bme unit v thermodynamics
TRANSCRIPT
UNIT V
Thermodynamics: Thermodynamics system (open, closed, isolated), Thermodynamic
Properties: Definition and Units of – Temperature, Pressure (atmospheric, absolute,
gauge), Volume, Internal Energy, Enthalpy, Concept of Mechanical Work,
Thermodynamics Laws with examples – Zeroth, First, Limitation of First Law,
Concept of Heat Sink, Source, Heat Engine, Heat Pump, Refrigeration Engine, 2nd law
of thermodynamics statement (Kelvin Plank, Claussius), Numerical of 2nd law only
Measurements: Measurement of Temperature (Thermocouple – Type according to
temperature range and application), Measurement of Pressure (Barometer, Bourdon
Pressure Gauge, Simple U Tube Manometer with numerical)
Thermal Engineering
IntroductionDefinition of Thermodynamics: Branch of physical science which deals with
study of energy transfer and its effects on properties of system and surrounding
Application of thermodynamics:
Thermodynamic System, Surrounding and Boundary
Thermodynamic System• A quantity of matter or region in
space chosen for study
Surroundings• Mass or region outside the system
Boundary• The real or imaginary surface that
separates the system from the surroundings
Types of Thermodynamics Systems
Closed System (control mass)
• Consists of a fixed amount of mass
• Mass can NOT cross the boundary
• Energy can cross the boundary
• Volume does not have to be fixed
Isolated System
• Special case of a closed system
• No energy is allowed to cross the boundary
mass = constantVolume = constantEnergy = constant
Open System (control volume)• Any arbitrary region in space• Usually involves a device through which
mass flows• Mass and energy can cross the boundary• Can be fixed in space or have a moving
boundary• Control volumes can change in actual
volume with time
Properties of a SystemProperty
• Any characteristic of a system• E.g. temperature, pressure, volume
Intensive Property• Independent of the size of the system• E.g. temperature, pressure, density
Extensive Property• Value depends on the size (or extent) of
the system• E.g. mass, volume, total energy
Specific Properties• Extensive properties per unit mass• E.g. specific volume, specific energy
mVv mEe
Property Unit ConversionTemperature Kelvin ( K),
Celcius1 = 273 K
Pressure N/m2, bar, Pascal(pa), mm of Hg
1 bar = 10 5 Pa
1 atm = 1.01325 bars
Volume m3, Litre 1 litre = 10-3 m3
Internal energy
Joule (J), N.m 1J = 1 N.m
Enthalpy Joule (J)
Units & Conversion
Zeroth Law of Thermodynamics Observation
• When a body is brought into contact with another body that is at a different temperature, heat is transferred from the body at higher temperature to the one at lower temperature until both bodies attain the same temperature (thermal equilibrium)
Zeroth Law of Thermodynamics• If two bodies are in thermal equilibrium with a
third body, they are also in equilibrium with each other
• Cannot be concluded from the other laws of thermodynamics
• Serves as a validity of temperature measurement• Restated: Two bodies are in thermal equilibrium if
both have the same temperature reading, even if they are not in contact
When closed system executes a cyclic process the algebric sum of work transfers is propertional to the algebraic sum of heat transfer
CYCLECYCLEQW
Example: Automobile Engine
Chemical Kinetic
Heater/Furnace Chemical HeatHydroelectric Gravitational
ElectricalSolar Optical ElectricalNuclear Nuclear Heat,
Kinetic, Optical
Battery Chemical ElectricalFood Chemical Heat,
KineticPhotosynthesis Optical Chemical
Ist Law of a Thermodynamics
Limitation of First Law of Thermodynamics
A cup of hot coffee left in a cooler room eventually cools off. The reverse of this process- coffee getting hotter as a result of heat transfer from a cooler room does not take place.Consider heating of a room by passage of electric current through an electric resistor. Transferring of heat from room will not cause electrical energy to be generated through the wire. Consider a paddle-wheel mechanism operated by fall of mass. Potential energy of mass decreases and internal energy of the fluid increases. Reverse process does not happen, although this would not violate first law. Water flows down hill where by potential energy is converted into K.E. Reverse of this process does not occur in nature.
Processes proceed in a certain direction and not in the reverse direction. The first law places no restriction on direction.
A process will not occur unless it satisfies both the first and second laws of thermodynamics.
Second law not only identifies the direction of process, it also asserts that energy has quality as well as quantity.
Need Of Second Law of Thermodynamics
Source and SinkA reservoir that supplies energy in the form of heat is called a source and one that absorbs energy in the form of heat is called a sink. For example, atmospheric air is a source for heat pumps and a sink for air conditioners.
Thermal ReservoirA thermal reservoir is a large system (very high mass x specific heat value) from which a quantity of energy can be absorbed or added as heat without changing its temperature. The atmosphere and sea are examples of thermal reservoirs.
W= Q1-Q2
Heat Engine
Q1
Q2
Refrigerator and Heat Pump
Statement of Second law of thermodynamics
The Kelvin-Planck Statement:It is impossible for any device that
operates on a cycle to receive heat from single reservior and produce net amount of work.
Thermal energy reservoir
Engine
Q1= 100 kw
Q2=0
W = 100kw
Heat engine violetes the Kelvin – Planck statement
Clausius Statement
It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a low temperature body to higher temperature body.
Warm environment
REFRIGERATOR
Cold refrigerated space
Q1 = 5 KJ
Q2 = 5 kJ
W = 0
Temperature Measurement
What is a Thermocouple?A thermocouple consists of two dissimilar metals, joined together at one end, which produce a small voltage when heated (or cooled). This voltage is measured and used to determine the temperature of the heated metals. The voltage for any one temperature is unique to the combination of metals used.
Summary of Different Types of Thermocouple
Pressure Measurement Devices
Absolute pressure: The actual pressure at a
given position
Gage pressure: Difference between absolute
and atmospheric pressure.
Vacuum pressure: Pressure below
atmospheric pressure.
Barometer
Bourdon Pressure Gauge
Simple U Tube Manometer