Exam Details
| Subject | mechanical engineering | |
| Paper | paper 2 | |
| Exam / Course | indian forest service | |
| Department | ||
| Organization | union public service commission | |
| Position | ||
| Exam Date | 2014 | |
| City, State | central government, |
Question Paper
SECTION-A
1. Answer all of the following
A balloon is filled ..th hydrogen. It has 1000 m3 volume at temperature 300 K and pressure 100 ikPa. Determine the payload that can be lifted with the balloon. 8
two-stroke CI engine delivers a brake power of 368 kW, while 73·6 kW is Iused to overcome friction losses. It consumes 180 kg/hr of fuel at an air-fuel ratio of 20; 1. The heating value of the fuel is 42000 kJ /kg. Calculate indicated power, mechanical efficiency, air consumption, indicated thermal efficiency imd brake thermal efficiency.
(c)Explain with T-s jd P-h diagrams the effect of superheating and subcooling in vapour compressiob cycle of refrigeration.
Prove the Clapeyron equation saturation =hfg/t.vfg, where the terms have their usual meanings. .
What do you mean by effectiveness of heat exchangers? Derive the expression for effectiveness ofi parallel-flow heat exchanger.
A circumferentially finned tube with inner diameter 3 cm, outer diameter 3·2 cm, having thermal conductivity 15 W is provided with 2 fins/cm. The outer diameter of circumferential fin is 4·5 cm and its thickness is 1 mm and is made of the same I material as the tube.
The finned tube is used to cool water, which flows inside the tube, by transferring heat to air which flows across the finned tube. Heat transfer coefficients on ak and water side are 15 W m 2 and 500 W m 2-K respectively. Effectiveness of fins is 0·65. Calculate the value of overall heat transfer coefficient for the finned tube based on inside surface ..ea of the tube. Three thinned-wall infinitely long hollow cylinders 2 and having radii I 7·5 cm, 12·5 cm ahd 17·5 cm respectively, are arranged concentrically. The surface of the cylidder 1 is maintained at 727 while the surface of cylinder I3 is maintained at 27 °C. Emissivities of all surfaces of the cylinders are 0·08. ICalculate the steady-state temperature of cylinder if there is high vacuum in 15 the annular spaces between the cylinders. 15
What is Fick's law1 On what factors does the rate of mass transfer depend in molecular diffusion? 10
3.(a)Discuss in brief various factors which affect the volumetric efficiency of an engine. 10
six-cylinder petrol engine develops 62 hp at 3000 RPM. The volumetric efficiency at NTP is 85%. The bore is equal to the stroke and thermal efficiency of 25% may be assumed. Calorific value of petrol is 10500 kcal/kg. Air-fuel ratio is to be 15: Calculate cylinder bore and stroke. 15
A four-cylinder four-stroke petrol engine was subjected to a laboratory test and the following data were obtained
Cylinder diameter 64 mm
Stroke length 90 mm
Fuel consumption 7·5 litres/hr
RPM 2400
Calorific value of fuel 11400 kcal/kg
Specific gravity of fuel 0·717
Brake drum diameter 73·5 cm
Rope diameter 2·5 cm
Load on brake drum running at one-third engine speed by belts, spring balances read 60 kg and 8 kg. Mechanical efficiency 80%
Determine brake thermal efficiency and indicated mean effective pressure. 15
Which is the more effective way to increase the efficiency of a Carnot engine-to decrease 7i keeping T2 constant or to decrease T2 keeping 7i constant? 10
The Joule-Kelvin coefficientµ j is a measure of the temperature change during a throttling process. A similar measure of the temperature change produced by an isentropic change of pressure is provided by the coefficient µs, where
µs . Prove that µs-µj v/cp.15
in a steam power station, steam flows steadily through a 0·2 m diameter pipeline from the boiler to the turbine. At the boiler end, the steam conditions are found to be p 4 MPa, t 400 h 3213·6 kJ /kg and u 0·073 m 3 /kg. At the turbine end, the conditions are found to be p 3·5 MPa, t 392 h 3202·6 kJ/kg and u 0·084 m 3 /kg. There is a heat loss of 8·5 kJ/kg from the pipeline. Calculate the steam flow rate.
SECTION-B
5. Answer all of the following
What is the fun..tion of a thermostatic expansion valve used in vapour compression refrigeration systems? Discuss with the help of neat sketch the working of an internally equalized thermostatic expansion valve. How does it 8
(b)Air enters an insulated pipe at Mach number 0·4 and leaves at M 0·6. What portion of t.H.e duct length in percentage is required for the flow to occur at
M 0·5? Take Fanno line parameters as
M fl/D
0·4 0·5770
0·5 0·2673
0· 1227
The moist air at 25 total pressure of 100 kPa and 50% RH is compressed to 50 total pressure of 300 kPa and then cooled at constant pressure. At what temperature will Jater begin to condense? Show the process on psychrometric chart. 8
Why is there a need of governing of steam turbines? With the help of a simple sketch, discuss thk working principle of hydromechanical speed-governing loopfor steam turbine.I
With the help of a bketch, discuss the working principle of temperature control in steam boiler. I ·
6. A simple R-12 pIJnt is to develop 5 tonnes of refrigeration. The condenser and evaporator t'emperatures are to be 40 °C and O°C Determine the following
Refrigerant nd1w rate
Volume flow rate handled by the compressor m m 3 s I
Compressor discharge temperature if the enthalpy of refrigerant compressor e..it is 209-4 1 kJ /kg
Pressure ratio
Heat rejected fo the condenser in kW
Flash gas percentage after throttling
COP
(viii} Power required to drive the compressor respectively. at
Saturated values of properties of R-12
<img src='./qimages/1279-6a.jpg'>
How does absorption refrigeration system differ from vapour compression
system? 10
What are the different types of compressors used in vapour compression plants? What are their limitations? 10
7. The following data relate to a small gas axial
Turbine inlet temperature, Toi 1100 K
Turbine inlet pressure, Po1 4 bars Pressure ratio (stage) Po1/po3 =1·873
Blade mean speed 340 s
Mean radius of rotating blade ring 0·216 m
Blade height 0·0612 m
For gas, take C=1-148 kJ/kg/K and y 1·333
Assuming isentropic efficiency of stage (total to total) as 0·9, flow coefficient as 0·8 and flow enters the jet nozzle without swirl from turbine, calculate direction of absolute and relative velocity vector at entry and exit of rotor with axial direction and loading coefficient and degree of reaction. 20
Mentioning the requirements of pulverized coal burners, explain the types of burners used for efficient utilization of different kinds of coal. 10
In context of a power plant, explain the following 10
Load curve
Load duration curve
Load factor
Plant factor
Reserve factor
Explain the working I principle of a supercritical boiler with the help of a diagram. What are tie advantages of a supercritical boiler? 20
What are the variou.. locations of ID and FD fans for producing draught? Discuss them. 10
Diesel plants are used as standby units in a grid system. Explain why. A power station supplies the following loads to consumers
Time I 0-6 6-9 9-12 12-16 16-20 20-22 22-24
Load I 20 50 60 50 80 70 40
Find the load fa..tor of the plant.
What is the load factor of a standby plant of 30 MW capacity, if it takes up all the loads above 50 MW? 10
1. Answer all of the following
A balloon is filled ..th hydrogen. It has 1000 m3 volume at temperature 300 K and pressure 100 ikPa. Determine the payload that can be lifted with the balloon. 8
two-stroke CI engine delivers a brake power of 368 kW, while 73·6 kW is Iused to overcome friction losses. It consumes 180 kg/hr of fuel at an air-fuel ratio of 20; 1. The heating value of the fuel is 42000 kJ /kg. Calculate indicated power, mechanical efficiency, air consumption, indicated thermal efficiency imd brake thermal efficiency.
(c)Explain with T-s jd P-h diagrams the effect of superheating and subcooling in vapour compressiob cycle of refrigeration.
Prove the Clapeyron equation saturation =hfg/t.vfg, where the terms have their usual meanings. .
What do you mean by effectiveness of heat exchangers? Derive the expression for effectiveness ofi parallel-flow heat exchanger.
A circumferentially finned tube with inner diameter 3 cm, outer diameter 3·2 cm, having thermal conductivity 15 W is provided with 2 fins/cm. The outer diameter of circumferential fin is 4·5 cm and its thickness is 1 mm and is made of the same I material as the tube.
The finned tube is used to cool water, which flows inside the tube, by transferring heat to air which flows across the finned tube. Heat transfer coefficients on ak and water side are 15 W m 2 and 500 W m 2-K respectively. Effectiveness of fins is 0·65. Calculate the value of overall heat transfer coefficient for the finned tube based on inside surface ..ea of the tube. Three thinned-wall infinitely long hollow cylinders 2 and having radii I 7·5 cm, 12·5 cm ahd 17·5 cm respectively, are arranged concentrically. The surface of the cylidder 1 is maintained at 727 while the surface of cylinder I3 is maintained at 27 °C. Emissivities of all surfaces of the cylinders are 0·08. ICalculate the steady-state temperature of cylinder if there is high vacuum in 15 the annular spaces between the cylinders. 15
What is Fick's law1 On what factors does the rate of mass transfer depend in molecular diffusion? 10
3.(a)Discuss in brief various factors which affect the volumetric efficiency of an engine. 10
six-cylinder petrol engine develops 62 hp at 3000 RPM. The volumetric efficiency at NTP is 85%. The bore is equal to the stroke and thermal efficiency of 25% may be assumed. Calorific value of petrol is 10500 kcal/kg. Air-fuel ratio is to be 15: Calculate cylinder bore and stroke. 15
A four-cylinder four-stroke petrol engine was subjected to a laboratory test and the following data were obtained
Cylinder diameter 64 mm
Stroke length 90 mm
Fuel consumption 7·5 litres/hr
RPM 2400
Calorific value of fuel 11400 kcal/kg
Specific gravity of fuel 0·717
Brake drum diameter 73·5 cm
Rope diameter 2·5 cm
Load on brake drum running at one-third engine speed by belts, spring balances read 60 kg and 8 kg. Mechanical efficiency 80%
Determine brake thermal efficiency and indicated mean effective pressure. 15
Which is the more effective way to increase the efficiency of a Carnot engine-to decrease 7i keeping T2 constant or to decrease T2 keeping 7i constant? 10
The Joule-Kelvin coefficientµ j is a measure of the temperature change during a throttling process. A similar measure of the temperature change produced by an isentropic change of pressure is provided by the coefficient µs, where
µs . Prove that µs-µj v/cp.15
in a steam power station, steam flows steadily through a 0·2 m diameter pipeline from the boiler to the turbine. At the boiler end, the steam conditions are found to be p 4 MPa, t 400 h 3213·6 kJ /kg and u 0·073 m 3 /kg. At the turbine end, the conditions are found to be p 3·5 MPa, t 392 h 3202·6 kJ/kg and u 0·084 m 3 /kg. There is a heat loss of 8·5 kJ/kg from the pipeline. Calculate the steam flow rate.
SECTION-B
5. Answer all of the following
What is the fun..tion of a thermostatic expansion valve used in vapour compression refrigeration systems? Discuss with the help of neat sketch the working of an internally equalized thermostatic expansion valve. How does it 8
(b)Air enters an insulated pipe at Mach number 0·4 and leaves at M 0·6. What portion of t.H.e duct length in percentage is required for the flow to occur at
M 0·5? Take Fanno line parameters as
M fl/D
0·4 0·5770
0·5 0·2673
0· 1227
The moist air at 25 total pressure of 100 kPa and 50% RH is compressed to 50 total pressure of 300 kPa and then cooled at constant pressure. At what temperature will Jater begin to condense? Show the process on psychrometric chart. 8
Why is there a need of governing of steam turbines? With the help of a simple sketch, discuss thk working principle of hydromechanical speed-governing loopfor steam turbine.I
With the help of a bketch, discuss the working principle of temperature control in steam boiler. I ·
6. A simple R-12 pIJnt is to develop 5 tonnes of refrigeration. The condenser and evaporator t'emperatures are to be 40 °C and O°C Determine the following
Refrigerant nd1w rate
Volume flow rate handled by the compressor m m 3 s I
Compressor discharge temperature if the enthalpy of refrigerant compressor e..it is 209-4 1 kJ /kg
Pressure ratio
Heat rejected fo the condenser in kW
Flash gas percentage after throttling
COP
(viii} Power required to drive the compressor respectively. at
Saturated values of properties of R-12
<img src='./qimages/1279-6a.jpg'>
How does absorption refrigeration system differ from vapour compression
system? 10
What are the different types of compressors used in vapour compression plants? What are their limitations? 10
7. The following data relate to a small gas axial
Turbine inlet temperature, Toi 1100 K
Turbine inlet pressure, Po1 4 bars Pressure ratio (stage) Po1/po3 =1·873
Blade mean speed 340 s
Mean radius of rotating blade ring 0·216 m
Blade height 0·0612 m
For gas, take C=1-148 kJ/kg/K and y 1·333
Assuming isentropic efficiency of stage (total to total) as 0·9, flow coefficient as 0·8 and flow enters the jet nozzle without swirl from turbine, calculate direction of absolute and relative velocity vector at entry and exit of rotor with axial direction and loading coefficient and degree of reaction. 20
Mentioning the requirements of pulverized coal burners, explain the types of burners used for efficient utilization of different kinds of coal. 10
In context of a power plant, explain the following 10
Load curve
Load duration curve
Load factor
Plant factor
Reserve factor
Explain the working I principle of a supercritical boiler with the help of a diagram. What are tie advantages of a supercritical boiler? 20
What are the variou.. locations of ID and FD fans for producing draught? Discuss them. 10
Diesel plants are used as standby units in a grid system. Explain why. A power station supplies the following loads to consumers
Time I 0-6 6-9 9-12 12-16 16-20 20-22 22-24
Load I 20 50 60 50 80 70 40
Find the load fa..tor of the plant.
What is the load factor of a standby plant of 30 MW capacity, if it takes up all the loads above 50 MW? 10