Study On Profiles Of The Temperature And Soot Concentration By The Radiative Imaging

With the rapid development of economy of China, the situation of energy supply is becoming tighter and tighter, and the environmental pollution produced by burned fuel is becoming more and more serious. Soot is one of pollutants produced by burned fossil fuel, and it represents unrealized chemical energy of fuel. The emission of soot by combustion processes is a source of atmospheric pollution and, due to its association with mutagenic and carcinogenic polycyclic aromatic hydrocarbons (PAH), has been affecting human health. Soot particles also affect the environment in many other ways including their contribution to the formation of photochemical smog and atmospheric acids. Soot particle presented in the atmosphere scatter and absorb solar radiation that can impede atmospheric visibility. In internal combustion engines and gas turbines, the deposition of soot has deleterious consequences for the maintenance and efficiency of the device, so the designer has many good reasons to avoid soot formation. This objective also applied in the case of fires, whose mechanism of propagation often involved radiant transfer from hot soot particles. On the other hand, this same ability to radiate is obviously desirable in a furnace. Therefore, it's critical to understand the physical and chemical mechanisms about soot formation, and make it possible to control the processes of soot nucleation, surface growth, and oxidation.Development of practical ways to numerically simulate flame environments, as a step toward developing computational combustion, requires significantly improved understanding of soot processes in flame. Clearly, improved understanding of soot formation in flames is needed to achieve effective methods of predicting flame radiation and pollutant emissions, as well as for developing practical methods of computational combustion.In this dissertation, the present situation of investigations about soot formation and oxidation, soot measurement, and visualization of physical properties in flame by flame image processing at home and abroad was analyzed in detail, and emphasis was put on the development of soot model and emission CT technique. The development of experimental use of chemiluminescence was also described. A numerical study of soot formation and oxidation in axisymmetric laminar coflow diffusion ethylene-air flame was conducted using detailed gas-phase chemistry and complex thermal and transport properties. A simple two-equation soot model was employed to predict soot formation, growth, and oxidation with interactions between the soot chemistry and the gas-phase chemistry taken into account. Radiation heat transfer by both soot and radiating gases was calculated using the discrete-ordinates method coupled with a statistical narrow-band correlated-k based band model. The governing equations in fully elliptic form were solved. Reasonable flame temperature and soot volume fraction distributions were given here.For visualizing non-uniform absorbing, emitting, non-scattering, axisymmetric sooting flames, conventional two-color emission methods are no longer suitable, so an emission CT method for the simultaneous estimation of temperature and soot volume fraction distributions is studied. The simulation results indicate that the emission CT method is suited for the reconstruction of flame structures with single peak or double peaks with small difference between the peak and valley. For a double-peaked flame structure with larger peak and valley difference, reasonable result can be obtained just when the mean square deviations of measurement data are small, for example, not more than 0.01. The simultaneous estimation of temperature and soot volume fraction distributions is one important innovation in this dissertation.The emission CT method is used to estimate temperatures and soot volume fractions simultaneously in a candle flame, a kerosene flame, and several ethylene flames from the knowledge of the monochromatic radiation intensities measured by a CCD in this dissertation. The results indicate that the greater soot concentration lies inside the higher flame temperature in both types of flame, both inside the flame front and outside the flame axis. In addition, the fuel flow rate of the kerosene flame is greater than that of the candle flame which increases the amount of soot in the flame, thus increasing radiation losses. This, in turn, causes a lower flame temperature. This is observed by other researchers.In computational combustion, it is generally assumed that all the energy released is transformed into the internal energy of the combustion medium. So the temperature of the medium increases, and then the thermal radiation emitted from it increases too. But it is opposed to the practical situation. Therefore, it was assumed in this paper that the total energy released in a combustion reaction is divided into two parts, one part is a self-absorbed heat, and the other is a directly-emitted heat. The former is absorbed immediately by the products, becomes the internal energy and then increases the temperature of the products as treated in the traditional way. The latter is emitted directly as radiation into the combustion domain and should be included in the radiation transfer equation (RTE) as a part of radiation source. For a simple, 2-D, gray, emitting-absorbing, rectangular system, the numerical study showed that the temperatures in reaction zones depended on the fraction of the directly-emitted energy, and the smaller the gas absorption coefficient was, the more strong the dependence appeared. Because the effect of the fraction of the directly-emitted heat on the temperature distribution in the reacting zones for gas combustion is significant, the measurement based on thermal radiation imaging need corresponding correction, the first problem to be solved is to determine the fraction of the directly-emitted heat. Experimental results show that the temperatures in reacting zones achieved by emission CT are greater than those measured by thermocouple. Besides computational and measuremental errors, the effect of chemiluminescence should be considered in experiment. Thus, a new clue is provided by consideration of chemiluminescence for combustion diagnosis.