Transfer by Convection

According to the method used to calculate heat transfer by convection, the convective mass transfer under steady state conditions is 

(1-158) describes generally the transition of a substance from within the bulk fluid to a phase boundary, or from a phase boundary into the bulk fluid (for the latter case, Eq. (1-158) must be appropriately modified). The concentration of the component in the kernel c,j is assumed to be constant throughout the fluid. A concentration gradient Ac, occurs only in the vicinity of the phase boundary (see Fig. 1-44). Furthermore, it is assumed that a laminar current exists in the boundary layer and that 

From this equation, the dependency of the mass transfer coefficient yff,- on the diffusion coefficient D, and the boundary layer thickness d of the fluid flow, may be seen. The laminar boundary layer and turbulent bulk cannot be distinguished exactly, due to the continous transition the boundary layer thickness 3 is, therefore, a formal complementary variable. 

The mass transfer coefficient P depends upon fluid mechanics (free flow, forced flow), the physical characteristics of the exchanging matter system and the properties of the substance in the fluid phase, and may 

Analogous variable Heat transfer Mass transfer 

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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... 

The heat is transferred by convection and conduction from the hot to the colder fluid through an infinitesimal surface area dA. The temperature of the hot fluid reduces by an amount dTf, and the temperature of the cold fluid increases bv an amount dT.. 

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. 

The products of the catalytic reaction are transferred by convection and diffusion to the bulk liquid phase. 

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. 

Nutrient Losses Associated With Biomass Burning. Nutrient losses associated with slash fires occur through volatilization and convective losses of ash. Elements with low temperatures of volatilization (e.g. N, K, S, and some organic forms of P) will be lost in the highest quantities (Table III) (57). Conversely, Ca and Mg have volatilization temperatures higher than that recorded during most vegetation fires. Almost all fire-induced losses of these elements are due to particulate transfer by convective processes. 

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. 

Another problem related to the validity of equation 9.9 is that equation 9.6 applies only to heat conduction. If T — 12 is large, some significant fraction of heat will be transferred by convection and radiation and thus will not be monitored by the thermopile. Consequently, the use of partial compensating Peltier or Joule effects was essential in the experiments involving Calvet s calorimeter, whose thermopiles had a fairly low thermal conductivity. 

For fires in open areas, heat is transferred by convection and rises away from the fire. However, for fires in building and other enclosed areas, the heat is transported through the building and collects on ceilings (NFPA, 1997). 

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. 

The mass transfer by convection and diffusion within the channel is precisely calculable. Numerical methods are used to fit measured concentrations at the detector electrode to the fluid flow in the cell, and to the reaction kinetics at the surface and in solution [24]. The technique was extensively developed during a study of the dissolution of calcite in dilute aqueous acid [25], and has latterly been applied to a number of organic reactions. 

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... 

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