Recirculation vortex

Figure 2. Radial-axial velocity field and temperature contours for a rotating-disk reactor at an operating condition where a buoyancy-driven recirculation vortex has developed. The disk temperature is HOOK, the Reynolds number is 1000, Gr/Re / = 6.2, fo/f = 1.28, and L/f = 2.16. The disk radius is 4.9 cm, the spin rate is 495 rpm. The maximum axial velocity is 55.3 cm/sec. The gas is helium.

In down-wash configuration, the flame is established in the wake of the burner tube. A recirculation vortex in the wake of a burner tube appears as a flame sheet. When R is further reduced, the flame tip is severely deflected by the crossflow. A small recirculation bubble was observed by Huang and Chang [16] atR = 0.04. For a value of R between 0.1 and 1, the impact of a cross-flow stream is dominant. An axisymmetric tail flame forms downstream of the recirculation vortex and the flame widens. This structure is characterized by several features such as flickering and bifurcation. In jet-dominated mode, the recirculation vortex disappears and only the tail part remains attached to the burner. The transition from crossflow-dominated to jet-dominated conditions occurs from 1 = 1 to 3. For R>3, the effect of crossflow becomes negligible the jet fluid mechanics dictate the flame characteristics. For R > 10, the flame detaches from the burner and stabilizes above the exit plane of the burner tip. Depending upon the jet exit velocity and burner diameter, the flame is either attached to the burner tip or stabilizes as a lifted flame until it blows out. 

Botros and Brzustowski [77] studied the velocity field of propane TDFCF experimentally and numerically. Their study revealed a pair of counter-rotating vortices in the flame. Gollahalli and coworkers [78-80] measured the flow field and turbulent characteristics of gas jet flames in crossflow. At very low values of R, the effects of crossflow stream are more dominant and the jet fluid burns in the wake of a model stack. A recirculation vortex is created and the flame stabilizes on the wall of the recirculation bubble. Figure 29.17a presents the velocity vector field and streamlines obtained by Huang and Wang [45] for down-wash... 

Song, Papanikolaou, and Mohamad [12] studied the stability of natural gas flames issued from circular and elliptic burners with different aspect ratios. When the minor axis was aligned with crossflow (Emin configuration of Pardiwalla [85]), the flare stack was able to withstand larger velocities before it blew out, as also observed by Pardiwalla [85]. In Emm configuration, the crossflow acts on a larger frontal area, which leads to the formation of a longer recirculation vortex. As discussed in Section... 

Figure 2.12 Capillary entry flow pattern for a branched polymer showing the flow cone and the recirculating vortex.

The high rates of NO, formation in hot zones surrounding the head vortex can be offset by introducing a diluent in the form of recirculated exhaust gas. 

Powders are dissolved either directly in the main mix tank or premix tank or indirectly using a vortex-type mixer (Figure 8.1) where powder is dropped into the vortex of a horizontally mounted pump head recirculating the fluid from and to the batch tank. Some specialised versions of this mixer can handle very viscous blending applications (50,000 cP or more). 

The examples presented in this chapter [308 320] are illustrations of the concepts presented in the previous chapters. They correspond to recent numerical analysis of burners which are typical of most modern high-power combustion chambers, especially of gas turbines the flame is stabilized by strongly swirled flows, the Reynolds numbers are large, the flow field sensitivity to boundary conditions is high, intense acoustic/combustion coupling can lead to self-sustained oscillations. Flames are stabilized by swirl. Swirl also creates specific flow patterns (a Central Toroidal Recirculation Zone called CTRZ) and instabilities (the Precessing Vortex Core called PVC). 

Limitations on temperatures of solid materials often cause the methods of stabilization by solid elements, discussed so far, to be impractical. In most applications of stabilization by solid elements the flame is attached in the wake behind the element, so that the solid is not fully exposed to the flame temperature. Representative examples are bluff-body flame stabilizers, such as the stabilizing rods or plates placed normal to the flow in ramjets and afterburners, which were mentioned in Sections 5.1.1 and 10.3.5. A distinctive feature of bluff-body flame stabilization is the presence of a recirculation zone behind the body. Unlike the alternate vortices shed from bluff bodies in cold flow over the Reynolds-number range of practical interest, a well-defined vortex, steady in the mean, is observed to exist just downstream from the stabilizer when combustion occurs. This is a toroidal vortex for an axisymmetric stabilizer or a pair of identical counterrotating line vortices for rodlike stabilizers. The reason for the drastic change in the... 

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