Petroleum & Coal ISSN 1337-7027 Available online at www.vurup.sk/petroleum-coal Petroleum & Coal 56(5) 562-571, 2014 HYDRODYNAMICS OF ISOTHERMAL UPWARD TWO PHASE FLOWS IN HELICAL COILS Mahmood Reza Rahimi 1 , Aboalfazl Askari 1 , Mehdi Ghanbari 1,2 1 Process Intensification Lab., Chemical Engineering Department, School of Engineering, Yasouj University, Yasouj 75918-74831, Iran; 2 Department of Petroleum and Chemical Engineering, Shiraz University, Shiraz, Iran. Received August 8, 2014, Accepted October 10, 2014 Abstract The hydrodynamic characteristics of isothermal upward turbulent two-phase flow of dispersed air-water in helical coils were investigated using computational fluid dynamics. Population Balance Modeling (PBM) approach was applied for size distribution of poly-dispersed air produced by coalescence and breakup of bubble groups. The main aim of this work is to examine the ability of proposed coupled CFD-PBM model, especially to show hydrodynamic characteristics as well as the influence of k-ε and Shear Stress Transport (SST) turbulence models on performance of CFD-PBM model. The detailed analysis of the flow in helical coils has been carried out using velocity, pressure, air volume fraction and secondary flow. The weakness of k-ε-CFD-PBM combined model is revealed in comparison to the SST-CFD-PBM combined model. In addition, the significance of replacing the helical pipes instead of directed-line pipes was briefly studied. It is found that the orientation of the dispersed phase in two phase flow through the kinds of bent tubes such as helical pipe and U-tubes maybe controls their applicabilites in heat exchanger tools. Keywords: helical coils; two phase flow; poly-dispersed flow; CFD; shear stress transport model. 1. Introduction Helically coiled heat exchangers have obtained very wide range of industrial applications such as, nuclear industry, steam generators, heat recovery systems, etc. [1-2] . This is due to the unique hydraulic and thermal characteristics of helical coils in comparison to the straight- line tubes. The study on hydraulic and thermal properties of helical coils is useful because of the fact that the results of such studies can be expanded to other similar geometry of instruments. For example, these studies may be easily extended to deal with a bent tube [3] embedded in a heat exchanger or a non-zero pitch bend tube in a riser tube of a high-pressure boiler. In many cases, the fluid which is flowing through the helical coil, is in the form of two-phase flow. The orientation of the dispersed phase in two-phase flow through the kinds of bent tubes such as helical coil and U-tubes may controls the unexpected failure occurred in heat exchanger tools. The report of unexpected failures of tubes through which two-phase flow is streamed is published in detail [4] . Although the results of studies on two-phase flow in straight pipes are widely reported [5-6] , for helical and U-tubes, the results are not widely available. Kasturi and Stepanek [7] , among others, carried out pressure drop and void fraction measurements for gas-liquid two-phase counter-flow in a helical coil. Czop et al. (1994) [8] carried out experiments on water-SF6 flow through a helically coiled tube of 19.8mm pipe diameter with 1170mm coil diameter. Several authors have proposed different models to describe the bubble coalescence and breakup processes [9-10] . Instead of arbitrarily considering two bubble classes, it may be useful to incorporate a coalescence and break-up model based on the Multiple Size Group (MUSIG) model framework in the Computational Fluid Dynamic (CFD) model. Jayakumar et al. [11] have studied the characteristics of two-phase flow in helical coils using two-fluid Eulerian-