Turbulence, vapor cloud explosions

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. 

Turbulence in a vapor cloud explosion accident scenario may arise in any of three ways ... 

Chapter 2 discussed the possible influence of atmospheric dispersion on vapor cloud explosion or flash fire effects. Factors such as flammable cloud size, homogeneity, and location are largely determined by the manner of flammable material released and turbulent dispersion into the atmosphere following release. Several models for calculating release and dispersion effects have been developed. Hanna and Drivas (1987) provide clear guidance on model selection for various accident scenarios. 

Generally, at any moment of time the concentration of components within a vapor cloud is highly nonhomogeneous and fluctuates considerably. The degree of homogeneity of a fuel-air mixture largely determines whether the fuel-air mixture is able to maintain a detonative combustion process. This factor is a primary determinant of possible blast effects produced by a vapor cloud explosion upon ignition. It is, therefore, important to understand the basic mechanism of turbulent dispersion. 

Experimental research has shown that a vapor cloud explosion can be described as a process of combustion-driven expansion flow with the turbulent structure of the flow acting as a positive feedback mechanism. Combustion, turbulence, and gas dynamics in this complicated process are closely interrelated. Computational research has explored the theoretical relations among burning speed, flame speed, combustion rates, geometry, and gas dynamics in gas explosions. 

The major mechanism of a vapor cloud explosion, the feedback in the interaction of combustion, flow, and turbulence, can be readily found in this mathematical model. The combustion rate, which is primarily determined by the turbulence properties, is a source term in the conservation equation for the fuel-mass fraction. The attendant energy release results in a distribution of internal energy which is described by the equation for conservation of energy. This internal energy distribution is translated into a pressure field which drives the flow field through momentum equations. The flow field acts as source term in the turbulence model, which results in a turbulent-flow structure. Finally, the turbulence properties, together with the composition, determine the rate of combustion. This completes the circle, the feedback in the process of turbulent, premixed combustion in gas explosions. The set of equations has been solved with various numerical methods e.g., SIMPLE (Patankar 1980) SOLA-ICE (Cloutman et al. 1976). 

Stock, M., W. Geiger, and H. Giesbrecht. 1989. Scaling of vapor cloud explosions after turbulent jet release. 12th Int. Symp. on the Dynamics of Explosions and Reactive Systems. Ann Arbor, MI. 

A flash fire is the nonexplosive combustion of a vapor cloud resulting from a release of flammable material into the open air, which, after mixing with air, ignites. In Section 4.1, experiments on vapor cloud explosions were reviewed. They showed that combustion in a vapor cloud develops an explosive intensity and attendant blast effects only in areas where intensely turbulent combustion develops and only if certain conditions are met. Where these conditions are not present, no blast should occur. The cloud then bums as a flash fire, and its major hazard is from the effect of heat from thermal radiation. 

The consequence of the second approach is that, if detonation of unconfined parts of a vapor cloud can be ruled out, the cloud s explosive potential is not primarily determined by the fuel-air mixture in itself, but instead by the nature of the fuel-release environment. The multienergy model is based on the concept that explosive combustion can develop only in an intensely turbulent mixture or in obstructed and/or partially confined areas of the cloud. Hence, a vapor cloud explosion is modeled as a number of subexplosions corresponding to the number of areas within the cloud which bum under intensely turbulent conditions. 

Potential centers of strong blast are found in areas in a cloud which are in intensely turbulent motion when reached by the flame. Such cloud areas are described in the introduction to this section. Practical examples of potential centers of strong blast in vapor cloud explosions are... 

If the release forms a vapor cloud that premixes with air before ignition occurs, and turbulence is developed (for example, by the flame front propagating through a process structure), the flame speed can accelerate sufficiently to cause a blast. This event is referred to as a vapor cloud explosion. In addition to blast effects, radiant heat and flame contact effects may also occur. Flashback to the source may cause a pool and/or jet fire. 

The amount of explosion overpressure is determined by the flame speed of the explosion. Flame speed is a function of the turbulence created within the vapor cloud that is released and the level of fuel mixture within the combustible limits. Maximum flame velocities in test conditions are usually obtained in mixtures that contain slightly more fuel than is required for stoichiometric combustion. Turbulence is created by the confinement and congestion within the particular area. Modem open air explosion theories suggest that all onshore hydrocarbon process plants have enough congestion and confinement to produce vapor cloud explosions. Certainly confinement and congestion are available on most offshore production platforms to some degree. 

Research on water explosion inhibiting systems is providing an avenue of future protection possibilities against vapor cloud explosions. British Gas experimentation on the mitigation of explosions by water sprays, shows that flame speeds of an explosion may be reduced by this method. The British Gas research indicates that small droplet spray systems can act to reduce the rate of flame speed acceleration and therefore the consequential damage that could be produced. Normal water deluge systems appear to produce too large a droplet size to be effective in explosion flame speed retardation and may increase the air turbulence in the areas. 

Finally, there must be a flame acceleration mechanism, such as congested areas, within the flammable portion of the vapor cloud. The overpressures produced by a vapor cloud explosion are determined by the speed of flame propagation through the cloud. Objects in the flame pathway (such as congested areas of piping, process equipment, etc.) enhance vapor and flame turbulence. This turbulence results in a much faster flame speed which, in turn, can produce significant overpressures. Confinement that limits flame expansion, such as solid decks in tnulti-lcvc process structures, also increases flame speed. Without flame acceleration, a large fireball or flash fire can result, but not an explosion. 

If ignition is delayed, the pool vaporizes and forms a flammable cloud. If ignition is encountered downwind, the vapor cloud can ignite and bum back to the source in the form of a vapor cloud fire and, if the release is still occurring, will cause a pool fire. If the flame encounters turbulence or significant blockage as it bums back to the source, a vapor cloud explosion can result. 

If the cloud is flammable and ignites, it could result in a flash fire or vapor cloud explosion, depending on the degree of confinement, the degree of turbulence and mixing, and the total flammable mass within the cloud. 

Scaling of Vapor Cloud Explosions After Turbulent... 

The potential hazard of vapor cloud explosions following the leakage of combustible gas under pressure has been studied in experiments with turbulent jet release of propane, natural gas and hydrogen through orifices of different size. 

Vapor cloud explosions in partially confined areas with obstacles exhibit a further increase of the maximum explosion pressure compared to unconfined explosions due to additional turbulent flame acceleration at obstacles. In experiments we found an increase up to a factor of four and a scaling behavior similar to that of the unconfined case. 

Cloud formation and the state of initial turbulence in the cloud are controlled by these release modes, leading to three different accident scenarios with vapor cloud explosions after ignition. 

Fig. 5 Peak e3q>losion pressures vs distance for unconfined vapor cloud explosions after turbulent jet release of propane through four different orifice diamters. For one point the corresponding pressure-time signal is shown.

Fig. 6 Peak pressure in vapor cloud explosions after turbulent jet release a) undisturbed propane jet, b) propane jet into obstacles and confinement, o Jackass Flats 1964 (hydrogen) and Flixborough 1974 (cyclohexane).

The hazard potential of a vapor cloud esqjlosion after turbulent jet release depends on maximum esqjlosion pressure Pmax inside the cloud and on the cloud size. For unconfined vapor cloud explosions the pressure wave measured at a distance r outside the cloud decays inversely proportional to r. The peak overpressure is proportional to the energy-scaled radius... 

Chapter I, Vapor Cloud Explosions, presents recent research on the safety hazards and explosion damage potential associated with the accidental release of combustible vapor clouds. Stock et al. report on experiments with explosive clouds formed by turbulent jets of propane, natural gas, or hydrogen released through various-sized orifices. They found significant scale effects, e.g., the maximum explosion pressure increased with the size of the vapor cloud and with the turbulence level in the jet. Desrosiet and coworkers have experimented with asymmetric explosions of vapoi clouds. They present results on how the ignition asymmetry of a hemi-... 

AIChE/CCPS (1994) provides an excellent summary of vapor cloud behavior. They describe four features which must be present in order for a VCE to occur. First, the release material must be flammable. Second, a cloud of sufficient size must form prior to ignition, with ignition delays of from 1 to 5 min considered the most probable for generating vapor cloud explosions. Lenoir and Davenport (1992) analyzed historical data on ignition delays, and found delay times firom 6 s to as long as 60 min. Third, a sufficient amount of the cloud must be within the flammable range. Fourth, sufficient confinement or turbulent mixing of a portion of the vapor cloud must be present. 

Qualitative studies26 have shown that (1) the ignition probability increases as the size of the vapor cloud increases, (2) vapor cloud fires are more common than explosions, (3) the explosion efficiency is usually small (approximately 2% of the combustion energy is converted into a blast wave), and (4) turbulent mixing of vapor and air and ignition of the cloud at a point remote from the release increases the impact of the explosion.27... 

The speed of the flame propagation must accelerate as the vapor cloud burns. This acceleration can be due to turbulence, as discussed in the section on confined explosions. Without this acceleration, only a flash fire will result. 

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