c. v. raman college of engineering bhubaneswar department of mechanical engineering thermodynamics 1

PowerPoint PresentationDEPARTMENT OF MECHANICAL ENGINEERING
DEPARTMENT OF MECHANICAL ENGINEERING
1. INTRODUCTION To Thermodynamics
What is Thermodynamics ?
The word thermodynamics comes from the Greek words therme (heat) and dynamis (power).
Thermodynamics is both a branch of physics and an engineering science.
Thermodynamics can be defined as the science of energy.
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What is Thermodynamics ?
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Application Areas of Thermodynamics
All activities in nature involve some interaction between energy and matter; thus, it is hard to imagine an area that does not relate to thermodynamics in some manner.
C. V. RAMAN COLLEGE OF ENGINEERING BHUBANESWAR
Macroscopic approach
Macroscopic Approach
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Macroscopic Approach
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Microscopic Approach
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System, Surrounding & Boundary
System: A quantity of matter or a region in space chosen for study.
Surroundings: The mass or region outside the system
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Types of system
Based on the mass transfer and energy transfer across the boundary the systems are divided in 3 types.
Closed system or control mass system.
Open system or control volume system.
Isolated system
Closed system
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Open system (control volume): A properly selected region in space.
A control volume can be fixed in size and shape or it may involve a moving boundary as shown in figure.
Most control volume however have fixed boundaries and thus don’t involve any moving boundaries.
Example : Compressor, turbine, IC engine, nozzle, boilers
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Isolated System
There is neither energy transfer nor mass transfer occurs the boundaries which is shown in Figure.
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An equilibrium state is one in which the properties of the system do not change with time.
In many cases, an equilibrium state has intensive variables which are uniform throughout the system.
A non-equilibrium state may contain intensive variables which vary in space and/or time.
An equation of state is a functional relationship between the state variables; e.g. if P,V and T are the state variables, then the equation of state has the form f(P, V, T) =0.
In 3-dimensional P-V-T space,
and the equation of state is represented by a surface.
PROPERTIES OF A SYSTEM
Property: Any characteristic of a system.
Some familiar properties are pressure P, temperature T, volume V, and mass m.
Properties are considered to be either intensive or extensive.
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A property is a macroscopic characteristic of a system such as mass, volume, energy, pressure, temperature, entropy etc.
It is measurable characteristics describing the system.
Measureable characteristics are temperature, pressure, Volume etc.
Properties are classified into two categories
Extensive property
Intensive Property
Intensive properties: Those that are independent of the mass of a system, such as temperature, pressure, and density.
Extensive properties: Those whose values depend on the size or extent of the system.
Specific properties: Extensive properties per unit mass.
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Extensive property & Intensive Property
Extensive property
A property is called extensive if its value for an overall system is the sum of its values for the parts into which the system is divided. Extensive properties depend on the size or extent of a system. Example: Mass, volume, energy etc.
Intensive Property
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State of a system
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State
At a given state, all the properties of a system have fixed values. If the value of even one properties changes, the state will change to a different one.
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State
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Steady state
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Path
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Process
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Thermodynamic cycle
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Reversible Process
Reversible Process:
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Irreversible Process
Irreversible process:
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HOMOGENEOUS AND HETEROGENEOUS SYSTEMS
A quantity of matter homogeneous throughout in chemical composition and physical structure is called a phase
Every substance can exist in any one of the three phases,i.e.,solid,liquid and gas
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Thermodynamic Equilibrium
A system is said to exist in a state of thermodynamic equilibrium when no spontaneous change in any macroscopic property is observed as the system is isolated from its surroundings.
Equilibrium means the state of balance of a system within itself and between system and surroundings. Thus for attending a state of thermodynamic equilibrium following three types of equilibrium states the system must achieve.
Thermal Equilibrium
Mechanical Equilibrium
Chemical equilibrium
Thermal Equilibrium
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Mechanical Equilibrium
It is related to pressure and a system is in mechanical equilibrium if there is no change in pressure at any point of the system with time
However, the pressure may vary with in the system with elevation as a result of gravitational effects. But there is no imbalance of forces
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Chemical equilibrium
A system is in chemical equilibrium if no chemical reaction or transfer of matter from one part to another within the system takes place either through diffusion or any other chemical process.
Chemical composition of the system doesn't change with time.
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Criteria for an equilibrium
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When a process proceeds in such a manner that the system remains infinitesimally close to an equilibrium state at all times, it is called a quasi-static or quasi-equilibrium process
It is a sufficiently slow process that allows the system to adjust itself internally so that properties in one part of the system do not change any faster than those at other parts
Pressure
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PRESSURE
The normal stress (or “pressure”) on the feet of a chubby person is much greater than on the feet of a slim person.
Some basic pressure gages.
Absolute pressure: The actual pressure at a given position. It is measured relative to absolute vacuum (i.e., absolute zero pressure).
Gage pressure: The difference between the absolute pressure and the local atmospheric pressure. Most pressure-measuring devices are calibrated to read zero in the atmosphere, and so they indicate gage pressure.
Vacuum pressures: Pressures below atmospheric pressure.
Throughout this text, the pressure P will denote absolute pressure unless specified otherwise.
Pascal’s law: The pressure applied to a confined fluid increases the pressure throughout by the same amount.
Lifting of a large weight by a small force by the application of Pascal’s law.
The area ratio A2/A1 is called the ideal mechanical advantage of the hydraulic lift.
The Manometer
Measuring the pressure drop across a flow section or a flow device by a differential manometer.
The basic manometer.
It is commonly used to measure small and moderate pressure differences. A manometer contains one or more fluids such as mercury, water, alcohol, or oil.
Other Pressure Measurement Devices
Bourdon tube: Consists of a hollow metal tube bent like a hook whose end is closed and connected to a dial indicator needle.
Pressure transducers: Use various techniques to convert the pressure effect to an electrical effect such as a change in voltage, resistance, or capacitance.
Pressure transducers are smaller and faster, and they can be more sensitive, reliable, and precise than their mechanical counterparts.
Strain-gage pressure transducers: Work by having a diaphragm deflect between two chambers open to the pressure inputs.
Piezoelectric transducers: Also called solid-state pressure transducers, work on the principle that an electric potential is generated in a crystalline substance when it is subjected to mechanical pressure.
THE BAROMETER AND ATMOSPHERIC PRESSURE
Atmospheric pressure is measured by a device called a barometer; thus, the atmospheric pressure is often referred to as the barometric pressure.
A frequently used pressure unit is the standard atmosphere, which is defined as the pressure produced by a column of mercury 760 mm in height at 0°C (Hg = 13,595 kg/m3) under standard gravitational acceleration (g = 9.807 m/s2).
The length or the cross-sectional area of the tube has no effect on the height of the fluid column of a barometer, provided that the tube diameter is large enough to avoid surface tension (capillary) effects.
Temperature
Temperature is defined as the degree of hotness or coldness of a body measured on a definite scale.
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Temperature
It is a measure of mean K.E. of the molecules of the system. A change in temperature of a system accounts for change in molecular motion and the K.E and the molecule.
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Temperature
When work or heat is supplied to a system it is not mandatory that the temperature of the system will increase. It may or may not increase until the molecular K.E. increase.
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Zeroth law of Thermodynamics
If two bodies A and B are separately in thermal equilibrium with third body „C then „A & B must be in thermal equilibrium with each other as shown in figure.
Adiabatic wall is a wall which does not allow the heat to pass through it.
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Fortunately, several properties of materials change with temperatures in repeatable & predictable way and this forms the basis for accurate temperature measurement.
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2. Gas thermometer
4. Resistance thermometer
Liquid in glass thermometer
It consists of a gas capillary tube connected with a bulb filled with a liquid such as mercury & alcohol and sealed at the other end.
Liquid in glass thermometer
The space above the liquid is occupied by the vapour of the same liquid or an inert gas.
As the temp increases the liquid expands in volume and rises in the capillary.
The length ‘L’ of the liquid in the capillary depends on the temp.
Accordingly the liquid is the thermometric substance and ‘L’ is the thermometric property.
Mercury freezes at -4°C at standard atmospheric pressure.
The glass deforms at 595°C.
Gas Thermometer
The constant volume gas thermometer is so exceptional in terms of precision & accuracy that it has been adopted internationally as the standard instrument for calibration.
Gas Thermometer
The thermometric substance is the gas (H2 or He) and the thermometric property is pressure exerted by the gas.
As shown in the Fig the gas is contained in a bulb and the pressure exerted by the gas is measured by an open tube mercury manometer.
As the temperature increases the gas expands forcing mercury up in the open tube.
The gas is kept at constant volume by raising or lowering the reservoir.
The gas thermometer is used as a standard worldwide by Bureau of standards.
Electric Resistance thermometer
In resistance thermometer as shown in Fig. the change in resistance of a metal wire due to change in its temperature is the thermometric property.
Electric Resistance thermometer
The wire frequently platinum may be incorporated in a Wheatstone bridge.
In a restricted range the following quadratic equation is often used
P/Q = R/S , R= R0 (1+At+Bt2)
Where R0 is the resistance of the platinum wire when it is surrounded by melting ice & A & B are constant.
Thermocouple
Thermocouple circuit made of from joining two wires A and B of dissimilar metals.
Thermocouple
According to See beck effects of the two junctions of two dissimilar metals are maintained at two different temperatures an emf will be generated and that emf is the thermometric property for measurement of temp using thermocouple.
Some commonly used material in thermocouple
(i) Copper-constantan (alloy of copper and nickel)
(ii) Chrome- Alumel
(iii) Platinum- (Platinum-Rhodium)
The advantages are: It comes with thermal equilibrium with the system whose temperature is to be measured quite rapidly because of its small mass.
Ideal gas temperatures scale
Let the bulb of the constant volume gas thermometer contains any one of the gas like O2, Air, N2, H2 etc.
Assuming that the pressure of the gas is approximately 1000 mm of Hg when it is in contact with triple point of water i.e.
Pt= 1000 mm of Hg
Let the bulb is surrounded by steam at atmospheric condition and let corresponding thermometric property is P1
Temperature reading will be
T1 = 273.16 X P1/Pt
Again if we remove some gas from the bulb so that when it is in contact with the triple point of H2O, then the Pt value is let 500 mm. Hence the new values of pressure and temperature are to be found out T2= 273.16 ( P2/500)
If we continue the process by reducing the amount of gas in the bulb then Pt and P will go on reducing and the corresponding temperature can be calculated.
If we consider constant pressure gas thermometer and a similar series of experiments will be carried out, we will get the same curves for different gases plot between T & V.
When Vt approaches zero, all gases, through follow different paths will meet at a point give rose to a common temperature T .
T= 273.16(V/Vt)
Standard scale temperature
In gas thermometer let the bulb containing the gas is kept contact initially with ice point and the pressure is Pi which is equivalent to 1000 mm of Hg. Then the bulb is in contact with steam point and the pressure is Ps.
Ratio Ps/Pi is determined and called (Ps/Pi) = 1000
Some gas is then removed such that Pi becomes 500 mm of Hg.
So (Ps/Pi)500 is obtained.
This may be repeated for more values of Pi.
If a graph is plotted between (Ps/Pi) and Pi for all gases (O2, Air, N2, H2, etc.) straight lines are achieved and they meet a common point when Pi approaches to zero.
On experiment Ps/Pi= 1.366
Ts= 373.16 K
Ti= 273.16 K
Where ti0C= 0°C
Thermodynamic Temperature scale
It is the temperature scale in which temperature value does not depend on the properties of any particular substance. It is known as Kelvin scale or absolute scale.
All thermodynamic calculations are based on absolute Kelvin scale.
T(K) = T°C+273.15
Rankine Scale
Absolute temperature scale in FPS unit. Relation between relative scale and absolute scale.
T(R) = T° (F) +459.67
a)pressure b)volume c)temperature d)energy
Open system is also known as ___________ system
In case of an isolated system there is interaction of energy with the surroundings (True/False)
In a thermodynamic cycle the final state is _________ with the initial state.
6. Mechanical equilibrium means equality of ______
7. Thermal equilibrium means equality of__________
8. A quasi-static process is also called a ________ Process.
9. In case of a constant pressure gas thermometer __________ is the thermometric property.
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(a)760mm Hg (b)101.32kN/m2 (c)1.0132 bar (d)All of these
12.The extensive property is defined as
(a) the property which depends on mass only
(b) the property which doesn't depend on mass
(c) the property which depends on various processes
(d) the property which depends on various thermodynamic cycles
13. which one of the following is false
(a)Volume is an extensive property
(b)Heat capacity is an extensive property
(c)pressure is an extensive property
(d)Entropy is an extensive property
14.A homogeneous system is defined as
(a)the system which consists of more than one phase
(b)the system which consist of single phase
(c)the system which consists of more than one pure substances
(d)none of these
16. Absolute pressure=Atmospheric pressure + _________________
17.Atmospheric pressure-absolute pressure=_________________
18.Which one of the following may be a variable in a closed system.
(a)mass of a system (b)boundary of a system
(c)Mass as well as boundary (d)none of these
19.Which one of the following is correct in an open system
(a)boundary of the system remain constant (b)mass remain constant
(c)boundary of the system varies (d)mass and boundary both varies
20.In an isolated system
(a)mass transfer takes place (b) energy transfer takes place (c)both mass and energy transfer takes place (d)neither mass transfer nor energy transfer takes place.
2. Explain in details about a quasi-static process.
3. Convert the following readings of pressure to Kpa, assuming that the barometer reads 760mm Hg.
a) 60cm Hg vacuum
b) 30cm Hg gauge
c) 2m H2O gauge
ASSIGNMENT QUESTIONS
4. The temperature t on a thermometric scale is defined in terms of a property K by the relation
t=a ln K + b
Where a and b are constants. The values of K are found to be 1.83 and 6.78 at the ice point and the steam point, the temperatures of which are assigned the numbers 0 and 100 respectively.Determine the temperature corresponding to a reading of K equal to 2.42 on the thermometer.
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ASSIGNMENT QUESTIONS
6.A pump discharges a liquid into a drum at the rate of 0.032m3/s.The drum, 1.50m in diameter and 4.20m in length can hold 3000kg of the liquid.Find the density of the liquid and the mass flow rate of the liquid handled by the pump.
7.A 30m high vertical column of a fluid of density 1898kg/m3 exists in a place where g=9.65m/s2.what is the pressure at the base of the column?
8.A turbine is supplied with steam at a gauge pressure of 1.4MPa.After expansion in the turbine the steam flows into a condenser which is maintained at a vacuum of 710mm Hg.The barometric pressure is 772mm Hg.Express the inlet and exhaust steam pressures in pascals(absolute).Take the density of mercury as 13.6 x 103 kg/m3.

c. v. raman college of engineering bhubaneswar department of mechanical engineering thermodynamics 1

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