Jadavpur University 2007 B.E Mechanical Engineering thermodynamics-II - Question Paper

Ref. No. Ex/ML22B/59/2006
Inter Mechanical Engineering, Examination, 2005
2nd Semester
SUBJECT - THERMODYNAMICS – II
Paper - ------
Time: 3 hours Full Marks-100
ans any 5 ques.. All ques. carry equal marks.
Use of steam and other thermodynamic tables is permitted. Data not given, may be
presumed consistent with the issue. ans must be brief and to the point.
various parts of the identical ques. must be answered together.
1. Consider a regenerative cycle using steam as the working fluid. Steam leaves the
boiler and enters the turbine at four MPa and 400°C. After expansion to 400 kPa, a few
of the steam is extracted from the turbine for the purpose of heating the feedwater in
an open feedwater heater. The pressure in the feedwater heater is 400 kPa and the
water leaving it is “saturated liquid” at 400 kPa. The steam, not extracted, expands
through the low-pressure turbine to a condenser pressure of 10 kPa. Determine the
cycle efficiency. (20)
2. discuss with the help of schematic and p-h (or, T-s) diagrams vapour compression
refrigeration system. Consider a vapour compression refrigeration system, operating
ranging from the pressure limits of 0.8 MPa and 0.14 MPa with R-134a as working fluid.
The mass flow rate of the refrigerant is 0.05 kg/s. Determine: i) the rate of heat
removal from the refrigerated space and the power input to compressors, ii) the
coefficient of performance of this refrigerator. Given:
At 0.14 MPa, hg=236.04kJ/kg, sg = 0.9322kJ / kgK,
At 0.8 MPa, h, =93.42kJ/kg and
T°(C) h(kJ/kg) s(kJ/kgK)
31.33 264.15 0.9066
40 273.66 0.9374
(20)
3. State the assumptions made for air standard cycle analysis. Show that the thermal
efficiency of an air standard dual combustion cycle is provided as;
where the symbols have their usual meaning. From the above expression obtain the
thermal efficiencies of Otto cycle and Diesel cycle. (20)
4. find an expression for the thermal efficiency of a gas turbine, working on an ideal
Brayton cycle, in terms of pressure ratio. A stationary power plant operating on an
ideal Brayton cycle has a pressure ratio of 8. The gas temperature is 300 K at the
compressor inlet and 1300 K at the turbine inlet. Utilizing the air standard
assumptions, determine i) the gas temperatures at the exits of the compressor and the
turbine, ii) the ratio of compressor work to the turbine work (back work ratio) and
iii) the thermal efficiency. Further, assuming the isentropic efficiencies of
compressor and turbine to be 80% and 85%, respectively, obtain out the thermal
efficiency of the plant. (20)
5.(a) Show that for identical index of expansion and compression, clearance has no effect on
the work needed to compress unit mass of air in a reciprocating air compressor. (8)
(b) For a single stage, single acting reciprocating air compressor, compression starts at
1 bar and 30°C. The stroke quantity is 10 m3/min and the delivery pressure is 7.5 bar.
The index of compression and expansion is 1.3. Assuming a clearance factor of 5%,
compute (i) the volumetric efficiency, (ii) true quantity handled, (iii) the mass of
air handled, and (iv) work needed to run the compressor. (12)
6.(a) discuss with the help of a T-s diagram, what is meant by “Dew Point” temperature.
In cold weather, condensation frequently occurs on the inner surfaces of the
windows due to the lower air temperature near the window surface. Consider a room
that contains air at 20°C and 75% relative humidity. What is the partial pressure of
water vapour in the air at this condition? At what window temperature will the
moisture in the air begin condensing on the inner surfaces of the windows? (6)
(b) An air conditioning system is to take in outdoor air at 10°C and 30% relative
humidity at a steady rate of 45 m3/min and to condition it to 25°C and 60% relative
humidity. The outdoor air is 1st heated to 22°C in the heating part and then
humidified by the injection of hot steam in the humidifying part. Assuming the
entire process takes place at a constant total pressure of 100 kPa, determine i) the
rate of heat supply in the heating part and ii) the mass flow rate of the steam
needed in the humidifying part. Plot the process on psychrometric chart and
show the control quantity you choose for analysis indicating different parts. Given:
hdry air = 1.005T kJ/kg dry air for dry air (T in °C)
hg(T)=2501.3 + 1.82T kJ/kg for water vapour (T in °C) (14)
7. discuss what is meant by adiabatic flame temperature. Liquid octane, C8H18(liquid)
at 25°C is burned with 400% theoretical air (also at 25°C) in a steady state steady
flow process. Determine the adiabatic flame temperature. presume complete
combustion. The stoichiometrically accurate complete combustion formula is provided
as:
C8H18(l)+12.5O2 +12.5×3.76N2
8CO2 + 9H2O + 47N2
begin with guessed outlet temperatures of 900K and 1000K. Interpolate your outcome to
find the adiabatic flame temperature. (20)
8. Write short notes or discuss briefly with the help of schematic diagram and/or
showing the processes on relevant thermodynamic plane (i) Cogeneration, (ii) Effect
of Reheating on vapour power cycle, (iii) Efficiency of Ericsson cycle with ideal
regeneration (iv) Adiabatic saturation process and wet bulb temperature of moist air.
4×5

Earning:   Approval pending.