Microwave temperature distributions

A piece of beef steak is cooked either in a microwave oven or a radiant heating oven. Sketch temperature distributions at specific times during the heating and the cooling processes in each oven. 

Figure 7.4 (A) Absorption spectra as a function of temperature for 30 il thymol blue measured during microwave heating (B) the respective absorbance, temperature vs. time ratiometric plot (C) Real-time temperature distributions of water on a SiFs-deposited sapphire substrate captured using a thermal camera (D) A thermal image of SiFs during microwave heating. Adapted from references 1 (Figure 2a-b) and 18 (Figure 2c-d). Adapted from references 1 (A, B) and 41 (C,D).

A key aspect of the uniformity of the temperature field in both low- and high-temperature processing is the nature of the thermal gradients within the material. Consider the temperature distributions within a flat ceramic slab of thickness L (Fig. 10). For microwave heating (top curve in Fig. 10), the temperature is relatively uniform within the bulk, with a drop in temperature near the specimen surface owing to heat losses. In contrast, for conventional heating from the specimen surfaces (bottom curve in Fig. 10), the temperature is highest at the surface and lowest near the specimen s midplane. 

Fig. 10 Schematic of temperature distributions in a flat ceramic slab of thickness L for volumetric microwave heating (top curve) and conventional heating from the slab surfaces (bottom curve). For conventional heating, the finite value of thermal conductivity, k, gives the highest temperatures near the specimen surface and the lowest temperature along the specimen s midplane. Conversely, for microwave heating the heating is more uniform, with decreasing temperature near the slab surface because of heat losses from the surfaces.

It is essential to provide accurate measurements of intratissue temperature distributions during normothermia and hyperthermia. New developments in the noninvasive thermometric techniques using MRI and microwave should alleviate some of these problems. 

It was found that the distribution of temperature in the materials processed by using microwave heating is generally non-uniform. The distribution is parabolic with a maximum at the center of the material, which is usually referred to as an inverse temperature profile and is a specific characteristic of microwave sintering. At short timescales, the temperature distribution follows the power distribution. Otherwise, it is determined by thermal conduction of the entire dimensions of the materials [70]. 

In the experimental study by Zhu et al. (1998), the heating pattern induced by a microwave antenna was quantified by solving the inverse problem of heat conduction in a tissue equivalent gel. In this approach, detailed temperature distribution in the gel is required and predicted by solving a two- or three-dimensional heat conduction equation in the gel. In the experimental study, all the temperature probes were not required to be placed in the near field of the catheter. Experiments were first performed in the gel to measure the temperature elevation induced by the applicator. An expression with several unknown parameters was proposed for the SAR distribution. Then, a theoretical heat transfer model was developed with appropriate boundary conditions and initial condition of the experiment to study the temperature distribution in the gel. The values of those unknown parameters in the proposed SAR expression were initially assumed and the temperatiue field in the gel was calculated by the model. The parameters were then adjusted to minimize the square error of the deviations theoretically predict from the experimentally measured temperatures at all temperature sensor locations. 

A comparison of thermal and microwave cure assumes a new dimension when the temperature distribution inside the sample is considered, and that is where the scientihc challenge lies. The fundamental difference in the heat transfer during re-... 

Discuss qualitatively the expected temperature distribution in a ceramic body with a linear dimension of 30 cm during microwave sintering at each frequency. How can the temperature distribution in the body be made more uniform ... 

The hterature describes that reaction occurs within hours or days, by microwave s)mthesis reaction time is greatly reduced - from 5 to 30 min [19,20,21]. Microwaves are distributed evenly in the reaction mixture, which makes the temperature field uniform (homogeneous) and radio frequency radiation provides the energy required for the reaction, and molecules get more energy (Ea) and the reaction speed increases manifold [20,21,22,23],... 

Figure 10.8 Microwave-promoted dielectric heating. Dipolar and conductive polarizations have been highlighted in the drawing. Furthermore two spots in the solution evidence an interior localized superheating and subsequent temperature homogenization. A color map has been reported in order to describe a possible temperature distribution.

One of the possible alternative methods for non-invasive temperature sensing and monitoring that is completely passive and inherently safe is microwave radiometry (MWR).. We proposed a multi-fl equency microwave radiometry as a non-invasive monitoring method of deep brain temperature and fabricated a five-band receiver system and reported its measurement performance of about 1.6 K 2o-confidence interval at 5 cm depth from the surface of a water-bath phantom with similar temperature distribution as infant s brain [6]. Because the clinical requirement is less than 1 K, further improvement of MWR system were essential for a successful hypothermia treatment. We have done a couple of actions to reduce background noise in order to obtain the better temperature resolutions of five microwave receivers and tried to retrieve the temperature profile in the phantom. This paper describes the current feasibility of the MWR system for clinical hypothermic treatment. 

Zhao Xiqiang et ak, Temperature distribution simulation of microwave heating process of straw b e, ... 

During the synthesis, the surface temperatures of the solid epoxy resin samples were monitored by means of a thermovision camera in order to observe the temperature distribution under microwave and conventional conditions for both stirred and nonstirred reaction mixtures (Figure 39). 

This ensures fast penetration of energy into the volume of materials transparent to microwave fields (like zeolites), that is, almost instantaneous heating (and cooling when the field is switched off) of materials. The spatial temperature distribution in the material is thus different from that observed under traditional convective or contact heating conditions. The most important differences related to the appearance of temperature gradients and nonequilibrium conditions are observed when a reaction medium or material (a catalyst) consists of several phases with different abilities to be heated by microwave radiation. 

Many researchers have identified the difference in the presence of hot spots (which locally enhance or promote some selected reactions or transformations). The narrow temperature distribution obtained by simulation can justify the formation of nanoparticles (having a narrower particle size distribution) with respect to conventionally heated synthetic routes in case of nucleation and growth of nanoparticles (microwave hydrothermal synthesis). The large-scale production of nanoparticles requires the development of microwave reactors, which can reflect the laboratory temperature profile homogeneity. It will provide a new dedicated eontinuous-flow reactor, made of two twin prismatic applicators for a microwave-assisted process in aqueous solution. The reactor can produce upto 1000 L/day of nanoparticles eolloi-dal suspension at ambient pressirre and relatively low temperature and henee, it ean be considered a green chemistry approach. 

The temperature distribution in ceramic materials heated by microwave energy can be obtained by solving the continuity equation for heat (equation... 

De Wagter, C., Computer simulation predicting temperature distributions generated by microwave absorption in multilayered media. J Microwave power 19 [2] 97-105 (1983). 

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