Channel flow, natural convection

Convection That form of mass transport in which the solution containing electroanalyte is moved natural convection occurs predominantly by heating of solution, while forced convection occurs by careful and deliberate movement of the solution, e.g. at a rotated disc electrode or by the controlled flow of analyte solution over a channel electrode. 

The reduction of the thickness of the flow channel, as discussed earlier, is equivalent to introducing more surface area per unit volume of medium. High surface areas inhibit all flow, including natural convective flow. One can increase relative surface areas by going to thinner tubes or channels, or by using a fine granular or porous support medium. Both approaches are used in electrophoresis as discussed in a subsequent chapter. 

In order to. illustrate how natural convection in a vertical channel can be analyzed, attention will be given to flow through a wide rectangular channel, i.e., to laminar, two-dimensional flow in a plane channel as shown in Fig. 8.15. This type of flow is a good model of a number of flows of practical importance. 

As previously discussed, there are two limiting cases for natural convective flow through a vertical channel. One of these occurs when /W is large and the Rayleigh number is low. Under these circumstances all the fluid will be heated to very near the wall temperature within a relatively short distance up the channel and a type of fully developed flow will exist in which the velocity profile is not changing with Z and in which the dimensionless cross-stream velocity component, V, is essentially zero, i.e., in this limiting solution ... 

Some of the more commonly used methods of obtaining solutions to problems involving natural convective flow have been discussed in this chapter. Attention has been given to laminar natural convective flows over the outside of bodies, to laminar natural convection through vertical open-ended channels, to laminar natural convection in a rectangular enclosure, and to turbulent natural convective boundary layer flow. Solutions to the boundary layer forms of the governing equations and to the full governing equations have been discussed. 

Air flows by natural convection through the channel formed between 2 20-cm high plates kept at a temperature of 50°C. If the distance between the 2 plates is 3 cm and if the ambient air temperature is 20°C, find the rate of heat transfer from the 2 plates to the air and the mean velocity of the air through the channel. 

Consider the natural convective flow of air at 10°C though a plane vertical channel with isothermal walls whose temperature is 40°C and whose height is 10 cm. Determine how the mean heat transfer rate from the heated walls varies with the gap between the walls. 

Sparrow. E.M. Chrysler. G.M., and Azevedo. L.F.. Observed Flow Reversals and Measured-Predicted Nusselt Numbers for Natural Convection in a One-Sided Heated Vertical Channel . J. Heat Transfer Vol. 106, No. 2. pp. 325-332. 1984. 

Heat transfer to a laminar flow in an annulus is complicated by the fact that both the velocity and thermal profiles are simultaneously developing near the entrance and, often, over the length of the heated channel. Natural convection may also be a factor. It is usually conservative (i.e., predicted heat-transfer coefficients are lower than those experienced) to use equations for the fully developed flow. 

W. W. Humphreys and J. R. Welty, Natural Convection With Mercury in a Uniformly Heated Vertical Channel During Unstable Laminar and Transitional Flow, AIChE Journal (21/2) 268-274, 1975. 

In the operation of PSDRS, core decay heat is transferred to the containment vessel, as shown in Fig 7 and the heat from the contamment vessel is dissipated to the air flow which is generated by the natural circulation from the density difference between the mr channel and the environment The heat dissipation to the air flow is made of two paths One is the direct convection heat transfer from the containment wall surfece and the other is an mdirect path to the air In the indirect path, heat is first transported from the containment vessel surface by radiation to the air separator which separates the hot air from the mcommg cold air Then the heat is dissipated to the air flow by convection... 

Electrochemical systems where the mass transport of chemical species is due to diffusion and electromigration were studied in previous chapters. In the present chapter, we are going to consider the occurrence of the third mechanism of mass transfer in solution convection. Although the modelling of natural convection has experienced some progress in recent years [1], this is usually avoided in electrochemical measurements. On the other hand, convection applied by an external source forced convection) is employed in valuable and popular electrochemical methods in order to enhance the mass transport of species towards the electrode surface. Some of these hydrodynamic methods are based on electrodes that move with respect to the electroljAic solution, as with rotating electrodes [2], whereas in other hydrodynamic systems the electrolytic solution flows over a static electrode, as in waU-jet [3] and channel electrodes [4]. 

Convection refers to fhe fransport of the reactant or product species by bulk fluid motion driven by natural or applied mechanical forces. The natural convection limitations are due to convective transport caused by differences in densities as a result of temperature or concentration. The species transport to the interface can also be limited by fhe fuel cell flow sfrucfures and fheir conditions. For example, in PEMFC, blockage of flow channels or pore structures in diffusion or elecfrode-cafalysf layers owing to the liquid phase can restrict the supply of fhe reactant to the interface. Accumulation of inert gases that do not participate in chemical reaction will limit the partial pressure of the reactant at the interface. This results to decreased reactions at the interface. The accumulation of chemical impurities at the reaction sites will prevent adsorption of desired reactant species. For example, in PEMFC, the presence of carbon monoxide degrades the platinum catalyst because the platinum preferentially adsorbs carbon monoxide, leaving few reaction sites for hydrogen adsorption and oxidation. This leads to high anodic overpotential. 

Figure 6.1.3. Bulk flow due to natural convection in a closed rectangular channel with two vertical plates at different temperatures.

Chatoorgoon, V., 2001. StabiUty of supercritical fluid flow in a single channel natural-convection loop. International Journal of Heat and Mass Transfer 44, 1963—1972. 

The conclusion to be drawn from the above examples and many others is that softness in a boiling system, preceding the boiling channel inlet, may cause flow oscillations of low frequency. It is probably the pressure perturbations arising from the explosive nature of nucleate boiling that initiates the oscillation, and the reduced burn-out flux which follows probably corresponds to the trough of the flow oscillation, as a reduction in flow rate always drops the burn-out flux in forced-convection boiling. 

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