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Thermodynamics Question Paper - Jun 17 - Mechanical Engineering (Semester 3) - Pune University (PU)
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Thermodynamics - Jun 17

Mechanical Engineering (Semester 3)

Total marks: 80
Total time: 3 Hours
INSTRUCTIONS
(1) Question 1 is compulsory.
(2) Attempt any three from the remaining questions.
(3) Draw neat diagrams wherever necessary.

1.a. Define a thermodynamics system and surroundings. Give classification of systems with example.
(6 marks) 00

1.b A fluid system contained in a piston and cylinder machine, passes through a complete cycle of four processes. The sum of all heat transferred during a cycle is -340kJ. The system completes 200 cycles per mi. Complete the following table showing the method for each item, and compute the net rate of work output in kW

Process Q (kJ/min) W (kJ/min) $\Delta$E (kJ/min)
1-2 0 4340 -
2-3 42000 0 -
3-4 -4200 - -73200
4-5 - - -

(6 marks) 00

OR

2.a Show that Cp-Cv =R. Derive the relation for heat transfer and work transfer for constant pressure process.
(6 marks) 00

2.b An iron cube at a temperature of 400degC is dropped into an insulated bath containing 10 kg water at steady state. Given that the specific heat of water is equal to 4186 J/kg K. Find the entropy changes for the iron cube and the water. Is the process reversible? If so why?
(6 marks) 00

3.a What is a available energy ? Define dead state, useful work and unavailable work.
(6 marks) 00

3.b An engine working on Otto cycle, air has pressure of 1 bar and temperature of 27°C. Air is compressed adiabatically with a compression ratio of 7 and then heat is added at constant volume till the temperature rises to 2000 K.Find the air standard efficiency , pressure at the end of compression and heat addition in process and the mean effective pressure of the cycle. Assume,Cv=0.718 kJ/kgK, γ=1.4 and R = 287 Nm/kgK.
(6 marks) 00

4.a Discuss the principle of separating calorimeter with a neat labeled diagram.
(6 marks) 00

4.b A thermal power plant works on Rankine cycle has a boiler pressure of 120 bar and condenser pressure of 5 kPa . Steam is superheated in the super heater to 400°C . Find per kg of steam generated by boiler: (i) Net work Output (ii) Rankine efficiency (iii) Specific steam consumption.
(6 marks) 00

5.a Give the function and location of any 3 : (i) Super heater (ii) Air preheater (iii) Fusible plug (iv) Water level indicator (v) Spring loaded safety valve.
(6 marks) 00

5.b The following results were obtained in a boiler trial : (i) Feed water per hour = 700 kg at 27degC. (ii) Steam produced at 8 bar and 0.97 dry. (iii0 Coal used = 100 kg/hr having CV of coal = 25000 kJ/kg (iv) Ash and unburnt coa; co;;ected = 7.5 kg/hr having CV = 2000 kJ/kg (v) Mass of flue gases produced per kg of fuel burnt = 17.3 kg (vi)Flue gas temperature = 327 deg C (vii) Room temperature = 16degC (viii)Specific heat of flue gases = 1.025 kJ/kgK Draw the energy balance on minute basis.
(5 marks) 00

6.a Derive the formula for:

(i) Equivalent evaporation and

(ii) Boiler efficiency.

(6 marks) 00

6.b The following readings wre recorded during a boiler trial of 6 hrs duration

Mean steam pressure = 12 bar Masss of the stream generated = 40000 kg

Mean dryness fraction = 0.85; mean feed water temperature = 30°C

Coal used = 4000 kg; Calorific value of coal = 33400 kJ/kg

Calculate:

(i) Factor of equivalent evaporation

(ii) Heat rate of boiler in kJ/kg

(iii) Equivalent evaporation form and at 100°C

(iv) Efficiency of boiler.

(7 marks) 00

7.a What are the factors affecting Human Comfort? Discuss in detail.
(6 marks) 00

7.b Affecting air at 30°C DBT and 18°C WBT is cooled to 20°C DBT without changing its moisture content

Find:

(i) Initial enthalpy and specific humidity of air

(ii) Final relative humidity of air and WBT

(iii) Sensible heat removed per kg of air.

OR

8.a Define and discuss the significance of the following:

(i) Wet Bulb temperature

(ii) Dew point temperature

(iii) Humidity ratio.

(6 marks) 00

8.b Air enters a window air conditioner at 1 atm and 30°C and 80% RH at a rate of 10 m³/min and leaves as saturated at 14°C. A part of moisture which condenses during the process is also removed at 14°C. Determine the heat flow rate and moisture removed from the air. Sow the process on psychrometric diagram.
(7 marks) 00

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