thermodyn..

MECHANICAL ENERGY OF FLUID FLOW
is the form of energy which can be converted to mechanical work completely and directly by an ideal mechanical device.
It consists of energy of flowing fluid (called flow work or flow energy), kinetic energy (KE) and potential energy (PE) .

flow KE PE
enegy

Mechanical energy of flowing fluid: Emech P V2 [kJ ]
Power (rate of energy transfer) = m + m + mgz

ρ2

Emech =  P V2 +   kJ  ∆E (2-11)
m + gz   s = kW  Emech = ∆t
ρ 2 

P V2  kJ 
emech = + + gz  kg  (2-10)
ρ2

Mechanical energy change of ∆ Emech = m  P2 − P1 + V22 − V12 + g ( z2 − z1 )  [kJ ]
incompressible flow ( ρ = const ):  ρ 2 
 

State 2  P2 − P1 V22 − V12   kJ 
 ρ 2  s kW 
∆ Emech = m  + + g ( z2 − z1 )  = (2-13)

State 1 P2 ,ρ ,m V2

P1 ,ρ ,m V1

z1 z2 P2 − P1 + V22 − V12 g ( z2 z1 )  kJ 
ρ 2  kg 
∆emech = + − (2-12)

ENERGY TRANSFER

Examples 2-4,2-5,2-6 Heating of a Potato in an Oven

Heat Transfer Q energy transfer due to temperature difference
energy interaction not caused by a temperature
transfer of energy carried by moving mass air
heat

Work Transfer W electrical
work
heat

Mass Flow m heating element

Work and Heat: 1. Work and heat are recognized at the boundary (boundary phenomena).
2. Systems possess energy, not work or heat.
3. Associated with a process, not with a state.
4. Path functions (depend on path of process), not point functions which depend on state.

+ -

SIGN CONVENTION positive system negative
to the system
from the system