Convected-mean

The description of mass transfer requires a separation of the contributions of convection and mutual diffusion. While convection means macroscopic motion of complete volume elements, mutual diffusion denotes the macroscopically perceptible relative motion of the individual particles due to concentration gradients. Hence, when measuring mutual diffusion coefficients, one has to avoid convection in the system or, at least has to take it into consideration. 

Any of various types of heat transfer equipment, whereby relatively cold water flowing over a surface will, by conduction and convection means, transfer heat away from a process. The most common types of heat exchangers are plate and frame and shell and tube designs. A boiler is also a type of heat exchanger. 

A first-order difference approximation for the axial derivative, 9(/0 z)/9j, is consistent with the first-order convergence of Euler s method. The convected-mean concentration is calculated from the dimensionless version of Equation (8.4) ... 

A second example of convective dissolution is the dissolution of a solid floor or roof. Forced convection means that the fluid is moving relative to the solid floor or roof such as magma convection in a magma chamber, or bottom current over ocean sediment. Free convection means that there is no bulk flow or convection, but the interface melt may be gravitationally unstable, leading to its rise or fall. 

The convected mean at position z is denoted by amix(z) and is found by multiplying the concentration on a streamline, a(r, z), by the volumetric flow rate associated with that streamline, dQ(r) = Vz(r)dAc, and by summing over all the streamlines. The result is the molar flow rate of component A. Dividing by the total volumetric flow, Q = uAc, gives the convected-mean concentration ... 

Assume that an industrial burner made of a tube of diameter D = 40 cm is internally exposed to a uniform heat flux q" = 24 kW/m2 resulting from combustion in the tube (Fig. 6P-2). The heat flux is to be transferred to the surrounding stagnant air at temperature Tx = 25 ° C by convective means. 

Bramley and Brewer [B19] and Martin and Kuhn [M4] proposed alternative thermal convective means of internal circulation. The system of internal scoops and bafBes shown in Fig. 14.10 was an important simplification introduced by Zippe [Z2]. 

Convection. Convection means, essentially, the mass transport of electroactive material to the electrode by gross physical movement— fluid or hydrodynamic flow— of the solution. Generally, fluid flow occurs because of natural convection (caused by density gradients) or forced convection (usually caused by stirring of some sort). 

It is important to stir the solution or rotate the electrode during the preconcentration stage. The purpose of this is to increase the analyte mass transport to the electrode by convective means, thereby enhancing preconcentration. In general, in electroanalysis one seeks to obtain proper conditions for difihision alone to permit mathematical expression of the process rate (the current). In this and controlled flow or rotation cases it is advantageous to purposely increase the quantity of material reaching the electrode surface. Pre-electrolysis times are typically 3 min or longer. 

In chemical processes, heat convection is much more important than heat conduction. Essentially, heat convection means that the heat transfer occurs between two phases, where at least one of them is a liquid or a gas. It is characterized by the fact that flows take place, where hot fluid flows into cold regions and vice versa. The convective heat transfer is therefore always related to flows, which can be generated by the temperature differences themselves (natural or free convection) or by a fluid flow engine, for example, a pump, a blower or a stirrer (forced convection). 

Hildebrand, P. H., Towery, N. G., Snell, M. R. (1979). Measurement of convective mean rainfall over small areas using high-density raingages and radar. Journal of Applied Meteorology, iS(lO), 1316-1326. 

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