Flames, appearance

Fig. 5. Effects of nozzle velocity on flame appearance in laminar and tuibulent flow (a), flame appearance (b), flame height and break-point height (40).

When he was about to replace the cap on the end of the filler pipe, a spark jumped from the cap to the pipe, and a flame appeared on the end of the pipe. It was soon extinguished. The flame could not travel back into the gasoline tank. The mixture of vapor and air in the tank was too rich to explode. 

The thermal radiation received by an object in the environment may now be computed if it is assumed that the flame appears as a flat plane, 33 m high, which propagates at a constant speed of 4.6 m/s during the full period of flame propagation (100/4.6 = 21.7 s). During this period, flame width varies from 0 to 100 m and back, according to Figure 8.1b ... 

The nature, or more precisely the structure, of a particular turbulent flame implied by these inequalities cannot be exactly established at this time. The reason is that values of <5L, Zk, A, or Z0 cannot be explicitly measured under a given flow condition or analytically estimated. Many of the early experiments with turbulent flames appear to have operated under the condition early theories that developed specified this condition in expressions for ST. The flow conditions under which <5L would indeed be less than Zk has been explored analytically in detail and will be discussed subsequently. 

The last point is worth considering in more detail. Most hydrocarbon diffusion flames are luminous, and this luminosity is due to carbon particulates that radiate strongly at the high combustion gas temperatures. As discussed in Chapter 6, most flames appear yellow when there is particulate formation. The solid-phase particulate cloud has a very high emissivity compared to a pure gaseous system thus, soot-laden flames appreciably increase the radiant heat transfer. In fact, some systems can approach black-body conditions. Thus, when the rate of heat transfer from the combustion gases to some surface, such as a melt, is important—as is the case in certain industrial furnaces—it is beneficial to operate the system in a particular diffusion flame mode to ensure formation of carbon particles. Such particles can later be burned off with additional air to meet emission standards. But some flames are not as luminous as others. Under certain conditions the very small particles that form are oxidized in the flame front and do not create a particulate cloud. 

The flash point is the lowest temperature at atmospheric pressure (760 mmHg, 101.3 kPa) at which application of a test flame will cause the vapor of a sample to ignite under specified test conditions. The sample is deemed to have reached the flash point when a large flame appears and propagates itself instantaneously over the surface of the sample. The flash point data is used in shipping and safety... 

A related term is flame speed. Flame speed is the speed with which a flame appears to move relative to a stationary observer. The flame speed can be much larger than the burning velocity due to expansion ofthe combustion products, instability and turbulent deformation ofthe flame. Flame speeds of 30-300 ft/sec (9-90 m/sec) are commonly observed for hydrocarbon-air mixtures. A gas phase detonation occurs when the flame speed exceeds the speed of sound in the burning vapor air mixture. 

When the same chemical compositions of the reactants are used to generate both types of flame, the chemical reaction rate is considered to be the same in both cases. However, the reaction surface area of the turbulent flame is increased due to the nature of eddies and the overall reaction rate at the combustion wave appears to be much higher than that in the case of the laminar flame. Furthermore, the heat transfer process from the burned gas to the unburned gas in the combustion wave is different because of the thermophysical properties specifically, the thermal diffu-sivity is higher for the turbulent flame than for the laminar flame. Thus, the flame speed of a turbulent flame appears to be much higher than that of a laminar flame. 

The compound is prepared in a retort attached to inlet tubes for dry chlorine and dry carbon dioxide and a distillation flask. White phosphorus is placed on sand in the retort. All air, moisture, and any phosphorus oxide vapors present in the apparatus are expelled by passing dry carbon dioxide. Dry chlorine is then introduced into the apparatus. If a flame appears on phosphorus it indicates presence of excess chlorine. In that event, the rate of chlorine introduction should be decreased. For obtaining phosphorus trichloride, flame should appear at the end of the chlorine-entry tube. The trichloride formed is collected by condensation in the distillation flask. A soda hme tube is attached to the apparatus to prevent moisture entering the flask. 

The former consists of an electrically heated vertical cylindrical tube (furnace), the top of which is connected to a small brass chamber containing the dust sample to be tested. A pneumatic system is used for blowing the dust downward thru the heated tube. In the usual ignition test, the lowest tube temp at which a flame appears at the lower mouth of the tube is taken as the ignition temp of the dust cloud. In the relative inflammability test the temp of the furnace is held constant at 700°C (1292°F) and only the amounts of Fuller s earth is decreased until no ignitions are observed... 

Combustion is a complex process involving many steps which depend on the properties of the combustible substances. At low temperatures, oxidation of combustible materials can occur very slowly, without the presence of a flame. When the temperature is raised, as for example by the application of external heat, the rate of oxidation is increased. If the temperature of the reactants is raised to above their ignition temperature , the heat generated will be greater than the heat lost to the surrounding medium and a flame will be observed. Thus, when a lighted match is applied to butane gas the temperature of the gas is raised to the ignition point and a flame appears. 

The bluish-violet layer observed against the glass surface of a flask when the latter is depressed on the upper part of a Bunsen flame appears to be due to the presence of hydrogen sulphide in the coal gas.5... 

There are three basic classifications (11 total calorific value types. (2) net calorific value types, and (3) inferential types. Net calorific value is less than the total calorific value by an amount equal to the latent heat of vaporization of the water formed during combustion. A net calorific value instrument uses a means that give results more nearly related to the net value. Thus, these types arc affected by gas composition and musl be calibrated for the gas to be tested. Inferential-type instruments depend upon such characteristics as flame appearance, maximum flame temperature, specific gravity, or gas analysis as indicative of the calorific value. 

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