JPSC Assistant Engineer 2021: Assistant Engineer Written Exam, Advt. No. 08/2018 and 08/2019, MECHANICAL ENGINEERING PAPER-I SUBJECT-CODE-10
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AEUD
MECHANICAL ENGINEERING
PAPER-I
SUBJECT CODE : 10
Full Marks : 200 Time : 3 Hours
Iustruetions For Candidates :
Please read each Of the following iustructioris carefully before a[templing questions.
(1) Ariswers must be written in ENGLISH only.
(2) The question paper contains a total Of 8 question in TWO Sections : Section-A
and Section-B. qTOUR questions in each section)
(3) Question No.1 Of section `A' and Question No. 5 of section `B' are compulsory.
(4) Candidates have to attempt another TIIPEE qu.estiorT from remaining SEA
questions choosing atleast ONE question from each section.
(5) Assume suitable data if f round necessary.
(6) Wherever any assumption be made for answering a question, they must be
clearly indicated.
q) Marks carried by a question / section is indicated agalusl it.
(8) Unless struck of f attempt of question shall be counted even if attempted partly.
(9) Any page -or portion of the page lef i blankin the A:nswer Boohiet must be clearly
struck off
SECTION-A
Marks
1. (i) 2 kg of an ideal gas is compressed adiabatically from pressure 100 kpa and
temperature 220 k to a final pressure of 400 kpa. Calculate (a) initial volume
a) final volume and temperature (c) work performed and (d) change in internal
energy.TakeCp=1.0kJkgKandCN=0.707kJkgK.
(ii) h a gas turbine, the gases flow at a rate of 5 kg/s. The gases enter the turbine at
a pressure of 7 bar with a velocity of 120 in/s and leaves at a pressure of 2 bar
with a velocity of 250 in/s. The turbine is insulated. If the enthalpy of the gas at
inlet is 900 kJthg and at the outlet is 600 kJkg, determine the capacity of the
turbine.
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(.lil) Establish the Inequality of clausius.
(iv) Two firs of 0.01 in diameter and 0.1 in length are used to dissipate heat from a
surface at 230 °C. The surroundings is at 30 °C. The thermal conductivity of the
material is 20 W/ink. The convective coefficient is 30 W/m2k. To check
alternate arrangement; it is proposed to replace the fins by a single fin of the
total area of the two fins with length and other conditions being the same as
before. Compare the heat transferred and the tip temperatures.
(v) An air conditioning unit receives an air water vapor mixture at 101 kpA, 35 °C
and 80°/o relative humidity. Determine the following :
(a) thedewpoint
a) the humidityratio
(c) the partial pressure of air
(d) the mass fi.action of water vapour
(i) (a) h a steam power station steam flows steadily throuch a o.2 in diangter
pipe line from the boiler to the turbine. At the boiler end, the slcan
conditions are found to be p = 4 Mpa, t = 400 °C, h = 3213.6 kJ/kg and
V = 0.073 m3ckg. At the turbine end, the conditions are found to be P = 35
. Mpa, t = 392 °C, h = 3202.6 kJAIg and ~ = 0.084 m3Acg. There is a hal
loss of 8.5 kJn{g from the pipe line. Calculate the steam flow rate.
a) A reversible heat engine operates between two reservoirs at temperafllle of
600 °C and 40 °C. The engine draws a reversible reffigerator Which
operates between reservoirs at temperatures of 40 °C and -20 °C. The hal
transfer to the heat engine is 2000 kJ and the net work output of the
combined engine - refrigerator plant is 360 kJ. 12
a Evaluate the heat transfer to the reffigerant and the net heat rmsfer
to the reservoir at 40 °C.
® Reconsider (a) given that the efficiency of the beat endne and the
COP of the refrigerator are each 40% of their maximum possible
values.
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Marks
(ii) A spark igm.tion engive working on ideal Otto cycle has the compression ratio 6.
The initial pressure and temperature in the cycle are 1 bar and 37 °C respectively.
The maxinun pressure in the cycle is 30 bar. For unit mass flow, calculate 10
(a) p, v and T atvarious salientpoints of the cycleand
(b) the ratio of heat supplied to the heat rejected.
Assume y = 1.4 and R = 8.314 kJ/K mol K.
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(iii) With the help of neat sketches explain the stages of combustion in CI engines.
3. (i) For an engine working on the ideal dual cycle, the compression ratio is 10 and
the maximum pressure is limited to 70 bar. If the heat supplied is 1680 kJftg,
find the pressures and temperatures at the various salient points of the cycle and
cycle efficiency. The pressure and temperature of air at the commencement of
compression ratio are 1 bar and 100 °C respectively. 12
Assume Cp = 1.004 kJthg K and
Cv = 0.717 kJckg K for air.
(ii) A heat flue of 1500 W/m2 is incident on the surface ofa slab of 10 cm thick with
thermal conductivity of 7.5 W/inK The hot side is found to be at 120 °C. On the
other side, the heat is passed on to the surroundings at 30 °C by convection and
radiation. If radiation is ideal, detemine the convective coefficient and also the
share of heat flow between the two processes.
(iii) (a) Ametal plate ofl0 mmthick at30 °C is suddenly exposed on one face to a
heat flux of 3000 W/m2 and the other side is expose to convection to a
fluid at 30 °C with a convective heat transfer coefficient of 50 W/inK.
Determine the heat transfer coefficient of 50 W/m2 K. 12
Determine the temperature variation with time and also steady state
temperatures and the time to reach 1 °C less than the steady state
temperature on the hot face.
(b) Air at 200 °C flows over a plane at 120 °C. The air pressure is at 1.8 atm.
The free strealn velocity is 15 in/s. The plate measures 0.2 in & 0.4 in. The
air flow was designed to be in the direction of 0.4 in side. Due to some
misunderstanding the equipment was fabricated with flow in the 0.2 in
direction. Determine the change in performance.
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Marks
4. (i) A steam condenser condensing at 70 °C has to have a capacity of 100 kw. Water
at 20 °C is used and the outlet temperature is limited to 45 °C. If the overall heat
transfer co-efficient has a value of 3100 W/m2 K, determine the area required. If
the inlet water temperature is increased to 30° C detemine the increased flow
rate of water for maintaining the same water outlet temperature. Also determine
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the flow rate for the same amount of heat transfer.
(ii) (a) The atmospheric air at a pressure of 1 bar and temperature -5 °C is drawn in
thecylinderofacompressoroftheBeuColemanrefrigeraringmachine.Itis
compressed isentropically to a pressure of 5 bar. In this cooler, the
compressed air is cooled to a 15 °C, pressure remaining the same. It is then
expanded to a pressure of 1 bar in an expansion cylinder, from where it is
passed to the wed chamber.
Find 0 The work done per kg of air and ¢1) The COP of the plant Assume
lawofexpansionforairispV`.2=coustanclawforcompressionispV'4=
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constant. The specific heat of air is 1 kMcg K
a) Derive an expression for the coefficient of performance of a vapour
compression refrigeration system with sub-cooling. Indicate the process in
P-h and T-S diagrams.
(in) A vapour compression refrigerator works between the pressure linits of 60 bar
and 25 bar. The working fluid is dry and the end of compression there is no
under cooling of the liquid before the expansion valve.
Detemine (a) COP of the cycle and a>) Capacity of the refrgerator if the fluid
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flow is at the rate of 5 kg/in.
Saturation Specificenthalpy, Specific entropy,
PressureOar)
temperature,K kJng K kJng K
Liquid Vapour Liquid Vapour
295 61.9 208.1 0.197 0.7030.896
60
35 261 -18.4 234.5 ro.075
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Marks
SECTION -8
5. (i) A circularplate of diameter 1.2 metre is placed vertically in water in such away
that the centre of the plate is 2.5 in below the free surface of water. Determine
the total pressure and the position of the centre of pressure.
(ii) A 200 mm x 120 mm venturimeter is installed in a horizontal pipe carrying
water. If the mercury differential manometer shows a reading of 200 mm, find
the discharge through the pipe. Take co-efficient of discharge Cd = 0.98.
(iii) The rate of flow of water through a horizontal pipe is 250 litre/s. The diameter of
the pipe which is 200 mm is suddenly enlarged to 400 mm. The pressure
intensity in the smaller pipe is 150 kpa. Find
(a) loss of head a]) pressure intensity in the larger pipe (c) loss of power.
(iv) A supersonic nozzle is to be designed for air flow with Mach number 3 at the
exit section which is 250 mm in diameter. The pressure and temperature of air at
the nozzle exit are 8.5 kN/m2 and 215 K Make the calculation for (a) reservoir
pressure and temperature and a) throat area.
(v) A three stage single acting reciprocating compressor has a perfect inter cooling.
The pressure and temperature at the end of suction stroke in LP cylinder is 1.013
bar and 15 °C respectively. If 8.4 m3 of free air is delivered by the compressor at
70 bar per minute and the work done is minimum, calculate;
(a) Lp and IIf delivery pressures
a) Ratio of cylinder volumes and
(c) Total indicated power -Neglect clearance and assume n = 1.2
6. (i) DeriveEuler'sequationofmotion.
(ii) Derive an expression for a discharge through a convergent -divergent nozzle.
(iii) A 30 long pipeline connects two reservoirs, both of which are open to the
atmosphere. The difference in their water level is 12 in. The pipe has three equal
sections of 10 in each. The first and last sections are 60 mm in diameter and the
. intemediate section is 40 mm in diameter. The value `f' for the pipes is 0.0054.
Calculate the flow rate and draw the total energy and hydraulic grade lines. 10
(iv) A trapezoidsl channel has side slopes 3 horizontal to 4 vertical and slope of its
bed is 1 in 1500. Determine the most economical dimeusious of the channel, if it
carries water at 10 m3/s. Take C = 60. 10
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Marks
7. (i) Air flows through a convergent divergent nozzle. At some section in the nozzle
pressure is 2 bar, Veto;ity 170 in/s and temperature 200 °C, and the cross
sectional area is 1000 m2. Assuming the isentropic conditions determine
(a) Stagnation temperature and stagnation pressure a) Sonic velocity and Mach
number at this section (c) velocity, Mach number and flow rate at outlet Section
when pressure is 1.1 bar (d) pressure, temperature, velocity and flow area of
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throat of the nozzle.
Take R = 287 JAcg K, Cp = 1000 Jckg K and y = 1.4
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HillH Defroe Famo line and Raleigh line with required equntious and plots.
(iii) In a carfage of Impulse turbine, the nozzle angle is 20° with the plane of the
wheel. The mean diameter of the blade ring is 2.8 metres. It develops 55 kw at
2400 Ipm. Four nozzles each of 100 mm diameter expand isentropically from 15
bar and 250 °C to 0.5 bar. The axial thust is 3.5 N. Calculate; (a) Blade angle at
entrance and exit and, (ii) The power lost in blade friction.
8. (i) A closed vessel of 0.2 m3 contains steam at a pressure of 10 bar and a
temperature of 250 °C. The vessel is cooled till the pressure of steam in the
vessel fuls to 3.5 bar. Find the final temperature, final dryness fraction, change
in internal energy, heat transferred and change in entropy.
(ii) In a gas turbine the air is compressed in .a single stage compressor from 300 K.
The same air is then heated to a temperature of 800 K and then expanded in the
turbine. The air is then reheated to a temperature of 800 K and then expanded in
the second turbine. Find the maximum power that can be obtained from the
installation,ifthemassofairiscirculatedpersecond.
(iii) Define degree of reaction of axial flow compressor. Derive an expression for it. 10
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Source: Jharkhand Public Service Commission. Question paper published by Jharkhand Public Service Commission.