IFS 2011 - II Question Paper

8/2/2019 IFS 2011 - II Question Paper

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SI. No. 171 D-VSF-L-ZRB

MECHANICAL ENGINEERINGPaper-II

I Time Allowed : Three Hours I I Maximum Marks 200 IINSTRUCTIONS

Candidates should attempt Question Nos. 1 and 5which are compulsory, and any THREE of theremaining questions, selecting at least ONE questionfrom each Section.All questions carry equal marks.

Marks allotted to parts of a question areindicated against each.Answers must be written in ENGLISH only.If any data is considered insufficient, assumesuitable value and indicate the same clearly.

Newton may be converted to kgf using the equality1 kilonewton (1 kN) = 100 kgf, if found necessary.All answers should be in 81 units.

Take 1 kcal = 4'187 kJ and 1kg/cm2 = 098 bar1bar = 105 pascals.

Universal gas constant = 8314,6 J/kmol-K.Neat sketches may be drawn, wherever required.. -The attached psychrometric chart is to be used foranswering Question No. 6(b). Then the psychrometricchart may be detached from the' question paper andattached firmly t~ your answer book.

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Section-A

1. Answer any four parts:(a J An imaginary engine receives heat and

does work on a slowly moving piston atsuch rates that the cycle of operation of1kg of working fluid can be representedas a circle 10 em In diameter on ap-v diagram on which 1em = 300 kPaand 1em=0.1m3 / kg.(i) Determine how much work is done

by each kg of working fluid foreach cycle of operation.

(ii) If the heat rejected by the enginein a cycle is 1000 kJ per kg ofworking fluid. find the thermalefficiency of the engine. 10

(b) Explain the principle of carburetion.Develop an expression for air-fuelratio for a simple carburetor takingcompressibility into account. 10

(c ) Explain the vanous mechanisms oflubrication bringing out their functions.What are the various desirable propertiesof lubricants? Explain how additiveshelp to achieve these. 10

(d) If a fin is thin and long and tiplosses are negligible, show that theheat transfer from the fin is given by

00 =mkA60 tanhml1where m = (hP / JcA)2.

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(e) For a balanced heat exchanger (R = 1),show that for counterflow arrangement

NTUE=---NTU+l 10

2. (a) An ideal gas cycle consists of threereversible processes m the followingsequence-(i) constant volume pressurerise, (ii) isentropic expansion to r timesthe initial volume and (iii) constantpressure decrease In volume. Sketchthe cycle on the p-u and T-s diagrams.Show that the efficiency of the cycle is

r Y - 1 - Y (r - 1 )llcycle = y 1r -

Evaluate the cycle efficiency) when y =43and r = 8. 15

(b) The following expressions for theequations of state and the specific heatCp are obeyed by a certain gas :

RT 2v=-+((T and Cp =A+BT+Cppwhere (X., A, B, C are constants. Obtainan expression for (i) the Joule-Thomsoncoefficient and (ii) the specific heat Cu' 15

(e) Show that the efficiency of a reversibleengine is independent of the nature oramount of the working substance goingthrough the cycle. 10

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3. fa) A petrol engine with a compressionratio of 7 uses a mixture of isooctaneand hexane as fuel. The pres sure andtemperature at the beginning of thecompression process are I bar and5522 C respectively. If the fuel-airmixture IS 1905% rich and themaximum pressure developed IS11526 bar, evaluate the compositionof the mixture (in percentageweight). Take, Cv = 0717 kJjkg-K,(CV)hexa~e = 43 MJ jkg, (CV) isooctane =42 MJjkg and PVI.31 1S constant forthe expansion and compressionprocesses.

(b) (i) Explain the stages of combustionin SI and CI engines.

(ii) Discuss the phenomenon of knockin SI and CI engines.

(iii) Discuss the important qualities ofSI and CI engine fuels. How arethese fuels rated? 7+7+6=20

20

4. (a) What do you mean by radiation shield?Where is it used?Two parallel discs of 1m diameter aresituated 2 m apart in the surroundingsat a temperature of 20 "C. One side ofa disc has an emissivity of 05 and ismaintained at 500C by electricalresistance heating and the other side isinsulated. The other disc is open toradiation on both sides. Determine theequilibrium temperature of the second

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disc and the heat flow rate from thefirst disc. Take, F12 =0,06 (for discs).Discuss the effect on the solution if bothsides of the second disc are perfectmirrors. 3+17=20

(b) (i) Show that for laminar flow of air(Pr = 0'714), the local and averagevalues of Nusselt number fornatural convection heat transferfrom or to a vertical plate aregiven by Nux =0378 Gr1/4 orNUL =0504 Grl/4,

(ii) A 15 em outer diameter steelpipe lies 2 m vertically and 8 mhorizontally in a large room withan ambient temperature of 30 "C.If,the pipe surface is at 250 "Candthe emissivity of the steel is 0'60,calculate the total rate of heat lossfrom the pipe to the atmosphere.Properties of air at 140C arep=O854kg/m 3, Cp =101kJ/kg-K,k =0035 W/m-K, Pr = 0684 andv ;: 278 x 10-6 m 2/ s. For verticalpart, use the formula

Nu =0 13 (Gr Pr)1/3and for horizontal part

Nu =O53 (Grd Pr)1/415+5=20

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Section-B

5. Answer any four parts:(a) With the help of a schematic diagram,

describe the working of a 'Bensonboiler'. What are its advantages? 10(b) How can the solar energy be used to

obtain refrigeration effect? Explain witha neat and clear sketch. 10

(e) Differentiate between air-conditioningand air cooling. Whether householdair-conditioners are properly named?Justify your answer. Show how airwasher can be used for year-roundair-conditioning. 10

(d) Prove that in a multistage turbinel1 t = 11s x RF

where llt = overall turbine efficiency,11s = small-stage efficiency and RF =reheat factor. 10

(e) What is the pressure coefficient of acentrifugal compressor? Derive

'V p =1- t P 2 cosP 2where < 1 > 2 ; : flow coefficien t. 10

6. fa) fi l Which of the two factors (increasein the upper pressure limit anddecrease in the lower pressurelimit) has more adverse effect onCOP of a refrigeration cycle?

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(ii) Show the variation of capacityfactor with varying evaporatortemperature In the vapour-compression system of refrigera-tion, the condenser pressureremaining the same.

(iii) Show the actual vapour-compression cycle on a p-hdiagram and explain the variousirreversibili ties with the help ofT-s diagram.

(iv) What is the difference betweenmultistage refrigeration andcascade refrigeration? 4+4+8+4=20

(b) (i) Define Effective Sensible HeatFactor (ESHF). How can ADP bedetermined with its help?The following data refer to a publichall:Total sensible heat = 105 kJ/hTotal latent heat = 60000 kJ jh'The room design conditionTdb= 27 C and I P ; ; ; ; 60% .The temperature rise of air (i.e.,difference between the roomdesign condition and supply airto" the room) is 8 "C.

Find the SHF, show the processon the psychrometric chart andalso calculate the volume of airsupplied to room using approxi-mate expression based on sensibleheat and enthalpy drop.

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(ii) To cu t cooling loads to minimizethe size of the air-conditioningplant, the energy conservationmeasures can help a lot. List suchmeasures and discuss each inbrief. Draw the schematic arrange-ment of a year-round air-conditioning system and explainits working with the help ofpsychrometric chart. 10+10::::20

7. (a) What is a Fanno line and a Rayleighline? Why do the end states of a normalshock lie on the Fanno line andRayleigh line? Show these lines on ah-s diagram for various conditions. Givethe physical meaning of this. 10

(b) A nozzle is designed assumingisentropic flow with an exit Machnumber of 26. Air flows through it witha stagnation pressure and temperatureof 2 MPa and 150C respectively. Themass flow rate is 5 kg/sec.(i) Determine the exit pressure,

temperature, area and throat area.(ii) If back pressure at the nozzle

exit is raised to 135 MPa, and theflow remains isentropic except fora norrnal shock wave. determinethe exit Mach number andtemperature, and the mass flowrate through the nozzle. Assumefor the value of P / Po of 0675.M :: 085 and T / To ;::::845 forisentropic flow. 15

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fe) A centrifugal compressor with 20m3 ofair per second at 1 bar and 15C is tobe compressed through a pressureratio of 15. The compression followsthe law pv1.

5 = constant. The velocityof flow at inlet and outlet remainsconstant and is equal to 60 m/s. Theinlet and outlet impeller diameters are06m and 12 m respectively, and thespeed of rotation is 5000 r.p.m.(i) Find the blade angles at inlet and

outlet of the impeller, and theangle at which the air from theimpeller enters the casing.

(ii} Find the breadth of impeller bladeat inlet and outlet.

It may be assumed that diffuser is notfitted and the whole pressure increaseoccurs in the impeller and that theblades have negligible thickness.

(iii) Sketch the velocity triangles atinlet and outlet of the impeiler. 15

8. fa) Draw a neat sketch of a Heavy Water-cooled Reactor (HWR) or CANDU-typereactor power plant. Clearly show thevarious components. What are theadvantages and disadvantages of thistype of nuclear reactor over others? 10

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(b) The yearly duration curve of a certainpO'Yer plant can be considered astraight line from 30 MW to 10 MW. Tomeet this load, three turbine generatorunits, two rated at 15MWeach and oneat 75 MW. are installed. Evaluate-

( iJ installed capacity;{ii} plant factor;(iii) maximum demand;(iv) load factor;(v) u tiliza tion factor. IS

(c) A turbine model of 1: 10 develops184kWunder a head of 5m ofwater at480 r.p.m. Find the power developed bythe prototype under a head of 40 m.Also find the speed of the prototype.Assume efficiency of both the turbinesto be same. Find and verify the specificspeeds. 15

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IFS 2011 - II Question Paper