THERMODYNAMICS
Thermodynamics – is that branch of the physical sciences that
treats of various phenomena of energy
and the related properties of matter,
especially of the laws of transformation of
heat into other forms of energy and vice
versa.
a. Pure substance - a working substance whose chemical
composition remains the same even if
there is a change in phase. (most
common example is water)
b. Ideal gas - a working substance which remains in gaseous
state during its operating cycle and whose
equation of state is PV = mRT. (most common
example is air)
Newton's law states that 'the acceleration of
a particular body is directly proportional to the
resultant force acting on it and inversely
proportional to its mass.”
a=
đđš
,
đ
F=
đđ
,
đ
k is a proportionality constant
k=
đđ
đš
System of units where k is unity but not dimensionless:
cgs system: 1 dyne force accelerates 1 gram at 1 cm/s2
mks system: 1 newton force accelerates 1 kg mass at 1 m/s2
fps system: 1 lb force accelerates 1 slug mass at 1 ft/s2
System of units where k is not unity:
If the same word is used for both mass and force in a
given system, k is either unity or dimensionless:
1 lb force accelerates a 1 lb mass at 32.174 ft/s2
1 g force accelerates a 1 g mass at 980.66 cm/s2
1 kg force accelerates a 1 kg mass at 9.8066 m/s2
The mass of body is absolute quantity of matter in it.
The weight of the body means the force of gravity Fg on
the body.
đ đš đšđ
= =
đ đ
đ
where:
g = acceleration produced by force Fg
a = acceleration produced by another force F
At or near the surface of the earth, k and g are
numerically equal, so are m and Fg.
Sample Problems
1. What is the weight of a 66-kgm man at standard condition?
2. The weight of an object is 50 lb. What is its mass at
standard condition?
Properties of Working Substance:
Formulas:
1. Pressure - is the ratio of force per unit area.
Gage pressure – is the pressure reading from the gage
pressure instrument which is higher or
lower than the atmospheric pressure.
Vacuum pressure – is a pressure that is less than
atmospheric pressure.
a. Pressure =
đđđđđ
, KN/m2, lb/in2
đ¨đđđ
đ
đˇ=
đ¨
Absolute pressure(Pabs) = Gauge pressure + Atmospheric Pressure
Pabs = Pg + Patm
b. 1 atm = 0 Kpag
= 101.325 Kpa
= 14.7 psi
= 29.92 in Hg
= 760 mm Hg
= 1.033 kg/ cm2
c. Hydrostatic Pressure, P:
P
h
đˇ = đđ
where:
đ¤ = density of fluid
= (đđē đĨ đ¤đ¤)
â = height of fluid
đ¤đ¤ = density of water
= 1000 kg/m3 = 9.81 KN/m3
= 62.4 lb/ft3 = 1 kg/li
2. Temperature - is the degree of hotness or coldness of a substance or
body.
Thermometer - is an instrument used to measure the temperature
of a body or a substance.
Pyrometer - is an instrument used to measure high temperature
gases.
a. Relation between ī°C and ī°F scales
đ
ī°C = (oF-32)
đ
đ
ī°F = đĒ + đđ
đ
b. Absolute temperatures K and R:
K = ī°C + 273
R = ī°F + 460
c. Temperature Difference:
īī°C = 5/9 īī°F
īī°F = 9/5 īī°C
īī°C = īK
īī°F = īR
3. Density, Specific Volume and Specific Gravity:
đ đ
a. đ =
=
đŊ đ
đ
đ
=
b. đ =
đĻ đ°
đđđ§đŦđĸđđ˛ đ¨đ đđ§đ˛ đđĨđŽđĸđ
c. đēđŽ =
đđđ§đŦđĸđđ˛ đ¨đ đ°đđđđĢ
D. If two or more fluids are mixed together:
đŽđ đđ + đđ + â¯
đžđ =
=
đŽđŊ
đŊđ + đŊđ + â¯
đđ đđ
+
+â¯
đ
đŽđŊ
đŊđ + đŊ đ + â¯
đžđ đžđ
đđ =
=
=
=
đžđ đŽđ đđ + đđ + ⯠đđ + đđ + â¯
đžđ
đēđŽđ =
đžđ
Where:
đžđ = đđđđ đđĄđĻ đđ đđđĨđĄđĸđđ
đđ = đ đđđđđđđ đŖđđđĸđđ đđ đđđĨđĄđĸđđ
đēđŽđ = đ đđđđđđđ đđđđŖđđĄđĻ đđ đđđĨđĄđĸđđ
Sample Problems
1. Find the pressure at the 100 fathom depth of water in kpag.
2. Convert 750°R to °K.
3. An iron block weighs 5 N and has a volume of 200 cm3.
What is the density of the block?
4. A storage tank contains oil with specific gravity of 0.88
and depth of 20 m. What is the hydrostatic pressure at the
bottom of the tank in kg/cm2.
4. Internal Energy(U) - heat energy due to the movement of
molecules within the system that is
brought about its temperature.
5. Flow Work,Wf = Pressure x Specific Volume
đ˛đą
đžđ = đˇ đđ − đđ ,
đ˛đ
đžđ = đˇ đŊđ − đŊđ , đ˛đą
where: P = pressure, KPA
đŊ = volume, m3
đ = specific volume, m3/Kg
6. Enthalpy, h:
đ= Internal Energy + Flow Work
đ = đŧ + đˇđŊ
đ = đŧ + (đˇâđŊâ − đˇâđŊâ)
7. Stagnation enthalpy, hs:
đŊđ đ đ˛đą
đđ = đđ +
,
đđđđ đđ
where:
đđ = initial velocity, m/s
đđ = ordinary enthalpy, KJ/kg
Sample Problems
1. 100 g of water are mixed with150 g of alcohol ( W= 790
kg/ m3). What is the specific volume of the resulting mixture,
assuming the fluids mixed completely?
2. Steam flows through a nozzle at 400 oC and 1 Mpa ( h= 3263.9
KJ/kg) with velocity of 300 m/s. Find the stagnation enthalpy.
3. Given the steam pressure of 900 lb/ft2 , the temperature of 300
oF, specific volume of 5.8 ft3/lb. If the specific enthalpy is 9500 ftlb/lb, what is the internal energy per lb of the system?
8. Entropy, s:
đ
đ¸
āļą đ
đ = āļą
đģ
đ¸
đ=
đģ
General Flow Equations:
1. Volume flow rate, (đ¸) đđ/đđđ:
đ¸ = đ¨đđđ đ đŊđđđđđđđ
= đ¨đđŊ
where:
đ
đ
A= đĢđ (đđđ đđđđđđđđ đđđđđ đđđđđđđ)
đ¨ = đŗ đ đ (đđđ đđđđđđđđđđđ)
2. Mass flow rate (m), kg/sec:
đ
đ=
đ
where:
đ = specific volume
đŊ= volume
3. If Q is constant:
đ¸đ = đ¸ đ
đ¨đ đŊ đ = đ¨đ đŊ đ
đ
đ
đ
đĢđ đŊđ =
đĢđ đ đŊđ
đ
đ
đĢđ đ đŊđ = đĢđ đ đŊđ
Where:
đ¨đ
đ¨đ
đŊđ
đŊđ
= đđđđđĄ đđđđ
= đđĨđđĄ đđđđ
= đđđđđĄ đŖđđđđđđĄđĻ
= đđĨđđĄ đŖđđđđđđĄđĻ
Sample Problems
1. Air enters a nozzle steadily at 2.21 kg/m3 and 30 m/s.
What is the mass flow rate through the nozzle if the inlet
area of the nozzle is 80 cm2?
2. A pipe has a diameter of 4” at section AA, and a
diameter of 2” at section BB. For an ideal fluid flow, the
velocity is given as 1 ft/sec at section AA. What is the
flow velocity at section BB?