Angular momentum equation
•
For angular momentum equation, B =HO the angular momentum vector about point O which moments are desired. •
Where β is
•
The Reynolds transport equation can be written as follows:
•
For one‐dimensional inlets and outlets, the flux terms become:
Integral Relations for CV
M. Bahrami
ENSC 283 Spring 2009
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Conservation of energy •
The conservation of energy principle or the first law of thermodynamics:
during an interaction, energy can change from one form to another but the
total amount of energy remains constant.
constant
(‐)
(+)
Q
System
(+)
(‐)
W
•
A system can exchange energy with its surroundings through heat and work transfer. In other words, work and heat are the forms that energy can be t
transferred across the system boundary.
f
d
th
t
b
d
•
Sign convention: work done by a system is positive, and the work done on a system is negative Heat transfer to the system is positive and from a system
system is negative. Heat transfer to the system is positive and from a system will be negative.
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ENSC 283 Spring 2009
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First law of thermodynamics
•
If δQ heat is added to a system and δW work is done by the system, the
system energy dE must change based on the first law of thermodynamics:
•
In a rate form:
•
Applying the Reynolds transport theorem to the first law of thermodynamics, the dummy variable B=E (energy) and β=dE⁄(dm)= e. The Reynolds transport eq ation for a fi ed control ol me becomes
equation for a fixed control volume becomes:
•
Energy per unit mass: can include: internal, potential, and kinetic:
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M. Bahrami
ENSC 283 Spring 2009
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Work rate •
Work rate can be divided into three parts:
•
Shaft work: is the work done by a machine (e.g. pump impeller, fan blade, piston, etc) and it involves a shaft that crosses the control surface. •
Pressure work: is done by pressure forces and occurs at the surface only; all work on internal portions of the material in the control volume is by equal and opposite forces and is self canceling
and opposite forces and is self‐canceling:
•
To find the total pressure work:
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M. Bahrami
ENSC 283 Spring 2009
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Work rate cont’d.
•
Shear work: is due to viscous stresses occurs at the control surface and
consists of the product of each viscous stress (one normal and two
tangential) and the respective velocity component:
•
Shear work is rarely important and may vanish or be negligible according to the particular type of surface. Note 1: that for all parts of control surface that are solid containing walls, V 0(
V=0 (no‐slip condition) thus W
li
di i ) h Wv=0 .
0
Note 2: for surface of a machine, the viscous work is contributed to the machine and we absorb it in the shaft work Ws. Note 3 the viscous work at inlet or outlets can be neglected due to negligible
Note 3: the viscous work at inlet or outlets can be neglected due to negligible amount of τnn Vn dA.
•
•
•
Integral Relations for CV
M. Bahrami
ENSC 283 Spring 2009
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Energy equation •
The control volume energy equation becomes:
•
From definition of enthalpy h=u+p/ρ, the final general form of the energy equation for a fixed control volume becomes:
equation for a fixed control volume becomes: •
For one‐dimensional inlets and outlets, the flux terms become:
Integral Relations for CV
M. Bahrami
ENSC 283 Spring 2009
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Example
•
A steady flow machine, shown in figure below, takes in air at section 1 and
discharges it at section 2 and 3. the properties at each section are as follows:
section 1 2 3 A, ft2
0.4
1.0
0.25
Q, ft3/s
100
40
50
T, F
70
100
200
p, lbf/in2 abs
20 30 ? z, ft
1.0
4.0
1.5
• Work is provided to the machine at the rate of 150 hp. Find the
pressure p3 in lbf/in2 absolute and heat transfer in Btu/s. Assume air
is a perfect gas with R=1716 and cp = 6003 ft. lbf/(slug.R).
/
Integral Relations for CV
M. Bahrami
ENSC 283 Spring 2009
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Steady flow energy equation
•
For steady flow with one inlet and one outlet, both assumed one‐
dimensional, reduces to a celebrated relation used in many engineering
analyses:
•
From conservation of mass
From conservation of mass we can re‐arrange it: we can re‐arrange it:
•
•
The following term is called stagnation enthalpy:
If we divide the steady energy equation by g, the dimension of each term becomes a length, called head in fluid mechanics and shown by symbol h
Integral Relations for CV
M. Bahrami
ENSC 283 Spring 2009
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Friction and shaft work
•
The energy equation for steady, low speed, incompressible flows through a
pipe or a duct with a pump or a turbine which has one inlet and one outlet
becomes:
•
Kinetic energy correction factor: The inlet and outlet to control volume often Kinetic
energy correction factor: The inlet and outlet to control volume often
do not have one‐dimensional flow, i.e., flow varies across the cross‐section. To use the average velocity in KE calculation, we should introduce:
•
Introducing the kinetic correction factor, the steady energy equation for incompressible flows becomes:
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M. Bahrami
ENSC 283 Spring 2009
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Bernoulli equation
• If flow is assumed frictionless, the steady energy equation reduced to
the Bernoulli equation.
•
The Bernoulli for steady frictionless incompressible flow along a streamline.
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M. Bahrami
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Application of Bernoulli equation
•
It should be noted that different streamlines may have different flow conditions and different Bernoulli constants.
Integral Relations for CV
M. Bahrami
ENSC 283 Spring 2009
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Bernoulli vs. energy equation
•
A comparison between the steady flow energy and Bernoulli equation
reveals that the energy equation is much more general than Bernoulli and
allows for i) friction, ii) heat transfer, iii)shaft work, and iv) various work.
•
Note that the Bernoulli equation is a momentum‐based force relation and is N
h h B
lli
i i
b df
l i
di
derived assuming:
i)
Steady flow
ii) Incompressible flow (applicable for Mach numbers less than 0.3)
iii) Frictionless flow (restrictive‐ where solid walls or mixing exist) iv)) Flow along a single streamline. g
g
•
•
•
•
Integral Relations for CV
M. Bahrami
ENSC 283 Spring 2009
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Hydraulic and energy line
•
The energy grade line (EGL) shows the height of the total Bernoulli constant, h0 = z + p⁄γ + V2⁄(2g)
•
The hydraulic grade line (HGL) shows the height corresponding to elevation and pressure head z + p⁄γ which is EGL minus the velocity head V2⁄(2g). Integral Relations for CV
M. Bahrami
ENSC 283 Spring 2009
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Example: Venturi tube
•
•
A constriction in a pipe will cause the velocity to rise and the pressure to fall
at section 2 in the throat. The pressure difference is a measure of the flow
rate through the pipe. The smoothly necked‐down system shown in figure
b l is
below
i called
ll d venturi
t i tube.
t b
Find an expression for the mass flux in the tube as a function of the pressure
change.
Integral Relations for CV
M. Bahrami
ENSC 283 Spring 2009
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