AC 2010-174: SOLAR COOKER DESIGN FOR THERMODYNAMICS LAB

Thomas Shepard, University of Minnesota, Twin CitiesThomas Shepard is a Mechanical Engineering Ph.D. candidate at the University of Minnesota. Hereceived an M.S. in Mechanical Engineering from Oregon State University and B.A. in Physicsfrom Colorado College. His teaching interests include undergraduate courses in the thermal/fluidsciences, experimental methods and renewable energy technologies. He has research interests inexperimental fluid mechanics, energy conversion, and engineering education.

Camille George, University of St. ThomasCamille George is an Associate Professor and the Program Director of Mechanical Engineering atthe University of St. Thomas in St. Paul, Minnesota. She teaches thermodynamics and maintainsa strong interest in technology literacy and international service-learning. Dr. George hasspearheaded several innovative international projects in collaboration with seven differentdepartments including Geology, Modern and Classical Languages, Sociology, Accounting andCommunications. She has also introduced a Peace Engineering track which combines MechanicalEngineering with a minor in Justice and Peace.

© American Society for Engineering Education, 2010

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Solar Cooker Design Project for Thermodynamics Lab

Abstract

A solar cooker design project was developed to teach energy conversion and the first law of thermodynamics. Student teams were provided with a solar concentrating device (either a 54” diameter spherical mirror or a 27”x36” Fresnel lens) with the task of utilizing it in the design of a safe and easily adjustable solar cooker which minimized convective heat loss. The solar cookers were then third-party tested during a lab involving transient first law analysis of the heating process for water in a pot and in a pressure cooker. Further, testing was done to compare student developed innovations for reducing heat losses. This allowed students to see different solutions to the same problem while comparing performance.

Another aim of this project was to provide junior level engineering students with a first hand experience combining lessons learned in lecture with the practical constraints of designing, building and testing a realistic application. The students were encouraged to be creative in this open-ended project that relied heavily on team-based learning. At the beginning of the semester a pre-project survey was used to allocate crucial skills (mechanical aptitude, writing ability, mathematical skills, etc.) evenly amongst the different teams. Assessment of how the project enhanced student learning was done via graded assignment and student survey. The project culminated in a final report incorporating several main components. The first component was a design analysis which included Solid Works drawings, a bill of materials, a user manual for safe operation of the cooker as well as a discussion on how the team reached its final design decision and compromises made. A lab analysis section incorporated the processing of data collected during the lab as well as theoretical calculations based on material learned in class. The final section called on students to reflect on the lessons learned throughout the process, suggest potential directions for future studies with the solar cookers and discuss the practicality of widespread solar cooker use.

The project appealed to a variety of learning styles and exposing the potential for global impact which can come from applying lessons in new or alternative ways added everyday relevance to the labs. Introduction

Perhaps the largest challenge faced in the instruction of introductory thermodynamics is bringing the material to life in a way that is both educational and exciting for the student. One strategy for this is to show students areas where thermodynamics is applied to alternative solutions for everyday problems. The growing area of solar-thermal technologies provides a wonderful opportunity for teachers to combine manageable thermodynamic analyses to an increasingly relevant field. This is especially convenient in a lab setting where a common heat source such as a hot plate can be replaced by a solar collector. As the thermodynamic analysis does not depend on the source of the heat, just

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the value, there is great potential for utilizing solar collectors as a way of bringing a unique element into the lab. An additional way to bring the thermal/fluid sciences to life for the students is via hand-on projects in which students design, build and test (DBT) a product1, 2, 3, 4. In DBT projects a student has the valuable opportunity to learn about the complications and compromises engineers must face while trying to make an idea a reality during the design and build phases. The testing phase supplies concrete feedback on how well a design works and generates discussion on any errors in judgment during the design process and how one might improve on the design. The thermodynamics course of which the following project was a part consisted of three lectures plus a two and a half hour lab each week. Incorporation of the project promoted one of the key learning outcomes for the course, i.e. demonstrate ability to apply thermodynamic knowledge to an open-ended design problem and develop practical skills, which aligned with multiple ABET specified program outcomes (a, b, c, e, j, k)5. While most of the lab periods involved structured experiments relevant to specific thermodynamic topics (heat capacity, reverse Rankine cycle, etc.), time dedicated to the design project was interspersed throughout the semester. In this paper we discuss the development of a solar cooker DBT project as part of an introductory thermodynamics course for junior level engineers including the implementation, project assessments, and lessons learned. Project Description

A pre-project survey, based off a survey by Coronella6, was used to assess the students’ perceived abilities in a number of areas in order to allocate skills evenly amongst the groups and avoid scheduling conflicts. The survey is presented in Appendix A. The class was then broken up into teams, each of which was given a solar concentrating device. Two of the teams were given a 54” diameter spherical mirror (EG Solar) while the third team was given two 27” x 36” Fresnel lenses (one from greenpowerscience.com, the manufacturer of the other was unknown). Teams were assigned the task of creating a user-friendly solar cooker design that would collect and focus incoming solar radiation onto a pot or pressure cooker. The key issues to be addressed included creating a stand that would allow the concentrator to track the sun, implementing a method for determining proper focus, and improvements in heat retention which would reduce convective heat losses while permitting radiation heat inputs. The key learning objectives for the project included: -how to manage a project -how to find and use outside information to help drive a design -how to communicate succinctly your progress and achievements -understanding the relationship between energy, pressure and temperature -appreciating the importance of solar power and the implications involved in harnessing it

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The project was open-ended and the performance of the design was minimally weighted in the grading to allow students to be as creative as they liked. The instructor was available for questions and to help recognize potential issues but refrained from offering suggestions during the design phase. As the students had yet to take a heat transfer course the project was not intended to be a capstone, but did provide a qualitative introduction to the different forms of heat transfer and how they occur. The final products can be seen in Figure 1; note the use of shaded glasses for protection from the brightness of the collected and focused light.

Figure 1. Solar cookers in action

Upon completion, the solar cookers were used in a lab utilizing a transient analysis of the first law of thermodynamics (Appendix B). The lab consisted of two main parts: comparing the heating process of water in an open pot versus a pressure cooker and secondly comparing the heating process of water in a pot exposed to wind versus a pot with a convective loss reduction device. To gather data students recorded the system temperature and pressure for the pot and pressure cooker while also monitoring the incoming solar radiation with a pyranometer (Apogee Instruments Inc.). The goal of the lab was to allow students to see how the internal energy of water being heated will differ depending on the pressure as well as give students experience working with the equations involved in a transient analysis, test their designs and see the other designs. During the lab each team had the opportunity to work with the other solar cookers and review their designs. To facilitate the operation by other teams and to force students to think critically about simplifying their design, teams created user manuals with instructions on how to properly and safely use their solar cooker. It is noteworthy to mention here one complication which arose due to the completely open-ended structure of the project. To have a fair comparison of the heating process between the pot and pressure cooker it is important that they receive a nominally identical heat input. As the design aspect was left open-ended the different solar cookers were adjusted and focused in different ways. For this reason one could not determine with any certainty whether the different cookers were collecting the same amount of radiation despite having similar collection areas as small changes in alignment can have a potentially large impact. To work around this problem a data set taken under more ideal conditions was given to the students to use for part of the lab analysis.

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To perform theoretical calculations the students were instructed to model the process as two distinct steps. A simplified model for free convective heat transfer was provided to the students which modeled the pot or pressure cooker as a cylindrical wall with a circular flat plate on the top. The students were told the assumptions and variables involved in the simplified model, but not shown the details as those are more appropriately introduced in a heat transfer course. First step: water is heated from ambient temperature to its boiling point -assume system is closed -heat into system is all of the collected solar radiation -heat out of system accounted for by simplified convection model Second step: water boils at constant temperature and pressure -assume system is open -heat into system is all of the collected solar radiation -heat out of the system is constant and due to convection only -the system’s specific internal energy (u) remains constant This last point is valid due to the fact that the system was at boiling conditions for a relatively short period of time and the change in system quality was minimal. By breaking the process up as such students could derive theoretical relations for how specific internal energy changes with time during the first step and how much water should be left after boiling for 10 minutes. The different components of the project were brought together with a final report which had three main sections: Design Analysis, Lab Analysis, and Reflective Analysis. For the Design Analysis students were to discuss the main problems their design addressed, alternative designs considered, compromises made, initial testing problems and solutions and any other relevant information that was considered. Also included in this section were the bill of materials for the project, Solid Works drawings of their design and their user manual which explained how to properly and safely use their solar cooker. The Lab Analysis section included the data collected during testing as well as the thermodynamic analyses and calculations asked for in the lab handout (Appendix B). For the Reflective Analysis section teams were asked to reflect on the process of designing and testing their cooker and suggest possible improvements to their design. This section further required students to discuss the use of solar cookers in a global context (regions/populations who might benefit) and the practicality of their widespread use. Representative Results

Figure 2 shows a comparison of how specific internal energy changes with time during the heating process in an open pot and pressure cooker. By plotting the data in such a way one can easily visualize the differences, something which is sometimes missed by simply comparing values from tables. Students can see that despite giving each system the same input energy the pressurized system sees a larger specific internal energy

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increase. This point is especially important when dealing with solar cookers as it displays a pressure cooker’s increased efficiency from an energy standpoint.

Figure 2. Comparison of specific internal energy while heating

Figure 3. Comparison of experimental data with theory for water heating in a pot

The comparison of experimental results with theoretical calculations is shown in Figure 3 and it is shown that the theoretical plot over-predicts the internal energy. There are a number of assumptions in the analysis which lead to this discrepancy. Assuming that the heat coming in is equal to all of the incoming radiation and that the heat leaving is due to just free convection present the greatest potential for causing the discrepancy. If one were to include the reflectivity of the collectors and pots and account for the fact that all of the radiation may not be directed accurately some gains could be made towards better agreement. Additionally, if there is any wind the heat lost due to forced convection will be much larger than that lost by free convection for the same set-up. While having good agreement would be nice, the fact that there are differences presented an opportunity to discuss why the differences may exist with the students.

0.00

100.00

200.00

300.00

400.00

500.00

600.00

0 5 10 15 20 25 30

time (min)

u (

kj/

kg

)

Pressure Cooker

Open Pot

0.00

200.00

400.00

600.00

0 5 10 15 20 25

time (min)

u (

kj/

kg

)

theoretical

experimental

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Figure 4. Comparison of heating with and without convection shield

From Figure 4 one clearly sees the difference in performance caused by the inclusion of a convection shield. For this specific case the convection shield blocked more incoming radiation than it prevented heat loss by convection resulting in an adverse effect on the heating process. Regardless, it provides a valuable opportunity to learn. At the completion of the project the class discussed each design in terms of what worked well and what did not work well, and brainstormed ideas for how the designs could be improved. Project Assessment

To assess the success of the project both subjective and objective feedback was gathered. The subjective feedback was taken using a post project survey (Appendix C) while the objective feedback came from the inclusion of subject specific questions (Appendix D) on the final exam for the course. As only half the class participated in the solar cooker project the other half was considered a control group against whom understanding of transient analysis could be compared. This control group did not use the solar cookers but did participate in a very similar transient analysis and experiment using hot plates. From Table 1 it can be seen that the project was successful in many of its goals, at least in the students’ eyes. There was a strong positive response to the group aspect of the project as well as the open-ended structure.

Table 1. Design project survey results Statement rated from 1-5 (1=strongly disagree, 5= strongly agree) average

The design project helped me to strengthen my understanding of material presented in lecture. 4.00

The design project helped me to better understand the practical constraints and compromises that exist in engineering projects.

4.55

I prefer the open-ended structure of the design project to one with a more rigid structure. 4.27

Group based projects are an effective tool for improving engineering skills. 4.45

My learning was improved by having multiple groups design for the same problem. 4.09

I understand the equations used to solve problems in which mass or energy are changing (transient problems).

4.09

The design project sparked my interest to learn more about the thermal/fluid sciences. 4.09

20

22

24

26

28

30

0 2 4 6 8 10 12 14

time (min)

Tem

pera

ture

(oC

)

Pot w/ Convection Shield

Open Pot

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A few key points can be summarized from the answers to the open-ended questions at the end of the survey. Students would have liked: better access to the materials outside of lab hours, some initial ideas to work from, more money and time for the design and build phase (they had a $30 per student budget and 7.5 hours of class time), more time testing their actual design and easier/less math for the lab portion. Students realized they could have gotten more out of the project if they had: spent more time on it, come up with and tested multiple designs, and done more research on potential designs. Students felt the best part was: the hands on building, testing their design, freedom in design, the relevance of the project, seeing their final product work. A statistical analysis on the objective feedback was used to determine if the design project enhanced student understanding of transient analysis. The solar cooker design students were compared to the control group using an unpaired two-sample t-test with equal variances (as verified by an F-test with P = 0.41). The results shown in Table 2 indicate that the mean performance between the groups do not differ in a conventionally statistically significant manner (i.e. P > 0.05). Table 2. Statistical analysis results of performance on transient analysis exam questions

Solar Design Group Control Group

Mean 0.473 0.615

# of students 11 13

two-tailed P-value 0.12

Though not statistically significant, it is noteworthy that the performance by the two groups was different. The students in the control group answered 61.5% of the transient analysis questions correctly compared to 47.3% for the solar design group. This is somewhat surprising given that the solar design students rated themselves as being more comfortable with transient analysis than the control group on the subjective survey (question 6, Appendix C). The average control group response to this question was 3.85 as compared to 4.09 for the solar design group. These results suggest the importance of including graded assessments in addition to student surveys as a means of providing unbiased feedback of student learning. Lessons Learned

As a first trial with a solar cooker design project plenty of valuable lessons were learned throughout the process. The idea for the project started in July but the designs were not tested until November. A three week window was scheduled within which testing was planned, though the lab only met once a week so the window was effectively three days.

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Of those three days one was rainy, one was completely overcast, and one was sunny. Without being able to plan some flexibility into the schedule this project would have been a large gamble as it is weather dependent. That being said, it was rewarding to see that even late in the afternoon in November enough solar radiation is available to boil water using a solar cooker. In the future, students would benefit by being able to test their design multiple times so as to iteratively improve their design. This could be facilitated by improving their access to the materials outside of lab and by scheduling more days of testing. Alternatively, the project could be simplified by perhaps removing the user manual and Solid Works portions to allow greater focus on the design and testing. From the assessments used it seems that the structure of the project did not enhance student learning of transient analysis. As each team turned in a single report it could be likely that not every team member contributed equally to each section. If this were the case then one could imagine a single group member performing the necessary lab analysis in which the students would be forced to work with the transient equations. By having each student individually hand in a lab analysis, as well as having it as part of the final report, every student would have greater exposure to the analysis. Additionally the feedback suggests that it may be useful to spend more time explaining the concepts and math behind transient analysis as part of the lab. Finally, it was clear how excited the students became upon witnessing the ability of the Fresnel lens to focus radiation. The lens would immediately start wood on fire (Figure 5) and even melted a hole through aluminum ducting when placed at the focal point. One truly gets an understanding for the power of the sun when beholding this. In addition to appealing to the students, many passers by were so intrigued as to stop and ask questions about the project and the class. It was a proud moment when it was expressed to one of the students “you engineers get to do the coolest projects.”

Figure 5. Fresnel lens igniting wood

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Conclusion

The incorporation of solar-thermal technologies into an introductory thermodynamics lab course shows great promise for exposing students to unique, innovative and relevant applications. An open-ended solar cooker design project was developed and implemented in a lab exposing the relationship between temperature, pressure and internal energy during a transient heating process. Through multiple assessments of the project and student learning the experience was shown to be a success on many fronts with students responding very positively to the open-ended and group nature of the project. Future projects with solar cookers are planned which will incorporate multiple identical collectors allowing students to focus on ideas for maximizing heat retention and permitting side by side parametric testing. Acknowledgements

The authors would like to thank the University of Minnesota Engineers Without Borders for allowing their solar concentrators to be used for this design project. We also thank John Angeli for his assistance to students during the design and build phase and for photographing the testing of the designs. We would also like to thank the students of the fall semester 2009 Thermodynamics course at the University of St. Thomas. References

1 Tao, Y., Cao, Y., “Implementation of Design, Build and Test Projects for Heat Exchanger and Air

Conditioning in Thermal Engineering Courses”, Proceedings of the 2007 American Society for

Engineering Education Annual Conference & Exposition, June 2007, Honolulu, HI. 2 Forsberg, C.H., “A Student-Centered Senior Capstone Project in Heat Exchanger Design”,

Proceedings of the 2004 American Society for Engineering Education Annual Conference &

Exposition, June 2004, Salt Lake City, UT. 3 Sherwin, K., Mavromihales, M., “Design, Fabrication and Testing a Heat Exchanger as a Student

Project”, Proceedings of the 1999 American Society for Engineering Education Annual

Conference & Exposition, June 1999, Charlotte, NC. 4 Dixon, G.W., “Three Thermal Systems Design-Build-Test Projects”, Proceedings of the 2004

American Society for Engineering Education Annual Conference & Exposition, June 2004, Salt Lake City, UT.

5 Accrediting Board for Engineering and Technology, Criteria for Accrediting Engineering

Programs, Baltimore, 2008. 6 Coronella, C., “Project-Based Learning in a First-Year Chemical Engineering Course”,

Proceedings of the 2006 American Society for Engineering Education Annual Conference &

Exposition, June 2007, Chicago, IL.

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Appendix A : Pre-Project Survey

Name

e-mail address

Major

Do you live on campus or off campus?

Best times to meet outside of class

Is there any student you cannot work with?

Rate your skills in the following areas from 1-10 :

Handiness (mechanical abilities with tools)

Math

Computers

Writing

Leadership (managing a team)

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Appendix B : Lab Description

Pressure Cooker Lab (solar concentrators)

In this lab a pressure cooker will be compared with a simple pot from a thermodynamic standpoint utilizing a transient analysis and first law principles. Solar concentrators will be used in place of a more typical heat source and design improvements for increasing their efficiency will be tested. Steps for testing pressure cooker vs. pot:

(your group may only be in charge of a pot or pressure cooker, not both, for this part) 1. Place 400 mL of water into the pot and 400 mL of water into the pressure cooker. 2. Place the pot and pressure cooker onto their solar concentrators (either the mirrors or lenses). 3. Every 2 minutes record the pyranometer reading, the temperatures of the water inside the pot and pressure cooker, and the pressure in the pressure cooker. 4. When the temperature of the water in the pot has remained constant for 10 minutes remove the pot from the burner, place it in the temperature bath to cool the water and measure the amount of water left in the pot. 5. When the temperature and pressure in the pressure cooker has remained constant for 10 minutes remove the pressure cooker from the burner, place it in the temperature bath to cool the water and measure the amount of water left in the pressure cooker. *If the pot or pressure cooker have not reached constant temperature or pressure conditions after an hour then remove vessels as in step 5; additional data will be provided as needed to complete the analysis. Analysis for pressure cooker vs. pot:

a). Using the data collected, plot the specific internal energy (u) vs. time for the water in the pot and the water in the pressure cooker on a single graph. b). Looking at this plot what differences can be seen? When would it make sense to use a pressure cooker?

Bringing the water in the pot to a boil is an example of a constant pressure process. During the heating the system can be assumed to be closed, though once the boiling starts liquid water will be converted into water vapor and leave the pot. With changes in potential and kinetic energy being negligible the 1st law of thermodynamics during the heating process is:

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WQdt

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dt

dU

dt

dE && −===

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find an average value for inQ& during the heating process:

Heat is transferred from the pot due to free convection which depends on the

temperature difference between the water and the surroundings. A theoretical estimate of this transfer can be determined to be:

ambientfinal

ambient

outTT

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−−

=&

For this equation C = 0.0181, the temperatures must be absolute (Kelvin or Rankine) and the units for the heat transfer rate will be in kW. Tfinal is the maximum temperature reached while Tambient is the ambient temperature. d). Using the incompressible model for liquids and assuming that the specific heat of water remains constant while heating determine a theoretical relation for how the specific

internal energy of the water depends on time. (Use the average value of inQ& found

previously). e). Use the data collected to determine the specific internal energy of the water for each measurement. Plot the experimental value and the theoretical value vs. time. f). What are the possible sources for any discrepancy seen in a comparison of experimental vs. theoretical values?

When heating the water in the pressure cooker the water initially experiences a constant volume process as a closed system followed by a (roughly) constant temperature and constant pressure process as an open system when the pressure is high enough for the vent to open. With changes in potential and kinetic energy being negligible the 1st Law of Thermodynamics during the constant T and P heating process is:

( )ee

cv

cv

cvcv hmWQdt

dmu

dt

mud

dt

dE&&& −−===

g). From the data collected, find an average value for inQ& when water vapor is venting:

h). Using the previous equation for convective heat transfer rate with C = 0.0552 and a mass balance, determine a theoretical value for the amount of water left in the pressure

1 The coefficient C was derived by the instructor and was based on the size of the open pot. 2 The coefficient C was derived by the instructor was based on the size of the pressure cooker.

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cooker. What are the possible sources for any discrepancy seen in a comparison of experimental vs. theoretical values? Steps for testing performance improvements:

1. Place 400 mL of water into an uncovered pot. 2. Place the pot onto the solar concentrator (either a mirror or lense), without including the convection shield. 3. Every 2 minutes record the pyranometer reading and the temperature in the pot. 4. After 30 minutes cool the contents of the pot in the bath. 5. Repeat steps 1-4 but include the convection shield Analysis for performance improvements:

j). Using the data collected, plot the specific internal energy (u) vs. time for the water in the vessel with and without the convective shield on a single graph.

k). Determine the average increase in totalQ& and compare the overall totalQ& /cost for the

case of improvements vs. no improvements.

Assessment of other teams improvements:

Third party testing and assessment is a part of your grade and each group will have a chance to use all the other set-ups. Peer feedback will be completely confidential, though a summary of the results may be presented.

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Peer Review of Performance

Please rate the improvements and user manual made by the other teams using this form. Reviewer: Team being reviewed:

Criteria scale of 1-10 (10 being highest)

User manual is clear

User manual is concise

Ease of concentrator adjustment (rotating up/down)

Ease of concentrator adjustment (rotating left/right)

Ease of focus determination

Sturdiness of concentrator

Ease of improvement implementation

Lack of safety issues

Any additional notes or feedback: Team being reviewed:

Criteria scale of 1-10 (10 being highest)

User manual is clear

User manual is concise

Ease of concentrator adjustment (rotating up/down)

Ease of concentrator adjustment (rotating left/right)

Ease of focus determination

Sturdiness of concentrator

Ease of improvement implementation

Lack of safety issues

Any additional notes or feedback:

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Appendix C : Assessment Survey and Questions

Design Project Survey 1. The design project helped me to strengthen my understanding of material presented in lecture.

strongly disagree disagree neutral agree strongly agree

1 2 3 4 5

2. The design project helped me to better understand the practical constraints and compromises that exist in engineering projects.

strongly disagree disagree neutral agree strongly agree

1 2 3 4 5

3. I prefer the open-ended structure of the design project to one with a more rigid structure.

strongly disagree disagree neutral agree strongly agree

1 2 3 4 5

4. Group based projects are an effective tool for improving engineering skills.

strongly disagree disagree neutral agree strongly agree

1 2 3 4 5

5. My learning was improved by having multiple groups design for the same problem.

strongly disagree disagree neutral agree strongly agree

1 2 3 4 5

6. I understand the equations used to solve problems in which mass or energy are changing (transient problems).

strongly disagree disagree neutral agree strongly agree

1 2 3 4 5

7. The design project sparked my interest to learn more about the thermal/fluid sciences.

strongly disagree disagree neutral agree strongly agree

1 2 3 4 5

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What could I have done as a student to get more out of the project? What was the worst part of the design project? What was the best part of the design project?

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