Thermodynamics and Marks

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First Year

Mechanical Engineering Science 3

??day ?? June 2007??:?? – ??:??

Answer TWO questions from each section A & B.
Section A

Q1. a) For a Newtonian fluid state an expression for the shear stress. [4 marks]

b) An ice hockey puck traverses the ice on a thin sheet of water. A puck struck by an ice hockey player Initially travels at a speed of 20 m/sec, the thin layer of water between the puck and the ice has a thickness of 0.2 mm calculate the shear stress if the viscosity of the water is 0.0018 kg/m sec. [8 marks]

A puck has an approximate diameter of 80 mm, calculate the initial drag force on the puck. [5 marks]

c) In the figures below the viscosity as a function of temperature is graphed for two fluids, Fluid A and Fluid B; given no other information, identify which fluid is a liquid and which is a gas. Justify your answer.

Fluid A Fluid B [pic]

Figure Q1
[8 marks]

Q2.Figure Q2a shows a mercury manometer, (mercury s.g.=13.6) connected to a pipeline transporting oil (oil s.g.=0.85), the other arm of the manometer is exposed to the atmosphere. What is the absolute pressure in the pipeline? [pic]

Figure Q2a
[10 marks]

Another mercury manometer is connected to two oil pipelines (oil s.g.=0.85) labelled A and B, as shown below in Figure Q2b. Calculate the pressure difference pB-pA.

Figure Q2b

[10 marks]

Suppose a pressure gauge was fitted to pipeline A and it measured a pressure of 2 bar. What would the absolute pressure in pipeline B be? Give your answer in bar. [5 marks]

Q3. a)A hot air balloon has a volume of 1,000 m3(1 thousand m3). Taking the surrounding air density to be 1.21 kg/m3 calculate the buoyancy force. [5 marks]

Taking the pressure and temperature of the hot air to be 1 bar and 200oC respectively calculate the mass of hot air (Specific gas constant R=287 J/kg K)? [5 marks]

If the balloon and basket have a mass of 100 kg combined and a typical passenger has a mass of 70 kg, how many people can ride in the balloon? [8 marks]

b) An experimental model boat has been constructed by a team of young engineers and it has been found to be unstable. A number of suggestions are made to improve the boats stability that all involve attaching buoyancy aids (blocks of polystyrene) to the outside of the boat, see the figure below.


Figure Q3

Of the three configurations rank them in order of most stable to least stable. Justify your ranking. [7marks]

Section B./
Section B

Q4.With the aid of a sketch on a pV diagram, describe the processes which occur in the cylinder of a 4-stroke, spark-ignition engine during its operation. [10 marks]

An engine test was carried out on a 6 cylinder 4 stroke engine with a total swept volume of 3000cm3. The following data were obtained:

Indicator diagram
Length (x axis on the indicator diagram)160mm
Spring constant75kN/m2 per mm
Net diagram area1600mm2

Engine speed2500 rev/min

a) the indicated work during one cyclefor one cylinder [5 marks] b) the indicated power output of the engine [4 marks] c) the brake power output of the engine [2 marks] d) the brake specific fuel consumption in kg/(kW h) if 150 cm3 of fuel is consumed in a 1 minute test (fuel density = 750 kg/m3) [4 marks]

Q5.(a) Coal fuelled power plant for generating electricity uses steam/water in a cycle. Name the thermodynamic devices in the cycle and show how they are linked together. [5 marks]

A power plant burns 0.5 metric tonnes of coal in an hour and in that time produces 6000 MJ of...
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