Enhancement of convective heat transfer by electrically-induced swirling effect in laminar and fully-developed internal flows Reza Baghaei Lakeh a, * , Majid Molki b, 1 a Mechanical and Aerospace Engineering Department, University of California, Los Angeles, 420 Westwood Plaza, Los Angeles, CA 90095-1597, USA b Department of Mechanical and Industrial Engineering, Southern Illinois University Edwardsville, Edwardsville, IL 62026-1805, USA article info Article history: Received 22 March 2013 Received in revised form 3 August 2013 Accepted 1 October 2013 Available online 24 October 2013 Keywords: Heat transfer enhancement Corona discharge Swirling flow abstract A computational and experimental approach is undertaken to study the enhancement of convective heat transfer in fully-developed internal flows by an electrically-induced secondary flow field. Using longi- tudinal flat electrodes along a parallel-plate configuration, the corona discharge is employed to generate an electrically induced secondary flow on the cross section of the flow passage. The electrically-induced secondary flow forms a swirling flow field in the fully-developed condition and enhances the heat transfer significantly. The flow field was solved computationally and the results were verified and validated by grid refinement study and computational error analysis. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction The heat transfer enhancement of internal flows has been the subject of many industrial and academic research studies. Laminar fully developed internal flows essentially lead to a constant Nusselt number and convection coefficient. The heat transfer enhancement techniques in fully developed regimes work based on disturbing the distribution of the velocity across the channels. Secondary flows consume part of the kinetic energy of the flow and may in- crease the required pumping power; however, they may be employed to improve convective heat or mass transfer concur- rently. The secondary flows can be generated by passive methods including bending and coiling or by active methods that require additional energy input. Electric field can be utilized to induce a secondary flow field on the cross section of the flow passage in internal flows to enhance heat transfer. When a high electric potential is applied to a conductor, an electric field will be generated in the vicinity of that conductor. The magnitude of the generated electric field can be beyond the limits of the partial breakdown of the surrounding gas. The high electric field may ionize the neutral molecules which are present in the neighborhood of the conductor and generates a low-temperature plasma, which is referred as Corona Discharge. Based on the po- larity of the applied potential and the chemical composition of the surrounding gas, various ions with positive or negative electric charges may be generated. The ionization process takes place in a very thin layer around the charged electrode which is called the “sheath layer.” The sheath layer is a bubbling low-temperature plasma in which tremendous amount of ionization reactions take place and positive and negative ions may be generated. If a suffi- ciently high positive potential is applied to the charged electrode, the free electrons in the space accelerate toward the positive electrode and collide with neutral molecules of the surrounding gas and ionize them to form positive ions. The ions accelerate away from the charged electrode in the direction of the electric field and escape from the sheath layer toward the grounded electrode. Hence the charged electrode acts as an ion-shooting region. The ions face the barrier of neutral species which do not interact with the electric field and have relatively smaller momentum. The collision of the injected ions with the neutral molecules of air cause momentum exchange between them and imposes a dragging effect on the neutral particles. The dragging effect of ions on the gas causes a bulk flow which is referred as “Ion-Drag flow.” The phenomenon of ion-drag flow was first introduced by Chattock [1] in 1899. He showed that when a high electric potential is applied to a sharp conductor, a flow field is induced by the electric field. The ion-drag flows have been studied with many different applications and objectives. The desired specifications of these flows can be obtained by adopting different geometries for the electric parts and the flow passage. Enhancement of heat * Corresponding author. Tel.: þ1 310 825 7793. E-mail addresses: rblakeh@ucla.edu (R. Baghaei Lakeh), mmolki@siue.edu (M. Molki). 1 Tel.: þ1 618 650 2372. Contents lists available at ScienceDirect Journal of Electrostatics journal homepage: www.elsevier.com/locate/elstat 0304-3886/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elstat.2013.10.002 Journal of Electrostatics 71 (2013) 1086e1099