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

Chapter 2 | 97

The energy flow rate associated with a fluid flowing at a rate
of m

.

is

which is analogous to E
me.
The mechanical energyis defined as the form of energy
that can be converted to mechanical work completely and
directly by a mechanical device such as an ideal turbine. It is
expressed on a unit mass basis and rate form as

and

where P/ris the flow energy,V^2 /2 is the kinetic energy, and
gzis the potential energyof the fluid per unit mass.
Energy can cross the boundaries of a closed system in the
form of heat or work. For control volumes, energy can also
be transported by mass. If the energy transfer is due to a tem-
perature difference between a closed system and its surround-
ings, it is heat;otherwise, it is work.
Work is the energy transferred as a force acts on a system
through a distance. Various forms of work are expressed as
follows:

Electrical work:

Shaft work:

Spring work:

The first law of thermodynamicsis essentially an expres-
sion of the conservation of energy principle, also called the
energy balance. The general mass and energy balances for
any systemundergoing any processcan be expressed as

Wspring

1

2

k 1 x^22 x^212

Wsh
 2 pnT

We
VI¢t

E

#

mech
m

#

emech
 m

#

a

P

r



V^2

2

gzb

emech

P

r



V^2

2

gz

E

#


m

#

e

Net energy transfer Change in internal, kinetic,

by heat, work, and mass potential, etc., energies

It can also be expressed in the rate formas

Rate of net energy transfer Rate of change in internal,

by heat, work, and mass kinetic, potential, etc., energies

The efficiencies of various devices are defined as

The conversion of energy from one form to another is often

associated with adverse effects on the environment, and envi-

ronmental impact should be an important consideration in the

conversion and utilization of energy.

hturbine–gen
hturbinehgenerator

W

#

elect,out

0 ¢E

#

mech,fluid^0

hpumpmotor
hpumphmotor

¢E

#

mech,fluid

W

#

elect,in

hgenerator

Electric power output

Mechanical power input

W

#

elect,out

W

#

shaft,in

hmotor

Mechanical power output

Electric power input

W

#

shaft,out

W

#

elect,in

hturbine

W

#

shaft,out

0 ¢E

#

mech,fluid^0

W

#

turbine

W

#

turbine,e

hpump

¢E

#

mech,fluid

W

#

shaft,in

W

#

pump,u

W

#

pump

E

.

inE

.

out¬^ 
¬^ dEsystem>dt¬¬^1 kW^2

EinEout¬


¬

⎭⎪⎬⎪⎫^ ¢Esystem¬¬^1 kJ^2

⎭⎪⎪⎬⎪⎪⎫

⎭⎪⎬⎪⎫

⎭⎪⎪⎬⎪⎪⎫

1.ASHRAE Handbook of Fundamentals. SI version.

Atlanta, GA: American Society of Heating, Refrigerating,

and Air-Conditioning Engineers, Inc., 1993.

REFERENCES AND SUGGESTED READINGS

2.Y. A. Çengel. “An Intuitive and Unified Approach to

Teaching Thermodynamics.” ASME International

Mechanical Engineering Congress and Exposition,

Atlanta, Georgia, AES-Vol. 36, pp. 251–260, November

17–22, 1996.