Natural convection current

Thermal currents Natural convection currents set up in a fluid due to density differences. 

The scan rate, u = EIAt, plays a very important role in sweep voltannnetry as it defines the time scale of the experiment and is typically in the range 5 mV s to 100 V s for nonnal macroelectrodes, although sweep rates of 10 V s are possible with microelectrodes (see later). The short time scales in which the experiments are carried out are the cause for the prevalence of non-steady-state diflfiision and the peak-shaped response. Wlien the scan rate is slow enough to maintain steady-state diflfiision, the concentration profiles with time are linear within the Nemst diflfiision layer which is fixed by natural convection, and the current-potential response reaches a plateau steady-state current. On reducing the time scale, the diflfiision layer caimot relax to its equilibrium state, the diffusion layer is thiimer and hence the currents in the non-steady-state will be higher. 

Dehberate stirring can be imposed on conductors with a transverse rotating magnetic field or by passage of electric current axially with a transverse magnetic field. Conversely, a constant magnetic field with no current imposed greatly reduces natural convection. 

We have estimated the likely heat that may be generated by a particular size of conductor and enclosure for a certain current rating and then have counterchecked whether the conductor and the enclosure so chosen can dissipate this heat by radiation and natural convection, and reach a state of thermal stability within permissible limits or we may have to increase the size of the conductor... 

Air infiltration The uncontrolled air interchange through structural imperfections and other openings into a space, due to natural convection, rising currents, or wind forces over a building. 

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. 

In forced convection, circulating currents are produced by an external agency such as an agitator in a reaction vessel or as a result of turbulent flow in a pipe. In general, the magnitude of the circulation in forced convection is greater, and higher rates of heat transfer are obtained than in natural convection. 

If a beaker containing water rests on a hot plate, the water at the bottom of the beaker becomes hotter than that at the top. Since the density of the hot water is lower than that of the cold, the water in the bottom rises and heat is transferred by natural convection. In the same way air in contact with a hot plate will be heated by natural convection currents, the air near the surface being hotter and of lower density than that some distance away. In both of these cases there is no external agency providing forced convection currents, and the transfer of heat occurs at a correspondingly lower rate since the natural convection currents move rather slowly. 

Countercurrent electrophoresis can be nsed to split a mixtnre of mobile species into two fractions by the electrical analog of elntria-tion. In such countercurrent electrophoresis, sometimes termed an ion still, a flow of the suspending flnid is maintained parallel to the direction of the voltage gradient. Species which do not migrate fast enough in the applied electric field will be physically swept out of the apparatus. An apparatus based mainly on this principle bnt nsing also natural convection currents has been developed (Bier, Electrophoresis, vol. II, Academic, New York, 1967). 

Both the identification of a species and the determination of the kinetics of its formation or decay can be achieved with longer pathlength cells, such as that depicted in Figure 2.103. In kinetic experiments, however, there is the proviso that the experiment can be performed before natural convection currents interfere with the measurements i.e. the operator must be certain that the removal of a chromophore from the optical path is due to reaction and not due to convection currents. It should be noted that the strength of UV-visible spectroscopy does not lie primarily in the identification of unknown species as the information it provides is not of a molecularly specific nature. 

The deposition mechanism of CBD CdZnS thin films under the current stirring conditions are dominated by convection mode (stirring or hydrodynamic transport). In a solution, fluid flow occurs by a natural convection mode... 

Very fine particles, particularly in the sub-micron range (d < I p,m), are very readily affected by natural convection currents in the fluid, and great care must be taken in making measurements to ensure that temperature gradients are eliminated. 

Some small extent of natural convection is always inevitable as caused, e.g. by warming of the solution around the current-bearing electrodes. The extent of this convection can be assumed to be negligible by comparison with... 

If the pipe is isolated by valves, then the heating of the pipe will heat the fluid inside and set up natural convection currents. The heat transfer coefficient is then low, and depends on the fluid temperature, which increases as the closed-in volume is heated... 

When a fluid is heated, the hot less-dense fluid rises and is replaced by cold material, thus setting up a natural convection current. When the fluid is agitated by some external means, then forced convection takes place. It is normally considered that there is a stationary film of fluid adjacent to the wall and that heat transfer takes place through this film by conduction. Because the thermal conductivity of most liquids is low, the main resistance to the flow of heat is in the film. Conduction through this film is given by the usual relation (74), but the value of h is not simply a property of the fluid but depends on many factors such as the geometry of the system and the flow dynamics for example, with tubes there are significant differences between the inside and outside film coefficients. 

A major fallacy is made when observations obeying a known physical law are subjected to trend-oriented tests, but without allowing for a specific behaviour predicted by the law in certain sub-domains of the observation set. This can be seen in Table 11 where a partial set of classical cathode polarization data has been reconstructed from a current versus total polarization graph [28], If all data pairs were equally treated, rank distribution analysis would lead to an erroneous conclusion, inasmuch as the (admittedly short) limiting-current plateau for cupric ion discharge, albeit included in the data, would be ignored. Along this plateau, the independence of current from polarization potential follows directly from the theory of natural convection at a flat plate, with ample empirical support from electrochemical mass transport experiments. 

The time variation of 8 before the onset of natural convection depends on how the diffusion process is provoked. If a constant current density is switched on at t = 0, then the time variation of the effective diffusion-layer thickness can be obtained from Eqs. (7.179) and (7.202)... 

C. Wagner, J. Electrochem. Soc. 95 161 (1949). Calculation of current at vertical electrodes with natural convection. 

So, here is the summary of what we can do to help the experimenter be sure that his or her measurement reflects interfacial and not transport control.3 (1) Working at short times (microseconds up to a millisecond, say), increases iL and therefore lengthens the current density range in which diffusion-free measurements can be made. (2) Working at times > about 10 s means that natural convection tends to make 8 constant, i. e., independent of time. However, this time-independent value can still be reduced (and hence iL helpfully increased by methods already reviewed (Chapter 7),... 

In Section (8.6.5) it was argued that the value of the sweep rate must be less than that which would cause the capacitative charging current to be significant compared with the total current and greater than a value that in the potential range of the sweep would cause 8t to reach a value at which natural convection would cause the breakdown of the relation 8t = VjiDt. 

We can see from Table 6 and eqns. (219) and (220) that, besides the suppression of natural convection, other advantages of LSV at the RDE are weak dependence on the physical properties of the electrolyte and the simultaneous determination of peak and limiting current in a single experiment. 

The transfer of heat by convection is also an important component of the indirect cooling of the process. Natural convection currents result from localized heating/cooling effects and the tendency of hot fluids to rise above colder fluids, while forced convection, utilizing a pump, enables higher rates of heat transfer to occur (within the limits of the heat-exchanger design). 

By introduction of a typical value for D0, 10 r> cm2 s 1, it is seen that the value of 8 after, for example, 5 seconds amounts to 0.1 mm. At times larger than 10-20 seconds, natural convection begins to interfere and the assumption of linear diffusion as the only means of mass transport is no longer strictly valid. At times larger than approximately 1 minute, the deviations from pure diffusion are so serious and unpredictable that the current observed experimentally cannot be related to a practical theoretical model. 

The studied BCAP0350 DLC has a D-cell battery shape factor which is defined in the standard with a 33 mm outside diameter and a 61.5mm length. The total external surface is about 80cm2. The production of losses inside the DLC is assumed to be uniform in the volume. In the case of a 30 A charge/discharge current the dissipated power is equal to 2.88 W. The measurements have been performed at room temperature 7 = 20°C which was constant during the experiment. The DLC is only cooled with a slowly moving airflow due to the natural convection. 

Natural convection generally has a negative effect on separation its countercurrent motion leads to the disruption and smearing of component zones. Electrophoresis is particularly subject to such convective flow because the passage of electrical current generates heat that in turn sets up temperature gradients in the electrophoretic medium. 

Large t. The spherical term dominates, which represents a steady-state current. However, due to the effects of natural convection this steady state is never reached at conventionally-sized electrodes. The smaller the electrode radius, the faster the steady state is achieved. It is possible to achieve a steady state at microelectrodes. These are described further in Section 5.5. 

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