Induced flow instabilities

Ishii, M., and N. Zuber, 1970, Thermally Induced Flow Instability in Two-Phase Mixtures, 4th Int. Heat Transfer Conf., Paris. (6)... 

Fig. 12.14. Methods for inducing flow instabilities (a) placing objects (protuberances, baffles) onto the membrane surface to form a rough surface, (b) placing objects into the flow channel away from...

In simulations for engineering problems involving turbulent combustion, a RANS description of the flow [1] and simplistic combustion models (e.g., [2]) are typically combined. This involves simulating only the mean flow-field features and modeling the effects of the entire range of turbulent scales. The restricted information provided by this approach, regarding the fluid dynamics, combustion, and their different interactions, precludes adequate prediction of the important phenomena required to achieve effective control of the combustion processes, such as combustion-induced flow instabilities, cycle-to-cycle variations, and combustion oscillations associated with unsteady vortex dynamics. 

W R Krigbaum and H J Lader. Electric field-induced flow instabilities in low molecular weight and polymeric nematics. Mol. 

Otherwise called pulses, induced flow instabilities at relatively low frequencies (<5 Hz) have been shown to improve heat transfer and have a positive effect on fouling. An analogy can be seen in pulse combustion systems, where the effect is to break down steady-state conditions and to create instantaneous changes in the fluid velocity and direction. The effect on fouling has been likened to coughing . 

Figure A3.14.15. The difTerential flow-induced chemical instability (DIFICI) in the BZ reaction. (Reprinted with pennission from [44], C5 The American Physical Society.)...

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. 

Another explanation of the lithium gap in the Hyades could be found in terms of turbulent diffusion and nuclear destruction. Turbulence is definitely needed to explain the lithium abundance decrease in G stars. If this turbulence is due to the shear flow instability induced by meridional circulation (Baglin, Morel, Schatzman 1985, Zahn 1983), turbulence should also occur in F stars, which rotate more rapidly than G stars. Fig. 2 shows a comparison between the turbulent diffusion coefficient needed for lithium nuclear destruction and the one induced by turbulence. Li should indeed be destroyed in F stars This effect gives an alternative scenario to account for the Li gap in the Hyades. The fact that Li is normal in the hottest observed F stars could be due to their slow rotation. 

We start with the ground state (°), fi(° defined by the simple shear flow y(°), Fig. 17. The principal effect is, as expected, the appearance of a small tilt of the director from the layer normal (flow alignment), predominantly in z direction (Fig. 18). Note that the configuration of layers is also modified by the shear (Figs. 19 and 20), i.e., the cylindrical symmetry is lost. This is analogous to the shear-flow-induced undulation instability of planar layers (wave vector of undulations in the... 

Any action in mitigating flow maldistribution must be preceded by an identification of possible reasons that may cause the performance deterioration and/or may affect mechanical characteristics of the heat exchanger. The possible reasons that affect the performance are [131,147] (1) deterioration in the heat exchanger effectiveness and pressure drop characteristics, (2) fluid freezing, as in viscous flow coolers, (3) fluid deterioration, (4) enhanced fouling, and (5) mechanical and tube vibration problems (flow-induced vibrations as a consequence of flow instabilities, wear, fretting, erosion, corrosion, and mechanical failure). 

Mixing in tnicrochannels in another major application where the transition to turbulent flows can be utilized for enhanced mixing possible in turbulent flows. The roughness elements then act to destabilize the flow and induce local instabilities. The localized transition to turbulence will enhance the mixing, and relaminarization process downstream will provide the laminar flow with its lower friction factor and associated lower pressure drop penalties. 

Viscoelastic micro-mixer elastically induced chaotic flow instabilities through contraction/ expansion geometrical configuration in a microchannel. 

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