980 Acta Chim. Slov. 2018, 65, 980–988 Sitar et al.: Oscillations During Flow Boiling ... DOI: 10.17344/acsi.2018.4678 Scientifc paper Oscillations During Flow Boiling in Single Microchannels Anže Sitar, 1,* Andrej Lebar, 1,2 Michele Crivellari, 3 Alvise Bagolini 3 and Iztok Golobič 1 1 University of Ljubljana, Faculty of Mechanical Engineering, Aškerčeva 6, 1000 Ljubljana, Slovenia. 2 University of Ljubljana, Faculty of Health Sciences, Zdravstvena pot 5, 1000 Ljubljana, Slovenia. 3 Fondazione Bruno Kessler, Center for Materials and Microsystems, Via Sommarive 18, I - 38123 Povo, Trento, Italy. * Corresponding author: E-mail: anze.sitar@fs.uni-lj.si phone: +386 1 4771 309 Received: 14-08-2018 Abstract Flow boiling of degassed double-distilled water in a single 50 × 50 μm and 100 × 50 μm microchannel was investigated on the basis of experimental measurements and high-speed visualization. Te visualized events during boiling were analyzed in terms of the bubble frequencies and boiling front oscillations in microchannels. A digital image sequence analysis algorithm was composed to determine the time dependence of bubble and meniscus locations. Te results show (i) the dynamic characteristics of boiling in microchannels, (ii) the increase of fundamental oscillation frequencies with increasing heat fux and temperature of the microchannel bottom, (iii) the amplitudes of the fow boiling oscillations are inversely proportional to the fundamental frequencies. Te outcomes of the study are important as the oscillations during boiling in single microchannels are experimentally confrmed to be predictable in terms of oscillation frequencies and amplitudes trends and dependencies. Tis knowledge is especially signifcant at constructing efcient two-phase micro heat exchangers, micro mixers or micro reactors, as the cross section and the length of the channel become ex- ceedingly important design parameters in micro devices with boiling. Keywords: Microchannels; fow boiling; oscillations; visualization; fundamental frequencies. 1. Introduction Flow boiling in microchannels has the potential for achieving higher heat transfer coefcients compared to the single-phase liquid fow. However, boiling in microchan- nels is hindered by instabilities and early dryouts, which prevent wide implementation and overall heat transfer en- hancement as presented by Kandlikar et al. 1 Moreover, the incipience of boiling in microchannels requires high ex- cess temperature diference between the channel wall and the bulk fuid as shown by Ghiaasiaan and Chedester 2 and also Kandlikar. 3 Accordingly, at the onset of nucleate boil- ing (ONB) the emerging bubble grows very rapidly due to the highly superheated liquid which was confrmed by Edel and Mukherjee. 4 As the vapor bubble flls up the cross section of the microchannel it encounters all the heated walls, which occurs shortly afer nucleation due to the small hydraulic diameters. Te additional heated surface accelerates the bubble growth, which results in a highly in- stable fow with oscillations. Te fuctuations of the work- ing fuid during boiling present a severe challenge, espe- cially when the vapor backfow becomes distinctive and hinders the forward fow of fresh liquid as shown by Bar- ber et al. 5 If the vapor backfow reaches the inlet manifold, it obstructs the liquid fow to all the channels connected to the manifold. It is therefore crucial to prevent pronounced vapor backfows in microchannels during fow boiling. One of the possible solutions for fow stabilization during boiling is the incorporation of inlet restrictors be- tween the inlet manifold and the microchannels (Kand- likar et al., 6 Kuo and Peles, 7 Wang et al., 8 Sitar et al. 9 and Szczukiewicz et al.), 10 which provide higher upstream pressure drop, promote the downstream fow of the vapor bubble and suppress the vapor backfow. Another approach to reduce fow instabilities is to fabricate potential nucleation sites. Tis precaution mea-