Study On Measuring Method Of The Temperature And Soot Volume Fraction In Laminar Co-Flow Axisymmetric Diffusion Flames

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 due to its association with mutagenic and carcinogenic polycyclic aromatic hydrocarbons (PAH), which would affect human health. Thus the control of combustion generated soot particles is not only related to the effective use of hydrocarbon fuel, but also to the protection of the environment and human health. For the aim of controlling the soot particle emissions effectively, the first task is making an intensive study of the physical and chemical mechanisms about soot formation. The combustion diagnostics technique is the key factor for fulfilling this task. Hence, this dissertation studied some combustion diagnostics techniques, including the measuring method for temperature and soot volume fraction in laminar co-flow ethylene/air diffusion flame. We improved the existing method and proposed a new method, which can provide a foundation for the further research on the control of soot particles in the future.In this dissertation, Thermocouple particle densitometry(TPD) is the first investigated method , it is a new method for measuring absolute soot volume fraction in flames which was suggested by Eisner and Rosner and has been successfully implemented in several laminar diffusion flames. This diagnostic relies on measuring the junction temperature history of a thermocouple rapidly inserted into a soot-containing flame region, then optimizing the fit between this history and one calculated from the principles of thermophoretic mass transfer. The TPD method is very simple to implement experimentally, yields spatially resolve volume fractions directly, can easily measure small volume fractions, and does not depend on the prevailing soot particle size, morphology, or optical characteristics. We measured the temperature and soot volume fraction in a laminar co-flow ethylene/air diffusion flame with this method and the results showed that it was powerful for the measurement of temperature and soot volume fraction in such laboratory-scale flame.The shortcomings of conventional TPD method is analyzed in detail by numerical simulation in the next section, In the quasi-steady energy balance of thermocouple junction, some important parameters, which have significant impact on the energy balance of junction such as the radiation heat from soot to junction, the conduction heat between the junction and wire, are ignored. This could introduce some error for the temperature and soot volume fraction measurement using TPD method. We compared the junction temperature history between with and without these parameters for finding the influence of them to the measurement result and the numerical simulation result showed that the radiation heat from soot to junction, the conduction heat between the junction and wire are two important souce of the junction's energy, their absence in the quasi-steady energy balance of thermocouple junction would cause the junction temperature to depart the flame's ture temperature significantly.Then the improved TPD method is proposed based on the analysis in the above section, which added these neglected parameters into the energy balance equation and deduced the mass flux of soot particle to the junction per unit time and surface area from the new energy balance equation directly. This new method is not based on the assumption that thermophoresis is the dominant mechanism of soot particle deposition on the junction surface, therefore it could be used to verify this assumption. And the result showed that this assumption is not valid during the variable-emissivity stage, what is the reason of this phenomenon should be further study. Because the soot volume fraction cannot be obtained with this new method, it also should continue to be improved in the future.In this dissertation, the second investigated method is the flame emission spectroscopy diagnostics technique, which is a non-intrusive method. We proposed a new algorithm based on the improved Tiknonov regularization method to solve the problem that the conventional algorithm cannot calculate the ill-conditioned matrix, which often issue when the structure of investigated flame is complicated. The numerical simulation result showed that our proposed method could calculate the ill-conditioned matrix perfectly. And we also improved the computing model by added the self-absorption term into the radiation transfer equation(RTE), which is often ignored in the conventional algorithms. The improved radiation transfer equation with self-absorption term could be solved with the iteration method presented in this section. The numerical simulation result showed that when the soot volume fraction in flame is not very high(below 10 ppm), the neglect of self-absorption term could not cause significant error for the measurement result, but when the soot volume fraction in flame is larger than the critical value(above 30ppm), the neglect of self-absorption term could cause the opposite result.The last section focus the application of the new method mentioned above to the laminar diffusion flame. The result shows that this method is powerful in the near-infrared range for accurate prediction of temperature and soot volume fraction. And to the self-absorption term, we found that it could be ignored for the laboratory-scale flames.