Oscillations, flow

Density-wave oscillations Pressure drop oscillations Flow regime-induced instability... 

An oscillated fuel flow was provided in the form of a central jet within the duct carrying the pilot stream. The dimensions of the tube carrying the oscillated flow implied that the mean velocity and equivalence ratio of the jet had to be larger than that of the pilot stream to enable the oscillation of at least 5% of the total fuel flow. An examination of the influence of the bulk mean velocities of the pilot stream and the central jet on the amplitude of oscillations in this flow arrangement showed that, for the present range of flow conditions, values of the bulk mean velocity of the pilot stream less than that of the annular flow had no effect on the amplitude of oscillations, although larger values led to a decrease in amplitude [20]. The amplitude was also insensitive to the bulk mean velocity of the oscillated jet for values up to 3.5 times... 

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. 

Figure 19.11 shows that, for an overall equivalence ratio of 0.73 and a swirl number of 0.6, the amplitude of oscillation increased with the proportion of oscillated fuel due to the unpremixedness caused by the higher value of mean fuel concentration in the oscillated flow. The attenuation also increased with the fuel flow to around 5.5 dB with oscillation of around 10% of the fuel, compared with around 7 dB by the oscillation of 7% of the fuel in the axial jet and the same overall equivalence ratio and swirl number. As expected, control was less effective with higher swirl numbers due to the greater dissipation of the oscillated input. 

Condensation Inside Horizontal Tubes. This mode is employed chiefly in air coolers where it is the only feasible mode. As condensation proceeds, liquid tends to build up in the tubes, then slugging and oscillating flow can occur. 

Chow, C.W.K., Kolev, S.D., Davey, D.E. and Mulcahy, D.E. (1996) Determination of copper in natural waters by batch and oscillating flow injection stripping potentiometry. Anal. Chim. Acta, 330, 79-87. 

An account of the behavior of acoustic wave propagation in a gas-solid suspension or particle movement in a turbulent eddy requires comprehensive knowledge of the dynamics of particle motion in an oscillating flow field. This oscillating flow can be analyzed in terms of one-dimensional simple harmonic oscillation represented by... 

Assume that the only force imposed on the particle in the oscillating flow field is the Stokes drag. Derive expressions for the particle velocity and the phase lag between the particle velocity and the gas velocity for the condition when the particle is placed in the flow field for sufficiently long time (i.e., t rs). 

Consider a gas-solid suspension which is in a state of steady dilute flow with no interparticle collision or contact. In this situation, the linear particle velocity is practically identical to the superficial particle velocity. The motion of a spherical particle in an oscillating flow field can thus be given by... 

The fluid oscillations can be detected by shuttle balls, ultrasonic and piezoelectric detectors. In noisy installations, dual piezoelectric elements provide noise compensation. The various detectors can measure one of the following (1) the oscillating flow across the face of the bluff body, (2) the oscillating pressure difference across the sides of the bluff body, (3) the flow through a passage drilled through the bluff body, (4) the oscillating flow or pressure at the rear of the bluff body, or (5) the presence of free vortices downstream to the bluff body. 

Subtle effects arise when the surface layer is modulated by the oscillating flow [116]. For example in response to the flow, the layer thickness may change or even deform. In the high-rate dissolution of copper in HC1 [92], a salt film of CuCl is deposited onto the surface. The film is formed at the electrode-film interface, limited by transport of Cl-, and dissolved at the same rate at the film-solution interface, limited by transport of CuClJ. Because the film thickness varies as 1/2, the presence of a film was only... 

Fig. 1. Typical flow curve of commercial LPE. There are five characteristic flow regimes (i) Newtonian (ii) shear thinning (iii) sharkskin (iv) flow discontinuity or stick-slip transition in controlled stress, and oscillating flow in controlled rate (v) slip flow. There are three leading types of extrudate distortion (a) sharkskin like, (b) alternating bamboo like in the shaded region, and (c) spiral like on the slip branch. Industrial extrusion of polyethylenes is most concerned with flow instabilities occurring in regimes (iii) to (v) where the three kinds of extrudate distortion must be dealt with. The unit shows the approximate levels of stress where the sharkskin and flow discontinuity occur respectively. There is appreciable molecular weight and temperature dependence of the critical stress for the discontinuity. Other highly entangled melts such as 1,4 polybutadienes also exhibit most of the features illustrated herein...

Many polymers exhibit neither a measurable stick-slip transition nor flow oscillation. For example, commercial polystyrene (PS), polypropylene (PP), and low density polyethylene (LDPE) usually do not undergo a flow discontinuity transition nor oscillating flow. This does not mean that their extrudate would remain smooth. The often observed spiral-like extrudate distortion of PS, LDPE and PP, among other polymer melts, normally arises from a secondary (vortex) flow in the barrel due to a sharp die entry and is unrelated to interfacial slip. Section 11 discusses this type of extrudate distortion in some detail. Here we focus on the question of why polymers such as PS often do not exhibit interfacial flow instabilities and flow discontinuity. The answer is contained in the celebrated formula Eqs. (3) or (5). For a polymer to show an observable wall slip on a length scale of 1 mm requires a viscosity ratio q/q equal to 105 or larger. In other words, there should be a sufficient level of bulk chain entanglement at the critical stress for an interfacial breakdown (i.e., disentanglement transition between adsorbed and unbound chains). The above-mentioned commercial polymers do not meet this criterion. 

Having unraveled the specific characteristics of the stick-slip transition, it is rather straightforward to describe the physical origin of the oscillating flow ob-... 

In summary, the origin of the oscillating flow (sometimes termed slip-stick regime) observed in the constant piston speed mode is the oscillation of the HBC between the no-slip and slip states due to a reversible coil-stretch transition of either adsorbed chains or the first layer of unbound chains entrapped with the adsorbed chains. The experimental demonstration of an abrupt large stick-slip... 

We have surveyed the most recent progress and presented a new molecular level understanding of melt flow instabilities and wall slip. This article can at best be regarded as a partial review because it advocates the molecular pictures emerging from our own work over the past few years [27-29,57,62,69]. Several results from many previous and current workers have been discussed to help illustrate, formulate and verify our own viewpoints. In our opinion, the emerging explicit molecular mechanisms have for the first time provided a unified and satisfactory understanding of the two major classes of interfacial melt flow instability phenomena (a) sharkskin-like extrudate distortion and (b) stick-slip (flow discontinuity) transition and oscillating flow. 

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