Upload
hadat
View
214
Download
1
Embed Size (px)
Citation preview
ENGR 102PROBLEM SOLVING FOR ENGINEERS
Lec
ture
# 1
3
March 6, 2018
I N T O / C S U P A R T N E R S H I P
ENERGY & THERMODYNAMICS
22
OBJECTIVES FOR ENGR 102
1. Work in a typical US university environment
2. Understand and solve engineering word problems
3. Analyze data and present engineering information
4. Understand several engineering concepts
5. Ability to use an engineering problem solving process
6. Use software tools: Microsoft Excel and MATLAB
7. Describe jobs in different engineering disciplines
8. Describe courses needed to graduate as an engineer
Source: athenadr.files.wordpress.com
Today’’’’s topics:
• Project:
• Energy storage
• Thermodynamics
• MATLAB
• Flowcharts
• Exam 2
33
ASSIGNMENT 11(Due before 12 noon on Tuesday, March 6)
1. Number of students in this class = __13__
2. Students with homework on time = __12__
3. Number of students with full credit = __8__
4. Students with all answers correct = __1 almost__
4
How to find a typical problem that you’re interested in.
1. Review the job description
2. Think of an interesting task a person working in this job might need to do, . . .
or, something that you think is interesting that demonstrates the skills required for this job.
3. Review your idea with Professor Bert, Cristian, Hein, Julio, or Akbar
Note: you cannot have the same problem as one of the samples.
PROJECT STEP 7, PART 1:Identify a typical problem that you would need to solve if you were hired to do the job you selected for this project.
YOUR PROJECT PROBLEM
NAME JOB TITLE 7-Step Problem (Question 7)
Abdulla Al Hajri Civil engineer/Hull & Associates Needs revision
Al Muataz Al-Mamari Electrical Eng / Boeing satellites Needs revision
Hamad Al-Mohannadi Software engineer/Apple Needs revision
Marzouq Al Kindi Mechanical CAD/ Calmax Maximum & minimum hole clearance
Sylvia (Yuxuan) Dai .NET Software developer Needs revision
Jean Dumas Organic medicinal chemist Needs revision
Haibing Huang Electrical Eng / Dredging robots Needs revision
Michael Liu Avionics for satellites / Boeing How system can fly without crashing
Yiyee Ooi Mechanical design / Tesla Air drag and electric motor efficiency
Fan Si Electrical design / Boise ID Phone can turn off home computer
Minh Tran Mech design / Boeing bombers Energy from hybrid airplane engine
Yuhe Wu Environmental Eng /Los Angeles What will you calculate?
Harry (Wenhan) Zhao Civil engineer / Transit systems Need revision
6
Energy: Power:Capacity to perform mechanical work Energy rate (per unit of time)
A. SI energy B. SI power C. Non-SI energy D. Non-SI power E. Other
Watt second (Ws)
Degree Kelvin (OK) Calorie (Cal or kcal) BTU (British Thermal Unit)
Horsepower (hp)
Newton (N)
Kilowatt hour (kWhr)
Foot pound (Ft lb) Joule (J)
Watt (W)
Quad (1015 BTUs)
Exajoule per year
?
7
1 kWhr = 3.6 MJ
= 860.4 kCal
= 3410 BTU
1 kg oil = 11.6 kWhr
1 gallon of gasoline = 32.8 kWhr
1 BTU = 1055 J
ENERGY and POWER
8
Which is smallest?A. JouleB. kWhrC. Gallon of gasolineD. kCalE. BTU
Which is largest?A. Kg of oilB. kWhrC. Gallon of gasolineD. kCalE. BTU
UNDERSTANDING ENERGY UNITS
1 kWhr = 3.6 MJ
= 860.4 kCal
= 3410 BTU
1 kg oil = 11.6 kWhr
1 gallon of gasoline = 32.8 kWhr
1 BTU = 1055 J
12
a. Potential energy:
b. Kinetic energy:
c. Internal energy:
d. Chemical energy:
e. Nuclear energy:
DEFINITIONS, EQUATIONS, AND EXAMPLES
13
DEFINITIONS, EQUATIONS, AND EXAMPLES
a. Potential energy: gravity, springs, magnets, capacitorsmgh (for gravity), ½kx2 (for a spring), (position can do work)
b. Kinetic energy: velocity can do work½mv2
c. Internal energy: temperature difference is usefulQ=mC(∆T) Internal energy can also be defined as “heat”
d. Chemical energy: burning, photosynthesis, batteriesExample: CH4 + 2O2 = CO2 + 2H2O + 802 kJ/mole
Changing molecular bonds can do work
e. Nuclear energy: atom bomb, fusion, fission
E=mc2
14
SAMPLE WORD DEFINITIONS
Internal energy
force x distance
Never created or destroyed
Entropy cannot decrease in closed system
Useless energy
Useful output/input
Energy/time
Useful cooling/input
Mass exchanged with environment
Mass does not cross boundary
1616
SOLVING THERMODYNAMICS PROBLEMS
1. Make sure you clearly understand the problem.
2. Draw a diagram of the energy flows.
3. Write the equations (theory).
4. List any assumptions.
5. Solve the equations.
6. Check answer for right units & significant digits.
7. Make any further comments, if needed.
17
EFFICIENCY
Figure 16.6Energy losses in a typical steam power plant
Overall Efficiency =Useful output
Total input
This steam plant uses 1 unit of fossil energy to produce 0.33 units of work available at the generator.
There are further efficiency losses in the electrical generator.
18
FIRST LAW OF THERMODYNAMICS(Conservation of Energy)
In a non-nuclear process, energy can never be created or destroyed.
Closed System Open System
Heat (Q)Work (W)
Heat (Q)Work (W)Mass (E2-E1)
Examples: Sealed balloon Example: Gasoline engine
Equation: 1Q2 = (U2-U1) + 1W2
Where: 1Q2 = heat added
1W2 = work done
(U2-U1) = change in internal energy
Equation: 1Q2 = (U2-U1) + (E2-E1) + 1W2
Where: 1Q2 = heat added
1W2 = work done
(U2-U1) = change in internal energy
(E2-E1) = change in energy from mass flow
20
Overall (System) Efficiency = (Engine Efficiency) x (Generator Efficiency)
About ½ of input power lost in
engine
About ½ of remaining power lost in generator
About ¼ of initial power is available
as electricity
Note: many light bulbs are less than 10% efficient in producing light.
21
SECOND LAW OF THERMODYNAMICS
The energy not available to do useful work can only increase.
Entropy
Give some examples of conserved energy, but increased entropy:
22
SECOND LAW OF THERMODYNAMICS
The energy not available to do useful work can only increase.
Entropy
Examples of processes with conserved energy and increased entropy:
• Friction (work converted to heat)
• Mixing materials at different temperatures without extracting work
• Creating heat by deforming a material (work converted to heat)
• Electrical resistance (batteries heat when charged because of this)
23
CARNOT EFFICIENCY, REFRIGERATION AND HEAT PUMPS
• Carnot process: an ideal process that converts thermal energy to work with no increase in entropy.
• Carnot efficiency is a function of the relative absolute temperatures between two objects.
• Carnot efficiency = 1 – TL/TH in degrees K
• What does this mean for heat engines, heat pumps, & refrigerators?
Source: universe-review.ca
24
CARNOT CYCLE(Theoretically most efficient thermodynamic process)
From 1 to 2:-- Reduce pressure
-- Keep temperature same (high)
-- Requires adding heat
-- Removes work from system
From 2 to 3:-- Reduce pressure
-- No heat added or removed
-- Pressure decreases
-- Removes work from system
From 3 to 4:-- Increase pressure
-- Keep temperature same (low)
-- Requires removing heat
-- Requires adding work to system
From 4 to 1:-- Increase pressure
-- No heat added or removed
-- Pressure increases
-- Requires adding work to system
25
Carnot engine efficiency formulas can calculate the maximum heat energy available to do useful work. They are based on temperature differences.
CarnotEngineHot side
(TH)
Work
Qheat2
Qheat1
Cold side (TL)
Maximum work: Work = Qheat1 (1 – TL/TH)
Maximum efficiency:
Work/Qheat1 = 1 – TL/TH
Conservation of energy:
Qheat1 = Work + Qheat2
ASSIGNMENT 11(Due before 12 noon on Tuesday, March 6)
27
ASSIGNMENT 22(Due before 12 noon Friday, October 25)
Gasoline engines operate like a Carnot engine, but aren’t as efficient. Efficiency = (rate of work out)/(rate of heat value in):
Engine
Rate of
work out
Qout
Rate of heat
value inEfficiency: Wout / Qin
Conservation of energy:
Qin = Qout + WoutQin
Generator
Electrical
Power out
Qout
Wout
Rate of
work in
Win Pout
Efficiency: Pout / Win
Conservation of energy:
Win = Qout2 + Pout
A generator is the same as an electric motor operating backwards:
28
MEANING OF CARNOT EFFICIENCY, REFRIGERATION AND HEAT PUMPS
• Heat engines: convert heat to mechanical work: Hotter engines are generally more efficient Efficiency limited by max material temp for turbine engines
• Refrigerators/air conditioners: work to heat flow Can produce more useful cooling out than the amount of
mechanical work put into them. Maximum ratio is a function of temperature Refrigeration Efficiency = RE = 1 / (TH/TL –1) Example between 0OC and 20OC: RE = 1 / (293/273 –1) = 14
• Heat pumps: convert mechanical work to heat flow Converting high temperature fuel to electricity and then using
an “air conditioner in reverse” is more efficient for heating a house if the outside temperature is not too cold.
Hot side
(TH)
Carnot engine efficiency formulas can calculate the maximum heat energy available to do useful work. They are based on temperature differences.
29
CARNOT ENGINE EFFICIENCY & REFRIGERATION EFFICIENCY
CarnotEngineHot side
(TH)
Work
Qheat2
Qheat1
Cold side (TL)
Maximum work = Qheat1 (1 – TL/TH)
Maximum efficiency:
Work/Qheat1 = 1 – TL/TH
Conservation of energy:
Qheat1 = Work + Qheat2
Refrigerator or Heat Pump
Work
Qheat2
Qheat1
Cold side (TL)
Maximum Qheat2 = Work / (1 – TL/TH)
Maximum refrigeration efficiency:
Qheat2/Work = 1/(1 – TL/TH)
Energy conservation:
Qheat1 = Work + Qheat2
Refrigerators and heat pumps run opposite of a Carnot engine. They take work and produce a temperature difference. The maximum efficiency calculations are also based on temperature differences.
An electric motor converts electrical energy to mechanical work. There are losses (friction, electrical resistance, etc) that result in heat.
30
ELECTRIC MOTOR EFFICIENCY & HEATER THERMODYNAMICS
ElectricMotorElectrical
energy
(Joules or kWhr)
Work
Qheat1
Qin Work = Efficiency • Qin
Conservation of energy:
Qheat1 = Qin- Work
Qheat1 = (1-Efficiency) Qin
Energy conservation:
Qheat = QinHeater
Chemical or
electrical
energy
(Joules or kWhr)
Qin Qheat
A normal heater, such as a natural gas heater, converts chemical energy to heat. An electric heater converts electrical energy to heat.