Orientation Effects on Flow Boiling Silicon Nanowire Microchannels

Orientation Effects on Flow Boiling Silicon Nanowire Microchannels Tamanna Alama, Wenming Lia, Fanghao Yangb, Jamil Khana, Chen Lia SC EPSCo. R Annual Conference January 25 th, 2016 Columbia, SC, USA a. Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, United States b. IBM T. J. Watson Research Center, Yorktown Heights, NY 10598, United States Results & Discussion Abstract Flow boiling in microchannels has been regarded as the most significant cooling method for various high heat density applications including 3 D ICs, power electronics of electric vehicles/hybrid electric vehicles (EV/HEV), avionics operations and numerous space systems (e. g. power systems, thermal control systems, and life support systems). However, in space applications, flow boiling in microchannels encounters some challenges because of weak buoyancy effects; microgravity is not favorable to enhance flow boiling due to the difficulty in detaching bubbles and the disappearance of the convection. Another area that requires attention is the effect of gravity (orientation effect) for terrestrial systems like military/avionics operations. Therefore, a gravity insensitive flow boiling system is necessary to enhance nucleate boiling and evaporation as well as to passively introduce advection by inducing high frequency liquid renewal on walls. A novel boiling surface, silicon nanowire (Si. NW) reduces the transitional flow boiling regimes (slug/churn/wavy) to a single annular flow and enhances rewetting as shown in our earlier studies [1 -3]. Therefore, Si. NW microchannels have potential to work as a gravity insensitive flow boiling system. In this study, the effects of heating surface orientation in flow boiling Si. NW microchannels have been investigated to reveal the underlying heat transfer phenomena and also to discover the applicability of this system in space applications. Comparison between Si. NW and Plainwall microchannels have been performed by experimental and visualization studies. Experiments were conducted in a forced convection loop with deionized water at mass flux range of 100 kg/m²s - 600 kg/m²s. Two different orientations were used to perform the test: upward facing (0° Orientation) and downward facing (180° Orientation). High speed visualization at a frame rate 5000 fps has been performed along with experimental investigation. Results for Plainwall show sensitivity to orientation, whereas, little effects of orientation have been observed for Si. NWs. Fig. 7. Comparison of Non-dimensional Numbers between nanowire and plainwall microchannel Motivations Fig. 9. Sequential images in flow boiling Si. NW at (a) q"eff =110 W/cm², (b) q"eff =155 W/cm² § Bo is the ratio of buoyancy force to surface tension force. v To identify new bubble nucleation and release mechanisms using Si. NWs. v To improve microchannel flow boiling performance for space missions. v To enhance NASA on-going research in two-phase transport for space applications. Fig. 8. Sequential images in flow boiling Plainwall at (a) =110 W/cm², (b) q"eff =155 W/cm² § Bo is higher for plainwall microchannel compare to nanowire. Research Aims § Evaporation dominates Si. NW. eff § Nucleate and intermittent flow boiling regime dominate. § Bubble growth period is much longer than evaporation period. § Therefore nanowire is more gravity insensitive and suitable for practical applications. v Effect of orientation in plainwall microchannels. v Effect of orientation in Si. NW microchannels. v Difference in bubble dynamics between plainwall and Si. NW microchannels. q" § Bubble growth period decreases and thin film evaporation period increases with the increasing heat flux. § Enhanced rewetting and thin film evaporation improve system performance Si. NW reduces the transitional flow boiling regimes (slug/churn/wavy) to a single annular flow Orientation Effects in Flow Boiling Plainwall Microchannel Experimental Study q. Heat transfer Strongly influenced by orientation in plainwall microchannel configuration. q. Dryout and CHF occur at low heat flux condition for plainwall downward facing microchannel due to flooding at upstream, blockage of liquid renewal and vapor stagnation. q∆P insensitive to orientation. q. Deviation in wall temperature for 180° Orientation are very high due to premature dryout and CHF. Fig. 1. Tested Microchannel Orientations q. Vapor stagnation leads to low deviation in pressure drop for 180° Orientation Fig. 2. Experimental Flow Loop and Test Sections Fig. 10. Effect of orientation on (a) boiling curve and (b) pressure drop curve Experimental Test Surfaces Fig. 11. Effect of orientation on (a) pressure drop instabilities and (b) wall temperature instabilities Orientation Effects in Flow Boiling Si. NW Microchannel a b c d Electrode Fig. 3. Static contact angle, θ for (a) Silicon plainwall surface and (b) Silicon nanowire surface. Fig. 12. Effect of orientation on (a) boiling curve and (b) pressure drop curve (c) pressure drop instabilities and (d) wall temperature instabilities Fig. 4. Scanning Electron Microscope (SEM) images of (a) Si. NW surfaces, (b) Plainwall surfaces Heat transfer, Pressure drop and Two-phase Flow instabilities show insensitivity to orientation for Si. NWs except very low mass flux condition Repeatability Test On-Going Research Current research is focusing on: Fig. 5. The repeat test for heat transfer data at different conditions Fig. 6. The repeat test for pressure drop data at different conditions The repeat boiling curves and pressure drop curves for different tests are nearly overlapped ! References [1] Yang, F. , et al. , Can multiple flow boiling regimes be reduced into a single one in microchannels? Applied Physics Letters, 2013. 103(4): p. 043122. [2] Yang, F. , et al. , Flow boiling phenomena in a single annular flow regime in microchannels (I): Characterization of flow boiling heat transfer. International Journal of Heat and Mass Transfer, 2014. 68: p. 703 -715. [3] Yang, F. , et al. , Flow boiling phenomena in a single annular flow regime in microchannels (II): Reduced pressure drop and enhanced critical heat flux. International Journal of Heat and Mass Transfer, 2014. 68: p. 716 -724. v Effect of orientation in flow boiling Si. NW microchannels at four different orientations. v upward facing (0°), vertical down flow (90°), downward facing (180°) and vertical up flow (270°). v Negligible orientation effects have been observed at Si. NW microchannels for all the orientation tested. Fig. 13. Tested orientations of Si. NW microchannel Fig. 14. Effect of orientation in flow boiling Si. NW microchannels at four different orientations Summary q. Effect of orientation on boiling curve, pressure drop curve and instabilities have been investigated for plainwall and Si. NW microchannels. q. Plainwall shows strong sensitivity to orientation, whereas, little effects of orientation have been observed on the Si. NW configuration. q. Comparative visualization studies for Si. NW and plainwall microchannels show distinguishable differences in flow boiling bubble dynamics. Acknowledgements NASA (Grant Award No NNX 14 AN 07 A) USC Microscopy Center Cornell Nanoscale Facilities (CNF) Institute of Electronics and Nanotechnology (IEN) in Georgia Tech