Convection, forced

In this expression, the numerator corresponds to the driving force and the denominator to the resistance . The mass transfer coefficient, km, has the dimensions of length divided by time which is implied in the unit by which it is expressed. 

All physical quantities are measured in appropriate units. These units can be divided into two categories fundamental or primary units and derived or secondary units. The latter can be expressed in terms of the former. The choice of primary units is based largely on convenience and may be quite arbitrary commonly used primary units include those pertaining to length, mass, time and temperature. 

A number of important dimensionless groups have been arrived at by dimensional analysis and by other means. The numerical value of such a dimensionless group for a given case is independent of the units chosen for the primary quantities as long as consistent units are used within that group. The units used in one group need not be consistent with those used in another. 

It must be emphasized that dimensional analysis is used to find the minimum number of dimensionless groups of all the variables known to be relevant to the description of a 

A theorem known as Buckingham s it theorem is very pertinent in the context of dimensionless groups. According to this theorem the number of dimensionless groups is equal to the difference between the number of variables and the number of dimensions used to express them. Any physical equation can be expressed in the form 

TABLE 8.7. Individual Rim Resistances (1/h) Including Fouling Effects, with fi in Btu/(hr)(sqft)(°F) 

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Convection is the movement of a species due to external mechanical forces. This can be of two types natural convection, which arises from thennal gradients or density differences within the solution, and forced convection, which can take the fomi of gas bubbling, pumping or stirrmg. The fomier is undesirable and can occur m any solution... 

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. 

In the forced convection heat transfer, the heat-transfer coefficient, mainly depends on the fluid velocity because the contribution from natural convection is negligibly small. The dependence of the heat-transfer coefficient, on fluid velocity, which has been observed empirically (1—3), for laminar flow inside tubes, is h for turbulent flow inside tubes, h and for flow outside tubes, h. Flow may be classified as laminar or... 

Fig. 14. (a) Workpiece heated by natural convection (b) workpiece heated by forced convection (80). 

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... 

The mathematical formulation of forced convection heat transfer from fuel rods is well described in the Hterature. Notable are the Dittus-Boelter correlation (26,31) for pressurized water reactors (PWRs) and gases, and the Jens-Lottes correlation (32) for boiling water reactors (BWRs) in nucleate boiling. 

Fig. 30. Aluminum—air power cell system. The design provides for forced convection of air and electrolyte, heat rejection, electrolyte concentration control...

The convective gas flow around a burning particle affects its burning rate. It has been postulated that in the absence of convection, the burning rate is independent of pressure. Forced convection, on the other hand, is befleved to increase the burning rate. 

Tank Cells. A direct extension of laboratory beaker cells is represented in the use of plate electrodes immersed into a lined, rectangular tank, which may be fitted with a cover for gas collection or vapor control. The tank cell, which is usually undivided, is used in batch or semibatch operations. The tank cell has the attraction of being both simple to design and usually inexpensive. However, it is not the most suitable for large-scale operation or where forced convection is needed. Rotating cylinders or rotating disks have been used to overcome mass-transfer problems in tank cells. An example for electroorganic synthesis is available (46). 

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 flmd is entirely the result of differences in density resiilting from temperature differences in forced convection, the motion is produced by mechanical means. When the forced velocity is relatively low, it should be reahzed that Tree-convection factors, such as density and temperature difference, may have an important influence. 

L. Other objects, including prisms, cuhes, hemispheres, spheres, and cylinders forced convection... 

Typical coil coefficients are listed in Table 11-2. More exact values can be calculated by using the methods for natural convection or forced convection given elsewhere in this section. 

Hot side Cold side Natural convection Forced convection Natural convection Forced convection... 

Maximum shell-side heat-transfer rates in forced convection are apparently obtained by cross-flow of the flmd at right angles to the tubes. In order to maximize this type of flow some heat exchangers are built with segmental-cut baffles and with no tubes in the window (or the baffle cutout). Maximum baffle spacing may thus equal maximum unsupported-tube span, while conventional baffle spacing is hmited to one-h f of this span. 

Forced-convection pit furnaces are employed for heat-treating small metal parts in biuk. Small pieces are suspended in a mesh-bottom basket, while larger pieces are placed on racks. Air heating is by means of Nichrome electric coils set in refractory walls around the... 

Experimental gas-solid mass-transfer data have been obtained for naphthalene in CO9 to develop correlations for mass-transfer coefficients [Lim et al., Am. Chem. Soc. Symp. Ser, 406, 379 (1989)]. The data were correlated over a wide range of conditions with the following equation for combined natural and forced convection ... 

Destabilization of the arc plasma is achieved by forced convection of gas created by the movement of the main and arcing contacts through a puffer piston. This is an integral part of the moving main and arcing contacts (both being concentric). In the light of more advanced techniques of arc extinction now available, the manufacture of such breaker is now limited to about 145 kV. 

By forced convection The factors that can influence the temperature of the enclosure, installed outdoors are wind and snow, other than forced cooling. But their effect on actual cooling may be small. Sometimes this happens and sometimes not. It is better to ignore this effect when estimating various thermal effects. Natural convection and radiation will take account of this. 

Dry Running Pump Again, no lubrication or dissipation of hear. Remove the heat with a double seal and barrier tank with forced convective flow. 

Gases and Liquids Tending to Gas Gases cannot lubricate the seal fltees. No dissipation of heat. Use a dual seal with forced convective flow. 

Critoph, R.E., A forced convection regenerative cycle using the ammonia-carbon pair. In proceedings of Solid Sorption Refrigeration, Paris, HR, 1992, pp. 80 85. 

Critoph, R.E. and Thorpe, R.N., Momentum and heat transfer by forced convection in fixed beds of granular active carbon. Applied Thermal Engineering, 1996, 16,419 427. 

Thorpe, R.N., Heat transfer by forced convection in beds of granular adsorbent material for solid adsorption heat pumps. Ph.D. Thesis, University of Warwick, UK, 1996. 

Specification for dedicated liquefied petroleum gas appliances - Mobile and portable non-domestic forced convection direct fired air heaters. Superseded BS 4096 1967... 

Estimation of the heat transfer coefficients for forced convection of a fluid in pipes is usually based on empirical expressions. The most well known expression for this purpose is ... 

Cooling towers are broadly classified on the basis of the type of draft natural draft (natural convection), mechanical draft (forced convection) and mechanical and natural. Further distinction is made based on (1) the type of flow i.e. - crossflow, counterflow, cocurrent flow (2) the type of heat dissipation-wet (evaporative cooling), dry, wet-dry and (3) the type of application-industrial or power plant. Each of the major types of cooling towers has a distinct configuration. The major designs are summarized in Figures 1 through 8 and a brief description of each follows. 

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