Enhanced flow boiling with HFE 7100 in microchannels

Enhanced flow boiling with HFE 7100 in microchannels coupling multiple nozzles with reentry cavities Wenming Li, Tamanna Alam, Wei Chang, Jamil Khan and Chen Li 1 Department of Mechanical Engineering, University of South Carolina, Columbia, SC Introduction: Bubble confinement can induce severe flow instability, cause liquid flow crisis, and retard thin film evaporation during flow boiling in microchannels. A microfluidic transistor (Fig. 1) is devised to timely collapse confined bubble slugs and also produce strong mixing through generating self-sustained two-phase oscillations [1]. • Extend mixing and facilitate rapid bubble collapse. • Promote nucleate boiling. • Sustain global liquid supply and local rewetting. Jetting Flow Aux. Channel (a) Gate Bubble 95% (a) The present configuration Fig. 4, A significant enhancement of CHF is achieved [2] Compared to four-nozzle configuration on HFE-7100 (1) High HTC (Fig. 5): Local HTC is significantly enhanced by increased nucleation sites. An enhancement of ~208% is achieved at a mass flux of 462 kg/m 2 s. (2) High CHF (Fig. 6): We achieve 216 W/cm 2 at mass flux of 1200 kg/m 2 s. (3) Reduced ∆P (Fig. 7): By integrating multiple nozzles, up to 82% pressure drop reduction is demonstrated in the this study compared to microchannel with orifice [3] at a mass flux of 462 kg/m 2 s. Vapor slug Micro-nozzle (b) (c) Bubble shrink 9. 5 ms Bubble coalesce (d) 11. 5 ms Bubble collapse 70% 91% Inlet Outlet Fig. 2, Geometric configurations of the two designs (a) 82% 27% Cavity filled with liquid Nucleate boiling Main channel Rewetting (c) 125% 100 μm (c) (d) 30 μm 57% Fig. 5, High HTC 25 μm 250 μm Fig. 3, Concept and structure of the microchannel with integrated multiple microscale nozzles and reentry cavities 81% 0 ms 4 ms (b) (d) Cavity gradually dryout Thin liquid film 3. 5 ms Cavity 60 μm 200 μm (c) [3] Fig. 6, High CHF Cavity and bottom dry-out 76% Auxiliary channel Cavity dryout (c) (b) 208% 100 µm Fig. 9, Enhanced two-phase oscillations (a) (b) 7 ms Bubble nucleation Four-nozzle configuration (c ) Jetting flow 0 ms (a) (a) 100 µm Fig. 8, Enhanced nucleate boiling DI-Water to extend mixing areas (Fig. 2), and (2) multiple jets combined with re-entry cavities to improve nucleate boiling and sustain global liquid supply and local rewetting (Fig. 3). (b ) Bubble nucleation Bubble Working Mechanism: Improved configurations: (1) multiple jets Auxi. channel 1016 W/cm 2 63% Fig. 1, The concept of microfluidic transistors [1] Reentry cavities 1016 W/cm 2 (b) Bubble coalesce (b) Compare to two-nozzle configuration on DI-water Off (a ) Bubble nucleation Results: On Aux. Channel Thermal Isolation Enhanced mechanisms: (a) Cavity rewetted 7 ms 100 µm Fig. 10, Delay local dryout Conclusions: • Extended two-phase oscillations and hence mixing have been achieved. • Global liquid supply to main channels is improved through auxiliary channels via nozzles. Local liquid spreading is enhanced by microscale reentry cavity-induced capillary flows. • Significant flow boiling enhancement in terms of CHF and HTC is achieved with a drastically pressure drop reduction. Acknowledgements: This work was supported by the Office of Naval Research (Program Officer Dr. Mark Spector) under Grant No. N 000141210724. References: 1 F. Yang, X. Dai, C. Li, Applied Physics Letters, Vol. 101, 2012; 2 W. Li, X. Qu, T. Alam, F. Yang, W. Chang, C. Li, Applied Physics Letters, Vol. 110, 2017; 3 C. J. Kuo, Y. Peles, Journal of Heat Transfer, Vol. 131, 2009. Our group is sponsored by