Steady-state Thermal Regime Research Articles

Field emission in vacuum resonant tunneling heterostructures with high current densities

We analyse the steady-state thermal regime of a one-dimensional triode resonant tunnelling structure. The high currents generated by resonant tunnelling produce a large amount of heat that could damage the structure. Establishing the conditions under which it can operate at optimum efficiency is therefore a problem of great relevance for applications. The tunnel current is found via eigenvalues of the Schrödinger equation in quantum wells. By calculating the current generated in the device and using the energy conservation law in the electrodes, the temperature reached is obtained for different types of electrodes and the importance of heat conduction and thermal radiation is analysed. In the cases discussed, conduction is dominant. When the electrode material is copper, the temperature reached is similar to that of the thermostat for a wide range of electrode lengths, whereas when the cathode material is diamond-graphite and the anode material is copper, the temperature increases significantly as a function of length. The results obtained allow the temperature to be controlled for optimum performance of the field-emitting triode structures.

Methodology of thermal loading estimation of the brake mechanism of a truck

The paper presents the results, conclusions of the assessment of friction pairs in the aspect of the phenomenon of fading. As you know, most of the kinetic energy of a car with ABS is extinguished due to the work of friction in the brake mechanism. Overheating the brake mechanism, namely its friction pairs, leads to the appearance of the phenomenon of critical fading, accompanied by a sharp decrease in the braking torque. When the metal friction elements of braking devices operate in a repeated-short-term mode, it is advisable to assess their operating parameters at a steady-state thermal regime, i.e. under the most severe temperature conditions. The proposed technique makes it possible to obtain a high convergence of the calculated values with the experimental data. Evaluation of thermally loaded using this method can be carried out at any stage of the design and does not require large computing power. Visualization of the results allows you to identify areas of increased heating to assess the need to change the design of the brake mechanism.

Concrete confinement with TRM versus FRP jackets at elevated temperatures

Despite the clear cost, ease of installation, and construction schedule advantages of confinement of concrete structural elements with fibre-reinforced polymers (FRPs) for strength and deformability enhancement, concerns as to their performance at elevated temperature, or in fire, remain. The results of a series of elevated temperature experiments on FRP and textile reinforced mortar (TRM) strengthening systems for confinement of circular concrete columns are presented. The behaviour and effectiveness of the respective confining systems is studied up to temperatures of 400 °C. A total of 24 concrete cylinders were wrapped in the hoop direction with different amounts of FRP or TRM, heated to steady-state temperatures between 20 and 400 °C, and loaded to failure in concentric axial compression under a steady-state thermal regime. The results indicate that the effectiveness of the FRP confining system bonded with epoxy decreased considerably, but did not vanish, with increasing temperatures, in particular within the region of the glass transition temperature of the epoxy resin/adhesive. Conversely, the TRM confining system, bonded with inorganic mortar rather than epoxy, demonstrated superior performance than the FRP confining system at 400 °C as compared against tests performed at ambient temperature.

Features and results of assessment the thermal conductivity of building materials and products by the active method of thermal non-destructive testing

Thermal control (passive and active) is a non-destructive testing method. During passive thermal control, the test object is characterized by a temperature field formed during its operation. In active thermal control, the test object is additionally thermally stimulated. This technique is widely used in various areas of construction, energy, mechanical engineering, transport. This paper proposes a variant of active thermal non-destructive assessment of the thermal conductivity coefficient of building materials and products on the example of a fragment of a building structure made of silicate bricks. The test object is subjected to thermal stimulation by an external source of thermal energy before reaching a steady-state thermal regime. Thermography of the test object surfaces is carried out. The average integral temperatures of surfaces or individual sections of the test object are calculated. The coefficient of thermal conductivity of the test object is determined, which is used to calculate its thermal resistance (resistance to heat transfer). After that, the coefficient of heat transfer is calculated. The method was implemented in laboratory conditions. It can be used in natural and operational conditions for accurate and quick determination of the key thermophysical properties of building materials and products.

New Thermal-Conductivity Constitutive Matrix in Fourier’s Law for Heat Transfer Using the Cell Method

In this paper, a new constitutive matrix Mτ for thermal conduction, for tetrahedral meshes, in a steady state thermal regime is developed through a new algebraic methodology, using the Cell Method as a computational method, which is included in the finite formulation. The constitutive matrix defines the behavior of solids when they are under a thermal potential. The results are compared with those obtained for the same problem by means of the constitutive matrix Mλ developed previously, taking in both cases with a 2D axisymmetric model as reference, calculated with the finite element method. The errors obtained with the new matrix Mτ are of the order of 0.0025%, much lower than those obtained with the matrix Mλ.

Effect of Hydrate Formation Conditions on Thermal Conductivity of Gas-Saturated Sediments

The paper summarizes experimental data on thermal conductivity variations in gas-saturated sediments exposed to hydrate formation at various conditions. All experiments were performed on a specially designed gas hydrate system maintaining a high gas pressure and a steady-state thermal regime, with a built-in unit for thermal conductivity measurements. The measurements were applied to natural samples of fine sand and silty sand collected from gas emanation sites in permafrost and to artificial sand and sand–clay mixtures. The results show that thermal conductivity can either increase or decrease depending upon hydrate formation conditions. Namely, it increases if gas hydrates form at positive temperatures (t > 0 °C) but decreases during hydrate formation in frozen samples. Freezing and thawing of hydrate-bearing sediments above the equilibrium pressure reduce their thermal conductivity as a result of additional hydrate formation. Experimental results are used to model hydrate and ice formation in gas-satur...

Longitudinal thermooptical aberration of the image in reflective telescopes

An analytical description of the dependence of the longitudinal thermal aberration of Gregorian and Cassegrain telescopes is obtained in the form of a sum of the components introduced by the thermal regime of each element of the telescope. Using a specific example, a technique is presented for determining the allowable ranges of temperature variation of the telescope housing and of the platform for attaching the primary mirror within which the thermal stability of the telescope in a steady-state thermal regime is ensured.

Methods of ensuring the thermal stability of a solar limbograph, taking into account all the thermal factors that affect it in near-earth space

It is shown by using a new calculational technique that it is possible to ensure the thermal stability of the SL-200 solar limbograph in the steady-state thermal regime. Such a regime is established in solar-stationary orbit at low altitudes by appropriately choosing the coating parameters of the quartz filter at the entrance pupil, taking into account the absorbed direct solar radiation and that reflected by the earth, as well as the earth’s thermal radiation.

Conditions for ensuring the thermal stability of the SL-200 solar limbograph in the steady-state thermal regime

A theoretical analysis is used to show that it is possible to ensure the thermal stability of the SL-200 solar limbograph in the steady-state thermal regime, implemented on solar-synchronous orbit, by appropriately choosing the parameters of the coatings of a quartz filter that attenuates the power of the solar radiation flux.

Kinematic, thermal and petrological model of the Central Alps: Lepontine metamorphism in the upper crust and eclogitisation of the lower crust

Seismic and seismological studies as well as gravimetric models indicate that a slab of European lithospheric mantle and lower crust is currently underthrust below the Apulian crust. We assume a simple kinematic model in which the lower and upper subducted European crusts are decoupled along a decollement. The lower crust goes into subduction without deformation. The upper crust deforms by pure shear with a horizontal compressional axis. The total erosional flux is adjusted to balance upper crust input so that the belt keeps the same geometry, different distributions of erosion being used. The computed temperature field is steady-state if the kinematic model applies during a minimum time of 40 Myr for a convergence rate of 8 mm/yr. Equilibrium mineral assemblages and densities are determined from the computed P, T conditions for a granodioritic chemical composition of the upper crust and an andesitic composition of the lower crust. Assuming local isostasy, the density model fits the average topographic profile across the Central Alps. The P-T-t paths obtained for the part of the upper crust initially at depths 10 to 16 km are compatible with the medium pressure Oligocene metamorphism in the Lepontine dome. The peak calculated temperature for the deepest non subducted crustal rocks is 600°C for a pressure of 0.8 GPa, near the lower limit of high-pressure amphibolites. We thus propose that the Lepontine metamorphism corresponds to the steady-state thermal regime. However, either faster erosion rates in the internal part of the belt or tectonic denudation are required for exhumation of the deeper portion of the belt. The computed temperature field implies eclogitisation of the lower crust at a depth of 55 to 60 km. We conclude that the Moho limiting the deepest part of the root may correspond to the eclogitisation phase change. Lower crust eclogites have a density comparable to or higher than that of the mantle, depending on their chemical composition (3.37 for andesitic eclogites, 3.56 for gabbroic eclogites). Thus, andesitic eclogites may stay in gravitational equilibrium in the mantle below the root whereas gabbroic eclogites are gravitationally unstable and should sink.

Calculation of a steady-state thermal regime in electron-beam autocrucible melting in the case of circular scanning with a beam

We suggest a mathematical model of a steady-state thermal regime in electron-beam autocrucible melting. We carry out a numerical investigation of the dependence of the thermal parameters of melting on the radius of circular scanning by the beam, the focal spot radius, and on the mean rate of mixing of the melt. The results of calculations are compared with experimental data.

Steady-state and quasi-static thermal results for bare and insulated pipes in permafrost

Analytical and semi-analytical results relating to the transient and steady-state thermal regime around uninsulated and insulated pipelines buried in permafrost are presented.A comparison of the results with some published numerical work is included.

Temperature field of a short tube with internal heat sources

An approximate solution is obtained for the problem of the temperature field of a short tube with internal heat sources in the steady-state thermal regime.