# A Review of Experimental Techniques for Measuring Micro- to Nano-Particle-Laden Gas Flows
## 3 Specific Measurement Methods
### Optical Particle Characterization
#### 3.1.4 Laser-Induced Fluorescence Techniques for Temperature and Composition Measurements  
LIF is derived from a process that begins with the absorption of the incident light by the fluorescent molecules, which are activated to a higher energy level. Some of the excited molecules return to the ground state by emitting fluorescence. As one of the most important inelastic scattering techniques, laser-induced fluorescence (LIF) is also used to measure particle temperature [36] and composition [38]. The basic principle behind such techniques rests on the temperature or molar fraction dependence of the fluorescence quantum yield.  
The fluorescence intensity detected on a spectral band from a droplet, which is previously seeded with a low concentration fluorescent tracer, is given by [36]:  
\[I_{\text{fi}}=\int_{\lambda_{\text{i}\text{i}\text{i}}}^{\lambda_{\text{i} \text{i}\text{i}\text{i}\text{2}}}K_{\text{opt}}(\lambda)K_{\text{spec}}( \lambda)V_{\text{C}}I_{0}\text{Ce}^{\beta(\lambda)/T}d\lambda\approx K_{\text{ opt}i}K_{\text{spec}i}V_{\text{C}}I_{0}\text{Ce}^{\frac{\text{eff}}{T}+\frac{ \text{bi}i}{T^{2}}} \tag{11}\]  
where f denotes the fluorescence, \(i\) denotes the spectral band, \(K_{\text{opt}}\) is an optical constant specific to the optical device, \(K_{\text{spec}}\) is a constant that depends solely on the spectroscopic properties of the fluorescent tracer in its environment, \(I_{0}\) is the laser excitation intensity, \(C\) is the molecular tracer concentration, \(T\) is the absolute temperature, \(V_{\text{C}}\) is the collection volume of the fluorescence photons, \(\beta(\lambda)\) describes the temperature dependence of the fluorescence intensity at the wavelength \(\lambda\) and \(a\) and \(b\) are the temperature sensitivity coefficients for the spectral band \(i\). However, the \(I_{0}\), \(V_{\text{C}}\) and \(C\) are strongly dependent on the droplet size, droplet position and on the probe volume dimensions. In order to address these problems, [PERSON] et al. determine the fluorescence intensity on two spectral bands (called two-band LIF) for which the temperature sensitivity is significantly different, and the selection of these spectral bands is optimized. Then, the temperature can be calculated from the fluorescence ratio between both fluorescence intensities collected on both optimal spectral bands [36]:  
\[R_{\text{f}}=\frac{I_{\text{f}\text{1}}}{I_{\text{f}\text{2}}}=\frac{K_{\text {opt}\text{1}}\ K_{\text{spec1}}}{K_{\text{opt}\text{2}}\ K_{\text{spec2}}} \text{e}^{\frac{\text{s}_{1}-\text{s}_{2}}{T}+\frac{\text{b}_{1}-\text{b}_{2} }{T^{2}}} \tag{12}\]  
Mean droplet temperature and velocity evolution in the flame as a function of time are plotted in Figure 10[36].  
Figure 10: Mean droplet temperature and velocity evolution in the flame as a function of time: mean droplet temperature measured using two-band LIF; droplet velocity measured using LDA [36].  
The fluorescence quantum yield \(g\) should rely on the liquid phase composition (i.e., fluorescent tracer fraction and temperature). However, [PERSON] et al. showed that the variation of \(g\) is almost negligible in the temperature range of the liquid phase [38]. Consequently, the total emitted fluorescence can be used to detect the number of molecules of the fluorescing component in a droplet, and the relation between the fluorescence signal and molar fraction of fluorescent tracer \(\chi_{a}\) (i.e., acetone), which is strongly related to \(g\), can be expressed by:  
\[I_{\mathrm{f}}=K_{\mathrm{opt}}K_{\mathrm{spec}}I_{\mathrm{I}}\mathrm{d}V\chi_ {a}\frac{1}{\chi_{a}\mu+1} \tag{13}\]  
where \(I_{\mathrm{I}}\) is the local intensity, which can be related to the laser incident intensity \(I_{0}\) by [PERSON]'s absorption law, \(\mathrm{d}V\) is an elementary volume and \(\mu\) is a dimensionless empirical constant, which can be properly calibrated in the experimental procedure.