is responsible for the amalgamation of confluent jets, but the static pressure reaches a minimum value between side jets and their neighboring jets, which results in the deflection of the side jets.

Place, publisher, year, edition, pages
2014. no PART 1, 271-285 p.
, ASHRAE Transactions, ISSN 0001-2505 ; 120
Keyword [en]
Air quality, Energy efficiency, Metals, Reynolds number, Turbulence models, Velocity, Ventilation, Air-supply system, Downstream distances, Low Reynolds number, Numerical predictions, Numerical results, Static pressure, Stream-wise velocities, Ventilation systems, Nozzles
National Category
Building Technologies
URN: urn:nbn:se:hig:diva-18480ScopusID: 2-s2.0-84902096944ISBN: 978-193650470-1OAI: oai:DiVA.org:hig-18480DiVA: diva2:770177
2014 ASHRAE Winter Conference, 18-22 January 2014, New York, NY
Available from: 2014-12-10 Created: 2014-12-10 Last updated: 2017-01-03Bibliographically 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  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 ( −  and shear stress transport [SST]  − ) give reasonable flow predictions for the single round jet with the prescribed inlet boundary conditions, while the transition models ( −  and transition SST ) are unable to predict the flow in the turbulent region. The results of numerical prediction (low Reynolds SST 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.
, Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1639
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
urn:nbn:se:hig:diva-18837 (URN)978-91-7519-161-4 (print) (ISBN)
Public defence
2015-02-06, C3, C-huset, Campus Valla, Linköping, 10:15 (English)
Available from: 2015-01-23 Created: 2015-01-23 Last updated: 2017-01-03Bibliographically approved

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