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Thermal management of high heat flux devices is becoming a key challenge, mainly due to dense packaging, chip miniaturization, high power density and enhanced output performance. Despite high heat removal rates achieved in phase change processes, such as the spray cooling, immense heat dissipation in high heat flux devices may not be thermally managed by conventional heat transfer fluids (such as water and dielectric fluids) due to their limited cooling capacity. This research is an effort to address heat dissipation issues in high heat flux devices (for instance, electric vehicle high power electronics) using an advanced thermal fluid, that is, the hybrid nanofluid, in a droplet based spray cooling process. This research shows that as hybrid nanofluid spray droplets evaporate, a porous residue comprising hybrid nanoparticles is formed over a substrate. As a main novelty of this thesis, wetting and phase change behavior of the subsequent hybrid nanofluid droplet over a residue formed by evaporation of the preceding hybrid nanofluid droplet for various mixing ratios and residue sizes are investigated. Analytical and semi-analytical mathematical models are developed to estimate the hybrid nanofluid droplet evaporation rate over plain copper and porous residue surfaces. This research indicates that both the residue size as well as the mixing ratio considerably affect the evaporation and boiling performances of the subsequent hybrid nanofluid droplet sitting over the residue from previously evaporated hybrid nanofluid droplet. Therefore, the residue effect on evaporation performance of subsequent droplets is important to consider in hybrid nanofluid spray cooling of high heat flux devices. In this thesis, the hybrid nanofluid spray cooling performance for various particle concentrations is investigated and compared with the benchmark fluid (water). Finally, it is demonstrated that the hybrid nanofluid spray cooling has a potential to keep high power electronics of curre

Flow boiling in a microchannel is an effective and attractive solution for thermal management. In this work, flow boiling was performed in microchannels with various wettability-patterned dotted surfaces. Pitch distances of dots were changed to study the wettability pattern effects on microchannel flow boiling. Glass-silicon-glass microchannels integrated with internal platinum heaters and various wettability patterns were fabricated and characterized using MEMS techniques. The transparent top of the channel allowed visualization of bubble dynamics and flow patterns. The hydraulic diameter of the microchannel is 338 μm, the side length of hydrophobic dots is 72 μm and the pitch distances of hydrophobic dots change from 122 μm to 172 μm. Bubble dynamics and flow patterns were visualized using a high-speed camera and a microscope. Heat transfer performance, including boiling curves and heat transfer coefficients (HTC), were explored experimentally under different mass flux. It is found that bubbles coalesce more frequently and flow pattern transitions are more intense with the decrease of pitch distance. In the low mass flux region, better heat transfer capability can be observed on the surface with larger pattern pitch distance, while with the increase of mass flux, a microchannel with smaller dot pitch distance presents better heat dissipation ability. Moreover, the critical heat flux (CHF) decreases with the decrease of dot pitch distance. The reasons behind these phenomena are related to the bubble coalescence, bubble density, and flow movement. © 2020, Avestia Publishing. All rights reserved.

Droplet evaporation-based cooling techniques, such as the spray cooling, give high heat transfer rates by utilizing latent energy and are usually preferred in thermal applications. However, with the significant rise in heat dissipation levels for high heat flux devices, these devices cannot be thermally managed due to the limited cooling capacity of existing thermal fluids. In this paper, we report the evaporation of the Cu-Al2O3 hybrid nanofluid (HNF) droplet on a copper surface as well as its own residue surface, developed from the evaporation of the first Cu-Al2O3 HNF droplet. As the main novelty, we identify the critical residue size and investigate the residue size effect, above and below the critical residue size, on evaporation rate of the succeeding Cu-Al2O3 HNF droplet resting over a residue surface. We also develop a new analytical model to estimate the Cu-Al2O3 HNF droplet evaporation rate and compare our results with other existing models. The results show that the Cu-Al2O3 HNF droplet gives 17% higher evaporation rate than a water droplet on a copper surface. Also, the evaporation rate of the Cu-Al2O3 HNF droplet on a residue surface sharply increases by 106% with increasing residue size up to the critical residue size. However, further increasing the residue size above its critical value has a negligible effect on the droplet evaporation rate. Moreover, the evaporation rate of the Cu-Al2O3 HNF droplet on its residue surface is enhanced up to 104% when compared to a copper surface. © 2021 American Society of Mechanical Engineers (ASME). All rights reserved.

The forewing and the hindwing of a dragonfly have different geometry that could be an evolutionary specialization for better aerodynamic performance via sophisticated wing pitch control. Under different extent of wing pitching by the wing root musculature, the fore- and hindwings could exhibit different shape deformation and aerodynamic characteristics as a result of passive shape deformation. We measured the flow around the flapping wings using time-resolved particle image velocimetry (TR-PIV) to investigate the consequences of shape and the pitching mechanisms of the wings on the aerodynamics of dragonflies. The flow fields and pitching angle variations of the naturally actuated wing of the dragonfly were compared with that of the same wing artificially actuated only by flapping motion. We found that the trailing edge vortex dynamics and the wake were affected by the wing shape only for the in-vivo experiment with muscle induced pitching. Under the in-vivo with muscle induced pitching, the hindwing took more part in generating horizontal momentum with larger pitching magnitude, due to the larger chord length compared with forewing. Meanwhile, when there was only pitching due to the wing membrane deformation of artificially actuated flapping, a slight difference in the surrounding flow structures was found between the hindwing and the forewing, and the net flow in one period was reduced nearly to zero. These results provided quantified evidence to the extent and importance of the pitching motion of the wings in dragonfly flight. The results of this work can be useful for the design of wings, their actuation mechanism, and the in-flight kinematics control of flapping wing micro air vehicles (MAVs).

In this paper, TGPs with thickness of around 0.53-0.6 mm are developed with copper plate as the casing material, nanostructured copper foam as the wick and electroplated copper pillar as the support of vapor core. The effects of charge amount of water, nanostructured wick, the number of copper foam layers and vapor core thickness on the thermal performance of the developed TGP are experimentally studied. It is found that increasing the number of copper foam layers or vapor core thickness can greatly lower the TGP thermal resistance. Specifically, a TGP with a double layers copper foam wick has a much better thermal performance than that with single layer even though the total wick thickness and porosity remain unchanged. Experimental results also show that the multiscale micro/nanostructured wick is beneficial for improving the TGP thermal performance. Small change of vapor core thickness can lead to great change of TGP thermal performance. The ultrathin TGP can reach an effective thermal conductivity of 2207 W/(m·K) at an input power of 22.66 W over a heating area of 8 mm × 8 mm. © 2020 The Authors.

Droplet evaporation is an efficient process as it removes a large amount of heat by using the latent energy, making it suitable for heat transfer applications. In this research, evaporation of the silver-graphene hybrid nanofluid (SGHF) droplet, because of its synergistic thermal conductivity, is investigated for substrate temperature in a range of 25-100 °C. The experiments for droplet evaporation were performed in an environmental facility for two droplet sizes, 3 µL and 30 µL volume, on a copper plate. A 100 W silicone heater mat was used to heat the copper plate from the underside, while two T-type thermocouples were used to monitor its surface temperature. As droplet evaporation ended, a porous residue was formed on the copper surface. Subsequently, a 3 µL volume of the SGHF droplet was dispensed on the porous residue surface. The results showed a tremendous rise in the evaporation rate (up to 160%) for the subsequent SGHF droplet sitting on the porous residue as compared to the non-wetted copper surface. Moreover, the evaporation rate of the SGHF droplet on the copper surface increased up to 56% as compared to the water droplet for a substrate temperature range of 25-100 °C. © 2020 ASME

The heat transfer performance of heat exchangers is extremely restricted due to the considerable thermal resistance of condensing droplets. By adopting the coalescence-induced droplet jumping phenomenon on a superhydrophobic coated surface, the condensing droplets can be spontaneously removed from the heat exchanger, and the heat transfer performance can be enhanced. However, under unfavorable conditions, the flooding effect, which restricts the droplet jumping rate and volume, occurs on heat exchangers and results in the degradation of heat transfer performance. In this study, novel heat transfer tubes with biphilic nanomorphology were fabricated and adopted to address the flooding issue and improve the heat transfer performance of heat exchangers. Through experiments we demonstrated that the droplet jumping effect could be sustained on a biphilic tube due to the formation of droplets in Cassie state during condensation. Because of the high droplet jumping and self-removal rate, the convective heat transfer coefficient was improved by 29% and 38% on a biphilic tube as compared with a typical copper tube and superhydrophobic tube, respectively. Besides, the condensation performance (i.e. water collection rate) on a biphilic heat exchanger prototype was also enhanced by 123% as compared with that on a copper heat exchanger prototype. This study not only enhances the condensation and convective heat transfer performance of heat exchangers by addressing the flooding issue, but also provides a practical strategy to improve the energy conversion efficiency of the thermal components with condensation effect in various thermal applications. © 2020 Elsevier Ltd

Droplet evaporation offers high heat rejection rates and is widely used in the form of spray cooling or dropwise cooling of various heat dissipating devices. However, due to the limiting heat flux removal capacity of conventional fluids, such as water, these cannot be used in thermal management of high heat flux devices. In this research, the evaporation of silver (Ag)-graphene (GNP) hybrid nanofluid droplet and its residue effects on the evaporation of following Ag-GNP hybrid nanofluid droplet, due to its synergistic thermal properties, is experimentally investigated for various mixing ratios, from MR-1 (0.1(Ag):0.9(GNP)) to MR-5 (0.9(Ag):0.1(GNP)), and different residue sizes. A theoretical model is also proposed for hybrid nanofluid droplet evaporation and semi-empirical relations are developed to estimate the hybrid nanofluid droplet spreading over its residue surface. The results show a substantial increase in the droplet evaporation rate with increasing residue size and decreasing mixing ratio. MR-1 hybrid nanofluid droplet gives the highest evaporation rate (up to 370%) on its highly wetted residue surface, while the evaporation rate significantly drops moving from MR-2 to MR-5 hybrid nanofluid droplets on their partially wetted residue surfaces. Moreover, the evaporation rate substantially increases (up to 240%) with increasing residue size for MR-1 hybrid nanofluid droplet resting on its residue surface, however, the effect of residue size on droplet evaporation rate considerably diminishes moving from MR-2 to MR-5 hybrid nanofluid droplets resting on their respective residues. © 2020

We present a quantitative characterization of the unsteady aerodynamic features of a live, free-flying dragonfly under a well-established flight condition. In particular, our investigations cover the span-wise features of vortex interactions between the fore- and hind-pairs of wings that could be a distinctive feature of a high aspect ratio tandem flapping wing pair. Flapping kinematics and dynamic wing-shape deformation of a dragonfly were measured by tracking painted landmarks on the wings. Using it as the input, computational fluid dynamics analyses were conducted, complemented with time-resolved particle image velocimetry flow measurements to better understand the aerodynamics associated with a dragonfly. The results show that the flow structures around hindwing’s inner region are influenced by forewing’s leading edge vortex, while those around hindwing’s outer region are more influenced by forewing’s shed trailing edge vortex. Using a span-resolved approach, we found that the forewing–hindwing interactions affect the horizontal force (thrust) generation of the hindwing most prominently and the modulation of the force generation is distributed evenly around the midspan. Compared to operating in isolation, the thrust of the hindwing is largely increased during upstroke, albeit the drag is also slightly increased during the downstroke. The vertical force generation is moderately affected by the forewing–hindwing interactions and the modulation takes place in the outer 40% of the hindwing span during the downstroke and in the inner 60% of the span during the upstroke.

Cost reduction and enhanced cooling performance are strongly demanded for daytime passive radiative cooling due to its attractive cooling strategy that does not require any energy input. Its potential application varies widely from air conditioning systems for buildings, photovoltaic cells, electronic device cooling and automobiles. However, recently proposed daytime passive radiative coolers are based on photonic structures which are high in cost. A relatively cheap metal oxide material, TiO2, which lowers the cost but is highly emissive in the mid-infrared range has been used, also improving the cooling performance of the photonic daytime passive radiative cooler. An optimized TiO2–SiO2 alternating multi-layered photonic daytime radiative cooler with average emissivity of 0.84 within 8–13 μm while reflecting 94% of incident solar energy is developed. Its net cooling power is estimated to be 136.3 W/m2 at ambient air temperature of 27 °C which shows an improvement of 90 W/m2 compared to that of the HfO2-SiO2 photonic radiative cooler. Last, a field test has been conducted in Hong Kong's subtropical climate (i.e. relative humidity = 60–70%) to investigate its feasibility, and with the help of solar shading, successfully demonstrated temperature reduction of 7.2 °C with a net cooling power of 14.3 W/m2 under direct sunlight.

Droplet evaporation is a complex phase change process with a wide range of cooling applications, such as spray cooling and dropwise hotspot cooling in microelectronics, to name a few. The hybrid nanofluid droplet evaporation and its residue effects on evaporation of the subsequent hybrid nanofluid droplet is investigated in this research. Silver-graphene (Ag-GNP) hybrid nanofluid exhibiting synergistic thermal properties is investigated and prepared by dispersing silver nanoparticles along with graphene nanoplatelets in water at 0.1% volume fraction and with different mixing ratios, followed by ultra-sonication. The evaporation rate and wetting characteristics of a 3 µl volume of Ag-GNP hybrid nanofluid droplet on a copper surface were studied using an optical tensiometer. Once dried, the nanoporous structure of the residue surface was examined using a scanning electron microscope, while the surface roughness was measured using an optical profiler. Experiments were continued to further investigate the evaporation rate and wetting effects of the subsequent Ag-GNP hybrid nanofluid droplet over the residue surface. The results showed improved wetting characteristics, with 88% reduction in initial static contact angle and 163-196% enhancement in evaporation rate of the subsequent Ag-GNP hybrid nanofluid droplets over the residue surfaces as compared to the copper surface. Copyright © 2019 ASME.

The investigation focused on the conversions of flow structures with a change in angle of attack (AOA) for a flexible cantilever wing, which experienced a self-excited vibration. Stereoscopic particle imaging velocimetry (Stereo-PIV) was utilized to measure the velocity field in the wing-tip region as AOA varied from 0 deg to 12 deg. At the Reynolds number (Re) of 3 × 104, instability waves shedding from the wing were amplified as they propagated and developed into Karman Vortex Street in the far downstream region at low AOAs (AOA = 4 deg and 6 deg). As AOA increased to 8 deg with the wing model was still steady, the Karman Vortex Street no longer existed. The wing started to vibrate at AOA = 10 deg owing to the self-excited vibration, and the Karman Vortex Street appeared again. The inception location of the Karman Vortex Street moved further upstream than in the cases at AOA = 4 deg and 6 deg. A new vortex structure, secondary vortex-pairs, appears outside the main wing-tip vortex (WTV).

Similar to an electrical diode, a thermal diode/switch is a device which allows heat to flow to a preferential direction. While a number of different types of thermal diodes/switches exist, a phase-change thermal diode/switch yields a greater diodicity performance compared to other types. However, most phase-change thermal diodes/switches have complex designs, complicated fabrication processes, and sophisticated working principles. Moreover, some materials used in thermal diodes/switches are rare, expensive and toxic. Thus, in this study, a simple water-vapor chamber thermal diode using latent heat of vaporization is designed, assembled and investigated, both experimentally and theoretically. The effects of the temperature difference between the hot side and the cold side and the water-air volume ratio on the effective thermal conductivity and diodicity of the water-vapor chamber thermal diode are also investigated. Mathematical models are also developed, not only for predicting one-dimensional phase change heat transfer performance in a rigid enclosure, but also for verifying the results from the experiment. This is the first study in which the water-vapor chamber thermal diode is studied both experimentally and theoretically. The experimental results show that an increase in temperature at the hot side of the thermal diode significantly enhances the performance of the thermal diode. The effective thermal conductivity at the hot side temperature of 70 °C shows a 50% improvement when compared with that at the hot side temperature of 40 °C in the forward direction. Additionally, with the water-air volume ratio of 0.5, the maximum diodicity of 1.43 is reported. Moreover, it is also found that the water-vapor volume ratio affects heat transfer performance and diodicity of the water-vapor chamber thermal diode. The results of these findings can further advance knowledge on phase-change heat transfer and can be applied to several applications, including heat transfer enh

A nanostructured surface is proposed for potential application of anti-icing. This surface integrates microcavities with doubly reentrant nanostructures which can simultaneously enhance static repellency and dynamic pressure resistance. This surface is inspired by the skin structure of a spring tail which can survive even in oil and ethenal. We fabricated nanostructures with overhanging edge which exhibited great wetting resistance ability. It is found that when a droplet impacting on different surfaces, the newly fabricated nanostructured doubly reentrant surface shows a shortest contact duration in comparison with other surfaces. Therefore, the nucleation for icing on a cold doubly reentrant nanostructures is depressed and the surface shows icephobic property. Our new structure showed great wetting resistance ability in both static and dynamic conditions, under low temperature and the theoretical analysis accorded with the existing simulation work. The decrease of contact time for the new surface also gave the potential in applications like anti-icing in the future. © 2019 Avestia Publishing.

Exposure to inhaling airborne particles in indoor and outdoor environments is a threat to public health. Indoor air passes through ventilation ducts and continuously circulates with the air outdoors, resulting in a higher concentration of suspended particles in indoor environments when compared to outdoor environments. Thus removing airborne particles in ventilation ducts becomes essential, especially in large buildings. Repeated surface ribs have been reported to greatly enhance the particle collection efficiency while it also causes a significant pressure drop, leading to higher energy consumption. In this study, an overall efficiency ratio is defined, taking into consideration the particle removal rate and the associated pressure drop, to evaluate the overall performance of different surface patterns in a ventilation duct. After the design and optimization processes, the semi-circular patterns are shown to have the best overall efficiency, i.e. a 1137 times increase when compared with having no patterned surface. The deposition velocity on the semi-circular surface found in simulation results were validated with a fully-developed wind tunnel experiment. This study shows that semi-circular surface patterns with a pitch-To-height ratio of 4 are recommended for the overall enhancement in the ventilation duct, especially for capturing submicron particles. © Published under licence by IOP Publishing Ltd.

Flow boiling heat transfer in microspaces is of significant interest for thermal management applications, where the latent heat of phase change offers an efficient method to dissipate large heat fluxes in a compact device, such as a heat spreader or a heat pipe. However, a significant challenge for the implementation of microscale phase change heat spreaders is associated with micro/nano flow instabilities due to insufficient micro/nano bubble removal, leading to local liquid dry-out that severely limits the heat removal efficiency. Furthermore, for the various heat transfer mechanisms involved, it is difficult to predict the location of nucleation sites at which the onset of nucleate boiling (ONB) occurs. In this paper, we study the bubble dynamics and heat transfer in a flow boiling microchannel with a wettability patterned channel surface and compare it with a homogeneous hydrophilic channel surface. The bubble nucleation site and coalescence are studied experimentally. The effects of wettability patterned surfaces on bubble nucleation and contact line dynamics in a microchannel will be investigated utilizing high speed visualization techniques. Effects of mass flux on flow boiling in a wettability patterned microchannel are studied. Wettability patterned surfaces are manufactured on glass/silicon wafers. It is found that the heat transfer coefficient can be significantly enhanced by wettability patterns in comparison to a hydrophilic surface. The surface temperature with wettability patterned surfaces is lower than hydrophilic surfaces. The detailed experimental setup and results are presented in the paper.

Flow boiling heat transfer in microchannels is of significant interest for thermal management applications, where the latent heat of phase change offers an efficient method to dissipate large heat fluxes in a compact device, such as a mciro heat spreader or a heat pipe. However, a significant challenge for the implementation of microscale phase change heat spreaders is associated with micro/nano flow instabilities due to insufficient micro/nano bubble removal, leading to local liquid dry-out that severely limits the heat removal efficiency. Furthermore, for the various heat transfer mechanisms involved, it is difficult to predict the location of nucleation sites at which the onset of nucleate boiling (ONB) occurs. In this paper, a chemcially patterned surface has been developed to manipulate nucleation boiling in a micro fluid flow channel. Bubble dynamics and heat transfer with shear force conditions in a heterogeneous wettability microchannel were studied experimentally and compared with a homogeneous hydrophilic microchannel. The bubble nucleation site and coalescence are studied experimentally. The effects of chemically patterned surfaces on bubble nucleation and bubble pinning and depinning in a microchannel will be investigated utilizing high speed visualization techniques and analytical modelling. Effects of mass flux on flow boiling in a wettability patterned microchannel are studied. Chemically patterned surfaces are manufactured on glass/silicon wafers. It is found that the heat transfer coefficient can be significantly enhanced by chemical patterns in comparison to a hydrophilic surface. The surface temperature with chemically patterned surfaces is lower than hydrophilic surfaces. © 2019, Avestia Publishing.

We investigated the characteristics of interwing aerodynamic interactions across the span of the high aspect ratio, flexible wings of dragonflies under tethered and free-flying conditions. This revealed that the effects of the interactions on the hindwings vary across four spanwise regions. (i) Close to the wing root, a trailing-edge vortex (TEV) is formed by each stroke, while the formation of a leading-edge vortex (LEV) is limited by the short translational distance of the hindwing and suppressed by the forewing-induced flow. (ii) In the region away from the wing root but not quite up to midspan, the formation of the hindwing LEV is influenced by that of the forewing LEV. This vortex synergy can increase the circulation of the hindwing LEV in the corresponding cross-section by 22% versus that of the hindwing in isolation. (iii) In the region about half-way between the wing root and wing tip there is a transition dominated by downwash from the forewing resulting in flow attached to the hindwing. (iv) A LEV is developed in the remaining, outer region of the wing at the end of a stroke when the hindwing captures the vortex shed by the forewing. The interaction effects depend not only on the wing phasing but also on the flapping offset and flight direction. The aerodynamics of the hindwings vary substantially from the wing root to the wing tip. For a given phasing, this spanwise variation in the aerodynamics can be exploited in the design of artificial wings to achieve greater agility and higher efficiency.