# A Review of Experimental Techniques for Measuring Micro- to Nano-Particle-Laden Gas Flows
## 3 Specific Measurement Methods
### Optical Particle Characterization  
One of the most important experimental characterization methods for microparticles (or larger particles of the continuum regime) is optical analysis. Optical analysis is especially popular in two-phase flow experiments, because it is carried out as a non-intrusive on-line test, for example PIV, LDA or light-scattering techniques. At first, optical analysis was only used to measure particle velocity and number density. Now, it can analyze particle size distribution (PSD) using new laser technology, such as the phase Doppler particle analyzer (PDPA). Such techniques measure particle temperature, shape and chemical components. In spite of such recent developments, one of the ongoing challenges of such advanced measurement technologies is to assess their measurement uncertainty. The four steps involved in particle characterization using optical analysis techniques are summarized by [PERSON] et al. in Figure 3[8].  
The main optical particle characterization techniques are comprehensively classified according to their measurement principles and measured quantities in [PERSON] et al. [8] and summarized here in Table 1. The measurement techniques listed here only cover techniques for particles larger than 500 microns (within the continuum regime or partly overlapping with the transition regime). Typically, both elastic scattering and inelastic scattering are used for optical particle characterization. Elastic scattering is used to detect particle size and velocity, while inelastic scattering is used to measure particle temperature, density and chemical species.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Measurement Principle** & **Measured Quantity** & **Technique** \\ \hline Imaging & Velocity, particle size, shape & PIV/PTV, shadowgraphy, glare-point separation \\ \hline Light intensity, light intensity ratio & Particle size, temperature, chemical composition & Extinction/absorption, modulation depth, Mie/LIF intensity ratio, dual band/three band LIF (laser-induced fluorescence) \\ \hline Interferometry & velocity, particle size, refraction index/temperature & ILIDS (interferometric laser imaging for droplet size)/IPI (interferometric particle imaging), diffraction, rainbow refractometry, holography \\ \hline Time shift & Particle size, velocity & The flying time, pulse displacement, time-shift technique \\ \hline Pulse delay & Particle size, temperature & Femtosecond laser method \\ \hline Raman scattering & Temperature, species concentration & Raman spectroscopy \\ \hline \hline \end{tabular}
\end{table}
Table 1: Optical analysis techniques for particle characterization.  
Figure 3: The four steps involved in particle characterization using optical techniques.  
In particle-laden gas flows, the relative velocity between particles and the carrier phase affects the particle concentration distribution and flux density, as well as the interphase transfer of mass and heat. This indicates that velocity measurement of both phases is indispensable. The velocity of the dispersed phase and the carrier phase can be acquired optically, either as a pointwise measurement, such as with an LDA [9], or as a field measurement, such as with PIV/PTV [3; 10]. The relative velocity between phases depends not only on the particle velocity, but also on the velocity of the carrier phase. This suggests that the velocity of the tracer particle used to deduce the carrier phase velocity and the velocity of the dispersed particle should be measured simultaneously. There are two effective solutions to this problem: (i) using a scattering light signal from the tracer and dispersed particle of a different wavelength; or (ii) ensuring there is a large size difference between the tracer and dispersed particle.