Orientation Effects on Flow Boiling Silicon Nanowire Microchannels Tamanna Alama, Wenming Lia, Fanghao Yangb, Jamil Khana, Chen Lia aDepartment of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, United States bIBM T. J. Watson Research Center,Yorktown Heights, NY 10598, United States SC EPSCoR Annual Conference January 25th, 2016 Columbia, SC, USA Abstract Results & Discussion Flow boiling in microchannels has been regarded as the most significant cooling method for various high heat density applications including 3D 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 (SiNW) 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, SiNW microchannels have potential to work as a gravity insensitive flow boiling system. In this study, the effects of heating surface orientation in flow boiling SiNW 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 SiNW 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 100kg/m²s - 600kg/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 5000fps has been performed along with experimental investigation. Results for Plainwall show sensitivity to orientation, whereas, little effects of orientation have been observed for SiNWs. Fig. 7. Comparison of Non-dimensional Numbers between nanowire and plainwall microchannel Fig. 9. Sequential images in flow boiling SiNW at (a) q"eff =110W/cm², (b) q"eff =155W/cm² Motivations Bo is the ratio of buoyancy force to surface tension force. Bo is higher for plainwall microchannel compare to nanowire. Therefore nanowire is more gravity insensitive and suitable for practical applications. Evaporation dominates SiNW. Bubble growth period decreases and thin film evaporation period increases with the increasing heat flux. Enhanced rewetting and thin film evaporation improve system performance To identify new bubble nucleation and release mechanisms using SiNWs. To improve microchannel flow boiling performance for space missions. To enhance NASA on-going research in two-phase transport for space applications. Fig. 8. Sequential images in flow boiling Plainwall at (a) q"eff =110W/cm², (b) q"eff =155W/cm² Nucleate and intermittent flow boiling regime dominate. Bubble growth period is much longer than evaporation period. Research Aims Effect of orientation in plainwall microchannels. Effect of orientation in SiNW microchannels. Difference in bubble dynamics between plainwall and SiNW microchannels. SiNW reduces the transitional flow boiling regimes (slug/churn/wavy) to a single annular flow Orientation Effects in Flow Boiling Plainwall Microchannel Experimental Study Heat transfer Strongly influenced by orientation in plainwall microchannel configuration. 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. ∆P insensitive to orientation. Deviation in wall temperature for 180° Orientation are very high due to premature dryout and CHF. Vapor stagnation leads to low deviation in pressure drop for 180° Orientation Fig. 1. Tested Microchannel Orientations Fig. 2. Experimental Flow Loop and Test Sections Fig. 10. Effect of orientation on (a) boiling curve and (b) pressure drop curve Fig. 11. Effect of orientation on (a) pressure drop instabilities and (b) wall temperature instabilities Experimental Test Surfaces Orientation Effects in Flow Boiling SiNW Microchannel a b c d Electrode Fig. 3. Static contact angle, θ for (a) Silicon plainwall surface and (b) Silicon nanowire surface. Fig. 4. Scanning Electron Microscope (SEM) images of (a) SiNW surfaces, (b) Plainwall surfaces Fig. 12. Effect of orientation on (a) boiling curve and (b) pressure drop curve (c) pressure drop instabilities and (d) wall temperature instabilities Heat transfer, Pressure drop and Two-phase Flow instabilities show insensitivity to orientation for SiNWs except very low mass flux condition Repeatability Test On-Going Research Current research is focusing on: Effect of orientation in flow boiling SiNW microchannels at four different orientations. upward facing (0°), vertical down flow (90°), downward facing (180°) and vertical up flow (270°). Negligible orientation effects have been observed at SiNW microchannels for all the orientation tested. Fig. 5. The repeat test for heat transfer data at different conditions Fig. 6. The repeat test for pressure drop data at different conditions Fig. 13. Tested orientations of SiNW microchannel The repeat boiling curves and pressure drop curves for different tests are nearly overlapped ! Fig. 14. Effect of orientation in flow boiling SiNW microchannels at four different orientations Summary Acknowledgements 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. NASA (Grant Award No NNX14AN07A) Effect of orientation on boiling curve, pressure drop curve and instabilities have been investigated for plainwall and SiNW microchannels. Plainwall shows strong sensitivity to orientation, whereas, little effects of orientation have been observed on the SiNW configuration. Comparative visualization studies for SiNW and plainwall microchannels show distinguishable differences in flow boiling bubble dynamics. USC Microscopy Center Cornell Nanoscale Facilities (CNF) Institute of Electronics and Nanotechnology (IEN) in Georgia Tech