Page 331 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

286 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

Impeller: A device on a shaft intended to cause motion of a called a ‘‘mixed flow reactor,’’ or a ‘‘backmix’’
fluid. Two broad categories of impellers are axial reactor, or a ‘‘CFSTR’’ (continuous flow stirred
flow and radial flow. tank reactor). See also CSTR.
Initial mixing: Injection of one or more chemicals (e.g., Mixing: Random distribution, into and through one another,
alum, a base, an acid, a polymer individually or of two or more initially separate phases (McCabe
in sequence) into a flow of raw water with mixing et al., 1993, p. 235). See also, agitation and
by turbulence. Synonymous with flash mixing, blending.
rapid mixing. The term has been used sometimes Mixing length: When a fluid mass with velocity, v, is dis-
in lieu of the term ‘‘rapid mix’’ (Stenquist and placed due to turbulent motion in the transverse
Kaufman, 1972, p. 7; Letterman et al. 1973, p. 716; direction, e.g., from y 1 to y 2 , with a change in vel-
Chao and Stone, 1979, p. 570, Kawamura, 2000, ocity Dv (the velocity fluctuation in the x-direction;
p. 307). the distance (y 2 y 1 ) is the mixing length, l m .
In-line mixers: An in-line mixer is placed in the raw-water Model: A representation of some portion of reality (see Chap-
pipe and has become used increasingly since the ter 4). With respect to a mixing impeller–basin sys-
early 1970s. tem, a model is a smaller version of the larger system
Jet: A high-velocity fluid flow that has significant penetration that it is intended to simulate.
into a fluid medium, the latter being of relatively Navier–Stokes Equation: Mathematically, the Navier–
large flow (for in-line mixing) or of large mass (for Stokes equation, an expansion of Newton’s second
mixing in a tank). law, is expressed as a partial differential equation
Jet Mixer: A nozzle, or a collection of nozzles, that emits (see, for example, Rouse (Ed.), 1959, p. 207;
high-velocity jets for mixing. The resulting sub- Munson et al., 1998, p. 361) that has the form, in
merged jet entrains the substance to be mixed within vector notation,
the subsequent zone of turbulence.
Kolmogorov microscale: The smallest range of eddies has
2
been termed the ‘‘universal equilibrium range. It has rp þ rg þ mr v ¼ r qv þ v rv (10:21)
qt
been further divided into a low eddy size region, the
viscous dissipation sub-range, and a larger size
where
region, the inertial convection sub-range. The two 2
p is the pressure (N=m )
sub-ranges are divided by the Kolmogorov micro- 2
g is the acceleration of gravity (m=s )
scale (Cleasby, 1984, p. 878).
x, y, z ¼ are coordinate directions (m)
Lagrangian coordinates: (1) A coordinate system in which
u, v, w are velocities of fluid in x, y, z directions,
the observer moves with the advection of a particle
respectively (m=s)
and describes the motion of particles relative to the
v is the velocity vector for infinitesimal mass
moving frame of reference. (2) Analysis of the fate
(m=s)
of a particular particle [Named after Joseph Louis
Lagrange (1736–1813). Lagrange was born in Turin
of French ancestry and in 1776 took Euler’s profes- The equation expresses all of the variables that
sorship at Berlin; he also succeeded Euler as the influence fluid motion, such as acceleration, pres-
world’s leading mathematician (from Rouse and sure, viscosity, surface energy, elastic energy, etc.
Ince, 1957, p. 107).] Its solution provides a full depiction of a flow field or
Marine impeller: An axial flow impeller with three large an acceleration field or a pressure field, etc. Analyt-
blades with constant pitch from tip to axis. ical solutions are not feasible but finite difference
Mariotte siphon: A closed container with a submerged air solutions started about the decade of the 1970s.
tube, which maintains constant head as a liquid sur- Since the mid-1990s, commercial software has been
face lowers in elevation, which maintains a constant available to provide solutions which, nevertheless,
flow from the container. Usually, the system is used still require knowledgeable persons to formulate.
in laboratory systems which can be sealed easily, e. Newtonian fluid: A fluid which conforms to the relationship
g., using a 19 L (5 gal) carboy. Any engineered that shear is proportional to velocity gradient, i.e.,
system for alum feed should be tested first using a t ¼ mdv=dy.
carboy with a section of tubing sized to give the Orthokinetic: (1) Collision of suspended particles due to
desired flow; in addition, clogging and cleaning fre- their motion as induced by fluid motion (Langelier
quency would be concerns. and Ludwig, 1949, p. 165; Argaman and Kaufman,
Mixed-flow reactor: See CSTR. 1968, p. 5). The latter stated that Smoluchowski
Mixed reactor: As given by Levenspiel (1972, p. 97), an was believed to have selected the term ‘‘ortho’’ to
‘‘ideal’’ reactor in which the contents are stirred describe the ordered nature of shear flocculation
and homogeneous; thus, the exit flow has the same in contrast to the disordered nature of Brownian
composition as the fluid within the reactor and is also or perikinetic flocculation. They use the term as