Turbulent mixing in supersonic combustion systems

Swithenbank, J.; Eames, I.; Chin, S. B.; Ewan, B. C. R.; Yang, Zhiyin; Cao, Jian

doi:10.2514/6.1989-260

Cited by 17 publications

(12 citation statements)

References 5 publications

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“…The combination of two issues makes the problem difficult to solve: (i) supersonic flow in the combustor of restricted length means very short residence time (e.g. order of tens of microseconds) for mixing and combustion (see, for example, Ferri 1973;Swithenbank et al 1991), and (ii) compressibility causes poor mixing relative to incompressible cases because of a reduced growth rate of the shear layers in the combustor. In order to solve the problem, techniques to keep mixing efficiency high with a minimum total pressure loss are required.…”

Section: Velocity Fields In Mixing-enhanced Compressible Shear Layersmentioning

confidence: 99%

Velocity fields in mixing-enhanced compressible shear layers

Watanabe

Mungal

2005

J. Fluid Mech.

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Planar velocity fields of mixing-enhanced compressible planar shear layers are measured via particle image velocimetry (PIV) in order to investigate the mechanism of mixing enhancement by sub-boundary-layer triangular disturbances. The measurements are conducted at convective Mach numbers, $M_{{c}}$, of 0.62 and 0.24 to examine compressibility effects on effectiveness of the mixing enhancement technique. Instantaneous side- and plan-view vector maps of the shear layers are obtained, and turbulence statistical quantities are derived from the instantaneous velocity data. Schlieren and planar laser Mie scattering (PLMS) techniques are also used to measure the shear-layer thickness and growth rate as well as surveying the qualitative flow fields. The velocity fields for several disturbance configurations with different shape, size, or thickness are compared in terms of the shear-layer thickness and growth rate in order to investigate the effects of the configuration variation on the mixing enhancement strategy. Configuration parameters include thickness, the semi-vertex angle of the triangular disturbance, and the streamwise offset of the disturbance from the splitter tip. The measured transverse profile of the mean streamwise velocity shows a characteristic shape with triple inflection points for the effective mixing-enhanced cases at the two different compressibility conditions, while periodic inflection points are observed in the spanwise direction. A pair of stationary counter-rotating streamwise vortices introduced by the subboundary-layer disturbances are also observed, even in the fully developed region of the shear layers. At $M_{{c}}\,{=}\,0.62$, it is found that in successfully enhanced cases, regardless of the disturbance configurations, the present mixing-enhancement strategy has the effect of increasing the turbulence intensity and Reynolds stress, and suppressing the turbulence anisotropy increase with increasing compressibility, i.e. alleviating the compressibility effect which intrinsically reduces pressure–strain-rate redistribution, leading to effective mixing enhancement. Comparison of the results at the two compressibility conditions reveals that the growth rate of the layer is almost constant in the streamwise direction for all cases at $M_{{c}}\,{=}\,0.62$, while for all disturbed cases at $M_{{c}}\,{=}\,0.24$, after an initial layer thickening, growth rate decreases with downstream distance to the value for the undisturbed case, indicating that the present mixing enhancement is less effective at nearly incompressible conditions.

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Section: Velocity Fields In Mixing-enhanced Compressible Shear Layersmentioning

confidence: 99%

Velocity fields in mixing-enhanced compressible shear layers

Watanabe

Mungal

2005

J. Fluid Mech.

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“…The methods of generating such vortices include tabs (Bradbury & Khadem 1975;Zaman, Reeder & Samimy 1994) and winglet-type vortex generators (Fiebig 1996;Carletti & Rogers 1995) inserted into the flow field, lobed wall geometry in exit flows (Crouch, Cooughlin & Paynter 1977;Presz, Gousy & Morin 1986;Tillman, Patrick & Paterson 1991;Eckerle, Sheibani & Awad 1992;McCormick 1992;McCormick & Bennett 1994;Yu, Yeo & Teh 1995;Tsui & Wu 1996), ramp nozzles (Yu et al 1992), and swirl generators (Naughton & Settles 1992). Secondary flows and lobed fuel injectors are useful in combustion systems (Schadow et al 1989;Swithenbank et al 1989;Rao & Heiba 1990), particularly to control the sequence of mixing and combustion in order to lower pollutant levels from air-breathing aero-engines (Smith et al 1997;Strickland, Selerland & Karagozian 1998).…”

Section: Introductionmentioning

confidence: 99%

Mixing enhancement via the release of strongly nonlinear longitudinal Görtler vortices and their secondary instabilities into the mixing region

Girgis

Liu²

2002

J. Fluid Mech.

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Mixing enhancement in a mixing layer is considered in terms of a ‘vortex generator’ that uses fluid dynamically generated counter-rotating longitudinal vortices rather than explicit winglets or similar devices. This view is reached through considering the centrifugal instability of weak initial Görtler vortices on a slightly concave wall that are allowed to develop to their various nonlinear stages through selecting the cutoff lengths of the trailing edge prior to their release into the mixing region. These vortices are released from one side of the (say, upper) stream in the present work. The quantitative entrainment properties of the longitudinal vortices are studied to select an optimal trailing-edge cutoff for fixed upstream conditions. As the vortices develop along the wall, they are intensified because of the centrifugal instability mechanism and because of the work done by the Reynolds stress of the vortices against the local mean flow rate of strain; simultaneously, the region of strong streamwise vorticity moves away from the wall. This selection process is explained through a balance between the vorticity strength and proximity to the lower stream when the trailing edge is cut off: it is shown, therefore, that vortices of relatively modest strength and kinetic energy that are close to the interface separating the two streams provide mixing properties superior to stronger vortices located too far from the interface. Energy-balancing mechanisms and the stretching of the initial interface are studied, as are the effects of the velocity ratio and the spanwise wavelengths other than the fundamental. In order further to enhance mixing by exploiting the inherent secondary instability of primary steady longitudinal vortices, the most amplified secondary instability of the optimal-trailing-edge cutoff situation, which is the sinuous mode, is studied in detail in terms of the nonlinear development and modification of the steady vortical flow. Local energy-exchange mechanisms are studied, as are the mixing properties of the modified steady flow, which are shown to be significantly improved compared to the unmodified steady flow. Though the initiation of steady longitudinal vortices relies on centrifugal instability upstream, such vortices are able to develop self-sustaining and amplifying properties through the Reynolds stresses in the mixing region even without centrifugal instability reinforcement. The secondary instability is initiated and sustained entirely through its own three-dimensional Reynolds stress properties, which work against the three-dimensional rates of strain in the entire steady flow. This contrasts with initially generated potential-like vortices that decay downstream in the presence of dissipative mechanisms without the production mechanisms due to the Reynolds stresses.

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“…Ramped injectors Enhancement of mixing using a ramped injection con figuration can be obtained by incorporating vortex-generating tabs at the trailing edges of the ramp between the fuel nozzle and the surrounding air flow (Swithenbank et al 1989). Fuel injection from swept and unswept wall-mounted ramps exhibited increased mixing while maintaining the fuel jet direction nearly parallel to the flow (Northam et a1 1989(Northam et a1 , 1991Drummond et a1 1989;Davis & Hingst 1991).…”

Section: Axial Vorticitymentioning

confidence: 99%