Convection Transfer Mechanisms

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... 

Convection The mechanism of heat transfer due to different temperatures, and hence different densities in fluids. It may be natural, dependent only on thermal forces, or forced, when use is made of a rotodynamic device to improve the rate of heat exchange. 

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. 

Essentially, except for once-through boilers, steam generation primarily involves two-phase nucleate boiling and convective boiling mechanisms (see Section 1.1). Any deposition at the heat transfer surfaces may disturb the thermal gradient resulting from the initial conduction of heat from the metal surface to the adjacent layer of slower and more laminar flow, inner-wall water and on to the higher velocity and more turbulent flow bulk water. 

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. 

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 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... 

Usually experimental methods are used to measure heat transfer. Basic transfer mechanisms commonly recognized are conduction and radiation. Convection is often used as a third classification. The convection classification is also used in the current work. 

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. 

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. 

Qi, C. and Ihab, H. Farag Heal Transfer Mechanism Due to Particle Convection in Circulating Fluidized Bed, in Circulating Fluidized Bed Technology IV (Amos A. Avidan, ed.), pp. 396-401. Somerset, Pennsylvania (1993). 

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... 

Figure 6.3 Mass transfer mechanism of natural convection between a placket and an adjacent medium. 1 placket or drying wall, 2 limit of adjacent medium,...

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. 

---