THERMODYNAMICS EXAMJULY 25TH, 7 PM - 10 PM EST
Université d’Ottawa Faculté de génie University of Ottawa Faculty of Engineering Department of Mechanical Engineering MCG 2131 Thermodynamics II Instructor: Nabel Sadek Final Exam Time Allowed: 3 Hours Date: July 31/2018 Closed Book Exam. Non-Programmable Calculators Allowed NOTE: Every student is allowed to use a one page, size A4, double sided formula sheet. The formula sheet has to be submitted with the answer booklet. Students should write their name on the formula sheet. 1. Concept questions, Use brief description only to answer them. (2 marks for each part). (a) Humid air A has the same moisture content as humid air B (ωA=ωB) but the dry-bulb temperature of B is higher than that of A. What would you say about the relative humidity and dew point temperature of A compared to those of B (higher, lower, or the same). (b) What is the importance of identifying the dew point temperature of the combustion products? (c) State three reasons that make the air-standard refrigeration cycle more suitable than vapor compression cycles for airplane cooling applications. (d) Explain why is the typical compression ratio of the compression-ignition engine much higher than that of the spark-ignition engine. What is the limiting factor in each case? (e) State two advantages of the open feed water heaters. 2. A gas turbine plant consists of one turbine for compressor drive and another for output power. Both turbines are served by one compressor and one combustion chamber (burner) as shown in figure 1. Air enters the compressor at 100 kPa and 15oC and is compressed with isentropic efficiency of 0.76. Gas inlet temperature and pressure in both turbines are 680oC and 500 kPa, respectively. Gas expands to 100 kPa in both turbines. The isentropic efficiency of each turbine is 0.86. Assuming cp=1.005 kJ/kg.K and k=1.4 for air and cp=1.128 kJ/kg.K and k=1.34 for gases, (a) Find T2 and T4 3 marks (b) For each kg of compressed air entering the burner, mf kg of fuel is added. The heat added in the combustion process 2-3 is 42000 kJ/kg of fuel. Use the energy balance over the combustion chamber to find mf and the air to fuel ratio. 4 marks (c) Find the fraction of flue gases that passes through the turbine driving the compressor, (shown as x in figure 1). 4 marks This study source was downloaded by 100000813526373 from CourseHero.com on 07-21-2022 13:49:12 GMT -05:00 https://www.coursehero.com/file/132696367/MCG2131Final-Summer2018-1pdf/ (d) If the mass flow rate of air through the compressor is 23 kg/s, find the power output of the plant (shown as �̇� in figure 1). 4 marks (e) State all your assumptions 1 mark 3. A steam power plant equipped with regenerative as well as reheat arrangement, as shown in figure 2, is supplied with steam to the H.P. turbine at 80 bar 470°C. For open feed water heating, a part of steam is extracted at 7 bar and the remainder of the steam is reheated to 350°C in a reheater and then expanded in L.P. turbine down to 0.03 bar. (a) Draw a T-S diagram of the cycle assuming isentropic processes throughout. 2 marks (b) Find h1, h2, h3, h4, and h5, using steam tables. 3 marks (c) If the isentropic efficiency of the pump is 0.8, find the specific work of each pump. 3 marks (d) Find the amount of steam bled-off for feed water heating (m) per kilogram of steam flow such that saturated liquid comes out of the regenerator. Assume adiabatic mixing in the regenerator. 3 marks (e) Find the cycle efficiency. 2 marks 4. Saturated air at 3ºC is required to be supplied to a room at 22ºC with a relative humidity of 55%. The air is heated and then water at 10ºC is adiabatically sprayed to give the required humidity. Neglect the fan power. Assume that the total pressure is constant at 1.0132 bar and for dry air consider cp=1.005 kJ/kg.K. Determine : (a) The mass of spray water required per kg of dry air. 3 marks (b) The mass of spray water required per m3 of dry air. 2 marks (c) The temperature to which the air must be heated. 3 marks 5. A four-cylinder petrol engine has a bore of 57 mm and a stroke of 90 mm. Its rated speed is 2800 rpm and it is tested at this speed against a brake which has a torque arm of 0.356 m. the net brake load is 155 N and the fuel consumption is 6.74 l/hr. the specific gravity of the fuel used is 0.735 and its lower heat value is 44200 kJ/kg. The mechanical efficiency at this speed is measured to be 82.3%. Calculate for this speed: (a) The brake mean effective pressure, 2 marks (b) The brake thermal efficiency, 2 marks (c) The brake specific fuel consumption in kg/kWh, 2 marks (d) The indicated mean effective pressure, 2 marks (e) If the engine is idling at this speed (brake load is removed), what would be its mechanical efficiency? 1 mark (f) The analysis of dry exhaust from this engine is as follows: Carbon dioxide (CO2) = 15 per cent Carbon monoxide (CO) = 3 per cent Methane (CH4) = 3 per cent, Hydrogen (H2) = 1 per cent Oxygen (O2) = 2 per cent Nitrogen (N2) = 76 per cent The molecular weight in kg/kmol is C=12, H=1, N=14, O=16 This study source was downloaded by 100000813526373 from CourseHero.com on 07-21-2022 13:49:12 GMT -05:00 https://www.coursehero.com/file/132696367/MCG2131Final-Summer2018-1pdf/ Calculate the proportions by mass of carbon to hydrogen in the fuel, assuming it to be a pure hydrocarbon. 4 marks Fig.1 Fig.2 This study source was downloaded by 100000813526373 from CourseHero.com on 07-21-2022 13:49:12 GMT -05:00 https://www.coursehero.com/file/132696367/MCG2131Final-Summer2018-1pdf/ This study source was downloaded by 100000813526373 from CourseHero.com on 07-21-2022 13:49:12 GMT -05:00 https://www.coursehero.com/file/132696367/MCG2131Final-Summer2018-1pdf/ This study source was downloaded by 100000813526373 from CourseHero.com on 07-21-2022 13:49:12 GMT -05:00 https://www.coursehero.com/file/132696367/MCG2131Final-Summer2018-1pdf/ This study source was downloaded by 100000813526373 from CourseHero.com on 07-21-2022 13:49:12 GMT -05:00 https://www.coursehero.com/file/132696367/MCG2131Final-Summer2018-1pdf/ Psychrometric chart This study source was downloaded by 100000813526373 from CourseHero.com on 07-21-2022 13:49:12 GMT -05:00 https://www.coursehero.com/file/132696367/MCG2131Final-Summer2018-1pdf/ Powered by TCPDF (www.tcpdf.org)