Convection, heat transfer mode

Heat transfer coefficient Thermal parameter that encompasses all of the complex effects occurring in the convection heat transfer mode, including the properties of the fluid gas/liquid, the nature of the fluid motion, and the geometry of the structure. 

Irrespective of this classification of convection heat transfer modes, the overall effect is given by a convection rate equation governed by Newton s law cf cooling expressed as... 

There are three heat-transfer modes, ie, conduction, convection, and radiation, each of which may play a role in the selection of a heat exchanger for a particular appHcation. The basic design principles of heat exchangers are also important, as are the analysis methods employed to determine the right size heat exchanger. 

Convective heat transfer is often nsed as an adjnuct to other modes, particnlarly to the coudnctive mode. It is often more convenient to consider the agitative effecl a performance-improvement iuflneuce on the thermal diffnsivity factor Ot, modifying it to Ot, the effective valne. 

Convective heat transfer occurs when a fluid (gas or liquid) is in contact with a body at a different temperature. As a simple example, consider that you are swimming in water at 21°C (70°F), you observe that your body feels cooler than it would if you were in still air at 21°C (70°F). Also, you have observed that you feel cooler in your automobile when the air-conditioner vent is blowing directly at you than when the air stream is directed away from you. Both ot these observations are directly related to convective heat transfer, and we might hypothesize that the rate of energy loss from our body due to this mode of heat transfer is dependent on not only the temperature difference but also the typie of surrounding fluid and the velocity of the fluid. We can thus define the unit heat transfer for convection, q/A, as follows ... 

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 ... 

Consider a tube heated uniformly at a heat flux q/A fed with saturated water at the base at a velocity Fo. For this velocity and heat flux, nucleate boiling will take place, and a temperature difference aTo will be established. At some distance up the tube vaporization will occur and increase the volumetric flow of material and hence the velocity to, say, Fi. The line for forced convective heat transfer meets the boiling curve below the heat flux of q/A and so nucleate boiling will still be the mode of heat transfer and the temperature difference AT, and hence the heat transfer... 

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. 

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. 

This book is concerned with a description of some methods of determining convective heat transfer rates in various flow situations, realizing that in many cases these methods will need to be combined with calculations for the other modes of heat transfer in order to predict the overall heat transfer rate. 

In some cases it is not possible to consider the modes separately. For example, if a gas, such as water vapor or carbon dioxide, which absorbs and generates thermal radiation, flows over a surface at a higher temperature, heat is transferred from the surface to the gas by both convection and radiation. In this case, the radiant heat exchange influences the temperature distribution in the fluid. Therefore, because the convective heat transfer rate depends on this temperature distribution in the fluid, the radiant and convective modes interact with each other and cannot be considered separately. However, even in cases such as this, the calculation procedures developed for convection by itself form the basis of the calculation of the convective part of the overall heat transfer rate. 

Forced convection heat transfer is probably the most common mode in the process industries. Forced flows may be internal or external. This subsection briefly introduces correlations for estimating heat-transfer coefficients for flows in tubes and ducts flows across plates, cylinders, and spheres flows through tube banks and packed beds heat transfer to nonevaporating falling films and rotating surfaces. Section 11 introduces several types of heat exchangers, design procedures, overall heat-transfer coefficients, and mean temperature differences. 

C In which mode of heat transfer is the convection heat transfer coefficient usually higher, natural convection or forced convection Why ... 

In most steady-state heat transfer problems, more than one heat transfer mode may be involved. The various thermal resistances due to thermal convection or conduction may be combined and described by an overall heat transfer coefficient, U. Using U, the heat transfer rate, Q, can be calculated from the terminal and/or system temperatures. The analysis of this problem is simplified when the concepts of thermal circuit and thermal resistance are employed. 

Sustained combustion requires a continuous supply of fresh reactants and a continuous removal of reaction products. This process is loosely known as mass transfer. Specifically, mass transfer is a consequence of three possible modes bulk fluid motion, molecular and turbulent diffusion, and reaction sources and sinks. Mass transfer due to bulk fluid motion is generally known as convection. It is similar to the convection heat transfer process. Mathematically, the rate of change for species / per unit volume, pYit via convection can be described as 3(pUjY ldxj, where p is fluid density, Yt is the mass fraction of species i, Uj is the / -component of the fluid velocity. 

Convection, conduction, radiation, electromagnetic fields, combination of heat transfer modes Intermittent or continuous ... 

It is possible, indeed desirable in some cases, to use combined heat transfer modes, e.g., convection and conduction, convection and radiation, convection and dielectric fields, etc., to reduce the need for increased gas flow that results in lower thermal efficiencies. Use of such combinations increases the capital costs, but these may be offset by reduced energy costs and enhanced product quality. No generalization can be made a priori without tests and economic evaluation. Finally, the heat input may be steady (continuous) or time-varying also, different heat transfer modes may be deployed simultaneously or consecutively depending on the individual application. In view of the significant increase in the number of design and operational parameters resulting from such complex operations, it is desirable to select the optimal conditions via a mathematical model. 

Direct Fired Furnaces. For direct fired furnaces, radiative heat transfer from the flame and combustion products as well as from the walls to the load is usually the dominant heat transfer mode. Convection from the combustion gases makes a much smaller contribution. The radiative transfer within the furnace is complicated by the nongray behavior of the combus-... 

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