Thermodynamic mid-term Test, Exercises of Thermodynamics

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Number of Pages: 3

INDIVIDUAL ASSIGNMENT

(74 marks)

COURSE: BTECH: MECHANICAL: MECHATRONICS

SUBJECT: THERMODYNAMICS 4 – THE400S

DUE DATE: 4 APRIL 2016 at 12:00 pm (via Blackboard)

CONTRIBUTION: 5 % of Final Mark

INSTRUCTIONS

1. Copying from friends or other sources will result in a mark of 0 % being

awarded for the assignment

2. Attempt all questions.

i

3. Assignment serves as preparation for Main Test. It is therefore important

that you complete the assignment by the due date so that I can post the

solutions on same day

4. An electronic copy of the assignment must be submitted via Blackboard.

The hardcopy will have to submitted on 13 April 2016

Note

The main test includes theory questions whereas this preparation

homework does not.

Question 1 [17 marks] 0.15 m^3 of an ideal gas at a pressure of 15 bar and 550 K is expanded isothermally to 4 times the initial volume. It is then cooled to 290 K at constant volume and then compressed back polytropically to its initial state. 1.1 Sketch the cycle on p-V diagram. 1.2 Calculate the net work done and heat transferred during the cycle. Question 2 [7 marks] Air flows steadily at the rate of 0.4 kg/s through an air compressor, entering at 6 m/s with a pressure of 1 bar and a specific volume of 0.85 m^3 /kg, and leaving at 4.5 m/s with a pressure of 6.9 bar and a specific volume of 0.16 m^3 /kg. The temperature of air leaving is 87.56 K greater than that of the air entering. Cooling water in a jacket surrounding the cylinder absorbs heat from the air at the rate of 59 kJ/s. 2.1 Calculate the power required to drive the compressor and 2.2 Determine the inlet and outlet pipe cross-sectional areas. For air Cp = 1005 J/kg-K, k = 1. Question 3 [12 marks] Steam at 10 bar and 0.95 dryness is available. Determine the final condition of steam in each of the following operations: 3.1 160 kJ of heat is removed at constant pressure. 3.2 It is cooled at constant volume till the temperature inside falls to 140°C. 3.3 Steam expands isentropically in a steam turbine to an exit pressure of steam of 0.5 bar.

Question 4 [23 marks] A reversible heat engine operates between two reservoirs at temperatures 700°C and 50°C. The engine drives a reversible refrigerator which operates between reservoirs at temperatures of 50°C and – 25°C. The heat transfer to the engine is 2500 kJ and the net work output of the combined engine refrigerator plant is 400 kJ. 4.1 Determine the heat transfer to the refrigerant and the net heat transfer to the reservoir at 50°C 4.2 Reconsider 4.1 given that the efficiency of the heat engine and the C.O.P. of the refrigerator are each 45 per cent of their maximum possible values. Question 5 [7 marks] An inventor claims that his engine has the following specifications: Temperature limits 750°C and 25°C Power developed 75 kW Fuel burned per hour 3.9 kg Heating value of the fuel 74500 kJ/kg Using calculations based of thermodynamics, state whether his claim is valid or not. Question 6 [8 marks] Five kg of water is contained in a piston–cylinder assembly, initially at 5 bar and 240°C. The water is slowly heated at constant pressure to a final state. If the heat transfer for the process is 2960 kJ, determine the temperature at the final state, in °C, and the work, in kJ. Kinetic and potential energy effects are negligible.