Download thermodynamics, heat and mass transfer

THERMODYNAMICS, HEAT AND MASS TRANSFER
TUTORIAL NO: 5
SECOND LAW OF THERMODYNAMICS
1. A refrigeration unit maintains – 100C in the refrigerator which is kept in a room
where the surrounding temperature is 250C and has a COP of 8.5. Check the validity
of this claim.
2. A cold storage is to be maintained at – 50C while the surroundings are at 350C. The
heat leakage from the surroundings into the cold storage is estimated to be 29 kW.
The actual COP of the refrigeration plant is one-third of an ideal plant working
between the same temperatures. Find the power required to drive the plant.
3. A heat engine receives a heat transfer rate of 1 MW at a high temperature of 550 0C
and rejects energy to the ambient at 300K. Work is produced at a rate of 450 kW.
Determine the energy discarded to the ambient and the engine efficiency. Compare
both of these to a Carnot heat engine operating between the same two reservoirs.
4. A reversible engine is supplied with heat from two reservoirs at 900 K and 600 K and
rejects heat to a sink at 300 K. The engine develops work equivalent to 90kJ/s and
rejects heat at the rate of 56 kJ/s. Estimate heat supplied by each source and thermal
efficiency of the engine.
5. Two blocks A and B which are initially at 950C and 5400C, respectively are brought
together into contact and isolated from the surroundings. They are allowed to reach a
final state of thermal equilibrium. Determine the entropy change of each block and of
the isolated system. Block A is aluminium (cP = 0.9 kJ/kg K) with mA = 0.45 kg, and
block B is copper (cP = 0.385 kJ/kg K) with mB = 0.9 kg.
6. Five kg of water at 300C is mixed with 1 kg of ice at 00C. The system is open to
atmosphere. Assuming the process of mixing is adiabatic, find the change of entropy.
Latent heat of ice = 336 kJ/kg, cP for water = 4.2 kJ/kg K.
7. Air initially at 155.50C and 1 bar, is composed reversibly and isothermally to a state
where the specific volume is 0.28 m3/kg. Find the change in internal energy, change
of entropy, and heat and work transfers per kg of air
8. A mass of air initially at 2060C and a pressure of 7 bar and having a volume of 0.03
m3. The air is expanded at constant pressure to 0.09 m3, a polytropic process with n =
1.5 is then carried out, followed by a constant temperature process which completes
the cycle. All processes are reversible. Sketch the cycle on P-V and T-S diagrams and
determine
(a) change in entropy during each process,
(b) heat received and heat rejected in the cycle, and
(c) efficiency of the cycle. [R = 287 J/kg K, CV = 718 J/kg K]
THERMODYNAMICS, HEAT & MASS TRANSFER
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TUTORIAL NO. 5
9. Steam enters a steam turbine at a pressure of 1 MPa, a temperature of 3000C, and a
velocity of 50 m/s. The steam leaves the turbine at a pressure of 150 kPa and a
velocity of 200 m/s. Determine the work per kg of steam flowing through the turbine,
assuming the process to be reversible and adiabatic.
10. Steam enters the nozzle at 1 MPa, 3000C, with a velocity of 30 m/s. The pressure of
the steam at the nozzle exit is 0.3 MPa. Determine the exit velocity of the steam from
the nozzle, assuming a reversible, adiabatic, steady state, steady flow process.
ANSWER
1. Invalid
2. 12.99 kW
3. 550 kW, 45 %, 63.5 %, 365 kW
4. 100kJ/s, 46 kJ; 61.6 %
5. 0.179 kJ/kg, - 0.121 kJ/kg, 0.0583 kJ/k
6. 0.0588 kJ/K
7. 0, - 0.425 kJ/kg K, - 182 kJ/kg, 182 kJ/kg
8. 0.168 kJ/K, 89.66 kJ, 38.84%
9. 377.5 kJ/kg
10. 737 m/s
THERMODYNAMICS, HEAT & MASS TRANSFER
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TUTORIAL NO. 5