Edge Heat Research Articles

Multi-field plasma sandpile model in tokamaks and applications

A multi-field sandpile model of tokamak plasmas is formulated for the first time to simulate the dynamic process with interaction between avalanche events on the fast/micro time-scale and diffusive transports on the slow/macro time-scale. The main characteristics of the model are that both particle and energy avalanches of sand grains are taken into account simultaneously. New redistribution rules of a sand-relaxing process are defined according to the transport properties of special turbulence which allows the uphill particle transport. Applying the model, we first simulate the steady-state plasma profile self-sustained by drift wave turbulences in the Ohmic discharge of a tokamak. A scaling law as f=aq0b+c for the relation of both center-density n(0) and electron (ion) temperatures Te(0)(Ti(0)) with the center-safety-factor q0 is found. Then interesting work about the nonlocal transport phenomenon observed in tokamak experiments proceeds. It is found that the core electron temperature increases rapidly in response to the edge cold pulse and inversely it decreases in response to the edge heat pulse. The results show that the nonlocal response of core electron temperature depending on the amplitudes of background plasma density and temperature is more remarkable in a range of gas injection rate. Analyses indicate that the avalanche transport caused by plasma drift instabilities with thresholds is a possible physical mechanism for the nonlocal transport in tokamaks. It is believed that the model is capable of being applied to more extensive questions occurring in the transport field.

A Tool for Predicting Heating Uniformity in Industrial Radio Frequency Processing

Radio frequency energy is utilised for heating in a wide range of applications, particularly in the food industry. A major challenge of RF processing is non-uniform heating in loads of variable and angular geometry, leading to reduced quality and product damage. In the study, the specific effects of geometry on the heating profiles of a range of geometrically variable loads in an industrial scale RF system are analysed, and the understanding used to derive a general tool to predict heating uniformity. Potato was selected as a test material for experimental work; dielectric properties were measured using a 44mm coaxial probe. Analysis of simulated and experimental surface temperature profiles and simulated power uniformity indices indicates that the presence of vertices and edges on angular particles, and their proximity to faces perpendicular to the RF electrodes increases localised heating; faces parallel to the electrodes heated less than those faces perpendicular to them. Comparison of the same geometrical shape in different orientations indicates that overall power absorption uniformity can be better even when localised heating of edges is greater. It is suggested, for the first time, that the rotation of angular shapes within a parallel plate electric field can improve heating uniformity, and that this can be achieved through the design of bespoke electrode systems. A Euler characteristic based shape factor is proposed, again for the first time, that can predict heating uniformity for solid, dielectrically homogenous shapes. This provides industry with a tool to quickly determine the feasibility for uniform RF heating of different three dimensional shapes based on geometry alone. This provides a screening method for food technologists developing new products, allowing rapid assessment of potential heating uniformity and reducing the need for early stage specialist computational modelling.

Change in Edge Fluctuations Synchronized with Heat Pulse in the JFT-2M Tokamak

The change in edge fluctuations synchronized with a heat pulse during Sawtooth oscillation has been investigated in the JFT-2M tokamak using a reciprocating Langmuir probe. Data window of a heat pulse is segmented into a number of time frames, and time traces of auto-power spectra at each frame are calculated. Conditional averaging over a number of heat pulse events in a single discharge is performed in order to reduce the variance of the fluctuation spectra. At each heat pulse event, a reduction in the geodesic acoustic mode fluctuation power and an increase in the turbulence fluctuation power are observed. The change in the fluctuation powers suggests that the edge heat pulse during Sawtooth instability could affect the edge turbulence structure.

Laser welding of components of different thickness made of dissimilar metals

The results of investigations into laser welding of components of different thicknesses produced from dissimilar metals are presented. The experimental results show that uniform heating of the weld edges is achieved by producing welded joints with a bead on the thin wall component, with the height of the bead 3–4 times greater than the thickness of the component. Experiments were carried out to investigate pulsed laser welding of components of different thicknesses made of dissimilar metals. The experimental results confirm that the mutual melting of the beads takes place without lack of fusion defects and burning in the welded joints only when the optimum dimensions of the beads are adhered to.

Thermal stress analysis of in-plane two-directional functionally graded plates subjected to in-plane edge heat fluxes

This study investigates the thermal stress and deformation states of bi-directional functionally graded clamped plates subjected to constant in-plane heat fluxes along two ceramic edges. The material properties of the functionally graded plates were assumed to vary with a power law along two in-plane directions not through the plate thickness direction. The spatial derivatives of thermal and mechanical properties of the material composition were considered, and the effects of the bi-directional composition variations and spatial derivative terms on the displacement, strain and stress distributions were also investigated. The heat conduction and Navier equations describing the two-dimensional thermo-elastic problem were discretized using finite-difference method, and the set of linear equations were solved using the pseudo singular value method. The compositional gradient exponents and the spatial derivatives of thermal and mechanical properties of the material composition were observed to play an important role especially on the heat transfer durations, the displacement and strain distributions, but had a minor effect on the temperature and stress distributions.

Influence of Mainstream Turbulence Intensity on Heat Transfer Characteristics of a High Pressure Turbine Stage With Inlet Hot Streak

An unsteady computational study was carried out on GE-E3 high pressure (HP) turbine at inflow turbulence intensities of 5%, 10%, and 20% accompanying with inlet hot streak (HS) at two circumferential positions (impinging and nonimpinging relative to vane leading edge) to analyze the interacted turbulence and HS influences. Turbulence decay mechanisms in turbine passage were presented, and the airfoil heat transfer behaviors were explored by means of adiabatic wall temperature, heat transfer coefficient (HTC), and wall heat flux. The results indicate that the elevated turbulence leads to favorable turbine airfoil temperature distributions, and turbulence induced HS attenuation mainly occurs in vane passage. In addition, the HS dispersion is related directly to the temperature gradients. Although the endwall temperature increases by more than 20 K (2.8% inlet mass-averaged temperature) and midregion temperature decreases by 16 K at blade leading edge, the hot region on blade pressure surface (PS) is only weakened by about 7 K, when turbulence intensity is increased from 5% to 20%. Higher turbulence significantly affects the airfoil HTC, excepting the regions secondary and leakage flow effects are dominating. Therefore, the tip and blade suction surface (SS) trailing edge heat flux is decreased for the temperature decline at higher turbulence, which is beneficial to tip cooling. HS position not only affects the airfoil surface temperature variations but also slightly affects the vane and blade midspan HTC for the variation of heat transfer driving temperature.

Improving the production of electrowelded straight-seam pipe. Part 2

For 12 years, specialists from Baikov Institute of Metallurgy and Materials Science and PAO Severstal’ have been focused on introducing and improving production technology for smalland medium-diameter welded straight-seam pipe at PAO Severstal’. The results are summarized in the present work. The adjustment of the supporting and welding system is discussed. Welding by high-frequency currents permits more efficient edge heating of the pipe blanks and the production of high-quality weld seams.

L-H transition physics in hydrogen and deuterium: key role of the edge radial electric field and ion heat flux

Previous work carried out in the ASDEX Upgrade tokamak on the role of the edge radial electric field and ion heat flux in the L-H transition physics in deuterium plasmas has been extended in hydrogen plasmas. Similar discharges were performed in the two gases providing a detailed comparison of the edge kinetic profiles and heat fluxes in L-mode up to the L-H transition, as the heating power is increased. At the L-H transition, the edge ion heat flux just inside the separatrix is about two times higher in hydrogen than in deuterium. However, the ion plasma parameters at the plasma edge, Ti and , as well as the radial electric field well, are found to be very similar in the two gases. The transport analysis based on this data reveals that, at the L-H transition, the ion heat transport at the plasma edge is about two times higher in hydrogen than in deuterium, in agreement with the well-known isotope effect of confinement and transport. This indicates that the higher power threshold in hydrogen is mainly due to the higher ion heat transport in this gas.

Tungsten dust impact on ITER-like plasma edge

The impact of tungsten dust originating from divertor plates on the performance of edge plasma in ITER-like discharge is evaluated using computer modeling with the coupled dust-plasma transport code DUSTT-UEDGE. Different dust injection parameters, including dust size and mass injection rates, are surveyed. It is found that tungsten dust injection with rates as low as a few mg/s can lead to dangerously high tungsten impurity concentrations in the plasma core. Dust injections with rates of a few tens of mg/s are shown to have a significant effect on edge plasma parameters and dynamics in ITER scale tokamaks. The large impact of certain phenomena, such as dust shielding by an ablation cloud and the thermal force on tungsten ions, on dust/impurity transport in edge plasma and consequently on core tungsten contamination level is demonstrated. It is also found that high-Z impurities provided by dust can induce macroscopic self-sustained plasma oscillations in plasma edge leading to large temporal variations of edge plasma parameters and heat load to divertor target plates.

Guiding Center Orbit Following Calculation of Edge Particle and Heat Transport in Stochastic Magnetic Field

AbstractParticle and heat transport in tokamak edge is investigated by guiding center orbit following calculation. The guiding center equation is solved for both electrons and ions in the presence of magnetic perturbation. It is suggested that the remnants of the magnetic islands play a role in characterizing the radial transport. The transport coefficient is estimated which also demonstrate local structure in the vicinity of magnetic islands. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Computer Aided Taguchi-Fuzzy Multi-Optimization of laser cutting process

Today's fast growing sheet metal industries are demanding overall improvement in cut quality rather than improving different quality characteristics individually. The laser cutting has proven to be the quality cutting process for almost any category of materials with few limitations. The multiple quality characteristics may be improved by controlling the process variables at optimum level. The paper presents a methodology for the simultaneous optimization of multiple quality characteristics in laser cutting process by developing the software 'CATFMO' Computer Aided Taguchi-Fuzzy Multi-Optimization. The software results have been validated by comparing the published experimental data and found suitable. This methodology has been applied for simultaneous optimization of kerf width, cut edge surface roughness and heat affected zone in laser cutting of difficult-to-laser-cut Aluminium alloy sheet.

Thermal residual stresses in in-plane functionally graded clamped hollow circular plates subjected to an edge heat flux

This study carries out the thermal residual stress analyses of functionally graded clamped hollow circular plates for in-plane constant inner and outer edge heat fluxes. The material properties of the functionally graded plates were assumed to vary with a power law along an in-plane direction not through the plate thickness direction. The transient heat conduction and Navier equations describing the two-dimensional thermoelastic problem were discretized using finite difference method, and the set of linear equations were solved using the pseudo singular value method. In order to determine the effect of the plate material properties on the thermal deformation and stress states the circular plates were designed in the way that their material compositions can vary from a pure ceramic (C) outer edge to a pure metal (M) inner edge and vice versa, such as ceramic-to-metal or metal-to-ceramic circular plates. The compositional gradient and direction affected considerably both in-plane temperature levels and heat transfer period whereas similar temperature distributions existed. The displacement components exhibited similar symmetrical distributions and were at lower levels in the metal-rich compositions. The normal strain components had similar distributions to those of the displacement components and were critical around both the inner and outer edges. A metal-rich composition resulted in lower normal strain levels. The equivalent strain distributions were always critical around the heat flux edge and became highest for a ceramic-rich composition. The normal stress components and equivalent stresses were critical in the ceramic-rich regions and the equivalent stress exhibited a sudden increase near the pure ceramic-edge subjected to the heat flux. The lower stress levels were also observed for a metal-rich composition variation. The ceramic material, a good thermal insulator, makes a metal-to-ceramic and a ceramic-to-metal material system suitable for outer and inner edge heat fluxes, respectively in practice. The metal-rich material compositions (with [Formula: see text] and 10.0) can prevent the local cracks in the ceramic-rich regions induced by high heat gradients.

Overview of recent and current research on the TCV tokamak

Through a diverse research programme, the Tokamak à Configuration Variable (TCV) addresses physics issues and develops tools for ITER and for the longer term goals of nuclear fusion, relying especially on its extreme plasma shaping and electron cyclotron resonance heating (ECRH) launching flexibility and preparing for an ECRH and NBI power upgrade. Localized edge heating was unexpectedly found to decrease the period and relative energy loss of edge localized modes (ELMs). Successful ELM pacing has been demonstrated by following individual ELM detection with an ECRH power cut before turning the power back up to trigger the next ELM, the duration of the cut determining the ELM period. Negative triangularity was also seen to reduce the ELM energy release. H-mode studies have focused on the L–H threshold dependence on the main ion species and on the divertor leg length. Both L- and H-modes have been explored in the snowflake configuration with emphasis on edge measurements, revealing that the heat flux to the strike points on the secondary separatrix increases as the X-points approach each other, well before they coalesce. In L-mode, a systematic scan of the auxiliary power deposition profile, with no effect on confinement, has ruled it out as the cause of confinement degradation. An ECRH power absorption observer based on transmitted stray radiation was validated for eventual polarization control. A new profile control methodology was introduced, relying on real-time modelling to supplement diagnostic information; the RAPTOR current transport code in particular has been employed for joint control of the internal inductance and central temperature. An internal inductance controller using the ohmic transformer has also been demonstrated. Fundamental investigations of neoclassical tearing mode (NTM) seed island formation by sawtooth crashes and of NTM destabilization in the absence of a sawtooth trigger were carried out. Both stabilizing and destabilizing agents (electron cyclotron current drive on or inside the q = 1 surface, respectively) were used to pace sawtooth oscillations, permitting precise control of their period. Locking of the sawtooth period to a pre-defined ECRH modulation period was also demonstrated. Sawtooth control has permitted nearly failsafe NTM prevention when combined with backup NTM stabilization by ECRH.

Static current profile control and RFP confinement

Static current profile control (CPC) is shown numerically to substantially enhance plasma confinement in the reversed-field pinch (RFP). By suitable application of an auxiliary electric field and adjustment of its internal location, width and amplitude, strongly decreased levels of dynamo fluctuations are obtained. The simulations are performed using a fully non-linear, resistive magnetohydrodynamic model, including the effects of ohmic heating as well as parallel and perpendicular heat conduction along stochastic field lines. The importance of controlling the parallel current profile in the core plasma to minimize the effects of tearing modes on confinement is thus confirmed. A near three-fold increase in energy confinement is found and poloidal plasma beta increases by 30% from 0.20 to 0.27. The edge heat flux is reduced to a third of that of the conventional RFP. The high-confinement phase is interrupted here by a crash, characterized by a rapid decrease in confinement. A detailed study of the crash phase is carried out by the standard Δ′ theory and a fully resistive linearized time-spectral method; the generalized weighted residual method. The analysis suggests that the instability is caused by pressure-driven, resistive g-modes. Inclusion of anisotropic thermal conduction reduces the linear growth rates. As compared with our earlier numerical studies of CPC in the RFP, employing feedback control, the present static control scheme should be more easily implemented experimentally.

Heat Transfer and Pressure Drop Measurements in High Solidity Pin Fin Cooling Arrays With Incremental Replenishment

Leading edge heat loads on turbine airfoils can be reduced by increasing the diameter of the leading edge. The lower external heat transfer and more generous curvature may allow for cooling this region internally. Large diameter leading edge regions are expected to exhibit a relatively broad region with nearly constant heat transfer. However, the ability of internal passages to cool a surface diminishes with distance as cooling air picks up thermal energy within the passage. Two novel internal cooling geometries have been designed and tested, which incrementally replenish cooling air by using impingement holes distributed along the array. These cooling methods have been compared to a baseline high solidity passage in terms of both array heat transfer and pressure drop. Heat transfer rates and pressure drop have been determined on a row by row basis to provide a means to assess their ability to sustain adequate cooling levels across the entire leading edge region. The authors believe turbine airfoil designs integrating large diameter leading edge regions with properly designed internal passages have the potential to eliminate showerhead cooling arrays in many industrial gas turbine applications. This change is especially beneficial in environments where fuel or air impurities have the potential to clog leading edge showerhead cooling arrays. Heat transfer and pressure drop measurements were acquired in a bench scale test rig. Reynolds numbers ranged from approximately 5000 to 60,000 for the constant height channel arrays based on the pin diameter and the local maximum average velocity across a row. The high solidity pin fin arrays have an axial spacing (X/D) of 1.074 and a cross channel spacing (S/D) of 1.625. The constant section pin fin arrays have channel height to diameter ratios of 0.5. Each array has eight rows of pins with six pins per row in a staggered arrangement. Heat transfer testing was conducted using a constant temperature boundary condition.

Development of a computer simulation model for processing food in a microwave assisted thermal sterilization (MATS) system

The microwave assisted thermal sterilization computer simulation model (MATS-CSM) was developed to improve the previous computer simulation model for the microwave assisted thermal sterilization (MATS) system. Development of the new MATS-CSM included determination of optimum heating time step, evaluation of electromagnetic field distribution and the resulting heating pattern in food, and experimental validation of heating patterns. It was determined that the minimum number of discretization that would not cause immediate divergence of the EM-heat transfer solution was 32 steps corresponding to 97mm and 5.6s of displacement and heating time for every step, respectively. Furthermore, this study successfully demonstrated the symmetrical electromagnetic field distribution between top and bottom microwave entry ports and a staggered electric field pattern from one cavity of the MATS to the next. In addition, MATS-CSM confirmed that incorporating heat diffusion in the simulation model reduces the difference in hot spot and cold spot temperature by 65%. It also confirmed that water circulation reduces the edge heating effect, as observed in experiments. The heating pattern generated by MATS-CSM was verified experimentally through a chemical marker method. Based on the percent areal cross section of the weighted average temperature, there were no noticeable differences between the heating zones generated by the MATS-CSM and by the chemical marker method. The percent areal cross section of the cold area 1, cold area 2, and hot area by MATS-CSM were 35%, 25%, and 40%, respectively, and the cold area 1, cold area 2, and hot area by chemical marker method were 35%, 30%, and 35%, respectively.

Application of Tamping Coke on Large-Scale BF Smelting

At present, domestic and international large-scale blast furnace applications of tamping coke is still in its exploratory stage. Part of the BF applications tamping coke, there are some problems, including high-temperature zone upward, feeding speed slows , air flow along edge enhancement and heat exchange being inadequate. Through analysis of tamping coke combustion performance and the effects of tamping coke on the material column permeability, The following measures are suggested; using a suitable charging system to open center airflow, stable gas distribution, combined to increase the oxygen-rich rate, increasing the amount of wind, improving coal injection rate. The stable operation of BF is realized, hot metal increased and coke conserved.

Numerical Study of Residual Stress Induced by Multi-steps Orthogonal Cutting

Residual stresses induced by material removal have a major influence on the lifetime of machined pieces especially in its corrosion resistance and fatigue life. For that reason, numerical prediction of residual stress profile was the subject of many works. But most of these models are developed only for a single step. They does not consider the effect of hardening and thermal softening on the residual stresses induced by the material removal which is, in the reality, a multi-steps operation such as milling and grinding. The cutting tool is in contact with a part of material that was the finished piece in the previous step. In this paper a multi-steps model for orthogonal cutting has been developed in order to study the influence of the cumulated strain and temperature induced by the different steps on the residual stresses. The effect of tool edge radius and heat generated by flank friction on the predicted stress profile is modeled. Commercial finite element software ABAQUS with its Explicit and Implicit modules was used. Computed Numerical predicted stress fields are compared against measured residual stresses obtained by X-Ray diffraction. Moreover, in order to take into account all the physics in the tool-work material interface, spring back simulation was performed using ABAQUS Implicit.

Wave modeling in a cylindrical non-uniform helicon discharge

A radio frequency field solver based on Maxwell's equations and a cold plasma dielectric tensor is employed to describe wave phenomena observed in a cylindrical non-uniform helicon discharge. The experiment is carried out on a recently built linear plasma-material interaction machine: The magnetized plasma interaction experiment [Blackwell et al., Plasma Sources Sci. Technol. (submitted)], in which both plasma density and static magnetic field are functions of axial position. The field strength increases by a factor of 15 from source to target plate, and the plasma density and electron temperature are radially non-uniform. With an enhancement factor of 9.5 to the electron-ion Coulomb collision frequency, a 12% reduction in the antenna radius, and the same other conditions as employed in the experiment, the solver produces axial and radial profiles of wave amplitude and phase that are consistent with measurements. A numerical study on the effects of axial gradient in plasma density and static magnetic field on wave propagations is performed, revealing that the helicon wave has weaker attenuation away from the antenna in a focused field compared to a uniform field. This may be consistent with observations of increased ionization efficiency and plasma production in a non-uniform field. We find that the relationship between plasma density, static magnetic field strength, and axial wavelength agrees well with a simple theory developed previously. A numerical scan of the enhancement factor to the electron-ion Coulomb collision frequency from 1 to 15 shows that the wave amplitude is lowered and the power deposited into the core plasma decreases as the enhancement factor increases, possibly due to the stronger edge heating for higher collision frequencies.

Numerical Study of Models for Estimating Edge Heat Loss in Solar Absorbers

Absorbers are the prime heat transfer components of solar energy devices. The effects of variable edge loss coefficients on heat transfer performance of flat-plate solar absorbers are examined. From fundamental energy relations, an edge loss coefficient is obtained which embodies the geometric and ambient parameters for estimating the edge heat loss. This model is similar in form to that given by Kaligrou (2009, Solar Energy Engineering Processes and Systems, Elsevier, New York, pp. 156–163) but differs from that obtained by Duffie and Beckman (1991, Solar Engineering of Thermal Processes, Wiley & Sons, Inc., New York), which can be corrected by employing a correction factor given here. The results confirm that the edge loss can significantly affect the absorber performance depending on Ae/Ap ratios, and the prevailing ambient conditions. However, previous models neglect these effects. It is found that the model given by Duffie and Beckman erroneously exceeds the total energy input and overestimates the loss by as much as a factor of 6, which goes against the first law of thermodynamics. When the correction factor is applied, the model gives a better approximation.