Longitudinal Temperature Gradient Research Articles

Atmospheric Retrievals of the Phase-resolved Spectra of Irradiated Brown Dwarfs WD-0137B and EPIC-2122B

We present an atmospheric retrieval analysis of Hubble Space Telescope/Wide Field Camera 3/G141 spectroscopic phase curve observations of two brown dwarfs, WD-0137B and EPIC-2122B, in ultrashort period orbits around white dwarf hosts. These systems are analogous to hot and ultra-hot Jupiter systems, enabling a unique and high-precision comparison to exoplanet systems. We use the PHOENIX Exoplanet Retrieval Algorithm retrieval suite to test various analysis setups, including joint-phase retrievals, multiple temperature structures, and nonuniform abundances. We find that WD-0137B has a dayside that closely resembles that of other ultra-hot Jupiters with inverted temperature structures and H− opacity, but quickly transitions to a mostly noninverted temperature structure on the nightside. Meanwhile, EPIC-2122B’s atmosphere remains inverted at all constrained longitudes, with dominant H− opacity. Retrievals with multiple temperature profiles and nonuniform vertical abundances were generally not statistically justified for this data set, but retrievals with dayside-dilution factors were found to be justified. Retrieving all phases simultaneously with a linear combination of a dayside and nightside atmosphere was found to be an adequate representation of the entire phase curve once a longitudinal temperature gradient free parameter was included in the retrieval. Comparing to global circulation models, we attribute behavior in the 1D retrievals to the inclined viewing geometry of the systems, which results in always-visible irradiated and inverted portions of the atmosphere contaminating spectra measured from the nightside hemisphere. This study sheds light on the similarities between these irradiated brown dwarf systems and hot and ultra-hot Jupiters, but also their unique differences, including the influence of the inclined viewing geometry.

Role of Longitudinal Temperature Gradients in Eliminating Interleaving Inclusions in Casting of Monocrystalline Silicon Ingots

Infrared analysis reveals the presence of interwoven inclusions, primarily comprised of silicon nitride and silicon carbide, in the casting process of monocrystalline silicon ingots. This study investigates how the longitudinal temperature gradient affects the removal of inclusions during the casting of monocrystalline silicon ingots through simulations and comparative experiments. Two monocrystalline silicon ingots were cast, each using different longitudinal temperature gradients: one employing smaller gradients and the other conventional gradients. CGSim (Version Basic CGSim 23.1) simulation software was utilized to analyze the melt flow and temperature distribution during the growth process of quasi–monocrystalline silicon ingots. The findings indicate that smaller longitudinal temperature gradients lead to a more robust upward flow of molten silicon at the solid–liquid interface, effectively carrying impurities away from this interface and preventing their inclusion formation. Analysis of experimental photoluminescence and IR results reveals that although inclusions may not be observed, impurities persist but are gradually displaced to the top of the silicon melt through a stable growth process.

Influence of Natural External Forcings on Interdecadal Variation of Global Land Monsoon over the last millennium in CESM-LME

Abstract Interdecadal variations of the global land monsoon have been previously attributed to internal fluctuations of the climate system, but the role of natural external forcings was under-explored. Here, we investigate this issue by using the Community Earth System Model ensemble simulations over the last millennium (950-1850 A.D.). Our analysis reveals that the surface temperature, with two dominant structures (global cooling/warming and longitudinal sea-surface temperature gradient in the tropical Pacific, which affects the Walker circulation), predominantly shapes the leading forced mode of the global land monsoon. This mode, representing 19% of the total variance, manifests as consistent features across South Asia, the southern part of East Asia, North Australia, South America, and western South Africa, contrasting with other monsoon regions. Under global cooling conditions, the monsoon intensity is enhanced in the northern parts of the East Asian and eastern parts of the North and South African monsoons, but it decreases in the other monsoon regions. Under weak Walker circulation conditions, changes in atmospheric circulation in response to the sea surface temperature gradient in the tropical Pacific are associated with a substantial attenuation of almost all land monsoon regions. It was further shown that the global mean surface temperature and the tropical Pacific temperature gradient jointly account for 75% of the total variance in the leading mode of the global land monsoon, with 29% and 46% as their respective contribution. Furthermore, our results suggest that volcanic eruptions are the dominant external forcing for these variations. These findings provide valuable insights for future research on global monsoon dynamics.

Numerical prediction of thermal conductivity and thermal expansion coefficient of glass fiber-reinforced polymer hybrid composites filled with hollow spheres

The optimal performance of composites enriched with hollow spheres has been reported in contemporary literature, whereas their thermal properties have received less attention. In this regard, a finite element method (FEM)-based micromechanical model has been developed systematically to investigate the role of intra-matrix embedding of hollow spheres on the thermal conductivity and coefficient of thermal expansion (CTE) of unidirectional fiber-reinforced hybrid composites. In so doing, the concept of representative volume element (RVE) considers microstructures comprising an epoxy matrix, E-glass fiber, and E-glass hollow spheres, assuming perfect bonding (ideal interface) between the components and modified approximate periodic boundary conditions. By computing the longitudinal and transverse temperature gradients generated due to the application of uniform heat flux as well as the geometrical variation in RVE owing to temperature enhancement, thermal conductivity and CTE have been respectively determined. Comprehensive evaluations have been conducted to examine the effects of microstructural-level features, including fiber volume content and orientation, plus volume content and thickness of hollow spheres, on the effective thermal conductivity and CTE of pseudo-porous ternary E-glass/epoxy composites.

The Specifics of PTC Thermistor Applications for Limiting Surge Currents

This paper investigates the possibilities of limiting emergency short current pulses by using materials with a positive temperature coefficient. A computational model and method for simulating this process were proposed. Barium titanate was considered a promising material for current-limiting devices due to its low cost, ease of manufacturing and autonomy. The current-limiting properties of a PTC thermistor device were estimated in relation to a significant pulse current overload (10 ms). The value of taking into account the varistor effect when modeling energy dissipation by a PTC thermistor is experimentally demonstrated. The results of the experiment were used in the development of computer models of the current-limiting properties of PTC thermistors. A method for taking into account the varistor effect in the developed models is proposed. It is noted that in order to increase the accuracy of the calculation, it is important to take into account the heat transfer between the PTC thermistor and its contact system. It is also shown that this heat transfer is capable of creating a significant longitudinal temperature gradient inside the PTC thermistor, which creates a high level of thermal stress and leads to mechanical failure. Destruction of this type has been observed during experiments. This research was carried out within the state assignment FSEG-2023-0012.

MODELING OF THREE-LAYER FLOWS WITH NON-UNIFORM EVAPORATION BASED ON THE EXACT SOLUTION OF CONVECTION EQUATIONS

Mathematical modeling of three-layer convective flows of two liquids and a gas-vapor mixture in a horizontal channel is based on the exact solution of the Navier-Stokes equations in the Boussinesq approximation. Thermocapillary interfaces are assumed to be non-deformable. The inhomogeneous mass transfer of the light liquid to the upper layer is considered. The Soret and Dufour effects are taken into account in the upper layer, and the gas flow rate is given. The influence of thermal load and flow geometry on the main characteristics of the flow was studied using the example of the water–benzine–air system. It is shown that the Soret effect has an impact on the nature of the flow. The influence of the effect of thermodiffusion, changes in the longitudinal temperature gradients, and thicknesses of the layers of the system on the dew point is revealed.

Monolithic SOI through-wafer Knudsen pumps with mechanically robust Si channels

Knudsen pumps (KPs) generate gas flow using thermal transpiration in narrow channels with a longitudinal temperature gradient. With no moving parts, KPs offer long lifetime and are attractive for complex microfluidic systems. Although high performance was previously reported in microfabricated KPs that utilized suspended dielectric membranes to construct the pumping channels, this approach can limit the mechanical robustness and complicate the fabrication process. In this work, we present monolithically microfabricated KPs without using suspended membranes. These types of structures significantly improve the mechanical robustness, allow a wider process window, and simplify the microfabrication process. The pumping channels in this work are 14 µm in length, 1.2 × 200 µm2 in cross-section, vertically oriented, and with rigid silicon and dielectric sidewalls; the overall structures are easily manufacturable. This work primarily focuses on two core KP designs, which incorporate 4 and 6 parallel pumping channels in ≈ 11 and 17 mm2 footprints, respectively. The scaling capability is also investigated preliminarily in a design with 45 parallel pumping channels. The KPs were fabricated on a silicon-on-insulator wafer by a six-mask lithographic process with standard fabrication steps. This process eliminated the use of complicated trench refill, polish, and sacrificial dry etch steps that were required for the previously reported KPs with suspended membranes. At atmospheric pressure, the 4-channel KPs provided a blocking pressure of 620 Pa and maximum flow rate of 0.041 sccm through 0.001 mm2 channel area, using 2 W applied power. This maximum flow rate per unit channel area (sccm/mm2) is superior to that of the KPs with suspended membranes. For all the KP designs, the experimentally measured pumping performance well matched the modeling results, with <15 % discrepancy. The fabrication simplicity and reliability of the mechanically robust KPs can enable monolithic integration onto complex lab-on-a-chip systems.

Casting of film-cooling holes for single-crystal blades

The flawless formation of film-cooling holes (FCHs) is critical for manufacturing single-crystal blade. Presently, high-energy beam drilling is mostly used to process gas film holes, which introduces the limitations of complex hole structures that are difficult to form and processing thermal defects that are difficult to eliminate. This study demonstrates an approach to form FCHs using casting, where an indirect additive manufacturing technology using a core/shell integral ceramic mold is applied for casting. Cylindrical 0.4, 0.8, and 4.0 mm-diameters hole are cast. The influence of fine ceramic cores on the microstructure of single-crystal blades during directional solidification is investigated using simulations and experiments. The fine ceramic cores of the FCHs affect the growth morphology of dendrites around the hole, which is determined by the hole diameter, primary dendrite spacing, and local temperature filed. Owing to the thermal conductivity difference between the alloy and ceramic, the local temperature filed around the holes changes abruptly, leading to the formation of shrinkage porosity and cavities. Furthermore, the process window for the casting of FCHs is determined using the longitudinal temperature gradient (GZ) and solidification rate (VZ). The proposed approach is of practical significance for the defect-free casting of tiny structures in single-crystal blades.

Absence of Spin-Orbit Torque and Discovery of Anisotropic Planar Nernst Effect in CoFe Single Crystal.

Exploration of exotic spin polarizations in single crystals is of increasing interest. A current of longitudinal spins, the so-called "Dresselhaus-like" spin current, which is forbidden in materials lacking certain inversion asymmetries, is implied to be generated by a charge current at the interface of single-crystal CoFe. This workreports unambiguous evidence that there is no indication of spin current of any spin polarizations from the interface or bulk of single-crystalline CoFe and that the sin2φ second harmonic Hall voltage, which ispreviously assumed to signify Dresselhaus-like spin current, is not related to any spin currents but rather a planar Nernst voltage induced by a longitudinal temperature gradient within the sample. Such sin2φ signal is independent of large applied magnetic fields and interfacial spin-orbit coupling, inversely correlated to the heat capacity of the substrates and overlayers, quadratic in charge current, and appears also in polycrystalline ferromagnets. Strikingly, the planar Nernst effect (PNE) in the CoFe single crystal has a strong fourfold anisotropy and varies with the crystalline orientation. Such strong, anisotropic PNE has widespread impacts on the analyses of a variety of spintronic experiments and opens a new avenue for the development of PNE-based thermoelectric battery and sensor applications.

Thermal Hall effect and the Wiedemann–Franz law in Chern insulator

Thermal Hall effect, where a transverse temperature difference is generated by implementing a longitudinal temperature gradient and an external magnetic field in the perpendicular direction to systems, is a useful tool to reveal transport properties of quantum materials. A systematic study of the thermal Hall effect in a Chern insulator is still lacking. Here, using the Landauer–Büttiker formula, we investigated the thermal Hall transport of the Harper–Hofstadter model with flux φ = 1/2 and its generalizations. We demonstrated that the Wiedemann–Franz law, which states that the thermal Hall conductivity is linearly proportional to the quantum Hall conductivity in the low temperature limit, is still valid in this Chern insulator, and that the thermal Hall conductivity can be used to characterize the topological properties of quantum materials.

Limitations of homogeneous and segmented single-crystal compact TEM00-mode Ho3+:YAG laser resonators

We present investigations on output power limitations of near-diffraction-limited compact hbox {Ho}^{3+}:YAG laser resonators employing a homogeneous or segmented laser crystal. An approach for designing a segmented crystal is presented. Maximum output powers of {57.6},hbox {W} and {51.9},hbox {W} are reached with the homogeneous and segmented crystal, respectively, resulting in pulse energies of {1.14},hbox {mJ} and {1.04},hbox {mJ} at a repetition rate of {50},hbox {kHz} in Q-switched operation. Interferometric experiments are conducted to derive the radial temperature profile for both crystals. Simulations based on a split-step beam propagation method are used to model the longitudinal temperature gradient in both crystals.

The Influence of Irregular Microwave Heating on the Intensity of Liquid-Liquid Chemical Process in a Microreactor

The influence of microwave heating of liquids in a microreactor using beam-type source on the intensity of the current chemical process is considered. It is shown that а non-uniform heating of the reagents over the microreactor length stimulates the development of the thermal capillary convection phenomenon and substantially accelerates the chemical process. A numerical evaluation of maximal velocities of the thermal capillary motion developed at the interface of contacting liquids depending on the specific power of internal heat sources and the longitudinal temperature gradient is given.

The Impact of Geometric Parameters on Three-Layer Flow Patterns in a Horizontal Channel: A Comprehensive Study

This paper focuses on studying stationary flows in horizontal layers, taking into consideration the effects of heat and mass transfer. A mathematical model is presented for a three-layer flow in an infinite channel with solid, impermeable walls. The gas-vapor mixture flows over one-component immiscible liquids in the lower and middle layers. The paper considers the processes of heat and mass transfer at the "liquid-gas" and "liquid-liquid" thermocapillary boundaries, respectively. The Dufour and Soret effects are also considered in the upper layer of the system. Mathematical modeling is based on exact solutions of a special form of the Navier-Stokes equations in the Boussinesq approximation. The procedure for determining unknown parameters is explained. The paper derives dependencies of the longitudinal temperature gradients at the system boundaries from each other. The influence of the flow region's geometry on the nature of the process is studied using the "silicone oil-water-air" system as an example. The paper presents longitudinal velocity profiles and temperature distribution for different values of the liquid layer heights while other parameters of the system remain fixed. The paper concludes that, in this case, the thickness of the lower layer is more influential than the thickness of the middle layer.

Gravity’s Impact on Flow Patterns in Liquid-Gas Systems with Mass Transfer Considerations

This paper investigates bilayer flows of liquid and cocurrent gas in an inclined channel with considerations for heat and mass transfer at the thermocapillary interface, which remains undeformable. The mathematical modeling is based on exact solutions of the special Ostroumov — Birikh type of Navier — Stokes equations in the Oberbek — Boussinesq approximation, accounting for the effect of thermodiffusion and diffusive thermal conductivity effect in the gas-vapor layer. Vapor in the gas is assumed to be a passive admixture that does not affect the properties of the gas. The paper considers the case with the vapor concentration function satisfying the condition of zero vapor flux on the upper wall of the channel. The working system chosen for this study is &lt;&lt;HFE-7l00-nitrogen&gt;&gt;, for which the paper provides examples of velocity profiles, temperature, and vapor concentration distribution in the upper layer. The study analyzes the effect of gravitational influence on the flow pattern and the value of the mass flow rate. The paper presents results for cases when the condition of closed liquid flow is fulfilled, at a negative value of the parameter determining the longitudinal temperature gradient, and at a given value of the gas flow rate.

Development of composite-phase change microcapsule coating and numerical investigation on its effect in ballastless track slabs

To solve the problems of interlayer cracking, up-arching and warping of ballastless track slab of high-speed railway under temperature load, a composite phase-change microcapsule (MPCM) was developed, and a series of physical and chemical experiments was conducted. The practicality and functional advantages of composite phase-change microcapsules were investigated by SEM, FTIR, DSC and other material tests. The cooling effects of the coating were evaluated and simulated by indoor tests and numerical simulations. The results show that the developed MPCM exhibits both strong reflectivity of TiO2 and the phase change energy storage property of paraffin, which can reflect 59%-63% of the near-infrared radiation in solar radiation. Moreover, 1 g MPCM can absorb and store 98.3–112.5 J of heat when the ambient temperature exceeds 40 °C. The MPCM coating can reduce the rate of temperature rise of the test specimen, and also decrease its overall temperature difference to 12–15 °C. Furthermore, it can reduce the longitudinal temperature gradient of the test specimen at least by 50%, and its surface temperature by 18%-28%. The temperature regulation ability of coating is positively correlated with the content of MPCM, and its cooling effect reaches the best when the content of MPCM is 20%. The coating reduces the vertical deformation of track slab by 25%-59.6% and the temperature difference inside the track slab by 12.2 °C under high-temperature condition.

The Study of Three-Dimensional Fluid Flow Regimes on the Basis of the Exact Solution to the Convection Equations

We consider the evaporation/condensation process of diffusion type in two-layer fluid flows under the assumption that both viscous heat-conducting incompressible fluids, a liquid and a gas-vapor mixture, have a common interface and occupy an infinite threedimensional rectangular channel. Based on the exact solution to the convection equations, we study the fluid flows arising in the case of a linearly distributed thermal load on the substrate and thermal insulation of the upper and lateral walls of the channel. It is observed that some complex vortex structures are formed under the action of the longitudinal temperature gradient and transversely directed gravity field. For the ethanol–nitrogen system we present the characteristics of hydrodynamic, thermal, and concentration fields, in particular, the profiles of the evaporative mass flow rates and interface tangential stresses, the topology of flows, and the temperature and vapor concentration distributions.

Двумерное течение Пуазейля ньютоновской жидкости с вязкостью, зависящей от температуры

The problem of a two-dimensional Poiseuille flow of a Newtonian fluid with a viscosity exponen-tially dependent on temperature under the influence of longitudinal pressure and temperature gra-dients is reduced to a two-parameter boundary value problem for a third-order ordinary differential equation with respect to a dimensionless unknown characterizing the change in temperature across the channel. In the range of negative values of the dimensionless longitudinal temperature gradient, the solution has two branches. By closing, the branches form a boundary on the parameter plane beyond which Poiseuille-type flows do not exist. The solutions belonging to different branches dif-fer in the flow rate and intensity of heat transfer by the liquid through solid boundaries. The latter circumstance entails deceleration of the flow in the near-wall areas. In the region of positive values of the dimensionless longitudinal temperature gradient, the solution is unique. With an increase in the mentioned parameter, the heat flow to the liquid from the walls increases, which entails the formation of a more bulged velocity profile compared to the isothermal case. At sufficiently large values of the longitudinal temperature gradient, there appears a ‘stick’ flow with a uniform velocity distribution everywhere except for narrow layers near solid boundaries. It is shown that the rate is proportional to the ratio of the densities of the transverse and longitudinal heat flows. Explicit ex-pressions for the pressure and shear stress fields are given. The properties of monotonicity of the velocity and temperature profiles as functions of the transverse coordinate have been studied. It is shown that, regardless of the values of the dimensionless parameters of the problem, the velocity has a positive maximum on the central axis of the channel and monotonically decreases to zero at the solid boundary.

The impact of technological parameters of the torch to physical and chemical properties of a gas-thermal burner for spraying ultra-high molecular weight polyethylene

The values of the heat flux density, transverse and longitudinal temperature gradients of the gas-thermal burner torch, the type of reaction during the combustion in various modes (reducing, neutral, oxidizing) depending on the mass fractions of the fuel and oxidizer burning have been determined in this paper. The significant role of the torch temperature as one of the most important parameters determining the quality of polymer coatings obtained by the gas-thermal method has been established. A spatial model of a gas-thermal burner has been designed, which allows to obtain thermal modes at specified technological parameters. A physical model of the interactions “torch-polymer particle”, “thermal jet – base” has been developed. The necessary density of the torch heat flux for the complete melting of ultrahigh molecular weight polyethylene (UHMWPE) particles not exceeding the temperature threshold of destruction has been determined. The required rate of introduction the UHMWPE powders into the burner flame has been also determined. Based on the calculations of the “torch-polymer particle interaction”, the optimal geometry of the torch, the particles trajectory in the torch and the spraying distances have been determined.

Multimodel evaluation of longitudinal stream temperature gradient and dominant influencing factors in Michigan streams

AbstractStream temperature is an important determinant of fish growth, migration, and survival and can thus impact the structure and function of stream ecosystems. Many streams in Michigan and elsewhere in North America receive groundwater inputs that help regulate instream conditions by stabilizing discharge as well as stream temperature. However, groundwater withdrawal can cause reductions in streamflow which typically results in increased summer stream temperatures. Other atmospheric and hydrologic variables (i.e., overland discharge) also impact the rate at which stream temperature changes as it flows downstream. We deployed paired up‐ and downstream water pressure and temperature loggers within 21 stream reaches throughout the state of Michigan to quantify and model relationships between stream discharge, air temperature, and longitudinal change in stream temperature (i.e., temperature gradient). Using multimodel selection criteria, we evaluated the performance of a hierarchical suite of models that predict temperature gradient as a function of potential driving variables. The multimodel selection criteria identified a best‐fitting model that was able to model the diurnal, seasonal, and annual variations in rates of longitudinal temperature fluctuations across most sample streams. Partial regression analysis indicated that proxy variables representing solar radiation at the stream surface were generally the most influential predictors of longitudinal changes in stream temperature, but air temperature and components of streamflow including groundwater input were significant predictors and important in many streams.

Nonlinear anomalous Hall effects probe topological phase-transitions in twisted double bilayer graphene

Nonlinear anomalous (NLA) Hall effect is the Berry curvature dipole induced second-order Hall voltage or temperature difference induced by a longitudinal electric field or temperature gradient. These are the prominent Hall responses in time-reversal symmetric systems. These band-geometry induced responses in recently realized twistronic platforms can probe their novel electronic band structure and topology. Here, we investigate the family (electrical, thermoelectric, and thermal) of second-order NLA Hall effects in the moiré system of twisted double bilayer graphene (TDBG). We combine the semiclassical transport framework with the continuum model of TDBG to demonstrate that the NLA Hall signals can probe topological phase transitions in moiré systems. We show that the whole family of NLA Hall responses undergo a sign reversal across a topological phase transition. Our study establishes a deeper connection between valley topology and nonlinear Hall effects in time-reversal symmetric systems.