Radiative Properties Of High Temperature Particle During Its Phase Transformation

The radiation of high temperature particle with phase transformation is encountered in numerous technologically important applications such as target diagnostics and investigation, temperature measurements of flame, remote sensing and suppression of forest fires, plasma spraying and many others. The corresponding problems, such as phase transformation of an irradiated particle, absorption and scattering of thermal radiation in combustion of fuel droplet, the local volumetric absorption of radiation in the particle with phase transformation and others, are of obvious interest to researchers in several fields. In this research, the alumina ( Al 2 O 3) particles in Solid Propellant Motor exhaust plume are taken as the primary research objects, and the radiative mechanisms of different phases and phase transformation kinetics are investigated, the models for simulating the thermal radiation of high temperature particles with phase transformation are established. The scope of present research contains five parts:(1) Three kinds of phases (liquid,γandαstates) that melted alumina particle in the exhaust is most likely to evolve into during its solidification are taken as research objects. The effect of different phase states of alumina particles on their optical constants and radiative properties is analyzed. Different mechanisms which are responsible for the alumina particles radiation are considered in the spectral range from ultraviolet to near infrared region. Based on the model for calculating absorption coefficient of alumina particles in different phases, the model for calculating radiative properties of isothermal particles is built up. Calculated results show satisfactory agreement with experimental data.(2) The optical constant of an alumina particle containing carbon impurity is simulated based on the effective medium theory, and phase transformation is considered when particle temperature goes through the melting point. The effective optical constants calculated by Maxwell-Garnett and Bruggeman mixing rules, volume-averaged complex refractive index and the concentric shell model for the particle are compared with each other, and the wave-dependence of effective optical constants calculated by the four mixing rules is studied. The effects of carbon content and mixing morphology on the particle's radiative properties are investigated. It is indicated that carbon impurity is an important factor that can reduce the discontinuity in the absorption index of pure alumina through the melting point.(3) Modeling for the transient radiative properties of alumina particles with phase transformation. First, the time dependence of the volume fraction of different phases is calculated based on the kinetic equation of alumina particles'crystallization process. Second, the transient effective optical constants of alumina particles with phase transformation are calculated by different mixing rules. It is indicated that results calculated by these mixing rules are very close. Third, two kinds of models are built up based on two possible solidification modes, including the multilayered sphere model and the effective-medium sphere model, and then the spectral radiance of alumina particle is calculated in the phase transformation. It can be summarized that the present results show a trend similar to the experimental results, and the results of the effective-medium sphere model which is more in accord with the experimental process shows better agreement with the experimental data.(4) The electromagnetic model is constructed to determine thermal radiation inside and outside the radially nonisothermal multiphase alumina particle. The downward recurrence relations for c i n and d in are developed to determine the values for coefficients a_in , b_in , c_in, d_in . Then the internal absorption factors are calculated, and the results of this paper are close to Mackowski D.W.'s results. Take two kinds of particles, which are multiphase alumina particle containing liquid,γ,αphases and particle containingγ,αphases, carbon soot impurity, as research objects, and the absorption and extinction factors are calculated by multilayered sphere model based on electromagnetic theory and effective-medium sphere model with Mie theory for these two kinds of particle. It is indicated that these two models'results show satisfactory agreement when the thickness of each layer is much less than the wavelength. The thermal radiation inside and outside the single phase and multiphase alumina particle is investigated by multilayered sphere model based on electromagnetic theory. The effects of constituent materials contents, the arrangement of constituent materials and radially temperature difference are also considered.(5) The influences of phase transformation and carbon soot impurity on the radiative properties of alumina particles flow are studied. The distribution of spectral radiance along the axis at 90o angle is calculated with backward Monte-Carlo method for axisymmetric high temperature alumina particles flow. Based on effective optical constant of the particle, the effect of alumina particle's phase transformation and carbon soot impurity on the spectral radiance along the particles flow axis is analyzed. The calculated results indicate that the spectral radiance along the particles flow axis is obviously changed when carbon soot impurity mixed into the high temperature alumina particle. The influence of multiphase state on the ultraviolet radiation of high temperature particles flow can not be neglected.