___________________________________________________________________________________ MTF1B Thermofluid Mechanics 1 – Academic Year 2006/2007 Page 1 of 4

University of Portsmouth

FACULTY OF TECHNOLOGY

Department of Mechanical and Design Engineering

MTF1B Thermofluid Mechanics 1 U08972C

Date: 25th May 2007 Time: 16.15 – 18.15

INSTRUCTIONS

Answer FOUR questions All questions carry equal marks

CLOSED BOOK

Calculators permitted to be used:

Casio FX 85 series Casio FX83 series

Examiners: Prof G Roberts, Dr Mike Waite

___________________________________________________________________________________ MTF1B Thermofluid Mechanics 1 – Academic Year 2006/2007 Page 2 of 4

Question 1. Air, initially at 400°C, undergoes two reversible non-flow processes.

Process 1 → 2 is an isothermal expansion process from 5 bar, 0.1 m3 to 2 bar, Process 2 → 3 is a constant pressure compression to the original volume.

Sketch the processes on a p-V diagram.

[2 marks] Determine: -

(i) the mass of air, [5 marks]

(ii) the volume at the end of process 1 → 2, [3 marks]

(iii) the temperature at the end of process 2 → 3, [3 marks]

(iv) the work transfer for each process stating whether work is done on or by the air.

[12 marks] Question 2. 1 kg/s of superheated steam at 5 bar, 230°C is expanded steadily in a steam turbine to 1 bar with a dryness fraction of 0.98. The outlet velocity is 60 m/s. If the expansion process can be assumed to be adiabatic and the inlet velocity is negligible, determine: -

(i) the power produced by the turbine, [18 marks]

(ii) the outlet area of the steam passage of the turbine. [7 marks]

Question 3. 0.05 kg of nitrogen, N2, is compressed polytropically and reversibly from 100 kPa,15°C to 1 MPa such that:

pV1.3 = constant . Determine: - (i) the final temperature,

[6 marks] (ii) the work done,

[7 marks] (iii) the change in internal energy

[7 marks} (iv) the heat transfer,

[5 marks] For nitrogen, the molar mass, m~ , is 28 kg/kmol and the ratio of specific heats, γ (cp/cv), is 1.4.

___________________________________________________________________________________ MTF1B Thermofluid Mechanics 1 – Academic Year 2006/2007 Page 3 of 4

Question 4. Water, density 1000kg/m3, is supplied to a horizontal nozzle at a pressure of 10 bar gauge by a pipe of 60 mm diameter. The outlet of the nozzle is 20 mm diameter and the resulting jet impacts on a stationary curved vane without rebound as in the diagram. The jet is deflected uniformly about the longitudinal axis through an angle of 120°. By ignoring friction, determine: -

(i) the mass flow rate of water leaving the nozzle, [13 marks]

(ii) the magnitude and direction of the reaction force acting on the curved vane. [12 marks]

Question 5. (a) Briefly summarise the relative merits of installation and running costs of the venturi-meter and the orifice meter.

[5 marks] (b) A horizontal venturi-meter has an inlet diameter of 12 cm and a throat diameter of 7 cm and its discharge coefficient is 0.99. Oil, relative density 0.8, flows through the meter. A vertical U-tube manometer containing mercury, relative density 13.6, is connected between the inlet and throat. Determine the volumetric flow rate of the oil when the reading on the manometer is 10.5 cm.

[20 marks]

Fig Q4

Water

120°

___________________________________________________________________________________ MTF1B Thermofluid Mechanics 1 – Academic Year 2006/2007 Page 4 of 4

Question 6. Oil, density 930kg/m3, is stored in a tank which is fitted with an inspection door of 1.1 m diameter. The door is inclined at an angle of 45° to the horizontal as in the diagram. When filled with oil, the surface of the oil is 2.1 m above the centre of the inspection door. The pressure in the space above the oil surface in the tank is atmospheric. Determine: -

(ii) the force acting on the inspection door, [10 marks]

(ii) the depth of the centre of pressure on the inspection door below the oil surface. [15 marks]

2.1 m

1.1 m diameter

Oil level

45°

Fig Q.6