Pressure drop oscillation

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... 

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. 

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

Pressure drop oscillations (Maulbetsch and Griffith, 1965) is the name given the instability mode in which Ledinegg-type stability and a compressible volume in the boiling system interact to produce a fairly low-frequency (0.1 Hz) oscillation. Although this instability is normally not a problem in modern BWRs, care frequently must be exercised to avoid its occurrence in natural-circulation loops or in downflow channels. 

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... 

Figure 6.4 Density wave and pressure drop oscillation. (From Stenning and Verizoglu, 1965. Reprinted with permission of Stanford University Press, Stanford, CA.)...

Computer codes Because of the computer s ability to handle the complicated mathematics, most of the compounded and feedback effects are built into computer codes for analyzing dynamic instabilities. Most of these codes can analyze one or more of the following instabilities density wave instability, compound dynamic instabilities such as BWR instability and parallel-channel instability, and pressure drop oscillations. 

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

Dynamic Pressure drop oscillations Very low-frequency periodical process [2,7]... 

Dynamic instabilities are driven by the inertia of the system the stationary state is not sufficient to predict the destabilization threshold of theses instabilities such as acoustic waves, pressure drop oscillations, or thermal oscillations. 

Qu et al. [2] observed in their experiment that hydrodynamic instabilities influence inlet and outlet pressures and can induce a degree an uncertainty in the measurement of pressiu-e drop. They recorded temporal pressure signals and made the following observations even with a small heat flux supplied to the parallel channels, the case of pressure drop oscillations presents pressure fluctuations with quite constant frequency, whereas in the case of instability in the parallel chaimels, the fluctuaticHis are small and random. 

Hetsroni et al. [3] also found evidence of a coupling phenomenon for an array of 17 parallel microchannels. In Fig. 7 from [3], they found evidence of two-phase flow oscillations. Only one channel is followed as a function of time. The water-steam flow is from the left to the right. Steam appears in the 5th picture. The liquid-vapor interface then moves to the exit or to the entrance. This interface movement is representative of a non-constant mass flow provided to the microchannel. The inlet condition before the plenum is a constant inlet pressure however, due to the plenum, the flow can come back and induce such coupling. The frequency of the interface oscillation can usually be related to the total pressure drop oscillation frequency. 

Characteristic pressure drop vs. flow rate instabilities Ledinegg instability Flow distribution instabrbty Flow pattern transition Pressure Drop Oscillation (PDO)... 

Manavela Chiapero et al. (2012, 2013a,b) have reviewed pressure drop oscillations in boiling systems. The exhaustive nature of the research at the Norwegian University of Science and Technology cannot be overemphasized. Reviews of instabilities in the case of single and parallel channels include Ozawa et al. (1989), Tadrist (2007), and Kakac and Bon (2008). The latter is especially complete. March-Leuba and Rey (1993) presented a state-of-the-art review for the case of coupled thermo-hydraulic-neutronic instabilities in BWRs. 

Guo, L.-J., Feng, Z.-P., Chen, X.-J., 2001. Pressure drop oscillation of steam-water two-phase flow in a helically coiled tube. International Journal of Heat and Mass Transfer 44, 1555—1564. 

Manavela Chiapero, M., Femandino, M., Dorao, C.A., 2012. Review on pressure drop oscillations in boiling systems. Nuclear Engineering and Design 250, 436—447. 

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