Heat Transfer Mechanisms conduction through solids

Heat can be conducted through solids, liquids, and gases. Conduction in solids is the most illustrative since it is the most common heat transfer mechanism in that type of medium. Conduction is the energy transfer between adjacent molecules or atomic particles at motion. The nature of the motion depends on the system and on the molecular and particle state. The motion can range from vibration of atoms in a crystal lattice of solids to the chaotic fluctuations of gas molecules. In metallic solids, movement of free electrons contributes to heat conduction. 

The mechanism of conduction is most easily understood by the study of conduction through homogeneous isotropic solids, because in this case convection is not present. As a simple illustration of heat transfer by conduction, let a flat parallel-sided plate of a uniform solid material, whose flat faces are maintained at temperatures Tt and T2 respectively (Tj > T2) be considered (Figure 3.15). Heat would be transferred from the face at the higher temperature (Tj) to that at the lower temperature (T2). Let the rate of this transfer be dQjdt, and the area of the plate perpendicular to the direction of heat flow be S. If L is the plate thickness, then it is found that dQ/dt is proportional to (Tt - T2) S/L. In other words,... 

The coupled heat and liquid moisture transport of nano-porous material has wide industrial applications in textile engineering and functional design of apparel products. Heat transfer mechanisms in nano-porous textiles include conduction by the solid material of fibers, conduction by intervening air, radiation, and convection. Meanwhile, liquid and moisture transfer mechanisms include vapor diffusion in the void space and moisture sorption by the fiber, evaporation, and capillary effects. Water vapor moves through textiles as a result of water vapor concentration differences. Fibers absorb water vapor due to their internal chemical compositions and structures. The flow of liquid moisture through the textiles is caused by flber-liquid molecular attraction at the surface of fiber materials, which is determined mainly by surface tension and effective capillary pore distribution and pathways. Evaporation and/or condensation take place, depending on the temperature and moisture distributions. The heat transfer process is coupled with the moisture transfer processes with phase changes such as moisture sorption and evaporation. 

Nonevacuated Powders. Insulation with gas-filled powders is achieved through the reduction or elimination of convective heat transfer due to the small gas voids present within the material. The presence of the particulate solid also reduces radiation (often by about 5% of the overall conductivity) and inhibits gas conduction thus, solid conduction and gas conduction through the voids serve as the predominant heat transfer mechanisms. Nusselt and Bayer developed the following expression for the apparent thermal conductivity of gas-filled powders ... 

Figure 6 Relative contribution of the heat transfer mechanism to the overall heat transfer. line 1, the conduction through the solid particles and stagnant fluid between non-contacted particles line 2, the conduction through the solid particles and stagnant fluid between contacted particles and line 3, the conduction through the contact area between contacted particles solid line, Model A dash line, Model B (c=0.5). Reprinted from Cheng et al. (1999) with permission from ACS.

Notably, the approach has taken into account almost aU the known heat transfer mechanisms including particle-fluid convection, particle-particle conduction, and radiative heat transfer between solid particles and surrounding environment. It can be tested through various comparisons of the predicted and measured results. The approach has a good capability in describing heat transfer in packed and fluidized beds at a particle scale. 

The value of the coefficient will depend on the mechanism by which heat is transferred, on the fluid dynamics of both the heated and the cooled fluids, on the properties of the materials through which the heat must pass, and on the geometry of the fluid paths. In solids, heat is normally transferred by conduction some materials such as metals have a high thermal conductivity, whilst others such as ceramics have a low conductivity. Transparent solids like glass also transmit radiant energy particularly in the visible part of the spectrum. 

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. 

Conduction is the primary mode of heat transfer through solid material. Conduction occurs by two mechanisms ... 

In conduction, heat is conducted by the transfer of energy of motion between adjacent molecules in a liquid, gas, or solid. In a gas, atoms transfer energy to one another through molecular collisions. In metallic solids, the process of energy transfer via free electrons is also important. In convection, heat is transferred by bulk transport and mixing of macroscopic fluid elements. Recall that there can be forced convection, where the fluid is forced to flow via mechanical means, or natural (free) convection, where density differences cause fluid elements to flow. Since convection is found only in fluids, we will deal with it on only a limited basis. Radiation differs from conduction and convection in that no medium is needed for its propagation. As a result, the form of Eq. (4.1) is inappropriate for describing radiative heat transfer. Radiation is... 

When a fluid is present in contact with each solid wall, there will be an additional resistance to heat transfer in each fluid boundary layer or film . The combined mechanism of heat transfer from a hot fluid through a dividing wall to a cold fluid has many similarities to conduction through a composite slab reviewed earlier. 

The transport of thermal energy can be broken down into one or more of three mechanisms conduction--heat transfer via atomic vibrations in solids or kinetic interaction amongst atoms in gases1 convection - - heat rapidly removed from a surface by a mobile fluid or gas and radiation—heat transferred through a vacuum by electromagnetic waves. The discussion will begin with brief explanations of each. These concepts are important background in the optical measurement of temperature (optical pyrometry) and in experimental measurement of the thermally conductive behavior of materials. 

Thermal conduction through electrically insulating solids depends on the vibration of atoms in their lattice sites, which, as discussed in section 3.7, is the mechanism of thermal energy storage. These vibrations act as the conduit for heat transfer by the propagation of waves ( phonons ) superimposed on these vibrations (schematically depicted in Figure 8.1). An analogy... 

Wc discussed the physical significance of the Biot number earlier and indicated that it is a measure of the relative magnitudes of the two heal transfer mechanisms convection at the surface and conduction through the solid. A small value of Bi indicates tliat the inner resistance of the body to heat conduction is smalt relative to the resistance to convection between the surface and the fluid. As a result, the temperature distribution within the solid becomes fairly uniform, and lumped system analysis becomes applicable. Recall that when Bi <0.1, the error in assuming the temperature within the body to be uniform is negligible. 

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