Radiation from Luminous Flames

In many process fires, heat transfer by radiation is the dominant form of heat transfer. The heat radiated from a flame is emitted by gases, in particular the products of combustion and by soot. Aflame in which the radiation is emitted solely from the gaseous products of combustion is termed nonluminous and a flame in which there is soot is termed luminous (i.e., yellow or visible). 

The test for luminometer number (ASTM D-1740) was developed because certain designs of jet engine have the potential for a shortened combustion chamber fife because of high liner temperatures caused by radiant heat from luminous flames.The test apparatus is a smoke point lamp modified to include a photoelectric cell for flame radiation measurement and a thermocouple to measure temperature rise across the flame. The fuel luminometer number (LN) is expressed on an arbitrary scale on which values of 0 to 100 are given to the reference fuels tetralin and iso-octane, respectively. 

See color insert following page 424.) Spectral emission of radiation from luminous and nonluminous flames. (From Baukal, C. E., The John Zink Combustion Handbook, Boca Raton, FL CRC Press, 2001.)... 

Steam-assist flares use high pressure steam to entrain surrounding air and inject it into the core of the flare gas stream. The rapid mixing of the steam and air with the flare gas helps reduce soot formation that tends to lower the flame radiant fraction. Figure 30.14 shows a steam-assisted flare operating under identical flare gas flow conditions with and without steam-assist. Notice without steam-assist, the flame is more luminous and contains more soot this results in higher radiant fractions. The fraction of heat radiated from a flame can also be greatly increased by the presence of liquid droplets in the gas. Droplets within a hot flame can easily be converted to soot [21]. 

There are two origins of radiation from products of combustion to solids (1) radiation from clear flame and from gases and (2) radiation from the micron-sized soot particles in luminous flame. 

Sherman, R.A. Radiation from Luminous and Nonluminous Natural Gas Flames, ASME Transactions, 1956, pp. 177-192. 

Furnaces for Oil and Natural Gas Firing. Natural gas furnaces are relatively small in size because of the ease of mixing the fuel and the air, hence the relatively rapid combustion of gas. Oil also bums rapidly with a luminous flame. To prevent excessive metal wall temperatures resulting from high radiation rates, oil-fired furnaces are designed slightly larger in size than gas-fired units in order to reduce the heat absorption rates. 

Maesawa et al, Radiation from the Luminous Flames of Liquid Fuel Jets in a Combustion Chamber , 12thSympCombstn (1969), pp 1229-37... 

In contrast to downward spread, upward spread of flames along vertical fuel surfaces usually is acceleratory. Rates of heat transfer to the fuel by radiation and conduction from the luminous flames of height /, that bathe the surface are so great that, in comparison, transfer elsewhere can be neglected in the first approximation. If q is the average normal energy flux from these flames to the surface, then from geometric considerations, q — (l/L)q for the q in equation (67), where L is the thickness of the fuel... 

The actual flame temperature is lower than the adiabatic equilibrium flame temperature because of heat loss from the flame. The actual flame temperature is determined by how well the flame radiates its heat and how well the combustion system, including the load and the refractory walls, absorbs that radiation. A highly luminous flame generally has a lower flame temperature than a highly nonluminous flame. The actual flame temperature will also be lower when the load and the walls are more radiatively absorptive. This occurs when the load and walls are at lower temperatures and have high radiant absorptivities. These effects are discussed in more detail in Chapter 4. As the gaseous combustion products exit the flame, they... 

In a combustion experiment of a luminous hydrocarbon flame, it is desired to measure the total irradiation received by a sensing device. The bright yellow color of the flame indicates that the radiation emitted by the small soot particles obeys the Planck distribution. The optical sensing device is a photomultiplier tube (PMT) which converts the photon interaction at the inlet (photocathode) to an electric current at the outlet (anode). The PMT has 3 cm2 of active sensor area facing the flame and is placed 2 m away from the flame axis. 

Flame radiation is a function of many variables C/H ratio of the fuel, air/fuel ratio, air and fuel temperatures, mixing and atomization of the fuel, and thickness of the flame—some of which may change with distance from the burner. Fuels with higher C/H ratio, such as oils, tend to make more soot, so they usually create luminous flames, although blue flames are possible with light oils. Many gases have a low C/H ratio, and tend to burn clear or blue. It is difficult to burn tar without luminosity. It is equally difficult to produce a visible flame with blast furnace gas or with hydrogen. 

Fig. 2.18 Spectographs of radiation from ciear and luminous flames. Nonlumlnous flames top graph) are blue luminous flames lower graph) are yellow and emit soot particle radiation. Both luminous and nonlumlnous flames and Invisible poo gases emit triatomic gas radiation. Courtesy of Ceramic Industry journal, Feb. 1994, and Air Products Chemicals, Inc. reference 13).

Fig. 5.6. Some relative values of refractory radiation, gas radiation, and particulate radiation intensities for a specific flame and furnace. Total radiation is 6.5% higher with a luminous flame than with a nonluminous flame. Multiply Btu/ft hr by 0.01136 to obtain MJ/m h. Multiply feet by 0.3048 to obtain meters. Adapted from a paper by Mr. K. Endo of Nippon Steel, presented at the International Flame Research Foundation, Ijmuiden, Netherlands, about 1980.

Heating by radiation is practiced by allowing combustion to take place in proximity to cooled surfaces. Radiation from flames and gases cannot be easily handled by Eq. (18-1) because (1) the irize of the-flame cannot be accurately determined, (2). the flame has a thickness so that radiation from the center the flame must penetrate the outer layers, and (3) the luminosity of flames varies with different fuels and conditions of combustion. In nonluminous flames, radiation is found to be dependent to a large extent on the percentage of carbon dioxide and water vapor that is present. However, radiation from such a flame is not effective, and the presence of partly burned carbon particles (luminous flames) greatly-increases radiation. The mechanism of radiation from flames is further complicated by the convection heat transfer that occurs by the circulation of gases within the furnace box. 

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 luminous zone in deton appears to have a small finite thickness This was given by Mitchell Paterson (Ref 1) as being less than 0.24 cm for NG, with duration of deton flame of less than 0.3 microseconds. Herzberg Walker (Ref 2) have measured the duration of actinic radiation by highspeed camera methods and from these calculated that the luminous zones in the deton of HE s were from 0.03 to 0.09 cm thick (Ref 5, pp 154-55)... 

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