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A study on proximal region of low reynolds confluent jets: Part 1: Evaluation of turbulence models in prediction of inlet boundary conditions
University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering. Department of Management and Engineering, Linköping University.
University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering. Department of Management and Engineering, Linköping University.ORCID iD: 0000-0003-3472-4210
2014 (English)In: ASHRAE Transactions, 2014, no PART 1, 256-270 p.Conference paper, Published paper (Refereed)
Abstract [en]

Conventional ventilation systems (mixing and displacement) produce low air quality in industrial premises. A new air supply system (confluent jets system) may improve the ventilation efficiency and the energy efficiency. When round jets issue from co-planar nozzles with enough spacing, they converge, merge, and combine at certain downstream distances, which are called confluent jets. In order to numerically predict confluent jets, it is crucial to provide inlet boundary conditions for these jets at the nozzles' exit. Numerical prediction of inlet boundary conditions of confluent jets was chosen due to two reasons: the difficulty of measurement at the nozzles' exit, and lack of information about the shape of the employed nozzles to make artificial inlet profiles. Numerical predictions by two turbulence models (Realizable k - ε and RSM) of the supply device producing the confluent jets was verified by hot-wire measurements at 0.26 d0 downstream of the nozzles' exit in both lateral and vertical direction. The verification was carried out for different nozzles in an array by measuring axial velocity and its turbulence intensity. The axial velocity profile at the nozzles exit has a saddle-back shape with two distinct off-centered overshoots. The convergence of the velocity profile shows the existence of Vena contracta phenomena. Low turbulence intensity at the central part of nozzles was found with narrow shear layer upstream of confluent jet flow. Differences of velocity components, turbulent kinetic energy (TKE), and turbulent dissipation rate (TDR) of the studied contraction nozzle were examined with a flow issuing from a typical long pipe. Reynolds number dependency in the studied range has been carried out and Re effects were observed on TKE but not on TDR. 

Place, publisher, year, edition, pages
2014. no PART 1, 256-270 p.
Series
ASHRAE Transactions, ISSN 0001-2505 ; 120
Keyword [en]
Air quality, Boundary conditions, Energy efficiency, Kinetics, Reynolds number, Turbulence models, Velocity, Ventilation, Wakes, Downstream distances, Hot-wire measurements, Numerical predictions, Turbulence intensity, Turbulent dissipation rates, Turbulent kinetic energy, Ventilation efficiency, Ventilation systems, Nozzles
National Category
Mechanical Engineering
Identifiers
URN: urn:nbn:se:hig:diva-18449Scopus ID: 2-s2.0-84902115837ISBN: 978-193650470-1 (print)OAI: oai:DiVA.org:hig-18449DiVA: diva2:770244
Conference
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.
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 −, 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: 2017-01-03Bibliographically approved

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