Venting of gas deflagrations through relief pipes
Vent devices for gas and dust explosions are often ducted to safety locations by means of relief pipes for the discharge of hot combustion products or blast waves (NFPA 68, 2002). The presence of the duct is likely to increase the severity of the explosion with respect to simply vented vessels posing a problem for the proper design of this venting configuration.
The phenomenology of the vented explosion is complicated as the interaction of combustion in the duct with primary combustion in the protected vessel intrudes the problem.
Several mechanisms have been proposed to account for the enhanced violence of the explosion in such a venting configuration but uncertainty still stands. The lack of both investigation and comprehension has so far prevented the development of reliable engineering guidelines for the sizing of the vent area.
Prior to any development of engineering correlations, a preparatory step should be the investigation of the nature of the interaction of the external combustion with the venting process.
In this work pilot scale experimental data are presented for an uncovered duct vented configuration.
Moreover, a CFD 2D axy-symmetric model based on the unsteady RANS approach is developed. The numerical model has been validated by comparison with available experimental results on a lab scale configuration and then tested on the pilot scale configuration.
Numerical and experimental results have evidenced that the severity of ducted explosion is mainly driven by the vigorous secondary explosion occurring in the duct (burn-up).
In particular the combustion in the duct has been found to interact with the internal combustion by means of purely mechanical effects. In dependence of the violence of the explosion in the duct, the mechanical effect acts as pre compression of the mixture in the vessel or venting flow rate reduction. No enhancement effects of the combustion in the vessel have been found to significantly affect the violence of the explosion.
Experimental data analysis - aided by the numerical results- have highlighted that as the scale of the explosion goes up, the external explosion moves toward more severe conditions.
A simple scaling law is proposed that accounts for many of the trends observed in literature but more data would be needed on scales of industrial interest to check its validity.