Radiation from combustion product

A quantitative understanding of certain primary combustion phenomena, e.g., liquid fuel-droplet vaporization and burning, gas phase chemical reaction kinetics, radiation heat transfer from combustion products, and mixing of reactants and combustion products, is required to develop a rational approach for the effective utilization of synfuels in industrial boiler/furnace systems. Those processes are defined by the interaction of a number of mechanisms which are conveniently described in terms of physical and chemical related processes. The physical processes are ... 

Other Gases The most extensive available data for gas emissivity are those for carbon dioxide and water vapor because of their importance in the radiation from the products of fossil fuel combustion. Selected data for other species present in combustion gases are provided in Table 5-6. 

Spectral interferences may also result from combustion products that exhibit broad-band absorption or particulate matter that causes scattering of the incident radiation. As both reduce the spectral radiance of the Hght source, they may erroneously lead to an overestimation of the absorbance and, consequently, the concentration. When the combustion or the particulate products arise from the fuel and oxidant mixture, they may be determined by measuring the absorbance while a blank is aspirated into the flame. The situation is more compHcated if the absorption or scattering arises from a product associated with the sample or its matrix. 

An important question to consider when using a flame as an atomization source, is how to correct for the absorption of radiation by the flame. The products of combustion consist of molecular species that may exhibit broad-band absorption, as well as particulate material that may scatter radiation from the source. If this spectral interference is not corrected, then the intensity of the transmitted radiation decreases. The result is an apparent increase in the sam-... 

The radiation from a flame is due to radiation from burning soot particles of microscopic andsubmicroscopic dimensions, from suspended larger particles of coal, coke, or ash, and from the water vapor and carbon dioxide in the hot gaseous combustion products. The contribution of radiation emitted by the combustion process itself, so-called chemiluminescence, is relatively neghgible. Common to these problems is the effect of the shape of the emitting volume on the radiative fliix this is considered first. 

Gaseous Combustion Products Radiation from water vapor and carbon dioxide occurs in spectral bands in the infrared. In magnitude it overshadows convection at furnace temperatures. 

The heat losss from the combustion products in Figure 4.10 is due to conduction and radiation. 

Radiation from flames and combustion products involve complex processes, and its determination depends on knowing the temporal and spatial distributions of temperature, soot size distribution and concentration, and emitting and absorbing gas species concentrations. While, in principle, it is possible to compute radiative heat transfer if... 

The inclusion of radiative heat transfer effects can be accommodated by the stagnant layer model. However, this can only be done if a priori we can prescribe or calculate these effects. The complications of radiative heat transfer in flames is illustrated in Figure 9.12. This illustration is only schematic and does not represent the spectral and continuum effects fully. A more complete overview on radiative heat transfer in flame can be found in Tien, Lee and Stretton [12]. In Figure 9.12, the heat fluxes are presented as incident (to a sensor at T,, ) and absorbed (at TV) at the surface. Any attempt to discriminate further for the radiant heating would prove tedious and pedantic. It should be clear from heat transfer principles that we have effects of surface and gas phase radiative emittance, reflectance, absorptance and transmittance. These are complicated by the spectral character of the radiation, the soot and combustion product temperature and concentration distributions, and the decomposition of the surface. Reasonable approximations that serve to simplify are ... 

Thermal or heat detectors respond to the energy emission from a fire in the form or heat. The normal means by which the detector is activated is by convention currents of heated air or combustion products or by radiation effects. Because this means of activation takes some time to achieve thermal detectors are slower to respond to a fire when compared to some other detection devices. 

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). 

Combustion of aluminum particle as fuel, and oxygen, air, or steam as oxidant provides an attractive propulsion strategy. In addition to hydrocarbon fuel combustion, research is focussed on determining the particle size and distribution and other relevant parameters for effectively combusting aluminum/oxygen and aluminum/steam in a laboratory-scale atmospheric dump combustor by John Foote at Engineering Research and Consulting, Inc. (Chapter 8). A Monte-Carlo numerical scheme was utilized to estimate the radiant heat loss rates from the combustion products, based on the measured radiation intensities and combustion temperatures. These results provide some of the basic information needed for realistic aluminum combustor development for underwater propulsion. 

Results of an experimental program in which aluminum particles were burned with steam and mixtures of oxygen and argon in small-scale atmospheric dump combustor are presented. Measurements of combustion temperature, radiation intensity in the wavelength interval from 400 to 800 nm, and combustion products particle size distribution and composition were made. A combustion temperature of about 2900 K was measured for combustion of aluminum particles with a mixture of 20%(wt.) O2 and 80%(wt.) Ar, while a combustion temperature of about 2500 K was measured for combustion of aluminum particles with steam. Combustion efficiency for aluminum particles with a mean size of 17 yum burned in steam with O/F) / 0/F)st 1-10 and with residence time after ignition estimated at 22 ms was about 95%. A Monte Carlo numerical method was used to estimate the radiant heat loss rates from the combustion products, based on the measured radiation intensities and combustion temperatures. A peak heat loss rate of 9.5 W/cm was calculated for the 02/Ar oxidizer case, while a peak heat loss rate of 4.8 W/cm was calculated for the H2O oxidizer case. 

Measurements of combustion temperatures, radiation intensity distributions in the range from 400 to 800 nm, and particle size distributions of combustion products have been made for the reaction of aluminum powder with both 02/Ar and H2O oxidizers in atmospheric dump combustors. The fraction of unburned aluminum in the combustion products was also determined for the H2O oxidizer case. An analytical study was performed to determine if the measurements are consistent with each other and with theory, and also to estimate the rate of heat loss from the combustion products. A Monte Carlo technique was used to determine the expected spectral energy distribution that would be emitted from a viewport located in the side of a combustion chamber containing products of aluminum combustion. 

Fig. 12.21 shows the combustion products of AP-HTPB and RDX-HTPB composite propellants. Large amounts of H2O, HCl, and CO2 are formed when an AP-HTPB propellant composed of a.p(0.85) is burnt. The molecules of H2O, HCl, and CO2 each emit infrared radiation. On the other hand, no COj or C(g) is formed when an RDX-HTPB propellant composed of ri3x(0.85) is burnt. Instead, large amounts of CO, H2, and Nj molecules are formed as its major combustion products. However, no infrared radiation is emitted from H2 or N2 molecules. Though CO molecules are formed at ri3x(0.85), the infrared radiation emitted from these is less than that from H2O or CO2 molecules. 

The thickness of Xs is very small and forms only about 5% of the total thickness Xb. Heat is transmitted by conductivity, convection and radiation. The rate of burning of smokeless powders is determined by the rate of transmission of the energy from the products of combustion to the powder itself. 

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