Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

5–15 Air enters a 28-cm diameter pipe steadily at 200 kPa
and 20°C with a velocity of 5 m/s. Air is heated as it flows,
and leaves the pipe at 180 kPa and 40°C. Determine (a) the
volume flow rate of air at the inlet, (b) the mass flow rate of
air, and (c) the velocity and volume flow rate at the exit.

258 | Thermodynamics

(b) the total and flow energies of the steam per unit mass, and

(c) the rate at which energy is leaving the cooker by steam.

5–22 Refrigerant-134a enters the compressor of a refrigera-

tion system as saturated vapor at 0.14 MPa, and leaves as

superheated vapor at 0.8 MPa and 60°C at a rate of 0.06 kg/s.

Determine the rates of energy transfers by mass into and out

of the compressor. Assume the kinetic and potential energies

to be negligible.

5–23 A house is maintained at 1 atm and 24°C, and

warm air inside a house is forced to leave the house at a rate

of 150 m^3 /h as a result of outdoor air at 5°C infiltrating

into the house through the cracks. Determine the rate of

net energy loss of the house due to mass transfer. Answer:

0.945 kW

5–24 Air flows steadily in a pipe at 300 kPa, 77°C, and 25

m/s at a rate of 18 kg/min. Determine (a) the diameter of the

pipe, (b) the rate of flow energy, (c) the rate of energy trans-

port by mass, and (d) also determine the error involved in

part (c) if the kinetic energy is neglected.

Steady-Flow Energy Balance:

Nozzles and Diffusers

5–25C How is a steady-flow system characterized?

5–26C Can a steady-flow system involve boundary work?

5–27C A diffuser is an adiabatic device that decreases the

kinetic energy of the fluid by slowing it down. What happens

to this lostkinetic energy?

5–28C The kinetic energy of a fluid increases as it is accel-

erated in an adiabatic nozzle. Where does this energy come

from?

5–29C Is heat transfer to or from the fluid desirable as it

flows through a nozzle? How will heat transfer affect the

fluid velocity at the nozzle exit?

5–30 Air enters an adiabatic nozzle steadily at 300 kPa,

200°C, and 30 m/s and leaves at 100 kPa and 180 m/s. The

inlet area of the nozzle is 80 cm^2. Determine (a) the mass

flow rate through the nozzle, (b) the exit temperature of the

air, and (c) the exit area of the nozzle. Answers:(a) 0.5304

kg/s, (b) 184.6°C, (c) 38.7 cm^2

Air

200 kPa

20 °C

5 m/s

180 kPa

40 °C

Q

FIGURE P5–15

5–16 Refrigerant-134a enters a 28-cm diameter pipe
steadily at 200 kPa and 20°C with a velocity of 5 m/s. The
refrigerant gains heat as it flows and leaves the pipe at 180
kPa and 40°C. Determine (a) the volume flow rate of the
refrigerant at the inlet, (b) the mass flow rate of the refriger-
ant, and (c) the velocity and volume flow rate at the exit.

5–17 Consider a 300-L storage tank of a solar water heat-
ing system initially filled with warm water at 45°C.
Warm water is withdrawn from the tank through a 2-cm
diameter hose at an average velocity of 0.5 m/s while cold
water enters the tank at 20°C at a rate of 5 L/min. Determine
the amount of water in the tank after a 20-minute period.
Assume the pressure in the tank remains constant at 1 atm.
Answer:212 kg

300 L

45 °C

Warm water

45 °C

0.5 m/s

Cold water

20 °C

5 L/min

FIGURE P5–17

Flow Work and Energy Transfer by Mass

5–18C What are the different mechanisms for transferring
energy to or from a control volume?

5–19C What is flow energy? Do fluids at rest possess any
flow energy?

5–20C How do the energies of a flowing fluid and a fluid
at rest compare? Name the specific forms of energy associ-
ated with each case.

5–21E Steam is leaving a pressure cooker whose operating
pressure is 30 psia. It is observed that the amount of liquid in
the cooker has decreased by 0.4 gal in 45 minutes after the
steady operating conditions are established, and the cross-
sectional area of the exit opening is 0.15 in^2. Determine
(a) the mass flow rate of the steam and the exit velocity,

AIR

V 1 = 30 m/s

T 1 = 200°C

P 1 = 300 kPa

A 1 = 80 cm^2

V 2 = 180 m/s

P 2 = 100 kPa

FIGURE P5–30