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ME 152Thermodynamics

G.A. Kallio

Dept. of Mechanical Engineering, Mechatronic Engineering & Manufacturing Technology

California State University, Chico

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Basic Concepts & Definitions

Reading: Cengel & Boles, Chapter 1

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Introduction

• Thermodynamics - science that deals with energy, matter, and the laws governing their interaction– general: all engineering systems

involve energy and matter

– fundamental: based upon primitive concepts (two primary laws)

– employs a unique vocabulary based upon precise definitions

– initially, it appears formal and abstract, but its significance and application will eventually be seen

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Introduction, cont.

• Classical Thermodynamics - macroscopic approach that deals with large systems, e.g., engines, power plants, refrigerators, etc.; studied and used by engineers

• Statistical Thermodynamics - microscopic approach that deals with the structure and properties of matter on an atomic/molecular level; studied and used by physicists and chemists

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Primary Laws of Thermodynamics

• First Law of Thermodynamics - quantitative conservation of energy principle; energy cannot be created nor destroyed

• Second Law of Thermodynamics - places qualitative restrictions on energy-related processes, e.g., direction of heat transfer, maximum performance of power plants

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Thermodynamic Applications

• See Figure 1-5 and class overhead slides

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Dimensions & Units

Dimension SI Englishmass kg lbmlength m fttime s stemperature(absolute)

K R

force N(= 1 kg-m/s2)

lbf(= 32.174lbm-ft/s2)

energy J(= 1 N-m)

Btu(= 778.169lbf-ft)

power W(= 1 J/s)

hp(= 0.7068Btu/s)

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Basic Thermodynamic Definitions• System - quantity of matter or

region of space chosen for study• Surroundings - mass or region

outside of system• Boundary - real or imaginary

surface that separates system from surroundings

• Closed System (Control Mass) - a fixed quantity of mass that can only experience energy transfer (no mass can enter or leave); an isolated system is a special case where no mass or energy transfer is allowed

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Basic Thermodynamic Definitions, cont.

• Control Volume (Open System) - region of space that can experience both energy and mass transfer across its boundary

• Property - a characteristic of a system that can be defined without knowledge of the system’s history

• Extensive Property - property that is dependent on system size

• Intensive Property - property that is independent of system size

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Basic Thermodynamic Definitions, cont.• State - a condition of a system that is

fully described by properties• Equilibrium - a state where there are

no imbalances due to mechanical, thermal, chemical, or phase effects

• State Postulate - gives the number of properties needed to fix the state of a system

• Simple Compressible System - a system where external force fields are negligible (i.e., electrical, magnetic, gravitational, motion, and surface tension effects)

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Basic Thermodynamic Definitions, cont.

• Process - a change that a system undergoes from one equilibrium state to another; the sequence of states through which the system passes is called the process path

• Quasi-equilibrium Process - a sufficiently slow process that allows the system to remain infinitesimally close to equilibrium

• Cycle - a sequence of processes that returns the system to its initial state

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Basic Thermodynamic Definitions, cont.

• Isothermal Process - a process where temperature remains constant

• Isobaric Process - a process where pressure remains constant

• Isochoric Process - a process where volume or density remains constant

• Steady-Flow Process - a control volume process where all properties at a fixed point remain constant with respect to time

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Some Basic Thermodynamic Properties

• Energy

• Density

• Specific Volume

• Pressure

• Temperature

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Energy

• Energy is an extensive property of a system; it is the capacity to do work or cause change– can be stored

– can be transferred

– can be transformed

– is always conserved

• Types of Energy– mechanical, kinetic, potential, thermal,

electric, magnetic, chemical, nuclear, latent, et al.

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Energy, cont.

• Macroscopic energy - forms of energy that a system possesses as a whole w.r.t. some external reference frame, e.g., kinetic and potential energies

• Microscopic energy - forms of energy related to the molecular and atomic structure of a system; the sum of all microscopic forms of energy is known as internal energy (U)

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Energy, cont.

• System energy can be stored as– Kinetic energy, KE = ½mV2

e.g., throwing a ball

– Gravitational potential energy, PE = mgz

e.g., raising a dumbbell

– Internal energy, U = ?

e.g., heating the air in a room

• In the absence of electric, magnetic, and surface tension effects, the total energy (E) of a system is

E = U + KE + PE

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Energy, cont.• Energy can be only be transferred

across a system boundary by– work interactions, due to a force acting

through some distance

– heat transfer, due to a temperature difference

– mass flow, due to fluid flow into or out of a control volume

• Energy can be transformed in many ways, e.g., – chemical-electrical (battery)

– electrical-thermal (resistor)

– potential-kinetic (dropping a rock)

– nuclear-thermal (nuclear reactor)

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Density and Specific Volume

• Density (kg/m3),

• Specific Volume (m3/kg),

– Specific Gravity

V

m

1

m

Vv

COHs

4@2

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Pressure

• Fluid Pressure (N/m2)

• Other units:

1 pascal (Pa) = 1 N/m2

1 kPa = 103 N/m2

1 bar = 105 N/m2

1 MPa = 106 N/m2

1 atm = 101.325 kPa

= 14.696 lbf/in2 (psi)

A

FP normal

A smalllim

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Pressure, cont.

• Absolute pressure - total pressure experienced by a fluid

• Gage pressure or vacuum pressure- difference between absolute pressure and atmospheric pressure (usually indicated by a measuring device):

Pgage = Pabs - Patm

Pvac = Patm - Pabs

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Pressure, cont.

• Pressure variation with depth:

• Pascal’s principle: a force applied to a confined fluid increases the pressure throughout by the same amount; since F = PA, mechanical advantage can be developed

ghPP atm

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Pressure Measurement

• Manometer – gravimetric device based upon liquid level deflection in a tube

• Bourdon tube – elliptical cross-section tube coil that straightens under under influence of gas pressure

• Mercury barometer – evacuated glass tube with open end submerged in mercury to measure atmospheric pressure

• Pressure transducer – converts pressure to electrical signal; i) flexible diaphragm w/strain gage ii) piezo-electric quartz crystal

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The U-tube Manometer

• Simple, accurate device for measuring small to moderate pressure differences

• Rules of manometry:– pressure change across a fluid column of

height h is gh

– pressure increases in the direction of gravity

– two points at the same elevation in a continuous static fluid have the same pressure (Pascal’s law)

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Temperature

• Temperature (ºC or K)– measure of a body’s “hotness” or

“coldness”

– indicative of a body’s internal energy

– used to determine when a system is in thermal equilibrium, i.e., when all points have the same temperature

– see zeroth law of thermodynamics, section 1-9

– unit conversions:

K = ºC + 273.15

R = ºF + 459.67

ºF = 1.8 ºC + 32

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Temperature Measurement

• Constant-P liquid-in-glass – utilizes volume change of mercury or alcohol in a tube

• Constant-V gas – utilizes pressure change of hydrogen or helium

• Bimetallic strip – utilizes differential CTE of adjoined dissimilar metals

• Thermistor, RTD – utilizes electrical resistance of metals and semiconductors

• Thermocouple - utilizes voltage produced from dissimilar metal junctions

• Optical pyrometer – utilizes infrared emission spectrum