Axial Temperature Gradient Research Articles

Magnetohidrodinamičko kočenje i Džulov efekat zagrevanja kod rotirajuće ograničene cilindrične šupljine punjene tečnim metalom

The present research work investigates the MHD braking and Joules heating effect in a confined rotating cylindrical cavity packed with liquid metal. All walls, except the top and bottom portion of the sidewall, of the cylindrical cavity, are made of electrically as well as thermally conducting material. The cavity is exposed to both axial magnetic fields along with the axial temperature gradient, packed with the incompressible electrically conducting liquid. The MHD braking effect is experienced within the rotating liquid metal flow due to the presence of a strong axial magnetic field. It is discerned that MHD braking governs the primary, as well as secondary flow, and, reduces Joules heating effect. Moreover, the internal heat generation due to Joules heating is governed by rotating speed, Hartmann strength, and temperature gradient.

Structural and electrical properties of (Bi0.88Sb0.12)0.86In0.14 crystal

The III-V groups of materials with narrow energy gap possessing favourable attributes have attracted the attention of researchers, as a special class of optoelectronic compounds. (Bi0.88Sb0.12)0.86In0.14 crystal has been grown using vertical Bridgman Technique by maintaining axial temperature gradient 60°C/cm with lowering growth rate 0.7 cm/h. The crystallinity of the material depends on the shape of the ampoule and diameters on the crystal and homogeneity was studied.Study of structural characterization using X-Ray Analysis, optical study characterized by UV–VIS spectroscopy and Electrical properties studied using Hall Measurement as well as I-V measurement. The results are reported and discuss in detail.

PRELIMINARY POWER TRANSIENT ANALYSIS OF THE SUPER FR WITH AXIALLY HETEROGENEOUS CORE

The Super FR is one of the SuperCritical Water cooled Reactor (SCWR) concepts with once-through direct cycle plant system. Recently, new design concept of axially heterogeneous core has been proposed, which consists of multiple layers of MOX and blanket fuels. To clarify the safety performance during power transient, safety analyses have been conducted for uncontrolled control rod (CR) withdrawal and CR ejection at full power. RELAP/SCDAPSIM code was used for the safety analysis. The results show that the peak cladding surface temperature (PCST) is high in the upper MOX fuel layer. It is also shown that axial temperature gradient of cladding greatly increases in a short period. Suppressing such large temperature gradient may be a design issue for the axially heterogeneous core from the viewpoint of ensuring fuel integrity.

Spatially and temporally resolved temperature measurements in counterflow flames using a single interband cascade laser.

Spatially and temporally resolved temperatures are measured in counterflow diffusion flames with a tunable diode laser absorption spectroscopy (TDLAS) technique based on direct absorption of CO2 near 4.2 µm. An important aspect of the present work is the reduction of the beam diameter to around 150 µm, thus providing high spatial resolution that is necessary to resolve the high axial temperature gradient in counterflow flames. The temperature non-uniformity was taken into account through both hyperspectral tomography and the multiline technique with profile fitting, with the latter one being capable of providing temporally resolved data. The proposed methods were used to measure four counterflow flames with peak temperature ranging from 1654 to 2720 K, including both non-sooting and sooting ones.

Exact solution for thermal–mechanical post-buckling of functionally graded micro-beams

In this paper, buckling and post-buckling analysis of functionally graded (FG) micro-beams subjected to the thermal gradient are presented. The FG micro-beam is embedded on an elastic substrate medium modeled by Winkler-Pasternak foundation. To capture the size effect, a modified couple stress theory is applied. Based on the minimum potential energy principle, and using Timoshenko beam hypothesis and von Karman strains, governing differential equations are derived. Both Fourier series solution and exact analytical method are presented for solving the system of coupled differential equations. Because Fourier series solution cannot satisfy all boundary conditions, it is not able to predict the post-buckling response of FG micro-beams, correctly. Moreover, in numerical result section, the effects of length-scale parameter, power-law index, transverse and axial load, temperature gradient, and elastic foundation constants on the post-buckling behavior of simply supported FG micro-beams are investigated. Obtained responses demonstrated that the buckling of the FG micro-beams are significantly sensitive to the variation of mentioned parameters.

The study of thermostressed condition of the hydrogen quartz reactor during the methane pyrolysis

Based on analytical and numerical methods, the detailed studies of the temperature and thermostressed condition of quartz reactor intended for production of hydrogen by pyrolysis of methane passed through the molten tin, are performed. Heating of tin contained in the reactor is realized by the application of electromagnetic field generated by the inductor. The analysis of the study results allows concluding that the values of radial, circular and axial stresses during a uniform movement of the reactor along its height even in the most unfavourable conditions when the temperature drop along the wall of the quartz reactor is maximum and is equal to the difference of temperatures between molten tin and environment (ΔT ≈ 1500 °C) are within the tensile strength limit for quartz σ p = 50Mpa (5 kg / mm 2). In case of non-uniform heating of the reactor along its height observed at its partial filling with molten tin, in the area of transfer from the filled part of the reactor to its non-filled part, there are high axial temperature gradients, which result in temperature stresses exceeding the tensile strength limit. By using the obtained results, the recommendations connected with the reactor heating rate and essential for ensuring its safe operation have been developed.

Thermal characteristics and thermal stress analysis of a superheated water/steam solar cavity receiver under non-uniform concentrated solar irradiation

Solar cavity receiver is a vital component in a solar power tower (SPT) system, where concentrated solar irradiation is absorbed, converted into heat and carried away by a kind of heat transfer fluid. The cavity receiver usually suffers from the high-flux and non-uniform concentrated solar irradiation, which would cause local overheating, large temperature gradient and even thermal stress fracture of the receiver and bring challenges for the safety of its daily operation. Therefore, it is necessary to investigate the thermal and mechanical performance of the solar cavity receiver. In the present study, a superheated water/steam solar cavity receiver with three boiling panels and a superheater was chosen as the physical model. An integrated simulation method, which couples the Monte-Carlo ray tracing (MCRT) method, the appropriate heat transfer correlations, the finite volume method (FVM) and the finite element method (FEM), was employed to simulate the complicated heat transfer process inside the receiver and conduct the thermal stress analysis for the absorber panels. With this method, the three-dimensional wall temperature and thermal stress distributions were gained both on the boiling panels and the superheater, and the causes of thermal stress induced by different temperature gradients were analyzed in detail. The following results were obtained: 1) The heat flux, wall temperature and equivalent thermal stress distributions all appear highly non-uniform on the absorber panels; 2) The hot spots can be found on the membrane wall between the elbows of the boiling panels, which can easily generate the high thermal stress due to the large temperature gradient with their surroundings; 3) The bi-axial stress at the outer and inner radius of the boiling tubes can be created by the large radial temperature gradient, and the thermal stress varies as a cosine distribution across the circumference of the superheated tubes, which is induced by the large temperature gradient in the circumferential direction. Besides, the spot-like areas with high thermal stress can also be found on the superheated tubes due to the considerable nonlinear axial temperature gradient. Moreover, the thermal stress on the membrane wall connected with both the boiling and the superheated tubes has the definite directionality, which shows parallel to the tubes or perpendicular to the tubes; 4) The thermal stress concentration always occurs at the high-temperature and structural discontinuity locations. Here, the thermal stress of the superheater is 360 MPa higher than that of the boiling panels. In general, the superheater has both the higher wall temperature and thermal stress, which is more crucial for the safe operation of the superheated water/steam solar cavity receiver.

Plasma Centrifuge With Crossed E × B Fields and Thermally Driven Countercurrent Flow

It is well established from gas centrifuge (GC) development that isotope separation in a rotating, multispecies gaseous medium increases with axial circulation of the rotating gas in a long separation chamber. This article aims at the development of a commercially viable plasma centrifuge (PC) for specific, nonuranium isotopes. The proposed model PC employs crossed constant radial electric and axial magnetic fields to provide radial separation of a partially ionized gas, combined with a thermal method to establish axial circulation in the rotating medium. The proposed thermal method creates an axial temperature gradient by cooling one end of the separation chamber. By calculation, the inclusion of circulation flow is shown to create axial redistribution of the target component concentration, increasing the longitudinal separation. An analysis of the hydrodynamic and thermal problems is performed for an extended end disk PC in the framework of the boundary layer theory. Radial velocity profiles of the conducting gas in the boundary layer are calculated with respect to the temperature ratio in the external flow ( $T_{1}$ ) and the disk surface ( $T_{0}$ ) for various Prandtl (Pr) numbers. The results demonstrate that for small values of the Pr number, cooling of an end surface within the PC separation chamber results in an intensive inflow of the conducting medium to the solid surface. It is shown that in a PC with crossed fields, cooling one of its ends achieves an effective multiplication of the separation effect along the length of the column.

Influence of Low Axial Temperature Gradient on Ga-Doped Germanium Crystal Growth by the Vertical Bridgman Method

The influence of a low axial temperature gradient on the growth of the Ge:Ga crystal by the vertical Bridgman technique was investigated. The axial temperature gradient of 3.3 K/cm is shown to provide conditions for polycrystalline growth of Ge:Ga crystals. The layers with the distances from 14 to 277 μm are found in the grown crystals with a concave of the solid-liquid interface. The motion of textolite particles in distilled water is visualized by the light cut method at different values of axial and radial temperature gradients. In the absence of the radial temperature gradient, no convection is observed when the axial temperature gradient increases from 0.1 to 6.5 K/cm. In this case, the particle motion velocity increases proportionally to the axial temperature gradient due to the occurrence of thermal diffusion. The convective flow appear in the liquid at the radial temperature gradient of 0.6 K/cm and an axial gradient above 0.1 K/cm.

Prospects of Bulk Growth of 3C-SiC Using Sublimation Growth

Free standing 3C-SiC wafers with a dimeter of 50 mm and a thickness of ca. 0.8 mm have been grown on a regular base using 3C-SiC CVD seed transfer from Si wafers to a poly-SiC-carrier and a sublimation epitaxy configuration. Up to the thickness of almost 1 mm, stable growth conditions of the cubic polytype have been achieved. The high supersaturation was kept stable by the proper design of the hot zone that enables a high axial temperature gradient at the growth interface. The Sirich gas phase was realized by the application of a Tantalum getter that was integrated into the graphitebased growth cell. Furthermore, an adaption of the growth setup allowed the growth of 3C material with a diameter of 95 mm and bulk material up to 3 mm on 25 mm diameter. Computer simulations were used to determine the supersaturation of the growth setup for different source-to-seed distances. The minimum supersaturation necessary for stable growth of cubic SiC was found to be higher 0.1 for seed already containing the required 3C polytype.

Non-isothermal flow of an electrolyte in a charged nanochannel

Electrokinetic flows are generally analyzed, assuming isothermal conditions even though such situations are hard to be achieved in practice. In this paper, the flow of a symmetric electrolyte in a charged nanochannel subjected to an axial temperature gradient is investigated using molecular dynamics simulations. We analyze the relative contribution of the Soret effect, the thermoelectric effect, and the double layer potential in the electrical double layer for various surface charges and temperature gradients. We find the flow driven by thermal gradient is analogous to electroosmotic flow. The thermophoretic motion of the electrolyte is significant for negative surface charge than the positive surface charge. The vibrational spectrum of graphene is calculated to delineate the effect of the surface charge polarity on the observed thermophoretic motion of the electrolyte. A unique structure of interfacial water layer is observed for the positive and negative surface charges. We attribute the presence of these structures to the differences in water-carbon interactions existing for various surface charge polarity. For an applied thermal gradient in the range 2.6 K nm−1 to 8 K nm−1, we observe a continuous net flow with average velocities reaching up to 9.4 m s−1 inside the channel for a negative surface charge of −0.101 C m−2. The results indicate that in a charged graphene-based nanochannel, temperature gradients can be employed to induce streaming current, depending on the relative influence of the Soret effect and the double layer potential.

Simulation and Experiment for Growth of High-Quality and Large-Size AlN Seed Crystals by Spontaneous Nucleation.

Seed crystals are the prerequisite for the growth of high quality and large size aluminum nitride (AlN) single crystal boules. The physical vapor transport (PVT) method is adopted to grow AlN seed crystal. However, this method is not available in nature. Herein, the temperature field distribution in the PVT furnace was simulated using the numerical analysis method to obtain free-standing and large-size seeds. The theoretical studies indicate that the temperature distribution in the crucible is related to the crucible height. According to the theory of growth dynamics and growth surface dynamics, the optimal thermal distribution was achieved through the design of a specific crucible structure, which is determined by the ratio of top-heater power to main-heater power. Moreover, in our experiment, a sole AlN single crystal seed with a length of 12 mm was obtained on the tungsten (W) substrate. The low axial temperature gradient between material source and substrate can decrease the nucleation rate and growth rate, and the high radial temperature gradient of the substrate can promote the expansion of crystal size. Additionally, the crystallinity of the crystals grown under different thermal field conditions are analyzed and compared. The Raman results manifest the superiority of the thermal inversion method in the growth of high quality AlN single crystal.

Photothermoelectric Response of Ti3C2Tx MXene Confined Ion Channels.

With recent growing interest in biomimetic smart nanochannels, a biological sensory transduction in response to external stimuli has been of particular interest in the development of biomimetic nanofluidic systems. Here we demonstrate the MXene-based subnanometer ion channels that convert external temperature changes to electric signals via preferential diffusion of cations under a thermal gradient. In particular, coupled with a photothermal conversion feature of MXenes, an array of the nanoconfined Ti3C2Tx ion channels can capture trans-nanochannel diffusion potentials under a light-driven axial temperature gradient. The nonisothermal open-circuit potential across channels is enhanced with increasing cationic permselectivity of confined channels, associated with the ionic concentration or pH of permeant fluids. The photothermoelectric ionic response (evaluated from the ionic Seebeck coefficient) reached up to 1 mV·K–1, which is comparable to biological thermosensory channels, and demonstrated stability and reproducibility in the absence and presence of an ionic concentration gradient. With advantages of physicochemical tunability and easy fabrication process, the lamellar ion conductors may be an important nanofluidic thermosensation platform possibly for biomimetic sensory systems.

Mathematical modeling of heat and mass transfer processes in the graphite thermal unit of the crystallization apparatus for Horizontal directional solidification method

The design of ECU has been created, which allows in the process of growing halide crystals to create in the growth and annealing zones, respectively, the high-gradient and low-gradient temperature regions necessary for each of the processes. The mathematical study of the influence of the processes of hydrodynamics, heat and mass transfer inside ECU, between ECU and the crystallization apparatus, is considered. For numerical calculations, the Navier-Stokes equation, the Menter differential k-ω turbulence model SST, the modified Stefan-Boltzmann law, the Fourier law are used. To describe the mechanism of heat transfer and heat conduction in solids, boundary conditions of the third kind were set. The distribution of the axial temperature gradient inside the carbon-graphite heat unit was studied. The mode of flow of the refrigerant inside the cooling jacket was determined. The radiative heat transfer between the surfaces, and the mechanisms of heat transfer and heat conduction in solids were studied.

Heating Characteristic and Thermal Optimization of Superconducting DC Induction Heater With Adjustable Air Gap Structure

The novel superconducting direct-current (DC) induction heater shows great advantages of high efficiency and heating quality compared to traditional alternating-current (AC) induction heater in aluminum extrusion. The efficiency of the superconducting DC induction heater can be up to 80–90%, which is much higher than the 40–50% efficiency of the AC induction heater. The uniform and gradient temperature distributions of the aluminum billet are the most significant parameters to evaluate the heating quality of this induction heater, which has a strong influence on the product quality of extrusion processing. In this paper, the superconducting magnet with adjustable air gap structure is designed to achieve controllable axial temperature uniformity and axial temperature gradient distribution. Each adjustable air-gap structure consists of 10 identical iron pieces whose position can be changed independently. Therefore, the uniformity and gradient temperature of the aluminum billet in the axial direction are controllable. The thermal optimization of the uniform and gradient temperature distributions for typical aluminum billets (Φ = 446 mm) is measured. A numerical finite-element method (FEM) model is built to analyze the heating behavior of the aluminum billet. The simulation and experimental results match well. The result of uniform and gradient axial temperatures are ±3 °C and the controllable range from 10 °C/m to 40 °C/m, respectively. The result obtained from the temperature-distribution optimization is useful for industrial applications.

Experimental investigation of temperature and heat distribution across 220 MWe Indian PHWR channel under boil-off condition

Under very low probability postulated accident like LOCA with unavailability of ECCS, the moderator heat sink maintains the channel integrity by providing adequate cooling. As the cooling is external to channel, the circumferential as well axial temperature gradient exists within the channel internals. An experimental set-up was fabricated to simulate the boil-off condition during a postulated accident in NPP. The experiments were conducted at average fuel bundle temperature 600 °C, 800 °C and 1100 °C with superheated mixture of steam and argon. The minimum and maximum temperature in CT, PT and fuel bundle were diametrically opposite to each other; the circumferential as well axial temperature gradient increased with an increase in bundle temperature. The rate of moderator heat absorption increases with increase in bundle temperature. The moderator was successfully able to prevent a breach in the channel; it proved to be the ultimate heat sink during a postulated LOCA with unavailability of ECCS.

Effect of rotating magnetic field on temperature field and flow field of Ga1−xInxSb crystal growth with traveling heater method

In order to optimize the process parameters and shorten the experimental period, Ga1−xInxSb crystal growth with travelling heating method applied rotating magnetic field (RMF) was simulated. The results show that RMF changed the melt convection pattern, firstly, from a single natural convection cell to two forced convection cells coupled each other, which then became independent of each other, then each of which involved into a couple of sub-convective cells as the RMF field intensity increased. Secondly, RMF significantly increased the convection intensity, and the maximum flow velocity increased about 52 times at the initial stage of crystal growth, and about 16 times in the last stage. Thirdly, RMF decreased the axial temperature gradient in the melting zone by 10% above, and decreased the radial temperature gradient about 25% at the same time. And RMF reduced the deflection of crystal growth interface about 21%–23% during the crystal growth.

Performance Study on Methanol Steam Reforming Rib Micro-Reactor with Waste Heat Recovery

Automobile exhaust heat recovery is considered to be an effective means to enhance fuel utilization. The catalytic production of hydrogen by methanol steam reforming is an attractive option for onboard mobile applications, due to its many advantages. However, the reformers of conventional packed bed type suffer from axial temperature gradients and cold spots resulting from severe limitations of mass and heat transfer. These disadvantages limit reformers to a low efficiency of catalyst utilization. A novel rib microreactor was designed for the hydrogen production from methanol steam reforming heated by automobile exhaust, and the effect of inlet exhaust and methanol steam on reactor performance was numerically analyzed in detail, with computational fluid dynamics. The results showed that the best operating parameters were the counter flow, water-to-alcohol (W/A) of 1.3, exhaust inlet velocity of 1.1 m/s, and exhaust inlet temperature of 773 K, when the inlet velocity and inlet temperature of the reactant were 0.1 m/s and 493 K, respectively. At this condition, a methanol conversion of 99.4% and thermal efficiency of 28% were achieved, together with a hydrogen content of 69.6%.

Modeling of coupled heat transfer in a windowed volumetric solar receiver

A two-dimensional coupled heat transfer model with Monte-Carlo ray tracing (MCRT) method is developed to analyze the heat transfer in windowed volumetric solar receiver. The local thermal non-equilibrium (LTNE) effects in ceramic foam absorber and spectral selectivity of quartz window are considered. With the solar absorption term calculated by the MCRT, the P1-approximation for thermal radiative exchange in the foam absorber is examined by comparing with the MCRT. In the model based on P1-approximation, net radiation method (NRM) and MCRT are applied to calculate the thermal radiative exchange between the window and absorber. The effects of porosity and cell size of the absorber foam on temperature distribution and thermal efficiency are investigated. The results show that for heat transfer in foam absorber at a highest temperature below 1727 K, the P1-approximation underestimates the absorber temperature by about 27 K and overestimates the window temperature by 106–133 K. And the thermal efficiency is underestimated by about 2.1% using the P1-approximation. There is small difference between the temperature in the absorber predicted by different models, but the NRM cannot predict the axial temperature gradient in the window. Under a constant air mass flow rate, within the absorber foam with the cell size range of 2–4 mm and porosity range of 0.7–0.9, a higher thermal efficiency can be reached at a larger cell size or a lower porosity.

Sulfur heat transfer behavior for uniform and non-uniform thermal charging of horizontally-oriented isochoric thermal energy storage systems

Elemental sulfur is a low-cost, chemically stable thermal storage medium suitable for many medium to high temperature applications. In this study, we investigate the heat transfer behavior of sulfur, isochorically stored in a horizontally-oriented thermal storage element (steel tube) using experimental, analytical, and computational methods. The sulfur container was uniformly and non-uniformly heated along its axis from 50 to 600 °C to simulate the potential operating conditions for the full-scale thermal energy storage systems. The results of the study reveal distinct sulfur heat transfer mechanisms based on the temperature range and mode of thermal charging. For temperatures from 50 to 200 °C, the sulfur heat transfer behavior is governed by two primary mechanisms; 1) solid–liquid phase change, and 2) sulfur viscosity that varies strongly with temperature. From 200 to 600 °C, the buoyancy-driven natural convection is the dominant heat transfer mechanism and facilitates significantly high thermal charge rates. For axially non-uniform thermal charging, the axial temperature gradient induces natural convection along the axis that rapidly redistributes the thermal energy within the sulfur mass. Such axial convection has a strong impact on the thermal characteristics, including thermal charge/discharge rate and exergetic efficiency of the thermal storage systems. These observations and the high-fidelity computational model used in this study provide important means to identify the design parameters and operating conditions for which sulfur-based thermal energy storage (SulfurTES) systems will provide desirable thermal performance at a low thermal storage cost.