midterm test sem thermodynamics 1

COLLEGE OF ENGINEERINGPUTRAJAYA CAMPUS

MIDTERM TEST

SEMESTER 1 2010 / 2011

PROGRAMME : Bachelor of Mechanical Engineering (Honours)

SUBJECT CODE : MEHB213

SUBJECT : Thermodynamics I

DATE : 25 August 2010

TIME : 8.30 pm – 10.00 pm (1½ hour)

INSTRUCTIONS TO CANDIDATES:

1. DO NOT open the question paper until you are allowed to do so.

2. This paper contains FOUR (4) questions in NINE (9) pages.

3. Answer ALL questions.

4. Write all answers and their solutions in this booklet.

5. The ‘Thermodynamics Property Tables and Formulae’ booklet is provided.Students are not allowed to write anything in this booklet.

THIS QUESTION PAPER CONSISTS OF 9 PRINTED PAGES INCLUDING THISCOVER PAGE.

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QUESTION 1 [15 marks]

(a) Define and explain the state postulate. [3 marks]

(b) A piston-cylinder device contains 0.1 m3 of liquid water and 0.9 m3 of water vapor in

equilibrium at 800 kPa. Heat is transferred at constant pressure until the temperature

reaches 350C. Determine:

(i) The initial temperature of the water (in C)? [2 marks]

(ii) The total mass of the water (in kg), [3 marks]

(iii) The final volume (in m3), [2 marks]

(iv) The initial quality of the water, [2 marks]

(v) Show the process on a P-v diagram with respect to saturation lines. [3 marks]


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A mass of 12 kg of saturated refrigerant-134a vapor is contained in a piston-cylinder device

at 240 kPa. Now 300 kJ of heat is supplied to the refrigerant at constant pressure while a 110-

V source supplies current to a resistor within the cylinder for 6 min. If the final temperature

of the refrigerant is 70C,

(a) Write the full energy balance equation of this system, [4 marks]

(b) Find the final phase of the refrigerant-134a, [3 marks]

(c) Evaluate the total enthalpy change (in kJ), [2 marks]

(d) Determine the current supplied (in A), [Hint: 1 V 1 J/C] [3 marks]

(e) Show the process on T-v diagram with respect to the saturation lines. [3 marks]

Assume the cylinder is stationary and the kinetic and potential energies are negligible.

We

Qin

R-134a


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Air enters an insulated diffuser operating at steady state with a pressure of 1 bar, a

temperature of 27C and a velocity of 250 m/s. At the exit, the pressure is 1.13 bar and the

velocity is 140 m/s. Potential energy effects can be neglected.

(a) Write the appropriate energy equation for the diffuser. [6 marks]

(b) Determine the exit temperature (in K), [4 marks]

(c) Calculate the ratio of the exit flow area to the inlet flow area. [5 marks]

Use Table A-17 for the ideal gas properties of air.


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(a) How is mass and volume flow rates related? [3 marks]

(b) An air conditioning system involves mixing of the cold air and warm outdoor air before

the mixture is routed to the conditioned room in steady operation. Cold air enters the

mixing chamber at 7C and 100 kPa at a rate of 1.25 m3/s, while warm air enters at 32C

and 100 kPa. The ratio of the mass flow rates of the hot to cold air streams is 1.6.

Determine:

(i) The mixture temperature at the inlet of the room (in C), [6 marks]

(ii) The mass flow rate at the inlet of the room (in kg/s). [6 marks]

Use Table A-17 for the ideal gas properties of air.


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-END OF QUESTION PAPER-


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