Study On Morphological Characteristics And Radiative Properties Of Soot In Flames

As the global energy crisis continues to intensify and environmental pollution problems become more serious,improving the combustion efficiency of fossil fuels and reducing pollutants emission during the combustion process of fossil fuels have become research hotspots.A deeper understanding of soot formation in flames is not only because it harms the natural environment and human health,but also because it plays a critical role in the formation of other pollutants.The development of advanced detection technique on the morphological characteristics and concentration of soot aggregates is one effective approach to reveal the primary mechanisms in soot formation processes.Futhermore,radiation dominants over conduction and convection in high-temperature combustion systems,thus,the radiative properties of participating media are the key to solving the radiative transfer equation(RTE).Since soot is an important participating media in the combustion system,obtaining the radiative properties of soot in the flames is of great significance for accurately calculating the radiative heat transfer in the combustion system.It follows that study on the morphological characteristics and radiative properties of soot in flames are necessary and urgent to carry out.In this paper,effective detection techniques are proposed for different flames,to study the morphological characteristics and radiative properties of soot.Firstly,an effective inversion method to detect the morphological characteristics of soot aggregates in hydrocarbon flames is proposed.The imaging method is used to measure the distribution of soot aggregates absorption coefficients in the flames,thereafter,the primary particle diameter,primary particle number and volume fraction of soot aggregates are retrieved from the detected absorption coefficients using the particle swarm optimization algorithm(PSO).Meanwhile,the thermophoretic sampling particle diagnostic-transmission electron microscopy(TSPD-TEM)technique is employed to verify the retrieved morphological parameters of soot aggregates by the proposed inversion technique.The morphological parameters of soot aggregates retrieved using the inversion method are in good agreement with that obtained by the TSPD-TEM technique,which suggests that the inversion method is robust and can accurately retrieve the morphological parameters.Then,the distributions of primary particle diameter,primary particle number and volume fraction in flames for two different cases are presented,and effect of combustion condition and soot formation mechanism on the distribution rules of these morphological parameters is discussed.Secondly,an inverse method is proposed to retrieve soot temperature and radiative properties in co-flow laminar diffusion ethylene/air flames.The proposed inverse method is first verified as capable of the simultaneous reconstruction of soot temperature and inhomogeneous radiative properties with and without scattering.Thereafter,the soot temperature,absorption and scattering coefficients in three atmospheric ethylene/air flames is experimentally inverted from the radiative intensity taken by the hyper-spectral imaging device by means of the inverse method.From the results,it is found that neglecting scattering has an insignificant influence on the retrieved flame radiative properties,due to the weak scattering ability in the atmospheric flames.Furthermore,the simultaneous reconstruction of the soot temperature and inhomogeneous radiative properties is also numerically conducted in two pressurized flames.In general,neglecting scattering in pressurized flames leads to significant underestimation of the retrieved temperature and obvious deviation of the retrieved absorption coefficients.It follows that the effect of scattering from high levels of soot in these pressurized flames is significant and should generally be considered in the reconstruction(measurement)of soot properties.Finally,a hyper-spectral imaging device that can simultaneously capture three-dimensional data information of spectra,space,and time is employed to experimentally investigate the combustion behavior of a single coal particle.Three types of single pulverized coal particles are injected and burned on a Hencken flat-flame burner,and their flames are captured by a high-speed camera and the hyper-spectral imaging device.The camera observations demonstrate that the three single coal particles mainly undergo particle heat-up,volatile release and combustion,and volatile and char oxidation during the combustion process.Furthermore,the image captured by the hyper-spectral imaging device records the spectral and spatial history of the single burning coal particles throughout the combustion process.Then,the time–space temperature and apparent spectral emissivity of single burning char particles are calculated from the time–space spectra radiative intensity obtained by the hyper-spectral imaging device.The gray radiation exhibited by the three single burning char particles in the longer wavelength range demonstrates the feasibility of calculating the temperature using the two-color method.Thereafter,the soot temperature and volume fraction distribution in the volatile flames are obtained,and the results show that an elongated tail-like soot cloud is formed around the two single bituminous coal particles during the combustion process,but a nearly spherical soot cloud is formed around the single lignite coal particle.