c.S. main 2009

MECHANICAL ENGINEERING
Paper II
Time Allowed Three Hours Maximum Marks 300

INSTRUCTIONS

Each question is printed both in Hindi and in English.

Answers must be written in the medium specified in the Admi.ssion Certificate issued to you, which must be stated clearly on the cover of the answer-book in the space provided for the purpose. No marks will be given for the answers written in a medium other than that specified in the Ad1ni.ssion Certificate.

Candidates should attempt Questions no. 1 and 5 which are compulsory, and any three of the remaining questions selecting at least one question from each Section.

The number of marks carried by each question is indicated at the end of the question.

If any data is considered insufficient, assume suitable value. Psychrometric chart is attached with this question paper. Wherever coordinate diagrams I graphs are to be drawn, these are to be plotted only on the answer book and not on separate graph sheets.

SECTION A

1. Answer any three of the following
Define 'Availability' with regard to a system. What is the other term by which this property is also referred to Also derive an expression for "A" (the availability) for a reversible cycle in which heat 1s withdrawn. The cycle works between temperatures T and T0 20

Describe briefly one-dimensional isentropic flow. Represent isentropic compression (diffusors) and expans1on (nozzles) processes 1n a Temperature Entropy coordinate diagram with nomenclature. 20

Draw the Meridional view of a single stage axial flow fan and a multistage axial ilow compressor. Describe an axial flow fan with its flow configuration. 20
The thermal conductivity of a hollow sphere of inside radius and outside radius is given <img src='./qimages/41-1d.jpg'> where, Ti= Inner surface temperature TO Outside surface temperature. Prove that The heat flow rate is given by <img src='./qimages/41-1d1.jpg'> Also determine the heat loss from a spherical shell whose Di 2·5 m and covered with 30 cm of insulation. The thermal conductivity of insulation is 0·3 W/mK and 0·2 W/m.K at inner and outer surface temperatures of 150° C and 15° C respectively. 20

2. Derive equations for the change in internal energy and entropy of a gas which obeys the van der Waals equation of state. 15

Define the Joule Thomson coefficient and prove that for an ideal gas, the value of Joule Thomson coefficient tends to zero. 10

2 kg of air is first compressed from state 1 at 13·75 N/cm2 and 5° C to state 2 at 48 N/cm2 and 283° C. It is then throttled to state 3 until its pressure is again 13·75 N/cm2Finally it is cooled at constant pressure to state 4 until its volume becomes 50% of that before the cooling process. Determine the net change in entropy. 0·291 N.m/g.K; CP 1·004 kJ/kg.K) 15

Write a short note on Redlich Kwong equatior of state. 15

3. Derive an expression for the Mach number after a normal shock wave occurring in a nozzle. Show the trend of this Mach number, (in the form of an x-y plot) with respect to the Mach number value before the shock. 15

Air enters a diffuser with a velocity of 250 mis and a temperature of 30° C. It leaves with a velocity of 90 mis. Neglecting friction and heat transfer determine exit temperature exit pressure if the inlet pressure is 125 kPa and area ratio between the exit and entrance. Will your answers change if there is friction present? Explain how. 30

For a circular tube, explain with the help of neat sketches Hydrodynamic entry region and hydrodynamically developed flows. Thermal entry region and thermally developed flows. 15

4. An oil is cooled to 100° C in a parallel flow heat exchanger by transferring its heat to cooling water, that leaves the exchanger at 30° C. However, it is now required that the oil must be cooled down to 75° C by increasing the length of heat exchanger, while oil and water flow rates, their inlet temperatures and other dimensions of the exchanger keeping constant. The inlet temperatures of water and oil are 15° C and 150° C respectively. If the original cooler was 1 metre long, determine outlet temperature of water in the ne,v cooler and length of the new cooler. 30

A hot plate of 15 cm2 area maintained at a temperature of 200° C is exposed to still air at 30° C temperature. When the smaller side of the plate is held vertical, convective heat transfer rate is 14% higher than when the bigger side of the plate is held vertical. Determine the dimensions of the plate. Neglect internal temperature gradient of the plate thickness. Also determine the heat transfer rates in both the cases. <img src='./qimages/41-4b.jpg'> 30


SECTIONB

5. Answer any three of the following

Draw a schematic of a pass-out turbine and explain its working. Represent the relevant process on Enthalpy- Entropy coordinates. 20

How is the heat balance of a CI engine often diagrammatically represented by means of the following? Pie chart Bar chart Sankey diagram Graph with le-axis as power output in percentage and y-axis as total heat input in percentage. 20

In what way is Velox boiler different from La Mont boiler Describe the working of the Velox boiler with a schematic. 20

What are the components to be considered for estimating cooling load, heating load for an air-conditioning system How do you calculate heat gain through ducts for an air-conditioning system 20

6. i Briefly explain 'Evaporative Cooling System' which 1s generally used for big-capacity stationary IC engmes, with a schematic diagram. List four advantages and disadvantages each of a water-cooled system in a CI engine. 20

Draw· a schematic of a Benson boiler and explain its working principle, pointing out its speciality. 20

Explain supersaturated flow in steam nozzles with the help of skeleton Mollier diagram inserting nomenclatures like dry saturated line, Wilson line and supersaturated zone. List also the five effects of supersaturation in the steam nozzles. 20

7. With the help of schematic and T s diagrams explain a reheating Rankine cycle. State its advantages over ordinary Rankine cycle and define the reheat factor. 20

Draw a schematic diagram of bubbling bed fluidized boiler and explain its working. 15

The fo1lowing observations refer to a surface condenser Mass flow rate of condensate 20 kg/min Mass flow rate of cooling water 800 kg/min Mean temperature of condensation 35° C Condenser vacuum 0-95 kg/cm2 Barometer reading 1·03 kg/cm2 Inlet cooling water temperature 20° C Outlet cooling water temperature 30° C Temperature of the hot well 29° C Calculate Weight of air per unit volume of condenser. Entering condition of steam to the condenser. Vacuum efficiency of the condenser. <img src='./qimages/41-7c.jpg'> 25

8. The outdoor summer condition for a Bank for one hundred persons is Tdb 310 and Twb 300 K The required inside conditions are T db 295 K and 60o/o. The room sensible heat is 4,00,000 kJ/hr. The room latent heat is 2,00,000 kJ/hr. Ventilation requirement per person is 0·0047 m3/hr. The By-pass factor is 0· 1 5 . Evaluate Grand total heat Effective sensible heat factor (ESHF) Apparatus dew point Volume flow rate of dehumidified air 40

20 m3 of air per mjnute at 30° C DBT and 60% RH is sensibly cooled to 22° C DBT. Take saturation pressure of water vapour at 30° C and 22° C temperatures to be 0·425 bar and 0·0265 bar respectively. Find heat removed from air. Take atmospheric air pressure pb 1 bar. 20