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ASSESSMENT MECHENG 713 Page 1 of 7 FIRST SEMESTER, 2020 Campus,
City ENERGY TECHNOLOGY SECTION 3: THERMODYNAMICS Time Allowed: This Final Assessment has been designed so that a well-prepared student could complete it within three hours. From the 1pm release time you will have 24 hours to complete and submit your assessment. No marks will be deducted for taking longer than three hours within that 24-hour period, but you must submit before the deadline. Book Designation: This assessment is open book, you are permitted to access your course manuals and other written material including online resources. The assessment must be your own work. Format: 1. This assessment carries a total of 90 marks and is weighted at 70% of your overall mark for this course. 2. It contains 3 sections as follows: a. Section 1: Energy Studies and Heat Transfer 30 marks b. Section 2: Mass Transfer 30 marks c. Section 3: Thermodynamics 30 marks 3. Each section is divided into two parts: a. Part A, 10 multiple-choice questions, of 1 mark each 10 marks b. Part B, 2 long answer questions, of 10 marks each 20 marks 4. Answer ALL questions including ALL parts of each question. 5. Some of the multiple-choice questions may have more than one correct answer, and you are to pick all the correct answers. 6. All the required data is provided in an Appendices booklet. 7. Obtain all the required fluid and material properties, and correlations, only from the Appendices booklets that are provided, or as specified in the question. Marks may be deducted if you obtain these from other sources. a. Appendices_Mecheng713_2020_HEAT-TRANSFER.pdf b. Appendices_Mecheng713_2020_MASS-TRANSFER.pdf c. Appendices_Mecheng713_2020_THERMODYNAMICS.pdf 8. Use linear interpolation to obtain the fluid or material properties whenever necessary. Do not round the properties to the nearest values shown in the tables. THE UNIVERSITY OF AUCKLAND FINAL ASSESSMENT MECHENG 713 Page 2 of 7 Submission: ‐ It is your responsibility to ensure your assessment is successfully submitted on time. ‐ We recommend you aim to submit a couple of hours in advance of the deadline, to allow time to deal with any technical issues that might arise. ‐ Upload your answer document as a .pdf, .doc or .docx file only. ‐ We STRONGLY recommend you download your submitted document from Canvas, after submitting it, to verify you have uploaded the correct document Support: ‐ If you have any concerns regarding your Final Assessment, please call the Contact Centre for advice, rather than your instructors ‐ If any corrections are announced during the 24 hours of the final assessment, you will be notified by a Canvas Announcement. Please ensure your notifications are turned on during this period. Question Interpretation: Please note that during the final assessment period you cannot contact your instructors for clarification on how to interpret the wording of any specific questions or to verify that your answer is correct. Interpreting wording and making appropriate assumptions is part of what is being assessed. You will need to interpret the question yourself and check your own answers. If you believe there is a typo, first re-read the question to check you have not misunderstood the question, as it is very common for students to misread questions. If you still believe there is a typo, please phone the Contact Centre. Academic Honesty Declaration: By completing this assessment, I agree to the following declaration: I understand the University expects all students to complete coursework with integrity and honesty. I promise to complete all online assessment with the same academic integrity standards and values. Any identified form of poor academic practice or academic misconduct will be followed up and may result in disciplinary action. As a member of the University’s student body, I will complete this assessment in a fair, honest, responsible and trustworthy manner. This means that: ‐ I declare that this assessment is my own work, except where acknowledged appropriately (e.g., use of referencing). ‐ I will not seek out any unauthorised help in completing this assessment. ‐ I declare that this work has not been submitted for academic credit in another University of Auckland course, or elsewhere. ‐ I am aware the University of Auckland may use Turnitin or any other plagiarism detecting methods to check my content. ‐ I will not discuss the content of the assessment with anyone else in any form, including, Canvas, Piazza, Facebook, Twitter or any other social media within the assessment period. ‐ I will not reproduce the content of this assessment anywhere in any form. ‐ I declare that I generated the calculations and data in this assessment independently, using only the tools and resources defined for use in this assessment. ‐ I declare that I composed the writing and/or translations in this assessment independently, using only the tools and resources defined for use in this assessment. THE UNIVERSITY OF AUCKLAND FINAL ASSESSMENT MECHENG 713 Page 3 of 7 SECTION 3: Thermodynamics (30 marks) Part A (10 marks) Note: There are 10 multiple-choice questions in this part, each worth 1 mark. Answer all questions. There may be more than one correct answer to each of these questions. Thus, pick all the correct answers. Your workings are not required for questions in Part A. Write only the letter(s) corresponding to your answer(s) to each question, against each question number, in your answer document. 1. Which of these technologies below is based on a thermal power cycle? a) Nuclear power station b) Wind farm c) Fuel cell d) Electric heater e) None of the above 2. Which of the cycles below do NOT use air as the working fluid? a) Brayton cycle b) Kalina cycle c) Organic Rankine cycle d) Absorption refrigeration cycle e) Gas refrigeration cycle 3. What is the maximum coefficient of performance of a refrigeration system that operates between 5°C and 50°C according to Carnot’s theorem? a) 0.14 b) 0.90 c) 1.11 d) 7.18 e) None of the above 4. Determine the pressure ratio at which a Brayton cycle with a regenerator and another without have the same thermal efficiency, if the air temperatures at the inlets of the compressor and the turbine are 25°C and 750°C, respectively. Please use the cold-air standard assumptions in your analysis. a) 2.0 b) 7.0 c) 8.7 d) 9.4 e) None of the above THE UNIVERSITY OF AUCKLAND FINAL ASSESSMENT MECHENG 713 Page 4 of 7 5. A cogeneration plant with a utilization factor of 20% takes in 1 MW of heat and produces 100 kW of heat at the process heater. The pumps consume 5 kW of power. Determine the power produced by the turbine. a) 95 kW b) 100 kW c) 105 kW d) 295 kW e) None of the above 6. Determine the isentropic efficiency of an expander that expands R-134a from an initial pressure and temperature of 0.4 MPa and 100°C to a final pressure and temperature of 0.06 MPa and 60°C. a) 0.7% b) 1.5% c) 4.7% d) 65.4% e) None of the above 7. Which of the fluids below is not used in modern commercial refrigeration systems? (Note: there may be more than one correct answer, pick all the correct answers) a) Ethanol b) Propane c) Carbon dioxide d) Ammonia e) Freon 8. A typical petrol automotive engine has a thermal efficiency of 25%. Assuming the burning temperature is 800°C and the ambient temperature is 20°C, determine the second law efficiency of such an engine. a) 25.6% b) 34.4% c) 72.7% d) 97.5% e) None of the above 9. An engine burns 1 liter of liquid decane (C10H22) completely. How much carbon-dioxide (in kg) is produced in this process? The molar mass of liquid decane is 142.3 kg/kmol and its density is 730 kg/m3. The molar mass of carbon dioxide is 44.01 kg/mol and its density at standard pressure and temperature is 1.97 kg/m3. a) 0.02 kg b) 1.2 kg c) 2.3 kg d) 7.3 kg e) None of the above THE UNIVERSITY OF AUCKLAND FINAL ASSESSMENT MECHENG 713 Page 5 of 7 10. Hot steam is flowing in a tube and releasing heat to a heated cabinet. The steam pressure is 15 MPa with negligible loss in the tube. The inlet and outlet steam temperatures are 500°C and 400°C, respectively. The room and ambient temperatures are 100°C and 20°C, respectively. Determine the exergy destruction of the steam. a) 0.7 kJ/kg b) 126.7 kJ/kg c) 161.2 kJ/kg d) 198.5 kJ/kg e) None of the above Part B (20 marks) Note: There are 2 long answer questions in this part. Answer all questions. Show all workings. 11. A simple ideal Rankine cycle with only one turbine is to be upgraded to that with two turbines and a reheater (as shown over the page). Steam leaves the boiler at 1.6 MPa and 400°C and is cooled in the condenser at a pressure of 100 kPa. It enters the pump as saturated liquid. The proposal is to use an intermediate pressure Preheat of 0.8 MPa and to have the fluid entering the secondary turbine at 400°C. Assume all the processes in the turbines and the pump are isentropic. Neglect pressure losses in the tubes and the heat exchangers. a) Determine the thermal efficiency of the original simple Rankine cycle. (4 marks) b) Determine the thermal efficiency of the modified cycle with two turbines and a reheater. (5 marks) c) Is the intermediate pressure properly selected to optimize the performance? If not, what should the pressure be? (1 mark) THE UNIVERSITY OF AUCKLAND FINAL ASSESSMENT MECHENG 713 Page 6 of 7 THE UNIVERSITY OF AUCKLAND FINAL ASSESSMENT MECHENG 713 Page 7 of 7 12. A cascade heat pump system is being considered as a high temperature heat source. The bottom cycle is an ideal R134a vapour compression system and the upper cycle is an ideal water vapour compression system. The evaporating and condensing pressures of the R134a cycle are 0.32 MPa and 1.60 MPa, respectively. The evaporating and condensing pressures of the water cycle are 0.01 MPa and 0.20 MPa, respectively. a) Determine the heat pump COP of the R134a cycle. (3 marks) b) Determine the heat pump COP of the water cycle. (3 marks) c) Determine the required water mass flow rate (in kg/s) if the R134a flow rate is 1 kg/s. (1 mark) d) Determine the total COP of this heat pump system. (3 marks) Water VCR cycle R134a VCR cycle
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