Gazi Universitesi/Gazi University Muhendislik-Mimarlik Fakijltesi/Faculty of Engineering and Architecture Makina Muhendisligi Bolumu/Mechanical

Engineering Department Ankara/Turkiye

Instructor: Dr. Atilla Bykolu

Assistant: Murat Erba

Q1 Q2 Q3 Q4 TOTAL

/25 /25 /25 /25 /100

MM204E THERMODYNAMICS-II

THE HOMEWORK I Due on: March 20th, 2014

(40%) Ql. The Rankine Cycle is given below. The mass flow rate of working fluid entering the closed feed

water heater (CFWH) is 10% of mass flow rate leaving the first turbine stage. The working fluid (3) leaving

open feed water heater (OFWH) is at saturated liquid state. The condenser pressure is 10 kPa. The

temperature of working fluid leaving the boiler (6) is at 700oC. The problem is specified for each student

according to the last two digit of the student number. For this purpose, t he boiler and OFWH pressures are

classified according to the last digit and the digit before the last one, respectively and presented in Table Q1.

The closed feed water heater pressure is half of boiler pressure. All the turbines and pumps are isentropic.

Prepare a table indicating state properties (temperature, pressure, quality, enthalpy, entropy) of the system

before solving the problem.

Table Q1. System pressures

Last two digit of the

student Number 0-9 1-8 2-7 3-6 4-5

(last digit)

Boiler Pressure (kPa)

10000 8000 6000 5000 4000

(the digit before the

last one)

OFWH Pressure(kPa)

1000 800 400 600 500

a) Find thermal efficiency of cycle.

b) Find the gas extraction ratio (y) to the OFWH at the exit of second turbine (y)?

c) What is mass flow rate entering the boiler for 1000 MW net power production?

Student's Name :

Student's Number :

(30%) Q2. A regenerative Brayton Cycle with intercooler is given below. The maximum cycle

temperature is 1600 K. The air entering compressor is at 95kPa and 300 K. The pressure ratio

of first compressor is 5. The intercooler cools air back to 300 K (3). The pressure ratio of

second compressor is 6. The regenerator is ideal. The isentropic efficiency of compressors are

78%, the isentropic efficiency of turbine is 85%.

a) What is thermal efficiency of the cycle?

b) How much work is recovered using intercooler with respect to using one compressor with

the same total pressure ratio?

c) What is back work ratio of the cycle?

(30%) Q3. Two stage refrigeration cycle is using R134 refrigerant. The temperature of vapor

leaving the condenser is 50oC. The evaporator temperatures are -25oC and 5oC, respectively. The

isentropic efficiency of compressors are 75%. The half of the mass flow rate of refrigerant is

sent to Evaporator 1 and the rest to the evaporator 2.

a) What is the coefficient of performance (COP) of system?

b) Can you give an example to such a system that is used at your daily life?

Please give your homework with an appropriate cover page and stapled.