Thermal Engineering AU Question STEAM NOZZLES AND TURBINES

UNIT III STEAM NOZZLES AND TURBINES

Part A Questions

1. Define coefficient of friction in nozzle or nozzle efficiency.

2. Define the term critical pressure ratio.

3. What is metastable flow?

4. Calculate critical pressure ratio for saturated and superheated steam.

5. What is the effect of supersaturated flow in steam nozzle.

6. State the relation between the velocity of steam and heat during any part of a steam nozzle.

7. List energy losses in steam turbine.

8. What are the principles of impulse and reaction turbine?

9. What are the factors reducing final velocity of steam in the nozzle flow?

10. What is the difference between impulse and reaction turbine?

Part B

1. Steam expands isentropically in a nozzle from 1MPa, 250C to 10KPa. The flow rate of the steam is 1 kg/s. Find the following when the inlet velocity is neglected.

a) Quality of steam b) velocity of steam at exit of the nozzle c) exit area of the nozzle

[Ans. X2 = 0.806; V2 = 1281 m/s ;  A2 = 0.00924m²]

2. What are the effects of friction in a nozzle? Explain

3. A CD nozzle is required to discharge 2 kg of steam per second. The nozzle is supplied with steam at 7 bar and 180 C. The expansion up to throat is isentropic and the frictional resistance between the throat and exit is equivalent to 63 kJ/kg of steam. Taking approach velocity of 75 m/s and throat of 4 bar estimate.

a) Suitable areas for the throat and exit and b) overall efficiency of the nozzle abased on the enthalpy drop between the actual inlet pressure and temperature and the exit pressure.

[Ans At= 19.87cm²; A2 = 0.0003356 m²; efficiency = 82%.  ].

4. Derive the condition for maximum discharge and expression for maximum discharge [16]

5. Explain physical significance of Wilson’s line as referred to supersaturated flow through steam nozzle [8]

6. Steam enters a nozzle in a dry saturated condition and expands from a pressure of 2 bar to a pressure of 1 bar. It is observed that supersaturated flow is taking place and the steam flow reverts to a normal flow at 1 bar. What is the degree of under cooling and increase in entropy and loss in the available heat drop due to irreversibility. [8]

[Ans. Degree of undercooling = 37.5C; dS = 0.053kJ/kgK;  Q = 100 kJ/kg]

7. Steam enters a CD nozzle at 2 MPa and 400C with a negligible velocity and mass flow rate of 2.5 kg/s and it exits at a pressure of 300 kPa. The flow is isentropic between the nozzle entrance and throat and overall nozzle efficiency is 93%. Find a) throat and exit area  .

[Ans. At = 17 cm²;  A2 = 20 cm²]

8. In a single stage of steam turbine the nozzle expands 9 kg/s steam from a pressure of 1.5MPa and 250C to 600kPa. The actual heat drop in the nozzle is 175 kJ/kg. Find the number of nozzles required to give an outlet area for each nozzle approximately 3.5cm² and adjust the outer dimension to suit the number. Assume isentropic flow in the nozzle.

[Ans. N =14; A2 = 3.356 cm²].

  

9A single active single stage reciprocating compressor takes 1m³ of air per min at 1.013 bar and 17C and deliver at 7 bar. The law of compression is PV^1.35 = C.  Clearance is neglected. Compressor runs at 300 rpm. Stroke to bore ratio is 1.5. Mechanical efficiency equals to  85% for compressor and motor transmission efficiency is 90%. Find mass of air delivered per min, indicated power, bore, stroke length and motor power.

[Ans. W = 288kw; m = 0.02kg/s; power = 6.3kw; L= 0.212m, D= 0.14m].