External forced convection heat transfer

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. 

Acrivos, A. 1958. Combined laminar free and forced convection heat transfer in external flows. AIChE Journal. 4. 285-289. 

Convective heat transfer is classified as forced convection and natural (or free) convection. The former results from the forced flow of fluid caused by an external means such as a pump, fan, blower, agitator, mixer, etc. In the natural convection, flow is caused by density difference resulting from a temperature gradient within the fluid. An example of the principle of natural convection is illustrated by a heated vertical plate in quiescent air. 

Convection is the heat transfer in the fluid from or to a surface (Fig. 11.28) or within the fluid itself. Convective heat transport from a solid is combined with a conductive heat transfer in the solid itself. We distinguish between free and forced convection. If the fluid flow is generated internally by density differences (buoyancy forces), the heat transfer is termed free convection. Typical examples are the cold down-draft along a cold wall or the thermal plume upward along a warm vertical surface. Forced convection takes place when fluid movement is produced by applied pressure differences due to external means such as a pump. A typical example is the flow in a duct or a pipe. 

Wind affects the convective heat transfer on external walls and is a driv -mg force for natural ventilation. 

C WTiat is external forced convection How does it differ from internal forced convection fian a heat transfer system involve both internal and external convection at the same time Give an example. 

Fluid mechanics and heat transfer correlations involved with internal and external forced convection and natural convection are summarized in this appendix. These correlations are organized in the following manner... 

Convective heat transfer takes place in a moving medium and is almost always associated with transfer between a solid and a moving fluid (such as air). Forced convection takes place when an external driving force, such as a wind or an air pump, moves the fluid. Free convection takes place when the temperature differences necessary for heat transfer produce density changes in the fluid and the warmer fluid rises as a result of increased buoyancy. 

In this section the correlations used to determine the heat and mass transfer rates are presented. The convection process may be either free or forced convection. In free convection fluid motion is created by buoyancy forces within the fluid. In most industrial processes, forced convection is necessary in order to achieve the most economic heat exchange. The heat transfer correlations for forced convection in external and internal flows are given in Tables 4.8 and 4.9, respectively, for different conditions and geometries. 

In forced convection, circulating currents are produced by an external agency such as an agitator in a reaction vessel or as a result of turbulent flow in a pipe. In general, the magnitude of the circulation in forced convection is greater, and higher rates of heat transfer are obtained than in natural convection. 

If a beaker containing water rests on a hot plate, the water at the bottom of the beaker becomes hotter than that at the top. Since the density of the hot water is lower than that of the cold, the water in the bottom rises and heat is transferred by natural convection. In the same way air in contact with a hot plate will be heated by natural convection currents, the air near the surface being hotter and of lower density than that some distance away. In both of these cases there is no external agency providing forced convection currents, and the transfer of heat occurs at a correspondingly lower rate since the natural convection currents move rather slowly. 

When a fluid is heated, the hot less-dense fluid rises and is replaced by cold material, thus setting up a natural convection current. When the fluid is agitated by some external means, then forced convection takes place. It is normally considered that there is a stationary film of fluid adjacent to the wall and that heat transfer takes place through this film by conduction. Because the thermal conductivity of most liquids is low, the main resistance to the flow of heat is in the film. Conduction through this film is given by the usual relation (74), but the value of h is not simply a property of the fluid but depends on many factors such as the geometry of the system and the flow dynamics for example, with tubes there are significant differences between the inside and outside film coefficients. 

Forced Convection. In forced convection, flow is generated by some external means, e.g. a pump, and heat transfer coefficients would be expected to be a function of... 

As with external mixed convection, the influence of buoyancy forces on the flow depends on the angle that the buoyancy forces makes to the direction of the forced flow. The heat transfer rate also, of course, depends on the duct cross-sectional shape as well as on whether the flow is laminar or turbulent. 

Convection is called forced convection if Ihe fluid is forced to flow over the surface by external means such as a fan, pump, or the wind. In contrast, convection is called natural (or free) convection if the fluid motion is caused by buoyancy forces that are induced by density differences due to the variation of temperature in the fluid (Fig. 1 33). For example, in the absence of a fan, heat transfer from the surface of the hot block in Fig. 1-32 is by natural convection since any motion in the air in this case is due to the rise of Ihe warmer (and thus lighter) air near the surface and the fall of the cooler (and thus heavier) air to fill its place. Heat transfer between the block and the surrounding air is by conduction if the temperature difference between Ihe air and the block is not large enough to overcome the resistance of air to movement and thus to initiate natural convection currents. 

In Chapters 7 and 8, we considered heat transfer by forced convection, where a fluid was forced to move over a surface or in a tube by external means such as a pump or a fan. In this chapter, we consider natural convection, where any fluid motion occurs by natural means such as buoyancy. The fluid motion in forced convection is quite noiicenhle, since a fan or a pump can transfer enough momentum to the fluid to move it in a certain direction. Tlie fluid motion in natural convection, however, is often not noticeable because of the low velocities involved. 

The magnitude of the natural convection heal transfer between a surface and a fluid is directly related to the flow rate of the fluid. The higher the flow rate, tbe higher the heat transfer rate. In fact, it is the very high flow rales that increase the heat transfer coefficient by orders of magnitude when forced convection is used. In natural convection, no blowers are used, and therefore the flow rale cannot be controlled externally. The flow rale in this case is established by the dynamic balance of buoyancy and friction. 

When a surface is subjected to external flow, the problem involves both natural and forced convection. The relative importance of each mode of heat transfer is determined by the value of the coefficient Gr /Ref Natural convection effects are negligible if GiJRel 1, free convection dominates and the forced convection effects are negligible if Gri/Re > 1, and both effects are significant atid must be considered if Grt/Re = 1. 

Heat is transferred from or to a region by the motion of fluids and the phenomenon of convection. In natural convection, the movement is caused by buoyancy forces induced by variations in the density of the fluid these variations are caused by differences in temperature. In forced convection, movement is created by an external agency such as a pump. 

Very often fluid motion near the solid is not only the result of the temperature gradient, but also the result of some outside forces. In such cases, heat transfer is by forced convection. Here, a pressure gradient appears as a result of an external force exerted by a pump, for example. Fluid mixing takes place as a result of the... 

Another distinction among flows is whether the flow is forced by an external means such as a pump (termed forced convection) or whether the flow arises as a result of a density difference developed in the fluid circuit as a result of the heat transfer (termed natural convection or thermosiphon action). Some cases include both mechanisms. 

The design of natural convection evaporators is difficult because a complex interrelationship between the liquid circulation rate due to density differences and heat transfer coefficients exists. The circulation flow rate depends on the amount of evaporated liquid and is not controlled by an external device as in forced circula-... 

The /th species mass flux, j, and the total heat flux, q, can be expressed in terms of transfer coefficients. This is useful in situations where the liquid or gas phase is not completely resolved, or when the flow conditions are not exactly known. Often, these transfer coefficients are determined experimentally for a particular flow situation. For instance, different expressions are used, depending on whether the transfer is due to pure conduction or whether it is dominated by ccaivection. Also, the type of convection plays a role, that is, if the convection is forced or non-forced. A forced convection has a non-zero relative velocity between droplet and environment, whereas for a non-forced convection, the relative drop-gas velocity is zero and only the Stefan flow dominates. Note that the natural convection due to gravity is taken to be zero since gravity is an external force, and external forces are neglected in this article. In addition, in forced convection, the nature of the flow, that is, whether the flow is laminar or turbulent, plays an important role. These issues will be discussed in more detail in the following subsections. 

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