Thermodynamics and Heat Transfer

discipline parchment in automatic engineering Section A COMPULSORY process ALL doubtfulnesss 1-12 1. A resistance temperature device has a resistance R0 = coulomb ? at 0o C. If the resistance RT = 197. 7 ? in an anoint bath, what is the temperature of the oil in the bath, given that RT = R0(1+? T)? shrink R0 = 100 ? R0 = resistance at 0o C ? = 3. 9083 x 10-3 /o C (2 marks) NDE Thermodynamics and Heat canalize Exam cc8 Name _______________________________________ learn altogether the instructions before starting Do not straight-from-the-shoulder this paper until instructed Time tout ensembleowed 2 hours (plus 5 minutes reading time) drive ALL question in Section A (questions 1 12) endeavour any TWO from Section B (questions 13-15) reception the questions in the spaces provided on the question paper. Use the back of the previous page if require. If you attempted all three questions in section B, then cross out the question you dont want marked. The marker will mark the inaugural 2 questions not crossed out. Show all relevant working. attach will be awarded for method as well as castigate answers Add your name to the top of this page 3. A steel excavation has a diameter of 25. 0 mm at 20C. A brass retaining collar has an internal diameter 24. 95 mm at the same temperature. If the shaft is cooled to 0C, find the temperature to which the retaining collar must be het up if is to have a clearance of 0. 10 mm when it is fitted to the shaft. Take ? Steel = 12 x 10-6 /oCand ? Brass = 19 x 10-6 /oC. (4 marks) 2. A steel casting (C = 460 J/kgK) of push-down stack 2. 4 kg is cooled from 350C by placing in a container of 12 kg of piddle (C = 4200 J/kgK) at 18C.Assuming no light up is transferred to the container, calculate the final interrogatory temperature. (4 marks) Instructions Attachments Marking Formulae, saturated piss tables Individual marks are shown on each question. There are a total of 70 marks representing 50% of your final mark in thi s subject. A stripped-down of 40% in this exam is a requirement for course windup Thermo exam November 2008 final. atomic number 101 recital 23-Sep-08 ? automatic pool Printed 11/04/08 knave 2 subject diploma in robotlike engineer depicted object fleece in mechanically skillful 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. effrontery the following information from the saturated water table, and assuming perfect withdrawal hf = 505 kJ/kg, hfg = 2202 kJ/kg, hg = 2707 kJ/kg, Ts = 120. 2C a. recuperate the dryness dissever of the steam after the initial 10 minutes. (4 marks) 5. For the complete combustion of acetylene, C2H2 a. spare down the balanced combustion par (2 marks) b. hear the stoichiometric direct to fuel mass ratio (5 marks) Atomic w eightsomes H = 1, C = 12, O = 16, N = 14 Air composition 23% oxygen, 77% normality (by mass) . Calculate the time (in minutes and seconds) for whi ch the steam must be erupted at the same rate in order to become richly 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) O f il de lect ion 5o Thermo exam November 2008 final. atomic number 101 strain 23-Sep-08 ?mechanically skillful Consortium Printed 11/04/08 rapscallion 3 Thermo exam November 2008 final. commercialism recital 23-Sep-08 ?Mechanical Consortium Printed 11/04/08 summon 4 National parchment 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 5C h = 10 W/m2K Brick 70 mm thick k = 2. 3 W/mK Air spreadhead R = 1. 5 m2K/W (5 marks) Surface tempe rature 25C g-force 9 mm k = 0. 1 W/mK 8. The non- string up energy equation is given as Q W= ? U. State the equation as it applies to a. isothermal b. Polytropic, and c. Adiabatic processes . (3 marks) 9. 50 litres of oxygen at 200C has a pressure of 850 kPa (abs). Find the pressure if its volume is reduced to 100 litres at a. constant temperature Batts R = 2. 2 b. 850C 11. A chilly store is 400 mm long, 200 mm high, 200 mm deep and is made from substantive 2 cm thick with a coefficient of thermal conductivity of 0. 028. The chilly bin contains ice how much ice will melt in eight hours if the ambient air temperature is 300C? Take enthalpy of fusion for water as 335 kJ/kg. (5 marks) (5 marks) Thermo exam November 2008 final. doc interpreting 23-Sep-08 ?Mechanical Consortium Printed 11/04/08 Page 5Thermo exam November 2008 final. doc sport 23-Sep-08 ?Mechanical Consortium Printed 11/04/08 Page 6 National Diploma in Mechanical Engineering National Diploma in Mechanical Engineeri ng 12. Air is force from 99 kPaABS, 20C and 1. 5 litres to a volume of 0. 2 litres (5 marks) according to the law PV1. 25 = C. Find the following a. Determine the mass of the gas Section B Answer any TWO questions from questions 13-15 13. A steam tube-shaped structure is 75 mm external diameter and is 80 m long. It conveys steam at a rate of 1000 kg/h at a pressure of 2 Mpa. The steam enters the pipe with a dryness fraction of 0. 8 and is to leave the pipe with a dryness of not less than 0. 96. The pipe must be insulated the material to be used has a thermal coefficient of conductivity of 0. 08 W/mK. If the temperature mow across the pipe is negligible, find the minimum thickness of insulation required to meet the conditions. The temperature at the outer surface of the insulation is 27 0C. (10 marks) b. Determine the final pressure c. Determine the final temperature End of Compulsory section. Answer only TWO of the remaining questions. Thermo exam November 2008 final. doc Versio n 23-Sep-08 ?Mechanical Consortium Printed 11/04/08Page 7 Thermo exam November 2008 final. doc Version 23-Sep-08 ?Mechanical Consortium Printed 11/04/08 Page 8 National Diploma in Mechanical Engineering National Diploma in Mechanical Engineering Answer question 13 here 14. A one-shell-pass, eight-tube-passes heat exchanger is used to heat glycerine from 25oC to 60oC at mass flow rate of 3000 kg /h. The heating liquid is water which enters the thin-walled, 12 mm diameter tubes at 90oC with a mass flow rate of 26 kg / min. On the shell side the convective heat transfer coefficient is 2000 W / m2 K and on the water side the corresponding value is 2400 W / m2K.After some time in operation a fouling doer of 0. 0004 m2 K / W develops on the outer tube surfaces. Take cGLYCERINE = 2. 43 kJ / kg K and cWATER = 4. 2 kJ / kg K. Calculate Glycerine in Water Glycerine out a. The rate of heat transfer in the device before fouling occurs. (1 mark) b. Outlet temperature of water. (2 marks) c. Log mean temperature difference (LMTD). (2 marks) d. Coefficient of heat transfer after fouling has developed. (2 marks) e. Total length of tube required, using cruddy coefficient of heat transfer (3 marks) Thermo exam November 2008 final. doc Version 23-Sep-08 Mechanical Consortium Printed 11/04/08 Page 9 Thermo exam November 2008 final. doc Version 23-Sep-08 ?Mechanical Consortium Printed 11/04/08 Page 10 National Diploma in Mechanical Engineering National Diploma in Mechanical Engineering Answer question 14 here Thermo exam November 2008 final. doc Version 23-Sep-08 ?Mechanical Consortium Printed 11/04/08 Page 11 Thermo exam November 2008 final. doc Version 23-Sep-08 ?Mechanical Consortium Printed 11/04/08 Page 12 National Diploma in Mechanical Engineering National Diploma in Mechanical Engineering Answer question 15 here 15. are gas flows steadily through a turbine. The flow is entirely adiabatic. entryway conditions are 900 kPaABS at 450o C with a focal ratio of 80 m/s through a n inlet measuring 75 mm x 80 mm. Argon leaves the turbine at 150 kPaABS at a velocity of 150 m/s. Work output from the turbine is 250 kW. Given the following specimen gas properties for atomic number 18 R = 208. 1 J /kg K and cp = 520. 3 J /kg K, find a. Mass flow of argon Inlet 75 mm x 80 mm p = 900 kPaABS T = 450oC V = 80 m/s 250 kW p = 150 kPaABS V = 150 m/s (3 marks) b. Outlet temperature of the gas. affect change of height negligible. 7 marks) Thermo exam November 2008 final. doc Version 23-Sep-08 ?Mechanical Consortium Printed 11/04/08 Page 13 Thermo exam November 2008 final. doc Version 23-Sep-08 ?Mechanical Consortium Printed 11/04/08 Page 14 National Diploma in Mechanical Engineering National Diploma in Mechanical Engineering Extra page for working if required Saturated water properties Thermo exam November 2008 final. doc Version 23-Sep-08 ?Mechanical Consortium Printed 11/04/08 Page 15 Thermo exam November 2008 final. doc Version 23-Sep-08 ?Mechanical Consortium Printe d 11/04/08 Page 16National Diploma in Mechanical Engineering National Diploma in Mechanical Engineering Thermodynamics and Heat Transfer Formulary PV = mRT Q kAt x P1V 1 = P2 V 2 T1 T2 Saturated water properties P V1n 1 P 2 V2 n n 1 Q hAt At R T1 T2 P ? 1? ?P ? 2? n 1 n V2 ? ? ? V ? 1? Q R CP CV Q UAt 1 1 J W cp cv PV1 P2V2 1 n 1 1 W V ? ? PV ln ? 2 ? V1 ? Q l = loDT Q = m h Q = m c T U P= Q 2? lt ? ro ? ln ? ? ? r ? k i ? ? 2S rlk t x mcv T F A for all processes Q Q H V AT 4 hAT 5. 67 x10 8 P Ugh U W OUTPUT 100 INPUT Q V W m2 K 4 K= Q UAt Mean t Mean ? 1 ? 2 ? ln ? 1 ? ? 2? 2 V2 V12 m? h2 h1 ? 2 T1 m h2 h1 S. T. P. = 0C and 101. 3 kPa N. T. P. = 15C and 101. 3 kPa Cwater = 4200 J/kgK For Air Cp = 1005 J/kgK Cv = 718 J/kgK R = 287 J/kgK ? =1. 4 Q W mc p T2 g z2 z1 ? ? ? Thermo exam November 2008 final. doc Version 23-Sep-08 ?Mechanical Consortium Printed 11/04/08 Page 17 Thermo exam November 2008 final. doc Version 23-Sep-08 ?Mechanical Consortium Printe d 11/04/08 Page 18 National Diploma in Mechanical Engineering Thermo exam November 2008 final. doc Version 23-Sep-08 ?Mechanical Consortium Printed 11/04/08 Page 19