Convection cell size

These observations consummated in a growth model that confers on the millions of aligned zone 1 nanotubes the role of field emitters, a role they play so effectively that they are the dominant source of electron injection into the plasma. In response, the plasma structure, in which current flow becomes concentrated above zone 1, enhances and sustains the growth of the field emission source —that is, zone 1 nanotubes. A convection cell is set up in order to allow the inert helium gas, which is swept down by collisions with carbon ions toward zone 1, to return to the plasma. The helium flow carries unreacted carbon feedstock out of zone 1, where it can add to the growing zone 2 nanotubes. In the model, it is the size and spacing of these convection cells in the plasma that determine the spacing of the zone 1 columns in a hexagonal lattice. 

With very low foam densities Ap is negligible, and A is mainly governed by Ag and by Ar (transfer by radiation plays an increasing role with decrease in density). Ac is, mostly, small, but may become of importance with larger cell size within the cells convection may play a role, which increases the heat transport of the gas in the cells considerably. With increasing foam density the contribution of Ap increases. In total, we find a minimum in the A-d curve, (Figure 8.6), the level of which is largely... 

All other conditions being the same (chemical composition, volumetric weight, closeness of cells), the cell size can considerably affect the properties of foamed plastics. For instance, the thermal conductivity coefficient of foams always increases when the cell size increases due to the growing number of radiation and convection paths of heat transfer (for more information see Sect. 12). An increase of the... 

However, since most of the plastic foams now produced commercially have a cell size below this value, the contribution of the convective heat exchange to the total heat... 

Four principal patterns of convection were distinguished when pure liquids were employed cells, streamers, ribs, and vermiculated rolls. These names were chosen in an attempt to describe the actual appearance of the convection patterns and in accordance with historical designations. Examples are shown in Fig. 21. The patterns depicted there were exhibited in all of the liquids under various conditions. In particular, cells appeared to be the dominant patterns in all liquids for depths of 2 mm or less, and the cell size for the various liquids at the 1-mm and 2-mm depths is shown in Table VI. For a thin (< 2 mm) layer of given liquid evaporating into still air, the cell size increased with the depth of the liquid layer, and the flow which the cellular schlieren pattern represented was the same as that observed by Benard (see Fig. 3). These cells were quite immobile and generally neither grew nor decayed in size with time. A direct stream of dry nitrogen onto the surface of the liquid sharpened the cell peripheries and tended to reduce the cell size. 

There are several contributions to the thermal conductivity of low-density, closed-cell foams thermal conductivity of the polymeric cell walls and the cell gas, plus convection and radiation in the cells. The thermal conductivity of most solid polymers is within a factor of 2 of 0.3 W m K . For foams of density 30kgm , the cell wall contribution, which is proportional to the foam relative density, is small. The contribution from convection inside the cells is negligible for cell diameters smaller than 10 mm. The radiation contribution is linearly proportional to the cell size, because infrared radiation is absorbed at each cell face then re-radiated. Figure 11.19 shows the effect of reducing the cell size of polyurethane foams on the total thermal conductivity. In polystyrene foams, the cells are rarely larger than 0.5 mm, so the radiation contribution to the foam conductivity is minimal. 

In systems with a low surface tension, the hole size of the sieve tray with downcomer should be smaller than 2 mm to prevent the continuous phase percolating through. For the treatment of such systems, dual flow trays without downcomers are more suitable. On a dual flow tray, the disperse and continuous phases flow in turn through the base plate holes and between the trays, producing strongly circulating convection cells. 

Hlgh>Temperature Thermal Insulation. A potential application of vitreous-carbon foam is high-temperature thermal insulation in vacuum or non-oxidizing atmosphere. Severai factors combine to make this structure an exceiient thermal insulator (a)Vne low volume fraction of the solid phase, which limits conduction (b) the small cell size, which virtually eliminates convection and reduces radiation through repeated absorption/reflection at the cell walls and (c) the poor conductivity of the enclosed gas (or vacuum). An additional advantage is its excellent thermal-shock resistance due to its... 

Because of the small proportion of sohd in the foam and the consequent large volume of gas, which has a much lower thermal conductivity, the resultant conductivity of the foams is much less than that of the sohd materials from which the foams are produced. The other reason for the low conductivity is the neghgible contribution of convection because of the small cell size (lower than 2-3 mm) [91]. 

The cone calorimeter used in this study (5) is a somewhat enlarged version of the model used at the National Institute of Standards and Technology in the United States. This particular equipment takes samples of size 20 cm x 20 cm mounted in a horisontal position on top of a load cell. Above the sample there is a cone heater and a spark ignitor. Gas samples are taken in fan ventilated exhaust duct mounted above the cone heater. The radiation used has been 50 kW/m2 and free convection ventilation over the sample. 

---