Flattening of boiling curves at post-CHF regime in the presence of localized electrostatic fields Seyed Reza Mahmoudi a,⇑ , Kazimierz Adamiak b , G.S. Peter Castle b a Department of Mechanical Engineering, Massachusetts Institute of Technology (MIT), Cambridge, MA, USA b Department of Electrical and Computer Engineering, University of Western Ontario, London, Ontario, Canada article info Article history: Received 15 April 2013 Received in revised form 13 September 2013 Accepted 13 September 2013 Available online 9 October 2013 Keywords: Boiling curve Electrostatic pressure Dielectric liquids Flattening of boiling curves Charge density abstract We studied two-phase cooling heat transfer enhancement from a heated disk through falling jets of dielectric fluid, HFE7100, in the presence of electrostatic fields. The field is established between the jet and the grounded target and the boiling curves are obtained. The boiling curves showed surprising devi- ation from typical boiling curves at post-critical heat flux (CHF) regimes. In the absence of electric field, the heat flux always falls suddenly as CHF regime reaches. The CHF phenomenon is a catastrophic phe- nomenon accompanying with overshooting the surface temperate. However, counter-intuitively, here we demonstrated that applying an appropriate electrostatic field, the curves became flat at post-CHF regimes and critical heat flux regime can be operated at a wide range of superheats. A hypothetical explanation is proposed to rationalize the flattened boiling curve observations at the CHF regime. Flattening of boiling curves in the presence of electric field promotes reliable operation of cooling device at CHF regime. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction The phase change phenomenon occurring during the boiling process is an important mode of heat transfer. Boiling heat transfer enhancement can be important to design more efficient evapora- tors, particularly for small-scale applications such as electronic cooling circuits and hot spot reduction for many applications [28,29]. Several passive and active techniques have been used to improve the localized heat removal capacity in the presence of phase charge boiling [24,28,32]. Among the enhancement meth- ods, electrohydrodynamic-boiling enhancement technique showed promises for readily control of heat transfer rates both in terrestrial and microgravity applications [26,27,38]. The study of EHD effects as an active technique of phase change heat transfer enhancements began about 40 years ago [33]. It was demonstrated that phase change phenomena during heat transfer can be directly or indirectly affected by electrostatic forces. EHD ef- fects were assumed to improve the heat transfer through enhance- ment of wettability over the heat transfer surface [1–3]. Such microscopic interfacial forces may cause macroscopic effects, which drastically change the thin liquid film wetting dynamics and control the overall heat transfer from the heated surface. Although EHD boiling heat transfer has been the subject of many studies [4–13], very few works have focused on exploring the EHD enhancement mechanisms [4,5,32,33]. Most of the previ- ous studies have concentrated on heat transfer enhancement at nucleation boiling [6–10]. Only limited literature exists on the ef- fect of electric field at the CHF and post-CHF regimes [5,13–15]. The proposed mechanisms, by which EHD techniques are be- lieved to enhance the boiling process, are briefly summarized as following: (1) movement of vapor bubbles on the heated surface due to Maxwell stress, (2) spreading of the vapor bubble base over the heat transfer surface due to electrostatic forces, (3) increasing the number of bubbles by breaking up large bub- bles by decreasing the bubble detachment diameter and cre- ating more turbulence, (4) elimination of boiling hysteresis, by decreasing the degree of the superheat required to start nucleate boiling, (5) improving the transitional and minimum film boiling condi- tions by destabilizing the blanketing vapor film, (6) improving the wetting of the heating surface due to the interfacial normal and shear stresses, arising from surface charge accumulation in the presence of a strong electric field, (7) generating EHD waves and perturbations at the surface of a boiling liquid, causing an increase in re-wetting of the heated surface due to the EHD interfacial instability of the vapor/liquid interface. 0017-9310/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2013.09.028 ⇑ Corresponding author. Tel.: +1 617 253 1000. E-mail address: mahmoudi@mit.edu (S.R. Mahmoudi). International Journal of Heat and Mass Transfer 68 (2014) 203–210 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt