SCHOOL OF COMPUTING, ENGINEERING AND MATHEMATICS SEMESTER 2 EXAMINATIONS 2014/2015 ME258. Thermodynamics

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1 s SCHOOL OF COMPUING, ENGINEERING AND MAHEMAICS SEMESER EXAMINAIONS 04/05 ME58 hermodynamics ime allowed: WO hours Answer: Any FOUR Questions Items permitted: Any approved calculator Items supplied: Steam ables Formula sheet Marks for whole and part questions are indicated in brackets ( ) ME58_SEM_04/05 Pae of 6 Printin date: 4/05/05

2 Question a) Describe the meanin of the system and control volume concepts in thermodynamics, ive examples. b) Describe the concepts of thermodynamic equilibrium. (6 marks) (3 marks) c) A piston-cylinder device initially contains 0.5 m 3 of air at 00 kpa and 50 C. he air is now compressed to 0. m 3 in such a way that the temperature inside the cylinder remains constant. Determine the work done durin this process (6 marks) Question a) Describe the concept of the first law of thermodynamics for closed systems. (7 marks) b) A closed system underoes a cycle consistin of two processes. Durin the first process, 50 kj of heat is added to the system while the system does 80 kj of work. Durin the second process, 35 kj of work is done on the system. Determine: i) he heat transfer durin the second process. (4 marks) ii) he network and heat transfer for the cycle. (4 marks) c) A cylinder fitted with a piston has a volume of 0.5 m 3 and contains 0.75 k of steam at 0.5 MPa. Heat is transferred to the steam until the temperature is 00 C, as a result the volume increases to three times and the chane in internal enery of 450 kj. Determine the work and heat transfer for this process. Nelect the chane in kinetic and potential eneries. (0 marks) ME58_SEM_04/05 Pae of 6 Printin date: 4/05/05

3 Question 3 a) Describe the meanin of the specific heats at constant volume and pressure. (7 marks) b) At constant pressure, non-flow as receives heat at a constant pressure of 400 kpa. he internal enery of the system increases by 00 kj while the temperature increases by 50 C and the work done is 5 kj. Determine cp and the chane in volume if there is.5 k of as in the cylinder (8 marks) Question 4 a) Describe the concepts of pure substance. (5 marks) b) Steam is contained at 0.6 MPa and 50 % quality in a 0.0 m 3 cylinder. he cylinder is fitted with a piston which maintains constant pressure. Calculate the amount of heat which must be added to the steam to increase the volume to 0.07 m 3 (0 marks) ME58_SEM_04/05 Pae 3 of 6 Printin date: 4/05/05

4 Question 5 Consider a ideal simple Rankine Cycle usin water (HO) as the workin fluid. he steam leaves the boiler and enters the turbine at 5 MPa, 400 C. he condenser pressure is 5 kpa. a) Draw a schematic diaram of the Rankine cycle and show it on the -s diaram. (8 marks) b) Determine the quality of steam at the exit of the turbine ( marks) c) Determine the cycle efficiency. (5 marks) Question 6 An air-standard diesel cycle has a compression ratio of 7. he pressure at the beinnin of the compression stroke is 00 kpa, and the temperature is 5 C. he maximum temperature is 500 K. a) Show the cycle on the p-v and t-s diaram. (7 marks) b) Determine the pressure and temperature at the end of each process of the cycle. i) Determine the thermal efficiency. (5 marks) ii) Determine the mean effective pressure for this cycle. (3 marks) ME58_SEM_04/05 Pae 4 of 6 Printin date: 4/05/05

5 Formula Sheet APPENDIX I First Law i) Closed system Q W ( U U) {( KE) ( KE) } {( PE) ( PE) } ii) Open System Q cv i e m ( hi zi ) m ( he ze ) W cv Work done: δw=pd Specific volume and internal eneries in saturated reions : v xv ( v f u xu ( u f s xs ( s f h xh ( h f Specific volume v m Polytropic process: ME58_SEM_04/05 Pae 5 of 6 Printin date: 4/05/05

6 ME58_SEM_04/05 Pae 6 of 6 Printin date: 4/05/05 pv n const n n p p / ) ( Heat transfer at constant pressure and volume are: Efficiency : And, mep = w net/(v v ) ) ( C q p L ) C ( q v L H net th q w k P P k

THERMODYNAMICS, FLUID AND PLANT PROCESSES. The tutorials are drawn from other subjects so the solutions are identified by the appropriate tutorial.

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