FACULTY OF ENGINEERING AND COMPUTER SCIENCEDEPARTMENT OF MECHANICAL AND INDUSTRIAL ENGINEERING

COURSE Thermodynamics II

NUMBER MECH 351

SECTION All Sections

EXAMINATION

FinalDATE

April 24, 2012TIME & PLACE Room:

9:00 – 12:00# OF PAGES

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PROFESSORS LAB INSTRUCTOR

I. Hassan and L. Kadem

MATERIALS ALLOWED NO YES (PLEASE SPECIFY)

CALCULATORS ALLOWED NO YES (non programmable)

SPECIAL INSTRUCTIONS:

Answer the following four questions.State clearly any assumptions you make.Draw a clear sketch of the problem.Return the Exam paper with the answers’ book.

Name: _______________________________ I.D.: ______________________Surname, given names

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Question no. 1 (30 Points)

Consider a cogeneration power plant modified with regeneration. Steam enters the turbine at 6 MPa and 450°C and expands to a pressure of 0.4 MPa. At this pressure, 60 percent of the steam is extracted from the turbine, and the remainder expands to 10 kPa. Part of the extracted steam is used to heat the feedwater in a feedwater heater. The rest of the extracted steam is used for process heating and leaves the process heater as a saturated liquid at 0.4 MPa. It is subsequently mixed with feedwater heater, and the mixture is pumped to the boiler pressure.

Assuming the turbine and the pumps to be isentropic,

a- Show the cycle on a T-s diagram with respect to saturation lines.b- Determine the mass flow rate of steam through the boiler for a net power output of 15 MW. c- Compute the utilization factor.

Question no. 2 (25Marks):

A stationary gas-turbine power plant, as shown in figure, operates on an ideal regenerative Brayton

cycle ( = 75 percent, ε=

qregen ,actqregen ,max ) with air as the working fluid. Air enters the compressor (state

1) at 100 kPa and 290 K and the turbine (state 3) at 610 kPa and 1100 K. The compressor and turbine isentropic efficiencies are 84 and 100%, respectively.

(a) Plot the T-s diagram of the cycle.(b) Determine the thermal efficiency of the cycle.

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(c) Determine the net work delivered by this plant.

Assume constant specific heats with temperature, Cv = 0.718 kJ/kg K and Cp= 1.005 kJ/kg K.

Question no. 3 (20 Marks)

Atmospheric air enters an air-conditioning system at 30oC and 70 percent relative humidity with a volume flow rate of 4 m3/min and is cooled to 20oC at a pressure of 1 atm, as shown in the figure. The system uses refrigerant-134a as the cooling fluid that enters the cooling section at 350kPa with a quality of 20 percent and leaves as a saturated vapor.

a- Sketch the process on the Psychrometric chart. b- What is the heat transfer from the air to the cooling coils, in kW? c- If any water is condensed from the air, how much water will be condensed from the

atmospheric air per min? d- Determine the mass flow ate of the refrigerant, in kg/min.

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Question no. 4 (25 Marks)

Benzene gas (C6H6) at 25°C is burned during a steady-flow combustion process with 95 percent theoretical air that enters the combustion chamber at 25°C and 1 atm. All the hydrogen in the fuel burns to H2O, but part of the carbon burns to CO. If the products leave at 1000 K, determine,

a- The mole fraction of the CO in the products.

b- The heat transfer from the combustion chamber during this process.

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THERMODYNAMICS DATA SHEET

Constants and Units R = 8.315 kJ/kmole°K1 bar = 100kPa1 gm/cm3 = 103 kg/m3

Molar masses and specific heats:Air: M = 29 kg/kmol Cp= 1.005kJ/kg K Note: Cp = Cv + R

Enthalpy of Formation:

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Formula Sheet – Thermodynamics II

where K= ;

MEP= ; ηcycle=

W net

Qin

Mole number: ; Mole fraction: ; Average molar mass = = ; gas constant:

; ; ; ;

Dalton’s law: ; Amagat’s law: ;

U = , U = U (same for h and s); ; , same for Cpm

Real gasses:Δh=(h2−h1 )ideal−RuT cr(Zh2−Zh1 ); ;

;

Air conditioning energy balance: ,

Air fuel ratio: ; m=NM ; ; Percentage of theoretical air:

mair , act¿

mair , th¿

or

N air ,act¿

N air , th¿

Enthalpy of combustion : ; ;

First law analysis of reacting systems:

Entropy generation of reacting systems:

Sgen=ΣN p s̄ p−ΣN r s̄ r

=ΣN p( s̄oi (T , Po)−Ru lny iPmPo )Pr od

−ΣN r ( s̄oi (T , Po )−Ru lny i PmPo )react

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Summary of Final Answers

Question No 1:

Required Numerical Final Answer (1) Mass flow rate

(2) Utilization factor

Question No 2:

Required Numerical Final AnswerThermal efficiency Work in kW

Question No 3:

Required Numerical Final AnswerHeat Transfer

Condensed water flow rateRefrigerant mass flow rate

Question No 4:

Required Numerical Final Answer (1) The mole fraction of CO (2) The heat transfer rate from the combustion chamber.

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