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HW # 6 /Tutorial # 6
WRF Chapter 20; WWWR Chapters 21 & 22
ID Chapters 10 & 11
• Tutorial # 6
• WRF#20.6; WWWR#21.14, WRF#20.7; WWWR#21.19, 22.3, 22.15.
• To be discussed 6 March, 2018.
• By either volunteer or class list.
Boiling
Two basic types of boiling:
• Pool boiling
– Occurs on heated surface submerged in a liquid pool
which is not agitated
• Flow boiling
– Occurs in flowing stream
– Boiling surface may be a portion of flow passage
– Flow of liquid and vapor important type of 2 phase
flow
Regime 1:
• Wire surface temperature is only a few degrees
higher than the surrounding saturated liquid
• Natural convection currents circulate the
superheated liquid
• Evaporation occurs at the free liquid surface as the
superheated liquid reaches that position
Regime 2:
• Increase in wire temperature is accompanied by
the formation of vapor bubbles on the wire surface
• These bubbles form at certain surface sites, where
vapor bubble nuclei are present, break off and
condense before reaching the free liquid surface
At a higher surface temperature, as in regime III, larger and more
numerous bubbles form, break away from the wire surface, rise,
and reach the free surface. Regimes II & III are associated with
nucleate boiling.
Regime IV:
• Beyond the peak of the curve the transition boiling
regime is entered.
• A vapor film forms around the wire, and portions
of this film break off and rise, briefly exposing a
portion of the wire surface
• This film collapse and reformation and this
unstable nature of the film is characteristic of the
transition regime.
• When present, the vapor film provides a
considerable resistance to heat transfer, thus the
heat flux decreases.
Condensation
• Occurs when a vapor contacts a surface
which is at a temperature below the
saturation temperature of the vapor.
• When the liquid condensate forms on the
surface, it will flow under the influence of
gravity.
• Film Condensation
•Normally the liquid wets the surface, spreads out and forms a
film.
• Dropwise Condensation
•If the surface is not wetted by the liquid, then droplets form and
run down the surface, coalescing as they contact other
condensate droplets.
Film Condensation:
Turbulent-Flow Analysis
• It is logical to expect the flow of the condensate
film to become turbulent for relatively long
surfaces or for high condensation rates.
• The criterion for turbulent flow is a Reynolds
number for the condensate film.
• In terms of an equivalent diameter, the
applicable Reynolds number is
Re = 4A Lu
P mf
41 ; 1; 4
44Re
L avg L avg
f f
AA P
P
v vA
P
m m
Dropwise Condensation
Dropwise Condensation
• Associated with higher heat-transfer
coefficients than filmwise condensation
phenomenon.
• Attractive phenomenon for applications
where extremely large heat-transfer rates
are desired.
Heat Transfer Equipment
• Single-pass heat exchanger – fluid flows through
only once.
• Parallel or Co-current flow – fluids flow in the
same direction.
• Countercurrent flow or Counterflow - fluids flow
in opposite directions.
• Crossflow – two fluids flow at right angles to one
another.
Shell-and-tube Arrangement
• E.g. Tube-side fluid makes two passes, shell-side fluid
makes one pass.
• Good mixing of the shell-side fluid makes one pass.
Log-Mean Temperature Difference (continued)
• First-law-of-thermodynamics
• Energy transfer between the two fluids
. .
p c p H
c H
q mC T mC T
. .
p c c c p H H H
c H
dq mC dT C dT mC dT C dT
( )
( )
H C
H C H C
dq UdA T T
T T T d T dT dT