Convection currents, effect

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... 

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). 

Right at the disc the convection current perpendicular to the surface vanishes. The transport to the surface is effected by diffusion so the particle current density jp of any species with concentration c and... 

Sedimentation analyses must be carried out at concentrations which are sufficiently low for interactive effects between particles to be negligible so that their terminal falling velocities can be taken as equal to those of isolated particles. Careful temperature control (preferably to 0.1 deg K) is necessary to suppress convection currents. The lower limit of particle size is set by the increasing importance of Brownian motion for progressively smaller particles. It is possible however, to replace gravitational forces by centrifugal forces and this reduces the lower size limit to about 0.05 p,m. 

Under submerged conditions, temperatures in the soil and water depend on the depth of the water and on the density of the plant canopy, as well as on meteorological conditions. The water transmits incident short-wave radiation to the soil but it also insulates the soil against emission of long wave radiation. The full plant canopy transmits 90 % of the short-wave infrared radiation (i.e. half the total short-wave). Hence there is a greenhouse effect and consequently the soil and water temperatures tend to be higher than the air temperature. Evaporative cooling reduces the surface water temperature and drives convection currents, so the water tends to be well mixed. 

Other factors do intervene. Significant solar heating of the soil surface, so that the soil becomes warmer than the air, causes vertical thermal convection currents to develop within the boundary layers. This introduces turbulence or instability that acts to move the chemical signature up into the free air. When the molecules are moved into the free flow of the air, the effect is to reduce the concentration by dilution. Conversely, when the soil surface is cooler than the air, thermal convection is inhibited, with the result that the molecules are effectively trapped in the boundary layer. This effect is strengthened by the cooling of the air adjacent to the surface, which increases its viscosity. Higher viscosity lowers the Reynold s number, thus decreasing boundary layer thickness. 

The number of thermocouples positioned within the container will be dependent on the container volume. A sufficient number of thermocouples should be positioned in areas representing the upper, middle, and lower portions of the container. Error in cold spot determinations may be introduced by employing an excessive number of thermocouples within the container. The error may be attributed to thermocouple mass and the resulting baffling effects may influence the normal convection currents of the liquid. 

The dye should become dispersed uniformly within the hot water first. The higher the temperature, the greater the average kinetic energy of the molecules. Because the molecules within the hot water are moving faster, they have a quicker effect on the dye of the Kool-aid crystals. Furthermore, the hot water will tend to have more convection currents that will also help to distribute the dye throughout the water. 

The basic construction of a horizontal thermal diffusion cell is sketched in Figure 19.14(a). When gases are to be separated, the distance between the plates can be several mm for liquids it is a fraction of a mm. The separation effects of thermal diffusion and convection currents are superimposed in the equipment of Figure 19.14(b), which is called a thermogravitational or Clusius-Dickel column after the inventors in 1938. A commercially available column used for analytical purposes is in Figure 19.14(c). Several such columns in series are needed for a high degree of separation. 

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). 

An ultracentrifuge is a high-speed centrifuge equipped with a suitable optical system (usually schlieren or interference optics, the latter being particularly useful when low concentrations are involved) for recording sedimentation behaviour and with facilities for eliminating the disturbing effects of convection currents and vibration. The sample is contained in a sector-shaped cell mounted in a rotor (usually c. 

The effect of surface films on the evaporation of ether from its aqueous solutions was very marked, but was traced to the film stopping the convection currents, which usually stir up the layers near the surface and eliminate the slow process of diffusion of ether through the water to the surface. There was no evidence of any considerable resistance, offered by the film itself, to the passage of ether molecules. 

Coke microstructure is determined by the conditions in the coalesced mesophase prior to solidification. It is determined by the combined effects of convection currents, bubble percolation, and imposed shear stresses, all of which tend to deform the mesophase. 

Vertical Propagation of Flame.4—In tins case the effect of convection currents becomes increasingly pronounced. The tube in which the flame travels becomes a chimney, and, with bottom ignition, the speed of the flame is enhanced by that of the draught. With top ignition the flame tends to bum at the mouth of the tube until all the combustible mixture has risen to the top. 

Thermophoretic forces produce very obvious effects near areas of significant temperature gradients. For instance, one can often observe a black deposit on the wall just above a hot-water radiator or pipe. Convection currents conduct the warm gas and particles over the radiator, but since the cooler surfaces nearer the radiator are not protected by a dust-free space, deposition takes place. On a ceiling or on walls of rooms heated by convection, one can often see a replica of the construction behind the plaster formed by deposited particles. Again, the dust is deposited on the cooler portions of the surface on spaces between the laths if the laths are poor heat conductors and directly opposite the laths if they are good conductors. In a room that is heated by direct radiation, such as by an open fire, the walls and furniture of the room are warmer than the air, so that particles suspended in the air are not deposited by thermal forces (Lodge, 1883 Gibbs, 1924). 

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