Turbulent large-scale

The turbulent fluidized bed has a similar or slightly lower soHds volume fraction than the vigorously bubbling bed. There is considerable transport of soHds out of the turbulent bed and the bed level is not very distinct. Large-scale cyclones are needed to return soHds to the bed. On average, the bed inventory passes through the cyclones several times per hour. 

With turbulent flow, shear stress also results from the behavior of transient random eddies, including large-scale eddies which decay to small eddies or fluctuations. The scale of the large eddies depends on equipment size. On the other hand, the scale of small eddies, which dissipate energy primarily through viscous shear, is almost independent of agitator and tank size. 

Zone 4 is a terminal zone in which the residual velocity decays quickly into large-scale turbulence. Within a few diameters, the maximum velocity subsides to below 0.25 m/s. Though this zone has been studied by several researchers, its characteristics are still not well understood. 

Turbulence is generated by wind shear in the surface layer and in the wake of obstacles and structures present on the earth s surface. Another powerful source of turbulent motion is an unstable temperature stratification in the atmosphere. The earth s surface, heated by sunshine, may generate buoyant motion of very large scale (thermals). 

Generally, accidental emissions take place close to the earth s surface. The scale of the turbulence in the surface layer is limited by the distance to the earth s surface, so the characteristic size of the large-scale turbulence decreases towards the surface. Therefore, some degree of homogeneity in a vapor cloud is first to be expected in a thin layer adjacent to the ground (Wilson et al. 1982b). The thickness of this layer will increase as the vertical dimension of the cloud grows. 

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. 

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. 

Moen, 1. O., J. H. S. Lee, B. H. Hjertager, K. Fuhre, and R. K. Eckhoff. 1982. Pressure development due to turbulent flame propagation in large-scale methane-air explosions. Comb, and Flame. 47 31-52. 

The calculation of heat transfer film coefficients in an air-lift bioreactor is more complex, as small reactors may operate under laminar flow conditions whereas large-scale vessels operate under turbulent flow conditions. It has been found that under laminar flow conditions, the fermentation broths show non-Newtonian behaviour, so the heat transfer coefficient can be evaluated with a modified form of the equation known as the Graetz-Leveque equation 9... 

On the large scale, air and water are brought into countercurrent contact in a cooling tower which may employ either natural draught or mechanical draught. The water flows down over a series of wooden slats which give a large interfacial area and promote turbulence in the liquid. The air is humidified and heated as i( rises, while the water is cooled mainly by evaporation. 

As a general rule, scaled-down reactors will more closely approach isothermal operation but will less closely approach ideal piston flow when the large reactor is turbulent. Large scaledowns will lead to laminar flow. If the large system is laminar, the scaled-down version will be laminar as well and will more closely approach piston flow due to greater radial diffusion. 

Although vortices of small scale, such as Kolmogorov scale or Taylor microscale, are significant in modeling turbulent combustion [4,6-9], vortices of large scale, in fhe order of millimeters, have been used in various experiments to determine the flame speed along a vorfex axis. 

P. Clavin and F.A. Williams. Theory of premixed-flame propagation in large-scale turbulence. Journal of Fluid Mechanics, 90(pt 3) 589-604, 1979. 

Clavin, P. and Williams, F.A., Effects of molecular diffusion and of thermal expansion on the structure and dynamics of premixed flames in turbulent flows of large scale and low intensity, /. Fluid Mech., 116, 251, 1982. 

In any circumstances, it can be expected that and (5x are algebraic functions of turbulence length scale and kinetic energy, as well as chemical and molecular quantities of the mixture. Of course, it is expedient to determine these in terms of relevant dimensionless quantities. The simplest possible formula, in the case of very fast chemistry, i.e., large Damkohler number Da = (Sl li)/ SiU ) and large Reynolds Re = ( Ij)/ (<5l Sl) and Peclet numbers, i.e., small Karlovitz number Ka = sjRej/Da will be Sj/Sl =f(u / Sl), but other ratios are also quite likely to play a role in the general case. 

Means for generating strong large-scale turbulence, thus a larger fraction of the average flow kinetic energy can be randomized... 

For most medium- and large-scale micromanifold structures, where one passage feeds multiple parallel channels, flow traverses through turbulent and transition flows in the micromanifold region. This fluid in turbulent to transition flow also turns in the micromanifold region as it drops flow into parallel microchannels, which are primarily in the laminar flow regime. 

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