High-temperature Mode Research Articles

Thermal decomposition and oxidation of coal processing waste

To expand the database of kinetic parameters used for modeling the ignition of coals and their processing waste, promising coal-water slurry and coal-water slurry containing petrochemicals, studies have been performed on an experimental set-up using thermal gravimetric analysis. The research into coals of various ranks (flame, gas, coking, low-caking, and non-baking) and their processing waste (filter cakes) has yielded the decomposition parameters of the organic matter of coal and the formation of volatile substances as well as the oxidation parameters of the coke residue of all the coals and filter cakes under study. The studies cover the temperature range of the burning processes: 450-1300 K (for low-, medium-, and high-temperature burning modes). We have ascertained the dependence of kinetic parameters (pre-exponential factor and activation energy) describing the thermal decomposition of the organic matter of coal on the rank of coals and filter cakes. The findings show that the kinetic parameters describing the thermal decomposition of the organic matter of coal and its processing waste are practically the same. The thermokinetic parameters of coke residue oxidation are close for all the coals under study but they differ significantly for coke residue of filter cakes. The values of thermokinetic parameters obtained in the research are necessary to devise adequate physical and mathematical models and perform numerical studies (for mathematical modeling) of fuel slurry combustion processes in the combustion chambers of power plants.

Optimization of the High-Temperature Induction Treatment Modes for Nonlinear Electroconductive Bodies

This paper proposes an approach to the computer simulation of electromagnetic, thermal and mechanical fields in ferromagnetic bodies based on the developed mathematical model for the description of thermo-mechanical processes in electroconductive bodies having different magnetizability and polarizability under electric field conditions. The results of the investigation on the modes of high-temperature induction treatment of bodies made of different ferromagnetic materials are presented. It is shown that by selecting the electric current frequency at the stage of heating and the subsequent cooling conditions, it is possible either to generate residual stresses in bodies, that are close to the given ones, or to reduce the duration of the heat treatment of workpieces under stress constraints.

Gas phase oxidation of furfural to maleic anhydride on V2O5/γ-Al2O3 catalysts: Reaction conditions to slow down the deactivation

An alumina-supported vanadium oxide catalyst (13.9wt.% vanadium oxide) has been characterized by different techniques and tested in the gas phase oxidation of furfural. These studies have shown that the catalyst unavoidably deactivates by deposition of maleates and resins over the surface. Full regeneration is accomplished by burning off these deposits at 773K. The studies have also demonstrated that if the primary contact occurs at temperatures at which furfural conversion is low and then the temperature is increased in a low- to high-temperature mode, intense deposition of maleates and resins takes place and the catalyst is rapidly deactivated. The increase of the temperature does not result in removal of deposits but accelerates the deposition. Under this protocol, the yield of maleic anhydride never exceeded 30%, irrespective of the reaction conditions (temperature and O2/furfural mole ratio). In contrast, if the catalyst first contacts the reaction mixture at high oxidizing potential, then the rate of maleate and resin deposition is much slower, and so is the deactivation rate, and the catalyst can display a higher yield of maleic anhydride for a longer period of time. A high oxidizing potential can be attained at a high reaction temperature (close to full conversion). A higher oxidizing potential at a given high temperature can be accomplished by increasing the O2/furfural mole ratio. Thus, for example, first contacting the catalyst at 593K (full conversion), 1vol.% of furfural, and O2/furfural mole ratio=10, obtained an initial maleic anhydride yield of 68%, and the yield was still greater than 50% after 15h on stream. On contacting at 573K with 1vol.% furfural and 20vol.% O2, the maleic anhydride yield was initially close to 75% and was above 60% after 15h.

Strong suppression of impurity accumulation in steady-state hydrogen discharges with high power NBI heating on LHD

Strong suppression of impurity accumulation is observed in long pulse hydrogen discharges with high power NBI (neutral beam injection) heating (Pnbi > 10 MW) on the large helical device (LHD), even in the impurity accumulation window where the intrinsic impurities such as Fe and C are always accumulated into the plasma core. Density scan experiments in these discharges demonstrate to vanish the window and a new operational regime without impurity accumulation is found in steady state hydrogen discharges. Impurity pinch decreases with increasing ion temperature gradient and carbon Mach number. The peaking of the measured carbon profiles shows strong anti-correlations with the Mach number and its radial gradient. An external torque has a big impact on impurity transport and strong co-current rotation leads to an extremely hollow carbon profile, so-called ‘impurity hole’ observed in high ion temperature modes. Impurity pinch in the plasmas with net zero torque input (balanced NBI injection) is also strongly reduced by increasing ion temperature gradient, which can drive turbulent modes. The combination effect of turbulence and toroidal rotation plays an important role in the impurity transport.

Gas dynamics and kinetics of the penetration of methane–oxygen flames through complex obstacles, as studied by 3D spectroscopy and high-speed cinematography

It was experimentally found that, on the penetration of a flame through obstacles, gas dynamic factors such as flame turbulization can determine the kinetics of combustion, for instance, a transition of low-temperature hydrocarbon combustion to the high-temperature mode.

Study of the features of BaF2 heteroepitaxy on CaF2/Si(100) layers obtained in the high-temperature growth mode

The surface morphology of BaF2 epitaxial films grown by MBE (molecular beam epitaxy) in various modes on the surface of CaF2/Si(100) is investigated by AFM. The CaF2 layers on Si(100) are obtained in the high-temperature growth mode (Т S = 750°C). It is shown that the epitaxy of BaF2 at a temperature of 600°C at the initial stage of growth leads to the formation of defects such as perforations in the epitaxial film, while epitaxy at a temperature of 750°C provides a defect-free film with a surface morphology suitable for the subsequent growth of semiconductors of IV–VI type and solid solutions based on them.

On a nonstationary theory of thermal explosion in a semi-batch reactor. Homogeneous systems

A thermal explosion in a semi-batch ideal-mixing reactor in which a homogeneous sequential exothermic reaction occurs is numerically simulated. It is shown that the critical condition of a thermal explosion, separating the low- and high-temperature modes of the reaction is realized only after the completion of the stage of introduction of the second reactant into the reactor. With increasing time of this stage, the interval of transition from the low- to the high-temperature mode expands with increasing value of the Semenov criterion, and eventually, the thermal explosion becomes degenerate.

Critical ignition conditions in exothermically reacting systems for arbitrary reaction kinetics

In this work, a universal method for determination of the critical ignition conditions taking into account the reactant consumption is proposed. Based on the analysis of the phase trajectories equation, the equation for maximal temperatures of exothermic reactions was obtained. In this case, the asymptotic criterion of ignition is determined by the impossibility of slow reaction mode realization with low value of maximum temperature. The method allows demarcating the regions of low- and high-temperature modes of exothermic reactions and to establish the criteria of transition to the region of high-temperature modes. The corresponding parametric diagrams can be characterized as the bifurcation ones (bistability). It was found that the region of thermal explosion (TE) existence is bounded by the classical TE conditions from below and by the degeneration conditions from above. The comparison of analytical calculation results with the results of numerical calculation gives a satisfactory agreement.

Large Eddy Simulation of a Swirl-Stabilized Pilot Combustor from Conventional to Flameless Mode

This paper investigates flame and flow structure of a swirl-stabilized pilot combustor in conventional, high temperature, and flameless modes by means of a partially stirred reactor combustion model to provide a better insight into designing lean premixed combustion devices with preheating system. Finite rate chemistry combustion model with one step tuned mechanism and large eddy simulation is used to numerically simulate six cases in these modes. Results show that moving towards high temperature mode by increasing the preheating level, the combustor is prone to formation of thermalNOxwith higher risks of flashback. In addition, the flame becomes shorter and thinner with higher turbulent kinetic energies. On the other hand, towards the flameless mode, leaning the preheated mixture leads to almost thermalNOx-free combustion with lower risk of flashback and thicker and longer flames. Simulations also show qualitative agreements with available experiments, indicating that the current combustion model with one step tuned mechanisms is capable of capturing main features of the turbulent flame in a wide range of mixture temperature and equivalence ratios.

Forming the Composition of Underground Coal Gasification Products in the Simulation of Various Heat and Mass Transfer Conditions in the Coal Seam

The mathematical model describing the heat and mass transfer processes in underground coal gasification is proposed. Numerical studies have allowed to determine the composition of gases depending on the temperature, pressure products of gasification, and the composition of the heated oxidant injected. Relations the composition of the concentration of combustible gas component of the oxidant injected: dry air, a mixture of oxygen, nitrogen and water vapor in different proportions were prepared. It is found that, depending on the oxygen content in the oxidizer low- temperature gasification mode is implemented (up to 15%). At higher values of the oxygen concentration in the oxidizer the high-temperature mode is realized, in which the fuel gas output increases significantly.

Morphological changes of nano-Al agglomerates during reaction and its effect on combustion

Agglomeration of aluminum nanoparticles (nano-Al) is inevitable due to the strong van der Waals forces. It is supposed to affect the combustion characteristics of nano-Al. However, the agglomeration effect is not fully understood yet due to the worse description of the morphological evolution of agglomerates during reaction. In this paper the morphological changes of agglomerated nano-Al, which mainly results from the oxide shell thickening when the particle temperature is below the melting point of alumina, are confirmed in the thermogravimetric analyzer (TGA) and Hencken experiments. Three factors (fA, fht, and fsp) are defined to describe the morphological changes quantitatively through proposed formulas. Thus the agglomerate could be equivalent to an effective spherical particle to help studying the effect of agglomeration on combustion. The results indicate two combustion modes for different agglomerates. The small Al agglomerate is hard to combust fully due to the strong heat transfer with ambience, corresponding to the high-temperature oxidation mode. However, at the same environment, as the agglomerate becomes larger, the maximum particle temperature (Tp,max) increases until that the agglomerate burns fully, corresponding to the fully-fledged combustion mode. At that mode, the burning particle temperature is mainly controlled by the viscous sintering of agglomerates.

Microwave ignition of the combustion synthesis of aluminides and field-related effects

Abstract Combustion synthesis of aluminide intermetallics starting from reactive powders mixtures comprising a ferromagnetic component (Fe, Co and Ni) has been performed by igniting the exothermic reactions in a microwave single mode applicator, in order to investigate possible effects related to the positioning of the reactive sample into the region of predominant electric or magnetic field. In addition to an increase of the reproducibility of the experiments and the possibility to avoid electric arcs generation, the ignition of the combustion reactions in predominant magnetic field allowed to significantly reduce the ignition times, with a global reduction of the power required to synthesize the intermetallics. This implies that the amount of the reactive volume under ignition conditions is reduced proportionally, thus moving from thermal explosion towards Self-propagating High-temperature Synthesis mode of combustion. Moreover, in the case of Fe–Al system, the disposition of the load into the region of predominant magnetic field increased the yield of the desired intermetallic compound, to the detriment of α-Fe(Al) solid solution co-product. Furthermore, the possibility to continue to furnish energy to the reactive Fe + Al sample after combustion synthesis ignition has been proven for both configurations. Results show that in case of exposure to the predominant magnetic field, the sample cooling rate resulted lower, hence the permanence at high temperatures after synthesis could be proficiently exploited to increase adhesion to an underlying substrate, in case combustion synthesis technique is used to obtain intermetallic-based coatings.

Field scale numerical modeling of low temperature air injection with propane for heavy-oil recovery from naturally fractured reservoirs

The alternatives for EOR in heavy oil deep fractured reservoirs are limited. Air injection is one of the options but running it in the high temperature mode, namely in-situ combustion, has technical limitations (poor areal distribution of injected air) because of highly heterogeneous characteristics of fractured reservoirs. This may also create a problem from safety point of view, i.e., there is a risk that oxygen is not completely consumed in reservoir reaching production wells quickly without fully consumed. Air, however, can be used as a pressurizing agent to recover matrix oil in the low temperature oxidation mode (LTO) even though it may result in oxidation of oil that forms asphaltenes. Our recent lab results showed that injecting a mixture of air–propane instead of pure air might reduce these negative effects. This enhanced diffusion of air and propane into matrix may also add to recovery. Hence, those lab results opened the window to think of air injection (LTO mode) as a way to produce some of the oil left in the matrix in addition to its pressurizing effect.In the present study, a single porosity numerical simulation model, with explicit representation of fractures, was developed to analyze the injection of a mixture of gases in a naturally fractured reservoir, involving the use of air in the LTO mode. A sector model of a hypothetical fractured heavy oil reservoir was created and several air–gas injection cases such as pure air, propane–air–propane and air–propane–air cycles were analyzed. The runs were performed using the diffusion data obtained from our previous experimental studies. Different scenarios of huff-and-puff options (cycle type and duration) were tested. Attention was paid to oxygen consumption in the matrix, while fracture to matrix oxygen transfer was mainly due to voidage replacement of oil by air, and some through diffusion of air into matrix during the injection and shut off periods of each cycle.The results obtained considering various shut in times and injection rates scenarios as well as safety aspects, showed benefits in the use of air in the LTO mode mixed with propane. This optimized air injection scheme could be considered as an alternative to develop naturally fractured heavy oil fields.

Relaxation dynamics and structural changes in DNA soft gels

Abstract Herein, we report relaxation dynamics and structural changes occurring in DNA during transition of its solution to soft gel phase using data obtained from rheology, dynamic light scattering (DLS) and small angle neutron scattering (SANS) experiments. The viscoelastic response showed this transition taking place above a crossover concentration (CDNA = 2% (w/v)). It was observed that at low concentration CDNA = 1% (w/v) solution possessed multiple relaxation dynamics owing to the presence of internal modes at all temperatures in the range, T = 30–80 °C. At higher concentration (CDNA = 3% (w/v)) the system showed single relaxation dynamics because of the systematic arrest of many of these modes. However, at crossover concentration (CDNA = 2% (w/v)), samples showed multiple relaxation modes at high temperature (T = 80 °C) and single mode relaxation at lower temperature (T = 50 °C) with sub diffusive behaviour showing transition from ergodic to arrested phase owing to ceased Brownian dynamics. Normal diffusion in CDNA = 1% (w/v) and sub-diffusive behaviour for CDNA = 2% and 3% (w/v) samples was observed. SANS studies revealed interparticle distance decreased and mesh like structures began to appear with increase in DNA concentration, 2% ≤ CDNA ≤ 5% (w/v). Thus, the observed relaxation dynamics was hierarchical in DNA concentration and the same was temperature dependent. The results augment our general understanding of relaxation dynamics of DNA in aqueous dispersions.

Dielectric relaxation kinetics at higher temperature mode in cellulose

In this work the complex dielectric permittivity was measured from 10 –1 to 1 MHz by dielectric dynamic spectroscopy (DDS). Thermally stimulated depolarisation current (TSDC) technique is used to investigate molecular mobility of cellulose. At high temperature, the various dielectric relaxation phenomena are separated by applying a recent analytical protocol. The comparison between the activation enthalpy values obtained by both techniques allows assignment of the α -mode to cooperative movements of nanometric sequences of the main chain. The strong/fragile model is used to explain the Arrhenius behaviour of this mode. The influence of water content on secondary and primary relaxation modes was examined.

Investigation of Evacuated Tube Collector Performance at High Temperature Mode Using TRNSYS Simulation Model

It has been proved that creation of vacuum between the absorber and the cover of a solar collector is resulting in a substantial improvement in the collector efficiency due to reduction in the heat loss through convection ad conduction. In this work, the performance of evacuated tube collectors is investigated using TRNSYS simulation model. Different levels of concentrations have been considered in the simulation to predict the power generation. The simulation results showed that the thermal performance of evacuated tube collectors with high concentration ratio can provide a good improvement to the receiver output.

Influence of the surface microroughness of a Si(001) substrate on the morphology of CaF2 epitaxial layers under high-temperature growth conditions

The influence of the surface microroughness of Si(001) substrates on the morphology of CaF2 epitaxial layers is investigated in the high-temperature growth mode. The atomic-force microscopy method is used to demonstrate that the silicon surface relief substantially affects the nucleation, growth, and formation of the morphology of CaF2 layers.

Aromatic products of radiation-thermal degradation of lignin and chitin

Lignin and chitin degradation in the high-temperature radiolysis mode at ≤295°C has been investigated. Experimental conditions ensured the removal of low-molecular-weight products from the irradiation zone via volatilization. The formation of the low-molecular-weight aromatic products methoxyphenols and pyridine derivatives from lignin and chitin, respectively, has been shown. The yield of the distillable aromatic products was much higher than in the case of conventional dry distillation. It is noted that the mechanism of high-temperature radiolytic transformations of lignin and chitin differs from the mechanism of thermal transformations, which is due to the thermally stimulated degradation of radiation-induced radicals. It has been shown that high-temperature radiolysis opens a possibility for producing aromatic hydrocarbons at a lower dose rate and at lower temperature in comparison with conventional radiolysis and pyrolysis.

Two modes of HVPE growth of GaN and related macrodefects

AbstractGaN films with thickness up to 3 mm were grown by halide vapour phase epitaxy method. Two growth modes were observed: the high temperature (HT) mode and the low temperature (LT) mode. Films grown in HT mode had smooth surface, however the growth stress was high and caused cracking. Films grown in LT mode had rough surface with high density of V‐defects (pits), however, such films were crack‐free. The influence of growth parameters on the pit shape and evolution was investigated. Origins of pits formation and process of pit overgrowth are discussed. Crack‐free films with smooth surface and reduced density of pits were grown using combination of the LT and HT growth modes. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Investigation of a MEMS-based capillary heat exchanger for thermal harvesting

This article describes theoretical and experimental studies of MEMS-based capillary heat exchangers, designed for use as the heat sink in a waste thermal energy harvesting system currently under development. Specific goals include the understanding of the impact of thermal conductivity and capillary dimensions. Experimental studies were done using silicon and SU-8 based heat exchangers each with rectangular capillaries having width of 100μm. The depth of silicon capillaries was maintained at either 64 or 100μm while that of SU-8 capillaries was varied from 48 to 134μm to 160 to 375μm. The overall footprint of each microdevice was 38mm by 13mm. The fluid used was 3M™ HFE 7200. Heat exchanger performance was characterized based on operating temperature, mass transfer rate, heat flux, and thermal resistance. Studies were done for two different operating temperatures, one below the boiling point and another at the boiling point of the working fluid. These two operating temperatures were referred to in this article as low-temperature and high-temperature operation mode. Based on the experimental study the fluid was found to wet both silicon and SU-8 with contact angle measured to be 6°. From the experimental studies conducted it was found that the pumping effect of the capillaries increased with increasing wall height. Based on the thermal tests it was observed that the highest mass transfer rate and heat flux under the low-temperature and high-temperature operation mode were obtained for an SU-8 heat exchanger with capillary height of 134μm and silicon heat exchanger with capillary height of 100μm, respectively. Based on this study it was found that under low-temperature operation mode the capillary dimension was more influential than thermal conductivity; however, under high-temperature operation mode the opposite was proved to be the case. The thermal resistances were found to vary from 0.011 to 0.09m2CW and from 0.0073 to 0.0096m2CW for low-temperature and high-temperature operation mode respectively. Also it was found based on comparison that the conceptualized MEMS-based capillary heat exchanger was found to have lower thermal resistance than existing heat sinks. Using the experimentally validated theoretical model developed in this work it was found that the performance of the MEMS-based capillary heat exchangers improved with increasing thermal conductivity of the substrate as well as the capillary dimensions. The model has helped conclude that developing SU-8 capillaries on aluminum or copper substrates would optimize the influence of thermal conductivity and capillary dimensions on the performance of the MEMS-based heat exchanger.