|
D. FLAMES,
LUMINOUS FLAMES, AND PARTICLE RADIATION Combustion consists of chemical
reactions in series and in parallel and involving various intermediate species.
The composition and concentration of these species cannot be predicted very well
unless knowledge is available of the flame reaction kinetics; this detailed
knowledge is not usually available or convenient to obtain. Because the flame
radiation properties depend on the distributions of temperature and species
within the flame, a detailed prediction of radiation from flames is not often
possible from knowledge of only the combustible constituents and the flame
geometry. It is usually necessary to resort to empirical methods for predicting
radiative transfer in systems involving combustion. Under certain conditions the
constituents in a flame emit much more radiation in the visible region than
would be expected from their gaseous absorption coefficients. For example, the
typical almost transparent blue flame of a Bunsen burner can become a more
highly emitting yellow-orange flame by changing the fuel-air ratio. Such
luminous emission is usually ascribed to incandescent soot (hot carbon)
particles formed because of incomplete combustion in hydrocarbon flames.
Alternatively, Echigo et al. (1967) and others have advanced
the hypothesis, supported by some experimental facts, that luminous emission
from some flames is by emission from vibration-rotation bands of chemical
species that appear during the combustion process prior to the formation
of soot particles. However, since soot formation is the most widely accepted
view, soot radiation will be emphasized in this discussion of luminous flames. D-1 Radiation from Nonluminous Flames Radiation from the nonluminous portion
of the combustion products is fairly well understood. For this the complexities
of the chemical reaction are not as important, since it is the gaseous end
products above the active burning region that are considered. Most instances are
for hydrocarbon combustion, and radiation is from the CO2 and H2O
absorption bands in the infrared. For flames a meter or more thick, as in
commercial furnaces, the emission leaving the flame within the CO2
and H2O vibration-rotation bands can approach blackbody emission in
the band spectral regions. The gas radiation properties in Chap. 9, and the
methods in this chapter, can be used to compute the radiative transfer. The
analysis is greatly simplified if the medium is well mixed and can be assumed
isothermal. A nonisothermal medium can be divided into approximately isothermal
zones, and convection can be included if the circulation pattern in the
combustion chamber is known. A nonisothermal analysis with convection was
carried out in Hottel and Sarofim (1965) for cylindrical flames. In Dayan and
Tien (1974), Edwards and Balakrishnan (1973), Modak (1975, 1977), Taylor and
Foster (1974), and Lefebvre (1984), radiation from various types of nonluminous
flames (laminar or turbulent, mixed or diffusion) is treated. The flame shape
for an open diffusion flame is considered in Annamali and Durbetaki (1975). The
local absorption coefficient in nonluminous flames is calculated in Grosshandler
and Thurlow (1992) as a function of mixture fraction and fuel composition.
Modest (2005) reviews models for radiative transfer in combustion gases. When considering the radiation from
flames, a characteristic parameter is the average temperature of a well-mixed
flame as a result of the addition of chemical energy. Well-developed methods
exist [Gaydon and Wolfhard (1979)] for computing the theoretical flame
temperature from thermodynamic data. The effect of preheating the fuel and/or
oxidizer can be included. An ideal theoretical flame temperature T is
computed using energy conservation assuming complete combustion, no dissociation
of combustion products, and no heat losses. The energy in the constituents
supplied to the combustion process, plus the energy of combustion, is equated to
the energy of the combustion products to give,
Energy losses by radiation and other
means, that would lower the flame temperature, are not included. Methods for
including these effects are in Gaydon and Wolfhard (1979); extinction of a flame
by radiative energy loss is analyzed in Ju et al. (2000). A list of ideal
theoretical flame temperatures (no radiation or other losses included) is in
Table 12-5 for various hydrocarbon flames, using data from Gaydon and Wolfhard
and from Barnett and Hibbard (1957). Results for complete combustion with dry
air are shown, followed by calculated results modified to allow for product
dissociation and ionization. The latter are compared with experimental results.
The heats of combustion of the substances are also given. Extensive tabulations
of similar data for more than 200 hydrocarbons are in Barnett and Hibbard and in
Perry et al. (2007). After its average temperature has been estimated, the
radiation emitted by a nonluminous flame can be considered, as illustrated by an
example. EXAMPLE D-1
As a result of combustion of ethane in 100% excess air, the
combustion products are 4 mol of CO2, 6 mol of H2O vapor,
33.3 mol of air, and 26.3 mol of N2. Assume these products are in a
cylindrical region 4 m high and 2 m in diameter, are uniformly mixed at a
theoretical flame temperature of 1853 K, and are at 1 atm pressure. Compute the
radiation from the gaseous region. The partial pressure of each constituent
is equal to its mole fraction:
The correction is Δε
= 0.031. Then the gas emittance is
TABLE 12-5
Heat of combustion and flame temperature for hydrocarbon
fuels [Gaydon and Wolfhard (1979); Barnett and Hibbard (1957); Lide (2008)]
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
