# A Review of Experimental Techniques for Measuring Micro- to Nano-Particle-Laden Gas Flows
## 2 Classification of Experimental Methods
### Classification of Characterization Methods
#### 2.2.2 Measuring Methods for the Carrier Phase  
Compared with the dispersed particle phase, less parameters can be directly measured in the carrier phase; these include velocity, pressure and temperature. Consequently, the number of measurement methods available for characterizing the dispersed particles is much larger than for the carrier phase. In fact, many widely-used non-intrusive velocity-measuring technologies for fluids involve determining a tracer particle velocity, which is treated as the equivalent fluid velocity at that position, for example, in techniques, such as particle image velocimetry (PIV), particle tracking velocimetry (PTV) and LDA. Clearly, these methods depend on a consistency in velocity between the fluid and tracer particles. Unfortunately, the smaller the tracer particle is, the better it can be tracked, but the larger the tracer particle is, the stronger its scattering intensity. Therefore, tracer particle size needs to be carefully selected. Typically, particles of several to dozens of microns in size are used.  
As soon as the particle is heavier than the carrier phase, the particle velocity follows an exponential law [3]:  
\[U_{p}(t)=\left[1-\exp\left(-\frac{t}{\tau_{\mathrm{s}}}\right)\right] \tag{1}\]  
where \(U_{p}\) is the particle velocity, \(t\) is the time, \(\tau_{\mathrm{S}}\) is the particle relaxation time, given by:  
\[\tau_{\mathrm{s}}=d_{p}^{2}\frac{\rho_{p}}{18\mu}\left(\simeq\frac{d_{p}^{2}}{ v}\right) \tag{2}\]  
where \(d_{p}\) is the particle diameter, \(\rho_{p}\) is the particle density and \(\mu\) and \(v\) are the dynamic and kinematic viscosity of the carrier phase, respectively. Figure 2 shows the time response of particles having different sizes in a decelerating gas flow, based on the relationship defined in Equation (1). It shows that particle response time decreases, as particle size decreases, which results in better tracking.  
It is typically more difficult to measure the characteristic parameters of the carrier phase, because of interference from the dispersed phase, especially when the dispersed particle size is close to or smaller than the tracer particles. Some researchers have tried to obtain the velocity of the carrier phase using intrusive measurements. [PERSON] and [PERSON] [4] measured the average velocity of an air flow using a specially-designed pitot tube, while [PERSON] and [PERSON] [5] used a hot-wire anemometer to determine air turbulence intensity. [PERSON] [6] used a hot-wire probe to simultaneously measure fluid velocity and dispersed bubble concentration. This produced sharp peaks in the velocity signal, when bubbles collided with the hot-wire. Hot-wires have been widely used in turbulence studies.  
Figure 2: Response time of oil particles having different diameters in a decelerating air flow [1].  
Despite this method being pointwise and intrusive, it has been able to detect the periodicity and refined vorticity in unsteady periodic flow, when combined with post-processing methods, such as a phase-averaged fast Fourier transform (FFT) methods [7].