Please post any questions relating to the final exam as comments on this blog post.

The comprehensive final covers CB chapters 1 - 7 and the assigned parts of chapters 9 - 11 or LT chapters 1 - 10.

The test will be a 2-hour, closed book test. You will be allowed to use FOUR 8.5"x11" cheat sheets. You can write or print on both sides of your cheat sheets.

It has been a great pleasure to work with you this quarter. I know it is a challenging course, but you cam through in good form. Best of luck to you in your career ahead.

# Thermodynamics

Learning undergraduate engineering thermodynamics might be less painful with a blog. I hope that students, faculty and interested observers will share their thoughts on the laws of thermodynamics, phase and chemical equilibrium and many related topics.

## Monday, June 06, 2011

## Tuesday, May 24, 2011

### ENGR 224 - HW #7

This HW covers CB chapters 9-11 or LT chapters 9 & 10.

The homework consists of 7 problems for a total of 48 pts.

Please begin your question with the problem number you are asking about.

9.116 - Brayton Cycle with Regeneration - 8 pts

11.76 - Helium Gas Refrigeration Cycle - 6 pts

WB-1 : Brayton Cycle with Variable Heat Capacities - 6 pts

WB-2 : Effect of Turbine Feed T on Rankine Cycle Efficiency - 6 pts

Wb-3 : Special Rankine Cycle with Reheat and Regeneration - 8 pts

Wb-4 : Ammonia Cascade Refrigeration Cycle - 8 pts

WB-5 : Vapor-Compression Heat Pump - 6 pts

The homework consists of 7 problems for a total of 48 pts.

Please begin your question with the problem number you are asking about.

9.116 - Brayton Cycle with Regeneration - 8 pts

11.76 - Helium Gas Refrigeration Cycle - 6 pts

WB-1 : Brayton Cycle with Variable Heat Capacities - 6 pts

WB-2 : Effect of Turbine Feed T on Rankine Cycle Efficiency - 6 pts

Wb-3 : Special Rankine Cycle with Reheat and Regeneration - 8 pts

Wb-4 : Ammonia Cascade Refrigeration Cycle - 8 pts

WB-5 : Vapor-Compression Heat Pump - 6 pts

## Friday, May 20, 2011

### ENGR 224 - Test #2 , May 24, 2011

Please post any questions relating to the second test as comments on this blog post.

Test 2 will focus on CB chapters 6 & 7 or LT chapters 6 - 8. Material from earlier chapters will also be part of this test, although it will not be the focus.

The test will be available for you to take between 8AM and 7PM in the Testing Center on Tuesday, 5/24/11. It is a 2-hour, closed book test. You will be allowed to use TWO 8.5"x11" cheat sheets. You can write or print on both sides of your cheat sheets. You will be penalized ONE point for every TWO minutes over 2 hours that you have the test in your possession. Do NOT forget the time stamps !

We will have an optional test-prep class on Mon, 5/23, and NO CLASS MEETING on Tue, 5/24. There is a QUIZ on 5/25. It is not my fault. The testing center situation messed up our schedule.

Best of luck to you !

Test 2 will focus on CB chapters 6 & 7 or LT chapters 6 - 8. Material from earlier chapters will also be part of this test, although it will not be the focus.

The test will be available for you to take between 8AM and 7PM in the Testing Center on Tuesday, 5/24/11. It is a 2-hour, closed book test. You will be allowed to use TWO 8.5"x11" cheat sheets. You can write or print on both sides of your cheat sheets. You will be penalized ONE point for every TWO minutes over 2 hours that you have the test in your possession. Do NOT forget the time stamps !

We will have an optional test-prep class on Mon, 5/23, and NO CLASS MEETING on Tue, 5/24. There is a QUIZ on 5/25. It is not my fault. The testing center situation messed up our schedule.

Best of luck to you !

## Monday, May 16, 2011

### ENGR 224 - HW #6

This HW covers CB chapter 7 or LT chapter 8.

The homework consists of 15 problems for a total of 71 pts.

Please begin your question with the problem number you are asking about.

Cengel & Boles: Ch 7:

7.127 - Power Requirement for an Air Compressor - 5 pts

7.131 - Analysis of an R-134a Compressor - 6 pts

7.146+ - Lost Work in a Heat Exchanger - 6 pts

Additional part c.) Determine the rate at which work is lost due to the irreversible nature of heat transfer in this process in kW. Assume the surroundings are at 20oC.

WB-1 - Back-Work Ratio of a Steam Power Cycle - 7 pts

Problem Statement :

Consider a steam power plant that operates between the pressure limits of 8 MPa and 20 kPa. Steam enters the pump as a saturated liquid and leaves the turbine as a saturated vapor. Determine the back work ratio (BWR is the ratio of the work delivered by the turbine to the work consumed by the pump). Assume the entire cycle to be reversible and the heat losses from the pump and the turbine to be negligible.

Hints :

Assume the cycle operates at steady-state and that the ump and turbine are reversible.

I suggest you make a table with 6 columns. The 1st column lists all the important properties you may need to evaluate for each steam in the cycle. Make a column for each of the 4 streams in the cycle. The last column should list the units for each variable. This table should help you keep track of what you know and what you still need to determine.

Make a nice process flow diagram. Make a nice TS Diagram. Include the 2-phase envelope and the two key isobars. Apply the 1st and 2nd Laws to the pump and turbine and you should be able to determine the BWR.

Two double-interpolations are required unless you use NIST or a plug-in for Excel or your calculator.

Ans.: BWR ~ 240

WB-2 - Polytropic Compression of N2 with Varying δ - 6 pts

Problem Statement : Nitrogen gas is compressed from 80 kPa and 27oC to 480 kPa by a 10 kW compressor. Determine the mass flow rate of nitrogen through the compressor assuming the compression process is …

a.) Isentropic, γ = 1.4

b.) Polytropic with δ = 1.3

c.) Isothermal

d.) Ideal, two-stage polytropic with δ = 1.3

Hints :

The compressor operates at steady-state.

Changes in kinetic and potential energies are negligible.

Flow work and shaft work are the only forms of work that cross the system boundary.

Nitrogen (N2) behaves as an ideal gas in all parts of this problem.

The heat capacities of N2 are constant and, therefore, γ is also constant.

The intercooler in part (d) cools the effluent from the first compressor back down to T1 before it enters the second compressor.

Parts (b), (c) and (d) should be plug-and-chug.

Ans.: a.) Mdot ~ 0.048 kg/s , c.) Mdot ~ 0.063 kg/s

WB-3 - Entropy Change, Heat Transfer and Irreversibilities - 7 pts

Problem Statement :

A closed system undergoes a process in which work is done on the system and heat transfer Q occurs only at temperature Tb. For each case listed below, determine whether the entropy change of the system is positive, negative, zero or indeterminate (you cannot tell for sure from the given information).

a.) Internally reversible process with Q > 0.

b.) Internally reversible process with Q = 0.

c.) Internally reversible process with Q < 0. d.) Internal irreversibilities present with Q > 0.

e.) Internal irreversibilities present with Q = 0.

f.) Internal irreversibilities present with Q < 0. Hints : Consider the sign of each term in the defining equation for entropy generation.

WB-4 - Entropy Generation and Lost Work in a Nozzle - 6 pts

Problem Statement :

Oxygen, O2, enters a nozzle operating at steady-state at 3.8 MPa, 387oC and 10 m/s. At the nozzle exit, the conditions are 150 kPa, 37oC and 790 m/s.

a.) For a system that encloses the nozzle only, determine the heat transfer (kJ/kg) and the change in specific entropy (kJ/kg-K), both per kg of oxygen flowing through the nozzle. What additional information would be required to evaluate the rate of entropy production in this process ?

b.) Using an enlarged system boundary that includes the nozzle and a portion of its immediate surroundings, evaluate the rate of entropy generation (kJ/kg-K) and the rate of lost work (kJ/kg), both per kg of oxygen flowing through the nozzle. Assume that heat exchange at the enlarged system boundary takes place at the ambient temperature, 20oC.

Treat O2 as an ideal gas with variable heat capacities. Verify that the ideal gas assumption is valid.

Hints :

In part (a), use the 1st Law and the ideal gas property tables to determine Q.

In part (b), evaluate the entropy generation from its definition, using the Ideal Gas Property Tables and Gibbs 2nd Equation.

Lost work is just the product of Tsurr and Sgen.

Ans.: a.) Q ~ -30 kJ/kg , b.) Sgen ~ 0.2 kJ/kg-K and Wlost ~ 61 kJ/kg

WB-5 - Lost Work in an Air Compressor and HEX - 7 pts

Problem Statement :

Air flows through the compressor and heat exchanger in the system shown in the diagram. A separate liquid water stream (CP,W = 4.18 kJ/kg-K) also flows through the heat exchanger. The data given on the diagram are based on steady-state operation. Consider the air to be an ideal gas and neglect heat exchange with the surroundings as well as changes in kinetic and potential energies. Determine...

a.) The compressor power requirement in kW and the mass flow rate of the cooling water in kg/s.

b.) The rate of entropy generation in kW/K and the rate at which work is lost in kW for the compressor. Assume the temperature of the surroundings is 300 K.

c.) The rate of entropy generation in kW/K and the rate at which work is lost in kW for the heat exchanger. Assume the temperature of the surroundings is 300 K.

Hints :

Use the ideal gas EOS and the volumetric flow rate to determine the mass flow rate.

Use an energy balance to determine the work for the compressor in kW.

To determine the water flow rate, draw the control volume enclosing the heat exchanger. This control volume has 4 mass flows entering or leaving but no Q or W. An energy balance on this control volume yields the water flow rate.

Very important point- the air and the water DO NOT MIX in the heat exchanger !

Determine the change in enthalpy of the water using: ΔH = Cp ΔT and determine the entropy change of the water using ΔS = CP Ln[T2/T1].

Part (b) Don't forget about the cooling water when you calaculate the entropy generated.

Ans.: a.) WS ~ -50 kW, b.) Sgen,comp ~ 0.020 kW/K , Wlost,comp ~ 6 kW , b.) Sgen,HEX ~ 0.015 kW/K

The homework consists of 15 problems for a total of 71 pts.

Please begin your question with the problem number you are asking about.

Cengel & Boles: Ch 7:

7.127 - Power Requirement for an Air Compressor - 5 pts

7.131 - Analysis of an R-134a Compressor - 6 pts

7.146+ - Lost Work in a Heat Exchanger - 6 pts

Additional part c.) Determine the rate at which work is lost due to the irreversible nature of heat transfer in this process in kW. Assume the surroundings are at 20oC.

WB-1 - Back-Work Ratio of a Steam Power Cycle - 7 pts

Problem Statement :

Consider a steam power plant that operates between the pressure limits of 8 MPa and 20 kPa. Steam enters the pump as a saturated liquid and leaves the turbine as a saturated vapor. Determine the back work ratio (BWR is the ratio of the work delivered by the turbine to the work consumed by the pump). Assume the entire cycle to be reversible and the heat losses from the pump and the turbine to be negligible.

Hints :

Assume the cycle operates at steady-state and that the ump and turbine are reversible.

I suggest you make a table with 6 columns. The 1st column lists all the important properties you may need to evaluate for each steam in the cycle. Make a column for each of the 4 streams in the cycle. The last column should list the units for each variable. This table should help you keep track of what you know and what you still need to determine.

Make a nice process flow diagram. Make a nice TS Diagram. Include the 2-phase envelope and the two key isobars. Apply the 1st and 2nd Laws to the pump and turbine and you should be able to determine the BWR.

Two double-interpolations are required unless you use NIST or a plug-in for Excel or your calculator.

Ans.: BWR ~ 240

WB-2 - Polytropic Compression of N2 with Varying δ - 6 pts

Problem Statement : Nitrogen gas is compressed from 80 kPa and 27oC to 480 kPa by a 10 kW compressor. Determine the mass flow rate of nitrogen through the compressor assuming the compression process is …

a.) Isentropic, γ = 1.4

b.) Polytropic with δ = 1.3

c.) Isothermal

d.) Ideal, two-stage polytropic with δ = 1.3

Hints :

The compressor operates at steady-state.

Changes in kinetic and potential energies are negligible.

Flow work and shaft work are the only forms of work that cross the system boundary.

Nitrogen (N2) behaves as an ideal gas in all parts of this problem.

The heat capacities of N2 are constant and, therefore, γ is also constant.

The intercooler in part (d) cools the effluent from the first compressor back down to T1 before it enters the second compressor.

Parts (b), (c) and (d) should be plug-and-chug.

Ans.: a.) Mdot ~ 0.048 kg/s , c.) Mdot ~ 0.063 kg/s

WB-3 - Entropy Change, Heat Transfer and Irreversibilities - 7 pts

Problem Statement :

A closed system undergoes a process in which work is done on the system and heat transfer Q occurs only at temperature Tb. For each case listed below, determine whether the entropy change of the system is positive, negative, zero or indeterminate (you cannot tell for sure from the given information).

a.) Internally reversible process with Q > 0.

b.) Internally reversible process with Q = 0.

c.) Internally reversible process with Q < 0. d.) Internal irreversibilities present with Q > 0.

e.) Internal irreversibilities present with Q = 0.

f.) Internal irreversibilities present with Q < 0. Hints : Consider the sign of each term in the defining equation for entropy generation.

WB-4 - Entropy Generation and Lost Work in a Nozzle - 6 pts

Problem Statement :

Oxygen, O2, enters a nozzle operating at steady-state at 3.8 MPa, 387oC and 10 m/s. At the nozzle exit, the conditions are 150 kPa, 37oC and 790 m/s.

a.) For a system that encloses the nozzle only, determine the heat transfer (kJ/kg) and the change in specific entropy (kJ/kg-K), both per kg of oxygen flowing through the nozzle. What additional information would be required to evaluate the rate of entropy production in this process ?

b.) Using an enlarged system boundary that includes the nozzle and a portion of its immediate surroundings, evaluate the rate of entropy generation (kJ/kg-K) and the rate of lost work (kJ/kg), both per kg of oxygen flowing through the nozzle. Assume that heat exchange at the enlarged system boundary takes place at the ambient temperature, 20oC.

Treat O2 as an ideal gas with variable heat capacities. Verify that the ideal gas assumption is valid.

Hints :

In part (a), use the 1st Law and the ideal gas property tables to determine Q.

In part (b), evaluate the entropy generation from its definition, using the Ideal Gas Property Tables and Gibbs 2nd Equation.

Lost work is just the product of Tsurr and Sgen.

Ans.: a.) Q ~ -30 kJ/kg , b.) Sgen ~ 0.2 kJ/kg-K and Wlost ~ 61 kJ/kg

WB-5 - Lost Work in an Air Compressor and HEX - 7 pts

Problem Statement :

Air flows through the compressor and heat exchanger in the system shown in the diagram. A separate liquid water stream (CP,W = 4.18 kJ/kg-K) also flows through the heat exchanger. The data given on the diagram are based on steady-state operation. Consider the air to be an ideal gas and neglect heat exchange with the surroundings as well as changes in kinetic and potential energies. Determine...

a.) The compressor power requirement in kW and the mass flow rate of the cooling water in kg/s.

b.) The rate of entropy generation in kW/K and the rate at which work is lost in kW for the compressor. Assume the temperature of the surroundings is 300 K.

c.) The rate of entropy generation in kW/K and the rate at which work is lost in kW for the heat exchanger. Assume the temperature of the surroundings is 300 K.

Hints :

Use the ideal gas EOS and the volumetric flow rate to determine the mass flow rate.

Use an energy balance to determine the work for the compressor in kW.

To determine the water flow rate, draw the control volume enclosing the heat exchanger. This control volume has 4 mass flows entering or leaving but no Q or W. An energy balance on this control volume yields the water flow rate.

Very important point- the air and the water DO NOT MIX in the heat exchanger !

Determine the change in enthalpy of the water using: ΔH = Cp ΔT and determine the entropy change of the water using ΔS = CP Ln[T2/T1].

Part (b) Don't forget about the cooling water when you calaculate the entropy generated.

Ans.: a.) WS ~ -50 kW, b.) Sgen,comp ~ 0.020 kW/K , Wlost,comp ~ 6 kW , b.) Sgen,HEX ~ 0.015 kW/K

## Wednesday, May 04, 2011

### ENGR 224 - HW #5

This HW covers CB chapter 7 or LT chapter 7.

The homework consists of 13 problems for a total of 59 pts.

Please begin your question with the problem number you are asking about.

Cengel & Boles: Ch 7: 14(2pts), 15(2pts), 18(2pts), 28E(4pts), 29(6pts), 66(6pts)

WB-1(4pts) , WB-2(6pts), WB-3(4pts) , WB-4(4pts), WB-5(8pts) , WB-6(5pts), WB-1(6pts)

The homework consists of 13 problems for a total of 59 pts.

Please begin your question with the problem number you are asking about.

Cengel & Boles: Ch 7: 14(2pts), 15(2pts), 18(2pts), 28E(4pts), 29(6pts), 66(6pts)

WB-1(4pts) , WB-2(6pts), WB-3(4pts) , WB-4(4pts), WB-5(8pts) , WB-6(5pts), WB-1(6pts)

## Saturday, April 23, 2011

### ENGR 224 - Test 1

Monday, 4/25/11 at 9AM in SMT 354.

Please post any questions you have relating to the 1st test as comments on this blog entry.

Test 1 covers Ch 1-5 in Cengel & Boles and in LearnThermo.com.

Best of luck !

Dr. B

Please post any questions you have relating to the 1st test as comments on this blog entry.

Test 1 covers Ch 1-5 in Cengel & Boles and in LearnThermo.com.

Best of luck !

Dr. B

## Wednesday, April 20, 2011

### ENGR 224 - HW #4

This HW covers CB chapter 6 or LT chapter 6.

The homework consists of 8 problems for a total of 45 pts.

Please begin your question with the problem number you are asking about.

Cengel & Boles: Ch 6:

6.79 - Effect of Source and Sink Temperatures on HE Efficiency - 6 pts

6.85 - Thermal Efficiency of a Geothermal Power Plant - 3 pts

6.107 - Carnot HE Used to Drive a Carnot Refrigerator - 6 pts

6.110 - Actual and Maximum COP of an Air-Conditioner - 8 pts

6.134 - Thermal Efficiency of Heat Engines in Series - 4 pts

Special Problems

WB-1 - "Show That" Problem Using the K-P Statement of the 2nd Law - 6 pts

WB-2 - Reversible, Irreversible and Impossible Power Cycles - 6 pts

WB-3 - A Reversible HE Used to Drive a Reversible Heat Pump - 6 pts

The homework consists of 8 problems for a total of 45 pts.

Please begin your question with the problem number you are asking about.

Cengel & Boles: Ch 6:

6.79 - Effect of Source and Sink Temperatures on HE Efficiency - 6 pts

6.85 - Thermal Efficiency of a Geothermal Power Plant - 3 pts

6.107 - Carnot HE Used to Drive a Carnot Refrigerator - 6 pts

6.110 - Actual and Maximum COP of an Air-Conditioner - 8 pts

6.134 - Thermal Efficiency of Heat Engines in Series - 4 pts

Special Problems

WB-1 - "Show That" Problem Using the K-P Statement of the 2nd Law - 6 pts

WB-2 - Reversible, Irreversible and Impossible Power Cycles - 6 pts

WB-3 - A Reversible HE Used to Drive a Reversible Heat Pump - 6 pts

## Wednesday, April 13, 2011

### ENGR 224 - HW #3

This HW covers CB chapter 5 or LT chapter 5.

The homework consists of 10 problems for a total of 65 pts.

Please begin your question with the problem number you are asking about.

Cengel & Boles, Ch 5:

5.35 - Adiabatic Steam Nozzle - 5 pts

5.54 - Adiabatic Gas Turbine - 5 pts

5.112 - Steam Flow in a HEX Tube - 5 pts

5.138 - Filling a Balloon with Helium - 10 pts

5.142 - Charging a Cylinder with a Spring-Loaded Piston - 8 pts

Special Problems

WB-1 - Effluent Pressure in a Non-Adiabatic Steam Diffuser - 5 pts

WB-2 - Steady-State, Polytropic Air Compressor - 5 pts

WB-3 - Analysis of a Two-Stage, Adiabatic Turbine - 6 pts

WB-4 - Analysis of an Adiabatic Steam De-Superheater - 8 pts

WB-5 - Waste Heat Steam Generator - 8 pts

The homework consists of 10 problems for a total of 65 pts.

Please begin your question with the problem number you are asking about.

Cengel & Boles, Ch 5:

5.35 - Adiabatic Steam Nozzle - 5 pts

5.54 - Adiabatic Gas Turbine - 5 pts

5.112 - Steam Flow in a HEX Tube - 5 pts

5.138 - Filling a Balloon with Helium - 10 pts

5.142 - Charging a Cylinder with a Spring-Loaded Piston - 8 pts

Special Problems

WB-1 - Effluent Pressure in a Non-Adiabatic Steam Diffuser - 5 pts

WB-2 - Steady-State, Polytropic Air Compressor - 5 pts

WB-3 - Analysis of a Two-Stage, Adiabatic Turbine - 6 pts

WB-4 - Analysis of an Adiabatic Steam De-Superheater - 8 pts

WB-5 - Waste Heat Steam Generator - 8 pts

## Wednesday, April 06, 2011

### ENGR 224 - HW #2

This HW covers CB chapters 2, 4 and parts of 6 or LT chapters 3 & 4.

The homework consists of 16 problems for a total of 57 pts.

Please begin your question with the problem number you are asking about.

Cengel & Boles: Ch 3:

3.26(2pts) -

Use either the NIST Webbook or the Thermal/Fluids Toolbox (TFT) Excel plug-in.

Use the default reference state for both the NIST and TFT.

3.29E(2pts) -

Use either the NIST Webbook or the Thermal/Fluids Toolbox (TFT) Excel plug-in.

Use the default reference state for both the NIST and TFT.

Cengel & Boles: Ch 4:

4.8(6pts) : W ~ -22 kJ

4.42(5pts) : Q ~ 12750 kJ

4.59E(4pts) : See textbook

4.60(4pts) : ΔU ~ 6200 Btu/lbm

Special Problems

WB-1(5pts) : ΔHvap ~ 575 to 600 Btu/lbm

WB-2(8pts) : ΔH14 ~ 9900 J

WB-3(6pts): Q23 ~ -85 Btu , Wcycle ~ -20 Btu

WB-4(3pts) : q ~ -97 Btu/h-ft2

WB-5(3pts) : Q ~ 3500 W

WB-6(3pts) : Lins ~ 6 cm

WB-7(3pts) : Wb ~ 350 kJ

Cengel & Boles: Ch 6: 6.23(2pts), 6.41(2pts), 6.55(2pts). These problems should be pretty easy.

The homework consists of 16 problems for a total of 57 pts.

Please begin your question with the problem number you are asking about.

Cengel & Boles: Ch 3:

3.26(2pts) -

Use either the NIST Webbook or the Thermal/Fluids Toolbox (TFT) Excel plug-in.

Use the default reference state for both the NIST and TFT.

3.29E(2pts) -

Use either the NIST Webbook or the Thermal/Fluids Toolbox (TFT) Excel plug-in.

Use the default reference state for both the NIST and TFT.

Cengel & Boles: Ch 4:

4.8(6pts) : W ~ -22 kJ

4.42(5pts) : Q ~ 12750 kJ

4.59E(4pts) : See textbook

4.60(4pts) : ΔU ~ 6200 Btu/lbm

Special Problems

WB-1(5pts) : ΔHvap ~ 575 to 600 Btu/lbm

WB-2(8pts) : ΔH14 ~ 9900 J

WB-3(6pts): Q23 ~ -85 Btu , Wcycle ~ -20 Btu

WB-4(3pts) : q ~ -97 Btu/h-ft2

WB-5(3pts) : Q ~ 3500 W

WB-6(3pts) : Lins ~ 6 cm

WB-7(3pts) : Wb ~ 350 kJ

Cengel & Boles: Ch 6: 6.23(2pts), 6.41(2pts), 6.55(2pts). These problems should be pretty easy.

## Friday, March 25, 2011

### ENGR 224 - Thermodynamics at GRCC

Welcome to the Thermodynamics course at Green River Community College in Auburn, WA, USA.

Feel free to post any general questions about the course as comments on this blog entry.

I have created a blog entry for each chapter we will cover in this course. If you think your question applies to a specific chapter, then please post your question as a comment on the appropriate blog entry.

I have created a blog entry for each of the 7 HW assignments in the course. If you have a question about a problem in the HW, please post it as a comment on the appropriate blog entry. Be sure to state which problem number your question refers to !

You can post your questions using your Google ID or a crazy fake name or your real name or you can post anonymously. Whatever makes you happy. Just don't hesitate to ASK !

Remember...

"It's not a miracle, mother, it's thermodynamics !"

- Harrison Ford in Mosquito Coast

Feel free to post any general questions about the course as comments on this blog entry.

I have created a blog entry for each chapter we will cover in this course. If you think your question applies to a specific chapter, then please post your question as a comment on the appropriate blog entry.

I have created a blog entry for each of the 7 HW assignments in the course. If you have a question about a problem in the HW, please post it as a comment on the appropriate blog entry. Be sure to state which problem number your question refers to !

You can post your questions using your Google ID or a crazy fake name or your real name or you can post anonymously. Whatever makes you happy. Just don't hesitate to ASK !

Remember...

"It's not a miracle, mother, it's thermodynamics !"

- Harrison Ford in Mosquito Coast

### ENGR 224 - HW #1

This HW covers CB chapters 1 & 3 or LT chapters 1 & 2.

The homework consists of 15 problems for a total of 71 pts.

Please begin your question with the problem number you are asking about.

Cengel & Boles: Ch 1: 7E(3pts), 11E(4pts), 37E(2pts), 41E(3pts), 78(6pts)

WB-1 (4pts) , WB-2 (5pts)

Cengel & Boles: Ch 3: 26(4pts), 29E(4pts), 37E(5pts), 53(6pts), 66(4pts),

83(6pts) - Assume that the air in the tire behaves as an ideal gas, but then check the validity of this assumption.

WB-3 (10pts) , WB-4 (5pts)

The homework consists of 15 problems for a total of 71 pts.

Please begin your question with the problem number you are asking about.

Cengel & Boles: Ch 1: 7E(3pts), 11E(4pts), 37E(2pts), 41E(3pts), 78(6pts)

WB-1 (4pts) , WB-2 (5pts)

Cengel & Boles: Ch 3: 26(4pts), 29E(4pts), 37E(5pts), 53(6pts), 66(4pts),

83(6pts) - Assume that the air in the tire behaves as an ideal gas, but then check the validity of this assumption.

WB-3 (10pts) , WB-4 (5pts)

## Friday, March 18, 2011

### HW 7B-16 - Isentropic Compression of High Quality Steam - 5 pts

Steam is compressed isentropically to a final pressure of 5 MPa. The steam is initially at 25oC and has a quality of 91%. How much work does this process require, in kJ/kg, if the process takes place in... (a) A closed system? (b) An open system ? (c) Explain why the answers to parts (a) and (b) are not the same.

For hints and answers visit:

http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B16

For hints and answers visit:

http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B16

## Wednesday, March 16, 2011

### HW 7B-15 - Heat and Work in an Isothermal Expansion of Steam - 5 pts

Saturated water vapor is contained in a piston-and-cylinder device that is positioned in a constant-temperature bath at 250oC. The 5.7 kg of steam inside the cylinder expands reversibly and isothermally to a final pressure of 1.2 MPa. Determine the heat transferred between the constant-temperature bath and the water inside the cylinder as well as the work done by the H2O inside the cylinder during this process.

For hints and answers visit: http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B15

For hints and answers visit: http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B15

### HW 7B-14 - Reversible Compression of Steam in a Piston-and-Cylinder Device - 4 pts

Superheated steam is reversibly compressed from 200 kPa to 1.4 MPa in a well-insulated piston-and-cylinder device. The cylinder initially contains 0.175 m3 of steam at 250°C. Determine the final temperature of the steam and the work done on the steam during this process.

For hints and answers visit: http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B14

For hints and answers visit: http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B14

### HW 7B-13 - Minimum Power Required for Adiabatic Compression of R-134a - 4 pts

An adiabatic compressor is used to increase the pressure from 105 kPa to 1.25 MPa in a stream of R-134a with a volumetric flow rate of 1800 L/min. Determine the minimum power that must be supplied to the compressor.

For hints and answers visit: http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B13

For hints and answers visit: http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B13

### HW 7B-12 - Reversible Expansion of R-134a in a Piston-and-Cylinder Device - 4 pts

The refrigerant R-134a expands reversibly from 2 MPa to 500 kPa in an insulated piston-and-cylinder device. The cylinder initially contains 15 L of R-134a with a quality of 1.0. Determine the final temperature of the R-134a in the cylinder and the work done during this process.

For hints and answers visit: http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B12

For hints and answers visit: http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B12

### HW 7B-11 - Entropy Change and Boundary Work for the Isobaric Expansion of Water - 10 pts

Electrical work is done by an electrical resistance heater on H2O contained in an insulated piston-and-cylinder device. The cylinder initially contains 2.4 L of saturated liquid water at a pressure of 500 kPa. Assuming the piston moves freely, determine the total entropy change of the H2O during a process in which 1875 kJ of heat is transferred into the water. What is the boundary work for this process?

For hints and answers visit: http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B11

For hints and answers visit: http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B11

### HW 7B-10 - Isobaric Condensation of R-134a - 5 pts

The refrigerant R-134a is cooled in a piston-and-cylinder device from 190°F to 75°F. The piston moves freely and the initial pressure is 200 psia. Determine the specific entropy change of the refrigerant, the work per lbm and heat transfer per lbm during this process.

For hints and answers visit:

http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B10

For hints and answers visit:

http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B10

## Tuesday, March 15, 2011

### HW 7B-9 - Expansion of Water into a Vacuum - 5 pts

A rigid tank is divided into two equal parts by a wall. One part of the tank contains 0.74 kg of water at 100 kPa and 25°C. The other part is a perfect vacuum. The wall is now removed and the water expands to fill the entire tank. A thermocouple indicates that the final equilibrium temperature of the H2O in the tank is 50oC. Determine the entropy change of water and the total heat transfer to the system during this process.

For hints and answers visit:

http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B9

For hints and answers visit:

http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B9

### HW 7B-8 - Vaporization of Water in a Rigid Tank - 5 pts

An electric resistance heater is used to heat the contents of an insulated rigid tank that holds 23.1 kg of H2O. Initially, the pressure in the tank is 150 kPa and 44% of the mass inside the tank is saturated liquid and the remaining 56% is saturated vapor. Power is applied to the heater until all of the H2O in the tank is saturated vapor. Determine the entropy change of the H2O and the total of heat transfer during this process.

For hints and answers visit:

http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B8

For hints and answers visit:

http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B8

### HW 7B-7 - Isentropic Expansion of R-134a - 4 pts

7-38E R-134a expands reversibly and adiabatically in a piston-and-cylinder device from 80 psia and 120°F to 5 psia. If the cylinder contains 17.4 lbm of R-134a, what are the total work done by the system during this process and final temperature of the R-134a ?

For hints and answers visit:

http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B7

For hints and answers visit:

http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B7

### HW 7B-6 - Isentropic Compression of Water Vapor - 3 pts

An isentropic compressor takes in H2O at 105 kPa and delivers it at 450 kPa. If the temperature of the feed is 180°C, determine the temperature and specific enthalpy of the H2O at the compressor outlet.

For hints and answers visit:

http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B6

For hints and answers visit:

http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B6

### HW 7B-5 - Entropy Change for an Isobaric Heating Process - 4 pts

A piston-and-cylinder device holds 9.7 lbm of water at 250 psia in a volume of 3.6 ft3. Heat is added to the water until the temperature reaches 425oF. During this process, the pressure in the cylinder remains constant because the piston moves freely. Determine the change in the entropy on the water in Btu/oR.

For hints and answers visit:

http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B5

For hints and answers visit:

http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B5

## Wednesday, March 09, 2011

### HW 7B-4 - Power Output of an Isentropic Turbine - 4pts

An isentropic turbine produces shaft work as R-134a at 275 psia and 225°F expands through the turbine and exits at 35°F. Assuming the turbine is adiabatic, determine the power output of the turbine per lbm of R-134a.

For hints and answers visit:

http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B4

For hints and answers visit:

http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B4

### HW 7B-3 - Entropy Changes Associated with the Evaporator of a Refrigerator - 4 pts

A refrigerator using R-134a as the refrigerant removes 200 kJ of heat from its refrigerated space. This heat is absorbed by a mixture of saturated liquid and saturated vapor that enters the evaporator at a pressure of 175 kPa. The R-134a leaves the evaporator as a saturated vapor. Determine...

a.) ... the entropy change of the R-134a,

b.) ... the entropy change of the refrigerated space assuming its temperature remains constant at -10°C,

c.) ... the entropy change of the universe for this process.

For hints and answers visit:

http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B3

a.) ... the entropy change of the R-134a,

b.) ... the entropy change of the refrigerated space assuming its temperature remains constant at -10°C,

c.) ... the entropy change of the universe for this process.

For hints and answers visit:

http://www.LearnThermo.com/T1-tutorial/ch07/lesson-B/pg20.php#P7B3

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