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Near-Field Study of Multiple Interacting Jets: Confluent Jets
University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering. Linköpings universitet, Institutionen för ekonomisk och industriell utveckling, Energisystem. (Energiteknik)
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 [en]
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: urn:nbn:se:hig:diva-18837ISBN: 978-91-7519-161-4 (print)OAI: oai:DiVA.org:hig-18837DiVA: diva2:782951
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
List of papers
1. A study on proximal region of low reynolds confluent jets: Part 1: Evaluation of turbulence models in prediction of inlet boundary conditions
Open this publication in new window or tab >>A study on proximal region of low reynolds confluent jets: Part 1: Evaluation of turbulence models in prediction of inlet boundary conditions
2014 (English)In: ASHRAE Transactions, 2014, no PART 1, 256-270 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 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. 

Series
, ASHRAE Transactions, ISSN 0001-2505 ; 120
Keyword
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:nbn:se:hig:diva-18449 (URN)2-s2.0-84902115837 (ScopusID)978-193650470-1 (ISBN)
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
2. Evaluation of RANS models in predicting low reynolds, free, turbulent round jet
Open this publication in new window or tab >>Evaluation of RANS models in predicting low reynolds, free, turbulent round jet
2014 (English)In: Journal of Fluids Engineering - Trancactions of The ASME, ISSN 0098-2202, Vol. 136, no 1, 011201Article in journal (Refereed) Published
Abstract [en]

In order to study the flow behavior of multiple jets, numerical prediction of the three-dimensional domain of round jets from the nozzle edge up to the turbulent region is essential. The previous numerical studies on the round jet are limited to either two-dimensional investigation with Reynolds-averaged Navier-Stokes (RANS) models or three-dimensional prediction with higher turbulence models such as large eddy simulation (LES) or direct numerical simulation (DNS). The present study tries to evaluate different RANS turbulence models in the three-dimensional simulation of the whole domain of an isothermal, low Re (Re = 2125, 3461, and 4555), free, turbulent round jet. For this evaluation the simulation results from two two-equation (low Re k-ε and low Re shear stress transport (SST) k-ω), a transition three-equation (k-kl-ω), and a transition four-equation (SST) eddy-viscosity turbulence models are compared with hot-wire anemometry measurements. Due to the importance of providing correct inlet boundary conditions, the inlet velocity profile, the turbulent kinetic energy (k), and its specific dissipation rate (ω) at the nozzle exit have been employed from an earlier verified numerical simulation. Two-equation RANS models with low Reynolds correction can predict the whole domain (initial, transition, and fully developed regions) of the round jet with prescribed inlet boundary conditions. The transition models could only reach to a good agreement with the measured mean axial velocities and its rms in the initial region. It worth mentioning that the round jet anomaly is still present in the turbulent region of the round jet predicted by the low Re k-ε. By comparing the k and the ω predicted by different turbulence models, the blending functions in the cross-diffusion term is found one of the reasons behind the more consistent prediction by the low Re SST k-ω. 

Keyword
Hot-wire anemometry, Low Reynolds, RANS models, Round jet, SST k-ω, Reynolds-Averaged Navier-Stokes, Round jets, Specific dissipation rate, Three dimensional simulations, Three-dimensional predictions, Anemometers, Blending, Boundary conditions, Computational fluid dynamics, Forecasting, Kinetics, Large eddy simulation, Nozzles, Three dimensional, Turbulence models, Wakes, Wire, Navier Stokes equations
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:hig:diva-18462 (URN)10.1115/1.4025363 (DOI)000327511000009 ()2-s2.0-84888105847 (ScopusID)
Available from: 2014-12-10 Created: 2014-12-10 Last updated: 2017-01-03Bibliographically approved
3. 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 x 6 round jets showed that the inward bending of streamwise velocity, which exists in the near field of the 6 x 6 jet array, does not occur in the single row of coplanar jets, although both setups have identical nozzle shape, spacing, and Reynolds number.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:hig:diva-18342 (URN)10.1007/s00348-014-1789-2 (DOI)000340838300014 ()2-s2.0-84904528875 (ScopusID)
Available from: 2014-12-09 Created: 2014-12-09 Last updated: 2017-01-03Bibliographically approved
4. A study on proximal region of low reynolds confluent jets: Part 2: Numerical prediction of the flow field
Open this publication in new window or tab >>A study on proximal region of low reynolds confluent jets: Part 2: Numerical prediction of the flow field
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.

Series
, ASHRAE Transactions, ISSN 0001-2505 ; 120
Keyword
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
Identifiers
urn:nbn:se:hig:diva-18480 (URN)2-s2.0-84902096944 (ScopusID)978-193650470-1 (ISBN)
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
5. 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 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. 

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
Engineering and Technology
Identifiers
urn:nbn:se:hig:diva-18353 (URN)10.1016/j.ijheatfluidflow.2014.01.007 (DOI)000336773700008 ()2-s2.0-84898077957 (ScopusID)
Available from: 2014-12-09 Created: 2014-12-09 Last updated: 2017-01-03Bibliographically approved
6. Investigation in the near-field of a row of interacting jets
Open this publication in new window or tab >>Investigation in the near-field of a row of interacting jets
2015 (English)In: Journal of Fluids Engineering - Trancactions of The ASME, ISSN 0098-2202, E-ISSN 1528-901X, Vol. 137, no 12, 121202Article in journal (Refereed) Published
Abstract [en]

Multiple interacting jets (confluent jets) are employed in many engineering applications, and the significant design factors must be investigated. Computational fluid dynamics (CFD) is used to numerically predict the flow field in the proximal region of a single row of round jets. The numerical results that are obtained when using the low Reynolds k-∈ are validated with the experimental data that are acquired by particle image velocimetry (PIV). PIV was used to measure mean velocity and turbulence properties in the proximal region of a row of six parallel coplanar round air jets with equidistant spacing at low Reynolds number (Re = 3290). The low Reynolds k-∈ underpredicts the streamwise velocity in the onset of the jets' decay. The characteristic points are determined for various regions between two neighboring jets. The comparison of the merging point (MP) and the combined point (CP) computed from measurements and simulations shows good agreement in the different regions between the jets. In this study, a computational parametric study is also conducted to determine the main effects of three design factors and the interactions between them on the flow field development using response surface method (RSM). The influences of the inlet velocity, the spacing between the nozzles, and the diameter of the nozzles on the locations of the characteristic points are presented in the form of correlations (regression equations). CFD is used to numerically predict the characteristic points for a set of required studies, for which the design values of the simulation cases are determined by the Box-Behnken method. The results indicate that the spacing between the nozzles has a major impact on the flow characteristics in the near-field region of multiple interacting jets. The RSM shows that the inlet velocity has a marginal effect on the merging and CPs. All of the square terms are removed from the response equations of MP, and only one two-way interaction term between inlet velocity and spacing remains in the regression model with a marginal effect. The square of the nozzle diameter contributes in the regression equations of CP in some regions between the jets.

Keyword
Multiple interacting jets, confluent jets, axisymmetric jet, Low Reynolds number jet, Particle Image Velocimetry (PIV), Low Reynolds k — ε, Response Surface Method
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:hig:diva-18838 (URN)10.1115/1.4031014 (DOI)000364791500010 ()2-s2.0-84939818106 (ScopusID)
Available from: 2015-01-13 Created: 2015-01-23 Last updated: 2017-01-03Bibliographically approved

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