Fanghao Yang

Enhanced Nucleate Boiling on Horizontal Hydrophobic-Hydrophilic Carbon Nanotube Coatings

Ideal hydrophobic-hydrophilic composite cavities are highly desired to enhance nucleate boiling. However, it is challenging and costly to fabricate these types of cavities by conventional micro/nano fabrication techniques. In this study, a type of hydrophobic-hydrophilic composite interfaces were synthesized from functionalized multiwall carbon nanotubes by introducing hydrophilic functional groups on the pristine multiwall carbon nanotubes. This type of carbon nanotube enabled hydrophobic-hydrophilic composite interfaces were systematically characterized. Ideal cavities created by the interfaces were experimentally demonstrated to be the primary reason to substantially enhance nucleate boiling.

Can multiple flow boiling regimes be reduced into a single one in microchannels

We report that multiple and transitional flow boiling regimes in microchannels can be reduced into a single annular flow from the onset of nucleate boiling to the critical heat flux condition. Hydrophilic silicon nanowires directly grown on inner walls of microchannels were tailored to create boiling surfaces with optimal submicron pores surrounded by nanogaps through controlling the height and density of silicon nanowires using the nanocarpet effect. A single two-phase regime can be realized by controlling the flow structure in two aspects: reducing bubble size and transforming the dominant surface tension force from the cross-sectional plane to the inner-wall plane.

High frequency microbubble-switched oscillations modulated by microfluidic transistors

Creating high frequency two-phase oscillations (HF-TPOs) remains an important goal in advancing microscale fluidic logic devices, micro-mixers, micro-actuators, and flow controls. However, thermally driven TPO frequency has been hindered by confinements of compressible vapor bubbles and low thermal diffusivity in microfluidic systems. In this study, a mechanism creating high frequency microbubbles growth/collapse cycle has been developed to achieve HF-TPOs. A “microfluidic transistor” was conceptualized and fabricated to passively sustain and modulate HF-TPOs. Three orders of magnitude higher TPO frequency has been achieved compared to TPOs reported in literatures under similar working conditions.

Enhanced flow boiling in microchannels through integrating multiple micro-nozzles and reentry microcavities

In a microchannel system, a higher mass velocity can lead to enhanced flow boiling performances, but at a cost of two-phase pressure drop. It is highly desirable to achieve a high heat transfer rate and critical heat flux (CHF) exceeding 1 kW/cm2 without elevating the pressure drop, particularly, at a reduced mass velocity. In this study, we developed a microchannel configuration that enables more efficient utilization of the coolant through integrating multiple microscale nozzles connected to auxiliary channels as well as microscale reentry cavities on sidewalls of main microchannels. We achieved a CHF of 1016 W/cm2 with a 50% less mass velocity, i.e., 680 kg/m2s, compared to the two-nozzle configuration developed in our previous studies. Two primary enhancement mechanisms are: (a) the enhanced global liquid supply by four evenly distributed micronozzles, particularly near the outlet region and (b) the effective management of local dryout by the capillary flow-induced sustainable thin liquid film resulti...

Single- and Two-Phase Thermal Transport in Microchannels With Embedded Staggered Herringbone Mixers

Improving mixing is an effective means to enhance singleand two-phase heat transfer in microchannels. However, it is challenging to induce since the flow in microchannels is laminar in the most working conditions. We report that heat transfer rate and critical heat flux (CHF) on 1-methoxyheptafluoropropane (HFE-7000) can be significantly enhanced by patterning embedded micromixers on the bottom walls in a parallel silicon microchannel array, which consists of five parallel channels (height, width, length: 250 μm × 220 μm × 10 mm). Compared with a plain-wall microchannel array at a mass flux range of 1018 to 2206 kg/m2· s and a heat flux range of 10 to 198 W/cm2, singlephase heat transfer rate, two-phase heat transfer rate, and CHF are enhanced up to 221%, 160%, and 61% using microscale staggered herringbone mixers in microchannels, respectively. These mixers consist of 7 or 3.5 Hz with 12 staggered herringbone grooves (50 μm in depth and width) with 90° between two asymmetric arms in each cycle. Its asymmetry is defined in accordance with the off center position of the apex of the herringbone groove. Finally, experimental results suggest that the locations and coverage of the micromixers have significant impacts on both single and two-phase heat transfer in microchannels.

Biphilic Nanoporous Surfaces Enabled Exceptional Drag Reduction and Capillary Evaporation Enhancement

Simultaneously achieving drag reduction and capillary evaporation enhancement is highly desired but challenging because of the trade-off between two distinct hydrophobic and hydrophilic wettabilities. Here, we report a strategy to synthesize nanoscale biphilic surfaces to endow exceptional drag reduction through creating a unique slip boundary condition and fast capillary wetting by inducing nanoscopic hydrophilic areas. The biphilic nanoporous surfaces are synthesized by decorating hydrophilic functional groups on hydrophobic pristine multiwalled carbon nanotubes. We demonstrate that the carbon nanotube-enabled biphilic nanoporous surfaces lead to a 63.1% reduction of the friction coefficient, a 61.7% wetting speed improvement, and up to 158.6% enhancement of capillary evaporation heat transfer coefficient. A peak evaporation heat transfer coefficient of 21.2 W/(cm2·K) is achieved on the biphilic surfaces in a vertical direction.

Characterization of Hybrid-Wicked Copper Heat Pipe

Thermal management of high power electronics is becoming a critical issue as the power density of semiconductors increasing. The flat heat pipe (FHP) is widely used in the electronic cooling because it is possible to interface with flat electronics packages without additional conductive and interface resistances. The heat flux of the next generation electronics may exceed 100 W/cm2 , which is significantly beyond the cooling capabilities of commercially available FHP today. A novel micro scale hybrid wick was developed in this study to improve the effective thermal conductivity and working heat flux of FHP. The hybrid wick consists of multilayer of sintered copper woven meshes to promote the capillary pressure and microchannels underneath to reduce the flow resistance. The analysis indicates that the effective thermal conductivity and the capillary limit of flat heat pipe (FHPs) with this novel micro scale hybrid wicking structure can be significantly enhanced as compared to the reported FHPs. In this paper, the design of this innovative micro scale hybrid wick is illustrated. The fabrication and charging processes are also outlined. The preliminary experimental results show that the effective thermal conductivity can approach 12,270 W/(m·K), which is more than 30 times better than pure copper at approximate 91.3 W input heat.Copyright

Enhanced Flow Boiling in Microchannels Using Auxiliary Channels and Multiple Micronozzles

Flow boiling in an array of five parallel microchannels (W=200 μm, H=250 μm, L=10 mm) can be dramatically enhanced using self-excited and self-sustained high frequency two-phase oscillations induced by two-nozzle configuration. However, the effect of the two-phase oscillations is confined to the downstream of the microchannels. In this study, four-nozzle microchannel configuration is developed with an aim to extend mixing to the entire channel. Flow boiling in the four-nozzle microchannel is experimentally studied with deionized water over a mass flux range of 120 to 600 kg/m2 s. Overall average heat transfer coefficient (HTC) is significantly enhanced approximately 54.5% by extending the enhanced multi-channel mixing to the whole channel. It is equally important that the pressure drop can be further reduced by approximately 50%. Compared with previous two-nozzle design, four-nozzle configuration not only extends the mixing to the whole channel but also increase nucleation sites, which has been confirmed by visualization study to promote nucleation boiling.Copyright

Orientation Effects on Flow Boiling Silicon Nanowire Microchannels

Flow boiling in Silicon Nanowire microchannel enhances heat transfer performance, CHF and reduces pressure drop compared to Plainwall microchannel. It is revealed by earlier studies that promoted nucleate boiling, liquid rewetting and enhanced thin film evaporation are the primary reasons behind these significant performance enchantments. Although flow regime plays a significant role to characterize the flow boiling Silicon Nanowire microchannel performances; surface characteristics, hydrodynamic phenomena, bubble contact angle and surface orientation are also some of the major influencing parameters in system performances. More importantly, effect of orientation (effect of gravity) draws a great attention in establishing the viability of flow boiling in microchannels in space applications. In this study, the effects of heating surface orientation in flow boiling Silicon Nanowire 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 Nanowire and Plainwall microchannels have been performed by experimental and visual studies. Experiments were conducted in a forced convection loop with deionized water at mass flux range of 100kg/m2s – 600kg/m2s. Micro devices consist of five parallel straight microchannels with Nanowire and without Nanowire (Plainwall) (200μm × 250μm × 10mm) were used to investigate the effects of orientation. Two different orientations were used to perform the test: upward facing (0° Orientation) and downward facing (180° Orientation). Results for Plainwall show sensitivity to orientation and mass flux, whereas, little effects of mass flux and orientation have been observed for Nanowire configuration.Copyright

Enhanced Flow Boiling of HFE 7000 by Chaotic Mixers in Microchannels

Improving mixing is an effective method to enhance flow boiling in microchannels. However, it is challenging to induce since the flow in microchannels is laminar under typical operating conditions. We report that flow boiling of 1-methoxyheptafluoropropane (HFE 7000) in a parallel microchannel array was significantly enhanced by chaotic mixers patterned on the bottom walls. The microchannel array consists of five parallel channels (height, width, length: 250 μm × 220 μm × 10 mm). The chaotic mixers consist of seven cycles with 12 staggered herringbone grooves (50 μm depth and width with 90° between two asymmetric arms) in each cycle. Its asymmetry is defined by the off center position of the apex of the herringbone groove. Compared with a smooth-wall microchannel array with identical channel dimensions, heat transfer coefficient and critical heat flux of flow boiling on HFE 7000 were enhanced up to 45 % and 61 % using chaotic mixer pairs in microchannels. Mass fluxes range from 1000 to 2200 kg/m2-s and wall heat fluxes from 10 to 198 W/cm2.Copyright