Diffusion Of Thermal Energy Research Articles

Investigation of heat transfer enhancement and thermal stratification phenomenon on supercritical hydrogen fuel flow

Thermal stratification is one of the major issues in rectangular regenerative cooling channels because of the asymmetric heating and limited turbulent diffusion of thermal energy of the fuel. As a result, the heat transfer performance is decreased. This paper uses Ansys fluent software to construct a novel cooling structure that combines wavy wall surfaces with cylindrical fins. In addition to this new configuration, this study also examined three other configurations (straight-smooth, cylindrical-ribs, and wavy-smooth) and compared them with each other. This structure aims to efficiently enhance hydrogen coolant flow and heat transfer characteristics while reducing the extent of the non-uniform distribution issue. The results show that the new structure’s Nusselt number and maximum wall temperature are enhanced by 46.89% and 22.71%, respectively, compared to the straight-smooth structure; in contrast, the friction factor increased only by 3.64% over the smooth channel. The novel design has the best thermal performance factor compared to the other three designs, which is improved by 43.33% over the smooth channel. Among all configurations, the new structure is the best fluid temperature distribution configuration, which shows an optimal distribution of the coolant temperatures between layers; thus, the stratification phenomena decreased significantly.

Thermal-Responsive Smart Windows with Passive Dimming and Thermal Energy Storage.

Chromogenic smart windows are one of the key components in improving the building energy efficiency. By simulation of the three-dimensional network of polymer hydrogels, thermal-responsive phase change materials (TRPCMs) are manufactured for energy-saving windows. For simulated polymer hydrogels, tetradecanol (TD) and a color changing dye (CCD) are filled in situ in poly(n-butyl isobutyrate) (PBB) networks. TRPCMs convert solar energy into thermal energy through a dark blue CCD. The TD phase change material (PCM) absorbs heat energy to become a transparent liquid. Simultaneously, the CCD changes from blue to colorless and transparent at around 45 °C. As a result, as-prepared TRPCMs transform from an opaque state at room temperature to a high-transparency state after melting (74.5%). TRPCMs also show a good thermal storage capacity, with a phase transition enthalpy exceeding 161.9 J g-1. As-prepared smart materials can simultaneously achieve photothermal conversion, thermal energy diffusion, latent heat storage, and resistance to liquid leakage at the phase interface between opaque and transparent states, providing more options for the design of energy-saving buildings.

Influence of welding-induced mechanical stress on the electromagnetic properties of magnetic lamination

PurposeThis paper aims to analyze the influence of welding-induced mechanical stress of a magnetic core material on the performance behavior of a permanent magnet excited synchronous machine (PMSM). Welding, interlocking, clinching and the use of adhesives are state-of-the-art packaging technologies used in the manufacturing of electrical machines. However, the packaging processes degrade the electromagnetic properties of the electric steel sheets, thereby decreasing the performance and achievable range of the electric vehicle.Design/methodology/approachIn this paper, an approach that maps the local changes in magnetic properties due to welding induced stress with the stress values is developed. The welding process induces internal stress inside the steel sheet due to the diffusion of thermal energy into the sheets. Other effects are the changes in the micro structures of the steel sheets (grain sizes). These induced mechanical stresses lead to significant deterioration of the electromagnetic properties. They also lead to an increase in iron loss attributed to steel lamination.FindingsA low speed (city), a high-speed (highway) and WLTC-c3 driving cycle will be used to analyze the effects of the induced stresses on the machine efficiency at the different operating conditions. A high-speed PMSM with a maximum speed of 26,000 min−1 and maximum torque of 130 Nm is designed for this study.Originality/valueThe value of this study is in the development of a local varying modeling approach that analyses the influence of weld-induced stress on the performance of electrical machines. Its originality is evident in the mapping methodology. This will enable an application dependent improvement possibilities due to the understanding of the impact of weld-induced stress on the electromagnetic properties of weld-packaged core.

Basic aspects of gold nanoparticle photo-functionalization using oxides and 2D materials: Control of light confinement, heat-generation, and charge separation in nanospace.

Although the optical properties of localized surface plasmon resonance and the relaxation processes of excited hot electrons in gold nanoparticles (AuNPs) have been well understood, the phenomena that occur when AuNPs relax on solid surfaces of semiconductors or insulators remain largely unknown. Thermal energy diffusion and electron transfer are relatively simple physical processes, but the phenomena they induce are interesting because of a variety of new application developments. In this Perspective, we introduce the fundamental aspects as well as advanced applications of several new physical phenomena induced by AuNPs-based hybrid materials with oxides or 2D materials. Localized heat can induce a great force on the surrounding medium to control mass transport, and plasmon-induced charge transfer reactions are expected to have applications in photocatalysis and solar cells. We also review increasing reports on the development of nano-optical sensors, transistors, and nano-light sources based on precisely controlled device structures utilizing AuNPs.

Density and atomic coordination dictate vibrational characteristics and thermal conductivity of amorphous silicon carbide

Silicon carbide coatings and thin films are used for a wide array of applications ranging from thermal barrier coatings to microelectronics. In this paper, we report on the role of mass density and atomic coordination on the fundamental vibrational characteristics and thermal conductivity of amorphous silicon carbide systems through a combination of experiments and systematic atomistic simulations. We use time domain thermoreflectance to show that the thermal conductivity of hydrogenated amorphous silicon carbide can be increased twofold with $\ensuremath{\sim}40%$ increase in the mass density. A simple description of thermal transport applicable to a range of amorphous solids where diffusion of thermal energy is predominantly driven by nonpropagating modes cannot fully describe our experimental measurements. Our molecular dynamics simulations in conjunction with our lattice dynamics calculations shed light on the intrinsic role of atomic coordination in dictating the contributions from both propagating and nonpropagating modes in amorphous silicon carbide structures. More specifically, we find that as the concentration of $s{p}^{3}$ hybridized carbon atoms is increased by up to $10%$ with increasing mass densities, the contribution from propagons can be increased from $\ensuremath{\sim}25%$ to $\ensuremath{\sim}40%$, after which further increments in the mass density and the $s{p}^{3}$ fraction does not lead to higher contributions from propagons. In contrast, contributions from the nonpropagating modes increases monotonically with increasing mass density and $s{p}^{3}$ hybridization. Our results pave a path forward to manipulate the thermal conductivity of amoprhous silicon carbide systems based on varying the atomic coordination.

Investigation on static and dynamic characteristics of high-speed and heavy-load tilting pad journal bearings

PurposeMultitilting-pad journal bearings (MTPJBs) used in large-scale hydraulic turbines often suffer from complex operating conditions, which greatly influence the overall performance of the rotating machine. The purpose of this study is to establish a thermal-elastic-hydrodynamic lubrication model for MTPJBs that can predict the static and dynamic characteristics of high-speed and heavy-load MTPJBs under different operating conditions.Design/methodology/approachA thermo-elasto-hydrodynamic lubrication model considering the turbulence effect is proposed for high-speed and heavy-load TPJBs, which is solved using the coupled finite difference method and finite element method. The model considered the turbulence effect, thermal energy diffusion, viscosity–temperature–pressure relationship and elastic deformation of the pads. The influences of the operating conditions on static and dynamic characteristics of tilting pad journal bearings were analyzed in depth.FindingsThe operating conditions have a strong effect on the static properties of the bearings. The dynamic characteristics of the TPJB were the most influenced by the shaft speed. The effects of the load direction on the dynamic properties of the TPJB were much stronger than those of the static characteristics.Originality/valueThis study used analytical methods and models to provide theoretical guidance for evaluating lubricating characteristics, assembling conditions and overall health.

Qualitative analysis of a 3D multiphysics model for nonlinear ultrasonics and vibration induced heating at closed defects

ABSTRACT Upon exciting a material using elastic waves, the locally induced deformation at the interfaces of internally closed defects may cause nonlinear wave mechanics and dynamics in the form of clapping and friction. As a result, both phenomena instigate spectral broadening of the excitation spectrum as well as the production of heat, directly originating from the defect. To better understand and account for the physics behind the dissipation of energy by internally closed defects as a result of the wave–interface interaction, dedicated models can be developed. In this work, we propose a 3D finite element multiphysics model that is capable of simultaneously describing the generation of nonlinearities and heating at a defect’s interface experiencing clapping and friction induced by elastic wave propagation. The model consists of three different modules. The first module describes the elastic wave propagation in a virgin/bulk material, whereas the second module captures the contact physics at the defect level. The third module is implemented to calculate the diffusion of thermal energy in the specimen. The contact physics module accounts for anharmonic and hysteretic effects, describing the nonlinear behavior of the defect’s interfaces, which is echoed in both the ultrasound spectrum and in the vibration-induced heating. A qualitative analysis of the computational model, integrating the three modules, is performed to validate the approach. Examples show that nonlinear spectral components are indeed observed as a result of the friction and the clapping experienced by the faces of the defect. At the same time, a localized temperature increase due to the induced friction is noted, and its response at the outer surface of the sample is examined. The qualitative validation approves that the model is ready to be tested further quantitively, and to compare its predictions to experiments.

Temperature field simulation of chalcogenide glass ablation by nanosecond pulsed laser-based on pump–probe technology

Chalcogenide glass (ChG) is one of the best candidate materials for the fabrication of micro-nano resolution photonic devices by laser direct writing (LDW). In this work, the thermodynamics and the surface structures evolution of As2S3 by nanosecond pulse are investigated by the dual-laser pump–probe system. Above the ablation threshold (124, 157, 196 mJ/cm2), the descend duration of sample surface reflectivity was observed to be 25, 48, and 80 ns, respectively. Furthermore, to research the effects of pulse irradiation and surface thermal energy diffusion on the sample surface temperature, the dual-temperature model is applied. At the lowest laser fluence, the temperature drops rapidly to the melting point in a short time (23 ns), which results in the formation of a crater in the pulse action area. At the highest fluence, the surface temperature decreases slowly to the melting point (77 ns). And during this period, the material surface is continuously heated, which leads to the deepening of melting depth. So, a deep crater (294 nm) is formed on the surface, and the reflectivity declines the slowest. Secondly, due to the highest temperature in the central region and the diffusion movement of heat energy, the rim structure and the small droplet structures will appear at a higher fluence.

Multi-Objective Optimization of Wire-Electric Discharge Machined Ultrathin Silicon Wafers Using Response Surface Methodology for Solar Cell Applications

Abstract Recent investigations on the fabrication of ultrathin silicon (Si) wafers using wire-electric discharge machining (wire-EDM) were observed to possess some inherent limitations. These include thermal damage (TD), kerf-loss (KL), and low slicing rate (SR), which constraints its industrial use. The extent of TD, KL, and SR largely depends on the process parameters such as open voltage (OV), servovoltage (SV), and pulse on-time (Ton). Therefore, optimizing the parameters that pertain to minimum TD and KL while maintaining a higher SR is the key to improvement in the fabrication of Si wafers using wire-EDM. Thus, this study is an effort to analyze and identify the optimal parameters that relate to the most effective Si slicing in wire-EDM. A central composite design (CCD)-based response surface methodology (RSM) was used for optimizing the process parameters. The capability to slice Si wafers in wire-EDM was observed to be influenced by the discharge energy, which significantly impacted the overall responses. The severities of TDs were observed to be mainly dominated by the variation in OV and Ton due to the diffusion of thermal energy into the workpiece, leading to melting and subsequent resolidification. For high productivity, the optimized parameters resulted in a SR of 0.72 mm/min, TD of 17.44 μm, and a kerf-loss of about 280 μm.

Measurement of angstrom-level laser induced protrusion using touchdown in heat-assisted magnetic recording

In heat-assisted magnetic recording (HAMR), a laser is employed above the read-write transducer to provide energy to the media, lowering its coercivity. However, the laser also brings thermal energy diffusion inside the slider and induces an extra angstrom-level protrusion, which we call laser-induced protrusion (LIP). The LIP needs to be taken into consideration in HAMR due to the significance of head-media spacing. This paper focuses on laser heating on the millisecond timescale during flying in the HAMR conditions. When the laser is turned ON for milliseconds, the LIP forms in the short term (∼μs) and fly height change (FHC) happens in the long term (∼ ms) due to the crown/camber change, resulting in a smaller touchdown power (TDP). Thus, the touchdown power change (ΔTDP) is measured and the LIP is isolated using the time constants. A component-level HAMR stage is used to study the effects of laser-on time, laser current, and linear velocity on the ΔTDP. The experimental results show that the FHC needs ∼ 28 ms to reach the steady state and that the protrusion size presents a two-stage linear relation with the laser current separated by a threshold. The LIP size is reduced by about half when operating from 12 m/s to 24 m/s.

Thermal Diffusion Flow Analysis in Unsaturated Loamy Soil using UPWIND and QUICK Numerical Simulation

Understanding of energy transport in saturated porous media and unsaturated porous media provides vital information regarding the development of thermal porous reservoirs, industrial filter applications, water reservoirs and low-temperature fluid flow applications. The present paper focus is on presenting the solution of thermal energy diffusion through unsaturated loamy soil using finite difference method approach. The unsteady temperature profile comparison as time function at different spatial locations and as function of spatial location at different time interval are compared using UPWIND and QUICK approach method using thermal diffusion case. MATLAB software is used to simulate above approach for simulation.

A Combined Non-Invasive Approach to the Study of A Mosaic Model: First Laboratory Experimental Results.

This paper presents first laboratory results of a combined approach carried out by the use of three different portable non-invasive electromagnetic methods: Digital holographic speckle pattern interferometry (DHSPI), stimulated infrared thermography (SIRT) and holographic subsurface radar (HSR), proposed for the analysis of a custom-built wall mosaic model. The model reproduces a series of defects (e.g., cracks, voids, detachments), simulating common deteriorated, restored or reshuffled areas in wall mosaics. DHSPI and SIRT, already well known in the field of non-destructive (NDT) methods, are full-field contactless techniques, providing complementary information on the subsurface hidden discontinuities. The use of DHSPI, based on optical imaging and interferometry, provides remote control and visualization of surface micro-deformation after induced thermal stress, while the use of SIRT allows visualization of thermal energy diffusion in the surface upon the induced thermal stress. DHSPI and SIRT data are complemented by the use of HSR, a contact method that provides localized information about the distribution of contrasts in dielectric permittivity and related possible anomalies. The experimental results, made by the combined use of these methods to the identification of the known anomalies in the mosaic model, are presented and discussed here as a contribution in the development of an efficient non-invasive approach to the in-situ subsurface analysis of ancient wall mosaics.

Dynamic localized turbulent diffusion and its impact on the galactic ecosystem

Modelling the turbulent diffusion of thermal energy, momentum, and metals is required in all galaxy evolution simulations due to the ubiquity of turbulence in galactic environments. The most commonly employed diffusion model, the Smagorinsky model, is known to be over-diffusive due to its strong dependence on the fluid velocity shear. We present a method for dynamically calculating a more accurate, locally appropriate, turbulent diffusivity: the dynamic localised Smagorinsky model. We investigate a set of standard astrophysically-relevant hydrodynamical tests, and demonstrate that the dynamic model curbs over-diffusion in non-turbulent shear flows and improves the density contrast in our driven turbulence experiments. In galactic discs, we find that the dynamic model maintains the stability of the disc by preventing excessive angular momentum transport, and increases the metal-mixing timescale in the interstellar medium. In both our isolated Milky Way-like galaxies and cosmological simulations, we find that the interstellar and circumgalactic media are particularly sensitive to the treatment of turbulent diffusion. We also examined the global gas enrichment fractions in our cosmological simulations, to gauge the potential effect on the formation sites and population statistics of Population III stars and supermassive black holes, since they are theorised to be sensitive to the metallicity of the gas out of which they form. The dynamic model is, however, not for galaxy evolution studies only. It can be applied to all astrophysical hydrodynamics simulations, including those modelling stellar interiors, planetary formation, and star formation.

Experimental and theoretical investigation of heat transfer in platform bed during polymer extrusion based additive manufacturing

Heat transfer plays a key role in polymer extrusion based additive manufacturing (AM) processes. Measurement and modeling of how temperature distribution on the platform bed changes with time during the dispensing of a polymer filament is of particular interest, as it determines the effectiveness of merging with neighboring filaments, and therefore, the properties of the built part. This paper reports infrared thermography based measurement of temperature field on the platform bed as a function of time during filament dispense and comparison with an analytical model based on moving heat source theory. Measurements identify two key heat transfer processes that influence temperature distribution on the bed. Data show that diffusion of thermal energy deposited on the bed with the filament and heat transfer from the hot nozzle tip both influence the temperature distribution on the bed. The relative contributions of these two sources of temperature rise on the bed are measured for different values of key process parameters. Measurement of temperature rise due to diffusion of thermal energy deposited on the bed is found to be in excellent agreement with an analytical model based on moving heat source theory. Evolution of the temperature field after completion of rastering of a line is measured. These data play a critical role in determining how effectively a filament merges with neighboring filaments. By identifying the key heat transfer processes that determine the temperature field on the platform bed, this work contributes towards thermal optimization of polymer-based additive manufacturing.

A Thermoelastohydrodynamic Analysis for the Static Performance of High-Speed—Heavy Load Tilting-Pad Journal Bearing Operating in the Turbulent Flow Regime and Comparisons to Test Data

Abstract Tilting-pad journal bearings (TPJBs) ensure rotordynamic stability that could otherwise produce dangerously large amplitude rotor oil-whirl/whip motions in high-speed rotating machinery. Currently, highly efficient turbo compressors demand an ever increasing rotor surface speed and specific load on its support bearings. The accurate prediction of bearing performance is vital to guarantee reliable products, specifically with regard to reducing maximum bearing pad temperature and drag power losses, and operating with the least flow rate while still maximizing load capacity. The hydrodynamic pressure and heat generation in an oil film acting on a bearing pad produce significant mechanical and thermal deformations that change the oil film geometry (clearance and preload) to largely affect the bearing performance, static, and dynamic. In addition, a high surface speed bearing often operates in the turbulent flow regime that produces a notable increase in power loss and a drop in maximum pad temperature. This paper details a thermoelastohydrodynamic (TEHD) analysis model applied to TPJBs, presents predictions for their steady-load performance, and discusses comparisons with experimental results to validate the model. The test bearing has four pads with a load between pads configuration; its length L = 76.2 mm and shaft diameter D = 101.6 mm (L/D = 0.75). The rotor top speed is 22.6 krpm, i.e., 120 m/s surface speed, and the maximum specific load is 2.94 MPa for an applied load of 23 kN. The test procedure records shaft speed and applied load, oil supply pressure/temperature and flow rate, and also measures the pads' temperature and shaft temperature, as well as the discharge oil (sump) temperature. The TEHD model couples a generalized Reynolds equation for the hydrodynamic pressure generation with a three-dimensional energy transport equation for the film temperature. The pad mechanical deformation due to pressure utilizes the finite elemental method, whereas an analytical model estimates thermally induced pad crowning deformations. For operation beyond the laminar flow regime, the analysis incorporates the eddy viscosity concept for fully developed turbulent flow operation. Current predictions demonstrate the influence of pressure and temperature fields on the pads mechanical and thermally induced deformation fields and also show static performance characteristics such as bearing power loss, flow rate, and pad temperatures. The comparisons of test results and analysis results reveal that turbulent flow effects significantly reduce the pads' maximum temperature while increasing the bearing power loss. Turbulent flow mixing increases the diffusion of thermal energy and makes more uniform the temperature profile across the film.

Analysis of premixed flame propagation between two closely-spaced parallel plates

Motivated by experimental observations on premixed-gas flame propagation in Hele-Shaw cells, this work analyzes quasi-isobaric flame propagation between two adiabatic parallel plates using a simple quasi-2D formulation based on averaging the flow properties across the cell gap. Instabilities associated with thermal expansion, buoyancy, viscosity change across the front and differential diffusion of thermal energy and reactants are investigated with one-step chemistry, constant heat capacity and variable transport coefficients through time-dependent computations of the flame front evolution in large domains. These instabilities are found to induce flame wrinkling which increases flame surface area and thus propagation speeds in ways different from those associated with freely propagating flames. The simulations are compared with experiments in Hele-Shaw cells; very good qualitative and (in some cases) quantitative agreement is found.

Thermal Energy Diffusion Incorporating Generalized Einstein Relation for Degenerate Semiconductors

The currently used generalized Einstein relation for degenerate semiconductors with isotropic nonparabolic energy bands produces physically improper results, as well as losing numerical accuracy for large values of nonparabolicity parameters at room temperature. Therefore, a new generalized Einstein relation (a macroscopic equation and a formula) is derived from the semiclassical momentum balance equation based on a drift-diffusion approximation, by introducing a new concept of the effective temperature of a carrier gas for generalization of the classical kinetic theory for nonideal gases of carriers in semiconductors. The proposed formula takes into account the carrier thermal energy diffusion effect completely, so that it can accurately reflect the effect of band nonparabolicity on the ratio of the diffusion coefficient to the mobility for carriers in degenerate semiconductors. From the results evaluated with the formula, new and critically important nonparabolicity effects are observed. It is shown that the new generalized Einstein relation is valid for applied electrical fields of the full linear regime. In addition, useful figures are also presented, from which the ratio of the diffusion coefficient to mobility, as well as the Fermi energy, can be easily determined from the electron concentration, or doping density, for a given semiconductor material.

Shock-powered light curves of luminous red novae as signatures of pre-dynamical mass-loss in stellar mergers

Luminous red novae (LRN) are a class of optical transients believed to originate from the mergers of binary stars, or "common envelope" events. Their light curves often show secondary maxima, which cannot be explained in the previous models of thermal energy diffusion or hydrogen recombination without invoking multiple independent shell ejections. We propose that double-peaked light curves are a natural consequence of a collision between dynamically-ejected fast shell and pre-existing equatorially-focused material, which was shed from the binary over many orbits preceding the dynamical event. The fast shell expands freely in the polar directions, powering the initial optical peak through cooling envelope emission. Radiative shocks from the collision in the equatorial plane power the secondary light curve peak on the radiative diffusion timescale of the deeper layers, similar to luminous Type IIn supernovae and some classical novae. Using a detailed 1D analytic model, informed by complementary 3D hydrodynamical simulations, we show that shock-powered emission can explain the observed range of peak timescales and luminosities of the secondary peaks in LRN for realistic variations in the binary parameters and fraction of the binary mass ejected. The dense shell created by the radiative shocks in the equatorial plane provides an ideal location for dust nucleation consistent with the the inferred aspherical geometry of dust in LRN. For giant stars, the ejecta forms dust when the shock-powered luminosity is still high, which could explain the infrared transients recently discovered by Spitzer. Our results suggest that pre-dynamical mass loss is common if not ubiquitous in stellar mergers, providing insight into the instabilities responsible for driving the binary merger.

Ultrasonic attenuation via energy diffusion channel in disordered conductors

We predict an existence of an new dissipation channel leading to attenuation of ultrasound in disordered conductors and superconductors with perfect electroneutrality. It is due to slow diffusion of thermal energy. We show that in doped silicon ultrasound attenuation may be enhanced by a factor about 100. Similar effect is also studied for s-wave and d-wave superconductors. The latter case is applied to BSCCO family where strong enhancement of ultrasound attenuation is predicted. For usual s-wave superconductors new dissipation channel might be important for very low-electron-density materials near the BCS-BEC crossover.

An exact thermodynamical model of power-law temperature time scaling

In this paper a physical model for the anomalous temperature time evolution (decay) observed in complex thermodynamical system in presence of uniform heat source is provided. Measures involving temperatures T with power-law variation in time as T(t)∝tβ with β∈R shows a different evolution of the temperature time rate Ṫ(t) with respect to the temperature time-dependence T(t). Indeed the temperature evolution is a power-law increasing function whereas the temperature time rate is a power-law decreasing function of time.Such a behavior may be captured by a physical model that allows for a fast thermal energy diffusion close to the insulated location but must offer more resistance to the thermal energy flux as soon as the distance increases. In this paper this idea has been exploited showing that such thermodynamical system is represented by an heterogeneous one-dimensional distributed mass one with power-law spatial scaling of its physical properties. The model yields, exactly a power-law evolution (decay) of the temperature field in terms of a real exponent as T∝tβ (or T∝t−β) that is related to the power-law spatial scaling of the thermodynamical property of the system. The obtained relation yields a physical ground to the formulation of fractional-order generalization of the Fourier diffusion equation.