Pick-up velocity

The mechanisms of a single particle-wall collision are given in Chapter 2. A particle-wall collision in pneumatic transport systems is a complex process. The bouncing characteristics depend on many parameters, including impact angle, translational and rotational velocities of the particle before collision, physical properties of the wall and particles, and wall roughness and particle shape. 

Saltation of solids occurs in the turbulent boundary layer where the wall effects on the particle motion must be accounted for. Such effects include the lift due to the imposed mean shear (Saffman lift, see 3.2.3) and particle rotation (Magnus effect, see 3.2.4), as well as an increase in drag force (Faxen effect). In pneumatic conveying, the motion of a particle in the boundary layer is primarily affected by the shear-induced lift. In addition, the added mass effect and Basset force can be neglected for most cases where the particle 

A single particle, initially at rest at the bottom wall of the conveying pipe, may undergo various states of motion when the gas velocity increases. Single-particle motion can occur in one or a combination of the following states [Zenz, 1964]  

The incipient gas velocities for each state have been analyzed by Halow (1973) and are elaborated in the following. 

The incipient gas velocity for a particle rolling along the wall can be determined by the torques exerted on the particle. Assuming that the drag force, gravitational force, and lift force are exerted through the central point of the sphere, the balance of the torques about the uppermost point of the particle-wall contact plane (see Fig. 11.8) can be given by 

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In specifying settling chamber dimensions, gas flow velocities must be maintained below the reentrainment velocity (pick-up velocity) of deposited particulate. As a general guideline, linear gas velocities are kept below 10 ft/sec (600 ft/min = 304.8 cm/sec). This is satisfactory for most materials however, some low-density particulates are reentrained at lower velocities. 

For a given type of gravity settler, the pick-up velocity and the minimum diameter of particles which can be collected must be defined. The pick-up velocity is the onset velocity for particle reentrainment and depends on the particle diameter. The gas flow rate is to be controlled in such a way that the average gas velocity in the settling chamber is much smaller than the pick-up velocity. The minimum diameter of collected particles can be evaluated by the following analysis. 

When the gas velocity in the settler is sufficiently high, the particles deposited on the collection plates can be picked up by the gas stream. This reentrainment can substantially reduce the collection efficiency. By neglecting interparticle friction, the pick-up velocity, f/pp, may be estimated as [Zenz and Othmer, 1960]... 

Narayanan et al. [2S] used a visual observation technique and have given relations for the minimum gas velocity to suspend the particles. To obtain a theoretical basis, they compared a pick-up velocity previously derived [2S] on the basis of a force balance ... 

Vacuum (negative-pressure) dilute-phase pneumatic conveying. Fig. 1, often is selected for reasons of convenience (e.g. several feed points, any leakage inwards), but without consideration of the relatively high rates of velocity increase and high pick-up velocities (i.e. compared to positive-pressure systems. Fig. 2). The fundamental differences between these two types of transport are demonstrated by tiie following examples. 

Dilute-phase pick-up velocities for positive- and negative-pressure systems, based on [14]... 

Liquid temperatures in the tubes of an LT evaporator are far from uniform and are difficult to predict. At the lower end, the hquid is usually not boihng, and the hquor picks up heat as sensible heat. Since entering hquid velocities are usually veiy low, true heat-transfer coef-... 

If the probe velocity is less than the stack velocity, particles will be picked up by the probe, which should have been carried past it by the gas streamlines. The inertia of the particles allows them to continue on their path and be intercepted. If the probe velocity exceeds the stack velocity, the inertia of the particles carries them around the probe tip even though the carrying gases are collected. Adjustment of particulate samples taken anisokinetically to the correct stack values is possible if all of the variables of the stack gas and particulate can be accounted for in the appropriate mathematical equations. 

In biphase systems velocity of the steam is often 10 times the velocity of the liquid. If condensate waves rise and fill a pipe, a seal is formed with the pressure of the steam behind it (Fig. 2). Since the steam cannot flow through the condensate seal, pressure drops on the downstream side. The condensate seal now becomes a piston accelerated downstream by this pressure differential. As it is driven downstream it picks up more liquid, which adds to the existing mass of the slug, and the velocity increases. 

The kick alert detects the free gas over the whole annulus length by comparing the pump noise in the standpipe to the pump noise picked up a few feet below the flowline. The phase shift is correlated to the annulus sonic velocity, itself a function of the free gas. 

Turbine flow meters are composed of some form of rotary device such as a helical rotor, Pelton wheel or a vane mounted in the flow stream. The fluid passing the rotor causes the rotor to turn at an angular velocity which is proportional to the flow velocity and hence the volumetric flowrate through the meter. The rotary motion of the rotor is sensed by some form of pick-up device that produces an electrical pulse output. The frequency of this signal is proportional to the flowrate and the total count of pulses is proportional to the total volume of liquid passed through the meter. 

The isotope effects shown by Figures 1 and 8 demonstrate the transition from the stripping or pick-up mechanism to the complex model for different chemical reactions. As far as we know this is the first evidence for such a transition. It may be emphasized again that this transition should also be detected in the velocity spectra. Therefore, it would be desirable to construct apparatuses which allow one to measure velocity spectra at ion energies of about 1 e.v. 

The clinical neuroscience community was quick to pick up on the importance of this discovery and Barker s Transcranial Magnetic Stimulation (TMS) was soon widely used to measure nerve conduction velocities in clinical and surgical settings. However, it is not in the clinical domain that magnetic stimulation provides the most excitement magnetic stimulation is a tool with which to discover new facts about brain function and it has already delivered in many areas. 

Cyclone separators are also frequently used for gas-liquid separation. They can be designed using the same methods for gas-solids cyclones. The inlet velocity should be kept below 30 m/s to avoid pick-up of liquid form the cyclone surfaces. 

As the reactant elements pass through the shear layer, they pick up and convect turbulent fluctuations to the flame. There is no constraint that the flame speed and radial velocity be uniform along the axis however, since the flame... 

The relationships developed from field measurements have been made dimensionless with the assumptions that v = 1.33 x 10 m /s and AijO = 2.6 x 10 m /s to facilitate comparisons between relations and avoid dimensional problems. They are given in Table 9.2. The early measurements were to investigate the loss of water from the reservoirs of the Colorado River in the United States, and the later measurements were designed to investigate heat loss from heated water bodies. A revelation occurred in 1969, when Shulyakovskyi brought in buoyancy forces as related to natural convection to explain the heat loss from heated water at low wind velocities. This was picked up by Ryan and Harleman (1973), who realized that natural convection could explain the need for a constant term in front of the relationship for gas film coefficient, as had been found by Brady et al. (1969), Kohler (1954), Rymsha and Dochenko (1958), and Shulyakovskyi (1969). Finally, Adams et al. (1990) rectified... 

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