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(a)lBDA30403/8DA3043

With an appropriate sketch, explain why is the Camot cycle not a realistic modelfor steam power plants.

(3 marks)(b) Explain why an excessive moisture in steam undesirable in steam turbines and give

the highest moisture content allowed.(2 marks)

(c) A steam power plant operates on an ideal reheat- regenerative Rankine cycle andhas a net power output of 80 MW. Steam enters the high-pressure turbine at 10MPa and 550'C and leaves at 0.8 MPa. Some steam is extracted at this pressure toheat the feedwater in an open feedwater heater. The rest of the steam is reheated to500oC and cooled in the condenser at a pressure of 10 kPa by running cooling waterfrom a lake through the tubes of the condenser at a rate of 2000 kg/s.

(D(ii)(iiD(iv)

Show the cycle on a T-s diagram with respect to saturation lines.Determine the mass flow rate of steam through the boiler.Calculate the net power output of the plantDetermine the temperature rise of the cooling water.

(20 marks)

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Q2 (a)

BDA30403/8DA3043

Define the effectiveness of a regenerator used in gas-turbine cycles.(2 marks)

For fixed maximum and minimum temperatures, describe the effect of the pressureratio on (i) the thermal effrciency and (ii) the net work output of a simple idealBrayton cycle?

(3 marks)

(c) Explain what the back work ratio is and what are typical back work ratio values forgas-turbine engines

(2 marks)A gas-turbine power plant operates on the regenerative Brayton cycle between thepressure limits of 100 and 700 kPa. Air enters the compressor at 30 C at a rate of12.6 kgls and leaves at 260 C. It is then heated in a regenerator to 400 C by the hotcombustion gases leaving the turbine. A diesel fuel with a heating value of 42,000kJ/kg is bumed in the combustion chamber with a combustion efficiency of 97percent. The combustion gases leave the combustion chamber at 87loC and enterthe turbine whose isentropic effrciency is 85 percent. Treating combustion gases asair and using constant specific heats at 500 C, determine

(b)

(d)

(i)(ii)(iii)(iv)(v)

the isentropic efficiency of the compressor,the effectiveness of the regenerator,the air-fuel ratio in the combustion chamber,the net power output and the back work ratio andthe thermal efficiencv

(18 marks)

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Q3 (a)

(c)

Q4 (a)

(b)

8DA30403/8DA3043

Explain the operating cycle of two-stage compressor and sketch thethermodynamics analysis on a p-v diagram for both compression processes.Explain the phenomena of the intercooling system that influences to the work savedduring the compression process.

(4 marks)Define the volumetric efficiency and discuss the influences of the effect ofclearance volume between head and TDC of piston during compression process.The explanations should be focused on the operating cycle, thermodynamicsaspects and their relatives merits and demerits.

(4 marks)A single-acting, two-stage reciprocating air compressor has a pressure ratio of 10 to1 and takes 5.5kg of air per minutes. The air is compressed from 1.013 bar and180C. During operation, the law of compression and expansion in both stages ispVt'' : constant and both stages have same pressure ratio. If intercooling iscomplete, calculate the indicated power and the cylinder swept volumes required.Assume that the clearance volumes of both stages are 6%o of their respective sweptvolumes and that the compressor runs at 350 rev/min. Sketch schematic and P-vdiagram for the above operation. (17 marks)

Using appropriate sketches draw a two-stage refrigeration system with a flashchamber. Explain briefly the function of flash chamber in this system.(5 marks)

(b) A two-stage compression refrigeration system operates with refrigerant-t34abetween the pressure limits of 1.4 and 0.10 MPa. The refrigerant leaves thecondenser as a saturated liquid and is throttled to a flash chamber operating at 0.6MPa. The refrigerant leaving the low-pressure compressor at 0.6 MPa is also routedto the flash chamber. The vapor in the flash chamber is then compressed to thecondenser pressure by the high-pressure compressor, and the liquid is throttled tothe evaporator pressure. Assuming the refrigerant leaves the evaporator assaturated vapor and both compressors are isentropic, determine(D(ii)(iii)

the fraction of the refrigerant that evaporates as it is throttled to the flashchamber,the rate of heat removed from the refrigerated space for a mass flow rate of0.25 kg/s through the condenser, andthe coefficient of perfornance

(20 marks)

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Q5 (a)

Q6 (a)

8DA30403/8DA3043

Sketch and label all four processes that make up the air-standard Otto cycle in a P-vdiagram or a T-s diagram.

(4 marks)The compression ratio of an air-standard Otto cycle is 9.6. This cycle is operatedwith 100 kPa, 30oC and 600 cm3 at the beginning of the isentropic compressionprocess. At the end of the polytropic expansion process with exponent n:l.35, thetemperature is 800K. Using the constant specific heats at room temperature,determine:(i) the maximum gas pressure;(ii) the maximum gas temperature;(iii) amount of heat added in kJ; and(iv) thermal efficiency.

(21 marks)In engineering applications, why do we need to treat dry air and air with moisturedifferently? How does the moisture content in atmospheric air affect air-conditioning application?

(5 marks)In Figure Q6, the cooling water from condenser of the power plant enters a wetcooling tower at 40oC at the rate of 75 kgls. The water is cooled to 28oC in thecooling tower by air that enters the tower at 1 atm, with dry and wet-bulbtemperatures 22 and l6oC, respectively. The air leaves at 32 C with relativehumidity of 90Yo. Determine:

(b)

(b)

(i)(ii) the volume flow rate of air into the cooling tower; andthe flow rate of the required makeup water.

(20 marks)

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8D430403/8DA3043

SEMESTER/ SESSIONCOURSE

FINAL EXAMINATIONSEM 2 12012-2013 PROGRAMME lTHERMODYNAMICS II COURSE CODE : 3BDD /4BDD8DA30403 / BDA3043

AIa. lTlEXIT \#

d$\{ r**s\ r\ IJTt - laanC.OOLI#ATER ? **un*rryrvater

FIGURE 06

END OF QUESTION