Search

Accurate analysis and control of thermoelastic damping in thin beams are crucial for the design of high-performance components such as interferometric gravitational-wave detectors and micro- and nanomechanical resonators. The nonlinear thermoelasticity due to temperature dependent material properties may be the dominant source of energy dissipation and thermal noise in some cases. In this paper, the effect of temperature dependent material properties on thermoelastic damping is investigated. Governing equations for nonlinear coupled thermoelasticity in a thin beam with temperature dependent material properties are described and the perturbation method is used to treat the governing equations. An analytical model of thermoelastic damping is derived from the definition of thermoelastic damping, and a coefficient ξ is introduced into the analytical model to represent the effect of temperature dependent material properties. Numerical results of the coefficient ξ in a silicon thin beam with temperature dependent material properties are presented and validated by the experimental results. From the obtained numerical results, one can suppress the effect of temperature dependent material properties by reducing the initial amplitude, cooling the beam and selecting suitable boundary conditions and mode shapes.

We present a new concept of a structured surface for enhanced boiling heat transfer that is capable of self-adapting to the local thermal conditions. An array of freestanding nanoscale bimorphs, a structure that consists of two adjoining materials with a large thermal expansion mismatch, is able to deform under local temperature change. Such a surface gradually deforms as the nucleate boiling progresses due to the increase in the wall superheat. The deformation caused by the heated surface is shown to be favorable for boiling heat transfer, leading to about 10% of increase in the critical heat flux compared to a regular nanowire surface. A recently developed theoretical model that accounts for the critical instability wavelength of the vapor film and the capillary wicking force successfully describes the critical heat flux enhancement for the nanobimorph surface with a good quantitative agreement.

The accurate information of the thermal stresses and temperature in isotropic elastic solids is the key for many engineering applications. At present the classical linear coupled theory of thermoelasticity deduced with the assumptions of small temperature changes is widely used to solve the thermoelastic problems in engineering. In this paper, to describe the thermoelastic behavior in isotropic solids undergoing large temperature changes more accurately, the novel coupled models of thermoelasticity and the corresponding finite element models have been presented explicitly and validated by experimental measurement. The effect of large temperature changes on the solutions of thermoelastic problems is discussed. For the heat transfer process, if the isotropic elastic solids will expand when heated and contract when cooled and the condition d E E d T · σ i j E − δ i j 1 − 2 ν α 0 can be met in the context of small deformations, the effect of large temperature changes can be regarded as increasing the specific heat. The proposed models are applied to solve two thermoelastic problems. From the obtained numerical results, the effect of large temperature changes will increase with the amplitude of temperature change and may be considerably even when the temperature changes slowly.

In this paper, we describe governing equations for modified coupled thermoelasticity in micro- and nanomechanical beam resonators, which can treat the effects of size by taking the relaxation time, the phonon mean-free path and the material length scale parameter into account. An analytical model of thermoelastic damping is derived using the complex-frequency approach. Numerical results of thermoelastic damping calculated using the proposed model are presented and compared to those calculated using the models proposed by Zener and Lifshitz and Roukes for a silicon thin beam. The results show that the nonlocal effect characterized by the length scale parameter is negligible even at sizes down to the phonon mean-free path. The size effects characterized by the relaxation time and the phonon mean-free path are significant for a micron-scale beam. Device miniaturization beyond the submicron scale will lead to increased energy dissipation due to thermoelastic damping considering size effects. The influence of size effects on thermoelastic damping can be suppressed by increasing aspect ratios. Finally, we present the range of geometry of a silicon beam resonator where the effects of size can be neglected by taking 10% as the permitted error bound.

Superhydrophilicity accompanying hemi-wicking driven by interfacial capillary force can be induced by constructing interfacial structures. We uncover the underlying mechanism for the morphologically driven hemi-wicking, and extend its impact into the practical heat transferring scheme of convective boiling system: the morphologically-driven hemi-wicking on a roughened interface can contribute greatly to the enhancement of boiling heat transfer performance of the convective heat dissipation capacity of critical heat flux (CHF). We present design prerequisites on controlling characteristic lengths of nanoscale interfacial structures that initiate hemi-wicking and consequently enhance boiling performance. Interfacial liquid refreshing through morphologically driven hemi-wicking leads to a greater than 100% increase in CHF by roughening surfaces using vertically aligned silicon nanowire structures (SiNWs). We confirm strong wicking characteristics are essential to increase CHF, however it must be differentiated from surface roughening. Even though the roughening is a prerequisite for leading to the wicking, it can even deteriorate CHF without involving advantage of the interfacial re-wetting. Dimensional prerequisites that initiate hemi-wicking by modulating the characteristic length of SiNWs can be design guidelines for pragmatic engineering applications to enhance feasibility and reliability. We use our findings to put forward a guideline to improve boiling performance, and suggest a way to make breakthrough in heat and energy transfer systems through the functionalized interface.

Critical heat flux (CHF) enhancement is necessary in order to ensure a high operating limit for two-phase cooling applications. As the boiling is developed, formation of vapor film layer becomes vigorous, which causes CHF. Here, a graphene-coated micropillar structure (GMS) is proposed in order to enhance boiling heat transfer by suppressing vapor film formation on the surface. The GMS is designed to separate the bubble nucleation region from the liquid supply region in order to enhance CHF. By controlling the height of the micropillar, we obtained a structure in which the rGO layer is coated at the top of the micropillar array with high aspect ratio of the micropillar. In particular, the GMS consists of a reduced graphene oxide (rGO) porous mesh layer and a micropillar array layer. The rGO porous structure facilitated bubble nucleation by providing a suitably sized cavity. The micropillar array, which has excellent wicking performance, is located below the rGO porous layer in order to provide a capillary pumping to the vapor bubbles. Consequently, the GMS provides a significantly improved heat transfer coefficient and CHF of 288% and 152%, respectively, compared to the plain surface.

The objective of this study is to design a broadband and wide-angle emitter based on metamaterials with a cut-off wavelength of 2.1 µm to improve the spectral efficiency of thermophotovoltaic emitters. To obtain broadband emission, we conducted the geometric parameter optimization of the number of stacked layers, the inner and outer radii of the nano-rings, and the thickness of the nano-rings. The numerical simulation results showed that the proposed emitter had an average emissivity of 0.97 within the targeted wavelength, which ranged from 0.2 µm to 2.1 µm. In addition, the presented multilayer nano-ring emitter obtained 79.6% spectral efficiency with an InGaAs band gap of 0.6 eV at 1400 K.

In this study, we investigate the interfacial droplet jetting characteristics and thermal stability of a focused surface acoustic wave device (F-SAW). An F-SAW device capable of generating a 20 MHz surface acoustic wave by applying sufficient radio frequency power (2&ndash, 19 W) on a 128&deg, rotated YX-cut piezoelectric lithium niobate substrate for interfacial droplet jetting is proposed. The interfacial droplet jetting characteristics were visualized by a shadowgraph method using a high-speed camera, and a heat transfer experiment was conducted using K-type thermocouples. The interfacial droplet jetting characteristics (jet angle and height) were analyzed for two different cases by applying a single interdigital transducer and two opposite interdigital transducers. Surface temperature variations were analyzed with radio frequency input power increases to evaluate the thermal stability of the F-SAW device in air and water environments. We demonstrate that the maximum temperature increase of the F-SAW device in the water was 1/20 of that in the air, owing to the very high convective heat transfer coefficient of the water, resulting in prevention of the performance degradation of the focused acoustic wave device.

Flow boiling accompanied impingement jet was highly desired to enhance convective heat transfer. The secondary jet impingement system was designed to get enhanced heat transfer performance. The fluidic behavior was analyzed through visualization, and the local heat transfer was evaluated using an array of resistance temperature detector (RTD) sensors. The dielectric fluid FC-72 was used as coolant, and flowed through the rectangular channel with flow rate of Re = 6000 and saturated condition. We confirmed that the jet blowing ratio significantly influenced to the fluidic structure and local heat transfer distributions. Reinforced convective motion by jet flow removed bubbles on the heating surface, and increased local heat transfer coefficient by 59% with decreased wall superheat by 11% at the jet blowing ratio of 1:5. Whereas more intensified convective flow could delay onset of nucleate boiling (ONB) by disturbing thermal boundary layer at the jet blowing ratio of 1:10. Critical heat flux (CHF) increased quasi-linearly by increasing of the jet blowing ratio leading to the reinforcement of total fluidic momentum. Based on the results of the various jet blowing ratios and consequent local/overall heat transfer data, we conclude that the jet blowing ratio of 1:5 is an optimized condition for enhancing heat transfer coefficient at a given exit quality in the tested blowing ratios.

Owing to the recent advances in nanotechnology, one significant progress in energy technology is increased cooling ability. It has recently been shown that nanowires can improve pool boiling heat transfer due to the unique features such as enhanced wetting and enlarged nucleation sites. Applying such nanowires on a flow boiling, which is another major class of boiling phenomenon that is associated with forced convection, is yet immature and scarce despite its importance in various applications such as liquid cooling of energy, electronics and refrigeration systems. Here, we investigate flow boiling heat transfer on surfaces that are coated with SiNWs (silicon nanowires). Also, we use highly-wetting dielectric liquid, FC-72, as a working fluid. An interesting wetting behavior is observed where the presence of SiNWs reduces wetting and wicking that in turn leads to significant decrease of CHF (critical heat flux) compared to the plain surface, which opposes the current consensus. Also, the effects of nanowire length and Reynolds number on the boiling heat transfer are shown to be highly nonmonotonic. We attempt to explain such an unusual behavior on the basis of wetting, nucleation and forced convection, and we show that such factors are highly coupled in a way that lead to unusual behavior.

Enhancing the critical heat flux (CHF), which is the capacity of heat dissipation, is important to secure high stability in two-phase cooling systems. Coolant supply to a dry hot spot is a major mechanism to prevent surface burn-out for enhancing the CHF. Here, we demonstrate a more ready supply of coolant using aligned silicon nanowires (A-SiNWs), with a high aspect ratio (10) compared to that of conventional random silicon nanowires (R-SiNWs), which have a disordered arrangement, for additional CHF improvement. We propose the volumetric wicking rate, which represents the coolant supply properties by considering both the liquid supply velocity and the amount of coolant (i.e., wicking coefficient and wetted volume, respectively). Through experimental approaches, we confirm that the CHF is enhanced as the volumetric wicking rate is increased. In good agreement with the fabrication hypothesis, A-SiNWs demonstrate higher coolant supply abilities than those of R-SiNWs. The longest (7 μm) A-SiNWs have the highest volumetric wicking rate (25.11 × 10

Interface-inspired convection is a key heat transfer scheme for hot spot cooling and thermal energy transfer. An unavoidable trade-off of the convective heat transfer is pressure loss caused by fluidic resistance on an interface. To overcome this limitation, we uncover that nano-inspired interfaces can trigger a peculiar fluidic interactivity, which can pursue all the two sides of the coin: heat transfer and fluidic friction. We demonstrate the validity of a quasi-fin effect of Si-based nanostructures based on conductive capability of heat dissipation valid under the interactivity with fluidic viscous sublayer. The exclusive fluid-interface friction is achieved when the height of the nanostructures is much less than the thickness of the viscous sublayers in the turbulent regime. The strategic nanostructures show an enhancement of heat transfer coefficients in the wall jet region by more than 21% without any significant macroscale pressure loss under single-phase impinging jet. Nanostructures guaranteeing fluid access via an equivalent vacancy larger than the diffusive path length of viscid flow lead to local heat transfer enhancement of more than 13% at a stagnation point. Functional nanostructures will give shape to possible breakthroughs in heat transfer and its optimization can be pursued for engineered systems.

Convective boiling heat transfer is an efficient cooling mechanism to dissipate amount of thermal energy by accompanying the phase transition of the working fluids. Particularly, the amount of heat dissipation capacity can be readily extensible by increasing the degree of subcooling due to initial demands requiring for coolant saturation. Under severely subcooled condition of 60°, we investigate boiling heat transfer phenomena regarding spatial heat transfer uniformity and stability on a planar surface. Severe subcooling can induce locally concentrated thermal loads due to poor spatial uniformity of the heat transfer. For reliable cooling, a high degree of spatial uniformity of the heat transfer should be guaranteed with minimized spatial deviation of heat transfer characteristics. Under pre-requisite safeguards below CHF, we experimentally elucidate the principal factors affecting the spatial uniformity of the heat transfer for a flow/thermal boundary layer considering heat transfer domains from a single-phase regime to a fully-developed boiling regime. Based on the local heat transfer evaluation, we demonstrate that full nucleation boiling over the entire heat transfer surface under subcooling conditions is favorable in terms of the uniformity of heat dissipation through the phase-change of the working fluid.

Boiling-favorable merits of nanowire arrays are discussed for the thermal stability of boiling heat transfer. Local and temporal heat transfer characteristics are evaluated on vertically aligned nanowire arrays using a devised temperature-array sensor. The effects of rough morphology and highly wetting characteristics of nanowires lead to the reliable heat transfer stability/uniformity as well as efficient heat dissipation performances in pool boiling environments. The easy re-wetting and by-productive cavity-like structures via long nanowires can stabilize nucleation dynamics that catalyzes bubble nucleation dispersely and detaches developed bubbles quickly. Nanowires-inspired boiling heat transfer can make a breakthrough in improvements of heat transfer uniformity/stability with spatial and temporal temperature variations less than 1.0 and 2.0 K, respectively. SiNWs can also guarantee enhancements of both heat dissipation capacity and efficiency by more than 100% compared to a plain surface.

The enhancement of boiling heat transfer, the most powerful energy-transferring technology, will lead to milestones in the development of high-efficiency, next-generation energy systems. Perceiving nano-inspired interface functionalities from their rough morphologies, we demonstrate interface-induced liquid refreshing is essential to improve heat transfer by intrinsically avoiding Leidenfrost phenomenon. High liquid accessibility of hemi-wicking and catalytic nucleation, triggered by the morphological and hydrodynamic peculiarities of nano-inspired interfaces, contribute to the critical heat flux (CHF) and the heat transfer coefficient (HTC). Our experiments show CHF is a function of universal hydrodynamic characteristics involving interfacial liquid accessibility and HTC is improved with a higher probability of smaller nuclei with less superheat. Considering the interface-induced and bulk liquid accessibility at boiling, we discuss functionalizing the interactivity between an interface and a counteracting fluid seeking to create a novel interface, a so-called smart interface, for a breakthrough in boiling and its pragmatic application in energy systems.

It is within bounds to say that Graphene is at the cutting edge of material science and various engineering fields. Representative research and applications can be readily found in composite material, semiconductor, display, and energy for its intrinsic and potential merits involving electrical, mechanical and physico-chemical characteristics. The powerful merits of Graphene have been adapted in lots of application fields through the development of reliable and feasible synthesis technology. Especially for heat transfer applications, we can also highlight its attractiveness on interfacial functionalization with extremely high thermal conductivity [1], which is one order of magnitude greater than even common conductive materials such as copper or aluminum (with the order of hundreds of W/m∙K).

Morphologically driven dynamic wickability is essential for determining the hydrodynamic status of solid-liquid interface. We demonstrate that the dynamic wicking can play an integral role in supplying and propagating liquid through the interface, and govern the critical heat flux (CHF) against surface dry-out during boiling heat transfer. For the quantitative control of wicking, we manipulate the characteristic lengths of hexagonally arranged nanopillars within sub-micron range through nanosphere lithography combined with top-down metal-assisted chemical etching. Strong hemi-wicking over the manipulated interface (i.e., wicking coefficients) of 1.28 mm/s0.5 leads to 164% improvement of CHF compared to no wicking. As a theoretical guideline, our wickability-CHF model can make a perfect agreement with improved CHF, which cannot be predicted by the classic models pertaining to just wettability and roughness effects, independently.

We report combined electrochemical double-layer capacitance (EDLC) and pseudocapacitance in reduced graphene oxide (rGO) thick film like paper due to annealing temperature variations. The influence of annealing temperature (from room temperature (RT) to 1000 °C) on the structural, morphological, electrical, and electrochemical properties of rGO paper was evaluated. Upon increasing the annealing temperature, shifting of the dominant (002) X-ray diffraction (XRD) peak to a higher degree, volume expansion, and red-shifting of the G band in Raman spectra were observed. High-resolution transmission electron microscopy (HRTEM) images showed a reduction in the interlayer distance in rGO sheets from 0.369 to 0.349 nm as the annealing temperature increased from RT to 1000 °C; these results were congruent with the XRD results. According to X-ray photoelectron spectroscopy (XPS), the presence of hydroxyl, carboxyl, and other oxygen-containing groups decreased in samples annealed at higher temperatures. The attached functional groups, the electrical conductivity, and the supercapacitance of rGO papers were found to be mutually interrelated and could be tuned by varying the annealing temperature. The rGO paper annealed at 200 °C in a 1 M H2SO4 electrolyte at a scan rate of 50 mV s−1 exhibited a maximum specific capacitance of 198 F g−1.

Micro-nano hybrid structure (MNHS) that comprises of microcavities and nanowires is a specific class of MNHS that is considered to be ideal for two-phase boiling heat transfer applications. Realizing MNHS with electrodeposition is favorable in boiling heat transfer, but the realization has been very difficult and time-consuming to achieve. Here, we demonstrate a simple, robust, rapid, and photolithography-free route to fabricate MNHS that consists of individual microcavities and copper nanowires on a large area. We show that this MNHS can be extremely beneficial in boiling heat transfer compared to the state-of-the-art nanowire surface.