In this article, a review of fundamental experimental studies on flow boiling in plain and surface enhanced microgaps along with new results of subcooled flow boiling of water through a micropin-fin array heat sink with outlet pressure below atmospheric are presented. Water is one of the most attractive coolants used in flow boiling experiments because of its large-specific heat and latent heat of vaporization, which enables the absorption of considerable amount of heat during both sensible heating and boiling. However, saturation temperature of water at atmospheric pressure is 100 °C, which may be unacceptably high for continuous operation of complementary metal oxide semiconductor (CMOS) electronic devices.
In plain microgaps, Flow patterns such as bubbly, intermittent, wavy, and annular are observed, and gap height is found to impact flow patterns and heat transfer characteristics. As gap height decreased between 2 mm to 0.2 mm, annular flow is dominant, and intermittent and wavy flow diminished. Generally, the two-phase heat transfer coefficients are higher in shorter microgaps than in taller ones, ranging from 10 kW/m2 K to 7.5 kW/m2 K for 110 μm and 500 μm microgaps, respectively. For uniform heating, at the same mass flux of 690 kg/ m2 s and heat flux range from 0 to 60 W/cm2, microgap cooled device demonstrated a smaller temperature gradient and smaller amplitude of pressure and temperature oscillation than microchannel. Also for microgap heights smaller than 500 μm, confined slug flow is the dominant flow pattern at low heat flux, while confined annular flow is the dominant flow pattern at higher heat flux; for microgap heights larger than 700 μm. It is concluded that confinement occurred in microgap heights smaller than 500 μm, and effect of confinement is negligible for microgap heights larger than 700 μm.
In Microgaps with Micropin-Fin Surface Enhancement, which contained an array of 20 x 12 or 13 micro channels (in tandem) staggered hydrofoil pin fins with a wetted perimeter of 1030 μm and chord thickness of 10 μm. The heat transfer coefficient is found to increase with increasing heat flux until a maximum is reached and then decreased monotonically with increasing heat flux until CHF for heat flux range between 19 to 312 W/cm2 and mass flux between 976 to 2349 kg/m2 s. The increasing trend at low quality is ascribed to nucleate boiling, and the decreasing trend at high quality to dominance of convective boiling heat transfer mechanism. The two-phase heat transfer coefficient is moderately dependent on mass flux, and independent of heat flux, for the range of mass flux between 346 kg/m2 s to 794 kg/m2 s and heat flux between 20W/cm2 to 350W/cm2 tested.
Results and discussion: The flow boiling experiment are conducted for 3 different operating conditions by changing parameters such as inlet temperature, vapor quality, heat flux, mass flux etc. Two-phase heat transfer coefficients for conditions where boiling is in the pin fin array are shown in Fig. 7(a), it is found out that the decreasing trend of two-phase heat transfer coefficient becomes less dependent on heat flux as heat flux increases. Figure 7(b) is a boiling curve and 7(d) represents the graph of exit quality versus effective heat flux, which shows that the wall superheat and estimated exit quality increases almost linearly with increasing heat flux. Also, from figure 7(c), it is concluded that the pressure drop increases with increasing heat flux because increase in pressure drop due to vapor phase is more prominent than decrease in pressure drop in single phase region due to decrease in viscosity.
Flow instabilities –
Flow instabilities are observed at high heat flux during the flow boiling experiment. The wall temperatures and pressure drop, for the mass flux 1351 kg/m2 s at 267 W/cm2, fluctuated over a large range, as seen in Fig. 8. This happened when vapor pressure increased enough to overcome inlet pressure and induced reverse flow. Liquid is pushed backward and not enough liquid flow occurred in the heat sink, causing dramatic increase in temperature. As liquid accumulated at inlet, and pressure is sufficient to overcome pressure drop across the pin-fin arrays, the liquid is forced into the microheat sink again, and the temperature decreased.
Han X, Fedorov A, Joshi Y. Flow Boiling in Microgaps for Thermal Management of High Heat Flux Microsystems. ASME. J. Electron. Packag. 2016;138(4):040801-040801-12. doi:10.1115/1.4034317.