•Variation in reported CHF values on flat metal surfaces cannot be explained by hydrocarbon contamination alone.•We studied CHF on flat Cu and Ni surfaces, accounting for surface oxidation during ...boiling.•Formation of rough Cu2O nanostructures was accompanied by an increase in CHF.•Flat NiO formation resulted in a relatively stable morphology and CHF over time.•Formation of hydroxide (Ni(OH)2) led to a notably higher CHF without an increased roughness.
The critical heat flux during pool boiling has been investigated for a range of applications including electrical power generation and thermal management. Reported experimental CHF values during pool boiling of water on flat metallic surfaces, however, show a large discrepancy across studies. Here, we address this discrepancy in CHF values by accounting for oxidation of metallic surfaces during boiling. We studied the effect of in situ oxidation on flat Cu and Ni surfaces by changing the duration that samples were held in saturated water before conducting boiling experiments. The morphology and chemical composition of surfaces after the boiling experiments were analyzed by atomic force microscopy and X-ray photoelectron spectroscopy, respectively. Cu surfaces showed gradually increasing CHF values as the duration in saturated water increased, which could be attributed to the increase in roughness due to the formation of Cu2O nanostructures. Conversely, Ni surfaces showed relatively stable CHF and morphology as a nearly flat layer of NiO formed, with one exception: formation of a highly wetting hydroxide, Ni(OH)2, on a Ni coupon held in saturated water for 24 h resulted in a uniquely high CHF value, signifying the importance of surface chemistry in addition to morphology. The fundamental mechanisms resulting in the wide spread of CHF values on metallic surfaces elucidated in this work will lead to more accurate estimation of CHF as well as a deeper mechanistic understanding of CHF values on engineered surfaces.
Advances in two-dimensional (2D) devices require innovative approaches for manipulating transport properties. Analogous to the electrical and optical responses, it has been predicted that thermal ...transport across 2D materials can have a similar strong twist-angle dependence. Here, we report experimental evidence deviating from this understanding. In contrast to the large tunability in electrical transport, we measured an unexpected weak twist-angle dependence of interfacial thermal transport in MoS2 bilayers, which is consistent with theoretical calculations. More notably, we confirmed the existence of distinct regimes with weak and strong twist-angle dependencies for thermal transport, where, for example, a much stronger change with twist angles is expected for graphene bilayers. With atomic simulations, the distinct twist-angle effects on different 2D materials are explained by the suppression of long-wavelength phonons via the moiré superlattice. These findings elucidate the unique feature of 2D thermal transport and enable a new design space for engineering thermal devices.
Two-dimensional (2D) materials and their heterogeneous integration have enabled promising electronic and photonic applications. However, significant thermal challenges arise due to numerous van der ...Waals (vdW) interfaces limiting the dissipation of heat generated in the device. In this work, we investigate the vdW binding effect on heat transport through a MoS2-amorphous silica heterostructure. We show using atomistic simulations that the cross-plane thermal conductance starts to saturate with the increase of vdW binding energy, which is attributed to substrate-induced localized phonons. With these atomistic insights, we perform device-level heat transfer optimizations. Accordingly, we identify a regime, characterized by the coupling of in-plane and cross-plane heat transport mediated by vdW binding energy, where maximal heat dissipation in the device is achieved. These results elucidate fundamental heat transport through the vdW heterostructure and provide a pathway toward optimizing thermal management in 2D nanoscale devices.
We describe a customized Capillary Breakup Extensional Rheometer (CaBER) with improved dynamic performance and added features for temperature control over the range from room temperature up to 250 ...°C. The system is aimed at characterizing the extensional rheological behavior of weakly rate-thickening fluids that are widely utilized in the automotive industry. We examine the shear rheology and filament-thinning dynamics of two commercially available automotive fluids with the same viscosity index. Comparisons of the rheological properties of the two samples reveal that although they have identical shear viscosities, they exhibit significant and distinct rate-thickening behavior in the strong extensional flow that is generated close to filament breakup. For the more elastic sample, the exponential filament-thinning dynamics are well-described by the Oldroyd-B model; however, this viscoelastic model poorly describes the response of the more weakly rate-thickening fluid. To address this limitation, we propose a simple Inelastic Rate-Thickening (IRT) model that more robustly describes the measured material response. The two constitutive parameters of the model represent the zero-shear-rate viscosity of the fluid and the rate of extensional thickening in the fluid. Numerical calculations with the IRT model show that the radii of thinning fluid filaments deviate from a linear decay in time and approach a quadratic dependence very close to breakup. By carefully fitting the measured temporal evolution of the mid-plane radius we can therefore systematically differentiate the extensional rheological response of the two oils. More generally, we show that the weakly rate-thickening regime can be distinguished from the well-known elasto-capillary response predicted by the Oldroyd-B model, via a constraint on the relaxation time (or more specifically the elasto-capillary number) of the fluid. The weakly rate-thickening behavior documented in these oils is representative of the relatively unstudied extensional rheology of many industrial fluids at large extensional strain rates (100–1000 s-1) and influences many complex industrial processes such as jetting, coating and stamping.
•An improved Capillary Breakup Extensional Rheometer is designed and constructed.•Two studied synthetic motor oils exhibit weakly extensional-thickening behavior.•A 2-parameter inelastic model is proposed to characterize rate-dependent responses.•A new dimensionless criterion is applied to determine the appropriate model.
•Multiscale porous ceramic heat exchanger for high temperature applications.•Hierarchical models to predict and optimize heat exchanger core.•2.8× enhancement of the surface area to volume ratio ...compared to existing designs.
The efficiency of a heat engine can be significantly improved by operating in a high-temperature and high-pressure environment, which is crucial for a wide range of applications such as hybrid and electric aviation as well as power generation. However, such extreme operating conditions pose severe challenges to the heat exchanger design. Although recently developed superalloys and ceramics can survive high-temperature and high-pressure loads, using these materials in a traditional heat exchanger design requires high cost and yields low power density. In this work, we propose an ultrahigh power density ceramic heat exchanger for high-temperature applications enabled by a multiscale porous design. By optimizing the design of centimeter-scale macrochannels and microchannels, significant improvement to both heat transfer and structural strength is predicted, with a negligible pressure drop penalty (< 1%). Based on finite element simulations, an optimized heat exchanger core design is expected to achieve power densities of 717 MW/m3 and 300 kW/kg, which indicates more than 2.5× enhancement in thermal performance compared to printed-circuit heat exchanger design. Furthermore, the heat exchanger design features low material costs and scalable fabrication, enabling highly customizable applications in aerospace and terrestrial power generation.
The critical heat flux during pool boiling has been investigated for a range of applications including electrical power generation and thermal management. Reported experimental CHF values during pool ...boiling of water on flat metallic surfaces, however, show a large discrepancy across studies. Here, we address this discrepancy in CHF values by accounting for oxidation of metallic surfaces during boiling. We studied the effect of in situ oxidation on flat Cu and Ni surfaces by changing the duration that samples were held in saturated water before conducting boiling experiments. The morphology and chemical composition of surfaces after the boiling experiments were analyzed by atomic force microscopy and X-ray photoelectron spectroscopy, respectively. Cu surfaces showed gradually increasing CHF values as the duration in saturated water increased, which could be attributed to the increase in roughness due to the formation of Cu2O nanostructures. Conversely, Ni surfaces showed relatively stable CHF and morphology as a nearly flat layer of NiO formed, with one exception: formation of a highly wetting hydroxide, Ni(OH)2, on a Ni coupon held in saturated water for 24 h resulted in a uniquely high CHF value, signifying the importance of surface chemistry in addition to morphology. Finally, the fundamental mechanisms resulting in the wide spread of CHF values on metallic surfaces elucidated in this work will lead to more accurate estimation of CHF as well as a deeper mechanistic understanding of CHF values on engineered surfaces.
In electrical discharge machining (EDM) process, tool wear is an inevitable phenomenon that adversely affects the geometrical accuracy of machined features. A theoretical model accounting for tool ...wear during EDM process is hence the basis study for high precision machining. However, in most modeling studies on tool wear and electrode shape, the sparking process is only factorized by the geometric configuration, i.e. the distance between electrodes. The real sparking process related to the fundamental physics is not addressed in these geometric models, which can produce large discrepancies with the experimental results. In this paper, a model of tool wear in EDM is proposed, which accounts for the electric field inside the dielectric fluid using electromagnetic (EM) theory. The spark is proposed to occur at the position where the local electric intensity reaches maximum and exceeds the breakdown strength of the dielectric fluid. This model is shown to provide the physical insight of the real EDM situation, and to give a more accurate prediction of tool wear compared with traditional geometric property based modeling. With these merits, this proposed model can be applied to predict tool wear in various machining processes. To evaluate this model, simulations of EDM die sinking and ED milling are carried out. The results by this electric field model were compared with both geometric model and experiments. By analyzing the profiles of the tool end, the differences in mechanism between the electric field and geometric model are identified. In addition, this electric field model is also applied to simulate the conic tool forming process in the fix-length compensation with micro-milling, which cannot be thoroughly addressed by the geometric model. The model presented in this paper is capable of capturing the key features of the tool wear in a variety of machining processes.
•The corona discharging is considered in the model with electromagnetic theory.•The effects of tool end curvatures on wearing processes are identified and explained.•The predictions of worn tool shape in various conditions are significantly improved.
Water vapor sorption is a ubiquitous phenomenon in nature and plays an important role in various applications, including humidity regulation, energy storage, thermal management, and water harvesting. ...In particular, capturing moisture at elevated temperatures is highly desirable to prevent dehydration and to enlarge the tunability of water uptake. However, owing to the thermodynamic limit of conventional materials, sorbents inevitably tend to capture less water vapor at higher temperatures, impeding their broad applications. Here, an inverse temperature dependence of water sorption in poly(ethylene glycol) (PEG) hydrogels, where their water uptake can be doubled with increasing temperature from 25 to 50 °C, is reported. With mechanistic modeling of water–polymer interactions, this unusual water sorption is attributed to the first‐order phase transformation of PEG structures, and the key parameters for a more generalized strategy in materials development are identified. This work elucidates a new regime of water sorption with an unusual temperature dependence, enabling a promising engineering space for harnessing moisture and heat.
Materials typically lose their water sorption capability with increasing temperature. Can this temperature effect be reversed? By leveraging the phase transformation of semicrystalline hydrogels, an unusual water sorption behavior is demonstrated where the water uptake can be doubled with an increased temperature of 25 °C. This finding will enable a new degree of freedom to harness moisture and heat.
Saturated steam (>121°C and >205 kPa) is widely used in the medical sterilization process known as autoclaving. However, solar-driven steam generation at such high temperature and pressure requires ...expensive optical concentrators. We demonstrate a passive solar thermal device mostly built from low-cost off-the-shelf components capable of delivering saturated and pressurized steam to drive sterilization cycles even under hazy and partly cloudy weather. Enabled by an optimized ultra-transparent silica aerogel, the device utilizes an efficient thermal concentration strategy to locally increase the heat flux and temperature obviating the need for active optical concentrators. With almost 2× higher energy efficiency (47%) than those previously reported at 100°C, the device demonstrated successful sterilization in a field test performed in Mumbai, India. In addition to enabling passive sterilization, this work promises the development of solar thermal energy systems for saturated steam generation in energy conversion, storage, and transport applications.
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•A passive solar thermal device that can generate steam at 128°C and 250 kPa•Efficient thermal concentration enabled by an ultra-transparent aerogel layer•Up to 56% efficiency when generating steam at 100°C with 0.7 kW/m2 solar flux•Effective medical sterilization demonstrated under realistic weather conditions
Healthcare-associated infections cause a massive burden for the health care system and the patients. Although the standard sterilization protocol with saturated steam (>121°C and >205 kPa) is effective, generating high-temperature and high-pressure steam is challenging without reliable access to electricity or fuel. While abundant solar energy is readily available, utilizing sunlight to generate steam beyond 100°C requires costly and bulky optomechanical components. In this work, we developed a stationary solar thermal device capable of providing the required saturated steam. Enabled by an optimized transparent aerogel layer, the device can efficiently convert solar energy into heat to drive the steam generation process. Successful sterilization cycles were demonstrated in a field test conducted in Mumbai, India. As a general approach, this work also promises further development of solar thermal technology in energy conversion, storage, and transport applications.
Solar steam generation at the sterilization condition suffers from low efficiency, especially in passive solar thermal devices. We developed a stationary solar collector with a transparent aerogel layer to achieve efficient solar steam generation via thermal concentration. In field tests performed in Mumbai, India, the device generated steam at 100°C with 56% efficiency and successfully powered a sterilization cycle following the standard sterilization protocol. Our work shows the potential of solar thermal technology in steam generation and other applications.
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