Interrogating the effect of an orifice on the upward two-phase gas–liquid flow behavior Ammar Zeghloul a , Abdelwahid Azzi a,⇑ , Faiza Saidj a , Barry J. Azzopardi b , Buddhika Hewakandamby b a University of Sciences and Technology Houari Boumedien (USTHB), FGMGP/LTPMP, Bab Ezzouar 16111, Algiers, Algeria b Faculty of Engineering, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom article info Article history: Received 5 February 2015 Received in revised form 26 April 2015 Accepted 28 April 2015 Available online 5 May 2015 Keywords: Orifice Two-phase Upward Void fraction Conductance Frequency abstract Experiments are reported on an air–water mixture flowing through an orifice in a vertical pipe. Time ser- ies of cross-sectionally averaged void fractions have been measured at nine axial positions by using a con- ductance probe technique. A series of six orifices with different thicknesses and apertures were employed. The Probability Density Function, the Power Spectral Density of the time series of cross sec- tionally averaged void fractions and the cross-correlation of time series from adjacent probes have been obtained to determine the effect of the orifice on the flow characteristics. The diameter area ratio and the thickness of the orifice have a higher influence on bubbly than on slug and churn flows. The recovery length is about 20, 10 and 7 pipe diameter downstream the orifice for these three flow patterns respec- tively. Homogenization effect needs a minimum liquid superficial velocity. Its position occurs depends on the value of this velocity and on the orifice fractional open area. Just downstream the orifice, the structure velocity increases for the bubbly and slug flows and decreases for churn flow. For bubble and slug flows, there is persistency of the frequency when passing through the orifice from the upstream to the downstream pipe. Ó 2015 Elsevier Ltd. All rights reserved. Introduction Gas–liquid two-phase flows through orifices are encountered in a variety of industrial plants. Some examples are: flow characteris- tics of rupture discs in engineering relief system of chemical reac- tors; leaks from ruptured vessels and pipes in power generation units; control of two phase flow using choke valves on oil produc- tion platforms; desalination process by multistage flash (MSF) and the metering of two-phase flows. The evaluation of the pressure drop caused by the orifice and the knowledge of its upstream and downstream influences is nec- essary for safe and adequate design of the equipment where ori- fices might occur. There has been significant effort in modeling single-phase flow through orifices and the corresponding pressure drops. Details of flow behavior and models of pressure drops can be found in fluid mechanics textbooks such as Idel’chik et al. (1994). In two-phase flow the flow mechanics are more complex due to the nature of the flow. These can exhibit a wide range of phase configurations as a consequence of the deformable interface. The majority of published work has been directed to the pressure drop as well as the pressure drop prediction models (Simpson et al., 1983; Chisholm, 1983; Morris, 1985; Fitzsimmons, 1964; Saadawi et al., 1999; Roul and Dash, 2012 and to a lesser extent to the flow behavior through orifices. Fossa et al. (2006) investigated the pressure profiles for slug flow through sharp edge orifices in horizontal pipes. Time series of cross-sectionally averaged void fraction were been measured using conductance probe technique upstream and downstream of the orifice which had dimensionless plate thickness of 0.023– 0.59 and area ratio of 0.54 and 0.73. They found that the void fraction usually reaches a maximum at a distance of about one diameter downstream of the throat. This maximum can be up to twice the value recorded in the fully developed flow regime far from the orifice. The flow in the developing region and the devel- oping length (downstream of the contraction) is also dependent on the upstream flow patterns and area ratio. Fossa and Guglielmini (2002) noticed that this behavior was observed irre- spective of the orifice thickness for high liquid flow rate and even more evident when the area ratio is low. Recently Roul and Dash (2012) investigated numerically the behavior of two-phase air–water flow through orifices placed in horizontal pipes. For their study, they used the same experimental conditions (flow conditions, orifice geometries) as those employed by of Fossa and Guglielmini (2002). They report findings similar to those of Fossa and Guglielmini. http://dx.doi.org/10.1016/j.ijmultiphaseflow.2015.04.013 0301-9322/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +213 (0) 771 45 89 26. E-mail address: azzi_abdelwahid@scientist.com (A. Azzi). International Journal of Multiphase Flow 74 (2015) 96–105 Contents lists available at ScienceDirect International Journal of Multiphase Flow journal homepage: www.elsevier.com/locate/ijmulflow