Interfacial structures and interfacial area transport in downward two-phase bubbly flow q M. Ishii * , S.S. Paranjape, S. Kim, X. Sun School of Nuclear Engineering, Purdue University, 400 Central Drive, West Lafayette, IN 47907-1290, USA Received 19 October 2003; received in revised form 14 April 2004 Abstract An adiabatic, air–water, co-current, vertically downward bubbly flow was studied to gain a better understanding of interfacial structures and flow characteristics. The experimental test sections were round pipes with internal diameters of 25.4 and 50.8 mm. Flow regime map was obtained using characteristic signalsobtainedfromanimpedancevoidmeter,andaneuralnetwork-basedidentificationmethodologyto minimize the subjective judgment in determining the flow regimes. A four-sensor conductivity probe was used to measure the local two-phase flow parameters that characterize the interfacial structures. The parametersmeasuredwere:voidfraction,interfacialareaconcentration,bubblevelocity,andbubbleSauter mean diameter. Furthermore, a laser Doppler anemometer (LDA) system was used to measure local axial liquid velocity and turbulence. The local profiles of these parameters as well as their axial development revealed the nature of the interfacial structures and the bubble interaction mechanisms occurring in the flow. Based on previous study of interfacial area transport for upward flows, the interfacial area transport equation applicable to downward flow was developed with certain modifications in bubble interaction terms. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Downward flow; Interfacial area concentration; Interfacial area transport equation; Four-sensor conductivity probe; Two-phase flow International Journal of Multiphase Flow 30 (2004) 779–801 www.elsevier.com/locate/ijmulflow q It is our great pleasure to celebrate Professor George Yadigaroglu’s 65th birthday and his many years of great contributions to the field of multiphase flow and heat transfer. Our exchanges of ideas and collaborations in research started very early during our Ph.D. student time since both of us worked on the density wave instabilities. We are fascinated to find out that Professor Yadigaroglu experimentally found the higher order density wave instabilities at MIT that was theoretically predicted by a model developed by us at Georgia Tech at one of the national heat transfer conference.Onthisoccasionwewouldliketocongratulatehimforhisgreataccomplishmentsandwonderfulleadership in the nuclear thermal-hydraulics. * Corresponding author. Tel.: +1-765-494-4587; fax: +1-765-494-9570. E-mail address: ishii@ecn.purdue.edu (M. Ishii). 0301-9322/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmultiphaseflow.2004.04.009