Numerical investigation on mass transfer enhancement downstream of an orifice Jinbiao Xiong ⇑ , Xu Cheng, Yanhua Yang School of Nuclear Science and Engineering, Shanghai Jiao Tong University, China article info Article history: Received 22 June 2012 Received in revised form 28 September 2012 Accepted 22 September 2013 Available online 16 October 2013 Keywords: Mass transfer enhancement Orifice Reattaching flow f–f model Correlation abstract Mass transfer enhancement is an indispensible element in flow accelerated corrosion (FAC). In order to investigate mass transfer enhancement downstream of an orifice, the two-dimensional computational fluid dynamics (CFD) calculation is conducted. First, validation of turbulence model, the f–f model, is con- ducted in the fully developed pipe flow, the flow through an orifice and the flow downstream of a sudden expansion. The validation shows that the model is adequate to predict mass transfer enhancement in all the tested flows. Effects of Reynolds number, orifice thickness and diameter ratio on mass transfer enhancement downstream of an orifice are then investigated based on the numerical calculation. The investigation shows that the mass transfer enhancement ratio decreases with the increasing Reynolds number. However, the locations of reattachment point and the peak transfer rate point are not affected by the Reynolds number. Parametric study on the orifice thickness shows that a thin orifice helps mass transfer enhancement in its downstream. This is attributed to more intense turbulence generation down- stream of a thin orifice. Moreover, the peak transfer point appear about 0.4L r (reattachment length) downstream of the orifice. The results of the parametric study are synthesized as a correlation, St max St fd ¼ 4:78Re 0:12 d D  1:16 1 þ 0:82ðL 1 =dÞ 3 1 þðL 1 =dÞ 3 which is expected to be valid in the range of 4.2  10 4 6 Re 6 1.3  10 5 , 0.4 6 d/D 6 0.75, 0.13 6 L 1 / d 6 2.61. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Mass transfer enhancement is regarded as one of the elements leading to flow-accelerated corrosion (FAC) in the piping system of nuclear and fossil power plants. The flow in the piping system of plants can hardly reach its fully developed condition because it keeps receiving disturbance from the components installed in the system, such as orifices. These components can change flow direction and consequently leads to flow impingement or reattach- ment on the pipe wall in their downstream. Peak local mass trans- fer rates usually appear in the neighborhood of impinged and reattaching point. These regions are identified as the FAC high-risk zone and demand careful examination. In order to assist the FAC analysis, this paper is dedicated to investigate the mass transfer enhancement downstream of an orifice based on the computa- tional fluid dynamics calculation. It is a well-known challenging task to model the turbulence near impinged or reattaching points with the eddy viscosity models because at these locations the wall shear stress, s w , and the friction velocity, u s ¼ ffiffiffiffiffiffiffiffiffiffiffi s w =q p , approaches to zero and, conse- quently, m/u s is not a valid wall characteristic length any more. The restriction eliminates application of the low-Reynolds number model which fits the wall behaviors based on y + . Partially moti- vated by the above mentioned challenge, models utilizing other dimensionless wall distances were developed, such as R y ¼ ffiffiffi k p y=m in the Lam–Bremhorst model [1] and R e = y/(m 3 /e) 1/4 in the Abe– Kondoh–Nagano model [2]. Another point worth noting is that the Schmidt number, Sc, is on the order of 10 3 in the ordinary mass transfer cases. Based on the relation between concentration and momentum boundary layer thickness, d c = Sc 0.3 d u , given by Shaw and Hanratty [3], the concentration boundary layer can be at least one order thinner than the momentum one. It can be expected that modeling of tur- bulent mass transfer is very sensitive to local model performance in the wall vicinity, if the model is designed to resolve the wall region. Most of the models based on the dimensionless wall dis- tances were developed via fitting the direct numerical simulation (DNS) data in the fully developed flow. Hence, these models should be carefully applied in the impinging and reattaching flows, 0017-9310/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2013.09.055 ⇑ Corresponding author. Tel.: +86 2134204917. E-mail address: xiongjinbiao@sjtu.edu.cn (J. Xiong). International Journal of Heat and Mass Transfer 68 (2014) 366–374 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt