Thermal Inhomogeneities Research Articles

Topographically Controlled Marangoni-Rayleigh-Bénard Convection in Liquid Metals

Convection induced in a layer of liquid with a top free surface by a distribution of heating elements at the bottom can be seen as a variant of standard Marangoni–Rayleigh–Bénard Convection where in place of a flat boundary at constant temperature delimiting the system from below, the underlying thermal inhomogeneity reflects the existence of a topography. In the present work, this problem is investigated numerically through solution of the governing equations for mass, momentum and energy in their complete, three-dimensional time-dependent and non-linear form. Emphasis is given to a class of liquids for which thermal diffusion is expected to dominate over viscous effects (liquid metals). Fixing the Rayleigh and Marangoni number to 104 and 5 × 103, respectively, the sensitivity of the problem to the geometrical, kinematic and thermal boundary conditions is investigated parametrically by changing: the number and spacing of heating elements, their vertical extension, the nature of the lateral boundary (solid walls or periodic boundary) and the thermal behavior of the portions of bottom wall between adjoining elements (assumed to be either adiabatic or at the same temperature of the hot blocks). It is shown that, like the parent phenomena, this type of thermal flow is extremely sensitive to the specific conditions considered. The topography can be used to exert a control on the emerging flow in terms of temporal response and patterning behavior.

Optical diagnostics in a detonation-driven direct-connected circular combustor fueled with hydrogen for mach 10 scramjet

Various optical methods are applied to study flame characteristics of a circular-cross-section combustor for a Mach 10 scramjet. Experiments are performed on a direct-connected test facility fueled by hydrogen and driven using hydrogen/oxygen detonation. The circular-cross-section combustor is advantageous in solving thermal inhomogeneity, corner boundary layer and aircraft integration problems. However, it is difficult to use traditional optical measurement methods, which require large-sized measurement windows. In this study, tunable diode laser spectroscopy and a multi-view imaging system are used with small windows. High repetition rate measurements allow the effective time of facility, which is approximately 5 ms in this case, to be obtained. The dynamic flame characteristics are diagnosed upstream and downstream of the cavity with/without strut injection. When combined with proper orthogonal decomposition to obtain the flame pulsation state during the effective time period, the method is expected to be effective for engine performance evaluation and numerical simulation verification.

Numerical study of the effect of droplet coagulation on the dynamics of a two-fraction aerosol in an acoustic resonator

The study is devoted to the study of the effect of coagulation of dispersed phase droplets on aerosol oscillations in an acoustic resonator. The mathematical model of aerosol dynamics implements a continuous mathematical model of the dynamics of a multiphase medium, taking into account the velocity and thermal inhomogeneity of the mixture components. To describe the dynamics of the carrier medium, a two-dimensional unsteady system of Navier – Stokes equations for a compressible gas is used, written taking into account the interphase force interaction and interphase heat transfer. To describe the dynamics of the dispersed phase, a system of equations is solved for each of its fractions, including the continuity equation for the «average density» of the fraction, the equations for the conservation of the spatial components of the momentum and the equation for the conservation of thermal energy of the fraction of the dispersed phase of the gas suspension. The interphase force interaction included the Archimedes force, the force of the added masses and the force of aerodynamic drag. The heat exchange between the carrier medium - gas and each of the dispersed phase fractions was also taken into account. The mathematical model of the dynamics of a polydisperse aerosol was supplemented by a mathematical model of collisional aerosol coagulation. For the velocity components of the mixture components, uniform Dirichlet boundary conditions were specified. For the remaining functions of the dynamics of the multiphase mixture, homogeneous Neumann boundary conditions were specified. The equations were solved by the explicit McCormack method with a nonlinear correction scheme that allows obtaining a monotonic solution. As a result of numerical calculations, it was determined that a region with an increased content of coarse particles is formed in the vicinity of the oscillating piston. The coagulation process leads to a monotonic increase in the volumetric content of the fraction of coarse particles and a monotonic decrease in the volumetric content of fine particles.

Elucidating the dominant mechanisms in burn rate increase of thermite nanolaminates incorporating nanoparticle inclusions

It was experimentally found that silica and gold particles can modify the combustion properties of nanothermites but the exact role of the thermal properties of these additives on the propagating combustion front relative to other potential contributions remains unknown. Gold and silica particles of different sizes and volume loadings were added into aluminum/copper oxide thermites. Their effects on the flame front dynamics were investigated experimentally using microscopic dynamic imaging techniques and theoretically via a reaction model coupling mass and heat diffusion processes. A detailed theoretical analysis of the local temperature and thermal gradients at the vicinity of these two additives shows that highly conductive inclusions do not accelerate the combustion front while poor conductive inclusions result in the distortion of the flame front (corrugation), and therefore produce high thermal gradients (up to 1010 K.m−1) at the inclusion/host material interface. This results in an overall slowing down of the combustion front. These theoretical findings contradict the experimental observations in which a net increase of the flame front velocity was found when Au and SiO2 particles are added into the thermite. This leads to the conclusion that the faster burn rate observed experimentally cannot be fully associated with thermal effects only, but rather on chemical (catalytic) and/or mechanical mechanisms: formation of highly-stressed zones around the inclusion promoting the reactant mixing. One additional experiment in which physical SiO2 particles were replaced by voids (filled with Ar during experiment) to cancel the potential mechanical effects while preserving the thermal inhomogeneity in the thermite structure confirms the hypothesis that instead of pure thermal conduction, it is the mechanical mechanisms that dominate the propagation velocity in our specific Al/CuO multilayered films.

STUDY OF SPATIAL HEAT TRANSFER IN A CORNER FRAGMENT OF AN EXTERNAL FENCE WITH CONNECTORS

Increasing energy efficiency and reducing heat loss in buildings are the main challenges of the modern construction sector. For this reason, it is recommended to design buildings using modern energy-saving materials and technologies. When using multilayer walls in housing construction, the problem of thermal bridges arises. Therefore, you need to understand how the thermal behavior of the enclosing structure changes when thermal engineering inhomogeneities are included in it. The aim of this work is to study the effect of heat-conducting inclusions located in external enclosing structures on the change in the values of temperature and heat flux density. The paper considers the importance of increasing the energy efficiency of outdoor fences. A numerical study of the heat transfer process for corner fragments of enclosing structures used in housing construction using the VELOX technology has been carried out. The influence of the design features of the considered corner fragments on the characteristics of temperature fields is shown. Mathematical modeling of spatial heat transfer in the corner fragment of the outer enclosure is carried out using a nonlinear system of differential equations of heat conduction with specified boundary conditions using the finite element method. The problem was solved using the ANSYS software package. The influence of metal and fiberglass connectors on the change in the fields of temperature and heat flux density in the enclosing structure has been investigated. The change in the thermal state is considered both in the thickness of the outer wall and along the inner surface of the fence from the corner area and along the smooth surface of the wall. Analysis of the calculations showed that the maximum disturbance is introduced by the connector made of metal, and the minimum — from fiberglass. The presence of highly heat-conducting inclusions in the thickness of the structure leads to a distortion of the density field of the heat flux of the fence.

Thermal damage effect on the thermal conductivity inhomogeneity of granite

In this work, thermal damage effect on the thermal conductivity inhomogeneity of granite was experimentally investigated. To study the influence of different cooling methods, granite samples were heated at different temperature levels (from 100°C to 1000 °C), then cooled in air and water, respectively. Distributed thermal conductivity of granite samples were measured using optical scanning method before and after thermal damage treatment. Besides, some non-destructive tests (open porosity and P-wave velocity) were performed. The results indicate that thermal damage will lead to the decrease of thermal conductivity and the increase of thermal inhomogeneity degree. The decrease degree of thermal conductivity values increases with the treatment temperature. The maximum decrease can be 54.0% (after heated at 1000 °C). When the treatment temperature is higher than the phase transition temperature of quartz (573 °C), the thermal inhomogeneity degree of the damaged samples increased obviously. After heated at each temperature level, the water cooled samples exhibit greater thermal conductivity decrease and greater thermal inhomogeneity factor increase than air cooled samples. This may be attributed to the fact that rapid cooling has extra effects on the damage evolution of rock, which is induced by the high temperature gradient generated when temperature changes occur in a very short time. A thermal damage model was proposed to describe the evolution characteristics of thermal conductivity with treatment temperature. Research results in this work can provide better knowledge to the evolution mechanism of distributed thermal conductivity of engineering rock masses experienced high temperature environment, and the influence of different cooling methods.

Influence of heat pipe assisted terminal cooling on the thermal behavior of a large prismatic lithium-ion cell during fast charging in electric vehicles

Lithium-ion based battery systems are a vital part of electric vehicles. Due to limited driving range accompanied by long charging times, manufactures focus on fast charging introducing serious challenges for the battery. Even if improved charging procedures have been discussed widely, little attention has been paid so far to the thermal behavior of larger automotive cells, especially in module arrangements. Therefore, the thermal behavior of a large prismatic cell during fast charging at electric vehicle conditions is investigated for common profiles such as constant-, pulse-, and boost charging. Revealing the necessity of an improved cooling, in this work, a novel heat pipe based terminal cooling concept in addition to conventional bottom cooling is investigated experimentally and by simulation. Therefore, a 3D electro-thermal model is developed predicting the cell temperature behavior at electric vehicle boundary conditions. After successful experimental validation with a prototype based on a prismatic 25 Ah cell, the effects of the proposed heat pipe cooling concept on the fast charging profiles are investigated in detail. Investigating various fast charging profiles and module scenarios, a decrease of the jelly roll temperature by a maximum of 4.3 °C and a reduction of thermal inhomogeneities are obtained, which reduces uneven aging.

Heat Engineering Heterogeneity Of The Outer Walls Of Earthquake-Resistant Buildings

When assessing the level of energy efficiency of civilian buildings, a special place is given to establishing the level of thermal protection of their external enclosing structures. Significant discrepancies in the results of theoretical and experimental studies of heat fluxes through the outer walls of buildings erected in seismic areas are associated with the design features of fences - the presence of reinforced concrete elements in them: anti-seismic belts at the level of floors, cores at intersections of walls and along the edges of large window openings ... In addition, in recent years, external walls have become widespread, which are filling of bricks or aerated concrete blocks between the main structural elements of the frame - monolithic reinforced concrete columns and crossbars. The introduction of reinforced concrete elements into the structure of the external wall fencing provides strength, rigidity and stability of buildings, guarantees its seismic resistance. At the same time, reinforced concrete inclusions are significant “cold bridges” in warmer brick or aerated concrete masonry. Such heat engineering heterogeneity of earthquake-resistant outer walls significantly complicates the process of determining their heat-shielding properties. This, in turn, leads to errors in the design of heating systems, which inevitably affects the thermal comfort of the premises, the formation of condensation and mold zones in the cold zones of the inner surface of the fences. The article presents the results of theoretical and experimental studies to determine the heat-shielding properties of external heat-engineering heterogeneous walls of earthquake-resistant buildings. The most reliable method for calculating the reduced resistance to heat transfer of an inhomogeneous external structure and the coefficient of its thermal inhomogeneity have been established.

Mechanistic underpinnings of thermal gradient induced inhomogeneity in lithium plating

In the pursuit to enable the rapid charging of lithium-ion batteries, lithium plating at the anode poses one of the most significant challenges. Additionally, the heat generation that accompanies high rate battery operation in conjunction with non-uniform cooling and localized heating at tabs is known to result in thermal inhomogeneity. Such thermal anomalies in the absence of proper thermal management can instigate accelerated degradation in the cell. In this work, a physics-based interrogation of the link between thermal gradient induced inhomogeneity and lithium plating during charging is presented. The relative importance of in-plane vs. through-plane (inter-electrode) thermal gradients to charging performance and cell degradation is necessary to intelligently design packaging and cooling systems for large-format cells. While in-plane thermal gradients strongly influence active material utilization, the lithium plating severity was found to be very similar to an isothermal case at the same mean temperature. By contrast, interelectrode thermal gradients cause a shifting on the solid phase potential at each electrode during charging, related to the increase or decrease in overpotential due to local temperature variation. When the cathode temperature exceeds the anode temperature, lithium plating is exacerbated, and accelerated degradation occurs.

Mechanistic Role of Thermal Gradients on Lithium Plating in Li-Ion Batteries

Unwanted lithium deposition on the surface of graphite particles tends to occur during charging of Li-ion batteries at low temperatures, high rates, and high states of charge. However, the intricacies of the coupled thermal-electrochemical interactions that govern Li plating behavior are yet to be fully uncovered. Thermal inhomogeneity naturally develops during the high rate operation of large-format cells, particularly when active cooling systems are employed; hence, a deeper investigation of the relationship between thermal gradients and Li plating is merited. This study employs a microstructure-aware modeling approach to probe the influences of both in-plane and inter-electrode thermal gradients on Li plating. Both types of thermal gradient were found to cause inhomogeneous current distribution within the electrodes. While inter-electrode gradients resulted in a variation in the Li plating onset point and severity, the Li plating induced by in-plane gradients was nearly the same as for isothermal cases at the same mean temperature. The sensitivity of charging performance to mean temperature and thermal gradient magnitude was analyzed across a range of practical conditions. Electrochemical experiments were performed under applied external thermal gradients to study the effect on plating and cell lifespan in various cell formats, and the electrode degradation was characterized post-mortem. By gaining a fundamental understanding of the Li plating phenomenon, this study aims to assist in the design of electrode structures and cell formats that will reduce the propensity for this mode of degradation.

Theoretical studies of atmospheric perturbations caused by the gravity field inhomogeneities

In solving atmospheric dynamics problems, the influence of the gravity field inhomogeneities (GFI) is neglected. The traditional basis for this is that the amplitude of gravity variations even in highly anomalous regions do not exceed 10−3 ms−2 in order of magnitude, i.e. 4 orders of magnitude less than average gravity. But in the presence of gravity anomalies, inhomogeneous forces of the same order act in the directions tangent to the mean Earth ellipsoid. The atmosphere dynamics is very sensitive to the forces of such directions. In highly anomalous regions, they are comparable with the main horizontal forces acting in the atmosphere, in particular with the forces of the pressure gradient and Coriolis forces. Therefore, it seems appropriate to analyze the possible effect of spatial variations of gravity on the dynamics of the atmosphere, primarily for mesoscale atmospheric disturbances. In a number of the authors’ previous works, theoretical studies of the possible effect of GFI on the atmosphere dynamics are carried out. This report contains some new relevant findings. A three-dimensional analytical model of geostrophic wind disturbances under the influence of GFI is developed. An analogy of atmospheric disturbances caused by thermal inhomogeneities of the underlying surface and GFI is shown. Attention is drawn to the possibility of “accumulation” of atmospheric effects associated with the gravitational field inhomogeneities.

Bowl piston geometry as an alternative to enlarged crevice pistons for rapid compression machines

Thermal inhomogeneity and physical processes like fluid dynamics reduce the utility of rapid compression machine (RCM) facilities to accurately study fuel combustion phenomenon relevant to internal combustion engines. Most current RCMs incorporate a large crevice volume in the piston to capture roll-up vortices that encroach into the combustion zone during compression. In this work, a bowl piston design similar to those used in diesel engines is proposed as an alternative to enlarged creviced pistons for creating a sufficiently thermally homogenous gas mixture prior to ignition without undesirable fluid motion found in flat piston configurations. The bowl piston also eliminates the possibility of cold unreacted gases entering the combustion chamber when the piston is retracted in rapid compression and expansion machines (RCEMs) like in creviced piston designs. In the work, a bowl piston was compared to creviced piston and flat piston configurations numerically and experimentally. Through non-reacting computational fluid dynamics simulations, the bowl piston reduced the roll-up vortex found for the flat piston and led to a higher temperature and more thermally uniform core of gas at peak compression compared to the enlarged crevice piston. Experimentally, three pistons were studied in a RCM facility with ethanol and n-butane as fuels. Results showed that the bowl piston yielded benefits over conventional piston geometries including: reduced heat loss due to lower surface area, higher turbulent Reynolds Number, stronger ignition, and higher heat release rate and combustion efficiency as estimated using heat release analysis. Based on the findings presented here, we conclude that bowl piston geometries are a promising alternative to creviced pistons for conducting fuel ignition studies in RCM and RCEM facilities.

Influence of Aggregates in Concrete on Fiber-Optic Based Thermal Integrity Profiling Analysis of Concrete Structures

Fiber-optic sensor has drawn wide attention in the nondestructive testing and evaluation of civil engineering materials due to its high accuracy and resolution as well as cost-efficiency. Currently, using optical fiber as the temperature sensor is proposed to conduct the thermal integrity profiling (TIP) of concrete structures. However, concrete is not a thermally homogeneous material as assumed in current studies of concrete TIP. Its essential components, such as aggregates may cause thermal inhomogeneity problems when implementing fiber-optic sensors for TIP. In this paper, we use the concrete structures with different grades of aggregates to conduct numerical simulation for nondestructive thermal testing. The goal is to investigate how the thermal inhomogeneity caused by aggregates would influence the testing result. Firstly, we establish three concrete structure models with three different grades of aggregates based on the scenarios where these concrete structures will be used. Then, we numerically simulate the thermal process on these models and extract the temperature at the location where optical fiber would be installed. The influence caused by the inhomogeneity of aggregate size and distribution as well as the possible method to minimize the effect are evaluated in the paper. Overall, aggregates of concrete have a significant influence on the accuracy of TIP analysis, and defects could be veiled if no proper treatment to the data is implemented for TIP analysis.

Acoustic-Wave-Induced Ferromagnetic-Resonance-Assisted Spin-Torque Switching of Perpendicular Magnetic Tunnel Junctions with Anisotropy Variation

We have investigated Surface Acoustic Wave (SAW) induced ferromagnetic resonance (FMR) assisted Spin Transfer Torque (STT) switching of perpendicular MTJ (p-MTJ) with inhomogeneities using micromagnetic simulations that include the effect of thermal noise. With suitable frequency excitation, the SAW can induce ferromagnetic resonance in magnetostrictive materials, and the magnetization can precesses in a cone with high deflection from the perpendicular direction. With incorporation of inhomogeneity via lateral anisotropy variation as well as room temperature thermal noise, the magnetization precession in different gains can be significantly incoherent. Interestingly, the precession in different grains are found to be in phase, even though the precession amplitude (angle of deflection from the perpendicular direction) vary across grains of different anisotropy. Nevertheless, the high mean deflection angle can complement the STT switching by reducing the STT current significantly; even though the applied stress induced change in anisotropy is much lower than the total anisotropy barrier. This work indicates that SAW assisted switching can improve energy efficiency while being scalable to very small dimensions, which is technologically important for STT-RAM and elucidates the physical mechanism for the potential robustness of this paradigm in realistic scenarios with thermal noise and material inhomogeneity

3D electron backscatter diffraction study of α lath morphology in additively manufactured Ti-6Al-4V

Titanium alloys exhibit complex, multi-phase microstructures which form during liquid-solid and solid-solid phase transformations. These phase transformations govern the microstructural evolution and are potentially more complex during additive manufacturing due to large thermal gradients and inhomogeneities. The prototypical fundamental unit of titanium microstructures are the α laths, and investigations into their three-dimensional morphology may provide new insights. A prior β-grain boundary, 3-variant clusters and interconnected laths were studied in 3D in electron-beam printed Ti-6Al-4V using a plasma FIB. These key features are of interest for studying variant selection in additive manufacturing.

Studies of a Microwave Radiometer Based on Integrated Circuits

Microwave thermometry provides noninvasive detection of thermal inhomogeneity in biological tissues, early diagnosis of oncological diseases, and correction of treatment based on changes in the parameters of the body’s electromagnetic radiation. We present an analytical review of developments in medical radiometers. Challenges hindering the development of radiometry are identified, as are proposed solutions.

Thermal versus mechanical topography: an experimental investigation in a rotating baroclinic annulus

We present a series of experimental investigations in which a differentially-heated annulus was used to investigate the effects of topography on rotating, stratified flows. In particular, we investigate blocking effects via azimuthally varying differential-heating and compare them to previous experiments utilising partial mechanical barriers. The thermal topography used consisted of a flat patch of heating elements covering a small azimuthal extent of the base, forming an equivalent of a partial barrier, to study the difference between blocked and unblocked flow. These azimuthally-varying heating experiments produced results with many similarities to our previous experiments with a mechanical barrier, despite the lack of a physical obstacle or formation of bottom-trapped waves. In particular, a unique flow structure was found when the drifting flow and the topography interacted in the form of an “interference” regime at low Taylor number, but forming an erratic “irregular” regime at higher Taylor number. This suggests that blocking may be induced by either or both of a thermal or mechanical inhomogeneity. Evidence of coherent/persistent resonant wave triads was noted in both kinds of experiment, though the component wavenumbers of the wave-triads and their impact on the flow was found to depend on the topography in question.

Performance of helically coiled gas heaters in supercritical CO2 Rankine cycles: A detailed assessment under convective boundary condition

Helical coil has drawn increasingly attention as the gas heater of supercritical Rankine cycles due to its high heat transfer rate and compact structure. This work focused on the thermal performance of supercritical CO2 helically coiled heater at a practical thermal boundary condition (i.e. convective boundary), under which the coil’s performance has been scarcely paid attention. Supercritical flow turbulence was solved by the Shear-Stress Transport k-ω model and heat transfer was dealt with in a solid-to-fluid conjugate manner. Influence of fluid temperature, mass flux, coil curvature and buoyancy was comprehensively analyzed over a wide range. Results reveal a significant self-regulation of local heat input in the region of Tb < Tpc, where heat flux gently varied along the coil axis. The mass flux and inlet temperature of heat source fluid had a strong impact on the local heat flux, the heat transfer coefficient and the thermal inhomogeneity, while coil curvature only had a weak influence. Comparison with results under uniform heat flux boundary shows that at convective boundary the influence of buoyancy on average heat transfer was weakened in the form of reduced affecting region and intensity, and local thermal inhomogeneity caused by buoyancy and centrifugal force was also relieved. Effect of thermal boundary condition is minimal in buoyancy-negligible cases like high mass flux condition. Further evaluation identifies that several Nusselt correlations obtained at uniform heat flux were capable of predicting average heat transfer at convective boundary, with a relative error within ±20%. Local heat transfer was characterized by a dimensionless number Ψ, which has three critical values of 0.1, 1 and 10 to quantitatively identify the relative importance of buoyancy and centrifugal effects. Finally, evaluation on the system design shows that helically coiled gas heater can contribute to improved compactness, reduced investment cost and a more flexible range of operating conditions for supercritical CO2 Rankine cycle.

Fast-Charging of Automotive Lithium-Ion Cells: In-Situ Lithium-Plating Detection and Comparison of Different Cell Designs

Fast-charging is a key requirement for customer acceptance of battery electric vehicles. In this work, various methods for lithium-plating detection are applied to prismatic hard-case lithium-ion cells to demonstrate their applicability for large-format automotive cells despite possible thermal and electrochemical inhomogeneities. Different fast-charging profiles with a charging time of less than 30 min from 10% to 80% state-of-charge are examined with methods based on coulombic efficiency, cell voltage analysis and swelling force. To validate these results, a lifetime cycling test with a subsequent cell opening is performed, followed by a discussion on lithium-plating detectability and the restrictions of each method.In order to further illustrate the applicability of the lithium-plating detection methods, the fast-charging ability of two cells with the same format but different cathode active materials, namely NMC622 and NMC811, are compared. Furthermore, a coupled electrochemical-thermal simulation is performed to study the effect of the cell design on the fast-charging ability and to elucidate why a higher cell energy density does not necessarily lead to a worse fast-charging ability.

To the theory of convective flows in a rotating stratified medium over a thermally inhomogeneous surface

A theoretical model of circulations over a thermally inhomogeneous horizontal surface in a gravity field is considered. The model is more general than that used in a number of previous works. It is free from the assumption about the relative thinness of the Ekman boundary layer, which (although not always justified) significantly simplified the calculations, since it was associated with the presence of a small parameter in the problem. On the basis of the proposed model, an analytical solution is found for a linear stationary two-dimensional convective flow problem in a semi-infinite stably stratified medium rotating around a vertical axis. Constitutive parameters are introduced - analogs of the Rayleigh and Taylor numbers, in which a given horizontal scale of thermal inhomogeneities appears as a spatial scale. For mesoscale atmospheric currents, which are characterized by very large values of these numbers, the consideration is limited to the case when the values of the Rayleigh numbers are much larger than the values of the Taylor numbers, but less than the latter to the 3/2 power (a situation typical for such atmospheric currents). Relationships for analyzing the dependences of the of velocity and helicity on the parameters of the problem are obtained. A number of general statements about the ratios of different helicity components in the discussed thermal circulations, in particular, in atmospheric currents with characteristic horizontal scales of the order of hundreds of kilometers, have been proved. Examples of numerical calculations of the vertical distribution of these are given.