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Trends towards miniaturizing the electronics devices and to increase the computational capability has driven the modern microelectronics industry toa state where efficient thermal management issue has become the most challenging and in some case limiting parameter for the further development of these devices. Research is going on around the globe to address this challenge and to come up with a solution that is economically viable and user level applicable to meet not only the current issue but also to address the forth coming heat removal challenges where 1000 W/cm 2 seems to be a feasible target specially for the defense related electronics. Exploiting high surface area to volume ratio and high latent heat of vaporization, flow boiling in microchannel possesses the highest opportunityto meet this challenge; although boiling instability and pressure fluctuation limit its implementation in any practical devices. To harness the full potential of flow boiling in microchannel, new schemes need to be implemented to control the boiling characteristics. This paper presents one active (implementation of synthetic jet in microchannel) and one passive (surface morphology modification with nanostructures) flow boiling control mechanism which can be implemented easily and can tune all the boiling performance parameters positively. Flow boiling in microchannel with reliable control mechanism could be the solution for the next generationthermal management challenges arising from the microelectronics industry. [ABSTRACT FROM AUTHOR]

Abstract: Flow boiling performance of a Cu microchannel coated with Al2O3 nanoparticle has been investigated experimentally. Experiments were performed in a single microchannel having hydraulic diameter of 672μm using de-ionized (DI) water as the coolant. Surface morphology of the microchannel was modified by depositing nanoparticles through pool boiling of ethanol based alumina (Al2O3) nanofluid. Flow boiling, along with single-phase convective heat transfer experiments were performed at different mass flux and different inlet sub-cooling. Results from the microchannel with bare surface are used as the reference data. Nanoparticles deposited surface was found effective in enhancing the critical heat flux (CHF); up to 39% enhancement in CHF was observed in this experimental range. However, the heat transfer coefficient (HTC) was observed to be reduced marginally for the nanoparticles coated surface compared with the bare surface. [Copyright &y& Elsevier]

Highlights • Five parallel microchannels are interconnected by micro-slots. • Micro-slots can enable high frequency nucleate boiling. • Highly coordinated, rapid and sustainable nucleate boiling are realized. • The HTC and CHF are significantly enhanced on the present design. • The enhancements of HTC and CHF are ∼175% and ∼76%, respectively. Abstract Flow boiling in microchannels using dielectric fluids is one of the most desirable cooling solutions for high power electronics. However, it is challenging to promote the flow boiling performances, particularly critical heat flux (CHF), due to their unfavorable thermophysical properties. Flow boiling in parallel and isolated microchannels has been extensively studied. In this study, five parallel microchannels (W = 200 µm, H = 250 µm, L = 10 mm) are interconnected by 4 × 28 micro-slots (20 µm wide and 250 µm deep) starting from the middle section to the channel outlet. Our visualization study shows that these micro-slots designed as artificial nucleation sites can enable high frequency nucleate boiling by drastically reducing the bubble waiting time and remaining fully activated, simultaneously. More importantly, such rapid switch on–off harmonically coordinated nucleate boiling in the neighboring channels creates a highly desirable periodic rewetting mechanism to substantially delay CHF conditions and enhance heat transfer rate. Flow boiling in this innovative microchannel configuration has been systematically characterized with mass flux ranging from 462 kg/m2∙s to 1617 kg/m2∙s. Compared to plain-wall microchannels with inlet restrictors (IRs), flow boiling heat transfer coefficient (HTC) is enhanced up to ∼172% at a mass flux of 462 kg/m2·s primarily owing to the enhanced latent heat transfer including nucleate boiling and thin film evaporation. The peak value of effective HTC is ~60 kW/m2·K in the fully developed boiling regime. Moreover, CHF is substantially enhanced by ∼76% at a mass flux of 1155 kg/m2·s owing to the rapid and periodic rewetting enabled by these micro-slots. Such drastic enhancements have been achieved without compromising two-phase pressure drop. [ABSTRACT FROM AUTHOR]

Extensive experimental investigations along with high speed visualizations have been performed to assess the flow boiling characteristics in Silicon Nanowire (SiNW) microchannels. Experiments have been also performed in Plainwall microchannels to compare their performances with SiNW configurations. HFE-7100 has been used as the working fluid and experiments are conducted in a forced convection loop at mass flux range of 400–1600 kg/m 2 s. Arrays of microchannel consist 5 (five) parallel straight microchannels with Width, Depth and Length dimension of 220 μm, 250 μm and 10 mm respectively. Flow boiling performances including heat transfer coefficient (HTC), pressure drop, two-phase flow instabilities and critical heat flux (CHF) have been studied in both the Silicon plainwall (smooth inner surface) and Silicon Nanowire (silicon nanostructured inner surface) microchannels. High speed flow visualizations have been performed at up to 70,000 frames per s (fps) to understand the difference in boiling mechanisms between Plainwall and SiNW. SiNW performs significantly enhanced HTC (up to 400% improvement), reduces flow boiling instabilities and pressure drop (up to 70% reduction) compared to Plainwall microchannels. However, little/insignificant effect of nanostructured surface has been observed on CHF. In addition, a major difference in two-phase flow regime development has been observed between the SiNW and Plainwall microchannels during flow visualization. Specifically, SiNWs introduce explosive bubble nucleation, reduce intermittent flow regimes (slug/churn), improve rewetting, maintain thin liquid film and thus, improve system performances. [ABSTRACT FROM AUTHOR]

Flow boiling of dielectric fluids in microchannels is one of the most desirable cooling solutions for high power electronics. However, the flow boiling of dielectric fluids is hindered by their unfavorable thermophysical properties. Specifically, without precooling dielectric fluids, it is challenging to promote critical heat flux (CHF) due to its high vapor density, low surface tension and the resulted superior wettability. In this study, each side wall of a five-parallel silicon microchannel array was structured with an array of microscale reentry cavities and four micronozzles bypassed by an auxiliary channel. The present microchannel configuration aims to significantly enhance CHF of HFE-7100 flow boiling by improving global liquid supply using auxiliary channels and micrononozzles as well as by sustaining liquid film using capillarity induced by reentry cavity array. Equally important, these structures can promote nucleate boiling at low heat flux, generate intense mixing, and promote thin film evaporation at high heat flux, resulting in high flow boiling heat transfer rate. Flow boiling of HFE-7100 in the present microchannel configuration is characterized with mass flux ranging from 231 kg/m 2  s to 1155 kg/m 2  s. The effective two-phase heat transfer coefficients (HTCs) are ranging from 6 kW/m 2  K to 117 kW/m 2  K. Compared to the four-nozzle plain-wall microchannels, for example, the effective HTC and CHF can be substantially enhanced up to 208% and 37%, respectively, without escalating pressure drop at a mass flux of 462 kg/m 2  s. Compared to plain microchannels with inlet restrictors, CHF is considerably enhanced up to 70% with a reduction of pressure drop ∼82% at a mass flux of 1155 kg/m 2  s. Significantly reduced pressure drop is achieved by integrating bypass and the enhanced confined bubble removal. A peak CHF value of 216 W/cm 2 is achieved at mass flux of 2772 kg/m 2  s in the present microchannel configuration with inlet temperature at room temperature. [ABSTRACT FROM AUTHOR]

Flow boiling in parallel microchannels can be dramatically enhanced through inducing self-excited and self-sustained high frequency two-phase oscillations as demonstrated in our previous studies using a two-nozzle microchannel configuration. Two-phase mixing induced by the rapid bubble collapse is shown to be the major enhancement mechanism. However, in the two-nozzle configuration microchannels, the mixing effect is limited to the downstream of the microchannels, meaning that only half length of the entire microchannel is functionalized as designated. In this study, a four-nozzle microchannel configuration is developed with an aim at extending the highly desirable mixing effect to the entire channel. Flow boiling in the four-nozzle configuration microchannels is experimentally studied with deionized water and the mass flux ranging from 120 kg/m 2 s to 600 kg/m 2 s. The onset of nucleate boiling temperature is considerably reduced by ∼14% because of more nucleation sites created by the multiple nozzles. Equally important, the improved microchannel configuration successfully extends the mixing to the entire channel as validated by the enhanced heat transfer rate and visualization study. Compared to the previous two-nozzle configuration, the overall heat transfer coefficient (HTC) is significantly improved primarily owing to the enhanced nucleate boiling. For example, the peak overall HTC of 262 kW/m 2 K is achieved at a mass flux of 150 kg/m 2 s, accounting for ∼83.7% enhancement. Additionally, the peak effective HTC reaches 97.6 kW/m 2 K, accounting for ∼67% enhancement. [ABSTRACT FROM AUTHOR]

Enhancing critical heat flux (CHF) of flow boiling without escalating pressure drop is highly desirable in thermal management of high power-density electronic devices. Usually, an improved CHF can be achieved by restricting flow or at a higher mass velocity, leading to a higher pressure drop. In this study, compared to the two-nozzle microchannel configuration, the improved microchannel configuration as detailed in the Part (I) of this study can enhance CHF without sacrificing pressure drop. In this part, CHF is experimentally evaluated together with the pressure drop with mass flux ranging from 120 kg/m 2 s to 600 kg/m 2 s. Compared to the two-nozzle configuration, our study shows that CHF can be enhanced up to 32% with a ∼53% reduction of pressure drop at a mass flux of 325 kg/m 2 s. The bubble collapse-removal process is significantly improved because more micronozzles are integrated. The enhanced pumping effect, which is created by rapid bubble collapse processes in the entire main channels, enables a more sustainable liquid supply and hence delays the CHF conditions. Moreover, two-phase flow in terms of pressure drop fluctuations is more stable owing to the effective management of bubble confinement in the entire channel. [ABSTRACT FROM AUTHOR]

In microchannel flow boiling, bubble nucleation, growth and flow regime development are highly influenced by channel cross-section and physical phenomena underlying this flow boiling mechanism are far from being well-established. Relative effects of different forces acting on wall-liquid and liquid–vapor interface of a confined bubble play an important role in heat transfer performances. Therefore, fundamental investigations are necessary to develop enhanced microchannel heat transfer surfaces. Force analysis of nucleating bubble and bubble dynamics in flow boiling silicon nanowire microchannels have been performed based on theoretical, experimental and visualization studies. The relative effects of different forces on flow regimes, instabilities and heat transfer performances of flow boiling in silicon nanowire microchannels have been identified. Inertia, surface tension, shear, buoyancy, and evaporation momentum forces have significant importance at liquid–vapor interface as discussed earlier by other researchers. However, no comparative study has been done for different surface properties till date. Detail analyses of these forces including contact angle effect, channel dimension effect, heat flux effect and mass flux effect in flow boiling microchannels have been conducted in this study. A comparative study between silicon nanowire and plainwall microchannels has been performed based on force analysis in the flow boiling microchannels. Compared to plainwall microchannels, enhanced surface rewetting and CHF are owing to higher surface tension force at liquid–vapor interface and Capillary dominance resulting from silicon nanowires. Whereas, low Weber number in silicon nanowire helps maintaining uniform and stable thin film and improves heat transfer performances. Moreover, results from these studies are compared with the literatures and great agreements have been observed. [ABSTRACT FROM AUTHOR]

A study on bubble growth mechanisms and underlying physical phenomena of flow boiling in silicon nanowire (SiNW) microchannels has been performed and compared these results with plainwall microchannels. A new approach in studying bubble dynamics and forces acting on liquid–vapor (L–V) interface of growing bubble has been proposed based on theoretical, experimental and visual studies. Bubble size, liquid film thickness, interfacial properties are measured and L–V interfaces are detected from high speed visualization data and results are analyzed by vision-based approach. Force analysis during instantaneous bubble growth from bubble nucleation to formation of annular flow regime has been performed for both the SiNW and plainwall microchannel configurations. Results from force analysis show the dominance of surface tension at L–V interface of growing bubble which resulted higher heat transfer contact area, lower thermal resistance and higher thin film evaporation. Whereas, inertia force is dominant at L–V interface of fully grown bubble and it helps in bubble removal process and rewetting before flow reversal. Significant differences between SiNW and plainwall microchannels have been observed in bubble growth mechanism, heat transfer mode and forces acting on L–V interface. [ABSTRACT FROM AUTHOR]

Critical heat flux ( CHF ) during flow boiling in silicon microchannels ( H = 250 μm, W = 200 μm, L = 10 mm) using self-excited and self-sustained high frequency two-phase oscillations is studied both experimentally and theoretically. Tests are performed on deionized water over a mass flux range of 200–1350 kg/m 2 s. An enhanced CHF of 1020 W/cm 2 is achieved experimentally at a mass flux of 1350 kg/m 2 s in the present study. Since no existing CHF models and correlations on parallel mini/microchannels considered high frequency two-phase oscillations, hence are not applicable to predict CHF in the present microchannel configuration. Adopting Helmholtz and Rayleigh instability theories and based on experimental study of liquid thin film dry-out phenomena in two-phase oscillations, a semi-theoretical CHF model is proposed. The proposed theoretical predictions show satisfactory agreement with experimental data with a reasonable low mean absolute error ( MAE ) of 25–32%. [ABSTRACT FROM AUTHOR]