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Research Papers: Fundamental Issues and Canonical Flows

# On the Influence of Array Size and Jet Spacing on Jet Interactions and Confluence in Round Jet Arrays

[+] Author and Article Information
Klas Svensson

Division of Energy Systems,
Department of Management and Engineering,
e-mail. klas.svensson@liu.se

Patrik Rohdin, Bahram Moshfegh

Division of Energy Systems,
Department of Management and Engineering,

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received March 25, 2015; final manuscript received February 22, 2016; published online May 20, 2016. Assoc. Editor: Elias Balaras.

J. Fluids Eng 138(8), 081206 (May 20, 2016) (19 pages) Paper No: FE-15-1206; doi: 10.1115/1.4033024 History: Received March 25, 2015; Revised February 22, 2016

## Abstract

This work uses computational models to study the effects of confluence and jet-to-jet interactions for inline matrices of confluent round jets. In total, 12 different confluent jet arrangements, having various jet array sizes and dimensionless jet spacing, $S/d0$, have been investigated. The array size varies from 6 × 6 to 10 × 10 jets, while $S/d0$ varies between $1.75≤S/d0≤4.0$. The Reynolds number, based on the nozzle exit diameter, is between 2200 and 6600. The results show that both jet spacing and jet array size largely influence the jet-to-jet interactions and flow field development in confluent jet arrays. The jet interactions in the investigated setups result in regions of negative static pressure between jets, jet deformation, high spanwise velocity, and jet displacement. Generally, smaller jet spacing and larger array size result in stronger influence of jet interactions. After the jets have combined, the confluent jets form a zone with constant maximum streamwise velocity and decay of turbulence intensity, called a confluent core zone (CCZ). During the CCZ, the combined jet will have asymmetric spreading rates leading to axis-switching. The entrainment rate of the CCZ is constant, but the volumetric flow rate of the combined jet is substantially affected by the degree of entrainment before the jets have combined.

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## Figures

Fig. 5

Comparison between PIV measurements (symbols) and numerical simulations (solid lines) for (a)–(c): streamwise velocity, (d)–(f): turbulent kinetic energy, and (g)–(i): turbulence intensity

Fig. 4

Displacement in the y-direction of central streamlines for three different jets. Comparison between experimental PIV data (dashed lines) and CFD simulations (solid lines).

Fig. 6

Contour plot of streamwise velocity, U/Ub, overlaid with vectors of spanwise movement (left) and contour plot of spanwise velocity magnitude, V2+W2/Ub, (right) for cases 1 and 3, at x/d0 = 1.5. The marked square area represents area of zoom-in plot presented in Fig.8.

Fig. 7

Contour plot of streamwise velocity, U/Ub, overlaid with vectors or spanwise movement (left) and contour plot of spanwise velocity magnitude, V2+W2/Ub, (right) for cases 9 and 11, at x/d0 = 1.5

Fig. 8

Zoom-in of the SJ located at zc = 0 for cases 1 and 3 at x/d0 = 1.5

Fig. 12

Shape of U0.5,J contour for the SJ located at zc = 0, at x/d0 = 3

Fig. 9

Contour plot of streamwise velocity, U/Ub, overlaid with vectors or spanwise movement (left) and contour plot of spanwise velocity magnitude, V2+W2/Ub, (right) for cases 1 and 3, at x/d0 = 5

Fig. 10

Contour plot of streamwise velocity, U/Ub, overlaid with vectors or spanwise movement (left) and contour plot of spanwise velocity magnitude,  V2+W2/Ub, (right) for cases 9 and 11, at x/d0 = 5

Fig. 11

Characteristic lengths of jet cross sections for a round jet in a cross-flow

Fig. 13

Ratio between maximum and minimum jet width Rmin/Rmax for different jets

Fig. 14

Isocontours of static pressure. Semitransparent green represents a pressure coefficient of −0.05 and red −0.2. The figure also shows the position of the CoJ and SJs next to the vertical (yc = 0) and horizontal (zc = 0) symmetry plane.

Fig. 15

Horizontal displacement of central streamlines for different jets (left). Decay of maximum jet velocity (right).

Fig. 3

Comparison of the numerical results and PIV measurements [29]

Fig. 2

Mesh configuration and location of PIV measurements at planes z=0 and z=2S

Fig. 1

Layout of the confluent jet array (left) and overview of computational domain (right) in case 9

Fig. 16

Development of Ub/Umax and TU,min in case 3 [32]

Fig. 18

Cross-sectional contour plots of U/Ub overlaid by the velocity field in the lateral directions (V and W) at three downstream locations. Lower right: development of r0.5/d0 along y = z and z=−0.5S. Data from case 3.

Fig. 17

Illustration of CCZ prevalence in case 3. Top: isocontours of U/Ub=0.345 (green) and U/Ub=0.335 (red) bottom: isocontours of TU=0.35 (red) and TU=0.1 (green).

Fig. 19

Development of ratio between half-width along array diagonal (y = z) and horizontal symmetry plane (z = −0.5S)

Fig. 20

Volumetric flow rate of the whole jet array (left). Volumetric flow rate in array element (AJ) next to array center.

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