BOILING HEAT TRANSFER – Boiling curve

BOILING HEAT TRANSFER – Boiling curve

WALL TEMPERATURE

HE

AT

FLU

X

Fully Developed Boiling Region

Partial Boiling RegionVarious curves representvarious flow rates

Single-Phase Forced Convection

Extension of the FDBBoiling CurveExtension of the

Single-Phase Curve

TSAT

F

B B’

BOILING HEAT TRANSFER – Nucleate boiling

POOL BOILING 1. Mechanistic models

a) Partially Developed Nucleate Boiling

Main heat transfer mechanism: Transient conduction into liquid (with bubbles acting like pumps – proposed by Forster and Grief 1959) Additional contributions: Microlayer evaporation and natural convection

b) Fully Developed Nucleate Boiling Main heat transfer mechanism: Evaporation (microlayer and macrolayer)

2. Correlations

a) Simplified form (yet useful in practice)

nsatTq ∆∝" n is between 3 and 3.33

what is the heat transfer coefficient in this case?

BOILING HEAT TRANSFER – Nucleate boiling

b) Standard form: Rohsenow (1952)

),(k

cpULfkhL µ

µρ=

What is U? What is L?

BOILING HEAT TRANSFER – Nucleate boiling

Final form of the correlation:

( ) n

f

ff

fg

satfs

fgf

gf

kcp

iTcp

Ci

gq

∆=

− − µµ

ρρσ

33

"

n is 3 for water and 5.1 for all other common liquids

BOILING HEAT TRANSFER – Nucleate boiling

FORCED CONVECTION BOILING – CORRELATIONS: Chen type correlation

ShFhh poolFCtp ⋅+⋅=

4.08.0 PrRe023.0Dk

h fFC =

75.024.024.024.029.05.0

49.045.079.0

00122.0 satsatgfgf

fffpool pT

icpk

h ∆∆

=

ρµσρ

where: ( ) 25.11Re FDxGf

−=µ

is the two phase Reynolds number

BOILING HEAT TRANSFER – Nucleate boiling

FORCED CONVECTION BOILING – CORRELATIONS: Chen type correlation

17.16

736.0

Re1056.211

1.01213.0135.2

1.010.1

tp

tttt

tt

S

Xif

XF

XifF

−⋅+=

>

+=

≤=

1.05.09.01

−=g

f

f

gtt x

xXµµ

ρρ ;

fg

g

mmm

xDD

D

+= , x is the mass quality