Page 639 - Dust Explosions in the Process Industries

606 Dust Explosions in the Process Industries

room for considerableimprovement of the combustion submodelin the code to account
for the pyrolysis of organic dust particles and the effect of turbulence on the combus-
tion rate.
Detkovskii et al. (1996) developed a numerical model describing transient flame prop-
agation following ignition of a free turbulent jet of combustible dust in premixed
methanehr. Different secondarycombustion-inducedflow patterns can result, depending
on the initialjet parameters.Thepredicted courses of events were found to be in satisfactory
agreement with results from careful experimental investigation of the same process.
Zhong, Deng, and Li (1998); Zhong et al. (2001); and Zhong and Deng (2000) devel-
oped a comprehensive numerical CFD-based code for explosions of clouds of corn-
starch in air. The scheme comprises the gas phase flow,includingthe k-E model, the move-
ment of dust particles in the gas flow, evaporation of water from the particles, flux of
combustible vapors from the particles, combustion of the vapor phase, and combustion
of carbon residue of particles. The effect on the combustion rate of incomplete dust dis-
persion, that is, particle agglomerates, was accounted for. Numerical simulation of maize
starch explosions in the essentially closed 12m3silo, in which Hauert, Vogl, and Radandt
(1994) had carried out concentration and turbulence measurements, were performed,
adopting the dust cloud structuresmeasured by these workers as preignition or initial con-
ditions. Unfortunately no experimental explosion data were available for validating the
simulations.
Zhong et al. (2002) developed a CFD-basedEulerianLagrangian model for coal dust
explosionscomprisingthe following features: a nonelementalArrhenius model and eddy
breakup model for the chemical reactions, detailed models of water vaporization,
volatile decomposition, and surface oxidation of carbon, and the k-E turbulence model
for the gas phase, and a random trajectory model for the particle phase. The model was
tested against results from coal dust explosions in closed vessels.
Bielert and Sichel (1999, 2001) developed a numerical model for simulation dust
explosions in pneumatic conveyors. The model combines a front-tracking method with
a solver for the Euler equations. The combined effects of chemical reactions and flow
turbulence were represented by the turbulent burning velocity of the dust cloud.
Pascaud and Brossard (2000) presented a mathematical model for the combustion of
hybrid systems (e.g., cornstach/propane/air)in closed vessels.

9.2.4.8
Simplified Approximate Models
Having first emphasized the central role that turbulence must play in any relevant dust
explosion model, Tamanini (1996b, 1998a, 2001, 2002) suggested that scientifically
based approximate design tools, which also account for turbulence effects, can be read-
ily obtained by adopting lumped-parameter descriptions of the system of concern, as
is done in dimensional analysis. His main argument is that comprehensive models han-
dling the entire reactive flow field in complex geometries may not be generally acces-
sible for some time, and when available, they will be expensive and time consuming to
use. Tamanini (1995a, 1995b) used a lumped-parameter approach to establish a corre-
lation for predicting the effect of vent ducts on the venting efficiency, whereas Tamanini
(1996a)used it for sizing dust explosionvents in spray dryers, where the explosivecloud
only occupies part of the dryer volume. Tamanini (1996c) also presented a simplified