Mass transfer in S-shape dual pipe bends under annular two phase flow conditions H. Mazhar, J.S. Cotton, D. Ewing, C.Y. Ching ⇑ Dept. of Mechanical Engineering, McMaster University, Hamilton, ON, Canada article info Article history: Received 5 September 2013 Received in revised form 8 November 2013 Accepted 3 January 2014 Available online 31 January 2014 Keywords: Mass transfer Bends Two phase flow Annular flow S-bends abstract The mass transfer in S-shape dual in-plane short radius bends is investigated under annular air–water flow conditions. Experiments were performed for a Schmidt number (Sc) of 1280 for different lengths of straight pipe (L/D) between the bends. Standard 90° bends with radius of curvature (r/D) of 1.5 were tested for L/D from 0 to 40 for water (J L ) and air (J V ) superficial velocities of 0.28 and 29 m/s, respectively. The maximum mass transfer was found to occur on the outer wall of the first bend in all cases. The region of high mass transfer in the second bend is located near the outlet of the bend outer wall and was approx- imately 60% of that in the first bend for the L/D = 0 case. This value increased and the location moved upstream as the separation distance was increased, with a magnitude of approximately 85% of that in the first bend at L/D of 40. The effect of J V and J L on the mass transfer for L/D = 0 was determined for J V and J L in the range 0.17–0.4 m/s and 22–30 m/s, respectively. The local mass transfer distribution was similar in all cases, with J V having a more significant effect on the mass transfer enhancement. The phase redistribution within the dual bend for L/D = 0 was visualized using laser induced fluorescence. The high mass transfer locations were found to correlate well with the locations of liquid impingement. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Flow accelerated corrosion (FAC) is a piping degradation mech- anism that develops over several years and results in pipe wall thinning with potential for abrupt pipe failure. FAC occurs in two stages: (i) oxidation of the pipe metal known as corrosion followed by (ii) the mass transfer of the formed corrosion layer to the adja- cent fluid by dissolution. Mass transfer is the rate limiting factor in FAC, and is affected by flow velocity and turbulence level. The fre- quency of pipe failure due to FAC is reported to be higher under two phase flow conditions than under single phase flow [1]. Two phase flow through pipe bends results in significant changes in flow direction leading to phase redistribution due to the difference in density between the two phases [2,3]. The phase redistribution is more severe under high void fraction flow condi- tions and can result in high mass transfer enhancement levels [4]. Thus, FAC studies have primarily focused on annular two phase flow conditions. Several investigations focused on understanding the annular two phase flow in bends through pressure drop mea- surements and flow visualizations [5–9], and through heat and mass transfer in the bends [10–12]. The phase redistribution is reported to cause a significant mass transfer enhancement on the pipe bend outer wall and develop distinctive mass transfer distribution features. Continuous liquid flow separation from the first bend inner wall and deposition on the second bend outer wall was observed by Maddock et al. [7] and Da Silva Lima and Thome [13]. A high mass transfer enhancement was reported at the loca- tion of the anticipated liquid deposition [12]. Comparison of mass transfer in straight pipes under annular two phase flow and single phase flow at an equivalent surface shear stress yielded an enhancement of 1–1.6 for the two phase flow over single phase flow [11]. The mass transfer enhancement for slug two phase flow relative to single phase flow at similar Rey- nolds number was reported to be approximately 3 times [14]. This enhancement was attributed to the effect of the entrained bubbles and the effect of liquid slugs on disturbing the near wall liquid region. The mass transfer in 180° bends was measured under annular flow conditions for radius of curvature r/D of 3 and 7.3 by Poulson [10,11]. Only centerline measurements were performed along the bend outer wall. The location of maximum mass transfer was on the bend outer wall and corresponded to the location of the en- trained liquid droplet deposition. The high mass transfer rate was attributed to the interplay of liquid droplets within the core impinging on the outer wall and the film re-distribution due to the centripetal forces generated by the bend curvature. The inter- play of these two mechanisms is expected to vary with the annular flow condition. The air superficial velocity had a significant effect on the mass transfer but the role of the water velocity was unclear 0017-9310/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2014.01.015 ⇑ Corresponding author. Tel.: +1 905 525 9140x24998; fax: +1 905 572 7944. E-mail address: chingcy@mcmaster.ca (C.Y. Ching). International Journal of Heat and Mass Transfer 72 (2014) 308–318 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt