Flow boiling with deionized water in silicon (Si) microchannels was drastically enhanced in a sin... more Flow boiling with deionized water in silicon (Si) microchannels was drastically enhanced in a single annular flow boiling regime enabled by superhydrophilic Si nanowire inner walls. Part I of this study focuses on characterizing enhanced flow boiling heat transfer. Part II focuses on revealing mechanisms in governing pressure drop and critical heat flux (CHF). Compared to flow boiling in plain-wall microchannels without using inlet restrictors (IRs), the average heat transfer coefficient (HTC) and CHF were enhanced by up to 326% and 317% at a mass flux of 389 kg/m 2 s, respectively. Additionally, compared with flow boiling in microchannels with IRs, HTC of flow boiling in the single annular flow was enhanced by up to 248%; while CHF in the new flow boiling regime was 6.4-25.8% lower. The maximum HTC reached 125.4 kW/m 2 K at a mass flux of 404 kg/m 2 s near the exits of microchannels. The significantly promoted nucleate boiling, induced liquid film renewal, and enhanced thin-film evaporation in the self-stabilized and single flow boiling regime are the primary reasons behind the significant heat transfer enhancements during flow boiling.
Enhancing the mobility of liquid droplets on rough surfaces is of great interest in industry, wit... more Enhancing the mobility of liquid droplets on rough surfaces is of great interest in industry, with applications ranging from condensation heat transfer to water harvesting to the prevention of icing and frosting. The mobility of a liquid droplet on a rough solid surface has long been associated with its wetting state. When liquid drops are sitting on the top of the solid textures and air is trapped underneath, they are in the Cassie state. When the drops impregnate the solid textures, they are in the Wenzel state. While the Cassie state has long been associated with high droplet mobility and the Wenzel state with droplet pinning, our work challenges this existing convention by showing that both Cassie and Wenzel state droplets can be highly mobile on nanotexture-enabled slippery rough surfaces. Our surfaces were developed by engineering hierachical nano- and microscale textures and infusing liquid lubricant into the nanotextures alone to create a highly slippery rough surface. We have shown that droplet mobility can be maintained even after the Cassie-to-Wenzel transition. Moreover, the discovery of the slippery Wenzel state allows us to assess the fundamental limits of the classical and recent Wenzel models at the highest experimental precision to date, which could not be achieved by any other conventional rough surface. Our results show that the classical Wenzel eq (1936) cannot predict the wetting behaviors of highly wetting liquids in the Wenzel state.
Thermal management of high power electronics is becoming a critical issue as the power density of... more 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/cm 2 , 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
International Journal of Heat and Mass Transfer, 2014
In Part II of this study, we report that pressure drop was reduced by approximately 48% and criti... more In Part II of this study, we report that pressure drop was reduced by approximately 48% and critical heat flux (CHF) was increased by approximately 300% in SiNW microchannels compared to these in smooth wall microchannels. The hydraulic characteristics of the single annular flow were systematically investigated to reveal the mechanisms responsible for the reduced pressure drop and enhanced CHF. In the single annular regime, the liquid and vapor flows were nearly fully separated during the entire flow boiling process (i.e., from the onset of nucleate boiling to the CHF conditions). Moreover, the entrainment droplets were reduced by flattening the profile of the liquid-vapor interfaces using the high capillary pressure generated by SiNWs. These two factors, i.e., flow separation and reduced entrainment droplets, lead to a dramatic reduction of frictional pressure drop. The separation of liquid and vapor flows as well as the improved global and local liquid supply result in a significant CHF enhancement without using inlet restrictors (IR). Reynolds number based the vapor flow at the exit ranged from 0.1 to 2100.
International Journal of Heat and Mass Transfer, 2013
Micromembrane-enhanced evaporating surfaces were developed to enhance capillary evaporation heat ... more Micromembrane-enhanced evaporating surfaces were developed to enhance capillary evaporation heat transfer coefficient (HTC) and critical heat flux (CHF). Micromembranes made of sintered single-layer copper mesh screen were diffusion bonded on microchannels to effectively promote capillary pressure and reduce flow resistance. Compared with mono-porous evaporating surfaces such as microchannels and copper woven mesh laminates in the same thickness under the similar working conditions, CHF was substantially increased by 83% and 198%, respectively, because of the separation of the capillary pressure generation and fluid transport process that was enabled by the micromembrane. The major features such as ''M''-shaped capillary evaporation heat transfer curves and the associated heat transfer regions were identified. Oscillating flows induced by the bubble growth and collapse as well as the capillary flows induced by the receding menisci were observed and believed to play imperative roles in enhancing the heat transfer by inducing advections and improving evaporation and nucleate boiling.
International Journal of Heat and Mass Transfer, 2013
Experimental study of flow boiling heat transfer in a microchannel array consisting of main chann... more Experimental study of flow boiling heat transfer in a microchannel array consisting of main channels connected to two auxiliary channels (each) was conducted. A microbubble-excited actuation mechanism, powered by high frequency vapor bubble growth and collapse, was established to create and sustain strong mixing in the microchannels. It was shown to significantly enhance flow boiling heat transfer in microchannels. Experimental studies were conducted at mass fluxes ranged from 150 to 480 kg/ m 2 s with de-ionized (DI) water as the working fluids. Compared with microchannels with inlet restrictors (IRs), the average two-phase heat transfer coefficient was improved by up to 149%. More importantly, a 71-90% reduction in pressure drop at moderate mass fluxes ranged from 400 to 1400 kg/m 2 s was observed. Heat flux up to 552 W/cm 2 at a mass flux of 480 kg/m 2 s was demonstrated. Flow and heat transfer mechanisms were studied and discussed.
h i g h l i g h t s < The temperature difference on a heating wall between the inlet and outlet w... more h i g h l i g h t s < The temperature difference on a heating wall between the inlet and outlet was reduced by a flow separation technique. < The pressured drop was reduced by a flow separation technique. < The single-and two-phase convection heat transfer coefficients were enhanced by a flow separation technique. < The enhancement mechanisms were studied.
Creating high frequency two-phase oscillations (HF-TPOs) remains an important goal in advancing m... more 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. V C 2012 American Institute of Physics.
We report that multiple and transitional flow boiling regimes in microchannels can be reduced int... more 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. V C 2013 AIP Publishing LLC. [http://dx.
Due to the difficulty in depositing conformal coatings on high aspect ratio surfaces, capillary e... more Due to the difficulty in depositing conformal coatings on high aspect ratio surfaces, capillary evaporation on superhydrophilic porous structures have not been well studied. In this work, superhydrophilic hybrid wick was fabricated by coating micromembrane-enhanced microchannels with 20 nm-thick silica (SiO 2 ) using the atomic layer deposited (ALD) technique. Rapid ALD SiO 2 coatings improve thin film evaporation of water on hybrid wicks by up to 56%. An appreciable enhancement of critical heat flux was not obtained in this study because of a compromise between the increased capillary pressure and viscous drag resulting from superhydrophilic ALD SiO 2 coatings. V C 2013 AIP Publishing LLC. [http://dx.
Flow boiling of dielectric fluids in microchannels is among the most promising
embedded cooling s... more Flow boiling of dielectric fluids in microchannels is among the most promising embedded cooling solutions for high power electronics. However, it is normally limited by their poor thermal conductivity and small latent heat. To promote thin film evaporation and nucleate boiling, the side and bottom walls of five parallel microchannels were structured with nanowires in a silicon chip. A 10-mm-long thin-film heater were built-in to simulate heat source. Wall temperatures were measured from adiabatic condition to critical heat flux (CHF) conditions. Compared to the plain-wall microchannels with identical channel dimensions, heat transfer coefficient of HFE 7000 can be substantially enhanced up to 344% at the mass flux ranging from 1018 kg/m2∙s to 2206 kg/m2∙s as promoted evaporation and nucleate boiling. Moreover, pumping power was reduced up to 40% owing to the capillarity-enhanced phase separation. CHF was achieved from 92 to 120 W/cm2 and enhanced up to 14.9% at moderate mass flux of 1018 kg/m2∙s as a result of annular liquid supply. However, interestingly, this trend is non-monotonic and CHF is reduced at higher mass fluxes. This experimental study is trying to explore an optimal range of working conditions using nanostructures in flow boiling on highly-wetting dielectric fluids.
Flow boiling with deionized water in silicon (Si) microchannels was drastically enhanced in a sin... more Flow boiling with deionized water in silicon (Si) microchannels was drastically enhanced in a single annular flow boiling regime enabled by superhydrophilic Si nanowire inner walls. Part I of this study focuses on characterizing enhanced flow boiling heat transfer. Part II focuses on revealing mechanisms in governing pressure drop and critical heat flux (CHF). Compared to flow boiling in plain-wall microchannels without using inlet restrictors (IRs), the average heat transfer coefficient (HTC) and CHF were enhanced by up to 326% and 317% at a mass flux of 389 kg/m 2 s, respectively. Additionally, compared with flow boiling in microchannels with IRs, HTC of flow boiling in the single annular flow was enhanced by up to 248%; while CHF in the new flow boiling regime was 6.4-25.8% lower. The maximum HTC reached 125.4 kW/m 2 K at a mass flux of 404 kg/m 2 s near the exits of microchannels. The significantly promoted nucleate boiling, induced liquid film renewal, and enhanced thin-film evaporation in the self-stabilized and single flow boiling regime are the primary reasons behind the significant heat transfer enhancements during flow boiling.
Enhancing the mobility of liquid droplets on rough surfaces is of great interest in industry, wit... more Enhancing the mobility of liquid droplets on rough surfaces is of great interest in industry, with applications ranging from condensation heat transfer to water harvesting to the prevention of icing and frosting. The mobility of a liquid droplet on a rough solid surface has long been associated with its wetting state. When liquid drops are sitting on the top of the solid textures and air is trapped underneath, they are in the Cassie state. When the drops impregnate the solid textures, they are in the Wenzel state. While the Cassie state has long been associated with high droplet mobility and the Wenzel state with droplet pinning, our work challenges this existing convention by showing that both Cassie and Wenzel state droplets can be highly mobile on nanotexture-enabled slippery rough surfaces. Our surfaces were developed by engineering hierachical nano- and microscale textures and infusing liquid lubricant into the nanotextures alone to create a highly slippery rough surface. We have shown that droplet mobility can be maintained even after the Cassie-to-Wenzel transition. Moreover, the discovery of the slippery Wenzel state allows us to assess the fundamental limits of the classical and recent Wenzel models at the highest experimental precision to date, which could not be achieved by any other conventional rough surface. Our results show that the classical Wenzel eq (1936) cannot predict the wetting behaviors of highly wetting liquids in the Wenzel state.
Thermal management of high power electronics is becoming a critical issue as the power density of... more 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/cm 2 , 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
International Journal of Heat and Mass Transfer, 2014
In Part II of this study, we report that pressure drop was reduced by approximately 48% and criti... more In Part II of this study, we report that pressure drop was reduced by approximately 48% and critical heat flux (CHF) was increased by approximately 300% in SiNW microchannels compared to these in smooth wall microchannels. The hydraulic characteristics of the single annular flow were systematically investigated to reveal the mechanisms responsible for the reduced pressure drop and enhanced CHF. In the single annular regime, the liquid and vapor flows were nearly fully separated during the entire flow boiling process (i.e., from the onset of nucleate boiling to the CHF conditions). Moreover, the entrainment droplets were reduced by flattening the profile of the liquid-vapor interfaces using the high capillary pressure generated by SiNWs. These two factors, i.e., flow separation and reduced entrainment droplets, lead to a dramatic reduction of frictional pressure drop. The separation of liquid and vapor flows as well as the improved global and local liquid supply result in a significant CHF enhancement without using inlet restrictors (IR). Reynolds number based the vapor flow at the exit ranged from 0.1 to 2100.
International Journal of Heat and Mass Transfer, 2013
Micromembrane-enhanced evaporating surfaces were developed to enhance capillary evaporation heat ... more Micromembrane-enhanced evaporating surfaces were developed to enhance capillary evaporation heat transfer coefficient (HTC) and critical heat flux (CHF). Micromembranes made of sintered single-layer copper mesh screen were diffusion bonded on microchannels to effectively promote capillary pressure and reduce flow resistance. Compared with mono-porous evaporating surfaces such as microchannels and copper woven mesh laminates in the same thickness under the similar working conditions, CHF was substantially increased by 83% and 198%, respectively, because of the separation of the capillary pressure generation and fluid transport process that was enabled by the micromembrane. The major features such as ''M''-shaped capillary evaporation heat transfer curves and the associated heat transfer regions were identified. Oscillating flows induced by the bubble growth and collapse as well as the capillary flows induced by the receding menisci were observed and believed to play imperative roles in enhancing the heat transfer by inducing advections and improving evaporation and nucleate boiling.
International Journal of Heat and Mass Transfer, 2013
Experimental study of flow boiling heat transfer in a microchannel array consisting of main chann... more Experimental study of flow boiling heat transfer in a microchannel array consisting of main channels connected to two auxiliary channels (each) was conducted. A microbubble-excited actuation mechanism, powered by high frequency vapor bubble growth and collapse, was established to create and sustain strong mixing in the microchannels. It was shown to significantly enhance flow boiling heat transfer in microchannels. Experimental studies were conducted at mass fluxes ranged from 150 to 480 kg/ m 2 s with de-ionized (DI) water as the working fluids. Compared with microchannels with inlet restrictors (IRs), the average two-phase heat transfer coefficient was improved by up to 149%. More importantly, a 71-90% reduction in pressure drop at moderate mass fluxes ranged from 400 to 1400 kg/m 2 s was observed. Heat flux up to 552 W/cm 2 at a mass flux of 480 kg/m 2 s was demonstrated. Flow and heat transfer mechanisms were studied and discussed.
h i g h l i g h t s < The temperature difference on a heating wall between the inlet and outlet w... more h i g h l i g h t s < The temperature difference on a heating wall between the inlet and outlet was reduced by a flow separation technique. < The pressured drop was reduced by a flow separation technique. < The single-and two-phase convection heat transfer coefficients were enhanced by a flow separation technique. < The enhancement mechanisms were studied.
Creating high frequency two-phase oscillations (HF-TPOs) remains an important goal in advancing m... more 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. V C 2012 American Institute of Physics.
We report that multiple and transitional flow boiling regimes in microchannels can be reduced int... more 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. V C 2013 AIP Publishing LLC. [http://dx.
Due to the difficulty in depositing conformal coatings on high aspect ratio surfaces, capillary e... more Due to the difficulty in depositing conformal coatings on high aspect ratio surfaces, capillary evaporation on superhydrophilic porous structures have not been well studied. In this work, superhydrophilic hybrid wick was fabricated by coating micromembrane-enhanced microchannels with 20 nm-thick silica (SiO 2 ) using the atomic layer deposited (ALD) technique. Rapid ALD SiO 2 coatings improve thin film evaporation of water on hybrid wicks by up to 56%. An appreciable enhancement of critical heat flux was not obtained in this study because of a compromise between the increased capillary pressure and viscous drag resulting from superhydrophilic ALD SiO 2 coatings. V C 2013 AIP Publishing LLC. [http://dx.
Flow boiling of dielectric fluids in microchannels is among the most promising
embedded cooling s... more Flow boiling of dielectric fluids in microchannels is among the most promising embedded cooling solutions for high power electronics. However, it is normally limited by their poor thermal conductivity and small latent heat. To promote thin film evaporation and nucleate boiling, the side and bottom walls of five parallel microchannels were structured with nanowires in a silicon chip. A 10-mm-long thin-film heater were built-in to simulate heat source. Wall temperatures were measured from adiabatic condition to critical heat flux (CHF) conditions. Compared to the plain-wall microchannels with identical channel dimensions, heat transfer coefficient of HFE 7000 can be substantially enhanced up to 344% at the mass flux ranging from 1018 kg/m2∙s to 2206 kg/m2∙s as promoted evaporation and nucleate boiling. Moreover, pumping power was reduced up to 40% owing to the capillarity-enhanced phase separation. CHF was achieved from 92 to 120 W/cm2 and enhanced up to 14.9% at moderate mass flux of 1018 kg/m2∙s as a result of annular liquid supply. However, interestingly, this trend is non-monotonic and CHF is reduced at higher mass fluxes. This experimental study is trying to explore an optimal range of working conditions using nanostructures in flow boiling on highly-wetting dielectric fluids.
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Papers by Xianming Dai
embedded cooling solutions for high power electronics. However, it is normally limited by their
poor thermal conductivity and small latent heat. To promote thin film evaporation and nucleate
boiling, the side and bottom walls of five parallel microchannels were structured with nanowires in
a silicon chip. A 10-mm-long thin-film heater were built-in to simulate heat source. Wall temperatures were measured from adiabatic condition to critical heat flux (CHF) conditions.
Compared to the plain-wall microchannels with identical channel dimensions, heat transfer
coefficient of HFE 7000 can be substantially enhanced up to 344% at the mass flux ranging from
1018 kg/m2∙s to 2206 kg/m2∙s as promoted evaporation and nucleate boiling. Moreover, pumping
power was reduced up to 40% owing to the capillarity-enhanced phase separation. CHF was
achieved from 92 to 120 W/cm2 and enhanced up to 14.9% at moderate mass flux of 1018 kg/m2∙s
as a result of annular liquid supply. However, interestingly, this trend is non-monotonic and CHF is
reduced at higher mass fluxes. This experimental study is trying to explore an optimal range of
working conditions using nanostructures in flow boiling on highly-wetting dielectric fluids.
embedded cooling solutions for high power electronics. However, it is normally limited by their
poor thermal conductivity and small latent heat. To promote thin film evaporation and nucleate
boiling, the side and bottom walls of five parallel microchannels were structured with nanowires in
a silicon chip. A 10-mm-long thin-film heater were built-in to simulate heat source. Wall temperatures were measured from adiabatic condition to critical heat flux (CHF) conditions.
Compared to the plain-wall microchannels with identical channel dimensions, heat transfer
coefficient of HFE 7000 can be substantially enhanced up to 344% at the mass flux ranging from
1018 kg/m2∙s to 2206 kg/m2∙s as promoted evaporation and nucleate boiling. Moreover, pumping
power was reduced up to 40% owing to the capillarity-enhanced phase separation. CHF was
achieved from 92 to 120 W/cm2 and enhanced up to 14.9% at moderate mass flux of 1018 kg/m2∙s
as a result of annular liquid supply. However, interestingly, this trend is non-monotonic and CHF is
reduced at higher mass fluxes. This experimental study is trying to explore an optimal range of
working conditions using nanostructures in flow boiling on highly-wetting dielectric fluids.