Convection spontaneous

If the acceleration is positive, our parcel is buoyant and spontaneous convection occurs. The atmospheric layer is said to be unstable. Negative acceleration implies that a small displacement, Az, results in the parcel accelerating back toward its initial position and therefore indicates stability. If dP /dz is that for an adiabatic test parcel dT /dz = —gM/Cp and dl/dz that of the existing layer, then for dP/dz > —9.8 K/km is stable and for d7/dz < — 9.8K/km is unstable. The 9.8 K/km figure then provides a simple benchmark for static stability of dry air. 

In electrochemical cells we often find convective transport of reaction components toward (or away from) the electrode surface. In this case the balance equation describing the supply and escape of the components should be written in the general form (1.38). However, this equation needs further explanation. At any current density during current flow, the migration and diffusion fluxes (or field strength and concentration gradients) will spontaneously settle at values such that condition (4.14) is satisfied. The convective flux, on the other hand, depends on the arbitrary values selected for the flow velocity v and for the component concentrations (i.e., is determined by factors independent of the values selected for the current density). Hence, in the balance equation (1.38), it is not the total convective flux that should appear, only the part that corresponds to the true consumption of reactants from the flux or true product release into the flux. This fraction is defined as tfie difference between the fluxes away from and to the electrode ... 

This discrepancy arises primarily from the fact that spontaneous liquid flows will always develop in any hquid even without artificial stirring (e.g., under the action of density gradients caused by local temperature or concentration fluctuations). This phenomenon has been termed natural convection. Electrochemical reactions reinforce natural convection, since the concentrations of substances involved in the reaction will change near the electrode surfaces, and also since heat is evolved. Gas evolution attending the reactions has a particularly strong effect on naturaf convection. 

Many of the electrochemical techniques described in this book fulfill all of these criteria. By using an external potential to drive a charge transfer process (electron or ion transfer), mass transport (typically by diffusion) is well-defined and calculable, and the current provides a direct measurement of the interfacial reaction rate [8]. However, there is a whole class of spontaneous reactions, which do not involve net interfacial charge transfer, where these criteria are more difficult to implement. For this type of process, hydro-dynamic techniques become important, where mass transport is controlled by convection as well as diffusion. 

Convection The transport of heat as a result of the physical movement of a carrier, such as air, water, or magma. Convection occurs spontaneously due to density differences. 

Consider the combustion of a coal pile of diameter D and height H. Before ignition the air flow is driven by wind, and after ignition by natural convection. The heat of combustion of carbon is -94 kcal/mole. Set up a qualitative energy balance (no numbers) to explain the dependence of combustion on weather and on D and H. What size and shape of coal piles do you recommend to minimize spontaneous combustion ... 

The importance of good housekeeping must be emphasized. Any accumulation of Zr dust or powder on work benches, floors or clothes of personnel must be removed promptly. The pyrophoric properties of Zr dust or powder requires the selection of either wet or dry methods of dust collection A limited amount of moisture can make Zr powder ignition sensitive, but attempts to collect the dust dry by suction or convection flow risk dust expins which are easily initiated by static discharge or spontaneous combustion. Because nitrogen will also react with Zr,... 

A more unstable form of convection occurs wrhen a denser fluid lies above a less dense fluid, corresponding to a situation where ihe medium is healed from below. IT v iscous forces outweigh the buoyant forces within ihe medium, this unstable condition can be maintained. If the buoyancy is greater than the viscosity, however, the volume element rises loo quickly, resulting in spontaneous flow-, which lakes the form of cells or voiles rolls. 

With convective heating of wood under laboratory conditions, spontaneous ignition is reported as low as 270°C and as high as 470°C (5,14,15). Spontaneous ignition of wood charcoal, which has excellent absorption of oxygen and radiant heat, occurs between 150°C and 250°C (5). In one experiment on ignition, oven-dried sticks of nine different species were ignited by pilot flame in 14.3 to 40 minutes when held at 180°C, in 4 to 9.5 minutes when held at 250°C and in 0.3 to 0.5 minutes when held at 430°C (16). 

Convective diffusion — The electrochemical - mass transport controlled by both -> convection and - diffusion is called a process by convective diffusion [i]. Convection is caused by externally controlled force or spontaneous force. Convective diffusion has been conventionally used in a strict sense for well-controlled flow such as for -> rotating disk electrodes [ii], - channel elec-... 

Systems that exchange entropy with their surroundings may undergo spontaneous transformation to dissipative structures and self-organization. The forces that exist in irreversible processes create these organized states, which range from convection patterns of Benard cells to biological cycles. 

One of the best-known physical ordering phenomena is the Benard cells, which occur during the heating a fluid held between two parallel horizontal plates separated by a small distance. The lower plate is heated, and the temperature is controlled. The upper plate is kept at a constant temperature. When the temperature difference between the two plates reaches a certain critical value, the elevating effect of expansion predominates, and the fluid starts to move in a structured way the fluid is divided into horizontal cylindrical convection cells, in which the fluid rotates in a vertical plane. At the lower hot plate, the hot fluid rises later, it is cooled at the upper plate, and its density increases again this induces a movement downward, as seen in Figure 13.2. The Benard cells are one of the best-known physical examples of spontaneous structurization as a result of sufficient distance from equilibrium, which is the large temperature difference between the plates. The critical temperature difference ( A 7 )c can be determined from the... 

The technique introduced by Citti et al. and then by Rotenberg used a vertical electrode heated by a laser beam or by an infrared diode. In this case, the same order of magnitude is obtained for the free convection and the thermal convection, and experiments can be performed with a sufficient accuracy. A fast redox reaction at a heated vertical electrode is considered. The motion of the solution is spontaneous and arises due to forces originating from heterogeneous reactions. Such forces result from density variation in the solution produced by both thermal and concentration gradients near the electrode. 

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