Transcript Heat Transfer

BOIL1.ppt
Boiling
(Chapter in Outline Notes –
“Boiling Heat Transfer ” )
Components of Course: What
Stage are We Up To?
• Types of exchangers, revision of OHTCs,
fouling factors.
• Heat exchanger selection.
• Thermal performance analysis (NTUs) for
co- & counter-current exchangers.
• Multi-pass exchangers (S&T).
• Condensation & boiling.
• Radiation.
Outline
• Important for reboilers on distillation
columns, steam boilers
• Mechanisms
• Example
Mechanisms
• Consider (first) situation in which wall
temperatures are controlled (by condensing
steam or other vapour)
• Different to controlled heat flux (controlled
by electrical, radiant, nuclear power input)
As Wall Temperature is Increased
• Temperature driving force for boiling is that
between wall & boiling point of liquid.
• Heat-transfer coefficient = ratio of heat flux
to temperature driving force.
• Initially, only forced convection heating
occurs; not boiling heat transfer
Two Basic Mechanisms
Nucleate boiling: strings of bubbles rise from
surface; liquid moves into surface; more
vapour generated
Film boiling: at higher heat fluxes, vapour
blankets surface, heat-transfer coefficient
decreases
Design Aim
• Keep heat fluxes low enough so that
nucleate boiling occurs, where heat-transfer
coefficients are best
• Critical heat flux; where bubbles start to
join to form vapour blanket
• Leidenfrost temperature; temperature
above which vapour blanket is continuous
across wall
Controlled Heat Flux
An Important Point
• Suppose that you are asked to calculate
heat-transfer rate for heat exchanger which
is required to do some boiling duty.
• All heat-transfer calculations are subject to
some uncertainty.
• Boiling heat-transfer calculations are more
uncertain than single-phase and boiling
calculations.
Mechanisms
“Why are heat-transfer coefficients for boiling
much more difficult to predict than those for
condensation?”
• Film-wise condensation occurs for any
normally rough or rougher surface.
• Drop-wise condensation is very rare (only
on very smooth surfaces).
• Once a film forms in condensation, surface
roughness does not matter much, since film
covers surface “bumps”.
• Contrast: With nucleate boiling, bubbles
always start at surface. Number, type &
shape of crevices, bumps, etc, always
governs ease with which bubbles leave
surface.
• Since bubbles insulate surface from liquid,
ease with which they leave surface is very
important in governing heat-transfer rate.
Pool Boiling vs Forced
Convection
Key distinctions:• Pool boiling - natural convection, flow
patterns driven by heat transfer
• Forced convection - fluid is moved across
surface
Forced Convection
Same basic processes as in pool boiling.
Some transitions:• single-phase forced convection
• sub-cooled nucleate boiling: Tw > Tsat, but
bulk fluid sub-cooled
• saturated nucleate boiling: vapour quality >
0
Dryout
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film is completely evaporated
quality < 1
droplets in gas
forced convection heat transfer
heat-transfer coefficient drops
wall temperature increases
dangerous if heat flux is constant
scale formation, corrosion are accelerated
Example: Pool Boiling, Pot on Stove
(presented in detail in Outline Notes –
“Boiling Heat Transfer ” chapter ))
• Critical heat flux: Zuber correlation
• For estimating wall temperature, heattransfer coefficient for pool boiling must be
estimated. The correlation of Cooper
(1984) is often used - see Outline Notes.
• Conclusions of example : nucleate boiling
will always occur, because pot would melt
before critical heat flux is reached.
Actual Designs
• Like condensers, mainly shell & tube, not
plate (high pressure drops in narrow plate
gaps, problems in finding suitable gasket
materials)
• Thermosyphon design common (vertical,
in-tube); difficult because it relies on
(strong) natural convection to drive fluid
flow, which drives heat transfer, etc
Conclusions
• Boiling more difficult than condensation to
predict accurately because (unpredictable)
nature of surface has a larger effect
• Behaviour is different for controlled heat
flux cf controlled wall temperature