Exam Details
| Subject | mechanical engineering | |
| Paper | paper 2 | |
| Exam / Course | indian forest service | |
| Department | ||
| Organization | union public service commission | |
| Position | ||
| Exam Date | 2016 | |
| City, State | central government, |
Question Paper
1. Derive the following thermodynamic relation
src='./qimages/8075-1a.jpg'>
Discuss the effect of the following on the flame speed during combustion in an SI engine
Compression ratio
Intake pressure
Engine speed
Air-fuel ratio
Engine load
A copper wire having a diameter of 2 mm is exposed to a convection environment with heat transfer coefficient, h 5000 and environment temperature, T 100°C. What current must be passed through the wire to produce a centre temperature of 150°C Specific electrical resistance of copper is l·67 µ ohm cm. Thermal conductivity of copper is 386W/m-K.
The following data are given for a single cylinder four stroke cycle oil P.nlrinP. Cylinder diameter 18 cm, stroke 36 cm, speed of the engine 286 rpm, brake torque 375 Nm, IMEP 7 bar, fuel consumption 3·85 litres/hr, specific gravity of fuel 0·8, calorificvalue of fuel 44·5 MJ/kg. A/F 25 ambient air temperature 21 cp of gases 1·2 exhaust gas temperature 415°C, cooling water circulated 4·2 kg/min, rise in temperature of cooling water 28·5°C. Find the mechanical and indicated thermal efficiency and draw a heat balance sheet on percentage basis.
2. The readings of two thermometers A and B agree at ice point and steam point as 0°C and 100°C. The two temperature readings are related by the following expression
tA a btB +ctB²
where b and c are constants. In a constant temperature bath, the temperature as shown 51 deg on thermometer A and 50 deg on thermometer B. Determine the reading on thermometer B when the thermometer A reads 65°C. Can you comment which of the two thermometers is correct
A 40 kg rigid steel tank of 1000 litre capacity contains air at 500 kPa pressure and both tank and air are at .20°C temperature. The tank is connected to a line flowing air at 2 MPa pressure and 20°C temperature.The valve is opened, allowing air to flow into the tank until the pressure reaches 1·5 MPa and it is then closed. Assume the air and tank are always at the same temperature and the final temperature is 35°C. Find the final air mass that entered the tank and the. heat transfer. Specific heat of steel 0·46 kJ/kg-K.
3. A piston-cylinder device is shown in the figure: The piston resting on top of a set of stops, initially· contains 3 kg of air at 200 kPa and 27°C temperature. The mass of piston is such that a pressure of 400 kPa is required to move it. Heat is now transferred to the air until the volume is doubled. Determine the work done by the air and the amount of heat transfer. and show the process on P-v diagram. The average Cp for air is 0·8 which remains constant throughout any process.
src='./qimages/8075-3a.jpg'>
A petrol engine having stroke volume of 0·0012 m3 and compression ratio of 5·5 compresses the mixture to 8·5 bar and 350°C. Ignition is started so that the pressure rises along a straight line during explosion and attains its higher value of28 bar after the piston has travelled J./30 of working stroke.The air-fuel ratio is 16 1.Calorific value of the fuel is 44 MJ/kg and Cp 0·962kJ/kg-K. Find the heat loss per kg of charge
4. Derive the following expression for the thermal efficiency of an air standard diesel cycle
r compression ratio; p cut-off ratio, y Cp/Cv
Calculate the proportion of the heat of fuel carried away by the flue gases for the following data Coal with CV of 29·6 MJ/kg of coal has a composition by mass C H S and remainder is ash. It is burnt in a furnace with 50% excess air. The flue gases leaving the chimney are at 327°C and the atmospheric temperature is 15°C. Assume perfect combustion. The specific heat, cp, for air and dry products is 1005J/kg-K and 1045J/kg-K respectively. The heat carried away per kg of moisture in the flue gases is 2990kJ/kg. Composition of air by mass O2 23% and N2=77%.
5. Air at 20 kPa and 5°C temperature enters a 25 mm diameter tube at a velocity of 1·5 m/s. Using a flat-plate analysis, estimate the distance from the entrance at which the flow becomes fully developed. Viscosity,µ= 1·864 x Pa-s.
Five grams of water vapour per kilogram of atmospheric air at 35°C dry bulb temperature and 60% relative humidity is removed and the dry bulb temperature of air after removing the water vapour becomes 25°C. Find for the air at final stage
the relative humidity and
the dew point temperature. Assume, condition of atmospheric air is 39°C dry bulb temperature and relative humidity is 60%. Atmospheric pressure is 100kPa. The saturation properties of water are given in the following table:
src='./qimages/8075-5b.jpg'>
Define blade efficiency of a 'single stage steam turbine and prove that the maximum blade efficiency for a single stage steam turbine, max= cos^2 where a is the blade inlet angle.
A solid sphere of diameter 10 cm is heated to 1000°Cand suspended in a room having room temperature of 30°C. Compute the time taken by the sphere to cool to 500°C assuming emissivity of sphere 0·l and density
8.68 gm/cc. Take specific heat 0.098 J/KG-K
6. Air at 200 kPa pressure and 200°0 is heated as it flows through a tube with a diameter of 25 mm at a velocity of 10 m/s. Calculate the heat transfer per unit length of tube, if a constant-heat-flux condition is maintained at the wall and the wall temperature is 20°C above the air temperature, all along the length of the tube. How much would the bulk temperature increase over a 3-m length of the tube For air at 200°0 temperature viscosity, µ 2·57 x 10-5 thermal conductivity, k 0·0386 constant specific heat, Cp= 1025J/kg-K.
A long cylinder having a diameter of 2 cm is maintained at 600°C and has an emissivity of 0·4. Surrounding the cylinder is another long, thin-walled concentric cylinder having a diameter of 6 cm .and an emissivity of 0·2 on both the inside and outside surfaces. The assembly is located in a large room having a temperature of27°C. Calculate the net radiant energy lost by the 2-cm-diameter cylinder per metre of length. Also calculate the temperature of the 6-cm-diameter cylinder. Stefan-Boltzmann Constant, sigma 5·67 x ·
7. A 25 TR capacity ammonia vapour compression refrigerating machine works between 26°C and 20°C. The ammonia leaves the compressor dry and saturated. Liquid ammonia is under cooled to 22·5°C, before throttling. Find the theoretical COP of the machine. The Cp of saturated ammonia liquidis 4·72 kJ/kg-K. The cp for ammonia vapour is 2·8 kJ/kg-K. Find the COP of the cycle and the mass flow rate of ammonia in kg/min.
src='./qimages/8075-7a.jpg'>
An open cycle gas turbine plant operates with a pressure ratio of 4·5 while using 82kg/min of air and 1 ·4 kg/min of fuel. The net output of the plant is 200 kW when 230 kW is needed to drive the compressor. Air enters the compressor at 100 bar and 15°C and combustion gases enter the turbine at 765°C. Assuming specific heat of air and combustion gases as 1·005 kJ/kg-K and 1·128 kJ/kg-K respectively, the index of compression 1·4, the index of expansion 1·34 and mechanical efficiency for both the compressor and turbine as 0·98, estimate
the isentropic efficiency of the compressor,
the isentropic efficiency of the turbine,and
the overall efficiency of the plant.
8. Steam flows from the nozzles of single row impulse turbine with a velocity of 450 m/s at a direction which is inclined at an angle of 16° to the peripheral velocity. Steam comes out of the moving blades with an absolute velocity of 100 m/s in the direction at 110° with the direction of blade motion. The blades are equiangular and the steam flow rate is 6 kg/s. Determine the power loss due to friction.
State Buckingham n-theorem and hence obtain an expression for the thrust developed by a propeller which depends upon the angular velocity co, approach velocity dynamic viscosity density propeller diameter D and the compressibility of the medium measured by the local velocity of sound c.
src='./qimages/8075-1a.jpg'>
Discuss the effect of the following on the flame speed during combustion in an SI engine
Compression ratio
Intake pressure
Engine speed
Air-fuel ratio
Engine load
A copper wire having a diameter of 2 mm is exposed to a convection environment with heat transfer coefficient, h 5000 and environment temperature, T 100°C. What current must be passed through the wire to produce a centre temperature of 150°C Specific electrical resistance of copper is l·67 µ ohm cm. Thermal conductivity of copper is 386W/m-K.
The following data are given for a single cylinder four stroke cycle oil P.nlrinP. Cylinder diameter 18 cm, stroke 36 cm, speed of the engine 286 rpm, brake torque 375 Nm, IMEP 7 bar, fuel consumption 3·85 litres/hr, specific gravity of fuel 0·8, calorificvalue of fuel 44·5 MJ/kg. A/F 25 ambient air temperature 21 cp of gases 1·2 exhaust gas temperature 415°C, cooling water circulated 4·2 kg/min, rise in temperature of cooling water 28·5°C. Find the mechanical and indicated thermal efficiency and draw a heat balance sheet on percentage basis.
2. The readings of two thermometers A and B agree at ice point and steam point as 0°C and 100°C. The two temperature readings are related by the following expression
tA a btB +ctB²
where b and c are constants. In a constant temperature bath, the temperature as shown 51 deg on thermometer A and 50 deg on thermometer B. Determine the reading on thermometer B when the thermometer A reads 65°C. Can you comment which of the two thermometers is correct
A 40 kg rigid steel tank of 1000 litre capacity contains air at 500 kPa pressure and both tank and air are at .20°C temperature. The tank is connected to a line flowing air at 2 MPa pressure and 20°C temperature.The valve is opened, allowing air to flow into the tank until the pressure reaches 1·5 MPa and it is then closed. Assume the air and tank are always at the same temperature and the final temperature is 35°C. Find the final air mass that entered the tank and the. heat transfer. Specific heat of steel 0·46 kJ/kg-K.
3. A piston-cylinder device is shown in the figure: The piston resting on top of a set of stops, initially· contains 3 kg of air at 200 kPa and 27°C temperature. The mass of piston is such that a pressure of 400 kPa is required to move it. Heat is now transferred to the air until the volume is doubled. Determine the work done by the air and the amount of heat transfer. and show the process on P-v diagram. The average Cp for air is 0·8 which remains constant throughout any process.
src='./qimages/8075-3a.jpg'>
A petrol engine having stroke volume of 0·0012 m3 and compression ratio of 5·5 compresses the mixture to 8·5 bar and 350°C. Ignition is started so that the pressure rises along a straight line during explosion and attains its higher value of28 bar after the piston has travelled J./30 of working stroke.The air-fuel ratio is 16 1.Calorific value of the fuel is 44 MJ/kg and Cp 0·962kJ/kg-K. Find the heat loss per kg of charge
4. Derive the following expression for the thermal efficiency of an air standard diesel cycle
r compression ratio; p cut-off ratio, y Cp/Cv
Calculate the proportion of the heat of fuel carried away by the flue gases for the following data Coal with CV of 29·6 MJ/kg of coal has a composition by mass C H S and remainder is ash. It is burnt in a furnace with 50% excess air. The flue gases leaving the chimney are at 327°C and the atmospheric temperature is 15°C. Assume perfect combustion. The specific heat, cp, for air and dry products is 1005J/kg-K and 1045J/kg-K respectively. The heat carried away per kg of moisture in the flue gases is 2990kJ/kg. Composition of air by mass O2 23% and N2=77%.
5. Air at 20 kPa and 5°C temperature enters a 25 mm diameter tube at a velocity of 1·5 m/s. Using a flat-plate analysis, estimate the distance from the entrance at which the flow becomes fully developed. Viscosity,µ= 1·864 x Pa-s.
Five grams of water vapour per kilogram of atmospheric air at 35°C dry bulb temperature and 60% relative humidity is removed and the dry bulb temperature of air after removing the water vapour becomes 25°C. Find for the air at final stage
the relative humidity and
the dew point temperature. Assume, condition of atmospheric air is 39°C dry bulb temperature and relative humidity is 60%. Atmospheric pressure is 100kPa. The saturation properties of water are given in the following table:
src='./qimages/8075-5b.jpg'>
Define blade efficiency of a 'single stage steam turbine and prove that the maximum blade efficiency for a single stage steam turbine, max= cos^2 where a is the blade inlet angle.
A solid sphere of diameter 10 cm is heated to 1000°Cand suspended in a room having room temperature of 30°C. Compute the time taken by the sphere to cool to 500°C assuming emissivity of sphere 0·l and density
8.68 gm/cc. Take specific heat 0.098 J/KG-K
6. Air at 200 kPa pressure and 200°0 is heated as it flows through a tube with a diameter of 25 mm at a velocity of 10 m/s. Calculate the heat transfer per unit length of tube, if a constant-heat-flux condition is maintained at the wall and the wall temperature is 20°C above the air temperature, all along the length of the tube. How much would the bulk temperature increase over a 3-m length of the tube For air at 200°0 temperature viscosity, µ 2·57 x 10-5 thermal conductivity, k 0·0386 constant specific heat, Cp= 1025J/kg-K.
A long cylinder having a diameter of 2 cm is maintained at 600°C and has an emissivity of 0·4. Surrounding the cylinder is another long, thin-walled concentric cylinder having a diameter of 6 cm .and an emissivity of 0·2 on both the inside and outside surfaces. The assembly is located in a large room having a temperature of27°C. Calculate the net radiant energy lost by the 2-cm-diameter cylinder per metre of length. Also calculate the temperature of the 6-cm-diameter cylinder. Stefan-Boltzmann Constant, sigma 5·67 x ·
7. A 25 TR capacity ammonia vapour compression refrigerating machine works between 26°C and 20°C. The ammonia leaves the compressor dry and saturated. Liquid ammonia is under cooled to 22·5°C, before throttling. Find the theoretical COP of the machine. The Cp of saturated ammonia liquidis 4·72 kJ/kg-K. The cp for ammonia vapour is 2·8 kJ/kg-K. Find the COP of the cycle and the mass flow rate of ammonia in kg/min.
src='./qimages/8075-7a.jpg'>
An open cycle gas turbine plant operates with a pressure ratio of 4·5 while using 82kg/min of air and 1 ·4 kg/min of fuel. The net output of the plant is 200 kW when 230 kW is needed to drive the compressor. Air enters the compressor at 100 bar and 15°C and combustion gases enter the turbine at 765°C. Assuming specific heat of air and combustion gases as 1·005 kJ/kg-K and 1·128 kJ/kg-K respectively, the index of compression 1·4, the index of expansion 1·34 and mechanical efficiency for both the compressor and turbine as 0·98, estimate
the isentropic efficiency of the compressor,
the isentropic efficiency of the turbine,and
the overall efficiency of the plant.
8. Steam flows from the nozzles of single row impulse turbine with a velocity of 450 m/s at a direction which is inclined at an angle of 16° to the peripheral velocity. Steam comes out of the moving blades with an absolute velocity of 100 m/s in the direction at 110° with the direction of blade motion. The blades are equiangular and the steam flow rate is 6 kg/s. Determine the power loss due to friction.
State Buckingham n-theorem and hence obtain an expression for the thrust developed by a propeller which depends upon the angular velocity co, approach velocity dynamic viscosity density propeller diameter D and the compressibility of the medium measured by the local velocity of sound c.