MAHALAKSHMI ENGINEERING COLLEGE

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MAHALAKSHMI ENGINEERING COLLEGE TIRUCHIRAPALLI-621213. Department: Mechanical Subject Code: ME2202 U N IT - 1 Semester: III Subject Name: ENGG. THERMODYNAMICS 1. 1 kg of gas at 1.

(2) When the gas is argon (Ar) w ith molar mass of 40kg/k mol and c p of 0.52 kj/kg K. (AUC DEC 05) Given: Polytropic process Mass (m) 1kg Pressure (P 1 ) 1.1bar 1.

1 MAHALAKSHMI ENGINEERING COLLEGE TIRUCHIRAPALLI Department: Mechanical Subject Code: ME2202 U N IT - 1 Semester: III Subject Name: ENGG. THERMODYNAMICS 1. 1 kg of gas at 1.1 bar, 27 o C is compressed to 6.6 bar as per the law pv 1.3 const. Calculate w ork and heat transfer, if (1) When the gas is ethane (C 2 H 6 ) w ith molar mass of 30kg/k mol and c p of2.1 kj/kg K. (2) When the gas is argon (Ar) w ith molar mass of 40kg/k mol and c p of 0.52 kj/kg K. (AUC DEC 05) Given: Polytropic process Mass (m) 1kg Pressure (P 1 ) 1.1bar KN/m 2 Temperature (T 1 ) 27 C K Pressure (P 2 ) 6.6bar KN/m 2 Molar mass of ethane (m) 30Kg/K mol C p of ethane 2.1KJ/K mol Molar mass of argon 40Kg/K mol C p of argon 0.52KJ/Kg.K PV 1.3 C L.VIJAYAKUMAR /A.P-MECH Page 1

2 To Find: 1. Work Transfer (W) 2. Heat Transfer (Q) Solution: Step:1 Case:1 (Ethane) Work transfer (W) P 1 V 1 mrt 1 V 1 V 1 V m 3 P 1 V 1 n P 2 V 2 V 2 V m 3 Work transfer (W) W KJ Heat transfer (Q) W Q KJ L.VIJAYAKUMAR /A.P-MECH Page 2

3 Case:2 (Argon) Work transfer (W) P 1 V 1 mrt 1 V 1 V 1 V m 3 P 1 V 1 n P 2 V 2 V 2 Work transfer (W) V m 3 Heat transfer (Q) W W KJ Q KJ L.VIJAYAKUMAR /A.P-MECH Page 3

4 2. In an air compressor, air flow s steadily at the rate of 0.5 kg/sec. At entry to the compressor, air has a pressure of 105kPa and specific volume of 0.86 m 3 /kg and at exit of the compressor those corresponding values are 705 kpa and 0.16 m 3 /kg. Neglect Kinetic and Potential energy change. The internal energy of air leaking the compressor is 95 kj/kg greater than that of air entering. The cooling w ater in the compressor absorbs 60kJ/sec. of heat from the air. Find pow er required to derive the compressor. (AUC MAY 06) Given: Mass (m) 0.5 kg/s Entering velocity (C 1 ) 7 m/s Entering Pressure (P 1 ) 100 KPa 100 KN/m 2 Entering volume (V 1 ) 0.95 m 3 /Kg Leaving velocity (C 2 ) 5 m/s Leaving Pressure (P 2 ) 700 KPa 700 KN/m 2 Leaving volume (V 2 ) 0.19 m 3 /Kg Change in internal energy (U 2 - U 1 ) 90 KJ/Kg Heat absorbs (Q) 58 KW To Find: 1. Compute the rate of shaft w ork input to the air in KW (or) Work input 2. Ratio of inlet pipe to outlet pipe ( ) Solution: Step:1 m (P 1 V 1 + +Z 1.g) + Q m ((U 2 - U 1 )+P 2 V 2 + +Z 2.g) + W Assume Z 1 Z (( )+ ) (90+( ) + ) + W W L.VIJAYAKUMAR /A.P-MECH Page 4

5 W W 5.994KW Note; - sign indicates that the w ork is done on the system Step:2 To find ratio of inlet to outlet dia ( ) From continuity equation The ratio of inlet to outlet dia of the pipe( ) L.VIJAYAKUMAR /A.P-MECH Page 5

6 3. In an isentropic flow through nozzle, air flow s at the rate of 600 kg/hr. At inlet to the nozzle, Pressure is 2 MPa and temperature is 127 o C. The exit pressure is 0.5 MPa. Initial air velocity is 300 m/s determines (i) Exit velocity of air (ii) Inlet and exit area of nozzle. (AUC DEC 06) To Find: Given: Mass of flow rate (m 1 ) 600Kg/hr Kg/sec Inlet pressure (P 1 ) 2MPa 10^6 N/m 2 Inlet temp (T 1 ) 127 C K Outlet pressure (P 2 ) 0.5MPa 10^6 N/ m 2 Inlet air velocity (C 1 ) 300m/s 1.Exit velocity of air (C 2 ) 2. inlet and exit area (d 1,d 2 ) Solution: Step:1 ( )^ T T 1 T L.VIJAYAKUMAR /A.P-MECH Page 6

7 T k Step:2 C 2 {2 m[c p (T 2 -T 1 ) + ] C 2 { [1.005( ) + ] C m/s Step:3 Inlet mass flow rate (m) P 1 V 1 mrt 1 V 1 V 1 V m 3 /Kg M 1 A 1 A 1 A ^-5 A 1 d 1^ ^-5 d 1^2 d 1 d m 1000 L.VIJAYAKUMAR /A.P-MECH Page 7

8 d mm Step:4 ( )^ ( )^ V V 1 V V m 3 /Kg A 2 A 2 A ^-5 d 2^ ^-5 d 2^2 D m 1000 D mm L.VIJAYAKUMAR /A.P-MECH Page 8

9 4. A centrifugal pump delivers 2750 kg of w ater per minute from initial pressure of 0.8 bar absolute to a final pressure of 2.8 bar absolute. The suction is 2 m below and the delivery is5 m above the centre of pump. If the suction and delivery pipes are of 15 cm and 10 cm diameter respectively, make calculation for pow er required to run the pump. (AUC DEC 06) Given: Mass (m) 2750 Kg/min Kg/s Initial pressure (P 1 ) 0.8bar KN/m 2 Final pressure (P 2 ) 2.8bar N/m 2 Z 1-2m (below the centre of pump) Z 2 5m (above the centre of pump) Dia, d 1 15cm m d 2 10cm m To Find: Pow er (P) or Work (W) Solution: Step:1 The steady flow energy equation is, m [ + +P 1 V 1 ] m [ + + P 2 V 2 ] + W L.VIJAYAKUMAR /A.P-MECH Page 9

10 W m [ + +(P 1 V 1 - P 2 V 2 )] M d 1 2 C 1 ρ C 1 C m/s C 2 C m/s Step:2 W [ + +( )] W [ ^3] W KJ/Kg W W KJ/Kg L.VIJAYAKUMAR /A.P-MECH Page 10

11 6. A blow er handles 1 kg/sec of air at 293 K and consumes a pow er of 15kw.Theinletandoutletvelocitiesofairarel00 m/sec and 150 m/sec respectively. Find the exit air temperature, assuming adiabatic conditions. Take C p of air as kj/kg-k. (AUC DEC 07) Given: Mass(m) 1Kg/s Temp (T 1 ) 293K Pow er (P) or Work (W) 15 KW Inlet velocity (C 1 ) 100m/s Outlet velocity (C 2 ) 150m/s To Find: Exit air temp (T 2 ) Solution: Step:1 Note; C p 1.005KJ/Kg.k m (h 1 + +Z 1.g) + Q m (h 2 + +Z 2.g) + W Neglect datum head (Z 1 ) (Z 2 ) 0 Q 0 (Adiapatic process m (h 1 + ) m (h 2 + ) + W 1 (h 1 + ) 1 (h 2 + ) + 15 (h 1 h 2 ) [ ] + 15 (h 1 h 2 ) C p (T 1 T 2 ) L.VIJAYAKUMAR /A.P-MECH Page 11

12 1.005(293 T 2 ) T T T 2 T K 7. A room for four persons has tw o fans, each consuming 0.18 kw pow er and three 100W lamps. Ventilation ant at the rate of kg/sec enters w ith an enthalpy of 84 kj/kg and leaves w ith an enthalpy of 59 kj/kg. If each person puts out heat at the rate of0.175 kj/sec, determine the rate at w hich heat is to be removed by a room cooler, so that a steady state is maintained in the room. (AUC DEC 07) Given: No of person(n p ) 4 No of fan (n f ) 2 (W f ) 0.18KW (each) (W l ) 0.1KW (each) Mass of air (m) 80Kg/hr Kg/s Enthalpy of air entering (h 1 ) 84KJ/Kg Enthalpy of air leaving (h 2 ) 59KJ/Kg Heat (Q p ) 630KJ/hr KJ/s To Find: Rate of heat is to be removed Solution: Step:1 Rate of energy increase Rate of energy in flow - Rate of energy out flow L.VIJAYAKUMAR /A.P-MECH Page 12

13 Q 1 -Ƞ p Q p Q 1 -( ) Q 1-0.7KJ/s or KW Step:2 M(h 1 - h 2 ) 0.022(84-59) 0.55KJ/s Step:3 W electrical energy input W Ƞ f W f + Ƞ l W l W (2 0.18)+( 3 0.1) W 0.66KW Step:4 Rate of heat is to be removed (Q) Q Q W Q Q KW L.VIJAYAKUMAR /A.P-MECH Page 13

14 8. One liter of hydrogen at 273 K is adiabatically compressed to one half of its initial volume. in the change in temperature of the gas, if the ratio of tw o specific heats for hydrogen is 1.4. (AUC DEC 07) Given: adiabatic process Initial volume (V 1 ) 1lit m 3 Temp (T 1 ) 273K Initial volume (V 2 ) one half of it s Initial volume (V 2 ) m 3 To Find: Change in temp of gas (T 2 T 1 ) Solution: Step:1 ( )^( ) ( )^( ) T T 1 T T K Step:2 To find change in temp (T 2 T 1 ) L.VIJAYAKUMAR /A.P-MECH Page 14

15 T 2 T T 2 T K 9. The velocity and enthalpy of fluid at the inlet of a certain nozzle are50 m/sec and2800 kj/kg respectively. The enthalpy at the exit of Nozzle is 2600 kj/kg. The nozzle is horizontal and insulated so that no heat transfer takes place from it' Find (1) Velocity of the fluid at exit of the nozzle (2) Mass flow rate, if the area at inlet of nozzle is 0.09 m 2 (3) Exit area of the nozzle, if the specific volume at the exit of the Nozzle is m 3 /kg. (AUC DEC 07) Given: Inlet velocity of nozzle (C 1 ) 50m/s Inlet enthalpy of nozzle (h 1 ) 2800KJ/Kg h ^3J/Kg no of heat transfer take place (Q) 0 exit enthalpy (h 2 ) 2600KJ/Kg h ^3J/Kg To Find: 1. C 2 2. m 3. A 2 Solution: Step:1 To find velocity of the fluid at exit of the nozzle (C 2 ) C 2 [2 (h 1 - h 2 ) + C 1^2] C 2 [2 ( ) ^2] L.VIJAYAKUMAR /A.P-MECH Page 15

16 C m/s Step:2 Tofind mass flow rate (m) m V 1 Specific volume is not given so it is assumed to be V 1 1m 3 /Kg m m 4.5Kg/s Step:3 To find exit area of nozzle (A 2 ) A 2 A m 2 L.VIJAYAKUMAR /A.P-MECH Page 16

17 10. A w ork done by substance in a reversible non-flow manner is in accordance w ith V (15/p) m 3, w here p is in bar. Evaluate the w ork done on or by the system as pressure increases from 10 to 100 bar. Indicate w hether it is a compression process or expansion process. If the change in internal energy is 500kJ, calculate the direction and magnitude of heat transfer. (AUC MAY 08) Given: reversible non-flow manner V (15/p) m 3 Pressure (P 1 ) 10bar Pressure (P 2 ) 100bar Change in internal energy (Δ u ) 500KJ To Find: The direction and magnitude of heat transfer (Q) Solution: Step:1 Heat transfer (Q) W+ Δ u L.VIJAYAKUMAR /A.P-MECH Page 17

18 Work done ʃ V dp ʃ 15/p dp 15 ʃ 1/p dp 15 [log p ] 15 [log 100- log 10] 15 [2-1] 15 [1] W 15KJ Direction: W 15 Positive sign indicates the expansion process Step:2 By first law of thermodynemics, Q W+ Δ u Q Q 515KJ Q 515 KJ Positive sign indicates the expansion process L.VIJAYAKUMAR /A.P-MECH Page 18

19 11. In a Gas turbine installation, the gases enter the turbine at the rate of 5 kg/sec w ith a velocity of 500 m/sec and enthalpy of 900 kj/kg and leave the turbine w ith 150 m/sec, and enthalpy of 400 kj/kg. The loss of heat from the gases to the surroundings is 25kJ/kg. Assume R kj/kg.k, Cp kj/kg.k and inlet conditions to be at 100 Kpa and 27 o C.Determine the diameter of the inlet pipe. (AUC MAY 08) Given: Mass flow rate (m) 5Kg/s Inlet velocity (C 1 ) 50m/s Inlet enthalpy (h 1 ) 900KJ/Kg Outlet enthalpy (h 1 ) 400KJ/Kg Outlet velocity (C 1 ) 150m/s Heat loss (Q) -25KJ/Kg R 0.285KJ/Kg C p 1.004KJ/Kg Inlet pressure (P 1 ) 100KPa KN/m 2 Inlet temperature (T 1 ) 27 C K L.VIJAYAKUMAR /A.P-MECH Page 19

20 To Find: Work done (W) Dia of the inlet pipe (d 1 ) Solution: Step: 1 m (h 1 + +Z 1.g) + Q m (h 2 + +Z 2.g) + W Z 2 Z 1 m (h 1 + ) + Q m (h 2 + ) + W 5 (900 + ) (400 + ) + W W W W 2425KW Step: 2 To find dia of the inlet pipe (d 1 ) m V 1? P 1 V 1 mrt 1 V 1 V 1 V m 2 /s L.VIJAYAKUMAR /A.P-MECH Page 20

21 m A 1 A 1 A m 2 2 A 1 d 1 2 d 1 d 1 d m d 1 d m 1000 d mm L.VIJAYAKUMAR /A.P-MECH Page 21

22 12. A frictionless piston-cylinder device contains 2 kg of nitrogen at 100 Kpa and 300 K. Nitrogen is now compressed slow ly according to the relation PV 1.4 C until it reaches a final temperature of 360 K. Calculate the w ork input during this process. (AUC DEC 09) Given: Polytropic process Molecular weight of nitrogen (m) 28 Ɣ 1.4 Inlet pressure (P 1 ) 100KN/m 2 Inlet temperature (T 1 ) 300K Outlet temperature (T 2 ) 360 K PV 1.4 C To Find: Work input (W) Solution: Step: 1 L.VIJAYAKUMAR /A.P-MECH Page 22

23 W Gas constant (R) 0.297KJ/ Kg.K W W W -89.1KJ Direction: W -89.1KJ Negative sign indicates that the work is done on the system. L.VIJAYAKUMAR /A.P-MECH Page 23

Two mark questions and answers UNIT I BASIC CONCEPT AND FIRST LAW SVCET

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