Oscillation flow-induced

Ruddick (1953) and Lowdermilk et al. (1958) found that flow oscillation can induce a premature boiling crisis. Moreover, in a boiling water reactor the flow oscillation may induce a nuclear instability. Thus, in designing a boiling system, it is imperative to predict and prevent those operational conditions that might create flow oscillation. 

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

Houcine et al. (64) used a non-intrusive laser-induced fluorescence method to study the mechanisms of mixing in a 20 dm CSTR with removable baffles, a conical bottom, a mechanical stirrer, and two incoming liquid jet streams. Under certain conditions, they observed an interaction between the flow induced by the stirrer and the incoming jets, which led to oscillations of the jet stream with a period of several seconds and corresponding switching of the recirculation flow between several metastable macroscopic patterns. These jet feedstream oscillations or intermittencies could strongly influence the kinetics of fast reactions, such as precipitation. The authors used dimensional analysis to demonstrate that the intermittence phenomenon would be less problematic in larger CSTRs. 

An active mixer based on an oscillating EOF induced by sinusoidal voltage ( 100 Hz, 100 V/mm) was devised and modeled for mixing of fluorescein with electrolyte solutions. This is termed as electrokinetic-instability micromixing, which is essentially a flow fluctuation phenomenon created by rapidly reversing the flow. Various microchips materials (PDMS, PMMA, and glass) and various electrolytes (borate, HEPES buffers) have been used to evaluate this method of micromixing [480]. 

If the oscillation of motion of fluid with respect to an electrode is imposed upon a stationary system and the perturbation is not small with respect to the steady-state then new effects can appear. For example, the flow induced by the motion can have both an oscillating and a steady component (the latter due to the interaction between viscous and inertial effects in the boundary layer) and the oscillating part may have components which are harmonics of the imposed oscillation. In some cases the stationary component of the induced flow can, indeed, dominate the oscillating component. These effects are particularly seen for vibrating electrodes. 

Blends of PI with PB were dynamically sheared at large amplitude (y = 0.8) and frequency CO = 0.63 and 6.3 rad/s [Matsuzaka et al., 1997]. After a temperature jump, the spinodal decomposition (SD) was in-situ observed at the lower frequency, but not at the higher. In the latter case, after stopping the oscillation, a modified SD pattern emerged. The authors postulated that the dynamic flow induced a structure in miscible system, quite different from that that exists in the non-sheared specimens. 

Reducing the fractional hole area (62, 108, 329, 425) This enhances the pressure drop and the dampening of oscillations. This technique has been powerful for overcoming flow-induced vibrations (62). 

Ultrasonic pumps are pumps that use acoustic streaming to create a fluid motion inside microchannels. Acoustic streaming is the time-averaged flow induced hy an ultrasonic wave (periodic pressure oscillation). Attenuation of the acoustic energy (via reflection and other distortion) generates a body force within the fluid and converts acoustic energy into kinetic energy of the fluid [1]. 

Donahue, S.W., Donahue, H.J., and Jacobs, C.R., Osteoblastic cells have refractory periods for fluid-flow-induced intracellular calcium oscillations for short bouts of flow and display multiple low-magnitude oscillations during long-term flow, /. Biomech., 36, 35, 2003. 

Bertram CD (2008) Flow-Induced Oscillation of Collapsed Tubes and Airway Structures. Respiratory Physiology Neurobiology 163 256-265. 

A second crosshead extruder was introduced to damp out the oscillating flow [86] induced by the oscillating screw (see Fig. 9.19). Gear pumps were also used. 

There have been few basic studies of flow in Buss Kokneters. Only in the 1990s do we have pubUcations of Elemans and Meijer [89] and Lyu and White [90 to 95] with which to seek to understand and model the flow mechanisms and the flow fields in the various machine elements. Certain things are clear, such as the fluid mechanics indicates the axially oscillating screw inducing an oscillatory output. The machine operates under starved conditions with alternating fully filled and starved sections. The secondary extruder into which compounds exit from the Kokneter is starved and the length of fill oscillates but the output is uniform. This would feed a palletizing die. 

It is interesting, however, that the instability of a system without activator may take place even if neither of its subsystems is a damped oscillator all the steady states are stable nodes. We argue that the mechanism of the instability is still of a resonant type. The heuristic arguments are as follows. When a system of at least three variables is split by a differential flow, one of the subsystems involves at least two variables. We assume that the steady state of this subsystem is a stable node. The response of such a system to a perturbation is a linear combination of at least two exponential functions [i.e. exp(—Aif) + 0 exp(—A2f) (-1-...) Aj > 0]. Although this response asymptotically decays, for certain a it may initially grow. Then its Fourier spectrum will have a maximum at a finite frequency. Therefore, the subsystem is most sensitive to a perturbation at this frequency. This can be interpreted as a resonance (although with a small quality factor). The instability is thus caused by the resonance which is induced by the differential flow. For this reason we call the instability of a system without an activator differential flow induced resonance instability . It becomes clear now why only modes with wavenumbers within a finite range [Equation (41)] may be unstable all other modes are out of resonance . 

It is supposed that the director is strongly anchored parallel to the bounding plates and that the lower plate is fixed while the upper plate is subject to sinusoidal oscillations parallel to the initial alignment, as indicated in Fig. 5.7. As pointed out by Leslie [169, 171, 174], in a preliminary investigation it appears reasonable to suppose that both the surface and inertial effects are of secondary importance and concentrate on the influence of the oscillatory flow induced by the oscillating upper plate. This is equivalent to supposing that the associated Reynolds number for this problem, defined by... 

Flow instabilities are undesirable in boiling, condensing, and other two-phase flow processes for several reasons. Sustained flow oscillations may cause forced mechanical vibration of components or system control problems. Flow oscillations affect the local heat transfer characteristics and may induce boiling crisis (see Sec. 5.4.8). Flow stability becomes of particular importance in water-cooled and watermoderated nuclear reactors and steam generators. It can disturb control systems, or cause mechanical damage. Thus, the designer of such equipment must be able to predict the threshold of flow instability in order to design around it or compensate for it. 

Saha, P., M. Ishii, and N. Zuber, 1976, An Experimental Investigation of the Thermally Induced Flow Oscillations in Two-Phase Systems, Trans. ASME, J. Heat Transfer 95 616-622. (6)... 

Ultrasonic atomization is sometimes also termed capillary-wave atomization. In its most common form, 142 a thin film of a molten metal is atomized by the vibrations of the surface on which it flows. Standing waves are induced in the thin film by an oscillator that vibrates vertically to the film surface at ultrasonic frequencies. The liquid metal film is broken up at the antinodes along the surface into fine droplets once the amplitude of the capillary wave exceeds a certain value. The most-frequent diameter of the droplets generated is approximately one fourth of the wavelength of the capillary wave,1 421 and thus decreases with increasing frequency. 

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