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Journal ArticleDOI

Experimental study of air–water countercurrent annular flow under post-flooding conditions

01 Jan 2002-International Journal of Multiphase Flow (Pergamon)-Vol. 28, Iss: 1, pp 51-67

AbstractExperiments have been conducted on countercurrent flow of air and water in tubes of 25, 67 and 99 mm diameter. Measurements of the pressure gradient, film thickness and down flow rate were made for a range of air and water flow rates under pre- and post-flooding conditions. The data showed that the pressure gradient did not increase appreciably until just before the onset of flooding. The mean film thickness under pre-flooding conditions was also predicted well by the falling film correlations. However, both quantities showed significant deviations from this pattern under post-flooding conditions. Existing correlations for predicting the pressure drop under post-flooding conditions gave poor prediction of the present data as well of the data from the literature. New correlations are presented for the pressure gradient and the mean film thickness under post-flooding conditions.

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Citations
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Journal ArticleDOI
Abstract: Results are reported of an experimental investigation of gas–liquid counter-current flow in a vertical rectangular channel with 10 mm gap, at rather short distances from liquid entry. Flooding experiments are carried out using air and various liquids (i.e., water, 1.5% and 2.5% aqueous butanol solutions) at liquid Reynolds numbers ReL < 350. Visual observations and fast recordings suggest that the onset of flooding at low ReL (<250) is associated with liquid entrainment from isolated waves, whereas ‘‘local bridging’’ is dominant at the higher ReL examined in this study. Significant reduction of flooding velocities is observed with decreasing interfacial tension, as expected. Instantaneous film thickness measurements show that under conditions approaching flooding, a sharp increase of the mean film thickness, of mean wave amplitude and of the corresponding RMS values takes place. Film thickness power spectra provide evidence that by increasing gas flow the wave structure is significantly affected; e.g., the dominant wave frequency is drastically reduced. These data are complemented by similar statistical information from instantaneous wall shear stress measurements made with an electrochemical technique. Power spectra of film thickness and of shear stress display similarities indicative of the strong effect of waves on wall stress; additional evidence of the drastic changes in the liquid flow field near the wall due to the imposed gas flow, even at conditions below flooding, is provided by the RMS values of the wall stress. A simple model is presented for predicting the mean film thickness and mean wall shear stress under counter-current gas–liquid flow, below critical flooding velocities.

65 citations

Journal ArticleDOI
Abstract: The liquid film characteristics at the onset of flooding in an inclined pipe (16 mm i.d. and 2.2 m in length) have been investigated experimentally. A constant electric current method and visual observation were utilized to elucidate the flow mechanisms at the onset of flooding. Two mechanisms are clarified to control the flooding in lower flooding and upper flooding, respectively. The lower flooding occurred at lower liquid flow rate and high pipe inclination angle. In this mechanism, the liquid film does not block the pipe cross-section. On the other hand, the upper flooding occurred at higher liquid flow rate and low pipe inclination angle. In this case, blocking of the pipe cross-section by large wave and entrainment plays an important role. The experimental data indicated that there was no reversal motion of liquid film at the onset of flooding during the operation of both lower flooding and upper flooding. The effects of pipe inclination angle on the onset of flooding are also discussed.

27 citations

Journal ArticleDOI
Abstract: The purpose of the present study is to investigate the effects of surface tension on flooding phenomena in counter-current two-phase flow in an inclined tube. Previous studies by other researchers have shown that surface tension has a stabilizing effect on the falling liquid film under certain conditions and a destabilizing or unclear trend under other conditions. Experimental results are reported herein for air–water systems in which a surfactant has been added to vary the liquid surface tension without altering other liquid properties. The flooding section is a tube of 16 mm in inner diameter and 1.1 m length, inclined at 30–60° from horizontal. The flooding mechanisms were observed by using two high-speed video cameras and by measuring the time variation of liquid hold-up along the test tube. The results show that effects of surface tension are significant. The gas velocity needed to induce flooding is lower for a lower surface tension. There was no upward motion of the air–water interfacial waves upon flooding occurrence, even for lower a surface tension. Observations on the liquid film behavior after flooding occurred suggest that the entrainment of liquid droplets plays an important role in the upward transport of liquid. Finally, an empirical correlation for flooding velocities is proposed that includes functional dependencies on surface tension and tube inclination.

24 citations

Journal ArticleDOI
Abstract: A model has been derived for interfacial wave propagation for a liquid film on the wall of a vertical pipe and for a flowing gas in the central core. An analytical study is presented for the stability of a flat interface, and for traveling waves on the interface. Long wave theory is applied to both phases and the resulting conservation equations are of the same form as a two-fluid model. Two situations are examined: the interface between a Taylor bubble and the liquid film, where the gas velocity is small, and the interface for cocurrent annular flow where the gas velocity is relatively large. The interface between a Taylor bubble and a liquid film was found to be dominated by waves, which can be destabilized by the inertia of the liquid phase. For annular flow the interface is subject to a Kelvin–Helmholtz instability. When the gas flow rate is small, and surface tension is negligible, the traveling wave has a shape similar to that of a Taylor bubble except near the tip and trailing edge. When surface tension is dominant, the solution is a soliton. This region and the receding part of the soliton appears to be related to the ripple waves seen near the trailing edge of Taylor bubbles.

13 citations

Journal ArticleDOI
Abstract: This paper aims to characterise the complex flow behaviour of counter-current gas-liquid flows in concentric vertical annuli over a wide range of gas and liquid flowrates. The experiments were performed with air and water in two different annulus sizes: (a) a 100 mm hydraulic diameter annulus with a 170 mm diameter outer pipe and a 70 mm diameter inner pipe; and (b) a 19 mm hydraulic diameter annulus between 44 mm and 25 mm pipes, to investigate the effect of flow geometry on flow structure. Flow regimes were identified quantitatively by applying fast Fourier transform (FFT) on the associated pressure fluctuation signals, collected at 10 Hz and 100 Hz frequency. Video images captured at 4,000 fps with a high-speed camera were used in a visual analysis of the flow regimes to verify the FFT results. Furthermore, flow regime transitions and their underlying mechanisms were determined by their associated pressure gradient and void fractions, as well as analysis of temporal pressure signals. The commonly described slug flow regime, consisting of stable Taylor bubbles traversing the length of the channel, was not observed in the larger annulus. However, the FFT of the pressure signals indicate that unstable Taylor bubbles form as a result of bubble coalescence but collapse due to instability at the gas-liquid interface, known as Rayleigh-Taylor instability. Therefore, the apparent slug-churn flow regime was classified as a highly turbulent heterogeneous flow, developing at superficial gas velocities from 0.265 to 3.968 m/s and superficial liquid velocities from 0.004 to 0.147 m/s. Interestingly, annular flow regime did not develop in either of the tested small and large annuli. The onsets of counter-current flow limitations, or flooding, were identified in the 100 mm hydraulic diameter annulus with gas flooding due to very high gas flow rates and liquid flooding due to very high liquid flow rates. The mechanism that initiates gas flooding was observed to be the formation of large waves flowing upward near the water inlet point, with counter-current flow observed below the water inlet point at the onset of gas flooding. Clarity was also provided on the concepts of flooding and zero liquid penetration for the cases of flow in a water filled channel and a falling film, and a new empirical correlation for the onset of flooding was developed.

10 citations


References
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Book
01 Jan 1955
Abstract: The flow laws of the actual flows at high Reynolds numbers differ considerably from those of the laminar flows treated in the preceding part. These actual flows show a special characteristic, denoted as turbulence. The character of a turbulent flow is most easily understood the case of the pipe flow. Consider the flow through a straight pipe of circular cross section and with a smooth wall. For laminar flow each fluid particle moves with uniform velocity along a rectilinear path. Because of viscosity, the velocity of the particles near the wall is smaller than that of the particles at the center. i% order to maintain the motion, a pressure decrease is required which, for laminar flow, is proportional to the first power of the mean flow velocity. Actually, however, one ob~erves that, for larger Reynolds numbers, the pressure drop increases almost with the square of the velocity and is very much larger then that given by the Hagen Poiseuille law. One may conclude that the actual flow is very different from that of the Poiseuille flow.

17,189 citations

Book
01 Aug 1969

3,801 citations

Journal ArticleDOI
H.J. Richter1
Abstract: The limitation of vertical countercurrent flow, called flooding, is important for the operation of Emergency Core Cooling Systems in Nuclear Reactors. A new flooding correlation is presented which solves the obvious contradiction between the Wallis correlation and the study by Pushkina and Sorokin concerning the scaling question at zero penetration of liquid. In addition, this flooding correlation is applicable for partial delivery in pipe and annuli experiments as long as the liquid penetrates in the form of a film along the walls.

142 citations

01 Sep 1979

99 citations

Journal ArticleDOI
Abstract: Experimental studies are reported on the flooding phenomenon and on the closely associated churn flow regime. Flooding experiments were carried out with air-water flow in a 32 mm dia vertical pipe with various forms of liquid outlet, namely a porous wall, a tapered outlet and a square-edged outlet. For the first time, downflow (penetration) measurements were made with the porous wall outlet and showed significant differences between penetration rates beyond flooding and the flooding rate. This contrasts with other types of injectors, where the mechanisms of flooding are somewhat different. Measurement of pressure drop and holdup in the churn flow regime were made both with and without a co-existing falling film below the liquid injector. These showed that the falling-film and churn flow regions were essentially decoupled. Analysis of the data for churn flow showed that the minimum pressure gradient does not, for this data, correspond to the condition of zero wall shear stress as had been suggested by some earlier analytical studies. Interfacial shear stresses in churn flow were compared with those used in current reactor safety codes (TRAC and RELAP) and it was found that, for the churn flow region, the relationships used in the RELAP code were more appropriate.

97 citations