Current convection

Although it is hard to draw a sharp distinction, emulsions and foams are somewhat different from systems normally referred to as colloidal. Thus, whereas ordinary cream is an oil-in-water emulsion, the very fine aqueous suspension of oil droplets that results from the condensation of oily steam is essentially colloidal and is called an oil hydrosol. In this case the oil occupies only a small fraction of the volume of the system, and the particles of oil are small enough that their natural sedimentation rate is so slow that even small thermal convection currents suffice to keep them suspended for a cream, on the other hand, as also is the case for foams, the inner phase constitutes a sizable fraction of the total volume, and the system consists of a network of interfaces that are prevented from collapsing or coalescing by virtue of adsorbed films or electrical repulsions. 

Although molecular diffusion itself is very slow, its effect is nearly always enhanced by turbulent eddies and convection currents. These provide almost perfect mixing in the bulk of each Hquid phase, but the effect is damped out in the vicinity of the interface. Thus the concentration profiles at each... 

Fig. 13. (a) Convection currents around electrically heated furnace tube (b) furnace construction to minimise convection (78). 

The life persistency of a smoke cloud is deterrnined chiefly by wind and convection currents in the air. Ambient temperature also plays a part in the continuance or disappearance of fog oil smokes. Water vapor in the air has an important role in the formation of most chemically generated smokes, and high relative humidity improves the performance of these smokes. The water vapor not only exerts effects through hydrolysis, but it also assists the growth of hygroscopic (deliquescent) smoke particles to an effective size by a process of hydration. Smoke may be generated by mechanical, thermal, or chemical means, or by a combination of these processes (7). 

The movement of these plates can ies with it continents, ocean basins and mountains. Scientists believe that convection currents are generated as a result of great heat within the earth, as illustrated in Figure 14. lO. Below the crust, the hot rocks and metal in liquid form rise to the crust, cool and sink into the mantle causing a turbulence through heat convection. The hot rocks become hardened at the surface of the mantle and push the crust which is part of the hug plates that are afloat the mantle. This movement of plates can cause the following ... 

In incompressible fluids, such as water, the vertical structure of temperature very simply reveals the stability of the fluid. When the lower layer is warmer and thus less dense than the upper layer, the fluid is unstable and convective currents will cause it to overturn. When the lower layer is cooler than the upper layer, the fluid is stable and vertical exchange is minimal. However, because air is compressible, the determination of stability is somewhat more complicated. The temperature and density of the atmosphere normally decrease with elevation density is also affected by moisture in the air. 

Free convection currents generated by air heating or cooling by surfaces (process equipment, external walls)... 

Figure 7.73 shows a plume in an open environment. The hot air from the source entrains ambient air into the convection current (the plume), thus making the air volume flow increase with height. 

Turbulent exchange between air in different zones due to energy-introduced by supply air jets, convective currents, or moving objects. In this case the resulting mass transferred between the zones equals 0 (Fig. 7.108d). 

Equipment and supplies should be placed in the enclosure before a contaminant-generating procedure commences. Unnecessary equipment should be removed. High-input heat sources within an enclosure will cause convection currents that can disturb the flow and should be avoided, if possible, or accounted for with correct placement of baffles and/or exhaust connections. 

Equipment and Processes Equipment and supplies should be placed in the cupboard before a procedure commences. Unnecessary equipment should be removed. High-input heat sources within a cupboard will cause convection currents that can disturb the flow and should be avoided if possible. Work should be carried out well within the cupboard, at least 150 mm from the plane of the sash whenever possible. It should not however be placed closer than 50 mm to the lower extract slot of a back baffle. Large pieces of equipment should if possible be raised 25 to 50 mm above the working surface of the cupboard to improve the flow in the cupboard. 

D. j. Nevrala, S. D. Probert. Modelling convective currents in rooms by means of wall jets. Building Services Engineering Research Technology, 2(1), 1981. 

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

A hot-water heating system forces water into pipes, or arrangements of pipes called registers that warm from contact with warm water. Air in the room warms from contact with the pipes. Usually, the pipes are on the floor of a room so that warmer, less dense air around the pipes rises somewhat like a helium-filled balloon rises in air. The warmer air cools as it mixes with cooler air near the ceiling and falls as its density increases. This process is called convection and the moving air is referred to as convection current. The process of convection described here is pipe-to-air and usually does a better job of heating evenly than in an air-to-air convection system—the circulation of air by fans as in a forced-air heating system. 

Oxygen from the atmosphere, dissolved in the electrolyte solution provides the cathode reactant in the corrosion process. Since the electrolyte solution is in the form of thin films or droplets, diffusion of oxygen from the atmosphere/electrolyte solution interface to the solution/metal interface is rapid. Moreover, convection currents within these thin films of solution may play a part in further decreasing concentration polarisation of this cathodic process . Oxygen may also oxidise soluble corrosion products to less soluble ones which form more or less protective barriers to further corrosion, e.g. the oxidation of ferrous species to the less soluble ferric forms in the rusting of iron and steel. 

Since thermogravimetry is a dynamic technique, convection currents arising in a furnace will cause a continuous change in the gas atmosphere. The exact nature of this change further depends upon the furnace characteristics so that widely differing thermogravimetric data may be obtained from different designs of thermobalance. 

A mechanism of heat transfer in which heat energy is transmitted by convection current motion through gases and liquids. Part of the heat-transfer process in a boiler is by convection whereby the circulation of water carries heat from the tube near the fire to the drum and surrounding areas. 

The low conductivity of heat insulating materials, such as cork, glass wool, and so on, is largely accounted for by their high proportion of air space. The flow of heat through such materials is governed mainly by the resistance of the air spaces, which should be sufficiently small for convection currents to be suppressed. 

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. 

A similar form of analysis has been given by Kra0SS0LD 44) for air between two concentric cylinders. It is important to note from this general analysis that a single layer of air will not be a good insulator because convection currents set in before it becomes 25 mm thick. The good insulating properties of porous materials are attributable to the... 

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