Influence Of Thermal Gradients Research Articles

Influence of Thermal Gradients and Arctic Temperatures on the Mechanical Properties and Fracture Behavior of Woven Carbon and Woven Kevlar® Composites

Influence of Thermal Gradients and Arctic Temperatures on the Mechanical Properties and Fracture Behavior of Woven Carbon and Woven Kevlar® Composites

ESTUDANDO A SUBLIMAÇÃO DE DIÓXIDO DE CARBONO

Background: The utilization of dry ice in cooling and storage units requires adjusting the intensity of sublimation due to the requirements of prudently using CO2 to maintain preset thermal conditions. Aim: When designing a carbon dioxide cycle, it is essential to consider the influence of thermal gradients on the adsorption and desorption of carbon dioxide. Methods: tests were conducted to study the production and sublimation of carbon dioxide. The testes were aimed to define the temperature relation of the dry ice sublimation period, the density of pressed СО2, and the humidity of the environment and concentration. Results and Discussion: According to the obtained test data, there was a linear relationship between the sublimation intensity and the ambient air temperature in the specified conditions. The effect of moisture condensation on the sublimation rate appeared weaker than expected, for the amount of moisture on the surface of the specimens was insignificant. The heat exchange was intensified by the fall of hoarfrost and the related surface expansion. However, much moisture froze out without reaching the dry ice surface, and the formed layer of ice formed a heat insulation surface, and the sublimation under that layer was less intensive. The direct influence of sublimation came from the pressure at which a specific specimen was formed; however, 75 kN pressure was optimal. Conclusion: Despite higher weight losses during the storage, the difference in spent energy is more critical than 90 kN. The factor no less important was the carbon dioxide storage temperature. The maximal sublimation time of a 55 g cylinder formed at 75 kN and stored at – 80°С was 135 hours, much higher than at similar parameters but at -60°С. That said, the amount of energy spent on operating a low-temperature chamber was almost identical.

STUDYING THE SUBLIMATION OF CARBON DIOXIDE

Background: The utilization of dry ice in cooling and storage units requires adjusting the intensity of sublimation due to the requirements of prudently using CO2 to maintain preset thermal conditions. Aim: When designing a carbon dioxide cycle, it is essential to consider the influence of thermal gradients on the adsorption and desorption of carbon dioxide. Methods: tests were conducted to study the production and sublimation of carbon dioxide. The testes were aimed to define the temperature relation of the dry ice sublimation period, the density of pressed СО2, and the humidity of the environment and concentration. Results and Discussion: According to the obtained test data, there was a linear relationship between the sublimation intensity and the ambient air temperature in the specified conditions. The effect of moisture condensation on the sublimation rate appeared weaker than expected, for the amount of moisture on the surface of the specimens was insignificant. The heat exchange was intensified by the fall of hoarfrost and the related surface expansion. However, much moisture froze out without reaching the dry ice surface, and the formed layer of ice formed a heat insulation surface, and the sublimation under that layer was less intensive. The direct influence of sublimation came from the pressure at which a specific specimen was formed; however, 75 kN pressure was optimal. Conclusion: Despite higher weight losses during the storage, the difference in spent energy is more critical than 90 kN. The factor no less important was the carbon dioxide storage temperature. The maximal sublimation time of a 55 g cylinder formed at 75 kN and stored at – 80°С was 135 hours, much higher than at similar parameters but at -60°С. That said, the amount of energy spent on operating a low-temperature chamber was almost identical.

Influence of thermal gradients on the invasion patterns during drying of porous media: A lattice Boltzmann method

Drying of porous media sounds simple yet complicated to study the multiphase flow counterparts in porous media with intricate pore geometries. In the past, we have discussed the Lattice Boltzmann Model (LBM) as a powerful multiphase solver for the drying of porous media. In this study, we extend our previous work on the Shan Chen representation of the multiphase LBM to drying of porous media with imposed thermal gradients. A linearly varied stationary temperature profile is imposed concerning the depth of the porous medium, i.e., free evolution of temperatures due to the phase change is neglected. The preferential heating is divided into two kinds of gradients: First, the positive thermal gradient where temperatures varies linearly on an increasing order from top to bottom (e.g., the contact heating mode of drying). Second, the negative thermal gradient opposite the former (e.g., convective mode of heating). It is observed that the thermal gradient can lead to stabilizing and destabilizing drying fronts, where the latter situation incurs two drying fronts in a later period of drying. The novelty of this work is the establishment of thermal aspects to the previously discussed LBM and introduces the concepts of evaporation–condensation of trapped clusters and liquid bridges. The characteristics of thermal drying for stabilized and destabilized drying fronts is re-established at a magnified level of study using the developed LBM.

Passive wine macromixing from 3D natural convection for different winery tank shapes: application to lees resuspension

The works presented in this paper aim at investigating the problems related to the clarification of wines when random passive resuspension of lees occurs. More precisely, resuspension is addressed when temperature variations occur between the wine stored in tanks and the external surroundings of the tanks. From in situ laboratory studies involving laser tomography techniques, it is shown that low temperature gradients between a wine containing light lees and its external environment induce mass transfer by natural convection, generating enough fluid energy to resuspend the light lees in the liquid phase. The experiments are then complemented by numerical, CFD-based simulations focused on the role played by the geometry of different commercial tanks in the intensity of internal mixing. Finally, the groundwork for a study on a new internal design of the tanks by helical grooving is presented. To the best of the authors' knowledge, no literature study mentions the influence of thermal gradients on the resuspension of light lees and the influence of winery tank shapes on the internal fluid mixing intensity.

The effect of cell-to-cell variations and thermal gradients on the performance and degradation of lithium-ion battery packs

The performance of lithium-ion battery packs are often extrapolated from single cell performance however uneven currents in parallel strings due to cell-to-cell variations, thermal gradients and/or cell interconnects can reduce the overall performance of a large scale lithium-ion battery pack. In this work, we investigate the performance implications caused by these factors by simulating six parallel connected batteries based on a thermally coupled single particle model with the solid electrolyte interphase growth degradation mechanism modelled. Experimentally validated simulations show that cells closest to the load points of a pack experience higher currents than cells further away due to uneven overpotentials caused by the interconnects. When a cell with a four times greater internal impedance was placed in the location with the higher currents this actually helped to equalise the cell-to-cell current distribution, however if this was placed at a location furthest from the load point this would cause a ∼6% reduction in accessible energy at 1.5 C. The influence of thermal gradients can further affect this current heterogeneity leading to accelerated aging. Simulations show that in all cases, cells degrade at different rates in a pack due to the uneven currents, with this being amplified by thermal gradients. In the presented work a 5.2% increase in degradation rate, from −7.71 mWh/cycle (isothermal) to −8.11 mWh/cycle (non-isothermal) can be observed. Therefore, the insights from this paper highlight the highly coupled nature of battery pack performance and can inform designs for higher performance and longer lasting battery packs.

Routes to chaos from axisymmetric vertical vortices in a rotating cylinder

• First complete transition route to chaos in a vertical cylinder non homogeneously heated from below. • Influence of thermal gradients and rotation rate on the generation and evolution of single and double vertical vortices. • Connection to the formation and evolution of some atmospheric vortices. In this work, we study several routes of the transition to chaos from a steady axisymmetric vertical vortex in a rotating cylinder depending on thermal gradients and rotation rates. The analysis is done using nonlinear simulations. For a fixed rotation rate, the chaotic regime appears, as thermal gradients increase, after a sequence of supercritical Hopf bifurcations to periodic, quasiperiodic and chaotic flows in a scenario similar to the Ruelle–Takens–Newhouse route to chaos. For moderate values of the rotation rate we find vortices that tilt and move away from the center of the cylinder in a periodic, quasiperiodic and finally chaotic movement around the central axis. For larger rotation rates the axisymmetric vortex splits into two symmetric vortices that move periodically around the central axis, and lose the symmetry merging again in one non-axisymmetric vortex that moves around the central axis quasiperiodically and later chaotically. The transitions to chaos when the rotation rate is varied at fixed thermal gradients reveal also the appearance of periodic, quasiperiodic and chaotic states in different routes. Tilted single vortices, double vortices and more complex structures with multiple vortices are reported in this case. The transitions are studied through a force balance analysis. Results are of interest as they connect to the behavior of some atmospheric vertical vortices.

Glass transition kinetics measured by fast scanning calorimetry

Enthalpy relaxation data obtained for amorphous selenium by means of fast scanning calorimetry were described in terms of Tool–Narayanaswamy–Moynihan model. The effect of thermal gradients formed within the measured sample was found to explain the marked difference between the FSC and conventional DSC data obtained for a-Se. If the influence of thermal gradients is accounted for, the similar set of TNM parameters as obtained for a-Se from conventional DSC can be used to describe the FSC data. Temperature profiles of the thermal gradients as well as sources of additional uncertainties occurring within the introduced conception are discussed in detail. In addition, a normalization procedure accounting for the difference between the slopes of heat capacity in the glass and undercooled liquid regions was introduced. Correct data preparation was found to have a critical influence on the applied enthalpy relaxation evaluation procedures. Due to the unresolved issue of the prediction of manifesting thermal gradients found in case of the recent study on amorphous selenium, the FSC technique appears, for the time being, to have limited usage for enthalpy relaxation studies.

Influence of thermal gradients on stress state of veneered restorations

ObjectivesTo assess transient and residual stresses within the porcelain of veneered restorations (zirconia and metal) as a result of cooling rate and porcelain thickness. MethodsPorcelain-on-zirconia (PZ) and porcelain-fused-to-metal (PFM) crowns were fabricated with 1 or 2mm of porcelain. Thermocouples were attached both internally and externally to the crowns to record transient temperatures. For fast cooling, the furnace was opened after the holding time and switched off. Slow cooling was accomplished by opening the furnace at 50°C below the glass transition temperature (Tg) of the material. An axially symmetric FEA model simulated thermal stresses. Time-dependent temperature equations from thermocouple readings were set as boundary conditions. Framework materials and the porcelain below Tg were considered to behave elastically. Visco-elastic behavior was assumed for porcelain above the Tg modeling properties as dependent on cooling rate. ResultsDifferences in residual stress were found for fast and slow cooled PZ and PFM crowns. Significant transient stress waves were observed within the porcelain when fast cooling through Tg. They are believed to be related to non-uniform volumetric changes originated from thermal gradients. Results were confirmed by modeling and physical testing of crowns containing a defect. SignificanceResidual stresses do not distinguish PZ from PFM. High magnitude transient stresses observed within the porcelain during fast cooling may explain clinical fractures involving internal defects. Stress waves may also originate internal micro-cracking which could grow under function. Therefore, slow cooling, especially for all-ceramic crowns with thick porcelain, is important to prevent thermal gradients and high-magnitude transient stresses.

Experimental Investigation of Drying in a Model Porous Medium: Influence of Thermal Gradients

In this study, isothermal and non-isothermal drying experiments with imposed constant temperature gradients have been conducted in a two-dimensional square pore network of borosilicate glass (SiO2) with interconnected etched channels. In experiments with temperature decreasing from the open surface of the pore network, a travelling stabilized gas-liquid region could be established, while in isothermal experiments with uniform temperature faster breakthrough and an extended two-phase zone were observed. Both findings are in good agreement with pore network simulations. However, numerical underestimation of drying rates (especially in the second period of drying) indicates that liquid films might play a major role in the experimental pore networks, even in the presence of thermal gradients.

Numerical modelling of the co-firing of pulverised coal and straw in a 300 MWe tangentially fired boiler

The co-firing of pulverised coal/biomass in power generation plants is receiving considerable attention due to its influence in reducing all forms of emissions. Unlike coal, milled biomass, such as straw, may contain large particles of different sizes and shapes, which can have an impact on the combustion characteristics and emissions. Computational fluid dynamics (CFD) is often used to understand the influence of large biomass particles in a furnace. However, most CFD sub-models simplify heat transfer effects within the particles during combustion. In this paper a particle heat-up model, which considers the influence of thermal gradients within large biomass particles, is applied to a co-firing coal/biomass simulation in a tangentially fired furnace with up to 12% thermal biomass loading. Different sizes of biomass particles of non-spherical shape and their impact on the combustion behavior have been investigated. The influence of the particle size and shape distribution on the combustion characteristics and emissions was found to be significant. The computed results were found to be in good agreement with the experimental data.

Influence of thermal gradients on stress state of veneered restorations

Influence of thermal gradients on stress state of veneered restorations

Tailoring laser-induced domain wall pinning

Field-induced domain wall (DW) motion in Permalloy microwires under the influence of thermal gradients is studied using magneto-optical Kerr-microscopy. The DW is found to be pinned at two positions in a laser spot focused onto the wire. By tuning the laser power, the pinning strength can be engineered. On increasing the laser power, the depinning field for the first pinning site in the first half of the laser spot is found to decrease, while for the second pinning site in the spot region located beyond the centre of the spot it increases. Below a threshold power W T , the pinning strength can be tuned reversibly while above W T irreversible increases in the pinning strength due to structural changes occur. This effect can be employed for flexible pinning sites for DW devices.

The influence of thermal gradients on AC losses in high-temperature superconducting coils

As high-temperature superconducting (HTS) tapes approach commercially acceptable performance levels, attention should be paid to how to utilize these tapes in real applications. One of the most important factors is the AC loss. When wound into a coil the operating conditions of the HTS tape vary in different parts of the coil. The optimum operating current, with respect to the AC losses, increases radially in the coil as the magnetic field decreases. With a constant current in the coil, large parts of the coil will not be optimally operated. The optimum operating current of the HTS tape also varies with temperature. In this paper we propose to use a controlled temperature distribution in the coil so that the coil can be operated at its optimum current in the entire coil. The potential for loss reduction with a controlled temperature distribution in a large AC HTS coil is about 10%.

Thermoelastic Modeling of a PWB With Simulated Circuit Traces Subjected to Infrared Reflow Soldering With Experimental Validation

A bare, four copper layer printed wiring board with simple trace patterns was built for modeling and experimental validation purposes. In-plane elastic properties of the core materials in the board were measured as a function of temperature. Thermoelastic lamination theory was utilized to predict the warpage of the board when subjected to an infrared reflow process, with emphasis on studying the influence of thermal gradients through the board, its support conditions and CTE differential on the warpage process. Board layers with traces were approximated with quasi-homogeneous effective properties obtained using micromechanics theory. An experimental system that employs the shadow moird technique in a simulated infrared reflow environment was used to evaluate the warpage for comparison to modeled results.

The Influence of Thermal Gradients on the Long-Term Evolution of the Near-Field Environment of High-Level Nuclear Wastes Disposal

ABSTRACTThe initial stage of some HLW disposal systems will be characterised by a large thermal pulse in the near-field environment, due to the heat of the radioactivity decay. This will lead to the development of a transient spatial thermal gradient between the hot canister and the cold geological medium, which could significantly affect the composition and the elemental distribution within the near-field environment. A coupled experimental and modelling work is presented in order to determine the influence of a thermal gradient on water-rock interaction processes. First experiments with a simulated nuclear glass evidenced mass transfer processes leading to chemical differentiation in the solid phases between the hot and the cold end of the system. The relevance of these experimental results to the case of a HLW disposal is strongly supported by in-situ experiments at Stripa, in which a realistic EBS under thermal gradient developed exactly the same mass transfers.In order to understand the driving force of these processes, we tried to model simplified experiments by using a mixing cell geochemical model built upon the geochemical code EQ3/EQ6. The discrepancies between modelling and experiments indicate the existence of coupled processes involving irreversible precipitation.Finally, thermal gradients were applied in nuclear glass-clay interaction experiments to enhance elemental migrations. The main results are: (i) a re-crystallisation of the initial clay toward a more silicic one through incorporation of elements released by the glass, (ii) a strong influence of clay chemistry on the nuclear glass secondary phases.

Design of a test structure to evaluate electro-thermomigration in power ICs

The design of a novel test structure to study the influence of electromigration and thermomigration in the large dimension metal lines used in power integrated circuits is reported. Incorporated into the test structure are a heating element and a number of temperature monitors, to enhance the thermal gradient and to measure it, respectively. Thermal simulations have been performed and compared with the electrical characterization carried out on a set of test structures. The agreement between simulation end electrical measurements has been proved confirming the validity of the test structure as a vehicle for a deeper understanding of the reliability of the line under the influence of thermal gradients.

Optimisation considerations for measurement of weak thermal lensing in laser media

A solid-state laser medium is known to behave as a weak lens under the influence of thermal gradients generated due to the nonuniform heating and cooling of the laser host material. The authors show that the fractional change in optical power due to induced thermal lensing can be greatly increased by careful choice of the size and position of an out-of-focus aperture. >

Diurnal variations in moisture and temperature of a desert soil under different management practices

A field study was conducted to find out the effect of some soil management practices on hydrothermal regime of soil under high radiation conditions of Jodhpur. Pulverization, compaction and pulverization + mulching formed the three treatments. The results showed vide variations in diurnal soil temperatures up to 15 cm depth. There were little differences in soil temperature under pulverization and compaction while pulverization + mulch reduced the maximum temperature by 4 to 10°C. Diurnal oscillations in soil moisture were observed under the influence of thermal gradients. Maximum loss of moisture was observed from the pulverized plots while minimum was recorded from the pulverized + mulched plots.