Analysis of flow patterns emerging during evaporation in parallel microchannels S. Hardt a , B. Schilder b , D. Tiemann b , G. Kolb b , V. Hessel b , P. Stephan a, * a Chair of Technical Thermodynamics, Darmstadt University of Technology, 64287 Darmstadt, Germany b Institute of Microtechnology Mainz (IMM), 55129 Mainz, Germany Received 19 December 2005; received in revised form 26 May 2006 Available online 10 October 2006 Abstract The evaporation processes of 2-propanol and water in cyclo olefin polymer (COP) and silicon microchannels of square cross-section are studied with a high-speed camera. The COP channels with a cross-section of 50 lm  50 lm are rather smooth, whereas the 30 lm  30 lm silicon channels have comparatively rough surfaces. For the COP channels, two different evaporation modes are iden- tified, both with oscillating liquid–vapor menisci. One of these modes is characterized by an extremely rapid evaporation and a corre- sponding discontinuous shift of the meniscus. In the silicon channels four different evaporation modes are observed. Oscillatory motion of the liquid fronts also dominates here, and depending on the total mass flow and the wall temperature the oscillations in dif- ferent channels are synchronized or desynchronized. Besides the flow patterns also the velocity trajectories of the evaporating liquid fronts are analyzed in detail and show a rather good reproducibility over different channels and different cycles. Compared to most other studies reported in this field, bubble nucleation is found to be of secondary importance for the evaporation processes. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Microchannels; Flow boiling; Flow patterns; Two-phase flow 1. Introduction Flow boiling in microchannels and corresponding microchannel heat sinks or evaporators have been in the spotlight of many research activities in the past few years. A main driver for these activities is the exponentially increasing power dissipation of microelectronic circuits, which demands for high performance cooling systems [1]. In addition to that, microchannel evaporators find their applications in fields such as fuel processing technology for fuel cells, where high mass flow rates of gaseous fuel have to be produced by evaporation from a compact vol- ume [2,3]. As discussed by Lasance and Simons [1], evaporation of water is among the mechanisms that promise the highest heat transfer coefficients for electronics cooling. When comparing single-phase liquid flows in microchannels with flow boiling, the latter usually requires less pumping power. In addition, the limit for the maximum achievable heat transfer coefficient is higher when allowing for phase change. These arguments underpin the considerable poten- tial of two-phase microchannel heat sinks for dissipating the high heat fluxes emerging from state-of-the-art miocro- electronic devices. Owing to the ever-increasing integration density of electronic circuits, the demand for efficient cool- ing technologies is expected to increase substantially in the near future. Fuel processing technology, a second application area for which flow boiling phenomena in microchannels have become relevant, poses similar demands on the heat trans- fer rates as electronics cooling technology. Corresponding micro evaporators are often designed for mobile fuel pro- cessing systems which means they should be compact and provide a vapor flow per unit volume as high as possible. 0017-9310/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2006.06.015 * Corresponding author. Tel.: +49 6151 163159; fax: +49 6151 166561. E-mail address: pstephan@ttd.tu-darmstadt.de (P. Stephan). www.elsevier.com/locate/ijhmt International Journal of Heat and Mass Transfer 50 (2007) 226–239