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].

UNIT II INTERNAL COMBUSTION ENGINES

UNIT II INTERNAL COMBUSTION ENGINES

Two marks questions

1.       What is scavenging?

2.       What are the effective characteristics of effective cooling system?

3.       The bore and stroke of a water cooled vertical single cylinder 4S diesel engine are 80mm and 110mm respectively and the torque is 23.5Nm. Find the mep of the engine.

4.       [Ans. Power = 2PiNT/60 =  mep Lank/60 => mpe = 534kPa.]

5.       Compare advantages and disadvantages of ic engines with external combustion engines

6.       Why cooling is necessary in ic engines

7.       State limitations of battery ignition system.

8.       State applications of 2S and 4S engines.

9.       Draw the valve timing diagram for CI engine.

10.   During peak power operation why petrol engine required rich mixtures?

11.   What do you mean by mist lubrications?

12.   Draw port timing diagram of petrol engine

13.   What is splash lubrication?

14.   What are the requirements of fuel injection system of a diesel engine?

Part B

1. In a laboratory experiment the following observations were noted during the test of 4Stroke SI engine. Area of indicator diagram = 510mm2; length of indicator diagram = 55 mm; spring index = 1.25 bar/mm; diameter of the piston =  20mm; length of the stroke = 180mm; engine rpm = 480 rpm; effective brake load = 25 kg; effective brake radius = 0.45m;  Find i) indicated mep ii) indicated power iii) brake power iv) mechanical efficiency.

[Ans;  i) Indicated mep = Pm=AS/L = 1159kN/m2;              ii)IP=Pm*l*a*n*k = 9.43kW;       iii) brake power BP = 2PiNWR = 5.54kW;            iv) mech effcy. = 58%;]

2. Discuss the construction and working principle of a four stroke engine with sketch

3. Explain the construction and working of battery coil ignition system with neat sketch

4. Explain the construction and working of fuel injector with a neat sketch and explain port timing diagram of 2S diesel engine

5. The following observations were taken during attest on a single cyinder 4S cycle engine having a bore of 300mm and stroke of 450mm.  Ambient temp= 22C; fuel consumption = 11kg/hr; CV of fuel = 42,000kJ/kg; N = 300rpm; mep = 6bar; net brake load W = 1KN; quantity of jacket cooling water = 590 kg/hr; cooling water entry temp = 22C; cooling water leaving temp = 70C; quantity of air measured = 225 kg/hr; Temp of gas =405C; rope diameter = 2cm;

Find ip; bp; mech efficiency; and draw a heat balance sheet on hour basis.

[Ans. IP= 47.7kW; mech effcy = 66.5%; Qs = 8.2kw; Qw = 1.7; Qg = 3.91; Qua = .75]

6. Compare SI and CI engine with respect to a) basic cycle b) fuel used c) introduction of fuel d) ignition e) compression ratio f) speed g) efficiency h) weight.

7. Explain the difference between 2S and 4S ic engine.

8. In a constant speed CI engine operating on 4S and fitted with band brake the following observations were taken.

Brake wheel diameter = 60cm; band thickness = 5mm; speed = 450 rpm; loan on band = 210N; spring balance reading = 30 N; area of indicator diagram = 4.15 cm2;  length of indicator diagram = 6.25 cm; spring no 11 is = 11 bar / cm; bore = 10cm; stroke = 15cm; SFC = 0.3 kg/kw-hr; heating value of fuel = 41800 kJ/kg; Find the brake power, indicated power, mechanical efficiency, indicated thermal efficiency and brake thermal efficiency.

Gas Power cycles Anna University Two mark question and Descriptive Problems

1.       Draw the actual PV diagram of two stroke engine

2.       What is meant by mean effective pressure?

3.       Name any four assumptions made for air standard cycle analysis

4.       Sketch the dual cycle on PV and TS coordinates

5.       Define actual and relative thermal efficiency

6.       What is an air standard cycle? Why such cycles are conceived?

7.       Define mep and stat its importance in reciprocating engines.

8.       For a given compression ratio, the air standard Diesel cycle is less efficient that air standard Otto cycle.

9.       In an engine working of an ideal Otto cycle the temperatures at the beginning at the end of compression are 27C and 327C respectively. Find the compression ratio and air standard efficiency of the engine. [ans : 5.67; 50%]

10.   Why Carnot cycle is not used practically?

       11. Draw the Pv diagram for a Dual cycle

12. How does the change in compression ratio affect air standard efficiency of an ideal cycle?

13. State thermodynamic process involved in Diesel cycle

14. Define air standard efficiency of Diesel cycle

15. In an Otto cycle pressure ratio during compression is 11. Find air standard efficiency. [ans. 49%]

Part B Questions

1.       An engine works on Otto cycle. The initial pressure and temperature of the air is 1 bar and 40C . 825kJ of heat is supplied per kg of air at the end of the compression. Find the temperature and pressure at all salient points if the compression ratio is 6. Also find the efficiency and mean effective pressure for the cycle. Assume air is used as working fluid and take all ideal conditions.  [au 14, 16]

2.       A gas turbine works on an air standard Brayton cycle. The initial condition of the air is 25C and 1 bar. The maximum pressure and temperature are limited to 3 bar and 650C. Find the following:  a) cycle efficiency b) heat supplied and rejected per kg of air  c) work output  d) exhaust temperature. [au14, 16]

3.       Derive an expression for air standard efficiency of diesel cycle. Explain why the efficiency of Otto cycle is more than that of the diesel cycle for the same compression ratio [au13, 16]

4.       In an oil engine working on Dual cycle the heat supplied at constant pressure is twice that of heat supplied at constant volume. The compression and expansion ratios are 8 and 5.3. The pressure and temperature at the beginning of cycle are 0.93 bar and 27C. Find the efficiency of the cycle and mean effective pressure. [au13,16]

[Ans. Effcy 53%; mep = 5.5 bar; r_p = 1.56]

5.       A SI engine working on an ideal Otto cycle has the compression ratio as 6. The initial pressure and temperature of air are 1 bar and 37C. The maximum pressure in the cycle is 30 bar. For unit mass flow find,

a)      p,V and T at various salient points of the heat rejected. B) the ratio of heat supplied to the heat rejected [au12 16]  [Ans : Qs /Qr  = 2.1 ; T4 = 9300K, P4 = 2.4bar]

6.       Air standard Dual cycle has a compression ratio of 18 and compression begins at 1 bar, 40C. The maximum pressure is 85 bar. The heat transferred to air at constant pressure is equal to that at constant volume. Find. A) the pressure and temperature at salient points  b) the cycle efficiency  c) mep  [au 12 , 16]

[Ans: p2 = 57 bar; T2 994K; p5 = 2 bar; T5 = 624K, effcy = 68%; mep = 5.5 bar]

7.       In an engine working on Dual cycle the temperature and pressure at the beginning of the cycle are 90c and 1 bar respectively. The compression ratio is 9. The maximum pressure is limited to 68 bar and total heat supplied per kg of air is 1750kJ. Find a) pressure and temperature at all salient points, efficiency and mep [au12, 16]

[ans; rp = 3.13; effcy = 58%;  mep = 11bar]

8.       A four stroke four cylinder petrol engine of 250mm bore and 375 m stroke works on an Otto cycle. The clearance volume is 0.01052 m3. The initial pressure and temperature are 1 bar and 47C. If the maximum pressure is limited to 25 bar, find the a) the air standard efficiency  b) mep. [au11, 16 ]

[ans: efficiency = 33%; mep = 9.9 bar]

9.       In an air standard Dual cycle the pressure and temperature at the beginning of compression are 1 bar and 57C respectively. The heat supplied in the cycle is 1250 kJ/kg, two third of this being added at constant volume and rest at constant pressure. If the compression ratio is 16, find the maximum pressure and temperature in the cycle, thermal efficiency and mep. [au 11, 16] 

 [ans: p3 = 106bar; re = 13.3; WD = 832kJ/kg; effcy = 66%; mep = 9.7 bar]

    10. A six cylinder four stroke petrol engine has a swept volume of 300 cubic cm per cylinder. A compression ratio of 10 and operates at a speed of 3500 rpm. If the engine is required to develop an output of 73.5kW at this speed find the cycle efficiency, the necessary rate of heat addition the mep, max pressure and temperature of the cycle efficiency ratio. The pressure and temperature before isentropic compression are 1 bar and 15C. [au10 16]

[Hint = W = mep*Vs*N/60*n; Ans; Qs=122kW; mep =0.7bar;]