Relaxation, aerodynamic

Electrical Single Particle Aerodynamic Relaxation Time (E-SPART) Analyzer simultaneously measures size in the range from submicron to 100 pm and particle charge distribution from zero to saturation levels [177,178]. 

Mazumder, M.K. E-spart analyzer its performance and applications to powder and particle technology processes. KONA 1993, 11, 105-118, KONA is produced by the Hosokawa Micron International Inc., which Markets the E-SPART Analyzer Literature is available from Micron Powder Systems (a member of the Hosokawa Micron group), 10 Chatham Rd., Summit, NJ 07901. (The term SPART Analyzer stands for Single Particle Aerodynamic Relaxation Time analyzer). 

Rapid aerodynamic flow past obstacles involves adiabatic compressions and rarefactions, and is influenced by relaxation of internal degrees of freedom in a way similar to shock phenomena. This effect has been quantitatively treated by Kan-trowitz18, who developed a method for obtaining relaxation times by measuring the pressure developed in a small Pitot tube which forms an obstacle in a rapid gas stream. This impact tube is not a very accurate technique, and requires a very large amount of gas it has been used to obtain a vibrational relaxation time for steam. 

Fig. 9.22 (a) Schematic view of E-Spart relaxation cell (b) principle of particle measurement. Individual particles are subjected to acoustic and/or electric excitation and the resultant response is measured by LDV to determine aerodynamic size and electrostatic charge. 

Laser Doppler velocimetry has been combined with acoustic excitation to allow the derivation of the relaxation time for particles, from which the aerodynamic diameter can be calculated [132-136], The particle relaxation time is derived from the velocity amplitude of the aerosol particle and that of the medium while the aerosol is subjected to acoustic excitation of a known frequency. A differential laser Doppler velocimeter is used to measure the velocity amplitude of the particle, and a microphone is used to measure the velocity amplitude of the medium. The aerodynamic diameter of the particle can be derived from the relaxation time and the known particle density. The method can be applied to real-time in situ measurement of the size distribution of an aerosol containing both solid and liquid droplets in the diameter range of 0.1 -10 pm. 

The cross-sectional area of the element is always normal to the frajectoiy of its center of mass. The initial radius of the cross-sectional area is denoted by ro. As the element travels in the gas, it is deformed and spreads due to the aerodynamic force. The extent of its deformation is governed by the interplay between aerodynamic, surface tension, and viscous forces. For simplicity, let us assume that the jet cross section changes from a circle to an ellipse as shown in Fig. 29.2c. We will relax this assumption later to obtain a more realistic shape. The elliptic element s aspect ratio, e, defined as the ratio of the ellipse minor axis b to its major axis a, decreases with time until the element reaches the CBL. 

In this section, we summarize important observations and facts about relaxation methods. These comments are based on the author s experience in developing aerodynamics and reservoir simulation models over two decades. 

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