Turbulent deflagration

Lee, J.H., Knystautas, R., and Freiman, A., High speed turbulent deflagrations and transition to detonation in Hj-air mixtures. Combust. Flame, 56,227,1984. 

Fast turbulent deflagrations often transit spontaneously to detonations. For fully developed self-sustained detonation, boundary conditions and confinement play minor roles. The Chapman-Jouguet velocity and overpressure are based on the energetics of the mixture and can be evaluated from equilibrium thermodynamic computations. During the onset of detonation, the transient peak overpressures developed can be much higher than the equilibrium detonation pressures. Transition from deflagration to detonation is to be avoided whenever possible because of this extremely high pressure transient at the onset of detonation. 

The flame propagates in the mixture from the zone around the spark-plug at a rate of the order of 50 m s l. This is a rate of turbulent deflagration, much higher than that of a laminar deflagration, which is of the order of 50 cm s l, but very much smaller than that of a detonation, which is of the order of 2000 m s . ... 

The rate of turbulent deflagration increases rapidly with the intensity of the turbulence, that is with the fluctuations in the local flow rate of the gases. 

The following typical combustion velocities have been observed the turbulent deflagration mode with a velocity of some dozens of meters per second in lean mixtures the sound deflagration mode, high-speed deflagration with 800-1,000 m/ s velocities, when the combustion front moves with the local sound speed relative to the reaction products the quasi-detonation mode when the velocity spectra exceeds 1,100 m/s, but is 200-500 m/s less than the CJ detonation velocity. The quasi-detonation mode velocity deficit, in comparison with thermodynamic defined values, is explained by impulse losses due to interactions with walls and obstacles. 

Deflagration to Detonation Transition A reaction front that starts out with velocities below the speed of sound and subsequently accelerates to velocities higher than the speed of sound in the unreacted material is said to have undergone a Deflagration to Detonation Transition. The possibility of transition is enhanced by confinement/turbulence generators in the path of the reaction front. 

If a large amount of a volatile flammable material is rapidly dispersed to the atmo vapor cloud forms. If this cloud is ignited before the cloud is diluted below its lower flammability limit, a UVCE occurs which can damage by overpressure or by thermal radiation. Rarely are UVCEs detonations it is believed that obstacles, turbulence, and possibly a critical cloud size are needed to transition from deflagration to detonation. 

GASFLOW models geometrically complex containments, buildings, and ventilation systems with multiple compartments and internal structures. It calculates gas and aerosol behavior of low-speed buoyancy driven flows, diffusion-dominated flows, and turbulent flows dunng deflagrations. It models condensation in the bulk fluid regions heat transfer to wall and internal stmetures by convection, radiation, and condensation chemical kinetics of combustion of hydrogen or hydrocarbon.s fluid turbulence and the transport, deposition, and entrainment of discrete particles. 

Deflagration-to-Detonation Transition (DDT) The transition phenomenon resulting from the acceleration of a deflagration flame to detonation via flame-generated turbulent flow and compressive heating... 

A deflagration can best be described as a combustion mode in which the propagation rate is dominated by both molecular and turbulent transport processes. In the absence of turbulence (i.e., under laminar or near-laminar conditions), flame speeds for normal hydrocarbons are in the order of 5 to 30 meters per second. Such speeds are too low to produce any significant blast overpressure. Thus, under near-laminar-flow conditions, the vapor cloud will merely bum, and the event would simply be described as a large fiash fire. Therefore, turbulence is always present in vapor cloud explosions. Research tests have shown that turbulence will significantly enhance the combustion rate in defiagrations. 

A deflagration-detonation transition was first observed in 1985 in a large-scale experiment with an acetylene-air mixture (Moen et al. 1985). More recent investigations (McKay et al. 1988 and Moen et al. 1989) showing that initiation of detonation in a fuel-air mixture by a burning, turbulent, gas jet is possible, provided the jet is large enough. Early indications are that the diameter of the jet must exceed five times the critical tube diameter, that is approximately 65 times the cell size. 

The preceding section described the state of transition expected in a deflagration process when the mixture in front of the flame is sufficiently preconditioned by a combination of compression effects and local quenching by turbulent mixing. However, additional factors determine whether the onset of detonation can actually occur and whether the onset of detonation will be followed by a self-sustaining detonation wave. 

Although the status of many 3D codes makes it possible to carry out detailed scenario calculations, further work is needed. This is particularly so for 1) development and verification of the porosity/distributed resistance model for explosion propagation in high density obstacle fields 2) improvement of the turbulent combustion model, and 3) development of a model for deflagration to detonation transition. More data are needed to enable verification of the model in high density geometries. This is particularly needed for onshore process plant geometries. 

Darrieus, G., Propagation d un Front de Flamme Essai de Theorie des Vitesses Anomales de Deflagration par Development Spontane de la Turbulence, unpublished manuscript of a paper presented at La Technique Modeme, 1938, and Le Congres de Mechanique Appliquee, Paris, 1945, 1938. 

Structure of the turbulent high-speed deflagration propagating in a very rough channel stoichiometric Hj/Oj mixture at 150 torn... 

Turbulence is required for the flame front to accelerate to the speeds required for a VCE otherwise, a flash fire will result. This turbulence is typically formed by the interaction between the flame front and obstacles such as process structures or equipment. Turbulence also results from material released explosively or via pressure jets. The blast effects produced by VCEs can vary greatly and are strongly dependent on flame speed. In most cases, the mode of flame propagation is deflagration. Under extraordinary conditions, a detonation with more severe blast effects might occur. In the absence of turbulence, under laminar or near-laminar conditions, flame speeds are too low to produce significant blast overpressure. In such a case, the cloud will merely bum as a flash fire. 

Recall that we are assuming faem "C faff (°r fax, if turbulent flow). Anyone who has carefully observed a laminar diffusion flame - preferably one with little soot, e.g. burning a small amount of alcohol, say, in a whiskey glass of Sambucca - can perceive of a thin flame (sheet) of blue incandescence from CH radicals or some yellow from heated soot in the reaction zone. As in the premixed flame (laminar deflagration), this flame is of the order of 1 mm in thickness. A quenched candle flame produced by the insertion of a metal screen would also reveal this thin yellow (soot) luminous cup-shaped sheet of flame. Although wind or turbulence would distort and convolute this flame sheet, locally its structure would be preserved provided that faem fax. As a consequence of the fast chemical kinetics time, we can idealize the flame sheet as an infinitessimal sheet. The reaction then occurs at y = yf in our one dimensional model. 

Figure 25.1 Regimes of turbulent combustion 1 — offshore flares, 2 — spark-ignition engines, 3 — supersonic combustion, Kl — turbulent kinetic energy referred to laminar ratio of kinematic viscocity to chemical time, — Damkohler number based on Kolmogorov scale, Ld — integral scale referred to thickness of laminar deflagration...

Under very light confinement in hemispherical geometry only deflagrations were observed when the fuels (see Table 3) were mixed with air and initiated by high-energy sparks. Flame propagation velocity was increased by turbulence... 

Darrieus G. (1944). Propagation d un front de flamme. Essai de theorie de vitesses anomales de deflagration par development spontane de la turbulence. Z. Phys. Chem., 88, 641. 

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