Heat transfer and mixing enhancement by free elastic flaps oscillation Samer Ali a,b,c,⇑ , Charbel Habchi a , Sébastien Menanteau b,c , Thierry Lemenand d , Jean-Luc Harion b,c a Lebanese International University LIU, Mechanical Engineering Dept., P.O. Box 146404 Mazraa, Beirut, Lebanon b Université Lille Nord de France, F-59000 Lille, France c Mines Douai, EI, F-59500 Douai, France d University of Angers – ISTIA, LARIS – EA 7315, Angers, France article info Article history: Received 12 March 2014 Received in revised form 22 January 2015 Accepted 26 January 2015 Keywords: Fluid–structure interaction Thermal performance Laminar mixing Numerical simulation Multifunctional heat exchangers/reactors abstract An original concept is proposed to enhance heat transfer and mixing quality performances by using flex- ible vortex generators (FVGs) for a static mixer configuration. The role of free elastic flaps oscillations on the mixing process and heat transfer in a two-dimensional laminar flow is numerically investigated. The computational domain consists of four distant FVGs mounted on two opposite walls. Two cases are stud- ied depending on the Reynolds numbers (based on the bulk velocity and the channel height) set to 1000 and 1850. FVGs efficiencies are compared to the corresponding cases with rigid vortex generators (RVGs). In the flexible cases, flaps oscillations increase the velocity gradients and generate an unsteady laminar flow with complex coherent vortices detaching from the tip of the flaps. The mixing efficiency is quanti- fied by the transport of a passive scalar through the channel. It is shown that oscillations in the elastic cases enhance the mixture quality up to 98% relative to that in the rigid cases. The heat transfer enhance- ment is also investigated showing up to a 96% increase in the Colburn factor, 56% increase in thermal per- formance factor and 134% increase in the overall heat transfer. As the FVGs oscillate freely without any additional external force other than that exerted by the flow itself, the implementation of such a tech- nique shows a great potential for the performance enhancement of multifunctional heat exchangers/ reactors. Ó 2015 Published by Elsevier Ltd. 1. Introduction Rigid vortex generators (RVGs) are frequently used for mixing and heat transfer enhancement due to their ability to disrupt the boundary layers and generate complex coherent vortices that destabilize the flow, enhancing thus the convective heat transfer property [1,2]. Various domains of applications of this technique can be found such as in flow jets, chemical reactors, static mixers, heat exchangers and systems in which continuous process is needed [3,4]. Vortex generation methods can be classified into passive and active control techniques [5]. On one hand, passive RVGs are effec- tive to enhance heat and mass transfer due to flow modification they induce. An overview of different RVG geometries is given by Fiebig [1]. Among them, trapezoidal shapes have successfully been used to enhance performances in the high efficiency vortex static mixer as described by Habchi et al. [6] in which a succession of 30° inclined trapezoidal RVGs arrays are mounted on the inner walls. For this configuration, performance enhancement can be achieved by optimizing RVGs shapes or locations in the pipe. Var- ious studies have already been conducted to this aim. In the turbu- lent regime range, Habchi et al. [7] studied several configurations in which arrays of RVGs were aligned or set with a 45° offset angle and oriented in the main flow direction or in the opposite direc- tion. They concluded that the RVGs orientation in the opposite direction of the main flow are the most efficient to increase heat transfer performance. In a more recent article, Habchi et al. [8] also studied tapped RVGs or coupled RVGs with downstream protru- sions that could noticeably affect heat transfer performances. As far as laminar regimes are concerned, RVGs have also been proven to be efficient to destructurate the boundary layers in the wake and to increase by this way the heat transfer and mixing properties of the flow. Indeed, experimental and numerical studies conducted on a single tab or on one raw of tabs showed that developing coun- ter-rotating vortex pair and hairpin-like vortices tend to enhance heat transfer in a laminar flow regime [6,9–11]. On the other hand, active control methods can be used to gen- erate vorticity with oscillating tabs or flapping wings with their displacements amplitude and frequency controlled by an external http://dx.doi.org/10.1016/j.ijheatmasstransfer.2015.01.122 0017-9310/Ó 2015 Published by Elsevier Ltd. ⇑ Corresponding author at: Lebanese International University LIU, Mechanical Engineering Dept., P.O. Box 146404 Mazraa, Beirut, Lebanon. Tel.: +961 1 70 68 81; fax: +961 1 30 60 44. E-mail address: samer.ali@liu.edu.lb (S. Ali). International Journal of Heat and Mass Transfer 85 (2015) 250–264 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt