Oscillations, flow pressure-drop

The pressure drop remains essentially constant as long as the liquid flow on tray remains steady during the period point A to point B on the diagram (the open balance point) [201]. At point B all valves are completely open off their seats, but are on the verge of closing and may be oscillating from open to closed. At point B the vapor velocity through the holes, opened balance point is ... 

Ozawa M, Akagawa K, Sakaguchi T, Tsukahara T, Fuji T (1979) Oscillatory flow instabilities in air-water two-phase flow systems. Report. Pressure drop oscillation. Bull JSME 22 1763-1770 Qu W, Yoon S-M, Mudawar 1 (2004) Two-phase flow and heat transfer in rectangular microchannels. J Electron Packag 126 288-300... 

The pressure spike introduces a disruption in the flow. Depending on the local conditions, the excess pressure inside the bubble may overcome the inertia of the incoming liquid and the pressure in the inlet manifold, and cause a reverse flow of varying intensity depending on the local conditions. There are two ways to reduce the flow instabilities reduce the local liquid superheat at the ONB and introduce a pressure drop element at the entrance of each channel, Kandlikar (2006). Kakac and Bon (2008) reported that density-wave oscillations were observed also in conventional size channels. Introduction of additional pressure drop at the inlet (small diameter orifices were employed for this purpose) stabilized the system. 

The flow patterns (expansion of the bubbly, slug and annular regions of flow) affect the local pressure drop, as well as the pressure oscillations in micro-channels (Kandlikar et al. 2001 Wu and Cheng 2003a,b, 2004 Qu and Mudawar 2003 Hetsroni et al. 2005 Lee and Mudawar 2005a). 

In the study by Hetsroni et al. (2006b) the test module was made from a squareshaped silicon substrate 15 x 15 mm, 530 pm thick, and utilized a Pyrex cover, 500 pm thick, which served as both an insulator and a transparent cover through which flow in the micro-channels could be observed. The Pyrex cover was anod-ically bonded to the silicon chip, in order to seal the channels. In the silicon substrate parallel micro-channels were etched, the cross-section of each channel was an isosceles triangle. The main parameters that affect the explosive boiling oscillations (EBO) in an individual channel of the heat sink such as hydraulic diameter, mass flux, and heat flux were studied. During EBO the pressure drop oscillations were always accompanied by wall temperature oscillations. The period of these oscillations was very short and the oscillation amplitude increased with an increase in heat input. This type of oscillation was found to occur at low vapor quality. 

The experimental investigations of boiling instability in parallel micro-channels have been carried out by simultaneous measurements of temporal variations of pressure drop, fluid and heater temperatures. The channel-to-channel interactions may affect pressure drop between the inlet and the outlet manifold as well as associated temperature of the fluid in the outlet manifold and heater temperature. Figure 6.37 illustrates this phenomenon for pressure drop in the heat sink that contains 13 micro-channels of d = 220 pm at mass flux G = 93.3kg/m s and heat flux q = 200kW/m. The temporal behavior of the pressure drop in the whole boiling system is shown in Fig. 6.37a. The considerable oscillations were caused by the flow pattern alternation, that is, by the liquid/two-phase alternating flow in the micro-channels. The pressure drop FFT is presented in Fig. 6.37b. Under... 

Aladyev et al. (1961) demonstrated that, with a compressible volume connected at the inlet of a test section, the flow oscillates and hence lowers the CHF. Flow fluctuation in the test section also depends on the compressibility of fluid upstream and on the pressure drop through the test section. Because the compressibility of water is approximately a function of temperature alone, the inlet temperature affects the boiling crisis. 

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

Compound dynamic instabilities as secondary phenomena. Pressure-drop oscillations are triggered by a static instability phenomenon. They occur in systems that have a compressible volume upsteam of, or within, the heated section. Maul-betsch and Griffith (1965, 1967), in their study of instabilities in subcooled boiling water, found that the instability was associated with operation on the negative-sloping portion of the pressure drop-versus-flow curve. Pressure drop oscillations were predicted by an analysis (discussed in the next section), but because of the... 

The increase of system pressure at a given power input reduces the void fraction and thus the two-phase flow friction and momentum pressure drops. These effects are similar to that of a decrease of power input or an increase of flow rate, and thus stabilize the system. The increase of pressure decreases the amplitude of the void response to disturbances. However, it does not affect the frequency of oscillation significantly. 

A particularly interesting feature of gas flow in a tube wetted by a wavy film is that the pressure drop for a given gas velocity is considerably larger than in the case of flow in a dry tube (M4), as shown very clearly by the data of Feind (F2). In an attempt to explain this effect, Laird (L3) investigated gas flows along tubes with flexible walls which performed sine wave oscillations. It was concluded that a large part of the increase in the... 

In recent years, a number of studies provided evidence to the sensitivity of two-phase micro-channel systems to flow instabilities. Oscillating pressure drops and wall temperatures, and visualizations showing cyclical backflow, were encountered in many experiments. Unfortunately, many of these data have been mingled in with stable data and thus create a confusing situation. 

Flow Maldistribution in Heat Exchangers with Phase Change. Two-phase flow maldistribution may be caused and/or influenced by phase separation, oscillating flows, variable pressure drops (density-wave instability), flow reversals, and other flow instabilities. For a review of pertinent literature, refer to Ref. 131. 

Plate efficiencies and HETP values are complex functions of measurable physical properties temperature, pressure, composition, density, viscosity, diflusivity, and surface tension measurable hydrodynamic factors pressure drop and liquid and vapor flow rates plus factors that cannot be predicted or measured accurately foaming tendency, liquid and gas turbulence, bubble and droplet sizes, flow oscillations, emulsification, contact time, froth formation, and others. Values for plate efficiency, HETP, or HTU, particularly those that purport to compare various devices, are usually taken over a limited range of concentration and liquid-to-vapor ratios. The crossovers in Fig. 2.5 and the rather strange behavior of the ethyl alcohol-water system, Fig. 2.6, demonstrate the critical need for test data under expected operating conditions. ... 

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). 

Outlet line This line is usually sized so that its cross-sectional area is at least the same as the total cross-sectional area of the reboiler tubes (253, 358, 360). Excessive pressure drop in this line promotes oscillations and elongates the preheat zone. Excessive velocity in this line may also be undesirable at the column inlet (see Sec. 4.1, guideline 5). On the other hand, if the outlet line rises vertically before bending toward the column, velocities should be kept above 15 ft/s (237), or slug flow may result. 

The system investigated by Teorell consists of two vessels containing electrolytes at different concentrations and in contact with each other via a sintered glass membrane (Figure 6). Normally, the liquid columns in the two vessels are different, so that there is a hydrostatic head drop across the membrane, p. In one of the vessels the surface area may be small, and then the passage of liquid from one vessel to another causes the pressure drop, p, to vary, as it does in the case of self-oscillations. Otherwise p remains fixed, and it will be assumed to be so below in our discussion of the excitation impulse. The flow of liquid through the membrane is caused not only by the head drop p, but also by the electroosmosis that can be initiated by applying a potential difference to the membrane with the help of an electrode pair. 

Flow Boiling Instability, Fig. 1 Top view of a severe pressure drop oscillation [2]... 

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