Showing papers on "Thermal published in 2011"

Azobenzene-Functionalized Carbon Nanotubes As High-Energy Density Solar Thermal Fuels

Abstract: Solar thermal fuels, which reversibly store solar energy in molecular bonds, are a tantalizing prospect for clean, renewable, and transportable energy conversion/storage. However, large-scale adoption requires enhanced energy storage capacity and thermal stability. Here we present a novel solar thermal fuel, composed of azobenzene-functionalized carbon nanotubes, with the volumetric energy density of Li-ion batteries. Our work also demonstrates that the inclusion of nanoscale templates is an effective strategy for design of highly cyclable, thermally stable, and energy-dense solar thermal fuels.

Light-activated shape memory of glassy, azobenzene liquid crystalline polymer networks

Abstract: Rapidly reconfigurable, adaptive materials are essential for the realization of “smart”, highly engineered technologies sought by aerospace, medicine, and other application areas. Shape memory observed in metal alloys and polymers (SMPs) is a primary example of shape change (adaptation). To date, nearly all shape adaptations in SMPs have been thermally triggered. A desire for isothermal, remotely cued shape adaptations of SMP has motivated examinations of other stimuli, such as light. Only a few reports document so-called light-activated SMP, in both cases exploiting photoinduced adjustments to the crosslink density of a polymer matrix with UV light of 365 nm (crosslinking) and <260 nm (decrosslinking). This work presents a distinctive approach to generating light-activated SMP by employing a glassy liquid crystal polymer network (LCN) material that is capable of rapid photo-fixing with short exposures (<5 min) of eye-safe 442 nm light. Here, linearly polarized 442 nm light is used to photo-fix temporary states in both cantilever and free-standing geometries which are then thermally or optically restored to the permanent shape. The combination of thermal and photo-fixable shape memory presented here yields substantial functionality in a single adaptive material that could reduce part count in applications. As a demonstration of the opportunities afforded by this functional material, the glassy, photoresponsive LCN is thermally fixed as a catapult and subsequently used to transduce light energy into mechanical work, demonstrated here in the “photo-fueled” launching of an object at a rate of 0.3 m s−1.

Prandtl and Rayleigh number dependence of heat transport in high Rayleigh number thermal convection

Abstract: Results from direct numerical simulation for three-dimensional Rayleigh–Benard convection in samples of aspect ratio and up to Rayleigh number are presented. The broad range of Prandtl numbers is considered. In contrast to some experiments, we do not see any increase in with increasing , neither due to an increasing , nor due to constant heat flux boundary conditions at the bottom plate instead of constant temperature boundary conditions. Even at these very high , both the thermal and kinetic boundary layer thicknesses obey Prandtl–Blasius scaling.

Mechanical properties of thermally-treated and recycled glass fibres

Abstract: This paper investigates the effects of temperature, heating time and atmosphere on the tensile modulus and strength of thermally-treated E-glass fibres. The heating conditions that were investigated are identical to those used in thermal recycling of waste polymer matrix composite materials, and therefore this study determines the effects of the recycling process conditions on the properties of reclaimed fibreglass. The loss in fibre strength is dependent on the temperature and time of the thermal process, and large strength loss occurs under the heating conditions used for high temperature incineration of polymer composites. A phenomenological model is presented for the residual fibre strength for the temperatures and heating time of the thermal recycling process. The reduction in fibre strength is dependent on the thermal recycling atmosphere under low temperature or short heating time conditions, but at high temperatures the strength loss is the same, regardless of furnace atmosphere (ambient air, dry air or inert gas). Quantitative fractographic analysis of the fibres shows that fracture for all heat treatments is caused by surface flaws. The strength loss is most probably due to structural relaxation during thermal annealing and a secondary effect of adsorbed surface water attacking the glass by thermally-activated stress-corrosion. It is shown that large reductions in fibre strength due to thermal recycling are not recovered during composite manufacture, therefore resulting in composite materials with significantly lower strength. The reduced strength of the composite matches the reduced fibre strength following thermal recycling.

Turbulent mixing and layer formation in double-diffusive convection: three-dimensional numerical simulations and theory

Abstract: Double-diffusive convection, often referred to as semi-convection in astrophysics, occurs in thermally and compositionally stratified systems which are stable according to the Ledoux criterion but unstable according to the Schwarzschild criterion. This process has been given relatively little attention so far, and its properties remain poorly constrained. In this paper, we present and analyze a set of three-dimensional simulations of this phenomenon in a Cartesian domain under the Boussinesq approximation. We find that in some cases the double-diffusive convection saturates into a state of homogeneous turbulence, but with turbulent fluxes several orders of magnitude smaller than those expected from direct overturning convection. In other cases, the system rapidly and spontaneously develops closely packed thermo-compositional layers, which later successively merge until a single layer is left. We compare the output of our simulations with an existing theory of layer formation in the oceanographic context and find very good agreement between the model and our results. The thermal and compositional mixing rates increase significantly during layer formation and increase even further with each merger. We find that the heat flux through the staircase is a simple function of the layer height. We conclude by proposing a new approach to studying transport by double-diffusive convection in astrophysics.

Heating and cooling of protons in the fast solar wind between 0.3 and 1 AU: Helios revisited

Abstract: The proton thermal energetics in the fast solar wind between 0.3 and 1 AU is re-investigated using the Helios 1 and 2 data. Closer to the Sun, it is estimated that, to ac- count for the observed radial profiles of the proton parallel and perpendicular temperature, non- negligible parallel cooling and perpendicular heating are necessary. Around 1 AU heating is needed in both directions. We also calculate the corresponding rates and find that in total sig- nificant interplanetary heating is necessary, in agreement with previous results. The possible influence that deceleration of fast solar wind streams due to interaction with slow ones has on the proton thermodynamics is evaluated.

Electrical-Thermal Co-Simulation of 3D Integrated Systems With Micro-Fluidic Cooling and Joule Heating Effects

Abstract: In this paper, the electrical-thermal co-simulation of 3D systems with Joule heating, fluidic cooling and air convection effects is proposed. The finite-volume method formulations of voltage distribution equation, heat equations for both fluid flow and solid medium with nonuniform mesh are explained in detail. Based on the proposed iterative co-simulation method, package temperature distribution and voltage drop with Joule heating and fluidic cooling effects can be estimated. Several packaging examples are simulated and the results show that with micro-channel fluidic cooling in high power density 3D integrated packages, the thermal effect on voltage drop is reduced by 10% which is much less than that of using a traditional heat sink.

Highly monodisperse core–shell particles created by solid-state reactions

Abstract: The size distribution of particles, which is essential for many properties of nanomaterials, is equally important for the mechanical behaviour of the class of alloys whose strength derives from a dispersion of nanoscale precipitates. However, particle size distributions formed by solid-state precipitation are generally not well controlled. Here we demonstrate, through the example of core-shell precipitates in Al-Sc-Li alloys, an approach to forming highly monodisperse particle size distributions by simple solid-state reactions. The approach involves the use of a two-step heat treatment, whereby the core formed at high temperature provides a template for growth of the shell at lower temperature. If the core is allowed to grow to a sufficient size, the shell develops in a 'size focusing' regime, where smaller particles grow faster than larger ones. These results suggest strategies for manipulating precipitate size distributions in similar systems through simple variations in thermal treatments.

Homogenization in finite thermoelasticity

Abstract: A homogenization framework is developed for the finite thermoelasticity analysis of heterogeneous media. The approach is based on the appropriate identifications of the macroscopic density, internal energy, entropy and thermal dissipation. Thermodynamical consistency that ensures standard thermoelasticity relationships among various macroscopic quantities is enforced through the explicit enforcement of the macroscopic temperature for all evaluations of temperature dependent microscale functionals. This enforcement induces a theoretical split of the accompanying micromechanical boundary value problem into two phases where a mechanical phase imposes the macroscopic deformation and temperature on a test sample while a subsequent purely thermal phase on the resulting deformed configuration imposes the macroscopic temperature gradient. In addition to consistently recovering standard scale transition criteria within this framework, a supplementary dissipation criterion is proposed based on alternative identifications for the macroscopic temperature gradient and heat flux. In order to complete the macroscale implementation of the overall homogenization methodology, methods of determining the constitutive tangents associated with the primary macroscopic variables are discussed. Aspects of the developed framework are demonstrated by numerical investigations on model microstructures.

Transport properties of CF3I thermal plasmas mixed with CO2, air or N2 as an alternative to SF6 plasmas in high-voltage circuit breakers

Abstract: This paper is devoted to the calculation of equilibrium compositions, thermodynamic properties (mass density, enthalpy and specific heat at constant pressure) and transport coefficients (viscosity, electrical conductivity and thermal conductivity) of air/CO2/N2–CF3I mixtures. These data are computed in the temperature range 300 K–50 kK and pressure between 1 and 32 bar. Results obtained for pure gases (CF3I, CO2, air and N2) are systematically compared with SF6. Transport coefficients for N2, CO2, CF3I and mixtures of CO2, N2 or air with CF3I are also confronted with previous published values. Particular attention is paid to the collision integral database by the use of the most accurate and recent cross-sections or interaction potentials available in the literature.

Thermal Response of Composite Materials to Elevated Temperatures

Abstract: Different decomposition models of varying complexity were developed to predict the heat and mass transfer through charring/reinforced materials that are undergoing decomposition. Models included a heat conduction based model, decomposition model neglecting internal pyrolysis gas convection, and decomposition model with internal convection. Experimental methods were developed to measure the decomposition kinetic parameters and thermal properties required for input into the different models. Model results compared well with experimental data. Agreement between the heat conduction model was further improved by modifying the heat of decomposition to account for the internal convection effects.

Natural Convection in a Nanofluid Saturated Rotating Porous Layer: A Nonlinear Study

Abstract: The present paper deals with linear and nonlinear analysis of thermal instability in a rotating porous layer saturated by a nanofluid. Momentum equation with Brinkman term, involving the Coriolis term and incorporating the effect of Brownian motion along with thermophoresis has been considered. Linear stability analysis is done using normal mode technique, while for nonlinear analysis, a minimal representation of the truncated Fourier series, involving only two terms, has been used. Stationary and oscillatory modes of convection have been studied. A weak nonlinear analysis is used to obtain the concentration and thermal Nusselt numbers. The behavior of the concentration and thermal Nusselt numbers is investigated by solving the finite amplitude equations using a numerical method. Obtained results have been presented graphically and discussed in details.

Quasiharmonic thermal elasticity of crystals: An analytical approach

Abstract: First-principles quasiharmonic calculations play a very important role in mineral physics because they can predict the structural and thermodynamic properties of materials at pressure and temperature conditions of the Earth’s interior that are still challenging for experiments. They also enable calculations of thermal elastic properties by providing second-order derivatives of free energies with respect to strain. The latter are essential to interpret seismic tomography of the mantle in terms of temperature, composition, and mineralogy, in the context of geophysical processes. However, these are exceedingly demanding computations requiring up to ∼10 3 parallel jobs running on tens or more processors each. Here we introduce an analytical and computationally simpler approach that requires only calculations of static elastic constants and phonon density of states for unstrained configurations. This approach, currently implemented for crystals with up to orthorhombic symmetry, decreases the computational effort, i.e., CPU time and human labor, by up to two orders of magnitude. Results for the major mantle phases periclase (MgO) and forsterite (α-Mg2SiO4) show excellent agreement with previous first-principles results and experimental data.