MECHAHICAL ENGINGGIeNG
PAPER 2 2002


Answer any THREE of the followmg parts (Answer to each part should not exceed 200 words) 20x3=60 A revem ble heat engme absorb s heat energy 110m a stream 0f hot gases whose temp erature lS falling linearly 110m to T,. The engme ects heat to a constant temperature sink at To Assummg that any reversible cycle can be conSidered to be eqUIvalent to an mfinite number ofCamot prove that the mffil1mum efficiency attamable for this heat enllne IS



A nlld msulated tank of 3 m3 volume lS diVided mto 2 compartments. One compartment of volume 1 m3 contams an Ideal gas at 0.1 MPa and 300 K while the second compartment of volume 2 m3 contams the same gas at 1 MPa and 1000 K. If the partition between the two compartments lS ruptured, calculate the final temperature and pressure 0f the gas

The actual amount of air supplied per kg of a fuel can be ascertained 110m the measured composition by volume of combustion products. Describe an apparatus to do the needful


Discuss why a carburellor lS bemg replaced by an Injection system m S.I. enllne these days A mass of air mitially at 760 KPa and 250°C occupies 0.026 m3 The air lS expanded at constant pressure to 0.07 m3 A polytrophic process with n I 52 lS then camed out followed


by an lSothennal process and the cycle lS thus completed, Assummg all the processes to be reversibl

show all the processes on pV and Ts planes


compute the heat received and rejected m the cycle


cal culate the efficiency 0f the cycle
Define availability E..lain the IIfference between useful work and the maXimum uselll work done m the context of availability of a closed system. Heat flows through a wall at the rate of 3 x kJih. The temperature of the two faces of the walllS 327°C and 207°C If the surroUllllngs are at 270°C, whatlS the loss m available ener..?

The exhaust gas analySiS of an enllne usmg Octane as fuel gives equal volumes of CO, and unused ..
Compute the actual and stOichiometric atr-fuel ratios
For eqUIvalence ratio of calculate


the volume of ffilxture per kg of fuel at pressure of I 00 kPa and temperature 70°C


the volume of the products of combustion per kg of fuel when the temperature of products of combustion lS 127°C at pressure I bar


"


Explain Cetane rating of a fuel. Discuss ItS Importance for the selection of fuels Justify the range 0f Cetane numbers of commercial lIesel fuels


Define scavengmg efficiency, trapping effiaency and scavenge factor m two-stroke I.C engmes. Develop a relati onship between them


"

A4-stroke petrol engme of 2 liters capaaty lS to develop maxlmum power at 4000 rpm The volumetric efficiency at this speed lS 0.75 and the airluel ratio lS 14 1 The ventun throat diameter lS 28mm The coefficient of discharge of venturi lS 0.85 and that for fuel JetlS 0.65 Calculate

the air velo a ty at the throat and


the diameter of the fuelJet


The specific gravity ofpetrollS 0.76. Atmosphenc pressure and temperature are 1 bar and 17°C respectively
SElmON B
AnsweranyTHREE0fthe followmgparts (Answerto eachpartshouldnotexceed 200words) 20x3=60

Show the folloWlng processes on the psychromethc chart


Heating andhmmdification


Cooling and dehmmdification


Co oling and humldi fication

Heating and delullmdification


Adiabatic saturation




HeatlS transferred along the axlS of a truncated corucal cylinder of length radius at the shorter end and radius CO at the bigger end. The circumference of the cylinder lS completely msulated. Develop an expreSSion to calculate heat transfer along the axlS of the cylinder Assume no vanation 0 f conductivity Wl th temperature


A thennal power plant of 200 MW capacity has the maXimum load of 160 MW and ItS annual

load factor lS 0 65 The coal consumption lS kg per kWh of energy generated and the cost of COallS Rs. 800 per ton. Other annual runrung expenses are Rs. 200 x Calculate the arulUal revenue earned If the energy lS sold at Rs. 1 5 per kWh and the capacity factor of the plant


A forced draught fan discharges 1200 m3 of air per nunute through the outlet of area and maintains a steady pressure of 110 nun of water The temperature of air lS 27°C. Calculate the power of the motor to dnve the fan If the efficiency of the fan lS 85%. Take the density of air at 27°C as 125kg/m3


A sphencalthennocouple of 2.5 nun diameter lS used to measure the temperature of air flowmg m a pipe. 1rutially both the thennocouple and the air are at a temperature of 30°C The air lS heated to a temp erature 0 f 235°C and maintained at this temp erature. Find the time reqUired for the thennocouple to reach 200°C. Also find out the time constant of the thermocouple and conunent on the suitability of this thennocouple to measure unsteady state temp erature. For thenno couple matenal take density 9000 kg/m3, sp eafi cheat 0.4 kJ I kg K and thenna! conductivity 30 W/m K

Convective heat transfer coefficient between thermocouple surface and the air lS 120


Air at 30°C flows over a honzonta! plate heated to 70°C at a speed of 3 m/s. Calculate the convective heat transfer co effi aent and the rate 0 f heat transfer between the pI ate and the air The length of the plate lS 2 m and Width 1 m Nusselt numb er lS given by


Nu 0 664 Re lQ Prw
The prop erties 0f alr at 50°C are
17.95 x 10.6
K 0.0283
p 1 093 kgim3,
Cp 1005 kJikgK



A vapour compressiOn refrigerator works between pressure limits of 10 bar and 3 bar The working flUid lS dly at the end of compreSSiOn and there lS no undercooking before the expanSion valve.1frefrigerant flow rate lS 10 kg/mm, determme COP and the capaaty ofthe refrigerant Table for prop erties of the refrigerantlS as under


DlScuss the dlfference between natural arculatiOn, forced clrculahon and one-through


Pressure b. " Saturation temp erature °C " LiqUid heat kJikg 298.90 Latent heat kJikg 1166.94 LiqUld entropy kJikg K 1 1242
" 135.37 1297.68 o 5443

boilers. Explain, Wlth the help of a sketch, the wor16n of a La Mont boller
Steam expands m a steam turbine lSentroplcally from lnlet to exhaust havmg an enthalpy drop 1200 kJ/kg. Assumlng ldeal conditions, delenmne the mean eli ameter ofthe wheellf the turbme were of

smgle lmpulse stage,


smgle 50% reaction stage,
one two-row Curtis stage and



ten 50% reaction stages


Take the nozzle angle as 18 and blade sp eed as 4000 rpm
An aXial flow compressor of50% reaction blading has lSentroplc efficlency of82% It draws alr at 17°C and compresses m the pressure ratio of 4 1 The mean blade speed and flow velocity are constant throughout the compressor. The mlet and outlet angles of blades are 15° and45°respectively(anglesmeasuredfrom ax::1aldirection).Blade speed=180wsandwork mput factor 0.84 Calculate flow velocity and numb er 0f stages