Peculiarities of volt-ampere characteristics of a tungsten cathode in a high-current arc

A model making it possible to calculate the emission currents in a cathode and its volt-ampere characteristic (VAC) taking the phenomena in the near-cathode plasma layer into account is proposed. The effect of plasma electrons and ions, thermal field-emission processes, and radiation from the plasma and evaporation of the cathode material are taken into account. The fields of the electrical potential and temperature in the cathode body, with the Joule heat release taken into account, are calculated. The numerical simulation yielded the conditions causing the volt-ampere characteristic ambiguity in the near-cathode layer in the absence of a liquid cathode spot. The obtained characteristics make it possible to estimate the lifetime and efficiency of a plasmotron with a tungsten cathode and argon serving as an actuating medium used in a number of plasma technologies.     

Influence of the heat exchange between the outer surface of a plane-parallel strip-foundation tribounit and the environment on its temperature

Asymptotics (for small and large times) for solving the thermal friction problem for a tribosystem consisting of a plane-parallel strip sliding at a constant velocity on the surface of a semi-infinite foundation have been found. The thermal strip-foundation contact is ideal, and on the outer surface of the strip convective heat exchange with the environment takes place. The corresponding thermal problem of friction under braking has also been solved. Numerical analysis has been performed for the materials of the frictional pair cermet layer–cast iron foundation.     

Convective cooling in contact problems of thermoelasticity with frictional heat generation

Contact problems with frictional heat generation are, as a rule, studied under idealized conditions of perfect thermal insulation of the surfaces of bodies outside the region of contact. We solve an axially symmetric contact problem of stationary thermoelasticity for semibounded bodies of revolution by taking into account the effect of convective cooling of the free surfaces. The problem is reduced to a system of Fredholm integral equations. An approximate solution of these equations is obtained in terms of simple analytic relations. We determine the conditions under which the influence of convective heat exchange on the level and distribution of contact stresses can be neglected. Franko L'viv State University. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 6, pp. 75–80, November–December, 1997.     

Radiative-conductive transmission of heat through optically dense media

The problem of heat transmission by conduction and radiation through a semiinfinite optically dense medium is analyzed, with incident external radiation and with convective heat transfer taken into account. An expression for the radiant thermal flux is derived from the solution to the equation of radiation flux propagation by the method of associative asymptotic expansions. The effect of the temperature gradient at the surface on the emissivity of the body is established for the medium range of absorptivity values.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 23, No. 3, pp. 459–464, September, 1972.     

Conjugate problem of natural convection in a vertical channel with heat release from a high-frequency electric current

It is shown that allowance for the finite thickness and finite thermal conductivity of the walls results in more intense convection and heat exchange. The region of parameters where the problem of convective heat exchange must be stated as conjugate is indicated.     

Study of resistance of metastable states under repeated thermal impulse loads

A digital unit aimed at controlling thermal loads on a wire heater and collect data on thermal exchange in liquids has been tested. Modes of high-speed photographic registration have been developed. A new mode of highly efficient convective heat transfer has been discovered.     

Entwicklung eines Ionisationsmanometers mit Feldemitterkathode

Development of an ion gauge with field emissionin cryogenic vacuum environments cathode for pressure measurements The measurement of UHV or even XHV pressures in low‐temperature vacuum systems has always been considered as a metrological problem. In principle, conventional hot‐cathode ion gauges can be used for pressure measurement in cryogenic vacuum environments. However, as a consequence of their high heat generation several disadvantages must be taken into account. With the development of an ion gauge of extractor‐type whose heat‐generating thermionic cathode is replaced by a non‐thermal field emission cathode, a promising approach to realize a reliable pressure gauge for cryogenic vacuum applications can be presented in this paper. The gauge equipped with a CNT cathode was investigated both experimentally and by numerical simulations in terms of their operating characteristics. It has been successfully demonstrated that the modified extractor gauge works reliably under low temperature conditions and provides meaningful pressure readings.     

Analytic–Numerical Solution of Contact Problems of Thermoelasticity for Thermosensitive Bodies

We propose a method for the construction of analytic–numerical solutions of nonstationary problems of heat conduction for the case of contact of thermosensitive bodies of simple shape (flat, cylindrical, or spherical) with heat exchange with the ambient medium through the surfaces of the bodies. It is assumed that the thermal diffusivity is constant. We construct the solution of a nonstationary problem of heat conduction for a system formed by a cylindrical body with thin coating heated by the ambient medium in the process of convective heat exchange through the surface of the coating. The thermal contact between the cylinder and the coating is regarded as perfect. The solution of the corresponding quasistatic problem of thermoelasticity is constructed by the perturbation method. The numerical analysis of the results is performed.     

基于热交换系数法的散热器等效建模方法研究

针对散热器结构复杂、建模难度大的问题,提出基于热交换系数法利用基座等效整个散热器,简化分析模型.首先由傅里叶定律和牛顿换热公式得到肋片传热的近似解和对流换热热流量,推导出自然对流和强迫对流两种情况下肋片对流换热系数的表达式,并根据能量守恒原则导出基座等效热交换系数关系式.然后用热分析软件Flotherm分别对等效前和等效后模型进行仿真,结果表明热交换系数法是一种行之有效的散热器简化建模方法.该方法同样适用于其它热仿真软件.     

Stabilization of Joule Heating in the Electropyroelectric Method

Recently the so-called electropyroelectric technique for thermal characterization of liquids has been proposed (Ivanov et?al., J. Phys. D: Appl. Phys. 43, 225501 (2010)). In this method a pyroelectric sensor, in good thermal contact with the investigated sample, is heated by passing an amplitude-modulated electrical current through the electrical contacts. As a result of the heat dissipated to the sample, the pyroelectric signal measured as a voltage drop across the electrical contacts changes in a periodical way. The amplitude and phase of this signal can be measured by lock-in detection as a function of the electrical current modulation frequency. Because the signal amplitude and phase depend on the thermal properties of the sample, these can be determined straightforwardly by fitting the experimental data to a theoretical model based on the solution of the heat diffusion equation with proper boundary conditions. In general, the experimental conditions are selected so that the thermal effusivity becomes the measured magnitude. The technique has the following handicap. As the result of heating and wear of the metal coating layers (previously etched to achieve a serpentine form) with time, their electrical resistance changes with time, so that the heat power dissipated by the Joule effect can vary, and thermal effusivity measurement can become inaccurate. To avoid this problem in this study, a method is proposed that allows maintaining stable the Joule dissipated power. An electronic circuit is designed whose stability and characteristics are investigated and discussed.     

Testing of the quality of solder joints through the analysis of their thermal behaviour with an interferometric laser probe

We present an optical contactless method for testing the quality of solder joints in surface mounted components by measuring their thermal dynamic behaviour. We detect surface normal displacements induced by Joule heating with a high resolution interferometnc laser probe. This probing method, based upon a homodyne Michelson interferometer, is an interesting tool for investigating the mechanisms of heat deposition and flow inside electronic devices. It allows the precise time evolution of the surface normal displacement to be measured (the laser probe has subnanometric resolution). This new approach of thermal behaviour laser testing is based upon the analysis of the diffusion of heat produced by the Joule effect in the structure (lead, solder and copper strip) from short current pulses and will influence heat diffusion. Solder joint failures (intermetallics, hidden voids, etc. ) behave as a thermal barrier. The optical test consists of measuring the dynamics of the solder joint expansion, and the variations from a standard response (good quality solder joints) will reveal defects. Important variations have been observed in solder joints that have undergone thermal cycling ageing tests. We have also investigated the thermal response of joints on IMS (insulated metallic substrate) and epoxy substrates. They show a very different time response.     

Conjugated Effect of Joule Heating and Magnetohydrodynamic on Laminar Convective Heat Transfer of Nanofluids Inside a Concentric Annulus in the Presence of Slip Condition

In the current study, the conjugated effect of Joule heating and magnetohydrodynamics (MHD) on the forced convective heat transfer of fully developed laminar nanofluid flows inside annular pipes, under the influence of MHD field, has been investigated. The temperature and nanoparticle distributions at both the inner and outer walls are assumed to vary in the direction of the fluid. Furthermore, owing to the nanoparticle migrations in the fluid, a slip condition becomes far more important than the no-slip condition of the fluid–solid interface, which appropriately represents the non-equilibrium region near the interface. The governing equations—obtained by employing the Buongiorno’s model for nanofluid in cylindrical coordinates—are converted into two-point ordinary boundary value differential equations and solved numerically. The effects of various controlling parameters on the flow characteristics, the average Nusselt number and the average Sherwood number have been assessed in detail. Additionally, the effect of the inner to outer diameter ratio on the heat and mass transfer rate has been studied. The results obtained indicate that, in the presence of a magnetic field when the fluid is electrically conductive, heat transfer will be reduced significantly due to the influences of Joule heating, while the average mass transfer rate experiences an opposite trend. Moreover, the increase in the slip velocity on both the walls causes the average heat transfer to rise and the average mass transfer to decrease.     

Diamond electrophoretic microchips—Joule heating effects

Microchip electrophoresis (MCE) has become a mature separation technique in the recent years. In the presented research, a polycrystalline diamond electrophoretic microchip was manufactured with a microwave plasma chemical vapour deposition (MPCVD) method. A replica technique (mould method) was used to manufacture microstructures in diamond. A numerical analysis with CoventorWare™ was used to compare thermal properties during chip electrophoresis of diamond and glass microchips of the same geometries. Temperature distributions in microchips were demonstrated. Thermal, electrical, optical, chemical and mechanical parameters of the polycrystalline diamond layers are advantageous over traditionally used materials for microfluidic devices. Especially, a very high thermal conductivity coefficient gives a possibility of very efficient dissipation of Joule heat from the diamond electrophoretic microchip. This enables manufacturing of a new generation of microdevices.     

Calculation of the Temperature Field of a Printed-Circuit Board with Account for Convective and Radiative Heat Exchange on the Board Surface

The temperature field of a printed-circuit board has been simulated numerically according to the two-dimensional thermal model with account for convective and radiative heat exchange on the board surface. The temperature fields in the actual printed-circuit board have been determined experimentally. The deviation of the results of the numerical analysis from the experimental data does not exceed ±4°C.     

A local thermal non-equilibrium model for transversely isotropic porothermoelastic materials

This work presents a local thermal non-equilibrium (LTNE) poroelasticity model for transversely isotropic porous media and applies the model to the problem of a cylindrical hole in an infinite porous medium subjected to convective cooling on the boundary of the cavity. The LTNE thermo-poroelasticity equations are solved using Laplace transform, and numerical examples are presented to examine the effects of LTNE and material anisotropy on the pore pressure and thermal stresses around the cavity. The results show that the thermal pore pressure and the magnitudes of thermal stresses increase significantly with an increase in the thermal expansion coefficient in the transverse direction. However, the elastic modulus anisotropy alone has only a marginal effect on the thermal pore pressure and stresses. The results also confirm that the LTNE effects become more pronounced when the convective heat transfer boundary conditions with moderate Biot numbers are considered.     

Dynamic regime of condensed substance ignition under boundary conditions of the third kind

The dynamic problem of ignition of a chemical condensed substance by a convective heat flux when the heat transfer coefficient α is a power function of time has been solved analytically. The critical conditions of the thermal ignition of a substance depending on the environmental medium temperature in application to the specific case of variable heat transfer coefficient were investigated. An engineering method of calculation of the basic parameters of the thermal ignition of a substance has been proposed.     

Axial temperature distribution of a thermionic cathode

A practical method for calculating the temperature distribution along a thermionic cathode based on numerical evaluation of the integral of the heat-balance equation is described. The radiation component of the heat flux, the Joule heat, the heat conduction along the cathode, and the temperature dependence of the electrical conductivity are taken into account.     

Continuous Subduction of a Lithospheric Platform near an Oceanic Trench

We investigate the convective heat exchange of a lithospheric platform in the zone of subduction during its submergence into the earth's mantle. We consider the effect of the angle of submergence, the velocity of motion of the lithospheric platform over the day surface of the ground, and the coefficient of thermal expansion on the heat exchange, the flow structure, and the submergence depth.     

A simulation tool for the AC-DC transfer difference of thermalconverters at low frequencies

To find the reason for the AC-DC transfer differences of the planar multijunction thermal converter (PMJTC) at low frequencies, where temperature oscillation is present due to the lack of integration of the oscillating Joule heat, the coupled electro-thermal mathematical problem has been solved. The dynamic nonlinear model includes the temperature dependence of all material parameters, of heat conduction through the air and radiation losses. The results of the simulation are compared with those of measurements using a sampling voltmeter with special software as a reference. A new design is proposed to improve the performance of the PMJTC at low frequencies     

液氮温区高温超导滤波器的漏热分析

为在液氮温区工作的高温超导滤波器的制冷机冷源选型,基于Sage 10软件对超导滤波器件的冷却装置进行了漏热分析,仿真计算了铜线和同轴线的几何参数对传导漏热量的影响,以及真空罩、冷盘的尺寸和发射率对辐射漏热量的影响,并综合上述分析计算了超导滤波系统的总漏热量。在仿真计算中发现,信号线的导热和真空罩中冷盘的辐射漏热在系统总漏热量中起主导作用。仿真计算结果表明,通过增大信号线长度、减小信号线直径的方式可将信号线漏热量降至0.72 W,约为初始导热量的1/3。与此同时,冷盘表面采用抛光镀金的方式减小表面发射率,使辐射漏热量降至原来的2/3。根据模拟计算的最优结果,选择制冷量为3 W@77 K的制冷机作为高温超导滤波器的冷源。