Hydraulic Machinery : Question Paper Dec 2013 - Mechanical Engineering (Semester 6)

3views

written 8.8 years ago by

teamques10 ★ 68k

Hydraulic Machinery - Dec 2013

Mechanical Engineering (Semester 6)

TOTAL MARKS: 100
TOTAL TIME: 3 HOURS (1) Question 1 is compulsory.
(2) Attempt any four from the remaining questions.
(3) Assume data wherever required.
(4) Figures to the right indicate full marks.

Attempt any four of the following:-

1 (a) A Kaplan turbine produces 60,000KW under a net head of 25m with a overall efficiency of 90%. Taking the value of speed ratio as 1.6, flow ratio as 0.5 and the hub diameter as 0.35 times the outer diameter and speed of the turbine.(5 marks) 1 (b) Prove that specific speed can be expressed as:

Where N_u and Q_u are the unit speed and unit discharge. η₀ is the overall efficiency.(5 marks) 1 (c) Discuss the conditions under which cavitations and negative slip occur and state why air vessels are used in reciprocating pump.(5 marks) 1 (d) Two geometrically similar pumps are to run at the same speed of 1200rpm. If one of the pumps which has an impeller diameter of 0.3m lifts water at the rate of 20lit/sec through a head of 16m, determine the head and size of the other pump to deliver half the quantity of water.(5 marks) 1 (e) Explain methods to balance axial thrust in centrifugal pump.(5 marks) 2 (a) A twin jet Pelton wheel is required to generate 7.5MW when the available head at the nozzle is 400m. Assuming generator efficiencyof 95%, overall efficiency of 80%, coefficient of velocity as 0.98, speed ratio as 0.46, reduction in relative velocity while passing through bucket as 15% and taking that the jet is deflected through 165° by the bucket, find:-
i) The diameter of each jet.
ii) Total flow in lit/sec.
iii) Force exerted by the jet on buckets in tangential direction.
If the jet ratio is not to be less than 10, find the synchronous speed at 50 cycles/sec and corresponding mean diameter of the runner.(16 marks) 2 (b) If a centrifugal pump does not deliver any water when started, what may be the probable causes and how can they be remedied.(4 marks) 3 (a) A model of a turbine 0.3m diameter when running at 960rpm under a head of 12m gear following results.

Percentage gate opening	40	60	80	100
Output Power (kW)	8.38	13.24	17.50	19.71

The actual turbine is required to develop 7360KW at 85% gate opening under a head of 35m. Assuming same efficiency at the corresponding gate opening for the model and the actual turbine. Find:-
i) Diameter and speed of the actual turbine
ii) Output of actual turbine at 50% gate opening(14 marks) 3 (b) What is NPSH? What is the difference between NPSH available and NPSH required?(6 marks) 4 (a) Write short note on performance characteristic of reciprocating pump.(6 marks) 4 (b) A reaction turbine and its draft tube has a vertical axis. Pressure head in the spiral casing at the inlet is 48m above the atmospheric pressure and velocity of water is 6m/s. The water flow through the tube is 2.1m³/s, the hydraulic and overall efficiencies are 0.83 and 0.80 respectively. The top of the draft tube is 1.2m below the central line of spiral casing while tailrace is 3.9m below top of the draft tube. The diameter of the draft tube at inlet is 0.75m and that at tailrace is 1.05m.(14 marks) 5 (a) The critical Thoma cavitation parameter for a certain type of turbine varies in the following manner:-

N_s	0	50	100	150	200	250
⁶C_r	0	0.04	0.1	0.18	0.28	0.41

The turbine runs at 300rpm under a net head of 50m and produces 2000KW power. If the runner is placed 4.7m above the tail water level, will the turbine cavitate? Take saturation vapour pressure to be 0.04bar (abs) and atmospheric pressure to be equivalent to 10.3m of water. What is the maximum safe height at which the turbine can be placed with reference to the tailrace level?(10 marks) 5 (b) The impeller of a centrifugal pump has 1.2m outside diameter. It is used to lift 1.8m³ of water to a height of 6m. Its blade make an angle of 150° with the direction of motion of outlet and runs at 200rpm. If the radial velocity of flow at outlet is 2.5m/s, find the useful power and efficiency. Find also the lowest speed to start the pump.(10 marks) 6 The table below shows head, discharge and efficiency of a centrifugal pump running at constant speed of 750rpm. It delivers water at a height of 15m through 1200m of 30cm pipe. Find discharge in LPM and input power.
i) When only one pump is working?
ii) When two such pumps are working in parallel?
Take coefficient of friction=0.008

Q(Lit/min)	0	0.0167	0.0334	0.0501	0.0668
H(m)	28.2	27.9	26.55	23.4	17.55
η(%)	0	48.5	72	75	71

(20 marks) 7 (a) A single acting reciprocating pump has a plunger of 80mm diameter and a stroke length of 150mm. It takes the supply of water from a sump 3m below the pump through a pipe 4.5m long and 30mm diameter. It delivers water to a tank 12m above the pump through a pipe 25mm diameter and 15m long. If separation occurs at 78.48KN/m² below atmospheric pressure, find the maximum speed at which the pump may be operated without separation. Assume the plunger to have simple harmonic motion.(10 marks) 7 (b) Write short notes on any two of the following:
i)Selection of turbines
ii) Application of CFD in turbines
iii) Variation of shape of impeller of centrifugal pump with specific speed(10 marks)

ADD COMMENT EDIT