Heat Transfer Mechanisms conduction through gases

In gas-filled windows there are three heat transfer mechanisms conduction and convection through the gas layer and radiation between the surroundings and the glass surfaces. The heat flow by conduction is minimized by using a fairly thick gas layer with a low conductivity. With even thicker layers, the effect of convection becomes important. Conduction and radiation cause similar heat fluxes, with heat transfer coefficients of a few watts per square metre per kelvin. 

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. 

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 ... 

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... 

Through virtually all of the filament length, the temperature is constant. But at the ends the temperature must drop almost to the cell body temperature. Heat is transferred to the body through the ends. Like thermal conductivity, this loss is proportional to the difference in temperature between filament and body, but unlike thermal conductivity, the heat transfer does not depend on conductivity of the gas. This amounts to 45 mW for the case cited. For the small-diameter wire and high-conductivity carrier gases used today, this heat loss mechanism can usually be neglected. 

The mechanism of heat transfer prevailing across an evacuated space (0.13 mPa or less) is by radiation and conduction through the residual gas. Radiation is generally the more predominant mechanism and can be approximated by ... 

The mechanism for heat transfer includes the following steps (1) conduction in the catalyst particle (2) convection from the particle to the gas phase (3) conduction at contact points between particles (4) convection between the gas and vessel wall (5) radiation heat transfer between the particles, the gas, and the vessel wall (6) conduction in the wall and (7) convection to the coolant. There are a number of ways, through reactor models, that these steps are correlated to provide design and analysis estimates and criteria for preventing runaway in exothermic reactors. 

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. 

The mass transfer in the boundary layers can be described by a mass transfer coefficient. In the membrane phase, the diffusion of water vapor can be described by either of the four mechanisms, namely molecular/Knudsen diffusion model, or Poiseuille flow, or by the dusty gas model (DGM). Heat transfer coefficients are used to describe the heat transfer in the boundary layer on either side of the membrane. In the membrane heat transfer occurs through the vapor and by conduction. These aspects have been explained in detail in the following sections. 

Heat Transfer in a Packed Bed (Effective Thermal Conductivity) In a bed of solid particles through which a reacting fluid is passing, heat can be transferred in the radial direction by a number of mechanisms. However, it is customary to consider that the bed of particles and the gas may be replaced by a hypothetical solid in which conduction is the only mechanism for heat transfer. The thermal conductivity of this solid has been termed the effective thermal conductivity k. With this scheme the temperature T of any point in the bed may be related to and the position parameters r and z by the differential equation... 

In order to avoid geometrical difficulties an ideal model of the packed bed will be employed to evaluate the heat transfer through the particle. The methods by which heat can enter a particle from its inner side are radiation, convection from the gas stream, and conduction through point contacts and stagnant fillets, as indicated in Fig. 13-8. Heat is transferred Through the particle and leaves the other side by the same three mechanisms. The three processes are in series, and the whole will be designated as the series mechanism. Hence... 

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