1 Copyright © 2010 by ASME Proceedings of the ASME 2011 9th International Conference on Nanochannels, Microchannels, and Minichannels ICNMM2011 June 19-22, 2011 Edmonton, Alberta, CANADA ICNMM2011-58168 EXPERIMENTAL STUDY ON THE EFFECT OF SYNTHETIC JET ON FLOW BOILING INSTABILITY IN A MICROCHANNEL Ruixian Fang, Wei Jiang, Jamil Khan Department of Mechanical Engineering University of South Carolina, Columbia, SC, USA ABSTRACT Two-phase flow instability in microchannel flow boiling exhibits pressure fluctuations with different frequencies and amplitudes. One possible way to suppress the dynamic instability is to introduce synthetic jet near the inlet of the channel. An important feature of synthetic jet is that it allows momentum transfer into the microchannel flow without net mass injection into it. The strength and frequency of the jet can be controlled by changing the driving voltage and frequency of the piezoelectric driven jet actuator. The effects of the synthetic jet together with upstream throttling are evaluated as a means of stabilizing the flow boiling instability. The results are compared with the case without synthetic jet. The pressure dynamics of the microchannel flow caused by the synthetic jet are also analyzed. Keywords: synthetic jet, microchannel, flow boiling instability, flow boiling, two phase flow 1. INTRODUCTION Flow boiling heat transfer in microchannels has shown great potential for removal of high heat flux in the thermal management of microsystems, micro-processors, high power density electronics, etc. However in practical applications, one great concern is the thermally induced two-phase flow instabilities. Oscillations of flow rate and system pressure are undesirable as they may cause problems of system control and in extreme circumstances, premature of critical heat flux condition so that the device may be overheated. This paper addresses one possible method to suppress the instabilities during flow boiling in microchannels by introducing synthetic jet into the channel flow. The jet stabilizing effects are studied alone and in conjunction with upstream throttling. Flow instabilities are characterized by visual observations and the pressure fluctuations measured between the inlet and outlet manifolds. Two-phase flow boiling in microchannels is different with that of conventional size channels. For bubble ebullition in microchannels, the surface tension becomes predominant and significantly reduces the vapour-liquid velocity slip. Thus the two-phase flow hydrodynamic characteristics are affected [1]. Bubble growth is limited in the radial direction by the narrow channel wall, and grows rapidly in the longitudinal direction, both upstream and downstream. Flow reversal may occur if the net force due to the evaporation momentum overcomes the surface tension and the inertia forces. Experimental studies on the onset of nucleate boiling [2-4] and flow patterns [5-6] have confirmed these differences. Flow boiling instabilities [1, 7-11] in microchannels have received increasing interest in recent years. Many types of flow instability modes, which have been investigated extensively and well identified in macroscale channels, are also presented in small-diameter channels, as discussed by Bergles and Kandlikar [11]. The flow excursion or Ledinegg instability is the most common static instability mentioned in the microchannel flow boiling works [1, 7]. It is resulted from the unique pressure drop characteristic of a boiling channel (the demand curve). When the heated channel is part of a forced circulation loop, operation at the portion of the demand curve with negative slope can be unstable. A well defined minimum point on the demand curve closely relates to the onset of flow instability, and it is a crucial operational threshold [1, 12]. Dynamic instabilities such as density wave oscillations and pressure drop oscillations may also occur in microchannel flow boiling. They are characterized by typical frequencies, amplitudes and physical mechanisms. In