Appearance Of Hot Spots Research Articles

Laboratory set-up to pilot plant investigations on vapour phase ammoxidation of 2,6-dichlorotoluene

The scaling-up of the gas phase catalytic ammoxidation of 2,6-dichlorotoluene (DCT) to 2,6-dichlorobenzonitrile (DCBN) over VPO catalysts in a pilot plant was investigated. The focus was led on the process variables that might be critical to the design of a commercial manufacturing facility. Furthermore, catalyst scale-up, catalyst stability and a development of a product separation section particularly for the solid reaction products was studied. The scaling-up of the gas phase catalytic ammoxidation of 2,6-dichlorotoluene (DCT) to 2,6-dichlorobenzonitrile (DCBN) over vanadium phosphate catalyst in a pilot plant was investigated. The focus was led on the process variables that might be critical for designing a commercial manufacturing unit, in particular on evaluation of reaction temperature that may cause hot spots, temperature gradients, etc. Furthermore, catalyst scale-up, catalyst stability and a development of a product separation section particularly for the solid reaction products was studied. The up-scaling of the process using 100 ml of catalyst was investigated both by reactor simulations and catalytic experiments. For reactor simulations, a pseudo-homogeneous model of a fixed bed tubular reactor with negligible axial and radial diffusion operating in the steady-state was applied. The kinetic model based upon the Langmuir–Hinshelwood mechanism was used. The results showed that the temperature control will be crucial in scaling-up of the process. The influence of the reactor geometry, reactor operating conditions and catalyst stability on the reactor performance is discussed. The possibility of an appearance of hot spots in the reactor is addressed. High selectivity of DCBN (85%) can be achieved for an extended period of time. A good agreement between experimental and simulation results is observed.

ICONE15-10776 COOLABILITY OF OXIDIZED PARTICULATE DEBRIS BED ACCUMULATED IN HORIZONTAL NARROW GAPS

When LOCA occurs in a nuclear reactor system, the coolability of the core would be kept as reported at a series of presentations in ICONE14. Therefore the probability of the core meltdown is negligible small. However, from the view point of defense in depth, it is necessary to be sure that the coolability of the bottom of reactor pressure vessel (RPV) is maintained even if a part of the core should melt and a substantial amount of debris should be deposited on the lower plenum. We carried out an experimental study in order to observe the coolability of particulate core-metal debris bed with 12mm thickness accompanied with rapid heat generation because of oxidization, which was reported at ICONE14. The coolability was assured by a small amount of coolant supply because of high capillary force of oxidized fine particulate debris produced. In the present study, we examined the coolability of particulate debris bed deposited in narrower gap of 1mm or 5mm that coolant supply is hard. The particulate debris beds were piled up on the stainless steel sheet with 0.1mm thickness, which was used to measure the bottom temperatures of particulate debris bed by using a thermo-video camera. We set up a heat supply section with heat input of 2.1kW, which simulates the hard debris bed deposited on the particulate debris bed as reported for the TMI-2 accident. We measured the temperatures of the bottom surface of the heat supply section and the heat fluxes released into debris bed as well as the temperatures at the bottom of debris bed on the stainless steel sheet. It is found that when only the upper surface of particulate debris bed is in the film boiling, capillary force causes coolant supply to the particulate debris bed. Therefore, in the condition of thicker gap with small particulate debris, coolability of debris bed is improved. We find out that smaller particulate debris is moved by vapor movement. As a result, the area that high capillary force is caused because of smaller debris particulates and the area that vapor is easily movable because of coarse debris particulates occurred in the gap. We observed appearance and disappearance of the hotspot with metal particulate debris bed with a MMD of 0.6mm and a gap of 1mm. We found that oxidization with heat generation contributes the appearance of hotspot and after oxidization completes hotspot disappears because of high capillary force in fine particulates produced through oxidization process.

Nuclear magnetic resonance imaging of an operating gas–liquid–solid catalytic fixed bed reactor

Abstract This work reports our pioneering application of the nuclear magnetic resonance imaging (MRI) technique to the dynamic in situ studies of gas–liquid–solid reactions carried out in a catalytic trickle bed reactor at elevated temperature. The major advance of these studies is that MRI experiments are performed under reactive conditions. We have applied MRI to map the distribution of liquid phase inside a catalyst pellet as well as in a catalyst bed in an operating trickle-bed reactor. In particular, our studies have revealed the existence of the oscillating regimes of the heterogeneous catalytic hydrogenation reaction caused by the oscillations of the catalyst temperature and directly demonstrated the existence of the coupling of mass and heat transport and phase transitions with chemical reaction. The existence of the partially wetted pellets in a catalyst bed which are potentially responsible for the appearance of hot spots in the reactor has been also visualized. The combination of NMR spectroscopy with MRI has been used to visualize the spatial distribution of the reactant-to-product conversion within an operating reactor.

PROGRESSIVE WAVINESS DISTORTION: A NEW APPROACH OF HOT SPOTTING IN DISC BRAKES

ABSTRACT This paper deals with the mechanisms of the appearance of hot spots in railway brake disks. From infrared measurements, a classification of hot spots is presented. The most damaging thermal gradients are identified as macroscopic hot spots uniformly distributed on the friction surface. An explanation of the origin of these macroscopic hot spots is presented based on a scenario of progressive distortion of the disk. Three major effects have to be considered: the structural response of the disk and the pads; realistic boundary conditions, both geometric and loading; and the thermoelastoplastic behavior of the material. Finite element simulations based on the proposed model agree with experimental observations.

Development of functional devices using hot spot in GdBa 2Cu 3O 7− δ-based composite ceramics

A hot spot, which is a local area glowing orange, appears in LnBa 2Cu 3O 7− δ (Ln: rare earth element) ceramic rod when a voltage exceeding a certain value is applied to the rod at room temperature. The rod with the hot spot shows various functional characteristics that give rise to applications in devices such as a constant-current generator, an oxygen sensor. The durability of the rod during a sustained presence of the hot spot is improved upon adding BaAl 2O 4 into the GdBa 2Cu 3O 7− δ . In this paper, GdBa 2Cu 3O 7− δ -based composite ceramic rod with various BaAl 2O 4 contents were prepared by solid-state reaction, and current–voltage characteristics and temperature dependences of resistivity were measured. The minimum electric power required for the hot spot to appear increased with increasing BaAl 2O 4 content. From the temperature dependence of the resistivity, temperature just before the hot spot appearance could be successfully estimated. The minimum electric power required for the hot spot to appear can be expected from the temperature dependence of the resistivity of the ceramics, which is useful to design the functional devices exploiting the hot spot.

Modelling of DC Electric Fields Induced by RF Sheath in Front of ICRF Antenna

Reducing the ICRH (ion cyclotron range frequency) antenna-plasma interaction is one of the key points for reaching very long tokamak discharges. One problem which limits such discharges, is the appearance of hot spots on the surface of the antenna: Radio Frequency (RF) sheaths modify the properties of the edge plasma by rectifying the RF potential along open magnetic field lines and can induce hot spots. This paper investigates the corrections to sheath potentials introduced by the interactions between adjacent flux tubes. Our theoretical study started from an oscillating double Langmuir probe model, in which a transverse influx of current was included. This model was confronted with 1D PIC simulations along a magnetic field line, and demonstrated that current exchanges can decrease mean potentials. A 2D electrostatic fluid code was then developed, which couples adjacent flux tubes in a poloidal cross section with collisional conductivity or polarization currents. It showed that transverse currents are able to smooth structures smaller than a characteristic size in the sheath potential maps (results for Tore Supra). These computed rectified potentials can be used to obtain the DC electric fields in front of the antenna. And then, it gives an estimate of the particle drift and the energy flux on the antenna structure, which can explain hot spots.

Fluidized‐bed reactor modeling for polyethylene production

AbstractGas‐phase technology for polyethylene production has been widely used by industries around the world. A good model for the reactor fluid dynamics is essential to properly set the operating conditions of the fluidized‐bed reactor. The fluidized‐bed model developed in this work is based on a steady‐state model, incorporating interactions between separate bubble, emulsion gas phase, and emulsion solid polymer particles. The model is capable not only of computing temperature and concentration gradients for bubble and emulsion phases, calculating polymer particle mean diameter throughout the bed and polyethylene production rate, but also of pinpointing the appearance of hot spots and polymer meltdown. The model differs from conventional well‐mixed fluidized‐bed models by assuming that the particles segregate within the bed according to size and weight differences. The model was validated using literature and patent data, presenting good representation of the behavior of the fluidized‐bed reactor used in ethylene polymerization. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 321–332, 2001

Study of propofol in bovine aortic endothelium: I. Inhibitory effect on bradykinin-induced intracellular calcium immobilization.

Propofol has been found to affect the intracellular calcium concentration with clinical manifestations of hypotension and bradycardia. The purpose of this study is to examine the effect of propofol on intracellular calcium immobilization in bovine aortic endothelium under the stimulation of bradykinin. In order to validate the effect of propofol on the alteration of intracellular calcium concentration, we used the cultured bovine endothelial cells (Gm 7372a) to measure the calcium immobilization within the cells preincubated with or without propofol of clinical concentration. Using Fluo-3 staining and a fluorescence spectrophotometer (confocal microscope), intracellular calcium immobilization was demonstrated by the appearance of "hot spots" within the cytoplasm and perinuclear regions after addition of bradykinin to the cells. The changes of fluorescence density measured within these areas versus the effect of time were analyzed and compared with the cells in control group. After addition of bradykinin, intracellular calcium hot spots increased dramatically within seconds and reached a maximal level within 20 seconds. The concentrations of calcium gradually decreased to a constant level after about 3 min following the addition of bradykinin to the cells. With pretreatment of propofol at 0.01 mM and 37 degrees C for 30 min, the immobilization of intracellular calcium from the intracellular stores were significantly inhibited that was demonstrated by the decreased appearance of hot spots when compared with control. Our data demonstrated that under the stimulation of bradykinin, propofol at 0.01 mM, could inhibit intracellular calcium release from the intracellular stores in bovine aortic endothelial cells. This phenomenon might explain the possible mechanism for the clinical manifestations of hypotension and/or bradycardia associated with propofol.

Use of infrared thermography for the evaluation of heat losses during coal storage

The exothermic processes during coal storage reduce the calorific value of the coal which in turn results in financial losses. An accurate and easy calculation of the losses may be an efficient tool to evaluate the effectiveness of the measures taken to reduce the spontaneous heating of coal and to predict the appearance of hot spots. This study was carried out with `Mezcla' coal used at Endesa power station in Andorra–Teruel (Spain). For this purpose, laboratory measurements relating to spontaneous heating properties and field measurements were carried out. Three experimental coal piles were built, a high porosity pile, a periodically compacted pile and a pile with a low angle slope in the prevailing wind direction. A method to determine the coefficient of total losses was developed and the coefficients of calorific losses and total losses were also found. Temperatures were measured by infrared thermography and by thermocouples and a good agreement between them was found. The use of a mathematical model, developed by ASINEL, allowed to calculate the heat fluxes due to spontaneous heating and a new coefficient of calorific losses, the similarity of this coefficient with the experimentally found revealed the effectiveness of this method to quantify heat emissions. The self-heating behaviour of the pile was also followed by an infrared thermographic camera which allowed the detection of heat fluxes.

Differential Diagnosis of Metastases in Bone Scans: Chemotherapy induced Bone Necrosis

Influenced by the incorrect diagnosis of a bone metastasis caused by bone necrosis we evaluated reasons and frequency of bone necrosis in patients referred for bone scanning in follow-up of tumors. Bone scans performed within two years on patients with primary bone tumors or tumors metastatic to bone were reviewed in respect to the final diagnosis bone necrosis. We found the cases of three young patients who presented the appearance of hot spots on bone scintigrams which were finally diagnosed as bone necrosis. In two cases the diagnosis was based on histological findings, in one case the diagnosis was made evident by follow-up. All the three patients had been treated by chemotherapy and presented no other reason for the development of bone necrosis. Enhanced tracer uptake in all sites decreased within eight weeks up to two years without therapy. Single and multiple hot spots after chemotherapy may be originated by bone necrosis but mimicry metastases.

Experimental observations of the spatial anisotropy of the neutron emission in a medium energy plasma focus

The results of experiments carried out on the DPF-78, a 60 kV, 28 kT plasma focus device are presented. The primary objective of these experiments was to investigate the spatial anisotropy of the neutron emission and to correlate the total neutron yield with hard X-ray emission and the presence of hot spots. To influence the plasma parameters, gas fillings of deuterium with doping of neon, argon, and krypton were used. An admixture of gases with an equivalent mass density of 5 mbar of D/sub 2/ was employed throughout the experiment. A novel technique for neutron detection was used, which allowed the recording of neutron signals at close distance to the focus (65 cm), with minimal dispersion of the signal due to time of flight and yet high X-ray rejection. A set of these detectors was placed at different angular positions to investigate the polar distribution in the neutron emission. It was identified for the first time that in a medium energy plasma focus device, the neutron emission is composed of two periods, similar to that reported in high energy devices. It was found that the first period occurs immediately before the maximum compression and lasts less than 50 ns while the second period lasts between 150 and 200 ns. It was found that the polar distribution of the neutron emission shows a strong anisotropy during the second period and depends on the Z of the doping gas. No correlation was found between the total neutron yield (about 10/sup 10/) and the hard X-ray emission or appearance of hot spots.

Hot Spot Formation on different Tokamak Wall Materials

AbstractThe thermal contraction phenomenon and generation of ‘hot spots’ due to thermoemission were described in [1–3]. The paper consider non‐linear stages of heat contraction on the graphite, beryllium, tungsten and vanadium wall. It is shown that on the beryllium surface hot spot can't appear due to strong cooling by sublimation. For other materials the conditions of hot spot appearance due to local superheating of the wall have been calculated and their parameters were found: critical surface temperature, size of spots and their temperature profiles, heat fluxes from plasma to the spots. It have been calculated fluxes of sublimating materials from spots to the plasma. It is noticed that nominal temperature of the grafite divertor plate, accepted in ITER's project to being equal 1500°C, is lower then critical temperature of the development heat contraction due to thermoemission.

Local structural instability and formation of hot spots in materials under mechanical loading

We have studied the formation of hot spot in materials under high-speed loading using the method of molecular dynamics. It is shown that the appearance of hot spots is associated with release of the elastic energy accumulated in the deficient region. The evolution of a hot spot is followed by intense energy release and by structural rearrangements near its location. The results obtained are of importance for understanding of the effects of mechanical activation of the components in solid-phase chemical reactions.

Formation and Limiting Mechanisms for Very High Sea Surface Temperature: Linking the Dynamics and the Thermodynamics

Abstract The present study composites atmosphere and ocean conditions associated with ocean hot spots. Ocean hot spots are defined as regions where SST exceeds 29.75°C and that have an area greater than 1 × 106 km2. The composite atmosphere includes surface flux parameters, deep convection and cloud amounts, cloud radiative forcing, and analysis fields from the National Meteorological Center (NMC) and European Centre for Medium-Range Weather Forecasts weather forecasting systems. The composite ocean includes sea level height and the temperature and velocity structures down to 720 m from the NMC ocean forecasting system. These fields are composited for the months before, during, and after the appearance of hot spots in order to develop a four-dimensional picture of the atmosphere and ocean conditions that are associated with the formation and the decay of these very high ocean surface temperatures. The analysis indicates that the formation of these hot spots is largely confined to the region within the 28°...

Chromospheric heating in late–type stars: evidence for magnetic and nonmagnetic surface structure

The aim of this paper is to evaluate recent observational and theoretical results concerning the physics of chromospheric heating as inferred from IUE, HST-GHRS and ROSAT data. These results are discussed in conjunction with theoretical model calculations based on acoustic and magnetic heating to infer some conclusions about the magnetic and nonmagnetic surface structure of cool luminous stars. I find that most types of stars may exhibit both magnetic and nonmagnetic structures. Candidates for pure nonmagnetic surface structure include M-type giants and supergiants. M-type supergiants are also ideal candidates for identifying direct links between the appearance of hot spots on the stellar surface (perhaps caused by large convective bubbles) and temporarily increased chromospheric heating and emission.

Modulation of an ultraviolet mutational hotspot in a shuttle vector in Xeroderma cells

Ultraviolet mutagenesis of the shuttle vector plasmid pZ189 in Xeroderma Pigmentosum cells yields a mutational pattern marked by hotspots at photoproduct sites on both strands of the supF marker gene. In order to test the influence of strand orientation on the appearance of hotspots the mutagenesis study was repeated on a vector with the supF gene in the inverted orientation. We recovered a pattern the same as that in the earlier work and conclude that the nature of the DNA polymerase involved in the replication of specific strands is not a primary determinant of hotspot occurrence in this system. One of the hotspots lies in an 8 base palindrome while the corresponding site on the other strand was not a hotspot. These results were obtained with calcium phosphate transfection of the UV treated vector. When DEAE dextran was used as a transfection agent both sites in the palindrome were hotspots. In a mixing experiment the calcium phosphate pattern was recovered. Our data suggest that the sequence determinants of mutational probability at these two sites lie outside the 8 bases of the palindrome and that mutagenesis at one, but not the other, site is sensitive to perturbation of cellular calcium levels.

A source of hot spots in frequency-tripled laser light

Laser light, frequency tripled to the ultraviolet (UV), shows numerous, strongly modulated hot spots in the target plane for the OMEGA laser used in laser-fusion experiments. However, at the fundamental frequency in the infrared (IR), the hot-spot modulation is relatively small. High-resolution measurements of the intensity and phase in the laser near field, combined with computer modeling of beam propagation to the target plane, show that the intensity fluctuations are dominated by the near-field phase errors. The appearance of hot spots in the UV and not in the IR are found to be the result of phase-error tripling caused by the frequency conversion.

Thermoelastic effects in brakes

A one-dimensional thermoelastic analysis is applied to the results of brake dynamometer experiments. The object is to determine if such an analysis can be used to predict the appearance of hot spots on the wheel surface. The results indicate that while the theory explains some of the experimental results, it is still far from quantitatively predicting the onset of hot spot behavior.

Second breakdown in simplified transistor structures and diodes

The use of high-frequency silicon power transistors with interdigitated comb emitter structure was successful for proving the essential features of the theory of lateral thermal instability and second breakdown. For a more detailed study of the appearance of hot spots and their relationship to the onset of second breakdown, the use of a simpler structure looked advisable. This paper describes an uncomplicated geometry which offers many possibilities for modifications. Calculations, experimental data, and photographs are presented which show the means by which second breakdown and possible burnout occurs after the formation of a hot spot. In particular, measurements demonstrate the localized thermal runaway in the collector-base diode and the relationship to the presence of the emitter. The role of the emitter and base layer resistances as well as the resistance of the contacting metal layers (series and contact resistance) in reducing the thermal instability is shown. The shorted emitter structure proved to be a successful method to postpone or even avoid the appearance of second breakdown. The tool for the determination of current and voltage distributions within the small silicon structures was a potential microprobing apparatus with a resolution of 2µ.