Cavity Effects and Hot Spot Formulation Inmicrowave Heated Ceramic Fibers

1996 ◽  
Vol 430 ◽  
Author(s):  
G. A. Kriegsmann
Keyword(s):  
Electric Field ◽  
Hot Spots ◽  
Hot Spot ◽  
Single Mode ◽  
Uniform Electric Field ◽  
Ceramic Fibers ◽  
Entire Sample ◽  
Mathematical Equations ◽  
Cavity Effects ◽  
Ceramic Cylinder

AbstractRecently the heating of a thin ceramic cylinder in a single mode applicator was modeled and analyzed assuming a small Biot number and a known uniform electric field through out the sample. The resulting simplified mathematical equations explained the mechanism for the generation and growth of localized regions of high temperature. The results predicted that a hot-spot, once formed, will grow until it consumes the entire sample. Most experimental observations show that the hot-spot stabilizes and moves no further.A new model is proposed which incorporates the effect of the cavity and the nlonuiniform character of the electric field along the axis of the sample. The resulting simplified mathematical equations indicate that these effects stabilize the growth of hot-spots.

Growth and Stabilization of Hot Spots in Microwave Heated Ceramic Fibers

1994 ◽  
Vol 347 ◽  
Author(s):  
Gregory A. Kriegsmann
Keyword(s):  
Electric Field ◽  
Hot Spots ◽  
Hot Spot ◽  
Single Mode ◽  
Uniform Electric Field ◽  
Fiber Axis ◽  
Ceramic Fibers ◽  
Mathematical Equations ◽  
Effective Heat Transfer Coefficient ◽  
Ceramic Cylinder

ABSTRACTRecently the heating of a thin ceramic cylinder in a single mode applicator was modeled and analyzed assuming a small Biot number and a known uniform electric field through out the sample. The resulting simplified mathematical equations explained the mechanism for the generation and growth of localized regions of high temperature. The results predicted that a hot-spot, once formed, will grow until it consumes the entire sample. Although this phenomenon is seen in some experiments, others show that the hot-spot stabilizes and moves no further.A new model is proposed which incorporates the dependence of the thermal conductivity and the effective heat transfer coefficient upon temperature, and the nonuniformity of the electric field along the fiber axis. The resulting simplified mathematical equations indicate that these effects may stabilize the growth of hot-spots.

Mathematical Model of Thermal Spikes in Microwave Heating of Oxide Ceramic Fibers

1994 ◽  
Vol 347 ◽  
Author(s):  
J. R. Thomas ◽  
Wesley P. Unruh ◽  
Gerald J. Vogt
Keyword(s):  
Mathematical Model ◽  
Hot Spots ◽  
Elevated Temperatures ◽  
Solid Phase ◽  
Microwave Sintering ◽  
Single Mode ◽  
Heat Transfer Process ◽  
Oxide Ceramic ◽  
Ceramic Fibers ◽  
Radiation Losses

ABSTRACTExperiments on microwave sintering of ceramic fibers in a single-mode cavity have revealed the presence of thermal spikes and “hot spots” which sometimes travel along the fiber and eventually disappear. They are triggered by relatively small increases in microwave power, and thus have obvious implications for the development of practical microwave-based fiber processing systems. These hot spots are conjectured to originate at slight irregularities in the tow morphology, and propagate as the result of solid phase transitions which take place at elevated temperatures and reduce the dielectric loss coefficient є”.An elementary mathematical model of the heat transfer process was developed which reproduces the essential features of the observed phenomena, thus lending support to our conjecture. This model is based on the assumption of one-dimensional heat conduction along the axis of the fiber tow, and radiation losses at the surface.

scholarly journals Experimental Observations of Thermal Spikes in Microwave Processing of Ceramic Oxide Fibers

1994 ◽  
Vol 347 ◽  
Author(s):  
Gerald J. Vogt ◽  
Wesley P. Unruh ◽  
J. R. Thomas
Keyword(s):  
Microwave Heating ◽  
Hot Spots ◽  
Hot Spot ◽  
Single Mode ◽  
Microwave Cavity ◽  
X Ray Diffraction ◽  
Thermal Spike ◽  
X Ray ◽  
Heating Behavior ◽  
Fiber Processing

ABSTRACTMicrowave heating of alumina/silica fiber tows in a single-mode microwave cavity at 2.45 GHz has produced a surprising thermal spike behavior on the fiber bundles. During a thermal spike, a “hot spot” on the tow brightens rapidly, persists for a few seconds, and then rapidly extinguishes. A hot spot can encompass the entire tow in the cavity or just a localized portion of the tow. Some local hot spots propagate along the fiber. Thermal spikes are triggered by relatively small (<15%) increases in power, thus having obvious implications for the development of practical microwave fiber processing systems. A tow can be heated through several successive thermal spikes, after which the tow is left substantially cooler than it was originally, although the applied microwave electric field is much larger. X-ray diffraction studies show that after each temperature spike there is a partial phase transformation of the tow material into mullite. After several excursions the tow has been largely transformed to the new, less lossy phase and is more difficult to heat. Heating experiments with Nextel 550 tows are examined for a plausible explanation of this microwave heating behavior.

Instability thresholds in the microwave heating model with exponential non-linearity

2011 ◽  
Vol 22 (3) ◽  
pp. 187-216 ◽  
Author(s):  
ADAM ALCOLADO ◽  
THEODORE KOLOKOLNIKOV ◽  
DAVID IRON
Keyword(s):  
Electric Field ◽  
Heat Conductivity ◽  
Hot Spots ◽  
Hot Spot ◽  
Two Dimensions ◽  
Microwave Cavity ◽  
Variable Electric Field ◽  
The Stability ◽  
Small Eigenvalues ◽  
Appl Math

When ceramics are heated inside a microwave cavity, a well-known phenomenon is the occurrence of hot spots – localised regions of high temperature. This phenomenon was modelled by Kriegsmann ((1997), IMA J. Appl. Math. 59(2), pp. 123–146; (2001), IMA J. Appl. Math. 66(1), pp. 1–32) using a non-local evolution PDE. We investigate profile and the stability of hot spots in one and two dimensions by using Kriegsmann's model with exponential non-linearity. The linearised problem associated with hot-spot-type solutions possesses two classes of eigenvalues. The first type is the large eigenvalues associated with the stability of the hot-spot profile and in this particular model there cannot be instability associated with these eigenvalues. The second type is the small eigenvalues associated with translation invariance. We show that the hot spots can become unstable due to the presence of small eigenvalues, and we characterise the instability thresholds. In particular, we show that for the material with low heat conductivity (such as ceramics), and in the presence of a variable electric field, the hot spots are typically stable inside a plate (in two dimensions) but can become unstable for a slab (in one dimension) provided that the microwave power is sufficiently large. On the other hand, for materials with high heat conductivity, the interior hot spots are unstable and move to the boundary of the domain in either one or two dimensions. For materials with moderate heat conductivity, the stability of hot spots is determined by both the geometry and the electric field inside the microwave cavity.

Bubble motion in liquid nitrogen under a non-uniform electric field in a microgravity environment

1997 ◽  
Vol 117 (11) ◽  
pp. 1109-1114
Author(s):  
Yoshiyuki Suda ◽  
Kenji Mutoh ◽  
Yosuke Sakai ◽  
Kiyotaka Matsuura ◽  
Norio Homma
Keyword(s):  
Electric Field ◽  
Liquid Nitrogen ◽  
Microgravity Environment ◽  
Uniform Electric Field ◽  
Bubble Motion

Discussion of Electrode Conditioning Mechanism Based on Pre-breakdown Current under Non-uniform Electric Field in Vacuum

2008 ◽  
Vol 128 (12) ◽  
pp. 1445-1451
Author(s):  
Takanori Yasuoka ◽  
Tomohiro Kato ◽  
Katsumi Kato ◽  
Hitoshi Okubo
Keyword(s):  
Electric Field ◽  
Uniform Electric Field

Spatial Hotspot Analysis of Bucharest’s Urban Heat Island (UHI) Using Modis Data

2018 ◽  
Vol 18 (1) ◽  
pp. 14-22 ◽  
Author(s):  
Georgiana Grigoraș ◽  
Bogdan Urițescu
Keyword(s):  
Surface Temperature ◽  
Air Temperature ◽  
Land Surface Temperature ◽  
Hot Spots ◽  
Land Surface ◽  
Hot Spot ◽  
Monitoring Network ◽  
Residential Areas ◽  
Hotspot Analysis ◽  
The City

Abstract The aim of the study is to find the relationship between the land surface temperature and air temperature and to determine the hot spots in the urban area of Bucharest, the capital of Romania. The analysis was based on images from both moderate-resolution imaging spectroradiometer (MODIS), located on both Terra and Aqua platforms, as well as on data recorded by the four automatic weather stations existing in the endowment of The National Air Quality Monitoring Network, from the summer of 2017. Correlation coefficients between land surface temperature and air temperature were higher at night (0.8-0.87) and slightly lower during the day (0.71-0.77). After the validation of satellite data with in-situ temperature measurements, the hot spots in the metropolitan area of Bucharest were identified using Getis-Ord spatial statistics analysis. It has been achieved that the “very hot” areas are grouped in the center of the city and along the main traffic streets and dense residential areas. During the day the "very hot spots” represent 33.2% of the city's surface, and during the night 31.6%. The area where the mentioned spots persist, falls into the "very hot spot" category both day and night, it represents 27.1% of the city’s surface and it is mainly represented by the city center.

The family Orthotrichaceae (Bryophyta) in flora of Russia: results of revision and biogeographical review

2018 ◽  
Vol 52 (2) ◽  
pp. 519-534 ◽  
Author(s):  
V. E. Fedosov
Keyword(s):  
Hot Spots ◽  
Latitudinal Gradient ◽  
Hot Spot ◽  
Spatial Differentiation ◽  
Taxonomic Composition ◽  
Undescribed Species ◽  
Longitudinal Gradient ◽  
Biogeographic Pattern ◽  
The Family ◽  
North Pacific Region

Recent studies on Orthotrichoid mosses in Russia are summarized genus by genus. Orthotrichum furcatum Otnyukova is synonymized with Nyholmiella obtusifolia. Orthotrichum vittii is excluded from the Russian moss flora. Description of O. dagestanicum is amended. Fifty four currently recognized species from 9 genera of the Orthotrichaceae are presently known to occur in Russia; list of species with common synonyms and brief review of distribution in Russia is presented. Numerous problematic specimens with unresolved taxonomy were omitted for future. Revealed taxonomical inconsistencies in the genera Zygodon, Ulota, Lewinskya, Nyholmiella, Orthotrichum are briefly discussed. Main regularities of spatial differentiation of the family Orthotrichaceae in Russia are considered. Recently presented novelties contribute to the certain biogeographic pattern, indicating three different centers of diversity of the family, changing along longitudinal gradient. Unlike European one, continental Asian diversity of Orthotrichaceae is still poorly known, the Siberian specimens which were previously referred to European species in most cases were found to represent other, poorly known or undescribed species. North Pacific Region houses peculiar and poorly understood hot spot of diversity of Orthotrichoid mosses. Thus, these hot spots are obligatory to be sampled in course of revisions of particular groups, since they likely comprise under-recorded cryptic- or semi-cryptic species. Latitudinal gradient also contributes to the spatial differentiation of the revealed taxonomic composition of Orthotrichaceae.

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