UNIVERSITI TEKNOLOGI MALAYSIA FACULTY OF SCIENCE

FINAL EXAMINATION SEMESTER I SESSION 2010/2011

SUBJECT CODE : SSP1123

SUBJECT THERMODYNAMICS

LECTURER DR. WAN NURULHUDA WAN SHAMSURI

COURSE 4SPM, 4SPF, 4SPP, 3SSF

DATE 3 DECEMBER 2010

TIME 2 hours 30 minutes

INSTRUCTION TO STUDENTS:

THIS EXAMINATION SCRIPT CONSISTS OF EIGHT(8) QUESTIONS.

STUDENTS ARE REQUIRED TO ANSWER ANY FIVE (5) QUESTIONS.

BEGIN ANSWERING EACH QUESTION ON A NEW PAGE IN THE ANSWER BOOK.

(THIS EXAMINATION SCRIPT CONSISTS OF 6 PRINTED PAGES)

SSP 1123

Answer ONLY any five (5) questions. If you answer more than five (5), only the first five (5) answers will be marked.

1. (a) Define the term 'quality X' in thermodynamics. In what region does quality X

significant?

(4 marks)

(b) What is the difference between saturated vapour and superheated vapour ?

(2 marks)

Table -1

40.201P T specific volume Internal energy Enthalpy(kPa) (°C) (m3 / k g ) (Kj/ kg) (Kj/kg)

u f u s h f h950 177.70 0.001124 0.2041 752.03 2581.3 753.10 2775.71000 179.92 0.001127 0.1944 761.75 2583.3 762.88 2775.71100 184.10 0.001133 0.1775 780.14 2586.0 781.38 2775.71200 188.00 0.001138 0.1633 797.31 2588.4 798.68 2775.7

(c) Table -1 shows data properties of water.

(i) Find the latent heat of vaporization for water at 950 kPa (4 marks)

(ii) Find the enthalpy, internal energy and volume of 3kg of steam at 1100 kPa

and quality 0.75 (10 marks)

2. (a) State the Van der waals equation of state of gas. Explain why it is considered better

than the ideal gas equation. (5 marks)

(b) Using the ideal gas law, show that c p = c v + R . (5 marks)

(c) 1-kg of carbon dioxide is compressed from 1 MPa and 200°C to 3 MPa in a piston -

cylinder device arranged to execute a polytropic process for which PV12 = constant.

Determine the final temperature treating the carbon dioxide as an ideal gas.

(Take the gas constant R = 0.1889 kPa.mi / k g . K )

(10 marks)

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SSP 1123

3. (a) What does the area under the process curve represent on a

(i) P- V diagram

(ii) T -s diagram

( b) P- V diagram for thermodynamic system is shown in Figure -1

P (MPa)

0.5

0.1

Figure -1

0.05 0.30

Process 1-2 is adiabatic and energy change is -50kJ;

Process 2-3 is at constant pressure;

Process 3-1 is at constant volume.

(a) Find the work done by each stage

(b) Find the work of the system for the complete cycle

(c) Find the heat interaction for the complete cycle

(2 marks)

(2marks)

a

ri

V (m3)

(6marks)

(5marks)

(5marks)

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SSP 1123

4. ( a ) A heat engine operates between two reservoirs, one at 1000°C and the other at

300°C. For every heat interaction (lkJ) in the high temperature reservoir it rejects 0.6

Kj to the low temperature reservoir.

(i) Draw the schematic diagram of this heat engine and label accordingly

(2 marks)

( i i ) Calculate the thermal efficiency of this heat engine

(2 marks)

( i i i ) Calculate the Carnot efficiency of this heat engine.

(2 marks)

( i v ) Hence determine whether this heat engine is reversible, irreversible or

impossible

(2 marks)

( v) If it rejects 0.3 kJ instead to the low temperature reservoir, is the heat

engine reversible, irreversible or impossible?

(2 marks)

( b ) A refrigerator extracts 291 kW from a cooled space at 253 K, while the ambient

temperature is 293 K. Find,

(i) the maximum COP (3 marks)

( i i ) the minimum power consumption (3 marks)

(iii )the heat interaction in the air cooler (hot source) (2 marks)

( c ) An inventor claims to have developed a heat engine that receives 750 kj of heat from

a source at 400 K and produces 250 kJ of net work while rejecting the waste heat to

a sink at 300 K. Is this a reasonable claim?

(2 marks)

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SSP 1123

5. (a) Draw the PV diagram of a Carnot cycle, label accordingly and indicate the heat in

(Qin) and heat out (Qout) of a thermodynamic system.

(4marks)

(b) (i) What is the Kelvin - Planck statement of the second law of thermodynamics?

(3marks)

( i i ) In the absence of any friction and other irreversibilities, can a heat engine

have an efficiency of 100 percent? Explain.

(4marks)

(c ) A commercial heat pump removes 10,000 kJ/min from the sink at -5°C, rejects

15,090kJ/h to the source at 60°C and requires 1.5kW of power.

(i) Draw a schematic diagram of this heat pump and label accordingly.

(3marks)

( i i ) Determine the heat pump's coefficient performance

(3marks)

( i i i ) What is the Carnot coefficient of performance of this heat pump?

(3marks)

6. ( a ) State two differences between an engine running using an Otto cycle and a Diesel

cycle. (5marks)

( b ) Draw a P-V diagram for an Otto cycle, label accordingly and show the heat addition

(Qin) and heat rejection process (QoUt) processes.

(5marks)

( c ) An ideal Otto cycle has a compression ratio of 12, takes in air at lOOkPa and 20°C.

Given that specific heat at constant pressure, c p = 1.005fc/1 kg.K and specific

heat constant volume, cv = 0.718fc71 kg.K, determine the thermal efficiency of

this cycle and the rate of heat input if the cycle is to produce 200kW of power.

(lOmarks)

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SSP 1123

7. (a) State the second - law efficiency of a heat engine, r j n l (4marks)

(b) Define exergy in thermodynamics. If 3 kJ enters a system as heat at 200°C, how much

exergy could be obtained with respect to a reservoir at 27°C? (4marks)

(c ) A freezer is maintained at 7°C by removing heat from it at a rate of 80 kJ/min. The:

power input to the freezer is 0.5kW, and the surrounding air is at 25°C. Determine :

(i) the reversible power (4marks)

( i i ) the irreversibility, and (4marks)

( i i i ) the second law - efficiency of this freezer (4marks)

8. ( a ) A system is a 0.5kg ice at 0°C and atmospheric pressure. The system is heated at

constant pressure to 393°C. Calculate the entropy change of this system.

(Given that the latent heat of fusion of water, Lf = 33.7kJ/kg;

latent heat of vaporization of water, Lv= 2,257 kJ/kg; specific heat capacity of

water at constant pressure c p = \ .%12?>kJ I kg.K) (lOmarks)

( b ) Using the equation of state P(V -a) = RT, verify

(i) the cyclic relation, and ( 6marks)

(ii )the reciprocity relation at constant V. (4marks)

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