Development of optical measuring techniques for soot particle size distributions

Motivation

Recently, nanotechnology became an important field for both, research and industry. Interesting physical, chemical and morphological properties of nano-scaled particles facilitate a wide range of potential applications. On the other hand, aerosols of ultra-fine particles emitted to the environment represent potential health hazards for human beings. In environmental analysis and industry reliable methods to characterize properties of nano-scaled aerosols and in particular particle size distributions are needed.  

In many cases formation of nano-scaled particles takes place in reactive flows. In turbulent reacting flow fields local temperatures, velocities and concentrations are subject to spatial and temporal fluctuations. On example is soot formation in turbulent flames. Spatially and temporally resolved measurements are essential for understanding the governing processes and development and validation of models. Inserting probes into reactive flows influences temperature and velocity fields. Therefore, a measuring technique should work preferably contact less. 

Optical techniques meet the above stated requirements. Primary soot particle sizes range from 4 to 60 nanometers. Therefore, a direct visualization of these particles using visible light is impossible. However, particles absorb and scatter light. 

Theoretical background of Laser- Induced Incandescence (LII)  

Particles are heated by absorbtion of an intense nanosecond laser pulse to temperatures far above ambient temperatures. The enhanced thermal radiation of these particles is detected using an appropriate detector (ICCD camera, photomultiplier, Streak camera). According to Planck’s law thermal radiation intensity is a function of particle temperature. Additionally, nano-scaled particles exhibit volumetric thermal radiation as long as particles have same temperatures independent of size. Therefore, LII can be used for soot volume fraction measurements. After the laser pulse heated particles cool due to heat transfer, thermal radiation and evaporation. The heat and mass transfer processes are size dependant. Small particles cool down faster than large particles. The temporal evolution of particle temperature or LII signal intensity is representative for particle size distribution and gas temperature in the measurement volume.   

Figure. 1: energy balance of a single particle 

RAYLIX - 2D Imaging of particle number densities, volume fractions and radii

The LII signal is induced using a homogenous laser sheet. The LII signal is calibrated for absolute values of soot volume fractions by the measurement of the integral extinction.  The soot volume fraction is a function of the third moment of the particle size distribution. For evaluation of medium particle sizes and number densities additional information is needed.  Rayleigh-Scattering is proportional to sixth moment of the particle size distribution. Lokal soot volume fractions, number densities and medium radii are evaluated from simultaneous detected extinction, Rayleigh scattering and LII signals. However, the shape of  the lokal particle size distribution has to be known.

From coagulation theory and ex-situ measurments it is known that primary soot particles in laminar premixed flames show a lognormal distribution with a width of 0.34.  In technical systems the distribution is influenced by mixing, coagulation surface growth and oxidation. Depending on actual process parameters  the size distribution might deviate from being lognormal.

Knowledge of local size distributions improves the accuracy of medium particle size measurements and aids understanding of particle formation. In principal LII signal decays contain information local particle size distributions.

Assessment of size distributions and gas temperatures

The LII signal is induced by a pulsed Nd-YAG laser. The LII is detected with one dimensional spatial and temporal resolution using a streak camera at two different wavelengths. From the ratio of LII signals particle temperature as a function of time is obtained (two colour pyrometry). LII model parameters can be evaluated based on these measurements, if particle size distributions and gas temperatures are measured by using independent methods. Finally, particle size distributions and gas temperatures are obtained from measured LII signal decays using multidimensional non-linear regression.

literature  

Figure 2: Streak camera: 1D spatially resolved measurement of LII signal decays in a laminar premixed flame at two 2 wavelengths