Falling particles

Another type of flocculation results from particle—particle collisions caused by differential settlement. This effect is quite pronounced in full-size plants where large rapidly falling particles capture small particles that settle more slowly. 

The drag force is exerted in a direction parallel to the fluid velocity. Equation (6-227) defines the drag coefficient. For some sohd bodies, such as aerofoils, a hft force component perpendicular to the liquid velocity is also exerted. For free-falling particles, hft forces are generally not important. However, even spherical particles experience lift forces in shear flows near solid surfaces. 

Working design equations for each of these flow rates follow. Hemeon developed equations for estimating the volumetric flow rate of induced air based on the power generated by the stream of falling particles (i.e., the work done per unit time by the drag force over the distance... 

Cunningham correction factor A factor used as a refinement to the Stokes equation for falling particles of small diameter. These tend to slip between the air molecules and, as a result, fall faster. Cup anemometer A device used by meteorologists for the measurement of wind speed. 

Relaxation time The time necessary for a moving particle to adjust from one given steady state velocity to another, e.g., the time for a falling particle to reach its terminal velocity. It is independent of the nature of the force applied to the particle. 

Open, Drip proof I, S 40°C 0 to 10% Dripping liquids or falling particles... 

Now that we have defined the biogenic model we can define two important properties. The first is the ratio of falling particles to the input to the surface. This property (g) is equivalent to the efficiency of bioremoval from surface waters ... 

There are several types of problems that we may encounter with falling particles, depending upon what is known and what is to be found. All of these problems involve the two primary dimensionless variables CD and ARe. The former is determined, for gravitation-driven motion, by Eq. (11-9), i.e.,... 

It often happens that we know or can measure the particle velocity and wish to know the size of the falling particle. In this case, we can form a dimensionless group that does not contain d ... 

PCB accumulation in the sediment is not a straightforward function of downward settling of falling particles. Several field and laboratory studies have shown that PCBs can be recycled back to the water column via biological... 

Jackson, G. A., Simulation of bacterial attraction and adhesion to falling particles in an aquatic environment, Limnol. Oceanogr., 34, 514, 1989. 

Since they tend to flatten out, they offer more resistance to falling and have lower terminal velocities than spherical particles. This effect is not important for freely falling particles having diameters less than a few hundred micrometers. Table 5.3 gives terminal settling velocity data for raindrops of various diameters. Above about 6 mm in diameter the drops fracture and break up while falling. 

A 10-p.m-diameter particle falls in still air with a velocity of 0.30 cm/s. If the drag coefficient is given by 24/Re, what is the force developed by the falling particle ... 

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