Enhancement of passive mixing tabs by the addition of secondary tabs

Bohl, Douglas; Foss, J. F.

doi:10.2514/6.1996-545

Cited by 26 publications

(32 citation statements)

References 2 publications

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“…That a tab produces a pair of counter rotating streamwise vortices had been measured in several previous experiments with incompressible flows (Zaman et al 1994;Bohl & Foss 1996;. That the corresponding vorticity distribution and dynamics are essentially similar in compressible flows had been conjectured from flow visualization of overall jet distortion (Zaman et al 1994).…”

Section: Streamwise Vorticity In the Tabbed Rectangular Jetmentioning

confidence: 96%

Spreading characteristics of compressible jets from nozzles of various geometries

1999

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The spreading characteristics of jets from several asymmetric nozzles, and a set of rectangular orifices are compared, covering a jet Mach number range of 0.3-2.0. The effect of 'tabs' for a rectangular and a round nozzle is also included in the comparison. Compared to a round jet, the jets from the asymmetric nozzles spread only slightly more at subsonic conditions whereas at supersonic conditions, when 'screech' occurs, they spread much more. The dynamics of the azimuthal vortical structures of the jet, organized and intensified under the screeching condition, are thought to be responsible for the observed effect at supersonic conditions. Curiously, the jet from a 'lobed' nozzle spreads much less at supersonic condition compared to all other cases; this is due to the absence of screech with this nozzle. Screech stages inducing flapping, rather than varicose or helical, flow oscillation cause a more pronounced jet spreading. At subsonic conditions, only a slight increase in jet spreading with the asymmetric nozzles contrasts previous observations by others. The present results show that the spreading of most asymmetric jets is not much different from that of a round jet. This inference is further supported by data from the rectangular orifices. In fact, jets from the orifices with small aspect ratio (AR) exhibit virtually no increase in the spreading. A noticeable increase commences only when AR is larger than about 10. Thus, 'shear layer perimeter stretching', achieved with a larger AR for a given cross-sectional area of the orifice, by itself, proves to be a relatively inefficient mechanism for increasing jet spreading. In contrast, the presence of streamwise vortices or 'natural excitation' can cause a significant increase -effects that might explain the observations in the previous investigations. Thus far, the biggest increase in jet spreading is observed with the tabs. This is true in the subsonic regime, as well as in the supersonic regime, in spite of the fact that screech is eliminated by the tabs. The characteristic spreading of the tabbed jets is explained by the induced motion of the tab-generated streamwise vortex pairs. The tabs, however, incur thrust loss; the flow blockage and loss in thrust coefficient, vis-à-vis the spreading increase, are evaluated for various configurations.

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Section: Streamwise Vorticity In the Tabbed Rectangular Jetmentioning

confidence: 96%

Spreading characteristics of compressible jets from nozzles of various geometries

1999

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“…The results of Bohl and Foss [3] have provided some insight into the effects of the secondary tabs. The object of the present investigation is therefore twofold.…”

Section: Introductionmentioning

confidence: 95%

“…Apart from the shape of the tabs, which has played an important role in the streamwise vorticity generation, a method to further enhance the tab's effect has also been proposed recently by Bohl and Foss [3] in which twò secondary' tabs were placed symmetrically on either side of the primary tab and directed into the ambient region.…”

Section: Introductionmentioning

confidence: 99%

The flow characteristics of a confined square jet with mixing tabs

Hou

Low

1998

Proceedings of the Institution of Mechanical Engineers, Part G:

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The flow characteristics of a confined square jet with mixing tabs have been determined by measurements obtained using a two-component laser Doppler anemometer at a Reynolds number of 1:026 3 10 5 (based on the exit hydraulic diameter, D H 60 mm, and bulk mean velocity, U r , of the stream at 1.71 m/s). Both tabs of rectangular and triangular shapes are considered with the same height±breadth ratio (h=b 1:35) and with their apex leaning downstream. Altogether four tabs have been used, with one tab each located at the centre of each side wall at the exit plane. Each tab is found to produce a dominant pair of counter-rotating streamwise vortices. The combined effects of the four tabs bifurcate the jet into four`fingers', resulting in a significant increase in entrainment at the downstream locations. The strength of the streamwise vorticity generated by the rectangular tabs is some 30 per cent higher than the triangular ones and decays faster with downstream distance. This appears due to a larger tab surface area which creates a larger pressure differential across the rectangular tab than the triangular tab. The region of high turbulent kinetic energy is found firstly at the locations where the streamwise vortices stretch the normal vortices and subsequently at locations where the streamwise vortices break down, resulting in significant mixing enhancement. Finally, the effects of the so-called secondary tabs have also been examined and are found to enhance the mixing further. The orientation of the secondary tabs is, however, crucial for the mixing enhancement to occur.

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“…Control of jets with passive control in the form of tabs of various shapes has been reported by large number of researchers in open literature. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] Control of jet by altering the nozzle boundary-layer thickness, turbulence level, and convergence had no strong influence on the jet development, compared to the mechanical tabs. 2 Control of supersonic jet revealed that the potential core length of the jet could be reduced from about 6D to less than 2D by using two diametrically opposite mechanical tabs.…”

Section: Introductionmentioning

confidence: 99%

Corrugated right-angled triangular tabs for supersonic jet control

Kumar

Rathakrishnan

2014

Proceedings of the Institution of Mechanical Engineers, Part G:

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This paper compares the mixing promoting efficiency of two identical sharp and truncated vertex right-angled triangular tabs provided with semicircular, triangular, and square corrugations along its edges, offering geometrical blockage of 2.5% each, placed at the exit of a Mach 2 circular nozzle was studied experimentally. To quantify the effectiveness of the controlled jets in mixing, Pitot pressure along the jet axis and the jet spread along and normal to the tabs were measured, covering highly overexpanded and marginal underexpanded levels at the nozzle exit. The results reveal that the corrugated tabs enhance mixing higher than the uncorrugated (plain) tabs, for both sharp and truncated vertices. For the jet controlled with sharp vertex right-angled triangular tabs, a maximum reduction of about 91% in core length is caused by the square corrugation at NPR7. The corresponding core length reduction caused by semicircle, triangular corrugated, and plain tabs is about 87, 85, and 65%, respectively. For the jet controlled with truncated vertex right-angled triangular tabs also, the core length reductions are of the same order as those for the sharp vertex triangular tabs. The jet mixing is strongly influenced by the combined effect of tab geometry, corrugation shape, tip effect, and expansion ratio. The shadowgraph pictures clearly show that the waves in the controlled jets are made weaker than those of the uncontrolled jet.

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Enhancement of passive mixing tabs by the addition of secondary tabs

Cited by 26 publications

References 2 publications

Spreading characteristics of compressible jets from nozzles of various geometries

Spreading characteristics of compressible jets from nozzles of various geometries

The flow characteristics of a confined square jet with mixing tabs

Corrugated right-angled triangular tabs for supersonic jet control

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