Page 267 - Dust Explosions in the Process Industries

Generation of Explosible Dust Clouds 239

the movement of the heated gas volumes is followed by another set of hot wires; poor
spatial resolution).
* Acoustic anemometer (poor spatial resolution).
* Electric discharge anemometer (corona and glow discharge).
* Cold-wire anemometer (for measurement of temperature fluctuations).
Durst et al. (1981) presented an in-depth discussion of one of the most versatile meth-
ods, the laser-doppler anemometer.
Beer et al. (1984) discussed the application of such methods in the study of turbulence
effects in burning dust clouds. Laser anemometers may be used for local particle velocity
measurements, particle sizing, and concentration fluctuationmeasurements. Very accurate
measurements of both mean and fluctuating particle temperatures are possible by other opti-
cal methods. As long as the flow is optically thin, which means low dust concentrations,
flow visualization is no more difficult in dust clouds than in gas flows. Some techniques,
like direct high-speed photography, are even simpler for two-phase combustion than for
gas flames, due to the strong radiation of the flames. However, the investigations are
extremely time consuming and difficult. Multipoint, conditionally sampled measurements
have to be performed for flame structure studies. Advanced data-reduction techniques
must be applied to evaluate and interpret and to extract information about individual events.
The development of controlled excitation studies provides the possibility of investigating
the details of the coherent structures through phase lock on the induced perturbation.
Hatta et al. (1989) extended the theoretical equations for flow of dust/gas mixtures
through nozzles to the complicated case of polysized particle systems.The equations cov-
ered both subsonic and supersonic gas flow. Some numerical solutions were discussed.
Fan Jiaren, Zhao Hua, and Cen Kefa (1989) studied the flow of polysized particledgas
mixtures in a coaxialjet system, both theoretically and experimentally. Advanced instru-
mentation was used to experimentally determine particle movement. Numerical com-
putations gave results in good agreement with the experiments.
Lockwood and Papadopoulos (1989) described a powerful method to calculate dis-
persion (not deagglomeration) of solid particles in a turbulent flow. An equation, which
correctly accounts for particle momentum conservation, was derived for the evolution
of the probability of particle velocity and position. The method enabled determination
of the position and velocity probability density functions for all cells within the com-
putational domain at a fraction the cost of conventional stochastic computations.

3.9
METHODS FOR GENERATING EXPERIMENTAL DUST
CLOUDS FOR DUST EXPLOSION RESEARCH

3.9.1
BACKGROUND

Over half a century ago Hartmann, Nagy, and Brown (1943) found it necessary, when
discussing research in the field of dust explosions, to make the following statement:
Over the past 30 years, various investigators have worked on means of producing uniform dust
clouds; comparison of results indicated that none of them has been wholly successful. The mecha-
nisms to produce such a cloud, of sufficient volume to be usable for test work, remain to be perfected.