3.1
CHAPTER 3
FLUID FLOW
Fluid Properties
..............................................................................................................................
3.1
Basic Relations of Fluid Dynamics
.................................................................................................
3.2
Basic Flow Processes
......................................................................................................................
3.3
Flow Analysis
..................................................................................................................................
3.5
Noise in Fluid Flow
.......................................................................................................................
3.13
Symbols
.........................................................................................................................................
3.14
LOWING fluids in HVAC&R systems can transfer heat, mass,
F
and momentum. This chapter introduces the basics of fluid
mechanics related to HVAC processes, reviews pertinent flow pro
cesses, and presents a general discussion of singlephase fluid flow
analysis.
FLUID PROPERTIES
Solids and fluids react differently to shear stress: solids deform
only a finite amount, whereas fluids deform continuously until the
stress is removed. Both liquids and gases are fluids, although the
natures of their molecular interactions differ strongly in both degree
of compressibility and formation of a free surface (interface) in liq
uid. In general, liquids are considered incompressible fluids; gases
may range from
compressible
to nearly
incompressible
. Liquids
have unbalanced molecular cohesive forces at or near the surface
(interface), so the liquid surface tends to contract and has properties
similar to a stretched elastic membrane. A liquid surface, therefore,
is under tension (
surface tension
).
Fluid motion can be described by several simplified models. The
simplest is the
idealfluid
model, which assumes that the fluid has
no resistance to shearing. Ideal fluid flow analysis is well developed
(e.g., Schlichting 1979), and may be valid for a wide range of appli
cations.
Viscosity
is a measure of a fluid’s resistance to shear. Viscous
effects are taken into account by categorizing a fluid as either New
tonian or nonNewtonian. In
Newtonian fluids
, the rate of defor
mation is directly proportional to the shearing stress; most fluids in
the HVAC industry (e.g., water, air, most refrigerants) can be treated
as Newtonian. In
nonNewtonian fluids
, the relationship between
the rate of deformation and shear stress is more complicated.
Density
The density
U
of a fluid is its mass per unit volume. The densities
of air and water (Fox et al. 2004) at standard indoor conditions of
68°F and 14.696 psi (sealevel atmospheric pressure) are
U
water
= 62.4 lb
m
/ft
3
U
air
= 0.0753 lb
m
/ft
3
Viscosity
Viscosity is the resistance of adjacent fluid layers to shear. A
classic example of shear is shown in Figure 1, where a fluid is
between two parallel plates, each of area
A
separated by distance
Y
.
The bottom plate is fixed and the top plate is moving, which induces
a shearing force in the fluid. For a Newtonian fluid, the tangential
force
F
per unit area required to slide one plate with velocity
V
par
allel to the other is proportional to
V
/
Y
:
F
/
A
=
P
(
V
/
Y
)
(1)
where the proportionality factor
P
is the
absolute
or
dynamic vis
cosity
of the fluid. The ratio of
F
to
A
is the
shearing stress
W
, and
V
/
Y
is the
lateral velocity gradient
(Figure 1A). In complex flows,
velocity and shear stress may vary across the flow field; this is
expressed by
(2)
The velocity gradient associated with viscous shear for a simple
case involving flow velocity in the
x
direction but of varying mag
nitude in the
y
direction is illustrated in Figure 1B.
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 Spring '11
 range
 Fluid Dynamics, ........., Laminar Flow, Turbulent Flow

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