Velocity turbulent

In modem Hquid-fuel combustion equipment the fuel is usually injected into a high velocity turbulent gas flow. Consequently, the complex turbulent flow and spray stmcture make the analysis of heterogeneous flows difficult and a detailed analysis requires the use of numerical methods (9). 

Erosion-corrosion of these components was caused by high-velocity turbulent flow resulting from incomplete opening of the valve. In this case, erosion is the dominant factor in the metal loss, corrosion being a minor contributing factor. 

Turbulence and high fluid velocities resulting from normal pump operation accelerated metal loss by abrading the soft, graphitically corroded surface (erosion-corrosion). The relatively rapid failure of this impeller is due to the erosive effects of the high-velocity, turbulent water coupled with the aggressiveness of the water. Erosion was aided in this case by solids suspended in the water. 

This corrosion is most pronounced in locations of high velocity, turbulence, and impingement, such as at elbows, weld reinforcements, pump impellers, steam injection nozzles, and locations where freshly condensed fractions drip upon or run down metal surfaces. 

Effort should be made to design shapes that will reduce the effects of high fluid velocity, turbulence and the formation of gas bubbles. 

TTie ability of the ventilation system to protect the worker efficiently can readily be determined by personal samples. The PIMEX method (see Chapter 12) can be used to determine the worker s exposure during various work phases. The capture efficiency as well as the supply air fraction can be measured using tracer gas techniques. Simple evaluation is carried out visually with smoke tube or pellet tests. Daily system evaluation is recommended using airflow or static pressure measurements at appropriate parts of the system. The air velocities, turbulence intensities, air temperature, mean radiant temperature, and air humidity should also be measured to provide an assessment ol thermal comfort. 

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. 

Seawater systems should be designed to avoid excessive water velocities, turbulence, aeration, particulates in suspension, rapid changes in piping section and direction. Likewise, extended periods of shutdown should also be avoided since stagnation of contained seawater, will result in bacterial activity and HjS production with consequential and perhaps serious corrosion and health and safety problems. 

Even with the velocity, turbulence and aeration of the water supplied to the equipment being within acceptable limits, if corrosion is to be avoided it is still necessary for the units themselves to be well designed. Some of the more important aspects involved are outlined below. 

Likewise, Newton s law for terminal velocity (turbulent flow) can be expressed in simplified form as... 

By exceeding a certain discharge velocity, turbulence forces increase to such an extent that film disruption takes place immediately at the orifice. Now the droplet size is independent of the film thickness. This state of atomization is described by the critical Weber number. Measuring data obtained with hollow cone nozzles of different geometry and pure liquids as well as lime-water suspensions are represented in Figure 19. Wep,crit... 

Imagine two parallel plates of area A between which is sandwiched a liquid of viscosity r). If a force F parallel to the x direction is applied to one of these plates, it will move in the x direction as shown in Figure 4.1. Our concern is the description of the velocity of the fluid enclosed between the two plates. In order to do this, it is convenient to visualize the fluid as consisting of a set of layers stacked parallel to the boundary plates. At the boundaries, those layers in contact with the plates are assumed to possess the same velocities as the plates themselves that is, v = 0 at the lower plate and equals the velocity of the moving plate at that surface. This is the nonslip condition that we described in Chapter 2, Section 2.3. Intervening layers have intermediate velocities. This condition is known as laminar flow and is limited to low velocities. At higher velocities, turbulence sets in, but we do not worry about this complication. 

Prandtl velocity ratio t>+ V velocity normalized by friction velocity Turbulent flow near a wall, friction... 

Practical flotation processes, however, take place under conditions of turbulence. Turbulent flow, as opposed to laminar flow (see Section 6.1), is characterized by rapid, almost random, fluctuations in flow velocity. Turbulence helps keep the solid particles suspended, helps disperse the injected air phase into bubbles, and helps induce particle-bubble collisions and attachments. With regard to the role of turbulence in mineral flotation, a rule of thumb for suspension stability is the one-second criterion which states that the particles in a suspension are sufficiently well dispersed for flotation if individual particles do not remain settled at the bottom of the flotation vessel for longer than one second [53]. 

In Table IV, we see that established techniques for velocity measurement allow us to determine the average momentum flux, average velocity, turbulent intensities, and shear stress. Next on the list, to complete the flow field description, is the fluctuation mass flux, and first on the combustion field list is the temperature and major species densities of the flame gases. 

By exceeding a certain discharge velocity, turbulence forces increase to such an extent that film disruption takes place immediately at the orifice. Now, the droplet... 

Figure 2.8 Typical mean velocity/turbulence distribution. Average profiles of the magnitude of wind speed, scaled by its value at z = h (canyon height) (a) averaged over all available data (b) approaching flow normal to the canyon (from SW, full line and stars) and parallel to the canyon (dashed line and circles) (c) approaching flow normal to the canyon from NE (full line and stars) and the same but only runs with wind speed larger than 3 m/s at position 21R [canyon roof] (dashed line and circles). (From Rotach, 1995 [547]).

Figure 2.9 Typical mean velocity/turbulence distribution. Variation of ut / U for neutral conditions with non-dimensional heights (a) zJZq and (b) zs/zH where zs is sensor height above ground zs = Zs - Zd, Zd is zero-plane displacement height. The line in (a) is based on the log-profile, and the line in (b) is an empirical fit. (From Roth, 2000 [550]).

The superposition of individual tree wakes results in the under-forest and aboveforest velocity features found in extensive areas of forests or woods. The initial growth of wake deficits and the subsequent decay at greater downwind distances are characteristics of both individual tree and forest measurements. Yano [663] developed a concept of momentum defect superposition in the wakes of an array of roughness elements to reproduce velocity, turbulence and shear distributions within and above canopies. 

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