ME 152 Thermodynamics

1
ME 152Thermodynamics

• G.A. Kallio
• Dept. of Mechanical Engineering, Mechatronic
  Engineering Manufacturing Technology
• California State University, Chico

2
Basic Concepts Definitions

• Reading Cengel Boles, Chapter 1

3
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

4
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

5
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

6
Thermodynamic Applications

• See Figure 1-5 and class overhead slides

7
Dimensions Units
8
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

9
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 systems
  history
• Extensive Property - property that is dependent
  on system size
• Intensive Property - property that is independent
  of system size

10
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)

11
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

12
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

13
Some Basic Thermodynamic Properties

• Energy
• Density
• Specific Volume
• Pressure
• Temperature

14
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.

15
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)

16
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

17
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)

18
Density and Specific Volume

• Density (kg/m3),
• Specific Volume (m3/kg),
• Specific Gravity

19
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)

20
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

21
Pressure, cont.

• Pressure variation with depth
• Pascals principle a force applied to a
  confined fluid increases the pressure throughout
  by the same amount since F PA, mechanical
  advantage can be developed

22
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

23
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 (Pascals law)

24
Temperature

• Temperature (ºC or K)
• measure of a bodys hotness or coldness
• indicative of a bodys 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

25
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