ME 200 L2: Introduction to Thermodynamics
Volume, Pressure, Temperature, Design
and Analysis, Problem Solving
Spring 2014 MWF 1030-1120 AM
L2 given by Indraneel Sircar
J. P. Gore, Reilly University Chair Professor
gore@purdue.edu
Gatewood Wing 3166, 765 494 0061
Office Hours: MWF 1130-1230
TAs: Robert Kapaku rkapaku@purdue.edu
Dong Han han193@purdue.edu
Resources for our learning
• Fundamentals of Engineering Thermodynamics, Moran,
Shapiro, Boettner and Bailey, Seventh Edition.
• Read assigned sections before coming to class.
• Group class email will be used frequently to
communicate. Also use http://www.purdue.edu/mixable
• Class participation welcome and essential.
• Given the size of the class, smaller groups of ~10
students to be formed soon. Special opportunities
offered to individual ME200 Peer Mentor to lead a group.
• Other Instructors, T. A. s, Classmates, Organized
Learning Groups such as www.purdue.edu/si
• Homework: Submission, grading, and return policies will
be announced in the class.
Mass, Specific Volume (
v ), Density (r)
►Matter is made up of “small,” and “homogeneous,” continua
distributed throughout “space.” Homogeneous refers to our
choice of defining averaged properties.
►When substances are treated as continua, it is possible to speak
of their intensive thermodynamic properties “at a point.”
►At any instant the density (r ) at a point is defined as (see text
book equations 1.6 and 1.7)
m 1
r  lim   
v
V V '  V 
m   rdV
V
m
 m 
r
|holdingall other properties fixed  lim 

V
V 0  V 
Molar Specific Volume (
v ), Number of mol (n)
►Avogadro’s Number represents the number of molecules in mass
containing one “gram mole” or “mole” or “mol.”
► Av=6.022x1023 #/gram mole. A “kmol,” will have 103 times more #
v  Mv, where M is molecular weight in g / mol or kg / kmol
m
n
M
Molecular/atomic weights M for substances generally
of interest in thermodynamics are in Table A-1: C =
12.01 kg/kmol, O2=32.00 kg/kmol, N2=28.01 kg/kmol. Air
is a mixture 1 kmol O2 and 3.76 kmol N2 plus small
amounts of CO2, Ar, H2O etc. Equivalent Molecular
Weight of air is given as 28.97 kg/kmol in Table A-1, p.
890 of text.
Pressure (p)
►Pressure within gases is force per unit area
resulting from molecular collisions with a
container wall and amongst molecules within a
gas.
►Visualize pressure within liquids and solids as
a force exerted by neighboring particles and
bonds The pressure (p) at the specified point
is defined as the limit:
 FNormal 
p  lim 

A 
A A ' 
Pressure Units, Absolute and Gauge Pressure
► SI unit of pressure is the pascal:
1 pascal = 1 N/m2
► Multiples of the pascal are frequently used:
► 1 kPa = 103 N/m2, 1 bar = 105 N/m2, 1 MPa = 106 N/m2
► English units for pressure are:
► pounds force per square foot, lbf/ft2 or pounds force per square inch
“psi”, lbf/in2
►When system pressure is greater than atmospheric pressure, the term
gage pressure is used.
p(gage) = p(absolute) – patm(absolute)
(Eq. 1.14)
►When atmospheric pressure is greater than system pressure, the term
vacuum pressure is used.
p(vacuum) = patm(absolute) – p(absolute)
(Eq. 1.15)
Hydrostatic Pressure
• The pressure throughout an uninterrupted fluid is constant at a fixed
depth.
Think about the difference in pressure between points H
and I, while we discuss the expressions for hydrostatic
pressures.
Pressure Measurement
• We can make use of the change in pressure with
elevation in a fluid to measure pressure.
• Examples of devices used to measure pressure are:
manometer
barometer
p=patm
p  patm  rgh
g
patm  pvapor  r M gh
Cortesy: Office of Basic Energy Sciences, U. S. Department of Energy
Temperature (T)
►If two substances (one warmer than the other) are
brought into contact and isolated from their surroundings, they
interact thermally with changes in observable properties.
►When changes in observable properties cease, the two
substances are in thermal equilibrium.
►Temperature is a physical property that determines
whether the two substances are in thermal equilibrium.
► A thermometer is used to measure temperature using
a change in a thermometric property of a thermometric
substance.
Thermometers for Temperature Measurements
► Liquid-in-glass thermometer
►A glass capillary tube connected to a bulb filled with
liquid and sealed. Space above liquid occupied by
vapor or an inert gas.
►As temperature increases, liquid expands and the
length (L) of the liquid in the capillary indicates the
temperature.
►The liquid is the thermometric substance. L is the
thermometric property.
►Other types of thermometers: Thermocouples,
Thermistors, Radiation thermometers and optical
pyrometers
Temperature Scales
ΔTºR = ΔTºF = 1.8 x (ΔTK = ΔTºC)
T(oC) = T(K) – 273.15 (Eq. 1.17)
T(oF) = T(oR) – 459.67 (Eq. 1.18)
USE ABSOLUTE TEMPERATURES IN
ALL YOUR PROBLEM SOLUTIONS.
oC
and oF Relationship
(100, 212)
oF
(-17.7, 0)
(0, 32)
oC
(-273, -460)
ΔTºR = ΔTºF = 1.8 x (ΔTK = ΔTºC)
oC
oF
-17.7
0
0
32
100
212
-273
-460
T(oC) = T(K) – 273.15 (Eq. 1.17)
T(oF) = T(oR) – 459.67 (Eq. 1.18)
USE ABSOLUTE TEMPERATURES IN YOUR PROBLEM SOLUTIONS
BECAUSE THE DIFFERENCES ARE IDENTICAL AND SOME
FORMULAE LIKE THE IDEAL GAS LAW REQUIRE TEMPERATURE IN
ABSOLUTE UNITS.
Temperatures of Interest
► Some temperatures and ranges of interest
► 0 K is the absolute lower limit of a temperature scale
► Ice temperature is 273 K
► Boiling point of water at atmospheric pressure is 373 K
► Healthy human body temperature ~ 309 K
► High fever human body temperature ~ 313 K
► Hydrocarbon “yellow flame” temperature 1400 K – 1800 K
► Hydrocarbon “blue flame” temperature 1900 K – 2400 K
► Effective solar temperature is considered to be 5500K
► Absolute upper limit of temperature is not defined yet but
engineering higher than flame temperature materials is
challenging. Materials with porous cooling walls may be
used.
Engineering Design and Analysis
Engineering Design
Engineering Analysis
Recognize (or create) a need
and define all requirements and
constraints associated with it.
Apply fundamental principles to
the functionality of the design.
Select “best design,” criteria:
cost, efficiency, size, weight, life.
Conservation of mass,
momentum species and energy
must be followed.
Consider product life cycle
reliability, manufacturability,
maintenance, sustainability.
Performance prediction, testing,
and suggestion of
improvements.
Customer and Business
considerations: Aesthetics,
appearance, color, customer
appeal, first to market, capital.
Limitations based on second law
of thermodynamics.
Compatibility with other
products, systems, policies.
Evaluate potential for scaling in
terms of product size or product
volume.
Problem Solving Techniques
A fairly straightforward problem:
•The system is easy to define (only one type).
•There are few basic equations.
Thermodynamics problems are more complicated
Exhaust
Plume
Steam
Exhaust
Fuel
Coal/Oil/Natural
Gas
Feed Water
Ash
Air
• For example, consider a small
portion of a coal/oil/natural
gas burning power plant.
• Many interconnected working
substances such as fuels, air,
exhaust, and ash.
• Many interconnected systems
and control volumes
• Additional working
substances such as liquid
water becoming saturated
water, saturated steam,
superheated steam etc.
Problem Solving Techniques
Important steps to solve any problem in a systematic
manner.
•Find: What quantities are of customer interest?
Usually dictates the definition of the System.
•System: Control mass/control volume choice indicates
what mass flow and energy interactions exist.
•Basic Equations: Which equations are required to
determine quantities of interest (related to Find)
•Given: What quantities are known?
•Assumptions: Do the number of unknowns and
available equations match? Otherwise, you need
appropriate simplifications and assumptions.
•Solution: Evaluate properties, employ correct units,
perform calculation, discuss result(s) if necessary.
Summary
►We defined temperature, different scales of
specifying temperature and their inter-relations.
►We summarized temperatures of engineering
interest.
► We identified the reasons behind the complexity of
thermodynamics problems and defined a
methodology for solutions.
► We are ready to do some example problems in
class.