Recirculation zones

The length of the recirculation zone, x, for a linear jet (the distance to the point of jet attachment to the surface) was studied by Sawyer,Miller and Comings, and Bourque and Newman.The results of these studies, summarized in Awbi, show that the length of the recirculation zone (Fig. 7.31) is proportional to the distance from the outlet to the surface and cait be described as... 

For W/H < 10, the length of this recirculating zone is reduced. The values of L, and can be calculated using the reduction coefficient C, from Table 7.30. 

The study shows, however, that the air curtain in the original design is bent off the wall due to the presence of the open oven with induced airflow inside the oven. This unexpected flow feature finally leads to a recirculation zone below the oven, and dust particles on the floor can be carried into the oven (class B quality). This effect was then also confirmed by smoke experiments in the real room and the existing ventilation system. 

Walter et al. studied the flow distribution in simple multichannel geometries by means of the finite-element method [112]. In order to reduce the computational effort, a 2-D model was set up to mimic the 3-D multichannel geometry. Even at a comparatively small Reynolds number of 30 they found recirculation zones in the flow distribution chamber and corresponding deviations from the mean flow rate inside the channels of about 20%. They also investigated the influence of contact time variation on a simple two-step reaction. 

Also a simulation of the flow field in the methanol-reforming reactor of Figure 2.21 by means of the finite-volume method shows that recirculation zones are formed in the flow distribution chamber (see Figure 2.22). One of the goals of the work focused on the development of a micro reformer was to design the flow manifold in such a way that the volume flows in the different reaction channels are approximately the same [113]. In spite of the recirculation zones found, for the chosen design a flow variation of about 2% between different channels was predicted from the CFD simulations. In the application under study a washcoat cata-... 

Compared with the use of arbitrary grid interfaces in combination with reduced-order flow models, the porous medium approach allows one to deal with an even larger multitude of micro channels. Furthermore, for comparatively simple geometries with only a limited number of channels, it represents a simple way to provide qualitative estimates of the flow distribution. However, as a coarse-grained description it does not reach the level of accuracy as reduced-order models. Compared with the macromodel approach as propagated by Commenge et al, the porous medium approach has a broader scope of applicability and can also be applied when recirculation zones appear in the flow distribution chamber. However, the macromodel approach is computationally less expensive and can ideally be used for optimization studies. 

The simulations of fluid flow and heat transfer in such microstructured geometries were carried out with an FVM solver. Air with an inlet temperature of 100 °C was considered as a fluid, and the channel walls were modeled as isothermal with a temperature of 0 °C. The streamline pattern is characterized by recirculation zones which develop behind the fins at comparatively high Reynolds numbers. The results of the heat transfer simulations are summarized in Figure 2.34, which shows the Nusselt number as a fimction of Reynolds number. For... 

Figure 2.40 Zigzag micro mixer with concentration field (left) and flow stream lines (right) obtained from a CFD simulation for a Reynolds number of 38. In [135] a sawtooth geometry of larger amplitude was considered and distinctive recirculation zones were found only at Reynolds numbers larger than 80.

We consider a point source axisymmetric example as illustrated in Figure 10.20. We shall examine the rise of the plume, zv, as a function of time. As the hot gases rise due to the source Q, initiated at time t = 0, the gases at the front or cap encounter cooler ambient air. The hot gases in the cap, impeded by the air, form a recirculating zone, as illustrated in Figure 10.20. Entrainment of air occurs over the vertical plume column and the cap. The warmed, entrained air forms the gases in the plume. 

As shown in Fig. 5.20, such regions normally occur only near the inlet zones where micromixing is poor. Further downstream, interaction between flamelets will become significant, and the assumptions on which the flamelet model is based will no longer apply.117 Reactors with recirculation zones are also problematic for flamelet models. For these reactors, partially reacted fluid is brought back to mix with the feed streams so that the simple non-premixed flow model no longer applies. 

FIGURE 4.54 Recirculation zone and flame-spreading region for a fully developed turbulent wake behind a bluff body (after Williams [57]). 

In either case, bluff body or aerodynamic, blowout is the primary concern. In ramjets, the smallest frontal dimension for the highest flow velocity to be used is desirable in turbojets, it is the smallest volume of the primary recirculation zone that is of concern and in dump combustors, it is the least severe step. 

Zukoski and Marble [70, 71] held that the wake of a flame holder establishes a critical ignition time. Their experiments, as indicated earlier, established that the length of the recirculating zone was determined by the characteristic dimension of the stabilizer. At the blowoff condition, they assumed that the free-stream combustible mixture flowing past the stabilizer had a contact time equal to the ignition time associated with the mixture that is, rw = ri( where rw is the flow contact time with the wake and r, is the ignition time. Since the flow contact time is given by... 

The ignition time is a function of the combustion (recirculating) zone temperature, which, in turn, is a function of the air-fuel ratio (A/F). Thus,... 

Theoretical studies are primarily concentrated on the treatment of flame blow-off phenomenon and the prediction of flame spreading rates. Dunskii [12] is apparently the first to put forward the phenomenological theory of flame stabilization. The theory is based on the characteristic residence and combustion times in adjoining elementary volumes of fresh mixture and combustion products in the recirculation zone. It was shown in [13] that the criteria of [1, 2, 5] reduce to Dunskii s criterion. Longwell et al. [14] suggested the theory of bluff-body stabilized flames assuming that the recirculation zone in the wake of the baffle is so intensely mixed that it becomes homogeneous. The combustion is described by a second-order rate equation for the reaction of fuel and air. 

So far, the flow patterns around bluff bodies in combustible flows are not understood completely. However, a recirculation zone in the immediate wake of the stabilizer which takes the form of a pair of eddies, similar to isothermal flows, is known to exist. The length Lrz of the recirculation zone differs for 2D and axisymmetric bluff bodies. For 2D bodies (V-gutters, rods, prisms), the measured values of Trz/H range from 3 to 6 depending on the operating conditions of combustor [11], which is considerably larger than for isothermal flows, where Lrz/H 2 [11]. For axisymmetric bluff bodies (discs, cones, cylinders), at low-blockage ratio Lrz/H 2 [32], which is similar to isothermal flows [32, 33], or Lrz/H 2.b-A [34], or even Lrz/H 10-11 [35]. 

As mentioned in section 12.1, Dunskii [12] was the first who put forward the phenomenological theory of flame stabilization. The theory is based on the characteristic residence time, L, and combustion time, tc, in adjoining elementary volumes of fresh mixture and combustion products in the recirculation zone behind the bluff body. Dunskii s condition for flame blow-off is U/tc = Mi, where Mi is the Mikhelson number close to unity (for example, for cone flame holder the measurements give Mi = 0.45 [36]). Residence time L is taken proportional to the flame holder size, H, and inversely proportional to the approach flow velocity, U, i.e., L = H/U. Combustion time is estimated as tc = at/Si, where... 

The results of Fig. 19.8 for a swirl number of 1.35 show that the attenuation increased to 10 dB with the velocity of the axial jet up to 42 m/s, and further increase to 47 m/s caused the amplitude to fall from around 6 kPa to less than 1.5 kPa and the attenuation to decrease from 10 dB to almost zero. Similar results were observed with the swirl number of 0.6 the attenuation improved with axial jet velocity up to 60 m/s, after which the amplitude and attenuation decreased. The decline in the amplitude of oscillation and its attenuation by active control was due to the interaction between the axial jet with a large velocity and the central recirculation zone, which caused the flame to move further downstream of the swirler and heat release to occur further from the pressure antinode. The consequent increase in the distance between the point of entry of the oscillated fuel and the active burning zone reduced the effectiveness of the oscillated input due to increased fluid dynamic damping and development of a large difference in phase between different parts of the oscillated flow, especially with swirl surrounding the oscillated axial jet. 

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