lecture 1 introduction of engineering thermodynamics

Engineering Thermodynamics

Course objective:The students will be given a comprehensive and rigorous treatment of engineering thermodynamics from the classical point of view. This course will prepare students to use thermodynamics in professional practice and gives them the necessary foundation for subsequent courses in thermodynamics, fluid mechanics and heat transfer.

Course materials:Required text: M.J. Moran and H.N. Shapiro, Fundamentals of

engineering Thermodynamics, 5th ed. John Wiley and sons, 2004.

Organization: 2 lectures per week (1.5 h/lecture).

Course evaluation: 2 term quizzes 10%

Assignments 10% Midterm exam 30%

Final exam 50%

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Topic and number of lectures TEXT SECTIONIntroductory Concepts and definitions: thermodynamics 1.1-1.7 Systems; property, state, process and equilibrium; unit for mass

length, time and force; specific volume and pressure; temperature;.

methodology for solving thermodynamics problems (2 lectures)

Energy and the First law of thermodynamics: Mechanical concepts 2.1-‐2.6

of energy; energy transfer by work; energy of a system; energy transfer

by heat; energy balance for closed systems; energy analysis of cycles.

(3 lectures)

Evaluating properties: State of a system; simple compressible system; 3.1-‐3.8

p-‐v-‐T realtion; thermodynamic property data; ideal gas model; polytropic

process of an ideal gas. ( 6 lectures)

Control Volume Energy Analysis: conservation of mass for a control volume, 4.1-‐4.3

Conservation of energy for a control volume; analysis of control volumes

At steady state. ( 4 lectures)

The second law of thermodynamics: Statements of the second law; 5.1-‐5.6

Irreversible and reversible processes; applying the second law to cycles;

Kelvin temperature scale; maximum performance for cycles operating

between two reservoirs; Carnot cycle. (3 lectures)

Entropy: Clausius inequality; entropy change definition; entropy 6.1-6.7, 6.9 of a pure, simple compressible substance; entropy change in internally reversible processes; entropy balance for closed systems;entropy rate balance for control volumes (steady state only); isentropic

processes; heat transfer and work in internally reversible, steady state

flow process. (7 lectures)

Thermodynamics

Study of heat and its interconversion to other kinds of energy

Heat?

Energy

Energy transformation

The principle of energy conservation

Energy formsChemical, mechanical, thermal, etc

Systems that transform energypower plants

refrigeration systems

Internal combustion engine

Fuel cells

Rockets

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Powerplant

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System: A quantity of matter

A system is a specific part of the universe that is of interest for study.

Surroundings: ” Everything else”

Concepts

System boundary: Separates system from surroundings

The boundary could be real or abstract. It is sometimes called control surface

All types of interaction between system and surroundings occur through the boundary.

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Example systems as defined by boundaryDefining the system boundary will specify the types of interactions between

system and surroundings

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Types of system

Closed system = Control mass

No mass crosses boundary Energy can cross the boundary

Control volume = open system

Mass and energy exchange

A gas in piston-cylinder assembly. example of a control volume ( open system. An

automobile engine

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Thermodynamics

Classical StatisticalVs

Macroscopic

Temperature pressure, etc

Overall behavior is a result of

Microscpic Atoms, molecules molecular interactions

Engineering thermodynamics

Energy Total Energy = E

E = U + KE + PE

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Engineering thermodynamicsClassical thermodynamics:

Concerned with overall behavior of a system. Do not deal with the structure of matter at atomic, molecular or subatomic level.

Objective: Evaluation important aspects of system behavior from observations of the overall system

Applications: Chemical engineering in general and many others

Statistical thermodynamics:

The microscopic approach to thermodynamics.

Objective: to characterize by statistical means the average behavior of particles making up a system.

Applications: Lasers, plasma, high speed gas flows, chemical kinetics, cryogenics ( very low temperature ) and others

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Thermodynamical concepts Property: Macroscopic characteristic of a system to which a numerical value can be assigned at a given time without knowledge of the previous behavior ( history) of the system.

Ex. Mass, volume, energy, pressure and temperature

If the value of a property for an overall system is the sum of its values for the parts into which the system is divided, it is called an Extensive property. Depend on the size or extent of the system.

Ex. Mass, volume, energy, and several others

Two types of properties:

Extensive properties

Intensive properties

If the value for property is independent of the size or extent of the system it is called Intensive property.

Ex. Temperature, density, ..

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State: refers to the condition of a system as described by its properties.

Steady state: A state in which the properties of the system do not change with time

Process: Transition from one state to another.

Thermodynamic cycle: a sequence of processes that begin and end at the same state.

How many properties do you need to describe the state of a system?

Thermodynamical concepts

State 1

State 2

Process path : The succession of states during a process

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P

V

(V1, P1)

(V2, P2)

Example of processes

Gas

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Process nomenclature

Isothermal processA process that occurs under constant temperature conditions.

example: melting of ice or evaporation of water

Isobaric processA process that occurs under constant pressure conditions.

example: melting of ice or evaporation of water

Isometric or isochoric process

A process that occurs under constant volume conditions.

Adiabatic process

A process that occurs with no heat transfer between the system and surroundings

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A condition in which all the competing influences are in balance.

Ex. Isolate a system and all its properties will go toward a uniform value

Quasiequilibrium process:

A process in which the departure from thermodynamic equilibrium is at most infinitesimal. i. e. a process that very close to equilibrium.

Ex. The crystallization of glass. Glass is a quasi state of SiO2 but very slowly it goes toward the equilibrium state which is the crystalline state of it.


Equilibrium

Can a process take place when you have a state at equilibrium?

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Phase:

A quantity of matter that is homogenous throughout in both chemical composition and physical structure

Ex. Air, homogenous liquid mixtures such as alcohol

Pure Substance:

A quantity of matter that is uniform and invariable in chemical composition.

It can exist in more than one phase but its chemical composition must remain the same in all phases.

Ex. Liquid water and water vapor


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SI unitsSI units: Systéme International d’Unités or International System of unites. Legally accepted system in most countries.

Quantity Dimensions Units symbol

Mass M kilogram kg

Length L meter m

time t second s

Quantity Dimensions Units symbol Name

Velocity Lt-‐1 m/s

Acceleration Lt-‐2 m/s2

Force MLt-‐2 kgm/s2 N newotons

Pressure ML-‐1t-‐2 kg/ms2 (N/m2) Pa pascal

Energy ML2t-‐2 kgm2/s2 (Nm) J joulePower ML2t-‐3 kgm2/s3 (J/s) W watt

Primary dimensions

Secondary dimensions

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Density and Specific volume

Continuum hypothesis: matter is described as distributed continuously throughout a region.

ρ = limV→ #V

mV$

%&

'

()

Density: When substances can be treated as continua, thus at any instant the density at a point is defined as.

V´ is the smallest volume for which the matter can be considered as continuum

Density or local mass per unit volume is an intensive property that might vary from point to point within a system.

Specific volume: defined as the reciprocal of the density. m3/kg Intensive property that may vary from point to point.

Specific molar volume: defined as the volume occupied by 1 mole of the substance. m3/kmol Intensive property that may vary from point to point.

v =Mv

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Pressure

Pressure: from the continuum viewpoint, the concept of pressure at any point is defined as

P= limA→ "A

FnormalA

#

$%

&

'(

A´ is the smallest area at a point for which the matter can be considered as continuum

The SI unit for pressure is Pascal (Pa) = 1 N/m2

Unit SI unit

1 Pa 1 Pa

760 mmHg (Torr) 1.0325x105Pa

1 atm 1.0325x105Pa

1 bar 105Pa

Ordinary vacuum gauge

△P = Patm - Pabs,2

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Absolute vs Gage pressure

Gage pressure: is the pressure measured with respect to the atmospheric pressure

0

Barometer reads atmospheric pressure Patm

Ordinary vacuum gauge

△P = Pabs,1 - Patm

Patm

Pabs,2

Pabs,1

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h

A

V=A*h W = mg = ⍴Vg = ⍴gAh

p = F/A = W/A = ⍴gh

PT

A B

PA = PB PT = PA

PB = Patm + ⍴gL

PT = Patm + ⍴gL

A

Pressure measurement

The gauge pressure is highly dependent on gravitational constant, so the gauge pressure on moon is completely different from that on Earth

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Absolute vs Gage pressure

Bourdon tube gage Manometer

P – Patm = ρgL

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Thermal equilibrium

When changes in temperature, electrical resistance and all properties related to the energy of the bodies cease to exist, then the two bodies are in thermal equilibrium.

Adiabatic process: a process with no thermal interaction with the surrounding.

Temperature is the indication to see the thermal equilibrium.

Isothermal process: a process in which the temperature of the system remains constant.

Zeroth law of Thermodynamics

When two bodies are in thermal equilibrium with a third body they are in thermal equilibrium.

A C B Then A B

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Temperature scale

Kelvin scale: the absolute thermodynamical temperature scale that provides a continues definition of temperature. Denotes as K

Celcius scale: uses the triple point of water as the standard fix point. The fix point is 273.16 K which is the 0 C.

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Methodology for solving thermodynamics problems

1. Known: state briefly in your own words what is known. Careful reading.

2. Find: Identify the objective. What has to be determined.

3. Schematic and given data: draw a sketch of the system and determine which system is appropriate for the analysis.

4. Assumptions: list all simplifying assumptions and idealizations made to reduce it to one that is manageable.

5. Analysis: use the assumptions and idealizations, reduce the appropriate governing equations and relationships to forms that will provide the desired results.

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Example

Vacuum system

IR spectrometer

A wind turbine-electric generator is mounted atop a tower. As wind blows steadily across the turbine blades, electricity is generated. The electrical output of the generator is fed to storage battery.

a) Considering only the wind turbine-electric generator as the system. Identify locations on the system boundary where system interacts with the surroundings. Describe changes occuring within the system with time.

b) Repeat for a system that includes only the storage battery.

Solution:

Known: A wind turbine-electric generator provides electricity to a storage battery.

Find: For a system of a) the wind turbine-electric generator b) the storage battery. Identify the locations where the system interacts with its surroundings, and describe the changes within the system with time.

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Example

Assumptions 1.In part (a) the system is the control volume shown by the dashed line. 2.In part (b) the system is closed system shown by dashed line 3.The wind is steady, i. e. blows at constant rate

Analysis (a)1st interaction between the system and surroundings is the air crossing the boundary of the control volume. 2d interaction is the electrical current passing through the wires. In terms of macroscopic interaction this is not a mass transfer. The changes of the system with time is none existent since the system reaches a steady state by the steady blowing wing. The rotational speed of the blades are constant thus the electricity generation also. (b)There is no macroscopic mass transfer. The system is closed. As the battery is charged and chemical reactions occur within it, the temperature of the battery surface may increase and a heat transfer to the surrounding may occur.

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Using specific volume and pressure

15 kg of carbon dioxide (CO2) gas is fed to a cylinder having a volume of 20 m3 and initially containing 15 kg of CO2 at a pressure of 10 bar. Later a pinhole develops and the gas slowly leaks from the cylinder.

a)Determine the specific volume in m3/kg of the CO2 in the cylinder initially. Repeat for the CO2 after the addition of 15 kg. b)Plot the amount of CO2 that has leaked from the cylinder in kg versus the specific volume of the CO2 remaining in the cylinder. consider v ranging up to 1.0 m3/kg

Solution:

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Patm

Patm

hB

hAh