Heat transfers By convection

So far, we have not dealt with the mechanism by which the heat is transferred from the surface of the fuel element to the coolant. For 

Because of viscosity effects, the velocity of the fluid is zero at the fuel surface. The velocity vectors of the fluid layers increase as one moves further away from the surface, as indicated in Fig. 6.7. Since the fluid layer in contact with the fuel is at rest, heat actually flows by conduction from the element to this layer, and the rate of heat flow will depend on the temperature gradient of the fluid at the surface. This in turn, however, depends on the rate at which heat is being carried away by the moving layers of the coolant, and so is a function of the coolant flow rate. The equation which gives the heat flux from the fuel plate to the coolant is 

By analogy with the terms in the denominator of equations (6.25) and (6.38), the term 1/hA represents the thermal resistance for convective heat transfer from the cladding of the fuel to the bulk coolant. Equation (6.25), for example, may then be extended to cover the whole heat transfer from the plate-type fuel element in terms of the difference in temperature between the centers of the fuel and the bulk coolant, yielding for the rate of heat flow out of each face the expression 

Equation (6.44) also applies to the cylindrical element, with the total thermal resistance given by (see equation 6.39) 

4nkfH 2%k,H 2nhH(R + c) where the surface area of the cladding has been taken as 2tiH (R + c). 

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Commercial dryers differ fundamentally by the methods of heat transfer employed (see classification of diyers, Fig. 12-45). These industrial-diyer operations may utihze heat transfer by convection, conduction, radiation, or a combination of these. In each case, however, heat must flow to the outer surface and then into the interior of the solid. The single exception is dielectric and microwave diying, in which high-frequency electricity generates heat internally and produces a high temperature within the material and on its surface. 

When heat is transferred to a wet sohd by convection to hot surfaces and heat transfer by convection is negligible, the solids approach the boiling-point temperature rather than the wet-bulb temperature. This method of heat transfer is utilized in indirect diyers (see classification... 

To reduce heat transfer by convection an insulant should have a structure of a cellular nature or with a high void content. Small cells or voids inhibit convection within them and are thus less prone to excite or agitate neighboring cells. 

Convection. Heat transfer by convection arises from the mixing of elements of fluid. If this mixing occurs as a result of density differences as, for example, when a pool of liquid is heated from below, the process is known as natural convection. If the mixing results from eddy movement in the fluid, for example when a fluid flows through a pipe heated on the outside, it is called forced convection. It is important to note that convection requires mixing of fluid elements, and is not governed by temperature difference alone as is the case in conduction and radiation. 

HEAT TRANSFER BY CONVECTION 9.4.1. Natural and forced convection... 

Heat transfer by convection occurs as a result of the movement of fluid on a macroscopic scale in the form of eddies or circulating currents. If the currents arise from the heat transfer process itself, natural convection occurs, such as in the heating of a vessel containing liquid by means of a heat source situated beneath it. The liquid at the bottom of the vessel becomes heated and expands and rises because its density has become less than that of the remaining liquid. Cold liquid of higher density takes its place and a circulating current is thus set up. 

Both h and ho are dependent on the equivalent gas film thickness, and thus any decrease in the thickness, as a result of increasing the gas velocity for example, increases both k and ho. At normal temperatures, (h/ho) is virtually independent of the gas velocity provided this is greater than about 5 m/s. Under these conditions, heat transfer by convection from the gas stream is large compared with that from the surroundings by radiation and conduction. 

Determination of the wet-bulb temperature. Equation 13.8 gives the humidity of a gas in terms of its temperature, its wet-bulb temperature, and various physical properties of the gas and vapour. The wet-bulb temperature is normally determined as the temperature attained by the bulb of a thermometer which is covered with a piece of material which is maintained saturated with the liquid. The gas should be passed over the surface of the wet bulb at a high enough velocity (>5 m/s) (a) for the condition of the gas stream not to be affected appreciably by the evaporation of liquid, (b) for the heat transfer by convection to be large compared with that by radiation and conduction from the surroundings, and... 

Under what conditions is heat transfer by convection important in Chemical Engineering ... 

Air flows at 12 m/s through a pipe of inside diameter 25 mm. The rate of heat transfer by convection between the pipe and the air is 60 W/m2K. Neglecting the effects of temperature variation, estimate the pressure drop per me Ire length of pipe. 

Let a simple case of heat transfer by convection be considered. Let it be assumed that the surface of a solid at temperature is in contact with a flowing fluid at temperature T2 (Tx > T2) as indicated in Figure 3.17. Let dQJdt represent the rate ofheat transfer by convection from the solid over a finite area of solid-fluid contact, S. In such a situation it is found in many cases that dQJdt is proportional to S (Tx - T2), so that... 

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. 

The Frank-Kamenetskii theory has been extended to allow for reactant consumption [16—18], other geometries [19, 20] and heat transfer by convection as well as conduction, this being particularly important in gaseous systems when the Rayleigh number Ra > 600 [21]. ... 

A. Ma ek, ChemRevs 62, 44-47 (1962) (Thermal decomposition of explosives including a thermal explosion theory) 14) A.M. Grishin O.M. Todes, DoklAkadN 151(2), 366-68 (1963) CA 59, 12585(1963) (Thermal explosion with heat transfer by convection and conduction) 15) P.G. Ashmore T.A.B. Wesley, "A Test of Thermal-Ignition Theory in Autocatalytic Reactions , lOthSympCombstn (1965), pp 217-226... 

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. 

Radiation is the main mode of heat transfer heat transfer by convection is negligible. 

For convection along a vertical plate, Ra > 109 indicates that turbulent flow is established and heat transfer by convection dominates. For smaller values of the Rayleigh number, Ra < 104, the flow is laminar and conduction dominates. Thus, the Rayleigh number discriminates between conduction and convection [2]. 

Consider the charge transfer between two spherical particles of diameters dpi and dp2. A direct analogy between the charge transfer by collisions and the heat transfer by convection appears to be in order. Thus, the current density through the contact area of these two... 

Fluid flow is often turbulent, and so heat transfer by convection is often complex and normally we have to resort to correlations of experimental data. Dimensional analysis will give us insight into the pertinent dimensionless groups see Chapter 6, Scale-Up in Chemical Engineering, Section 6.7.4. 

This example, taken from [12], belongs to the field of heat transfer by convection. Here the heat transfer coefficient, h, represents the target quantity. This quantity can be determined only via the general heat transfer equation... 

Heat transfer by convection may be added to the heat transfer by radiation. So the total heat transfer from a body 1 to the surrounding fluid f is the sum of the convective heat transfer and the radiative heat transfer between body 1 and body 2, say. The convective heat transfer from body 1 to the fluid f is... 

Problem For a body that is being considered, the convection heat transfer coefficient to the adjacent air is 33 W/(m2.°C), and the radiative heat transfer coefficient from this body to another body is approximately 36 W/(m2.°C). If the temperature of the first body is 188° C, that of the adjacent air is 22°C, determine the temperature of the second body so that the heat transferred by convection is equal in magnitude to the heat transferred by radiation. The area ratio Aconv Arad is 1 1.2. 

What happens in a flameproof enclosure with a short-circuit arc inside First of all, the power and energy converted in an arc into radiation, heat transfer by convection and melting or vaporizing heat for metals shall be calculated. 

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. 

The following equation is used as a basis for evaluating rates of heat transfer by convection ... 

Example 1 Combined heat transfer by convection and radiation. The OD of an... 

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