BOLING AND CONDENSATION HEAT TRANSFER

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BOILING AND

CONDENSATION

Boiling – The change from the liquid to the vapor state is sustained by heat

transfer from the solid surface.

Condensation – The change from the vapor to the liquid state results in heat

transfer to the solid surface.



CONVECTION PROCESSES OF BOILING ANDCONDENSATION

Boiling Modes

• When evaporation occurs at a solid-liquid interface, it is termed boiling• Heat is transferred from the solid surface to the liquid

q”s = h (Ts – Tsat) = h ΔTe , ΔTe = excess temp.

• The process is characterized by the formation of vapor bubbles

• Boiling may occur under various conditions:

i) Pool boiling- process in which the heating surface is submerged in a large

body of stagnant liquid

ii) Forced convection boiling

- fluid motion is induced by external means as well as by free

convection and bubble induced mixing

iii) Subcooled boiling

- the temp.of the liquid is below the saturation temp. and

bubbles formed at the surface may condensed in liquid

iv) Saturated boiling

- the temp. of the liquid slightly exceeds the saturation temp.



Homeworks

Describe briefly the boiling regimes as shown above

(Figure 10.4)

Explain briefly the forced convection boiling flow

regimes as shown above (Figure 10.8)

Submit: 23rd of September 2010



Pool Boiling Correlations

Nucleate Pool Boiling Correlation:

– The coefficient Cs,f and the exponent n depend on the solid-liquid combination and representative

experimentally determined values are presented in Table 10.1

• Critical Heat Flux

– For horizontal plates (C = 0.149)

– For sphere, large horizontal cylinders and large finite heated surfaces (C = 0.131)

• Minimum Heat Flux

C = 0.09

All properties are evaluated at the saturation temperature.



Pool Boiling Correlations

• Film Pool Boiling Correlation:

– For horizontal cylinders (C = 0.62)

–For spheres (C = 0.67)

Vapor properties are evaluated at the film temperature, Tf = (Ts + Tsat)/2 and the liquid density is evaluated

at the saturation temperature (Tsat).

At Ts ≥ 300°C, radiation across the vapor film becomes significant. To calculate total heat transfer

coefficient:

The effective radiation coefficient is expressed as

( )sats

4sat

4s

rad TT

TTεσ

=h

ε = emissivity of the solid (Table A.11)

σ = Stefan-Boltzman constant



PROBLEM 10.10 – Nucleate Boiling

PROBLEM 10.27 – Film Boiling

EXAMPLES:



CONDENSATION : PHYSICAL MECHANISMS

• Condensation occurs when the temperature of a vapor is reduced below its saturation temperature

• The latent energy of the vapor is released, heat is transferred to thesurface and the condensate is formed

• Modes of Condensation :

i) Film condensation (Figure 10.9 a)

- occurs in clean and uncontaminated surfaceii) Dropwise condensation (Figure 10.9 b)

- occurs in coated surface with a substance that inhibits wetting

- the drops form in crack, pits and cavities on the surface

iii) homogenous condensation (Figure 10.9 c)- vapor condenses out as droplets suspended in a gas phase to

form a fog

iv) direct contact condensation (Figure 10.9 d)

- occurs when vapor is brought into contact with a cold liquid



LAMINAR CONDENSATION ON A VERTICALPLATE

• For laminar film condensation, the total condensation rate may bedetermined from the relation :

m = q q = total heat transfer to the surface

h’fg • The total heat transfer to the surface is

q = hL A (Tsat – Ts)

• The average Nusselt number :NuL = hL L = 0.943 ρℓ g (ρℓ - ρ v ) h’fg L3 1/4

k μℓk ℓ (Tsat – Ts)

• Heat of vaporization, h’fg

h’fg = hfg (1 + 0.68Ja)Ja = CPℓ (Tsat – Ts)

hfg

* All liquid properties (ρℓ , μℓ , CPℓ , k ℓ) should be evaluated at the filmtemperature, Tf = (Tsat – Ts)/2 and ρ v and hfg should be evaluated at Tsat *



TURBULENT FILM CONDENSATION• To check the flow conditions, Reynolds number may be expressed as

Reδ = 4m = 4ρℓ Um δ

μℓ b μ ℓ

• For Laminar Wave – free Region:hL (υℓ

2/g)1/3 = 1.47 Reδ-1/3 , Reδ ≤ 30

k ℓ • For Laminar Wavy Region :

hL

(υℓ

2/g)1/3 = Reδ

, 30 ≤ Reδ ≤ 1800

k ℓ 1.08 Reδ1.22 – 5.2

• For Turbulent Region :

hL (υℓ2/g)1/3 = Reδ

, Reδ ≥ 1800

k ℓ 8750 + 58Pr-0.5 (Reδ

0.75 – 253)

L

b

Laminar wave-free region (Re ≤ 30)

Laminar wavy region (30 ≤ Re ≤ 1800)

Turbulent region (Re ≥ 1800)

υℓ = μℓ /ρℓ g = 9.81 m/s2



• To determine the value of Reynolds number,

Reδ :

( )

( )

( )

( )

( )( )1800 ≥Re 253+Pr 151Pr +

g/υhμ

TTLk069.0=Re

1800 ≤Re ≤30 8.4+g/υhμ

TTLk70.3=Re

30 ≤Re g/υhμ

TTLk78.3=Re

δ345.05.0

312'fg

ssatδ

δ

82.0

312'fg

ssatδ

δ

43

312'fg

ssatδ



FILM CONDENSATION ON RADIAL SYSTEMS

• Laminar film condensation on the outer surface of a sphere may be

expressed as :hD = 0.826 g ρℓ (ρℓ - ρ v ) k ℓ

3 h’fg ¼

μℓ (Tsat – Ts) D

• Laminar film condensation on the outer surface of a horizontal tube:

hD = 0.729 g ρℓ (ρℓ - ρ v ) k ℓ3 h’fg

¼


• For N horizontal tubes :

hD,N = 0.729 g ρℓ (ρℓ - ρ v ) k ℓ3 hfg

¼

Nμℓ (Tsat – Ts) D

hD,N = hDN-1/4 , hD = heat transfer coefficient for thefirst (upper) tube

h’fg = hfg [1 + 0.68 Ja] , Ja = CPℓ (Tsat – Ts)

hfg



FILM CONDENSATION IN HORIZONTAL TUBES

• For low vapor velocities :Reυ,i = ρυ um,υ D < 35,000

μυ i

• Laminar film condensation in the inner surface of a horizontal tubemay be expressed as :

hD = 0.555 g ρℓ (ρℓ - ρ v ) k ℓ3 h’fg ¼


h’fg = hfg + 3 CPℓ (Tsat – Ts)

8



DROPWISE CONDENSATION

• For drop wise condensation :hdc = 51104 + 2044 Tsat (°c) , 22°c ≤ Tsat ≤ 100°c

hdc = 255510 , Tsat ≥ 100°c

• All liquid properties are evaluated at the film temperature

Tf = (Tsat + Ts)/2

• ρυ and hfg are evaluated at Tsat



PROBLEM 10.47

Saturated ethylene glycol vapor at 1 atm is exposed to a vertical plate

300 mm high and 100 mm wide having a uniform temperature of 420 K.Estimate the heat transfer rate to the plate and the condensation rate. Approximate the liquid properties as those corresponding to saturated

conditions at 373 K (Table A.5)

Example – Film condensation on vertical plate

Example – Film condensation on radial system

PROBLEM 10.55

A horizontal tube of 50mm diameter with a surface temperature of 34°c isexposed to steam at 0.2 bar. Estimate the condensation rate and heat transferrate per unit length of the tube?

BOLING AND CONDENSATION HEAT TRANSFER

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