Turbulent fuel jets

Diffusion Flames in the Transition Region. As the velocity of the fuel jet increases in the laminar to turbulent transition region, an instabihty develops at the top of the flame and spreads down to its base. This is caused by the shear forces at the boundaries of the fuel jet. The flame length in the transition region is usually calculated by means of empirical formulas of the form (eq. 13) where I = length of the flame, m r = radius of the fuel jet, m v = fuel flow velocity, m/s and and are empirical constants. 

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. 

On the other hand, turbulence may also be generated by external sources. For example, fuels are often stored in vessels under pressure. In the event of a total vessel failure, the liquid will flash to vapor, expanding rapidly and producing fast, turbulent mixing. Should a small leak occur, fuel will be released as a high-velocity, turbulent jet in which the fuel is rapidly mixed with air. If such an intensely turbulent fuel-air mixture is ignited, explosive combustion and blast can result. 

The multienergy method is based on the concept that, if detonation of unconflned parts of a vapor cloud can be ruled out, strong blast is generated only by those cloud portions which bum under intensely turbulent conditions. Such cloud portions include, for instance, intensely turbulent fuel-air jets resulting from a high-pressure... 

A turbulent jet diffusion flame was investigated. The apparatus and experimental procedure are described in detail in the article by Rambach et al. (11). The fuel jet had the following properties diameter of 1.6 mm, Reynolds number of 4400, and a fuel composition of 37% methane and 63% hydrogen. 

Liquid fuel sprays are not yet fullj understood [310]. The atomization process of a liquid fuel jet [376 332 345 293 309], the turbulent dispersion of the resulting droplets [256 253 262 333 319], their interaction with walls [259 365], their evaporation and combustion [290] are phenomena occurring in LES at the subgrid scale and therefore require accurate modeling. 

G. J. Nathan, S. R. Turns, and R. V. Bandaru. "The influence of fuel jet pressess-ing on the global properties and emissions of unconfined turbulent flames," Combust Sci. Tech., 112, 211-230, 1996. 

After combustion, the rich burning mixture leaves the combustion zone and flows between the rows of air jets entering the liner. Each jet entrains air and burning fuel and carries it toward the combustor axis, forming torroidal recirculation patterns around each jet that result in intensive turbulence and mixing throughout the combustor. 

The above discussion holds for dispersion by atmospheric turbulence. In addition, a momentum release of fuel sometimes generates its own turbulence, e.g., when a fuel is released at high pressure in the form of a high-intensity turbulent jet. Fuel mixes rapidly with air within the jet. Large-scale eddy structures near the edges of the jet entrain surrounding air. Compositional homogeneity, in such cases, can be expected only downstream toward the jet s centerline. 

Fuel from a fiilly unobstructed jet would be diluted to a level below its lower flammability limit, and the flammable portion of the cloud would be limited to the jet itself. In practice, however, jets are usually somehow obstructed by objects such as the earth s surface, surrounding structures, or equipment. In such cases, a large cloud of flammable mixture will probably develop. Generally, such a cloud will be far from stagnant but rather in recirculating (turbulent) motion driven by the momentum of the jet. 

Turbulence may arise by two mechanisms. First, it may result either from a violent release of fuel from under high pressure in a jet or from explosive dispersion from a ruptured vessel. The maximum overpressures observed experimentally in jet combustion and explosively dispersed clouds have been relatively low (lower than 1(X) mbar). Second, turbulence can be generated by the gas flow caused by the combustion process itself an interacting with the boundary conditions. 

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