Answer the following questions:

1-a) Explain the effect of lowering the condenser pressure on the thermal

efficiency of the Rankien cycle?

b) A regenerative steam power plant has two feed-water heaters; one open

heater and the other is a closed that operates at a terminl temperature

different (TTD) of 2 οC. Steam leaves the boiler to the high pressure turbine

at 70 bar, 550 ο

C, condenser pressure 0.08 bar. Condensate from the closed

heater is throttled through a steam trap to the open heater, if the enthalpy

rise for feed water in both heaters is to be equal, evaluate the approximate

values of pressure for extracted steam to the heaters. Calculate the mas of

extracted steam per kg of mass in the boiler, the thermal efficiency and

draw the T-s diagram. Assume the isentropic efficiency of all turbine stages

is 95% and the pump operates isentropically.

2-a) For the simple gas turbine cycle, drive the following relation:

b) A gas turbine is used to generate 5000 kW. The unit is operating with two

compressor stages with an overall pressure ratio of 9:1 and an inter-cooling

installed between them. Isentropic efficiency of both compressors is 0.8. Power

input to both compressors is assumed to be equal. Temperature at both

compressors inlets is 15 οC. The unit consists of two stages turbine with a

reheat installed between them. The high pressure turbine is used to drive the

two compressors, whereas the low pressure turbine is used to drive the

generator. Isentropic efficiency of both turbines is 0.85. The temperature at

both turbines inlets is 650 οC. Assume mass flow rate through the cycle is

constant. Sketch the diagrammatically and present the cycle on T-S diagram.

Determine for the gas turbine.

1- Mass flow rate through the cycle.

2- Work ratio, and

3- The thermal efficiency.

Benha University Time: 3 Hrs

Benha Faculty of Engineering Department: Mechanical (Production )

January 2012 – Fall semester Course name: Thermo-fluid Machinery

Exam (regular) Year: Third

Ti me Allowed: Hours

1

p

cy

r

1 - 1 1

1-

p

3

1 r T

T - 1 2

wr

heat specific :

ratio pressure : r

ratio work : r

cycle in the re temperatumaximum :T

cycle in the re temperatuminimum :

p

w

3

1

T

3-a) Saturated steam at 250 οC flow inside a 10-cm inside diameter steel tube of

wall thickness 5-mm( k = 50 W /m οC ) . The tube is insulated with 1-cm

thick fiberglass ( k = 0.05 W /m οC ). The outer side of the insulated tube is

subjected to outside air at 25 ο

C. The convection heat transfer coefficient on

the steam and air sides are 8000 and 7 W / m2

οC, respectively. Determine:

1- The heat transfer rate per unit length of the tube

2- The temperature of the outer surface of insulation

3- The overall heat transfer coefficient.

b) A concentric tube heat exchanger for cooling lubricating oil is comprised in

a thin walled. The flow rate of cooling water through the inner tube

25 iD mm is 0.1 kg/s, while the flow rate of oil through the outer

annulus 45 oD mm is 0.1 kg/s. The water and oil enter at temperature

of 30 and 100 οC, respectively. The exchanger operates in counter-flow

with an overall heat transfer coefficient of 0.060 K 2/ . .W m K .The specific

heats of water and oil are 4.2 and 1.9 kJ/kg.K. If the outlet temperature of

the oil is to be 60 οC, determine

- The total heat transfer rate and the outlet temperature of the water

- The length required for the heat exchanger.

4-a) Prove that, the thermal efficiency of the Diesel cycle is.

1

111

1

cth

c

r

r r

b) A diesel engine has a state before compression of T = 290 K, P = 95 kPa, a

peak pressure of 6000 kPa, and a maximum temperature of 2400 K.

Determine.

1 - Draw the P-v and T-s diagrams

2 - The volumetric compression ratio.

3 - The cut of ratio.

4 - The thermal efficiency of the cycle.

Good Luck

Examiner: Dr. Samia Naser El Deen Abedo