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Engineering Thermodynamics Ii Question Paper
Engineering Thermodynamics Ii
Course:Bachelor Of Science In Engineering
Institution: Kenyatta University question papers
Exam Year:2008
KENYATTA UNIVERSITY
UNIVERSITY EXAMINATIONS 2007/2008
SECOND SEMESTER EXAMINATION FOR THE DEGREE OF BACHELOR OF SCIENCE (ENERGY AND MANUFACTURING ENGINEERING)
SET 307: ENGINEERING THERMODYNAMICS II
DATE: TUESDAY, 29TH APRIL 2008 TIME: 8.00 A.M. – 10.00 A.M.
______________________________________________________________________
INSTRUCTIONS:
1. This paper consists of FIVE Questions.
2. Answer any THREE Questions.
3. All Questions carry equal marks and mark distribution for each question
is shown.
4. You should have the following:
- Scientific Calculator
- Thermodynamic and Transport properties of fluids by Y.R. Mayhew and G.F.C. Rodgers
QUESTION 1 (20 MARKS)
(a) (i) State two ways which a gas turbine resembles a diesel engine. (2 marks)
(ii) With the use of a line diagram and a T-S diagram, show that
INTERCOOLING improves the Work ratio of a gas turbine. (6 marks)
(b) A gas turbine unit takes in air at 17oC and 1.01 bar and the pressure ratio
is 8:1. The H.P. turbine drives the compressor while the L.P. turbine drives
the output shaft. The isentropic efficiencies of the compressor, H.P. and
L.P. turbines are 0.8, 0.85 and 0.83 respectively. The maximum cycle
temperature is 620oC.
Using the given data, calculate the pressure and
temperature of the gases entering the power turbine, the net power
developed by the unit per Kg/s mass flow, the work ratio and the thermal
efficiency. Neglect the mass of fuel.
Cp = 1.005 KJ/Kg K and ? = 1.4 for the compression process
Cp = 1.15 KJ/Kg K and ? = 1.333 for the combustion and expansion process.
(12 marks)
QUESTION 2 (20 MARKS)
(a) State two main reasons which make the Carnot Cycle unsuitable for steam
plant operation. (2 marks)
(b) Draw a labelled diagram showing the basic components in a steam plant
layout. (3 marks)
(c) A steam power plant operates between a boiler pressure of 42 bar and a
condenser pressure of 0.035 bar. Calculate for these limits the cycle
efficiency, the work ratio and the specific steam consumption.
(i) for a Rankine cycle with dry saturated steam at entry to the turbine.
(ii) For a Rankine cycle with the steam superheated to 500oC. Neglect
the feed pump work for this case. (15 marks)
QUESTION 3 (20 MARKS)
(a) State both the Clausius Statement and the Kelvin – Planck Statement of the
2nd law of Thermodynamics. (2 marks)
(b) A diesel engine takes in air at 1.01 bar, 20oC and the maximum cycle
pressure is 69 bar. The compression ratio is 18:1. Calculate the air standard
thermal efficiency based on the dual combustion cycle. Assume that the heat
added at constant volume is equal to the heat added at constant pressure.
Take 1.4
C KJ Kg K v 0.717 /
C KJ Kg K p 1.005 /
(10 marks)
QUESTION 4 (20 MARKS)
(a) With the use of an energy flow diagram, differentiate a heat engine cycle
and a refrigeration cycle. (4 marks)
(b) The pressure in the evaporator of ammonia refrigerator is 1.902 bar and
the pressure in the condenser is 12.37 bar.
(i) Calculate the Ideal COP for a machine working between the
corresponding saturation temperatures and the Ideal refrigerating
effect per Kg of refrigerant.
(ii) Calculate the refrigerating effect per Kg of refrigerant and the COPr
for the practical cycle working between the same pressures when dry saturated
vapour is delivered to the condenser after Isentropic compression and there is no
undercooling.
(iii) Calculate also the refrigerating effect per Kg of refrigerant and the C.O.P.r
for the practical cycle for the above working within the same pressure with dry
saturated vapour being delivered to the compressor where it is compressed
Isentropically with the liquid being undercooled by IOK after condensation. (16 marks)
QUESTION 5
(a) Define the following fuel properties
(i) Flash point
(ii) Fire point
(iii) Volatility
(iv) Calorific value
(v) Octane Number (5 marks)
(b) A sample of coal having composition by mass of carbon 80%, hydrogen 4%,
Oxygen 5% sulphur 2%, Nitrogen 3% and the remainder ash is burnt in a
furnace with 60% excess air. The temperature of the flue gases entering the
stack is 320oC from an atmospheric temperature of 15oC. Determine:
(i) The actual mass of air supplied per Kg of fuel.
(ii) The percentage heat energy carried away by the flue gases.
Take calorific value of coal as 29.7 MJ/Kg and assume the combustion to be
complete. Heat energy carried away per Kg of moisture is 3020 KJ. For the
dry flue gases take
Cp = 1.05 KJ/Kg K. Air contains 23% oxygen and 77% nitrogen by mass. (15 marks)
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