Page 330 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

Mixing 285

desired conditions (raw-water flow and rotational a submerged jet. A radial-flow impeller in a mixing
velocity of the impeller) with measurements taken basin induces eddies by the submerged jet (the flow
from the effluent stream. If the blending is not effi- from the impeller), by the eddies induced by the flat
cient, the concentration of the outflow may be much blade as it rotates, and by the general circulatory
less than the potential. Once the conditions are motion induced by the pumping action (advective
attained to meet the blending criterion, additional flow). The terms, ‘‘eddy’’ and ‘‘vortex’’ are used
mixing may be necessary if reaction is involved. interchangeably (Hanson, 1989). The modern con-
Coagulation: (1) The driving together of colloidal particles cept of turbulence is, however, in terms of vortices.
by chemical forces (ASCE-AWWA, 1969, p. 65). See also vortex.
(2) Colloid destabilization due to reduction of zeta- Emulsion: A dispersion in which one liquid is not miscible
potential. (3) Destabilization of colloidal particles with water.
through the addition of coagulant to water (Stenquist Eulerian coordinates: (1) A coordinate system in which the
and Kaufman, 1972, p. 34). observer is stationary and the mathematics of motion
Complete-mix reactor: Continuous flow reactor, in which are with respect to a stationary reference. (2) The
the reactor contents are homogeneous and are the analysis of occurrences at a fixed point is usually called
same as the effluent concentration. Mathematically, the Eulerian method (Rouse and Ince, 1957, p. 107).
the distribution of residence times is declining «: Defined, « ¼ (P=Vr), which is energy dissipated by turbu-
exponential. If two or more reactors are arranged in lence per unit mass of fluid.
series the effluent distribution curve, i.e., C(t), Field: The loci of a given vector or scalar quantity, e.g.,
becomes S-shaped, with the steepness of the concentration, pressure, velocity, acceleration,
S-curve increasing as the number of reactors force, gravitation, magnetic that is usually repre-
increases. See also CSTR. sented graphically in space as a result of computa-
Computational fluid dynamics: The term computational tions or experimental measurements. If the field is
fluid dynamics, abbreviated, ‘‘CFD,’’ refers solution unchanging with time, then it is steady state; if
of the Navier–Stokes equation (along with the con- changing with time, then the field is unsteady.
tinuity equation) for a given set of boundary condi- Flash mix: Short residence time CSTR utilizing an impeller
tions by means of a finite element computation with high rotational velocity to produce homoge-
utilizing a computer. The solution is displayed, as a neous mixtures through advective dispersion and
rule, in terms of a velocity field or a pressure field or turbulent mixing (Stenquist and Kaufman, 1972,
as a flow net (depending on conditions) or by means p. 7). See also rapid mix.
of color coding. If the solution is unsteady, it may be Flash mixing: Mixing of coagulant chemicals with raw
displayed in animated format. water; a short detention time is implied, e.g., 1s.
CSTR: Constant flow stirred tank reactor; reactor contents Usually, a flash mixer is the same as an ‘‘in-line’’
are uniform throughout; also called ‘‘mixed reactor’’ mixer, i.e., an impeller in a pipe, but may be any
and ‘‘back-mix reactor’’ (Levenspiel, 1999, p. 91). device that distributes a coagulant and generates
Also, the reactor is called a ‘‘complete-mix’’ reactor. turbulence (e.g., a hydraulic jump, baffles, sub-
The essential feature is that the contents of the merged jets, an impeller–basin system) As a rule,
reactor are homogeneous at any instant in time and the effectiveness of the mixing depends on the inten-
the effluent concentration is the same as the reactor. sity of turbulence, a short detention time, high tur-
See also ‘‘complete-mix’’ reactor. bulence form of initial mixing, i.e., rapid mixing.
Dead zone: The residence time of a portion of the fluid is Flocculation: (1) Refers to the assimilation of coagulated
particles into floc particles (ASCE-AWWA, 1969,
much greater than the average, i.e., t p u, where, t p
is the residence time of any given parcel of fluid. See p. 65). (2) The purpose of flocculation is to remove
also, ‘‘short-circuiting.’’ particles in the 1 mm size range and place them in the
Disperse: (1) Exchange of mass between adjacent coordin- 10 mm size range so that the inertial and gravita-
ates in fluid flow. (2) To scatter,.. or to send in tional forces will transport the particles (Hanson and
different directions (Oxford American Dictionary, Cleasby, 1990, p. 68). (3) Collision and aggregation
Oxford University Press, New York, 1980). of destabilized particles into relatively large aggre-
Dispersion: (1) A mixture, (2) see glossary, Chapter 4. gates known as flocs. The term is applied to the
Draft tube: A tube with diameter about the same as the hydrodynamics of aggregation and floc formation
associated impeller, positioned with axis coincident (Stenquist and Kaufman, 1972, p. 34).
with the impeller that directs advective flow into the Flow number: The flow number is defined, Q ¼ Q(impeller)=
3
eye of the impeller. The impeller may be positioned (nD ).
1=2
above, within, or below the draft tube. G: Defined, G ¼ (P=Vm) , which has the dimensions of
1
Eddy: A circular motion within a fluid induced by a surface velocity gradient, e.g., s . An alternative equation
discontinuity, e.g., a flat plate normal to the velocity which is equivalent to G ¼ (P=Vm) 1=2 and used fre-
vector, or a turbulent shear, i.e., velocity gradient, or quently by researchers is G ¼ (e=n) 1=2 .