Phase-Doppler method

The phase Doppler method utilizes the wavelength of light as the basis of measurement. Hence, performance is not vulnerable to fluctuations in light intensity. The technique has been successfully appHed to dense sprays, highly turbulent flows, and combustion systems. It is capable of making simultaneous measurements of droplet size, velocity, number density, and volume flux. 

The phase-Doppler method is capable of accurately measuring particle size distribution and velocity J655] The most recent models ofphase-Doppler particle analyzer (PDPA) can generate data of droplet size and velocity simultaneously as a function of time, from that droplet drag can be calculated and clustering phenomenon can... 

In the phase-Doppler method, the number density, Nd, is determined from ... 

Typically, the phase Doppler method is good for the measurement of particle sizes ranging from 1 /u.m to 10 mm with a variation by a factor of 40 at one instrument setting. As a rule of thumb, the maximum measurable concentration is 1,000 particles per cubic millimeter (mm3). Commercial instruments using this technique are available, e.g., the phase Doppler particle analyzer (PDPA) (Aerometrics) and the Dantec particle dynamics analyzer (DPDA) (Dantec Electronics). 

For an actual application of the phase-Doppler method it is necessary to find off-axis-angles (p at which the measured value of the phase shift shows a linear or continuous and unambiguous dependence on the particle diameter d. This is directly related to the scattering behaviour of the liquid droplets that can be exactly described only, by the Mie scattering theory... 

Yokoi N, Aizu Y, Mishina H (2001) Unidirectional phase-Doppler method for particle-size measurements. Appl Opt 40(7) 1049-1064... 

Methods for analysis of the particle size distribution in the aerosol cloud include techniques such as time of flight measurement (TOE), inertial impaction and laser diffraction. Dynamic light scattering (photon correlation spectroscopy) is confined to particles (in suspension) in the submicron range. In addition to the size distribution, the particle velocity distribution can be measured with the Phase Doppler technique. 

Fig. 1. Set-up of the PTB laser system. The Nd YAG laser is frequency stabilized onto a selected iodine absorption line using the phase modulation method. The probe beam is modulated at 2.05 MHz by an electro-optic modulator (EOM), the pumb beam is frequency shifted by an acousto-optical modulator (AOM). The driving AOM rf power is chopped in order to cancel frequency offsets introduced by the Doppler background using a lock-in detection scheme. The transmitted probe beam signal is detected by a photodiode (PD) and mixed with the EOM rf in a double balanced mixer (DBM)...

A method which has received considerable attention over the past 15 years is phase-Doppler anemometry (PDA) which allows determination of the size of spherical particles, droplets or bubbles. Hence, local particle size distributions and size-velocity correlations can be obtained. Moreover, recent developments provide the basis for accurate particle concentration or mass flux measurements... 

The principle of phase-Doppler anemometry (PDA) relies on the Doppler difference method used for conventional laser-Doppler anemometry and was first introduced by Durst and Zare (1975). By using an extended receiving optical system with two or more photodetectors it is possible to measure simultaneously size and velocity of spherical particles. For obtaining the particle size the phase shift of the light scattered by refraction or reflection from the two intersecting laser beams is used. 

In order to demonstrate the performance of the Doppler-burst envelope integral value method for the estimation of the instantaneous particle velocity vector and the particle mass flux or concentration, measurements were performed in a liquid spray issuing from a hollow cone pressure atomizer (cone angle 60°) and a swirling flow which exhibits complex particle trajectories (Sommerfeld and Qiu 1993). All the measurements were conducted using the one-component phase-Doppler anemometer. The integration of the mass flux profiles provided the dispersed phase mass flow rate which agreed to 10 % with independent measurements of the mass flow rate (Sommerfeld and Qiu 1995). 

Bauckhage, K., The phase-Doppler-difference-method, a new laser-Doppler technique for simultaneous size and velocity measuremets. Part. Part. Syst. Charact., 5, 16-22 (1988)... 

Qiu, H.-H. and Sommerfeld, M., A reliable method for determining the measurement volume size and particle mass fluxes using phase-Doppler anemometry. Experiments in Fluids, Vol. 13, 393-404 (1992)... 

Qiu, H.-H., Sommerfeld, M. and Durst, F., Two novel Doppler signal detection methods for laser-Doppler and phase-Doppler anemometry. Meas. Sci. Techn., 5, 769-778 (1994)... 

Most experiments used Phase Doppler Interferometry (PDI), a method well suited for volatile solvents as used in ESI-MS [27]. A series of PDI measurements using various solvents are given in Table 1.2. One can deduce from this table that the dependence on the type of solvent is relatively small. Thus, droplets from all solvents experience Coulomb fissions close to, or at, the Rayleigh limit. The loss of mass on fission is between 2 and 5% of the parent droplet mass but the loss of charge is much larger, that is, some 15-25% of the charge of the parent droplet. 

Laser Diffractometry Phase Doppler Particle Analysis Systems (1) Broad measurement range for a given optical configuration (0.1 pm to several mm) (2) Excellent reproducibility between measurements of the same aerosol (3) No calibration required (1) Non-intrusive optical method (2) Measure the size or speed of spherical particles (liquid and solid) (1) Particles are assumed to be spherical (2) Bias due to high concentration or absence of the finest particles in the distribution... 

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