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THE OPEN UNIVERSITY OF SRI LANKA

DEPARTMENT OF MECHANICAL ENGINEERING

DIPLOMA IN TECHNOLOGY

MEX3235 –THERMO-FLUIDS

ACADEMIC YEAR 2014/2015

ASSIGNMENT 01

INSTRUCTIONS

(1) This assignment is based on the course material in Book 1 & 2

(2) Answer all questions.

(3) Write your registration number clearly on your answer script

(4) Submit your assignments as stipulated in the specimen format for the tutor marked

assignment.

(5) Send your answer scripts under registered cover to the address given below or put it into

the assignment box available in front of the Mechanical Engineering research lab,

science and technology building.

THE COURSE COODINATOR—MEX3235

DEPARTMENT OF MECHANICAL ENGINEERING

THE OPEN UNIVERSITY OF SRI LANKA

P.O. BOX 21, NAWALA, NUGEGODA.

(6) Last date of submission 06th

February 2015. Strictly no late submissions will be

accepted due to any reason. .

1. Air flows steadily at the rate of 0.4 kg/s through an air compressor, entering

at 6m/s with the pressure of 1 bar and a specific volume of 0.85 m3/kg, and

leaving at .5 m/s with the pressure of 6.9 bar and a specific volume of 0.16

m3/ kg. The internal energy of the air leaving is 88 kJ/kg greater than that of

the air entering. Cooling water in a jacket surrounding the cylinder absorbes

heat from the air at the rate of 59 kJ/s. Calculate

a) The power required to drive the compressor

b) The inlet and outlet pipe cross sectional areas

2. Air at 1.02 bar, 22oC, initially occupying a cylinder volume of 0.015m

3, is compressed

reversibly and adiabatically by a piston to a pressure of 6.8 bar. Calculate the final

temperature, the final volume, and the work done on the mass of air cylinder. ( ᵞ for air =

1.4)

2

3. A mass m of water at temperature T1 is isobarically and adiabatically mixed with an

equal mass of water at temperature T2. Show that the entropy change of the mixture is

given by

[

√ ]

Show that the change in entropy of one kg of gas when compressed according to the law

pVn =C in terms of temperatures T1 and T2 before and after compression respectively is

given by

[

]

( )

The value of specific heat is of the form

and where a,b, and k are constants.

4. An engine working on Otto cycle has a volume of 0.45m3, pressure 1 bar and

temperature 30 ºC at the beginning of compression stroke. At the end of compression

stroke the pressure is 11bar. 210 kJ of heat is added at constant volume. Determine

a. Pressures temperatures and volumes at salient points in the cycle.

b. Percentage clearance

c. Thermal efficiency

d. Mean Effective pressure

5. A 120mm radius steam flowing tube is covered with 50mm of high temperature

insulation (k=0.092W/m.K) and 40mm of low temperature insulation (k=0.062W/m.K).

The inner and outer surface temperatures are 390°C and 40°C respectively. Calculate the

total heat loss per unit length of the steam pipe.