Correlations for flame speed and explosion overpressure of dust clouds inside industrial enclosures
ABSTRACT Explosion relief vents on enclosures in powder-handling plants are currently designed according to technical standards that in some situations may overestimate the required vent area significantly. These technical standards sometimes do not take into account the real work conditions of industrial plants (e.g. turbulence intensity) and therefore explosion worst cases are not always foreseeable. The availability of methods either for the evaluation of explosion overpressure or sizing of relief vents, with involvement of the pre-ignition turbulence, could be very useful for a better estimate of these quantities. In this work two empirical correlations are presented: the first one allows the calculation of the flame speed and the burning velocity starting from the explosion indices KSt and Pmax of the standardized 20-l sphere test. The second allows either the calculation of the explosion overpressure or the sizing of relief vents of an enclosure.
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ABSTRACT: Using VDI 3673 or NFPA guidelines for venting we distinguish between a homogeneous and inhomogeneous dust distribution in principle. Experiments were conducted in a 12 m3-silo to measure the dust distribution, turbulence and the reduced explosion pressure using three different ways of generating the dust/air mixture: ring nozzles with pressurized dust chambers  (homogeneous distribution),  pneumatically fed vertically downwards  and tangentially . Additionally, the dust feeding rate, the conveying velocity and the ignition position was changed. The reduction of the conveying velocity and therefore the decrease of the RMS turbulence velocity will cause a strong reduction of the explosion pressure and the pressure rate, respectively. The results show that the dust concentration in the pneumatically filled silo is inhomogeneous. However, the RMS turbulence velocity and the reduced explosion pressure for the homogeneous distribution and the vertical filling are of the same order. In comparison, the tangential feeding results in lower values. The calculations for the pneumatical vertical filling according to VDI 3673 guideline (inhomogeneous distribution) underestimate the resulting pressure for small vent areas, but the equations for the homogeneous distribution cover the experimental data.Process Safety Progress 08/1996; 15(3):178 - 184. · 0.72 Impact Factor
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ABSTRACT: Propagation of starch-dust air flames has been investigated in a tube (3 m long; square cross section 0.2 m × 0.2 m). Laminar, cellular and slightly turbulent propagation regimes have been observed. Several analogies with premixed gaseous flames have been displayed. Conduction heat transfer through the flame appears to be predominant for preheating the mixture. The burning of the mixture occurs in the gas phase after complete pyrolysis of particles. The laminar burning velocity is found equal to 0.27 m/s, the quenching distance 7 mm, the flame thickness 3-4 mm and the lean flammability limit 70g/m. Some questions, such as the actual role of flame radiation, are not yet completely elucidated.Combustion Science and Technology - COMBUST SCI TECHNOL. 01/1988; 62:149-172.
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ABSTRACT: Following the quantitative determination of dust cloud parameters, this study investigates the flame propagation through cornstarch dust clouds in a vertical duct of 780 mm height and 160 × 160 mm square cross section, and gives particular attention to the effect of small scale turbulence and small turbulence intensity on flame characteristics. Dust suspensions in air were produced using an improved apparatus ensuring more uniform distribution and repeatable dust concentrations in the testing duct. The dispersion-induced turbulence was measured by means of a particle image velocimetry (PIV) system, and dust concentrations were estimated by direct weighing method. This quantitative assessment made it possible to correlate observed flame behaviors with the parameters of the dust cloud. Upward propagating dust flames, from both closed/open bottom end to open/closed top end of the duct, were visualized by direct light and shadow photography. From the observation of propagation regimes and the measurements of flame velocity, a critical value of the turbulence intensity can be specified below which laminar flame propagation would be established. This transition condition was determined to be 10 cm/s. Laminar flames propagated with oscillations from the closed bottom end to the open top end of the testing duct, while the turbulent flames accelerated continuously. Both laminar and turbulent flames propagated with steady velocity from the open bottom end to the closed top end of the duct. The measured propagation velocity of laminar flames appeared to be in the range of 0.45–0.56 m/s, and it was consistent with the measurements reported in the literature. In the present experimental study, the influence of dust concentration on flame propagation was also examined, and the flame propagation velocity was found weakly sensitive to the variations in dust concentration. Some information on the flame structure was revealed from the shadow records, showing the typical heterogeneous feature of the dust combustion process.Combustion Science and Technology - COMBUST SCI TECHNOL. 01/2006; 178:1957-1975.