Heat transfer mechanisms conduction

In order to achieve passive safety with reactive material, the radius of the reactor tube is designed to be small to avoid any thermal explosion inside the tube. Using the Frank-Kamenetskii approach (see Chapter 13), the radius remains below the critical radius. Thus, even assuming a purely conductive heat transfer mechanism, corresponding to a worst case, no instable temperature profile can develop inside the reaction mass. The reactor can be shut down and restarted safely. 

Conduction heat transfer mechanisms in flame impingement (a) steady-state, and (b) transient. 

These reactors contain an additional conductive heat transfer mechanism across the outer wall at / outside that must be included in the cooling fluid s thermal energy balance. If one adopts equations (4-55), (4-57), and (4-58) for cocurrent cooling,... 

Step-response curves are presented on Figure 11.21. If the thermocouple had been used bare, a single time constant of 1.9 seconds would have been obtained for F = 152 ft/sec. In view of the large annular clearance, dierc may be some error here in assuming a purely conductive heat-transfer mechanism for the annular fill. 

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 ... 

Radiative heat transfer is perhaps the most difficult of the heat transfer mechanisms to understand because so many factors influence this heat transfer mode. Radiative heat transfer does not require a medium through which the heat is transferred, unlike both conduction and convection. The most apparent example of radiative heat transfer is the solar energy we receive from the Sun. The sunlight comes to Earth across 150,000,000 km (93,000,000 miles) through the vacuum of space. FIcat transfer by radiation is also not a linear function of temperature, as are both conduction and convection. Radiative energy emission is proportional to the fourth power of the absolute temperature of a body, and radiative heat transfer occurs in proportion to the difference between the fourth power of the absolute temperatures of the two surfaces. In equation form, q/A is defined as ... 

Many everyday heat flows, such as those through windows and walls, involve all three heat transfer mechanisms—conduction, convection, and radiation. In these situations, engineers often approximate the calculation of these heat flows using the concept of R values, or resistance to heat flow. The R value combines the effects of all three mechanisms into a single coefficient. 

The vial heat transfer coefficient is the sum of heat transfer coefficients for three parallel heat transfer mechanisms (1) direct conduction between glass and shelf surface at the few points of actual physical contact, Kc (2) radiation heat exchange, Kr, which has contributions from the shelf above the vial array to the top of the vials, Krt, and from the shelf upon which the vial is resting, Krb and (3) conduction via gas-surface collisions between the gas and the two surfaces, shelf and vial bottom, Kg ... 

In this simulation the model assumption is that the melt film is transported into the pores of the bed. Thus the melt film is relatively thin even for this mechanism due to the reducing depth of the channel. The model as developed does not account for any conductive heat transfer into the solid particles from the liquid infiltrate. For most extrusions, the center of the solid bed may be porous while the edges exposed to the melt film are not. The sealed edges prevent the melt from infiltrating the solid bed and the melting process occurs via conventional melting. Conventional melting was observed for the case with the measured bulk density data in Fig. 4.1. 

Properties of peroxide cross-linked polyethylene foams manufactured by a nitrogen solution process, were examined for thermal conductivity, cellular structure and matrix polymer morphology. Theoretical models were used to determine the relative contributions of each heat transfer mechanism to the total thermal conductivity. Thermal radiation was found to contribute some 22-34% of the total and this was related to the foam s mean cell structure and the presence of any carbon black filler. There was no clear trend of thermal conductivity with density, but mainly by cell size. 27 refs. 

In these examples, the degree of confinement increases from the first to last case. In general, when the heat transfer mechanism is conduction, it contributes significantly to the resistance. Thus, the nature of the reacting mass and its contents must be considered first in the assessment of a confinement situation. 

When a solid is exposed to a thermal environment, it will either absorb or release heat. This thermal energy is delivered via heat transfer mechanisms of conduction, convection, and radiation. Typical thermal properties of a solid include specific heat, thermal conductivity, thermal expansion, and thermal radiation properties. 

The governing heat transfer modes in gas-solid flow systems include gas-particle heat transfer, particle-particle heat transfer, and suspension-surface heat transfer by conduction, convection, and/or radiation. The basic heat and mass transfer modes of a single particle in a gas medium are introduced in Chapter 4. This chapter deals with the modeling approaches in describing the heat and mass transfer processes in gas-solid flows. In multiparticle systems, as in the fluidization systems with spherical or nearly spherical particles, the conductive heat transfer due to particle collisions is usually negligible. Hence, this chapter is mainly concerned with the heat and mass transfer from suspension to the wall, from suspension to an immersed surface, and from gas to solids for multiparticle systems. The heat and mass transfer mechanisms due to particle convection and gas convection are illustrated. In addition, heat transfer due to radiation is discussed. 

The suspension-to-wall surface heat transfer mechanism in a circulating fluidized bed (see Chapter 10) comprises various modes, including conduction due to particle clusters on the surface or particles falling along the walls, thermal radiation, and convection due to... 

Conduction Heat transfer within a substance by molecular motion (and also by electron flow in electrical conductors). The molecular motion may be actual displacement of molecules (the predominant mechanism in gases) or may be collisions between adjacent vibrating molecules (the predominant mechanism in liquids and nonmetallic solids). 

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. 

Metals, on the other hand, have an additional mechanism of conductive heat transfer—electron motion—which can be envisioned to transfer heat in an analogous fashion to that of the kinetic behavior a gas. Good electrical conductors tend to be good thermal conductors. However, the thermal conductivity of metals decreases with increasing temperature because of increased electron-electron scattering. 

Conduction is the heat transfer mechanism that takes place when the media is stationary. It can take place in solids, gases and liquids. It may be thought of as the transfer of energy from the more energetic particles of a medium to nearby particles that are less energetic owing to particle interactions. Conduction heat transfer is described macroscopically by Fourier s law, which is... 

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. 

There are three heat transfer mechanisms used to dry textiles. Conduction methods involve direct contact of the wet textile with heated surfaces. These are the most efficient heat transfer methods, but do not allow for control of fabric width during drying. Steam heated cylinders are examples of conduction drying methods... 

The only significant mass and heat transfer mechanisms in the membrane and the support layers are radial diffusion and conduction and reaction effects. The membrane or the support matrix containing the fluid is treated as a continuum with effective diffusivities and thermal conductivities. 

Some people do not consider convection to be a fundamental mechanism of heat transfer since it is essentially heat conduction in the presence of fluid motion. But we still need to give this combined phenomenon a name, unless we are willing to keep referring to it as conduction with fluid motion." 3 hus, it is practical to recognize convection as a separate heat transfer mechanism despite the valid arguments to the contrary. 

Radiation heat transfer to or from a surface sui rounded by a gas such as air occurs parallel to conduction (or convection, if there is bulk gas motion) between the surface and the gas. Thus the total heat transfer is determined by adding the contributions of both heat transfer mechanisms. For simplicity and convenience, this is often done by defining a combined heat transfer cu-eflicicnt hcombiiKd that includes the effects of both convection and radiation. Then the total heat transfer rate to or from a surface by convection and radiation is expressed as... 

Convection and conduction are the two major heat transfer mechanisms that have been investigated at microscale. Convective heat transfer in microchannels has been intensively analyzed by both experimental and analytical means. Conduction studies have focused mostly on thin films in recent years to address such questions as How is the heat transferred How does it differ from large-scale conduction ... 

Also, the pyrolyser construction plays an important role in the rate of attaining the Teq temperature. This construction may determine the way in which heat is transferred to the sample. This transfer process can be understood by evaluating the heat transfer mechanisms, which are conduction, convection, and radiation. The heat transfer rate q in J/s = W (watts) by unidirectional conduction for a small element of a material having the area A and thickness dx is given by Fourier s law ... 

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