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 R1 and R2 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{symbol} ≥ 0.45) whereas alumina-metallic composite support may be suitable for smaller porosities (φ{symbol} ≤ 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. © 2006 Elsevier Ltd. All rights reserved.

Tanmay Basak

Department Of Chemical Engineering

K. Aparna

Arcot R. Balakrishnan

Alumina

Composite materials

Dielectric materials

Resonance

POWER ABSORPTION

SLAB DEPTH PLOTS

spatial distribution

Microwave heating

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

International Journal of Heat and Mass Transfer

Susceptor-assisted microwave processing is a rapidly growing technology due to its superiority over the conventional processing. In contrast to the conventional heating from the surface, the microwave heating occurs volumetrically via direct interaction with the material. Correspondingly, the microwave heating rates are in general much faster than the heating rates in the conventional furnaces, where heat has to be transferred from the heat sources to the material via conduction, convection and radiation. The need for the susceptor stems from the fact that the majority of the ceramics are low lossy materials and they cannot couple well with the microwave at room temperatures. The susceptor provides an easy and non-invasive technique to exploit the rapid microwave processing even for the highly microwave transparent ceramics, such as alumina, silicon nitride, quartz, etc. This article critically evaluates the susceptor-assisted microwave sintering and solid state synthesis of ceramics which have been reported over the last two decades. A wide range of ceramics has been considered and each case has been analyzed in terms of the enhancement of the processing rates and product qualities (grain structure, material properties, etc.) compared to the conventional processing. It has been shown that the susceptor-assisted microwave processing can greatly reduce the processing time while providing an easy pathway to achieve the desired product qualities. The use of the appropriate susceptor is the key to achieve the fast, smooth, and reliable microwave processing of ceramics and this article provides the required database for the appropriate design of the susceptor based on the process requirement. © 2017 Taylor & Francis Group, LLC.

Critical Reviews in Solid State and Materials Sciences

Susceptor-Assisted Enhanced Microwave Processing of Ceramics - A Review

Microwave heating has vast applications in the field of food processing such as cooking, drying, pasteurization and preservation of food materials. In this article, various applications of microwave food processing such as microwave cooking, microwave pasteurization and microwave assisted drying were extensively reviewed. The advantages and the factors affecting the microwave cooking of food materials have been reviewed. Microwave pasteurization of fresh juices, milk and various food products has been elaborately discussed. Microwave pasteurization has the ability to achieve destruction of microorganisms at temperatures lesser than that of conventional pasteurization due to significant enhancement or magnification of thermal effects. Applications of microwave drying include microwave assisted hot air drying, microwave vacuum drying and microwave freeze drying. Microwave drying combined with other conventional methods of drying enhances the drying characteristics of the sole effect of microwave drying. Modeling of microwave heating of food materials based on Maxwell's equations and Lambert's law equations have been reviewed along with their applications. Microwave modeling can be used to predict the temperature and moisture distributions during microwave heating of food materials. © 2013 Elsevier Ltd.

Food Research International

Microwave food processing-A review

The choice of material for a crucible is essential in hybrid microwave heating and graphite is one of the strong microwave absorbers which can sustain high temperatures. Experiments were carried out to investigate the microwave heating behavior of graphite combined with other materials to enhance efficiency. Mixtures comprising of graphite and magnetite or graphite and silicon carbide were found to have better microwave heating properties than pure components. The enhancement was likely due to microplasma generated by graphite in a microwave environment. Microwave heating of graphite-silicon carbide mixtures provided the best results in terms of high heating rates without arcing. The optimal combination of graphite-silicon carbide was identified and can be used as a starting material for making efficient crucibles and susceptors. © 2013 Elsevier Ltd.

International Communications in Heat and Mass Transfer

Microwave heating characteristics of graphite based powder mixtures

Microwave heating is caused by the ability of the materials to absorb microwave energy and convert it to heat. This article represents a review on fundamentals of microwave heating and their interaction with materials for various applications in a comprehensive manner. Experimental studies of single, multimode, and variable frequency microwave processing were reviewed along with their applications. Modeling of microwave heating based on Lambert's law and Maxwell's electromagnetic field equations have also been reviewed along with their applications. Modeling approaches were used to predict the effect of resonances on microwave power absorption, the role of supports for microwave heating, and to determine the nonuniformity on heating rates. Various industrial applications on thermal processing have been reviewed. There is tremendous scope for theoretical and experimental studies on the athermal effects of microwaves. Some of the unresolved problems are identified and directions for further research are also suggested. © 2011 American Institute of Chemical Engineers (AIChE).

AIChE Journal

Microwave material processing-a review

A theoretical analysis has been carried out to analyze efficient microwave assisted heating processes of food cylinders with various shapes of metallic annuli in presence of lateral and radial irradiation. Lateral irradiation represents the sample incident at one direction with source at infinity whereas the radial irradiation represents the situation where the sample is incident with microwave radiations from co-axial cylindrical cavity at infinity. A generic finite element method has been used to obtain computational mesh for all shapes of metallic annulus and Galerkin finite element method has been used to solve electric field and energy balance equations. Influence of metallic annulus has been illustrated for three representative sample lengths or regimes (I, II and III) as a function of wave number (Nw). Sample radius with regime I is smallest and largest sample radius corresponds to regime III. In general, power absorption due to lateral irradiation is larger for regime I whereas identical power with lateral or radial irradiation or larger power absorption with radial irradiation corresponds to regime II in absence of metallic annulus. The metallic annuli are found to enhance power absorption or heating rate for specific aspect ratios of both bread and beef samples. The aspect ratios are defined as the ratio of the dimension of metallic annulus and outer radius of the food sample. Lateral irradiation is found to give larger heating rates especially for regimes II and III for bread samples (low dielectric loss), but radial irradiation gives smaller degree of thermal runaway. On the other hand, radial irradiation gives significantly large heating rates with minimal thermal runaway for beef samples (high dielectric loss) with all regimes. It is interesting to observe that the metallic annuli enhance two to three times microwave power for beef samples involving regimes II and III. Various shapes of metallic annuli are also found to play dominant role either to focus or to optimize heating effects. © 2010 Elsevier Ltd. All rights reserved.

Theoretical analysis on the role of annular metallic shapes for microwave processing of food dielectric cylinders with various irradiations

A detailed theoretical analysis has been carried out to study efficient heating of one-dimensional (1-D) composite dielectric food slabs due to microwaves. Current study involves processing of beef with oil layers with/without ceramic support for various cases, such as the support and/or oil layer is directly exposed to microwaves (case 1), the beef layer is directly exposed to microwaves (case 2) and oil-beef layers and the ceramic supports are exposed to microwaves at both the sides (case 3). A preliminary analysis on microwave heating has been carried out for beef-oil composite without any support and the maxima in average power corresponding to resonances occur at various sample thicknesses for all cases. The detailed spatial distribution for various cases illustrates that for small oil thicknesses, heating rate is less and greater heating rate would correspond to larger oil thicknesses specially for case 1. The similar analysis has been extended for all cases with oil layer directly attached with ceramic supports (Al2O3 and SiC). The detailed analysis on spatial distribution of temperature and power during one side incidence illustrate that the greater thermal runaway occurs for case 2 with Al2O3 support corresponding to smaller oil thickness whereas SiC support may cause thermal runaway for larger oil thicknesses. For distributed microwave incidences (case 3), it is observed that Al 2O3 support may cause thermal runaway whereas SiC support may reduce the thermal runaway, based on suitable distribution of microwaves at both sides. © 2006 American Institute of Chemical Engineers.

Influence of ceramic supports on microwave heating for composite dielectric food slabs

A detailed analysis has been carried out to study efficient heating processes due to microwaves for one dimensional food samples placed on ceramic plates with various distributions of microwave incidence. The enhanced heating effects due to microwave propagation within samples have been illustrated via average power within a sample vs sample thickness diagram. The maxima in power, also termed as 'resonances' is observed for specific sample thicknesses. The spatial temperature distributions illustrate the presence of local thermal heating effects for various cases. Either one side incidence or both sides incidence with suitable distribution of microwave incidence may be preferred for various food materials attached with ceramic plates. One side incidence may cause overall larger heating rates for beef samples and the heating rates for oil samples would remain invariant irrespective of various distributions of microwave incidence. It is observed that beef samples would exhibit greater thermal runaway specially with SiC plate in presence of both sides distributed microwave incidence whereas oil sample would exhibit smaller thermal runaway effects with both sides equi-distributed microwave incidence irrespective of any ceramic plates. Current analysis has been illustrated for low and high dielectric food materials (beef and oil) and based on our analysis, a generalized heating strategy for any materials due to uniform plane waves may be derived. © 2006 Elsevier Ltd. All rights reserved.

A theoretical analysis on microwave heating of food slabs attached with ceramic plates: Role of distributed microwave incidence

A theoretical analysis has been carried out to study efficient microwave heating of porous dielectrics. The heating effects are analyzed for two types of porous material: beef-air (b/a) and beef-oil (b/o) with and without ceramic supports (Al2O3 and SiC). Three test cases for porosities (φ{symbol}) 0.3, 0.45 and 0.6 are considered. The maxima in average power corresponding to resonances occur at various sample thicknesses for all porous materials with and without supports and two dominant resonance modes R1 and R2 are considered where the average power at R1 is larger than that at R2. It is interesting to observe that average power absorption is enhanced for samples (b/a and b/o) in presence of Al2O3 support whereas the average power is smaller with SiC support. From the analysis on spatial distribution of electric field, power and temperature, it is seen that runaway heating is observed at the face which is not attached with support for b/a samples, and the intensity of thermal runaway increases with porosity whereas lower thermal runaway is observed for b/o samples at all porosity values. An efficient heating strategy has been investigated for various distributions of microwave incidences. It is observed that one side incidence may correspond to the largest heating rates whereas distributed sources may correspond to smaller thermal runaway for both beef-air (b/a) and beef-oil (b/o) samples. © 2006 Elsevier Ltd. All rights reserved.

Effect of ceramic supports on microwave processing of porous food samples

A detailed analysis has been carried out to study efficient heating due to microwaves for one-dimensional samples placed on ceramic supports (Al2 O3, SiC). The greater effects on microwave heating of samples have been illustrated via average power within a sample versus sample thickness diagram for various cases. The maxima in power, also termed as "resonances," is observed for specific sample thicknesses and the two consecutive maxima in average power are termed as R1 and R2 modes. The greater heating effects leading to hot spots would occur in water samples during both-sides incidence when the sample is kept on Al2 O3 support. SiC support may be recommended for water samples due to uniform heating throughout the sample. In contrast, SiC support could cause local hot spots or thermal runaway for oil samples. The localized hot spots are more pronounced for the samples exposed to microwaves on both faces. The choice of support may not be trivial due to the complex dielectric response of sample-support assembly. Current analysis has been illustrated for low- and high-dielectric materials (water and oil) and a representative case study has also been shown for beef samples. Based on such observations, a generalized heating strategy for materials due to uniform plane waves has been derived. © 2005 American Institute of Physics.

Fulltext

Journal of Applied Physics

Role of ceramic supports on microwave heating of materials

A detailed analysis on enhanced heating effects attributed to resonance of microwaves has been carried out to study the efficient heating methodologies for oil-water emulsions. Studies on heating have been carried out for samples incident with microwaves at one side and at both sides. The maxima in average power corresponding to resonances occur at various sample thicknesses for all emulsions and we consider two dominant resonance modes R1 and R 2, where the average power at R1 is larger than that at R2. During one side incidence, it is observed that processing rates are greater at the R2 mode for both oil-in-water (o/w) and water-in-oil (w/o) emulsions, whereas for both-sides incidence, the R 1 mode is favored for o/w emulsion and the R2 mode is advantageous for w/o emulsion. The greater rates in thermal processing are observed when the emulsions (o/w and w/o) are incident at both sides. Current analysis recommends on the efficient way to use microwaves in a customized plane-wave oven for greater heating effects and determines the optimal sample thicknesses vs. various oil-water contents for thermal processing. The emulsion system is a combination of materials with very low dielectric loss (oil) and high dielectric loss (water), and the effective dielectric response is highly nontrivial to predict the efficient way to heat an emulsion, either o/w or w/o. © 2004 American Institute of Chemical Engineers.

Journal	International Journal of Heat and Mass Transfer
ISSN	00179310
Open Access	No