2nd yr. thermodynamics

7/31/2019 2nd Yr. Thermodynamics

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Ex/IT/PE/T/213/50/2010

BACHELOROF INFORMATION TECH. ENGG. EXAMINATION, 2010

(2nd Year1st Semester)

THERMODYNAMICS

Time : Three hours Full Marks : 100

Answer any five questions.

Missing data, if any, may be assumed.

1. (a) Discuss the salient features of Carnot cycle and reversed

Carnot cycle with a perfect gas as the working substance.

10

(b) What are the conditions of reversibility? 2

(c) Show that the C.O.P. of a heat pump in greater than the

C.O.P. of a refrigerator by unity, when both devices are

associated with the same heat and work interactions. 4

(d) A cyclic machine receives 325 kJ from a 1000 K energy

reservoir. It rejects 200 kJ to a 400 K energy reservoir, and

the cycle produces 125 kJ of work output. If this cycle

reversible, irreversible, or impossible? Justify. 4

2. (a) State Kelvin-Planck statement of 2nd law of thermodynamics.

2

(b) Write a brief note on entropy. 6


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(c) Prove that the change of specific entropy of a perfect gas

when changing its state from p1, v

1, T

1to a state p

2, v

2, T

2

is S2S

1= C

pln (T

2/T

1)R ln (p

2/p

1), where the symbols have

their usual meanings. 6

(d) Show that the change of specific entropy of an ideal gas

during expansion or compression according to the general law

pVn=constant is

S2S

1= C

v

n

n 1ln (T

1/ T

2).

the symbols bear their usual meaning. 6

3. (a) A gas initially at 1 MPa, 300C is constrained in a frictionless

piston-cylinder arrangement with an initial volume of 0.1 m3.

The gas is expanded in a reversible process according to the

relation pVn=constant, until a final pressure of 100 kPa is

reached.

Sketch the processes on a p-Vdiagram for the cases when

n = 1, n=1.35, n=1.4 and n . 4

(b) When a gas expands in a cylinder polytropically to the general

law pVn=constant, show that the heat interchanged through

the cylinder-walls is given by

Q =

n

1 polytropic work,

where = adiabatic index

and n = polytropic index. 6

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7. (a) A two-stage reciprocating air compressor with perfect

intercooling takes in air at 1 bar and 27C. The law of

compression in both the stages in pV1.3 = constant. The

compressed air is delivered at 9 bar from H.P. cylinder to an

air reservoir. Calculate per kg of air

(i) the minimum work done,

(ii) the heat rejected to intercooler,

(iii) the minimum work done in a three stage compressor

working under the same operating conditions.

Take Cp

= 1.005 kJ/kg-K. 12

(b) What will be the loss in air standard efficiency of a diesel

engine with compression ratio 14, if the fuel cut-off is delayed

from 6% to 9% of stroke? 8


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(c) A 4-stroke engine working on Otto cycle has a stroke volume

of 0.1 m3. The compression ratio is 7. The condition at the

start of the cycle is pressure 0.1 MPa and 90C. The heat

addition at constant volume is 100 kJ/cycle.

Determine

(i) ideal efficiency,

(ii) temperatures at key points in the cycle, and

(iii) mean effective pressure.

Assume air as the working substance with Cv

= 0.718 kJ/kgK

and = 1.4. 10

6. (a) Explain with the help of an indicator the working of a single

stage reciprocating air compressor, and derive the equation

for work done by the compressor without clearance when

the law of compression followed is (i) pVn=constant,

(ii) isothermal. 10

(b) Prove that for maximum efficiency of a two-state

reciprocating air compressor with perfect intercooling

pi

= p ps d ,

where pi

= intermediate pressure,

ps

= suction pressure,

pd

= delivery pressure. 10

( 3 )

(c) State Clausius Theorem and explain why heat transfer

across a finite temperature difference makes a process

irreversible. 6

(d) An inventor claims to have developed a heat engine which

has the following particulars :

Power developed = 76 KW

Fuel consumption = 4 kg/hr

C.V. of fuel = 75,000 kJ/kg

Temperature limits = 727C and 27C.

Comment on his claim. 4

4. (a) Write the steady flow energy equation for a single stream

entering and a single stream leaving a control volume and

breifly explain the various terms in it. State the conditions to

be satisfied for the validity of this equation. 10

(b) In a gas turbine the flow rate of air is 4 kg/s. The velocity

and enthalpy of air at entrance are 250 m/s and 6910 kJ/kg

respectively. At exit, the velocity is 170 m/s and the enthalpy

is 5025 kJ/kg. As the air passes through the turbine a loss

of heat equal to 40 kJ/kg occurs. Find the power developed

by the turbine. 10

5. (a) Prove that for the same compression ratio and the same heat

input, Otto cycle is more efficient than Diesel cycle. 4

(b) Briefly give a comparative analysis of the spark ignition and

compression ignition engines. 6

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2nd yr. thermodynamics

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