Chapter 10 · Develop a good understanding of the boiling curve, and the different boiling regimes corresponding to different regions of he boiling curve Calculate the heat flux and

Chapter 10BOILING AND CONDENSATION

Heat Transfer

Universitry of Technology Materials Engineering DepartmentMaE216: Heat Transfer and Fluid

ObjectivesDifferentiate between evaporation and boiling, and gain familiarity with different types of boilingDevelop a good understanding of the boiling curve, and the different boiling regimes corresponding to different regions of he boiling curve

Calculate the heat flux and its critical value associated with nucleate boiling, and examine the methods of boiling heat ransfer enhancement

Derive a relation for the heat transfer coefficient in laminar film condensation over a vertical plateCalculate the heat flux associated with condensation on nclined and horizontal plates, vertical and horizontal cylinders or spheres, and tube bundlesExamine dropwise condensation and understand the

LING HEAT TRANSFERaporation occurs at the liquid–vapor interfaceen the vapor pressure is less than the saturation ssure of the liquid at a given temperature.iling occurs at the solid–liquid interface when a id is brought into contact with a surface intained at a temperature sufficiently above the uration temperature of the liquid.

ssification of boiling

iling is called pool boiling in e absence of bulk fluid flow.y motion of the fluid is due to tural convection currents and

e motion of the bubbles under e influence of buoyancy.iling is called flow boiling in

e presence of bulk fluid flow.flow boiling, the fluid is forced to ove in a heated pipe or over a rface by external means such

excess temperature

ing heat flux from a solid surface to the fluid

bcooled BoilingWhen the

mperature of the ain body of the

quid is below the aturation mperature.

aturated BoilingWhen the

mperature of the quid is equal to e saturation mperature.

OL BOILING

g takes different forms, depending the Texcess = Ts Tsat

l boiling, the fluid is not forced to flowmover such as a pump.

otion of the fluid is due to naturalction currents and the motion of the es under the influence of buoyancy.

ng Regimes and Boiling Curve

tural Convection BoilingPoint A on the Boiling Curve)

ubbles do not form on the heating surface until the liquid is heated few degrees above the saturation temperature (about 2 to 6°C for ater)

he liquid is slightly superheated in this case (metastable state).

he fluid motion in this mode of boiling is governed by natural onvection currents.

eat transfer from the eating surface to the fluid by natural convection.

or the conditions of Fig. 0–6, natural convection oiling ends at an excessmperature of about 5°C.

e bubbles form at an reasing rate at an increasing mber of nucleation sites as we

ove along the boiling curve ward point C.

cleate Boiling (between nts A and C)

gion A–B ─ isolated bbles.

gion B–C ─ merous continuous lumns of vapor in the uid.

t A is referred to as

region A–B the stirring and agitation caused by the entrainment of the uid to the heater surface is primarily responsible for the increased heat nsfer coefficient.region A–B the large heat fluxes obtainable in this region are caused by e combined effect of liquid entrainment and evaporation.r the entire nucleate boiling range, the heat transfer coefficient ranges m about 2000 to 30,000 W/m2·K.

er point B the heat x increases at a wer rate with creasing Texcess, and aches a maximum at int C. e heat flux at this int is called the tical (or maximum)at flux, and is of

nsition Boiling tween Points C and D)

en Texcess is increased past point he heat flux decreases.s is because a large fraction of the ter surface is covered by a vapor , which acts as an insulation.

he transition boiling me, both nucleate and boiling partially occur.

eration in the transition ing regime, which is

o called the unstable m boiling regime, is

ided in practice.water, transition boiling urs over the excess

m Boiling (beyond Point Dyond point D the ater surface is mpletely covered by a tinuous stable vapor .

nt D, where the heat reaches a minimum

alled the Leidenfrost nt.

e presence of a vapor between the heater

face and the liquid is ponsible for the low

at transfer rates in the boiling region.

e heat transfer rate reases with increasing

nout Phenomenon

A typical boiling process oes not follow the boiling urve beyond point C.

When the power applied to he heated surface exceeded he value at point C even lightly, the surface emperature increased uddenly to point E.

When the power is reduced radually starting from point

E the cooling curve follows ig. 10–8 with a sudden drop

n excess temperature when oint D is reached.

attempt to increase the heat beyond qmax will cause the ration point on the boiling e to jump suddenly from t C to point E.

wever, surface temperature corresponds to point E is

ond the melting point of mostter materials, and burnouturs. refore, point C on the boiling e is also called the burnout

nt, and the heat flux at this t the burnout heat flux.t boiling heat transfer pment in practice operate

htly below q to avoid any

at Transfer Correlations in Pool Boilingoiling regimes differ considerably in their character.

Different heat transfer relations need to be used for different boiling regimes.

n the natural convection boiling regime heat transfer rates can be accurately etermined using natural convection relations.

o general theoretical relations for heat ansfer in the nucleate boiling regime is vailable.

xperimental based correlations are sed.

he rate of heat transfer strongly epends on the nature of nucleation nd the type and the condition of the eated surface.

cleate Boiling

r nucleate boiling a widely used rrelation proposed in 1952 by Rohsenow:

ak Heat Fluxe maximum (or critical) heat flux (CHF) in nucleate pool boiling:

is a constant whose value depends on the heater geometry, but generally is out 0.15. e CHF is independent of the fluid–heating surface combination, as well as

e viscosity, thermal conductivity, and the specific heat of the liquid.e CHF increases with pressure up to about one-third of the critical pressure, d then starts to decrease and becomes zero at the critical pressure.e CHF is proportional to hfg, and large maximum heat fluxes can be obtained ng fluids with a large enthalpy of vaporization, such as water.

inimum Heat Fluxnimum heat flux, which occurs the Leidenfrost point, is of actical interest since it presents the lower limit for the at flux in the film boiling regime.

ber derived the following pression for the minimum heat x for a large horizontal plate

is relation above can be in error 50% or more.

ransition

m Boiling

t high surface temperatures ypically above 300°C), heat ansfer across the vapor film by

adiation becomes significant and eeds to be considered.

heat flux for film boiling on a horizontal cylinder or re of diameter D is given by

ancement of Heat Transfer in Pool Boilinghe rate of heat transfer in the ucleate boiling regime strongly epends on the number of ctive nucleation sites on the urface, and the rate of bubble rmation at each site.

herefore, modification that nhances nucleation on the eating surface will also nhance heat transfer in ucleate boiling.

regularities on the heating urface, including roughness nd dirt, serve as additional ucleation sites during boiling.

h ff t f f h

rfaces that provide enhanced heat nsfer in nucleate boiling permanently

e being manufactured and are available the market.

eat transfer can be enhanced by a ctor of up to 10 during nucleate boiling, d the critical heat flux by a factor of 3.e use of finned surfaces is also known enhance nucleate boiling heat transfer d the maximum heat flux.iling heat transfer can also be hanced by other techniques such as

echanical agitation and surface bration. ese techniques are not practical,wever, because of the complications

volved.

OW BOILINGlow boiling, the fluid is forced to move an external source such as a pump as it ergoes a phase-change process.

xhibits the combined effects of vection and pool boiling.

ternal flow boiling over a plate or nder is similar to pool boiling, but the ed motion increases both the nucleate ing heat flux and the maximum heat considerably.

e higher the velocity, the higher the leate boiling heat flux and the critical t flux.

ernal flow boiling, commonly referred to two-phase flow, is much more mplicated in nature because there is no

e two-phase flow in a tube hibits different flow boiling imes, depending on the

ative amounts of the liquid d the vapor phases.

te that the tube contains a id before the bubbly flow ime and a vapor after the

st-flow regime.

at transfer in those two ses can be determined ng the appropriate ations for single-phase nvection heat transfer.

ug flowBubbles coalesce into slugs of vapor.Moderate mass qualities

nular flowCore of the flow consists of vapor only, and liquid adjacent to the walls. Very high heat transfer coefficients

st flowA sharp decrease in the heat transfer coefficient

por single-phase flow

uid single-phase flowIn the inlet region the liquid is subcooled and heat transfer to the liquid is by forced convection (assuming no subcooled boiling).

bbly flowIndividual bubblesLow mass qualities

condensationhe condensate wets the surface and rms a liquid film.he surface is blanketed by a liquid m which serves as a resistance to eat transfer.wise condensationhe condensed vapor forms droplets n the surface.he droplets slide down when they ach a certain size.o liquid film to resist heat transfer.s a result, heat transfer rates that

densation occurs when the temperature of a vapor is reduced belowaturation temperature.

ONDENSATION HEAT TRANSFER

quid film starts forming at the top the plate and flows downward

nder the influence of gravity.

increases in the flow direction xeat in the amount hfg is released uring condensation and is ansferred through the film to the ate surface.

s must be below the saturation mperature for condensation.

he temperature of the condensate Tsat at the interface and decreases

radually to Ts at the wall.

LM CONDENSATION

Heat transfer in condensation depends on whether the condensate flow is laminar or turbulent. The criterion for the flow regime is provided by the Reynoldsnumber.

Modified latent heat of vaporization

vapor that enters the condenser as superheated or at a temperature Tv instead of as saturated vapor:

n the final state is subcooled liquid instead of saturated liquid:

Rate of heat transfer

This relation is convenient to use to determine the Reynolds number when thecondensation heat transfer coefficient or the rate of heat transfer is known.

The properties of the liquid should beevaluated at the film temperature

The hfg should be evaluated at Tsat

low Regimes he dimensionless parameter ontrolling the transition between gimes is the Reynolds number

efined as:

hree prime flow regimes:

Re < 30 ─ Laminar (wave-free)

30 < Re < 1800 ─ Laminar (wavy)

Re > 1800 ─ Turbulent

he Reynolds number increases in e flow direction.

t Transfer Correlations for Film Condensation

mptions:th the plate and the vapor are maintained constant temperatures of Ts and Tsat, spectively, and the temperature across the uid film varies linearly.at transfer across the liquid film is by pure

onduction.

e velocity of the vapor is low (or zero) so at it exerts no drag on the condensate (no cous shear on the liquid–vapor interface).

e flow of the condensate is laminare<30) and the properties of the liquid are nstant.

rtical Plates

average heat transfer coefficient for laminar film ensation over a vertical flat plate of height L is

All properties of the liquid are to be

(10-22)

vy Laminar Flow on Vertical Platesaverage heat transfer coefficient in y laminar condensate flow for

A simpler alternative to the relation above

bulent Flow on Vertical Platesbulent flow of condensate on vertical plates:

physical properties of the condensate are again to be uated at the film temperature Tf = (Tsat + Ts)/2.

clined Platestion 10–22 was developed for vertical plates, can also be used for laminar film condensation e upper surfaces of plates that are inclinedangle from the vertical, by replacing g in that

tion by g cos.

ertical Tubes

ation 10–22 for vertical plates can also be to calculate the average heat transfer

ficient for laminar film condensation on the r surfaces of vertical tubes provided that the

(10-22)

orizontal Tubes and Spheresaverage heat transfer coefficient for film condensation

he outer surfaces of a horizontal tube is

For a sphere, replace the constant 0.729 by 0.815.

omparison of the heat transfer coefficient relations for a vertical tube ofght L and a horizontal tube of diameter D yields

tube whose length is 2.77 times its diameter, the average heat transfer coefficient for ar film condensation will be the same whether the tube is positioned horizontally or vertically.> 2.77D, the heat transfer coefficient is higher in the horizontal position.dering that the length of a tube in any practical application is several times its diameter, it is

mon practice to place the tubes in a condenser horizontally to maximize the condensation ransfer coefficient on the outer surfaces of the tubes.

orizontal Tube BanksThe average thickness of the liquid film at the lower tubes is much larger as a result of condensate falling on top of themfrom the tubes directly above.

Therefore, the average heat transfer coefficient at the lower tubes in such arrangements is smaller.

Assuming the condensate from the tubes above to the ones below drain smoothly, the average film condensation heat transfer coefficient for all tubes in a vertical tier can beexpressed as

This relation does not account for the increase in heat transfer due to the ripple

ect of Vapor Velocityhe analysis above we assumed the vapor velocity to be small and s the vapor drag exerted on the liquid film to be negligible, which sually the case.

wever, when the vapor velocity is high, the vapor will “pull” the id at the interface along since the vapor velocity at the interface st drop to the value of the liquid velocity.

e vapor flows downward (i.e., in the same direction as the liquid), additional force will increase the average velocity of the liquid thus decrease the film thickness.

s, in turn, will decrease the thermal resistance of the liquid film thus increase heat transfer.

ward vapor flow has the opposite effects: the vapor exerts a force the liquid in the opposite direction to flow, thickens the liquid film, thus decreases heat transfer.

Presence of Noncondensable Gases in Condensersmental studies show that the presence of ndensable gases in the vapor has a ental effect on condensation heat transfer. small amounts of a noncondensable gas inpor cause significant drops in heat transfer

cient during condensation.ommon practice to periodically vent out the ndensable gases that accumulate in the nsers to ensure proper operation.ransfer in the presence of a noncondensablerongly depends on the nature of the vapor nd the flow velocity.

h flow velocity is more likely to remove the ant noncondensable gas from the vicinity of rface, and thus improve heat transfer.

M CONDENSATION INSIDERIZONTAL TUBEScondensation processes encountered in ration and air-conditioning applications e condensation on the inner surfaces of ntal or vertical tubes. ransfer analysis of condensation inside is complicated by the fact that it is strongly

nced by the vapor velocity and the rate of accumulation on the walls of the tubes.

w vapor velocities:

OPWISE CONDENSATIONse condensation, characterized by

ess droplets of varying diameters on the nsing surface instead of a continuous lm and extremely large heat transferents can be achieved with this nism.

mall droplets that form at the nucleationn the surface grow as a result of ed condensation, coalesce into large s, and slide down when they reach a size, clearing the surface and exposing it

or. There is no liquid film in this case to eat transfer.

sult, with dropwise condensation, heat r coefficients can be achieved that are han 10 times larger than those associated m condensation.

allenge in dropwise condensation is not eve it, but rather, to sustain it for

Dropwise condensation of steam on copper surfaces:

Boiling heat transfer

Pool boiling Boiling regimes and the boiling curve

Flow boiling

Condensation heat transfer

Film condensation

Film condensation inside horizontal tubes

Dropwise condensation

Summary