Thermodynamics and Heat Transfer

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National Diploma in Mechanical Engineering

Section A: COMPULSORY Answer ALL questions 1-12 1. A resistance temperature device has a resistance R0 = 100 Ω at 0o C. If the resistance RT = 197.7 Ω in an oil bath, what is the temperature of the oil in the bath, given that RT = R0(1+αT)? Take R0 = 100 Ω R0 = resistance at 0o C α = 3.9083 x 10-3 /o C (2 marks)

NDE Thermodynamics and Heat Transfer Exam 2008
Name: _______________________________________
Read all the instructions before starting Do not open this paper until instructed Time allowed: 2 hours (plus 5 minutes reading time) Attempt ALL question in Section A (questions 1 – 12) Attempt any TWO from Section B (questions 13-15) Answer the questions in the spaces provided on the question paper. Use the back of the previous page if required. If you attempted all three questions in section B, then cross out the question you don’t want marked. The marker will mark the first 2 questions not crossed out. Show all relevant working. Marks will be awarded for method as well as correct answers Add your name to the top of this page 3. A steel shaft has a diameter of 25.0 mm at 20°C. A brass retaining collar has an internal diameter 24.95 mm at the same temperature. If the shaft is cooled to 0°C, find the temperature to which the retaining collar must be heated if is to have a clearance of 0.10 mm when it is fitted to the shaft. Take αSteel = 12 x 10-6 /oCand αBrass = 19 x 10-6 /oC. (4 marks) 2. A steel casting (C = 460 J/kgK) of mass 2.4 kg is cooled from 350°C by placing in a container of 12 kg of water (C = 4200 J/kgK) at 18°C. Assuming no heat is transferred to the container, calculate the final temperature. (4 marks)

Instructions:

Attachments: Marking:

Formulae, saturated water tables

Individual marks are shown on each question. There are a total of 70 marks representing 50% of your final mark in this subject. A minimum of 40% in this exam is a requirement for course completion

Thermo exam November 2008 final.doc
Version 23-Sep-08

”Mechanical Consortium
Printed 11/04/08

Page 2

National Diploma in Mechanical Engineering

National Diploma in Mechanical Engineering

4. Heat was added to 10 kg of saturated liquid water at a pressure of 2 bar at the rate of 20 kW for 10 minutes. Given the following information from the saturated water table, and assuming perfect insulation: hf = 505 kJ/kg, hfg = 2202 kJ/kg, hg = 2707 kJ/kg, Ts = 120.2°C a. Find the dryness fraction of the steam after the initial 10 minutes. (4 marks)

5. For the complete combustion of acetylene, C2H2: a. Write down the balanced combustion equation (2 marks)

b. Determine the stoichiometric air to fuel mass ratio (5 marks) Atomic weights: H = 1, C = 12, O = 16, N = 14 Air composition: 23% oxygen, 77% nitrogen (by mass)

b. Calculate the time (in minutes and seconds) for which the steam must be heated at the same rate in order to become fully dry. (3 marks) 6. Prepare a labelled sketch of a manometer inclined at 5o to a horizontal datum. If the manometer contains oil with a density of 800 kg / m3, and a change in the pressure applied to it is 50 Pa, find the oil deflection in mm. (4 marks)

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Thermo exam November 2008 final.doc
Version 23-Sep-08

”Mechanical Consortium
Printed 11/04/08

Page 3

Thermo exam November 2008 final.doc
Version 23-Sep-08

”Mechanical Consortium
Printed 11/04/08

Page 4

National Diploma in Mechanical Engineering

National Diploma in Mechanical Engineering

7. Explain for the Seebeck effect and how it may be applied to temperature measurement. (3 marks)

10. Find the total heat transfer, per m2, through the wall shown below. Neglect radiation. Air temperature –5°C h = 10 W/m2K Brick 70 mm thick k = 2.3 W/mK Air Gap R = 1.5 m2K/W

(5 marks)

Surface temperature 25°C Gib 9 mm k = 0.1 W/mK 8. The non-flow energy equation is given as Q - W= ΔU. State the equation as it applies to: a....
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