Thermodynamics revision

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System:
A quantity of matter or a region in space chosen for study.
Surroundings.
The mass or region outside the system
Boundary
The real or imaginary surface that separates the system from its surroundings
System
Closed
(Fixed Mass)
(Control Mass)
No Mass Transfer Cross Boundry
Energy Can be Transfered
Open
(Fixed Volume)
(Control Volume)
Mass Transfer CrossBoundry
Energy can be Transfered

Isolated system
Energy is not allowed to cross the boundary
State
A set of properties uniquely defining the conditions of the substance
Process
Any change from one equilibrium state to another
Cycle
A series of connected processes in which the initial state is the final state.
Special Types of Processes:
 Isothermal process
A process during which the temperature T remains constant.
 Isobaric process
A process during which the pressure P remains constant.
 Isochoric (or isometric) process
A process during which the specific volume (v) remains constant.
 Adiabatic process
No heat transfer occurs
 Isentropic Process
No entropy change occurs
Density is defined as mass per unit volume

Specific Volume v
The reciprocal of density is, which is defined as volume per unit mass
Pressure
Force per unit area is called pressure, and its unit is the Pascal, N/m2
in the SI system
Special Types of Processes:
Isothermal process (C): A process during which the temperature T remains constant.
Isobaric process (D):A process during which the pressure P remains constant.
Isochoric (or isometric) process (A):A process during which the specific volume v remains
constant.
Adiabatic, B

Properties of Pure Substances
Pure Substance
A substance that has a fixed chemical composition throughout.
Compressed Liquid (Sub-Cooled Liquid)
No vapor, all liquid, NOT about to vaporize
Saturated Liquid
About to vaporize
Saturated Vapor:
About to condense
Superheated Vapor:
No longer any liquid, NOT about to condense.
Saturation Temperature:
The boiling temperature at a given pressure
Saturation Pressure:
The pressure at which boiling occurs at a given T
Enthalpy, H
 another property of pure substances (like internal energy, U)
 U is a function ONLY of the temperature of the substance, f(T)
 H is formed by combining the internal energy, U, and work done, f(T,P)
 on a per unit mass basis: h = u + Pv (specific enthalpy)

PROPERTY TABLES
 used to look up the properties for various substances at many temperatures and
pressures
 vf = specific volume of the saturated LIQUID
 vg = specific volume of the saturated VAPOR
 vfg = the difference between the specific volume of the saturated vapor and
saturated liquid = (vg - vf)
QUALITY, x, is never used to describe compressed liquid or superheated vapor, it may
be expressed as a percentage: from 0% to 100%, where 0% is a saturated liquid and
100% is a saturated vapor.
Quality has significance for saturated mixtures only. It has no meaning in the
compressed liquid or superheated vapor regions. Its value is between 0 and 1.

1- For saturated
The values of the average properties of the mixtures are always between the values of
the saturated liquid and the saturated vapor properties
2- For Compressed Liquid
Usually we approximate compressed liquid behavior evaluated at the given
TEMPERATURE (don’t use the pressure)
v = vf @ T h = hf @ T u = uf @ T s = sf @ T

3- For Super heated vapor
Ideal Gases:
The Ideal Gas Equation is:
where: Ru …… the universal gas constant
Ru = 8.314 kJ/kmole-K (for all gases)
M …… the molecular weight of the gas (kg/kmole )
N……..the number of moles of the gas molecules
For changes of state (with a fixed mass of Ideal gas; closed system):
P1 v1 = R T1 and P2 v2 = R T2

Specific heat at constant volume (Cv)
The amount of energy needed to raise the temperature of a unit of mass of a
substance by one degree in a constant-volume process
Specific heat at constant volume (Cp)
The amount of energy needed to raise the temperature of a unit of mass of a
substance by one degree in a constant-pressure process

FIRST LAW OF THERMODYNAMICS
Law of Conservation of Energy: "Energy cannot be created or destroyed, it can only
change forms"
The First Law of
Thermodynamics
Closed
(Fixed Mass)
(Control Mass)
no mass transfer (in/from the
System
Examples:
- Tank
- Vessile
- Piston Cylinder Device
Open
(Fixed Volume)
(Control Volume)
Examples:
- Compressor
- Turbine
- Nozzle
- Diffuser
- Mixing Chamber
- Throttle vavle
- Heat Exchanger
- Pump

For a closed system:
Energy can cross the boundaries of a closed system in the form of heat or work.
Energy transfer across the boundaries of a closed system is due to a temperature
difference, it is heat; otherwise, it is work.
Remember, SIGN CONVENTIONS:
+Q is heat ADDED TO a system FROM the surroundings (Energy IN)
+W is work done BY a system ON the surroundings (Energy OUT)
Q - W = ∆E = ∆U + ∆KE + ∆PE
∆U = m ( u2 - u1)
∆KE = ½ m ( V2
2
- V1
2
)
∆PE = mg ( z2 - z1)
If NO Work is done ON or BY the system (W = 0):
Q = ∆E
If NO Heat is exchanged between the system and its surroundings:
(Q = 0, ADIABATIC = perfectly insulated)
-W = ∆E
If the system is stationary (no motion), ∆KE = ∆PE = 0
Q - W = ∆U
ALTERNATE Form (more convenient):
Q - WOTHER - Wb = ∆E
In terms of RATE:
- = dE/dt
In terms of per unit mass:
q - w = ∆e
In a CYCLE, the system returns exactly to its initial state:
∆E = 0 because E1 = E2 Q - W = 0
For an Open System:
A control volume differs from a closed system in that it involves mass transfer
MASS CONSERVATION: The Continuity Equation
Σ m in - Σ m out = Σ m cv
(mass coming in) - (mass going out) = (change of mass in the C.V.)

= amount of mass/unit time
If the flow velocity, V, and the cross-sectional area, A, of the channel are given:
= .V(Velocity). A since  = 1 / V
Flow is also measured as a VOLUMETRIC Flow Rate (m3/sec)
= V(Velocity). A
Giving: =  = / v
MASS CONSERVATION (the amount of mass in the C.V. is CONSTANT):
I = E
( V A) I = (  V A) E

Steady Flow Devices
NOZZLE
̇ ̇
For adiabatic
̇ ̇
DIFFUSER
̇ ̇
For adiabatic
̇ ̇
TURBINES:
̇
For adiabatic
̇ ̇
COMPRESSORS (or PUMPS):
Require Work (Work Done ON Fluid)
̇
For adiabatic
̇ ̇
THROTTLING VALVES:
MIXING CHAMBERS:

Heat Engine
Air conditioning,
Refrigerator
Heat Pump

CARNOT PRINCIPLES
(Use ABSOLUTE T)
THE CARNOT HEAT ENGINE
for a reversible heat engine
The Carnot efficiency is the highest possible efficiency for a heat engine operating
between thermal energy reservoirs at TH and TL.
CARNOT Refrigeration and Heat Pumps

WHAT IS ENTROPY?
Entropy is a thermodynamic property that measures the degree of randomness,
disorder or uncertainty
S = total entropy in kJ/o
K
s = specific entropy in kJ/kg o
K
For a process, the change in entropy is
Entropy Change in Ideal Gases (Assuming Constant Specific Heats):
Using ideal gas relationships: CP = CV + R and k = CP/CV
For isentropic process:

The Efficiency of Compressor and Turbine

Thermodynamics revision

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