Large Heating Rates Research Articles

Draw ratio enhancement in nonisothermal melt spinning

AbstractNonisothermal melt spinning of materials having a step‐like viscosity variation with temperature is studied in this work. A set of nonlinear equations is used to describe the fiber behavior and to obtain the draw ratio, the square of the ratio of the fiber diameter at the entrance to that at the exit of the fiber‐spinning device. The fluid‐flow equation is based on a slender‐jet approximation, and external heating and cooling have been accounted for with a one‐dimensional model in order to obtain the fiber temperature and viscosity along the fiber length. The model is similar to that used by Wylie et al. (J Fluid Mech. 2007;570:1–16) but accounts for inertia, shear stress at the fiber surface, surface tension, gravity, cooling, and larger heating rates. Steady‐state analysis reveals that the draw ratio increases with an increase in the pulling force, passes through a maximum, and then starts increasing again, resulting in three possible pulling forces for the same draw ratio. However, linear stability analysis reveals that depending on the strength of heating and/or cooling, at most two of the steady states are stable. The stability analysis also predicts complicated oscillatory and nonoscillatory dynamical behavior as the pulling force varies. Nonlinear simulations reveal that an unstable system always tends to limit‐cycle behavior. Systems predicted as stable by the linear stability analysis are also stable for large‐amplitude perturbations. External heating is found to dramatically enhance the draw ratio of the melt‐spinning process. The addition of a cooling section suppresses the draw ratio, but this can be compensated for with a higher heating strength. © 2009 American Institute of Chemical Engineers AIChE J, 2009

Enhanced microwave processing of oil-water emulsions confined within ceramic and ceramic/metallic or composite plates

A detailed theoretical analysis has been carried out on enhanced microwave heating of one-dimensional emulsion samples confined within ceramic and ceramic/metallic or composite plates. A preliminary study has been carried out via average power vs emulsion thickness diagram to estimate microwave power absorption within emulsion samples (o/w and w/o) for various plate assemblies. Two maxima in average power or resonances ( R 1 and R 2 modes) have been considered corresponding to specific emulsion thicknesses for efficient processing of samples. It is observed that, microwave power absorption is enhanced for both o/w and w/o emulsions in presence of metallic (single/composite) plates during both R 1 and R 2 modes. However, efficient heating strategies are characterized by ‘large heating rates’ with ‘minimal thermal runaway effects’ within the sample. Therefore, based on a number of case studies for detailed distributions of power and temperature, suitable heating strategies have been recommended for various compositions of o/w and w/o emulsion samples.

Theoretical analysis on microwave heating of oil–water emulsions supported on ceramic, metallic or composite plates

A detailed theoretical analysis has been carried out to study efficient heating due to microwaves for one-dimensional (1D) oil–water emulsion samples placed on various ceramic, metallic (reflective) and ceramic–metallic composite supports. Two typical emulsion systems are considered such as oil-in-water (o/w) and water-in-oil (w/o). A preliminary study has been carried out via average power vs emulsion thickness diagram to estimate microwave power absorption within emulsion samples for various cases. The maxima in average power, also termed as ‘resonances’, are observed for specific emulsion thicknesses and the two consecutive resonances of significant magnitudes are termed as R 1 and R 2 modes. For both o/w and w/o emulsions, it is observed that microwave power absorption is enhanced in presence of metallic and composite supports during both R 1 and R 2 modes. The efficient heating strategies characterized by ‘large heating rates’ with ‘minimal thermal runaway’ i.e. uniform temperature distributions within the sample have been assessed for each type of emulsion. Based on the detailed spatial distributions of power and temperature for various cases, SiC-metallic composite support may be recommended as an optimal heating strategy for o/w samples with higher oil fractions ( ϕ ⩾ 0.45 ) whereas metallic and Alumina-metallic composite supports may be favored for samples with smaller oil fractions ( ϕ = 0.3 ) during R 1 mode. For w/o samples, SiC-metallic composite support may be suitable heating strategy for all ranges of water fractions during R 1 mode. During R 2 mode, metallic and Alumina-metallic composite supports are favored for both o/w and w/o emulsion samples. Current study recommends the efficient way to use microwaves in a single mode waveguide and the heating strategy can be suitably extended for heating of any other emulsions for which dielectric properties are easily measurable or available in the literature.

A novel concept on discrete samples for efficient microwave processing of materials

A novel concept on discrete samples has been introduced to analyze the efficient heating processes due to microwave radiations. A preliminary investigation on average power absorption within samples vs sample thickness has been carried out for continuous samples and various regimes have been identified such that these regimes may correspond to minima or maxima of average power. Two discrete sample pieces replace a continuous sample for each of these regimes such that the total thickness of the discrete samples is identical with the thickness of the continuous sample. Electric fields and temperature have been solved for each discrete sample layer and the intermediate air layer. The thickness of air layer plays a significant role to dramatically alter the interference of waves and power absorption within each sample layer. It is observed that the discrete sample layers exhibit larger heating rate than that within continuous sample especially for smaller sample thicknesses. Suitable ratios of discrete sample thickness and the thickness of air layer also show large heating rate for larger sample thicknesses. The discrete samples also play important role to optimize thermal runaway. Based on two factors: ‘large heating rate’ and ‘optimal thermal runaway’, the efficient heating strategy has been recommended for highly lossy food substances, and case studies are also demonstrated for low lossy substances.

Theoretical analysis on efficient microwave heating of materials with various square cross sections in the presence of lateral and radial irradiation

A theoretical analysis has been carried out to analyse the efficient heating process of long rectangular samples with various orientations of square cross sections in the presence of lateral and radial irradiation. Lateral irradiation represents the sample incident at one direction with the source at infinity whereas radial irradiation represents the situation where the sample is incident with microwave radiation from the coaxial cylindrical cavity at infinity. Electric field equations have been solved with a hypothetical circular domain which surrounds the square cross sections and facilitates the solution of field equations with the radiation boundary condition. The electric field and temperature have been solved using the finite element method for the composite domain. Generalized characteristics on power absorption and temperature distribution as functions of the wave number (Nw) and the penetration number (Np) have been obtained. Radial irradiation gives a larger power absorption for Nw ≤ 0.56 and either lateral or radial irradiation is favoured for Nw ≥ 0.56 based on various Np values. The aligned square cross section is found to give larger heating rates in the presence of dominant lateral irradiation. The detailed spatial distributions of power and temperature are extensively studied and the suitability of either radial or lateral irradiation for a specific cross section has been recommended. The large heating rate as well as minimal thermal runaway become the competing factors for the selection of a specific heating strategy. The case studies are demonstrated for high and low lossy substances (beef and bread).

Role of metallic and composite (ceramic–metallic) supports on microwave heating of porous dielectrics

The heating of porous dielectrics, beef–air (b/a) and beef–oil (b/o), attached with ceramic, metallic or ceramic–metallic composite supports, has been studied analytically using microwaves. Three test porosities were considered, 0.3, 0.45 and 0.6. A preliminary understanding of enhanced heating and power absorption within the material was obtained from the average power vs slab depth plots. The maxima in power, also termed as ‘resonances’, are observed for specific sample thicknesses and the two consecutive resonances are termed as R 1 and R 2 modes. It is observed that, for both b/a and b/o samples, higher intensity of resonances for power absorption occur in the presence of metallic and alumina–metallic composite supports. The detailed spatial distribution of power illustrates that the sample attached with the metallic support attains lower power distribution for both b/a and b/o samples. The low power absorption near the unexposed face has been enhanced by ceramic–metallic composite support. The overall heating efficiency is also enhanced with ceramic–metallic composite supports. The optimal heating strategy has been derived based on large heating rate with small thermal runaway. Metallic support may be recommended as optimal heating strategy for higher porosities ( ϕ ⩾ 0.45 ) whereas alumina–metallic composite support may be suitable for smaller porosities ( ϕ ⩽ 0.45 ) for b/a samples. Metallic or alumina–metallic composite supports give optimal heating effects for b/o samples. For both b/a and b/o samples, the thermal runaway is larger at higher porosities.

Application of graphite-based sacrificial layers for fabrication of LTCC (low temperature co-fired ceramic) membranes and micro-channels

Fabrication of sensors and micro-fluidic structures from low temperature co-fired ceramic (LTCC) sheets is a growing interest in the micro-packaging community. Such devices usually have inner cavities, whose production is quite complicated. The most elegant method to build such structures so far achieved is by a fugitive phase that is introduced into the multilayer and removed during firing. This paper, therefore, is aimed to introduce the graphite-based sacrificial paste developed for this purpose, and it is constructed in two sections: (i) selection of paste and determination of LTCC open-porosity elimination temperature, and (ii) fabrication and characterization of pressure sensitive LTCC membranes. In the former section, it is shown that increased heating rates (and decreasing tape thickness) shift the open porosity elimination temperature of LTCC by 20 °C, which is small compared to the shift of graphite oxidation temperature (about 100 °C). In the latter section, three parameters affecting the balance between the graphite oxidation and LTCC sintering are studied: heating rate, graphite phase thickness and width of the membrane inlet/outlet channels. As expected, larger heating rates and narrow inlet/outlet channels are found to hinder the oxidation of graphite and evacuation of the resulting products, which results in swollen membranes. Large graphite thickness, through the increased channel height, results in lower swelling in spite of the larger amount of graphite to be oxidized. Membranes with low swelling are found to exhibit excellent pressure sensing characteristics, whereas those with high swelling display hysteretic behavior.

Marangoni effect and heat pipe dry-out

In this study, Marangoni flows effects on heat pipe performances are explored. Recent results dealing with computations of the meniscus shape in a wedge are extended to take into account a nonzero flow rate, caused by the liquid mass lost at the evaporator and supplied by the vapor condensation at the opposite side. Moreover “inverse Marangoni effects” (prevailing in some binary mixtures) are investigated. Dry-out conditions have been computed for different flow rates and surface tension gradients. Results show that while dry-out occurrence is anticipated by the usual Marangoni effect, as illustrated by Yang and Homsy [Phys. Fluids18, 042107 (2006)], an inverse Marangoni effect is beneficial and dry-out conditions are delayed at much larger heating rates.

Thermal hazard of iron picrate

Iron picrate (FePic) was synthesized under conditions similar to those that result in the natural deterioration of chemical weapons. Its thermal hazard was investigated by comparing it with iron picrate obtained by the chemical synthesis method (FePic(Ba)). FePic has eight or more water molecules of crystallization and consists of a mixture of various hydrates. It shows low sensitivity to friction and drop hammer tests due to the large number of water molecules of crystallization. Under the experimental conditions of the thermal analysis, the hydrated iron picrates began to decompose before being dehydrated to form the anhydrous salt. Prolonged holding under natural environmental conditions does not appear to result in formation of the dangerous anhydrous salt. Based on the observed heating rate dependence, it is thought that the hydrated iron picrate should start to decompose before dehydration to the anhydrous salt occurs when subjected to a large heating rate.

Bounds on convection driven by internal heating

Bounds are derived for the space–time averaged temperature 〈T〉 of a fluid layer in the Boussinesq approximation between fixed-temperature horizontal boundaries subject to uniform heating H throughout the volume. The analysis is carried out for both finite and infinite Prandtl number fluids. While the average temperature 〈T〉∼H in the purely conductive state, convection enhances the heat transport beyond static conduction reducing the temperature. Lower bounds to the average temperature of the layer scale with the magnitude of the imposed heat flux, with one scaling exponent for the arbitrary Prandtl number case and another for the infinite Prandtl number model. Specifically, it is proven here that at large heating rates where convection is important, 〈T〉⩾c1H2/3 for finite Prandtl number fluids and 〈T〉⩾c2H5/7 for infinite Prandtl number fluids. Explicit prefactors c1 and c2 for the scaling bounds are computed as well.

Oxidative pyrolysis of solid fuels

This study addresses the dependence of the rate and pattern of pyrolysis of solid fuels from the oxidizing versus inert nature of the gaseous atmosphere. A selection of four solid fuels is considered in the study, namely two plastics (polyethylene and polyethylene terephthalate), one lignocellulosic material (Robinia Pseudoacacia) and a South African bituminous coal. Fuels are pyrolyzed in a thermogravimetric apparatus at different heating rates, under inert conditions or in the presence of oxygen at different concentration. Results indicate that the action exerted by oxygen during pyrolysis depends on the nature of the fuel and on the process conditions such as heating rate and oxygen concentration. Larger heating rates and larger oxygen concentration may indeed emphasize differences between inert and oxidative pyrolysis. Further analysis is directed to check the adequacy of a power low kinetic expression to describe the dependence of the rate of oxidative pyrolysis from the level of oxygen concentration.

Analysis of a temperature inversion event in the lower mesosphere

Rayleigh lidar measurements of stratospheric and lower mesospheric temperatures obtained at the Urbana Atmospheric Observatory (40.1°N, 88.1°W) on 17/18 November 1997 revealed large temperature inversions at altitudes between 55 to 65 km. Prior to and during a large increase (by up to about 50K) in the amplitude of the mesosphere inversion layer (MIL), a clear and persistent vertical wave structure between 30 and 65 km was observed. The wave has a vertical wavelength of about 12 km and an apparent period of about 12 hours. However, the intrinsic characteristics of the wave are uncertain due to the lack of information regarding the background wind profile and the relative direction of the wave propagation vector with respect to the background wind vector. Two different cases, corresponding to small and large background wind speeds projected onto the horizontal direction of wave propagation, are studied numerically to represent two scenarios with different intrinsic wave characteristics devised to explain the development of the MIL event observed. When the projected background wind is small, the wave is likely to be an inertial‐gravity wave. It is shown that the breaking of such a wave does not produce the large heating rate observed. However, the numerical modeling shows that such an inertial‐gravity wave can modulate the stability of a separate internal gravity wave, and the breaking of this internal gravity wave produces a heating rate similar to the observed rate. When the projected background wind is large, however, the observed wave could be an internal gravity wave with a large intrinsic phase speed. The analysis shows that the breaking of this wave can generate a large heating rate and a MIL that is similar to the observed event. We close with a discussion of the observational implications of these two scenarios. Possible wave sources are also discussed, and it appears that the observed MIL event might be related to a developing frontal system in the tropopause region observed just prior to the onset of the observed event.

Seasonal variation and heat preference of the south asia high

By use of the NCEP/ NCAR reanalysis data, the seasonal variation of the South Asia high (SAH) is analyzed. The influences of temporal and spatial variations of the middle and upper level atmospheric temperatures, the visible heat sources, and the diabatic heating rates in the whole atmospheric column on the seasonal variation of the SAH are discussed. Results show that the SAH has two seasonal balancing modes, one of which is the land high in summer and the other the ocean high in winter. The land high itself can be divided into two patterns as well, that is the Tibetan high and the Iranian high. Heating fields have important impacts on the seasonal variation of the SAH. The SAH is a warm high and its center has the property of heat preference, usually locating over or moving to an area with relatively larger heating rates. The annual cycle of the SAH is mainly controlled by the seasonal process of the latent and sensible heating in South Asia. Strong shortwave radiative heating in the north at high latitudes and over the Tibetan Plateau also has an effects on the northward movement and maintenance of the SAH. The cooling effect of infrared radiation is an important cause in weakening the SAH.

Effect of the Supercooled Liquid Region on Al85Ni7Gd8 Metallic Glass Crystallization Products

ABSTRACTThe effect of the supercooled liquid region on the primary crystallization of Al85Ni7Gd8 metallic glass, which exhibits a clear glass transition before its primary crystallization, was evaluated systematically under different annealing conditions, including isothermal annealing at different temperatures and non-isothermal annealing employing different heating rates. It was found that isothermal annealing within the supercooled liquid region and non-isothermal annealing at small heating rates (≤ 5 °C/min) result in the co-precipitation of fcc-Al and Al compound(s). When isothermal annealing is done at temperatures where partial crystallization is involved, or non-isothermal annealing is carried out at a larger heating rate, the primary crystallization product is a single phase of fcc-Al. The effect of the supercooled liquid region on the crystallization product is discussed in detail.

Fission-Product Release during Transient Heating of Irradiated CANDU Fuel

A direct-electric-heating (DEH) apparatus was developed to heat Zircaloy-sheathed irradiated fuel samples. The apparatus was used in the temperature gradient 1 (TG1) experiment to measure fission product releases from Zircaloy-sheathed irradiated Canada deuterium uranium (CANDU) UO2 fuel samples during fast temperature ramps in the presence of a radial temperature gradient in the fuel. The ohmic heating of the UO2, combined with surface heat removal by the surrounding helium coolant flow, produced a radial temperature profile that approximates the profile for fission- or decay-heated fuel.The 11 tests conducted in the TG1 experiment simulated various transient heating rates and high-temperature annealing conditions. The results indicate that the DEH technique can produce large radial temperature gradients and rapid heating rates. Ceramographic examinations showed columnar grain growth and evidence of UO2 melting. Chemical interactions between the tungsten electrodes and the UO2 were also observed. Releases of krypton, and release and redistribution of cesium were measured. Fission product release and redistribution results from some of the tests are also reported.The Kr measurements indicated that the amount of Kr released was highly dependent upon the peak dwell power: The higher the dwell power, the higher the cumulative release. The redistribution of cesium was mapped using emission gamma radiography of the fuel specimen after the test. Cesium was released from the center of the fuel sample where temperatures were the highest. A well-defined area was confirmed near the center where the Cs activity was depleted. The measured Kr releases were in good agreement with the Cs migration and release.

Radiative heating rates and direct radiative forcing by mineral dust in cloudy atmospheric conditions

We explore several issues relevant to assessments of solar and infrared radiative effects due to mineral aerosols. One issue is the importance of the vertical distribution of dust for calculations of dust radiative heating rates. Another issue is the role that clouds may play in augmenting the radiative forcing by dust. We also explore the importance of the composition of mineral aerosols by employing spectral optical properties for dust that comes from two different regions of the globe, the Saharan and Afghan deserts. A combined longwave and shortwave radiative transfer model was used to determine the instantaneous radiative forcing in the atmosphere, radiative fluxes at the surface, and radiative heating rates by airborne mineral aerosols for clear‐sky and cloudy atmospheric conditions. Extensive calculations with our model show that increasing dust loading results in increasing both solar heating rates and infrared cooling rates. However, the net instantaneous rates during the day are always positive, yielding net radiative heating of the dust layer. With similar atmospheric conditions and dust loading, Saharan dust causes larger heating rates than Afghan dust. The magnitudes of the Saharan dust heating rates can easily be 25% larger than Afghan dust heating rates at high Sun angles and over bright surfaces. Also, Saharan dust yields more positive values of TOA (top of the atmosphere) radiative forcing than Afghan dust; and for a diurnal average, this can lead to a change of sign of the TOA radiative forcing from negative to positive just due to mineralogical composition. Clouds significantly influence the direct radiative impact of dust depending on cloud altitude and optical depth. Moreover, this influence is strongly dependent on Sun position and surface albedo.

Investigation of the Thermoexfoliation Process in Different Acceptor GICs

The studies of thermoexfoliation process in intercalation compounds Gr-SbCl5, Gr-ICI, and Gr-H2SO4 have shown that thermoexfoliation has a vivid non-linear threshold character. Experimental data are discussed within the thermodynamic theory of graphite intercalation compounds. Thermoexfoliation process takes place in the temperature range from T* to Tcr in which intercalant in GIC layers becomes unstable and the transition from the state “intercalant in GIC” into the state of pure intercalant occurs. The maximal effect of thermoexfoliation is obtained at high temperatures of thermal shock Te, i.e. at large heating rates of the sample.

Release of CO2 from carbonate rocks during regional metamorphism of lithologically heterogeneous crust

Prograde regional metamorphism drives CO 2 from carbonate rock to crustal fluids that ascend and ultimately interact with the atmosphere and oceans. The observed loss of CO 2 from metamorphic belts remains problematic, however, because the cooling that accompanies fluid ascent favors reactions that add CO 2 to metacarbonate rock by removing CO 2 from fluids. A new two-dimensional model of coupled mass transfer, chemical reaction, and heat transport was developed to assess how rock devolatilization proceeds along the upward escape paths of crustal fluids during prograde metamorphism. The model is based on upper greenschist to lower amphibolite facies growth of amphibole in metacarbonate layers and garnet and biotite in intercalated metapelite layers of the Wepawaug Schist, Connecticut (Acadian orogeny). The modeling indicates that during heating, CO 2 concentrations were larger in metacarbonate layers than in adjacent metapelite layers because amphibole growth in metacarbonates produced CO 2 , whereas garnet and biotite growth in metapelites produced H 2 O. The resulting cross-layer concentration gradients drove H 2 O into the metacarbonate layers and CO 2 out by diffusion and the transverse component of mechanical dispersion. Such cross-layer mass transfer can continually force rock decarbonation while fluids ascend, dominating the effects of cooling, unless fluid fluxes are large and prograde heating rates are small. Consequently, prograde metamorphism of carbonate-bearing sedimentary sequences containing significant amounts of pelitic rock will release CO 2 to regionally migrating fluids in a wide range of orogenic settings, regardless of whether flow is in a direction of increasing or decreasing temperature. Regional CO 2 release can be driven by outcrop-scale processes of volatile exchange between contrasting lithologies.

A fast heating-rate thermogravimetric study of the pyrolysis of scrap tyres

Pyrolysis of scrap tyres has been investigated by means of non-isothermal thermogravimetric analysis, at heating rates ranging from 5 to 900°C/min and particle sizes of 1 and 3mm. The occurrence of primary and secondary pyrolysis stages has been recognized at low heating rates. Based on experimental findings and on the relevant literature on pyrolysis of rubber components, a series–parallel reaction network and a kinetic model are proposed. Accordingly, the first stage of weight loss is related to depolymerization of the rubber components. The residue, upon cyclization and crosslinking, degrades in the secondary stage at larger temperatures. Kinetic parameters of primary and secondary pyrolysis and of the cyclization/crosslinking stages are determined on the basis of experimental results obtained under chemical kinetic controlled pyrolysis conditions.At large heating rates distinction between primary and secondary pyrolysis is no longer observed and a single peak of the pyrolysis rate is observed. This phenomenology can be partly related to decreasing extent of cyclization/crosslinking that take place as the heating rate is raised. Moreover, radial non-uniformities of either temperature, overall volatile pressure or concentration can be established at larger heating rates. These can be further responsible for the modification of the pyrolysis pattern.

Magnetic reconnection: flares and coronal heating in active galactic nuclei

ABSTRA C T A magnetically structured accretion disc corona, generated by buoyancy instability in the disc, can account for observations of flare-like events in active galactic nuclei. We examine how Petschek magnetic reconnection, associated with MHD turbulence, can result in a violent release of energy and heat the magnetically closed regions of the corona up to canonical X-ray emitting temperatures. X-ray magnetic flares, the after effect of the energy released in slow shocks, can account for the bulk of the X-ray luminosity from Seyfert galaxies and consistently explain the observed short-time-scale variability. process directly into the corona outside the disc. Magnetic recon- nection can be responsible for the rapid dissipation of magnetic energy though a field-aligned electric potential in thin current sheets. We, here, describe how the observationally required large heating rates per unit mass, in the optically thin gas of a hot coronal region, can be accounted for by an ion-acoustic instability in the context of slow shocks associated with Petschek-type reconnection and give rise to flare-like events. We estimate that such a process may be responsible for the heating of the coronal plasma in the active regions of an AGN to a level where it emits X-rays at a temperature of ,10 9 K even in the presence of inverse Compton and synchro- tron cooling processes. Magnetic buoyancy, which drives magnetic flux out of the disc, naturally decreases the plasma density to the point where the active region can reconnect very efficiently. X-ray observations imply that the coronal plasma is very tenuous with a density much lower than that of the underlying disc. We show that the energetics of such flares are consistent with the X-ray lumin- osities of typical Seyfert galaxies. The onset of magnetic flares through slow shocks associated with Petschek reconnection, should occur over time-scales comparable to those required to explain