Radiation conductivity mechanism

Contact Drying. Contact drying occurs when wet material contacts a warm surface in an indirect-heat dryer (15—18). A sphere resting on a flat heated surface is a simple model. The heat-transfer mechanisms across the gap between the surface and the sphere are conduction and radiation. Conduction heat transfer is calculated, approximately, by recognizing that the effective conductivity of a gas approaches 0, as the gap width approaches 0. The gas is no longer a continuum and the rarified gas effect is accounted for in a formula that also defines the conduction heat-transfer coefficient ... 

One early program carried out at AUied-Signal, Inc. proposed the use of conductive polymers in remotely readable indicators (210). Conductivity changes induced in the conductive polymer could be read externally and the history of the sample known. Systems designed to detect time—temperature, temperature limit, radiation dosage, mechanical abuse, and chemical exposure were developed. 

In normal atmospheric conditions, fire usually is initialed by a combustible material coming in contact with a heat source. The spread of fire occurs due to direct flame impingement or the transfer of heat to the surrounding combustible materials. Heat transfer occurs by three principal mechanisms - conduction, convection, and radiation. Conduction is the movement of heat through a stationary medium, such as solids, liquids or gases. Steel is a good conductor of heat as is aluminum, therefore they can pass the heat of a fire if left unprotected. 

Microwave radiation, as all radiation of an electromagnetic nature, consists of two components, i.e. magnetic and electric field components (Fig. 1.3). The electric field component is responsible for dielectric heating mechanism since it can cause molecular motion either by migration of ionic species (conduction mechanism) or rotation of dipolar species (dipolar polarization mechanism). In a microwave field, the electric field component oscillates very quickly (at 2.45 GHz the field oscillates 4.9 x 109 times per second), and the strong agitation, provided by cyclic reorientation of molecules, can result in an... 

There are several possible mechanisms for the heat exchange between a reacting medium and a heat carrier radiation, conduction and forced or natural convection. Here we shall consider convection only. Other mechanisms are considered in the chapter on heat accumulation. The heat exchanged with a heat carrier (q ) across the reactor wall by forced convection is proportional to the heat exchange area (A) and to the driving force, that is, the temperature difference between the reaction medium and the heat carrier. The proportionality coefficient is the overall heat transfer coefficient (U) ... 

They subsequently (2) developed a one-dimensional mathematical model in the form of coupled differential and integro-differential equations, based on a gross mechanism for the chemical kinetics and on thermal feedback by wall-to-wall radiation, conduction in the tube wall, and convection between the gas stream and the wall. This model yielded results by numerical integration which were in good agreement with the experimental measurements for the 9.53-mm tube. For this tube diameter, the flows of unbumed gas for stable flames were in the turbulent regime. 

We mentioned in Chapter 1 that there are three basic mechanisms of heat trans-fet conduction, convection, and radiation. Conduction and convection are similar in that both mechanisms require the presence of a material medium. But they are different in that convection requires the presence of fluid motion. 

In the case of solid crystalline oxides, thermal conductivity decreases with increasing temperature but begins to rise above 1500— 1600 °C because transmission of heat by radiation (photons) begins to take a significant part besides the conduction of heat (phonon mechanism). In completely transparent materials (the coefficient of absorption a = O), no interaction with the radiation occurs in an opaque body (a = oo) the heat is transferred by conduction alone. With translucent materials, each element of the substance absorbs some of the incident radiation, and emits simultaneously,This internal radiation mechanism of heat transmission is characteristic for glasses. At high temperatures, a considerable proportion of heat is therefore transmitted by radiation the so-called apparent thermal conductivity is a sum of true conductivity with radiation conductivity ... 

High thermal conductance Low leachability Chemical and radiation stability Mechanical ruggedness Noncorrosiveness to container Minimum volume Minimum cost... 

Heat is transferred through slags by a variety of mechanisms which include convection, radiation and various thermal conduction processes, viz. thermal ("phonon") conductivity, (k ), electronic conductivity (kg ) and radiation conductivity (k ). Methods for estimating the various physical properties involved in these processes are considered below. 

Thermal insulation improves (Figure 7) because closed cells reduce conduction by convection thus, only radiation and solid conduction remain as conductive mechanisms. 

During exposure of ion-permeable membranes to radiation, their mechanical firmness and flexibihty decreases. Ion-exchange capacity, crosshnking, conductivity and selectivity decrease as well. 

It is often claimed in the literature that heat transmission from a body is based on three mechanism, radiation, conduction, and convection. This subdivision of heat transmission in conduction and convection gives the impression as if we have two independent phenomena. From this one had to conclude, that heat might also be transferred by convection without any contribution of conduction. This, however, is not true. 

Devices that use for the detection of moisture, radiation, and mechanical abuse can also be fabricated using the same rf antitheft tag technology. In the case of moisture detection, the antenna is again covered with a doped, highly conductive layer of PA, and the device is always in an OFF state. When the tag is exposed to moisture, the conductivity is reduced, which eventually turns the device ON. In the cases of radiation and mechanical abuse detection, a similar design can be employed. Again, the external stimuli (radiation or mechanical forces) serve to release the dopant, and the increased in conductivity leads to a lack of rf response by the tag. 

There are several mechanisms for heat exchange between a reacting medium and a heat carrier radiation, conduction, and forced or natural convection. Here we shall... 

Temperature-control problems are really heat transfer problems, whether the mechanism is radiation, conduction, or convection. Al-... 

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