Analysis and active control of pressure-drop flow instabilities in boiling microchannel systems TieJun Zhang a, * , Yoav Peles a , John T. Wen a , Tao Tong b,d , Je-Young Chang d , Ravi Prasher c,d , Michael K. Jensen a a Center for Automation Technologies and Systems, Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180, USA b Department of Mechanical Engineering, University of California, Berkeley, CA 94720, USA c Department of Mechanical and Aerospace Engineering, Arizona State University, Tempe, AZ 85287, USA d Intel Corporation, 5000 W. Chandler Boulevard, Chandler, AZ 85226, USA article info Article history: Received 9 October 2009 Received in revised form 1 February 2010 Keywords: Boiling Microchannel Pressure-drop oscillation Two-phase flow instability Active control abstract Pressure-drop oscillations are one of the most severe dynamic instabilities for boiling flow especially in microchannel systems. This paper presents a systematic framework for the transient analysis and active control of microchannel flow oscillations at a system-level view. To quantify the upstream compressibil- ity and the associated oscillatory transients in an experimental microchannel boiling system, a lumped oscillator model is derived from the momentum balance equation, and both analytical and numerical nonlinear parameter identification methods are proposed. The predictions from the flow oscillation model agree well with the experimental pressure-drop observations across a flow meter and a micro- channel heat sink. Based on the identified nonlinear oscillator model, a virtual state observer is designed to estimate the mass flow acceleration from the mass flowrate measurement in the boiling channel. Then a family of state and dynamic output-feedback active flow controllers is developed and evaluated for the dynamic pressure-drop instability suppression. Further analysis and simulations show the flow oscilla- tion amplitude can be regulated to whatever level is desired under certain conditions. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Thermal challenges in next-generation electronic systems are attracting more attention due to the rapidly increasing demands of high-power density electronics [1]. Advanced reliable/effective cooling technologies are particularly desirable in military, space, and automotive applications. The peak heat dissipation rate of de- fense radars, directed-energy lasers, and electromagnetic weapons will exceed 1000 W/cm 2 in the near future [2], while the surface temperatures of chips and devices need to be maintained below 85 °C in naval all-electric surface ships [3]. Air cooling solutions are not capable of dissipating heat fluxes above 100 W/cm 2 , and the high heat flux level of 1000 W/cm 2 pushes the capabilities of single-phase liquid cooling solutions [4,5]. However, flow boiling options can be considered in these high heat flux ranges since they can utilize the latent heat of vaporization with a lower mass flowrate. In recent years, microchannel cooling has become a very popu- lar scheme in high heat flux electronics cooling [2,4,6,7]. With a small hydraulic diameter, microchannel heat sinks can enhance the convective heat transfer performance significantly, and such heat sinks also have important attributes of low thermal resis- tance, compact dimensions, minimal coolant usage, and fairly uni- form stream-wise temperatures, thus making them suitable for thermal management of high-power electronics. However, two- phase microchannel heat sinks have a potential shortcoming: excessive pressure drop and, therefore, high pumping power con- sumption. A more severe operational problem is that cooling sys- tems with microchannel heat sinks are prone to various boiling flow instabilities (for example, pressure-drop and thermal oscilla- tions [6,7]). In fact, two-phase flow instabilities occur in various boiling and condensing flow systems including steam generators, nuclear boiling water reactors, conventional power plants, refriger- ation equipment, and heat exchangers widely used in chemical process units and oil refineries [8–11]. These undesirable flow instabilities can result in thermal limits being exceeded, forced mechanical vibrations, fatigue and failure of system components, poor system control, and difficult normal operations and system safety [10]. In particular, sustained flow oscillations may cause the local heat transfer characteristics to deteriorate and induce the boiling crisis, also called the critical heat flux (CHF) condition [9,11]. These critical operational issues have been officially recog- nized and recommended in [1] for future research: New ‘‘concepts 0017-9310/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2010.02.005 * Corresponding author. Tel.: +1 518 276 2125; fax: +1 518 276 4897. E-mail addresses: zhangt6@rpi.edu, tjzhang@ieee.org (T.J. Zhang). International Journal of Heat and Mass Transfer 53 (2010) 2347–2360 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt