Exam 20220720 Eng

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ENERGY CONVERSION Note: you can keep this text for self-evaluation at the end of the exam. Exam 20/07/2022 Please, write your person code and surname-name on all sheets you hand in. Exercise 1 (10 points) A race ship has two fast propellers, each driven by a gasoline-fired gas turbine. Each gas turbine is in a two-shaft configuration: there is an expander driving the compressor on one shaft and there is another expander driving the propeller on a separated power shaft. The second expander is placed downstream of the first one. Every gas turbine is characterized at nominal conditions by: • fuel: gasoline (for simplicity, taken as pure C8H18 with a Lower Heating Value of 44.8 MJ/kg), • oxygen content in the exhaust gas: 15% on a dry volume basis, • expander driving the propeller: non cooled, • power to the propeller: 2000 kW, • mechanical-drive efficiency: 35% (referred to the power to the propeller as well as to the LHV of the fuel). Please: i. motivate clearly the solution procedure and highlight neatly the assumptions (1 point); ii. sketch the process flow diagram for the propulsion system and the Ts diagram of the cycle (2 points); iii. calculate the air-to-fuel mass ratio (2 points); iv. determine the temperature of exhaust entering the expander of the propeller power shaft (3 points); v. comment without calculating how the temperature of the exhaust entering the expander of the power shaft varies at low speed (2 points). Exercise 2 (12 points) Consider a low temperature recuperated organic Rankine cycle in which the heat source is a flow of a heat transfer oil. The inlet temperature of the oil is 240 °C, the specific heat is equal to 900 J/(kg K), and the mass flow is of 25 kg/s. The thermal power to the cycle is equal to 2.5 MW. The cycle has the following…