High-Speed Visualization of Two-Phase Flow in a Micro-Scale Pin-Fin Heat Exchanger

Abstract

Flow regimes and bubble growth are observed in a pin-fin micro-scale heat exchanger with R-11 as the working fluid. The heat exchanger is machined in silicon and derived from a DNA micro-array consisting of 150 µm-square fins separated by 50 µm-square passages. The fins are staggered and oriented 45 degrees to the flow direction such that approximately 750 channel intersections occur within the volume of the exchanger. The purpose of the study is to determine if this multiplyconnected geometry produces the flow blockage, reversal, and other instabilities observed in single and parallel microchannel configurations. The upper surface of the exchanger is a glass plate that provides optical access. High-speed digital photography and microscope optics are used to obtain real-time images of the flow at a framing rate of 5 kHz. The lower surface is electrically heated and instrumented with a heat flux gage. Inlet and outlet temperatures and pressures, heater and wall temperatures, and volumetric flow rate are monitored. Nucleation is observed near the entrance of the heat exchanger. In the central section, developed vapor regions are composed of broad slug-like vapor fronts immediately followed by a slowly growing bubbly flow. An annular regime dominates the downstream section of the exchanger with drop-like liquid structures appearing at the downstream edge of fins. The heat transfer coefficient decreases with exit quality as in other micro-scale exchangers; however, the flow instability present in parallel channel exchangers is not observed in this configuration.

The Advantages of Evaporation in Micro-Scale Channels to Cool Microeletronic Devices

“…Recently, instead of parallel channels, new multi-channels configurations aiming the confinement of the effects of the "explosive boiling" to restrict regions of the heat sink are under study. Cognata et al (2006) performed experiments using a multi-channel heat sink machined in silicon consisting of 150-μm square fins separated by 50-μm square passages. The fins were staggered and oriented 45 degrees to the flow direction such that approximately 750 channel intersections occur within the volume of the heat exchanger.…”

“…Besides all these aspects, Thome (2006) enumerated the following targets to be pursued to the development of heat sinks based on evaporating fluids: (i) the definition of the most appropriate working fluid; (ii) a clear characterization of the conditions for the critical heat flux in order to guarantee a safe operation; (iii) the optimization of the format and dimensions of the channel taken into account effects of heat conduction through the heat sink structure, viz. Consolini and Thome (2005) pointed out that two consecutives rows of microscale channels presents certain advantages over single rows configurations; (iv) guarantee a safe operation in case of non-uniform heating; and (v) guarantee a safe operation also under transient conditions as the microprocessor and cooling system start up.…”