Heat transfer mechanisms convection

In the second heat transfer mechanism, convection, molecular scale transfers of kinetic energy are augmented by the macroscopic movement of a fluid transfer medium. Convection is most important as the mechanism of transfer between the solid surface of a static bed or an individual suspended particle and the gaseous medium that surrounds it. Convection has been modeled classically using the following relationship generally attributed to Isaac Newton ... 

In injection molding there are two major heat transfer mechanisms— convection and conduction. 

DropletHea.tup, A relation for the time required for droplet heatup, T can be derived based on the assumption that forced convection is the primary heat-transfer mechanism, and that the Ran2-MarshaH equation for heat transfer to submerged spheres holds (34). The result is... 

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 transfer of heat by radiation in general can be said to occur simultaneously with heat transfer by convection and conduction. Transfer by radiation tends to become more important than that by the other two mechanisms as the temperature increases. It is useful to gain an appreciation of the basic definitions of the energy flux terms, the surface property terms and their relationships while discussing radiative heat transfer. With this objective, reference may be made to Table 3.4 in which these are presented. 

At later times, solar heat fluxes and convective heat transfer from the atmosphere become important. For a spill onto an insulated dike floor these fluxes may be the only energy contributions. This approach seems to work adequately for liquefied natural gas (LNG) and perhaps for ethane and ethylene. The higher hydrocarbons (C3 and above) require a more detailed heat transfer mechanism. This model also neglects possible water freezing effects in the ground, which can significantly alter the heat transfer behavior. More details on boiling pools is provided elsewhere.19... 

The interest in heat-transfer for high-pressure systems is related to the extraction of a valuable solute with a compressed gas. The compressed fluid is usually a high-pressure gas-often a supercritical fluid, that is, a gas above its critical state. In this scenario, the prevalent heat-transfer mechanism is convection. 

Conductive and Convective Heat Transfer, Thermo Explosion by. There are three fundamental types of heat transfer conduction, convection radiation. All three types may occur at the same time, but it is advisable to consider the heat thransfer by each type in any particular case. Conduction is the transfer of heat from one part of a body to another part of the same body, or from one body to another in physical contact with it, without appreciable displacement of the particles of either body. Convection is the transfer of heat from one point to another within a fluid, gas or liquid, by the mixing of one portion of the fluid with another. In natural convection, the motion of the fluid is entirely the result of differences in density resulting from temp differences in forced convection, the motion is produced by mechanical means. Radiation is the transfer of heat from one body to another, not in contact with it, by means of wave motion thru space (Ref 5)... 

Note that Equation 5.16 gives us the opportunity to see which heat transfer mechanism is dominant. That is, it provides the ratio of the heat transferred by convection to that transferred by diffusion. 

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 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... 

It is also clear that Newton s law of cooling is a special case of Fourier s law. The foregoing provides the reason for only two commonly recognized basic heat transfer mechanisms. But owing to the complexity of fluid motion, convection is often treated as a separate heat transfer mode. 

Most readers will be familiar with the terms used to denote the three modes of heat transfer conduction, convection, and radiation. In this chapter we seek to explain the mechanism of these modes qualitatively so that each may be considered in its proper perspective. Subsequent chapters treat the three types of heat transfer in detail. 

Transfer of heat by physical mixing of the hot and cold portions of a fluid is known as heat transfer by convection The mixing can occur as a result of density differences alone, as in natural convection, or as a result of mechanically induced agitation, as in forced convection. 

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. 

Diffusion combustion of small samples (<0.2-0.3 m) is mostly laminar, but as the combustion zone and, correspondingly, the flame height, increase the flame may become turbulent. Turbulent convective heat transfer is essential in the case of such flames. And finally, in medium and large-sized samples (>0.2-0.3 m) radiation is the prevailing heat transfer mechanism... 

Convection is a heat transfer mechanism involving bulk movement of fluid at an elevated temperature to a region of lower temperature. 

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... 

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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