Pressure drop in two phase slug/bubble flows in mini scale capillaries Ed Walsh a, * , Yuri Muzychka b,c,1 , Patrick Walsh a , Vanessa Egan a , Jeff Punch b a Stokes Institute, Mechanical & Aeronautical Engineering Dept., University of Limerick, Ireland b CTVR, Stokes Institute, Mechanical & Aeronautical Engineering Dept. University of Limerick, Ireland c Faculty of Engineering and Applied Science, Memorial University of Newfoundland, St. John’s, NL, Canada A1B 3X5 article info Article history: Received 1 May 2009 Received in revised form 27 June 2009 Accepted 29 June 2009 Available online 5 July 2009 Keywords: Taylor flow Plug flow Segmented flow Pressure drop Bio-fluidics Micro-channels abstract A segmented two phase slug/bubble flow occurs where a liquid and a gas are pumped into the same tube over a range of Reynolds numbers. This segmented two phase flow regime is accompanied by an increase in pressure drop relative to the single phase flow where only one fluid is flowing in a capillary. This work experimentally and theoretically examines the pressure drop encountered by the slug/bubble flow with varying slug lengths in mini channels. In the experimental work the dimensionless parameters of Rey- nolds number and Capillary number span over three orders of magnitude, and dimensionless slug length ranges over two orders of magnitude to represent flows typical of mini- and micro-scale systems. It is found, in agreement with previous work, that these dimensionless groups provide the correct scaling to represent the pressure drop in two phase slug/bubble flow, although the additional pressure drop caused by the interface regions was found to be 40% less than previously reported. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Many different regimes of two phase flow have been studied from pressure drop, mass transfer and heat transfer perspectives, and are typically characterized by the terminology of bubbly, slug, mist, annular, wavy or stratified flow regimes. Many authors have formulated charts in an attempt to identify which type of flow re- gime is most likely under defined gas and liquid flow rates without giving much consideration to the local flow-field. For example, in the slug flow regime the actual length of the slugs is not typically measured although it was shown, some time ago, to be important on the resultant pressure drop, mass and heat transfer by Horvath et al. (1973). A two phase non-boiling slug flow regime is the focus of the current study and a diagrammatic sketch is shown in Fig. 1, where a gaseous flow is used to segment a continuous liquid stream, to create a well ordered train of segmented slugs. This two phase flow can provide significant heat transfer improvements over low profile single phase systems such as those studied by Walsh et al. (2008), Walsh and Grimes (2007), Egan et al. (2009a,b). As seen from this figure a large number of different configura- tions are possible for the same gas and liquid flow rates depending on the ratio of slug length to channel diameter (L s /d). These non- boiling slug flows are commonly found in many industrial applica- tions such as monoliths catalyst structures (Kreutzer et al., 2005), petroleum industry (Lin and Tavlarides, 2009; Kim and Ghajar, 2002), micro-fluidic biological processing (Gunther et al., 2004; Walsh et al., 2006, 2007; King et al., 2007) and micro-reactors (Waelchli and von Rohr, 2006). The latter resulting in bio-compati- bility issues between the two phases employed (Walsh et al., 2005). In recent works by the authors, we report new experimental data, which considers the slug/bubble lengths, for two phase slug flows in circular tubes with a constant heat flux boundary condition (Walsh et al., 2009), and developed a model which addresses the ef- fect of slug length on Nusselt number (Muzychka et al., 2009). The present work addressed the pressure drop encountered by the slug/bubble flow with varying slug lengths in mini channels. The present model, in conjunction with proper heat and mass transfer models, allows for concise design and analysis of slug flow systems for a wide range of applications, such as those noted above. Pressure drop in such systems is a key parameter in terms of, flow rates, stability of parallel channel, sizing of pumps and overall design of any two phase system. The simplest approach to modelling two phase flows is the separated flow model, which considers both phases independently and predicts the resultant pressure drop to be the sum of their single phase contributions. This model does not account for any additional pressure drop associated with the inter- faces between the two phases. In an attempt to account for this addi- tional interfacial pressure drop, Lockhart and Martinelli (1949) extended the separated flow model to include an empirical 0301-9322/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmultiphaseflow.2009.06.007 * Corresponding author. Tel.: +353 61 213181; fax: +353 61 202393. E-mail addresses: edmond.walsh@ul.ie (E. Walsh), y.s.muzychka@gmail.com (Y. Muzychka), Pat.walsh@ul.ie (P. Walsh), Vanessa.egan@ul.ie (V. Egan), jeff. punch@ul.ie (J. Punch). 1 Sabbatical address: Stokes Institute, Mechanical and Aeronautical Engineering Department, University of Limerick, Ireland. International Journal of Multiphase Flow 35 (2009) 879–884 Contents lists available at ScienceDirect International Journal of Multiphase Flow journal homepage: www.elsevier.com/locate/ijmulflow