thermo syphons

7/31/2019 Thermo Syphons

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1

Heat pipes and Thermosyphons

Cold end

Hot end

Inside the system, there is a fluid (usually termed refrigerant)

Heat pipes and Thermosyphons Heat is transferred as latent

heat of evaporation which

means that the fluid insidethe system is continuouslychanging phase from liquidto gas.

The fluid is evaporating atthe hot end, therebyabsorbing heat from thecomponent.

At the cold end, the fluid iscondensed and the heat isdissipated to a heat sink(usually ambient air).

Hot end

Cold end

Heat pipes and Thermosyphons Heat pipes

Heat pipes

In Heat Pipes, capillary forces in the wick

ensures the liquid return from the hot end to

the cold end.

This means that a Heat Pipe can operate

independent of gravity. The heat pipe was

actually developed for zero gravity (i.e.

space) applications.

Heat pipes


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2

Heat pipes Heat pipes - Applications

Heat pipes - Applications Thermosyphons

Are always gravity driven!

Loop system enables enhancement of heat

transfer and minimization of flow losses

(pressure drop).

Generally have better performance compared to

Heat Pipes working with gravity.

Schematic of a Thermosyphon

PCB

LiquidHot

Component

Liquid-

Vapor

Mixture

Evaporator

Condenser

Air

Example of a

Thermosyphon

cooling three

components in

parallel

1200

988

Falling tube length=1750mm

Rising tube height=1200 mm

27

Liquid head:988+27=1015 mm

Condenser

101510

273

Fallingtube

5 hole with d_f=1.5 mm

Evaporator

Risingtube


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3

Example of aThermosyphon

cooling three

components in

series

Areas in a thermosyphon

Component, 1 cm2

Evaporator, front, 2.2 cm2

Evaporator, inside, 3.5 cm2

Condenser, inside, 108 cm2

Condenser, facing air,

(heat sink included), 5400 cm2

4 times

Advantages with Thermosyphon cooling:

Large heat fluxes can be dissipated from small

areas with small temperature differences

(150 W/cm2)

Heat can be transferred long distances without

any (or with very small) decrease in

temperature.

Hot side Cold side

Temp

Saturation tempBoiling

Condensation

Temperatures obtained experimentally in aThermosyphon system that has three evaporators thateach cool one component. The total heat dissipation is170 W.

Component Contact

resistance

Evaporation

Saturation

temperature

Condensation

Contact

resistance

Thermosyphon

Fin to

air

Air

Condenser

Evaporator

Temperature difference as a function of the

heat dissipation(Prototype C, Condenser is fan cooled)

Data:P8F2MAX.STA10v * 23c

P (W)

Temp.d

ifference(C)

0

2

4

6

8

10

12

0 40 80 120 160

Filling Ratio = 39% Evaporator2

Condenser

R142b

Evaporator geometries

14.7 mm

d=1.1 mm

10mm

d=1.5 mm

Tc, d=0.8 mm

d=2.5 mm d=3.5 mm


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4

Cooling of Power Amplifiers in a

Radio Base Station

Thermosyphons - Applications

Thermosyphons - Applications Thermosyphons - Applications

Immersion cooling Two phase flow in a

large diameter tube:

Flow regimes determine heat transfermechanism


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5

Classification and application of

thermosyphon systems.

Open thermosyphon Closed thermosyphon

Pipe thermosyphon

Single-phase flow

Two-phase flow

Simple loop Thermosyphon

Single-phase flow

Two-phase flow

Closed advanced two-phase flow thermosyphon loop

Thermosyphon is a circulating fluid system whose motion is

caused by density difference in a body force field which result

from heat transfer.

Thermosyphon can be categorized according to:

1. The nature of boundaries (Is the system open or closed to mass flow)

2. The regime of heat transfer (convection, boiling or both)

3. The number of type of phases present (single-or two-phase state)

4. The nature of the body force (is it gravitational or rotational)

All thermosyphon systems removes heat from prescribed source

and transporting heat and mass over a specific path and rejecting the

heat or mass to a prescribed sink.

gas turbin blade cooling

electrical machine rotor cooling

transformer cooling

nuclear reactor cooling

steam tubes for bakers oven cooling for internal combustion engines

electronics cooling.

The most common industrial thermosyphon

applications include: Open Thermosyphon:Single-phase, natural-

convection open system in the

form of a tube open at the top

and closed at the bottom.

For open thermosyphon

Nua=C1Raam(a/L)C2,

Nua=(ha)/k

a: based on radius

Closed Thermosyphon

(simple pipe)A simple single-phase natural-

convection closed system in the form

of a tube closed at both ends.

It has been found that the closed

single-phase thermosyphon can be

treated as two simple open

thermosyphon appropriately joined at

the midtube exchange region.

The primary problem is that of

modeling the exchange region.

It has been found that the exchange

mechanism is basically convective.

Simple thermosyphon

loop

Advanced thermosyphon

loop

Evaporator

Condenser

Thermosyphon pipe


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Closed loop thermosyphon

Two distinct advantages make the closed-loop

thermosyphon profitable to study:

1. Natural geometric configuration which can be found or

created in many industrial situation.

2. It avoid the entry choking or mixing that occurs in the

pipe thermosyphon

3. For single phase loop:

4. NuL=0.245(GrPr2L/d)0.5 can be used

Two-phase thermosyphon

The advantages of operating two-phase

thermosyphons are:

1. The ability to dissipate high heat fluxes due to

the latent heat of evaporation and condensation

2. The much lower temperature gradients

associated with these process.

3. Reduced weight and volume with smaller heat

transfer area compared to other systems.

Heat pipe and thermosyphon

Thermosyphon and heat pipe cooling both rely on

evaporation and condensation. The difference between

the two types is that in a heat pipe the liquid is

returned from the condenser to the evaporator by

surface tension acting in a wick, but thermosyphon

rely on gravity for the liquid return to the evaporator .

However the cooling capacity of heat pipes are lowerin general compared to the thermosyphon with the

same tube diameter.

Closed advanced two-phase thermosyphonloop

Thermosyphon cooling offers passive circulationand the ability to dissipate high heat fluxes withlow temperature differences between evaporatorwall and coolant when implemented with surfaceenhancement.

An advanced two-phase loop has the possibility of

reducing the total cross section area of connectingtubes and better possibility of close contactbetween the component and the refrigerantchannels than a thermosyphon pipe or a heat pipe.

Thermosyphons

Heat Transfer and Pressure Drop

Rahmatollah Khodabandeh

Heat Transfer Coefficient

At least two different mechanisms behind flow boiling heattransfer: convective and nucleate boiling heat transfer.

General accepted that the convective boiling increasesalong a tube with increasing vapor fraction and mass flux.Increasing convective boiling reduces the wall superheatand suppresses the nucleate boiling. When heat transferincreases with heat flux with almost constant vaporfraction and mass flux, the nucleate boiling dominates theflow boiling process. Due to the fact that the mechanism ofconvective and nucleate boiling can coexist, a goodprocedure for calculating flow boiling must have bothelements.


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7

all heat transfer correlations can be divided into three basic

models: 1) Superposition model 2) Enhancement model 3)

Asymptotic model

In the superposition model, the two contributions are

simply added to each other, while in the enhancement

model the contribution of nucleate and convective boiling

are multiplied to obtain a single-phase model. In the

asymptotic model the two mechanisms are respectively

dominant in opposite regions.

The local heat transfer coefficient as sum of the two

contributions

Where n is an asymptotic factor equal to 1 for the

superposition model and above 1 for the asymptotic model

( ) ( )nnbnL

nb

ncb

ntp hFhEhhh +=+=

With larger n, the htp is implying more asymptotic behavior

in the respectively dominant region. hL and hnb are the heat

transfer coefficients for one-phase liquid flow and poolboiling respectively. E and F are enhancement and

suppression factors.

Chen, Gungor-Winterton [1986] and Jungs correlations

are based on superposition model.

Shah, Kandlikar and Gungor-Wintertons [1987]

correlations are based on enhancement model.

Liu-Winterton, Steiner-Taborek and VDI-Wrmeatlas are

based on asymptotic model.

Lazarek-Black, Tran and Crnwell-Kew have developed heat

transfer correlations for small diameter channel.

Coopers pool boiling correlation or Liu-Wintertons flow

boiling correlation can be used for heat transfer coefficient in

an advanced closed two-phase flow thermosyphon loop.

Liu-Winterton correlation( ) ( )[ ]

( )( )( ) ( )

( )

( )[ ]( )

( ) ( ) 4.0l8.0

ll

l

116.0l

1.0

35.0

g

ll

67.05.055.0r

12.0rpool

5.02pool

2ltp

PrRed

k023.0h

ReE055.01s

1Prx1E

qMp10logp55h

hshEh

=

+=

+=

=

+=

Total thermal resistance in an advanced closed two-phase

flow thermosyphon loop

The thermosyphons thermal resistance can be considered to the sum

of four major component resistances:

Rtot=Rcr+Rbo+Rco+Rcv

(K/W)

Rcr

is the contact resistance between the simulated component and the

evaporator front wall. In order to reduce Rcr

a thermally conductive

epoxy can be used.

Rbo

, is the boiling resistance.

Rco, is the condensing resistance. This resistance is in fact very low dueto the high heat transfer coefficient in condensation and the large

condensing area.

Rcv

is the convection resistance between the condenser wall and the air.

Heat transfer depends on pressure level, vapor fraction,flow rate, geometry of evaporator and thermal properties of

refrigerant.

The influence of pressure level, choice of working fluid,

geometry of evaporator, pressure drop, heat transfer

coefficient, critical heat flux and overall thermal resistance

were investigated during the present project.

Considerations when choosing refrigerant

A fluid which needs small diameter of

tubing

A fluid which gives low temp. diff. in

boiling and condensation

A fluid which allows high heat fluxes in the

evaporator.


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8

For turbulent single-

phase we can derive

pressure drop as:

For a certain tube

length, diameter and

cooling capacity, the

pressure drop is a

function of viscosity,

density and heat of

vaporization.

4/7

4/1

4/19

4/7

2

2

2

4/11

21

241.0

4

4

/

4

Re

Re158.0

fg

fg

fg

hd

QLp

dh

Q

d

hQ

d

m

A

Vw

dw

f

d

Lwfp

=

===

=

=

=

Fig. shows ratio of viscosity

to density and heat of

vaporization vs. Saturated

pressure, we find that the

general trend is decreasingpressure drop with increasing

pressure and decreasing

molcular weights.

The Two-phase pressure

drops expected to follow the

same trends.

For Saturated temperature

between 0-60 C.

0.00E+00

5.00E-09

1.00E-08

1.50E-08

2.00E-08

2.50E-08

0 5 10 15 20 25 30 35 40

Pressure (bar)

Figureofmerit(Dp)

R32, M=52.02

NH3, M=17.03

R12, M=120.9

R134a, M=102

R22, M=86.47

R600a, M=58.12

Coopers pool boilingcorrelation is plottedversus saturatedpressure for differentfluids: (for saturatedtemp. between 0-60C)

As can been seen heattransfer coefficientgenerally increases

with increasingpressure anddecreasing themolecular weights.

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

0 5 10 15 20 25 30 35 40

Ps(bar)

h-Coo

erW/mK

NH3, M=17.03

R32, M=52.02

R600a, M=58.12

R134a, M=102

R12, M=120.9

R22, M=86.47

R11, M=137.4

Another important

parameter when choosing

working fluid is the critical

heat flux.

Figure shows calculation of

Kutateladze CHF correlation

versus reduced pressure for

pool boiling.

As can been seen ammonia

once again shows

outstanding properties with3-4 times higher than the

other fluids.

0

300

600

900

1200

1500

1800

2100

2400

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Reducedpressure

CHF(W)

R600a, M=58.12

R11, M=137.4

NH3, M=17.03

R134a, M=102

R12, M=120.9

R22, M=86.47

R32, M=52.02

FC fluids In immersion boiling FC fluids have been used

FC fluids generally have poor heat transferproperties:

-Low thermal conductivity

-Low specific heat

-Low heat of vaporization

-Low surface tension

-Low critical heat flux

-Large temperature overshoot at boilingincipience

Influence of system pressure and

threaded surface

R600a (Isobutane)

Tests were done at five reduced pressures ;

; 0.02, 0.05, 0.1, 0.2 and 0.3.

Two types of evaporators: smooth and

threaded tube surfaces.

crr

p

pp =


9/14

9

The picture shows heat flux vs.

temperature difference between

inside wall temperature and

refrigerant.

As can be seen, the temperature

difference increases with

increasing heat flux, but with

different slopes, depending on the

saturation pressure in the system

As the heat transfer coefficient is

the heat flux divided by the temp.

difference, this indicates higher

heat transfer coefficient with

increasing pressure

0

50000

100000

150000

200000

250000

300000

350000

0 5 10 15 20 25

DT (C)

q(W/m)

pr=0.02pr=0.3

Isobutane

Smooth tube

The Fig. shows temperature

difference between inside wall

temperature and refrigerant vs. heat

input.

As can be seen, the temperaturedifference increases with

increasing heat input, but with

different slopes, depending on the

saturation pressure in the system

As the heat transfer coefficient is

the heat flux divided by the temp.

difference, this indicates higher

heat transfer coefficient with

increasing pressure

0

2

4

6

8

10

12

14

16

18

2022

24

0 20 40 60 80 100 120Q (W)

DT(C)

pr=0.3

pr=0.2

pr=0.1

pr=0.05

pr=0.02

The Fig. shows, heat transfer

coeff. vs. reduced pressure for 110

W heat input to each one of the

evaporators.

The dependence of heat transfer

coefficient on reduced pressure are

often expressed in the form of h=f

(prm), in which m is generally

between 0.2-0.35.

In the present case, m=0.317,

correlates the experimental data

well for the smooth tube with

Isobutane as refrigerant.

h = constantpr0.317

R2 = 0.9957

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

pr

h(W/m.K)

Q=110 W

Effect of threaded surface

at different reduced

pressure on heat transfer

coefficient

The fig. shows temp. diff. vs.

reduced pressure from 10 to 110 W

heat input for each one of

evaporators on threaded surface.

Relatively low temp. diff can be

achieved.

Temp. diff. In the most points willbe reduced to less than a third by

increasing the reduced pressure

from 0.02 to 0.3.

0

1

2

3

4

5

6

7

8

9

10

0 0.1 0.2 0.3 0.4

pr

DT(C)

10 W

30 W

50 W

70 W

90 W

110 W

Effect of heat flux on heat

transfer coefficient

Figur shows the relation

between heat transfer

coefficient and heat flux for

Pr=0.1, with smooth tube.

The dependence of heat

transfer coefficient on heat

flux can be expressed as h=f

(qn), n, in most cases varies

between 0.6-0.8

Presented data follows h=f

(q0.57)

y = 0.8761x0.5755

R2 = 0.9984

0

5

10

15

20

25

0 40 80 120 1 60 2 00 2 40 2 80

q (kW/m)

h(kW/m.K)

R600a

h=f (qn)

h=f (q0.57

)

Comparison between

Coopers correlation andexperimental results

The Fig. shows heat transfer coeff.

comparison between Coopers pool

boiling correlation versus

experimental results for smooth

tube surfaces at different reduced

pressure.

As can be seen the heat transfer

coeff. calculated by Coopers

correlation is in good agreement

with the experimental results

For the most points the deviation

is less than 25 percent.

0

10000

20000

30000

40000

50000

0 10000 20000 30000 400 00 50000

h-exp (W/mK)

h-Cooper(W/mK)

Q=10 W

Q=30 W

Q=50 W

Q=70 W

Q=90 W

Q=110 W

25%

25%


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10

Comparison between Liu-

Wintertons correlation and

experimental results

The Fig. shows heat transfer coeff.,

comparison between Liu-Wintertons

correlation versus experimental

results for smooth tube surfaces at

different reduced pressure.

As can be seen the heat transfer

coeff. calculated by Liu-Wintertons

correlation is in good agreement with

the experimental results

For the most points the deviation is

less than 25 percent.

0

10000

20000

30000

40000

50000

0 10 00 0 2 000 0 300 00 40 00 0 50 000

h-exp (W/mK)

h-LW(W/mK)

10 W30 W

50 W

70 W

90 W

110 W

25%

25%

Influence of diameter

Testing condition

R600a as refrigerant

Tests were done with 7, 5,4, 3, 2 and 1 vertical

channels with diameter of 1.1, 1.5,1.9, 2.5 3.5 and

6 mm.

Smooth surface

At reduced pressure 0.1 (p/pcr)

Influence of diameter

Heat transfer coefficient vs.

heat flux at different diameters.

The influence of diameter on

the heat transfer coefficients for

these small diameter channels

was found to be small and no

clear trends could be seen.

0

5

10

15

20

25

30

0 50 100 150 200 250 300 350

Heat flux (kW/m)

h-exp.

(kW/mK) d=6 mm

d=3.5 mm

d=2.5 mm

d=1.9 mm

1.5 m m

d=1.1mm

Conclusions

Heat transfer coefficients and CHF can be expected toIncrease with increasing reduced pressure and withdecreasing molecular weight

The effects of pressure, and threaded surface on heattransfer coefficient have been investigated.

The pressure level has a significant effect on heattransfer coefficient.

h=f (prm) m=0.317

h=f (qn) where n=0.57

Conclusion

Heat transfer coefficient can be improved by usingthreaded surfaces.

Heat transfer coefficient at a given heat fluxis more than three times larger at the reducedpressure 0.3 than 0.02 on threaded surfaces.

The experimental heat transfer coefficients arein relatively good agreement with CoopersPool boiling and Liu-Wintertons correlations.

Conclusion

The effects of pressure, mass flow, vapor quality, andenhanced surface on CHF have been investigated.

Threaded surface has a minor effect on CHF.

The pressure level has a significant effect on CHF.

The CHF can be increased by using a higher pressure.

The influence of diameter on the heat transfer coefficientsfor these small diameter channels was found to be smalland no clear trends could be seen.


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11

Operation condition of an advanced two-phase

thermosyphon loop

The net driving head caused by the difference in densitybetween the liquid in the downcomer and the vapor/liquid

mixture in the riser must be able to overcome the pressure

drop caused by mass flow, for maintaining fluid circulation.

The pressure changes along the thermosyphon loop due to

gravitation, friction, acceleration, bends, enlargements and

contractions.

In design of a compact two-phase thermosyphon system,

the dimensions of connecting tubing and evaporator,

affects the packaging and thermal performance of thesystem.

The pressure drop is a limiting factor for small tubing

diameter and compact evaporator design.

By determining the magnitude of pressure drops at

different parts of a thermosyphon, it may be possible to

reduce the most critical one, therby optimizing the

performance of the thermosyphon system.

Single-phase flow pressure drop in downcomer

The total pressure drop in the downcomer consists of two

components: frictional pressure drop and pressure drop due

to bends respectively.

For fully developed laminar flow in circular tubes, the

frictional pressure drop can be calculated by:

For the turbulent flow regime, the Blasius correlation forthe friction factor can used:

ll

d

LGp

=2

Re

16

l

ld

LGp

= 2

Re079.0 25.0

The pressure loss around bends can be calculated by:

where is an empirical constant which is a function of

curvature and inner diameter.

In the downcomer section, the pressure drop due to friction

is much larger than the pressure loss around bends.

l

lb

Gp

=

2

Two-phase flow pressure drop Two-phase flow in the riser and evaporator:

The total two-phase flow pressure drop consists of six

components:

1. Acceleration pressure drop

2. Friction pressure drop

3. Gravitational pressure drop

4. Contraction pressure drop

5. Enlargement pressure drop

6. Pressure drop due to the bends

7. Frictional and gravitational pressure drop are most important

pressure drops in the riser

Method of analysis two-phase flow pressure drop

The methods used to analyse a two-phase flow are often

based on extensions of single-phase flows.

The procedure is based on writing conservation of mass,

momentum and energy equations.

To solve these equations, often needs simplifying

assumptions, which give rise different models.


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12

Homogeneous flow model

One of the simplest predictions of pressure drop in two-

phase flow is a homogeneous flow approximation.

Homogeneous predictions treat the two-phase mixture as a

single fluid with mixture properties.

In the homogeneous flow model it is assumed that the two

phases are well mixed and therefore have equal actual

vapor and liquid velocities.

In other words in this model, the frictional pressure drop is

evaluated as if the flow were a single-phase flow, by

introducing modified properties in the single-phase friction

coefficient.

Separated flow model

The separated flow model is based on assumption that two

phases are segregated into two separated flows that have

constant but not necessarily equal velocities.

Drift flux model

This model is a type of separated flow model, which looks

particularly at the relative motion of the phases. The model

is most applicable when there is a well-defined velocity in

the gas phase

Pressure drop in the riser

The total two-phase flow pressure drop in the riser is

mainly the sum of two contributions: the gravitational-

and the frictional pressure drop.

The most used correlations for calculation of frictional

pressure drop are:

1. Lockhart-Martinelli correlation

2. CESNEF-2 correlation

3. Friedel correlation

4. Homogeneous flow model correlation

In the homogeneous model, the analysis for single-phaseflow is valid for homogeneous density and viscosity. Thehomogeneous density is given by:

Several different correlations have been proposed forestimation of two-phase viscosity, such as:

Cicchitti et al.

Beattie- Whalley

Mc Adams et al.

Dukler et al.

Lgh

xx

+=

11

( ) Lgh x1x +=

)5.21)(1( gLh ++=

Lgh

xx

+=

11

( )

L

hL

g

hg

h

xx

1 +=

g

hx

=

Gravitational pressure drop

The gravitational or head pressure change at the riser

The momentum equation gives:

Where is void fraction A: total cross-section area (m2)

Ag: average cross-section area occupied by the gas phase (m2)

Void fraction can be calculated by:

1. Homogeneous model

2. Zivimodel [1963]

3. Turner& Wallis two-cylinder model [1965]

4. Lockhart-Martinelli correlation [1949]

5. Thom correlation [1964]

6. Baroczy correlation [1963]

rmRG Hgp , =

Lgm )1( +=

A

Ag= For the homogeneous flow the phase velocities are equal,

uL=ug, , where Sis the slip ratio.

+

=

L

g

L

g

x

x

u

u

)1(1

1

L

g

u

uS=

+

=

L

g

h

x

x

)1(1

1


13/14

13

Acceleration pressure drop

Acceleration pressure drop in the evaporator, resulting

from the expansion due to the heat input during the

evaporation process can be calculated:

(homogeneous model)

v specific volume

xvvGp Lg )(2 =

Experimental setup

Not to scale

939

974

Condenser

8

95

10

15

10

Evaporator

Downcomer

5 hl med d_ f=1.5 mm5 hole with d_f=1.5 mm

ID=6.1 mm

Abs.pressuretransduc

er

1160

186

255

150

glass tube

77

Fig. 1

B

C

All dimensions in the figureare in mm

CHFTesting condition

R600a (Isobutane)

Tests were done at three reduced pressures;

0.035, 0.1, and 0.2.

Two types of evaporators: smooth and

threaded tube surfaces.

CHF=f(pr, G, x)Effect of pressure onCHF:The Fig shows temperaturedifference between inside walltemperature and refrigerant for threeevaporators, vs CHF.

For pr =0.2 the CHF is 690 W whichcorrespond to 230 W/cm front area ofthe component which correspond to650 kW/m heat flux for smoothchannels.

As can be seen, the saturationpressure strongly affected the temp.diff. With increased pressure thetemp. diff. decreases in the total rangeof heat load up to CHF.

0

5

10

15

20

25

30

35

350 400 450 500 550 600 650 700 750

Qtot (W)

DT(C)

0.035

0.1

0.2

pr=0.2

pr=0.1pr=0.035

smooth channel

Effect of mass flow on

CHF

The mass flow is a function ofboth heat flux and system pressure.

As can be seen simulations atCHF shows that mass flowincreases with increasing reducedpressure.

This is believed to be theexplanation for the higher CHF.

Higher pressure gives higher massflow on CHF, which facilitates thedeposition and replenishment ofliquid film.

0

0.001

0.002

0.003

0.004

0.005

0.006

0 100 200 300 400 500 600 700

Qcri

(W)

m_

dot(kg/s)

pr=0.035

pr=0.1

pr=0.2

smooth channel

Effect of vapor

quality on CHF

The Fig. shows, vapor

quality vs. CHF for three

evaporators.

According to the

simulations the vapor

quality at different

pressure on CHF is almost

constant.

00.1

0.20.30.40.50.60.7

0.80.9

1

0 100 200 300 400 500 600 700

Qcri (W)

x

pr=0.035

pr=0.1

pr=0.2

smooth channel


14/14

14

Effect of enhanced surfaceon CHF

Generally at enhanced surfaces

increases the heat transfer.In this study threaded surfaceshave been used to investigate theeffect of surface structure on CHF.

The picture shows the CHF versusreduced pressure for both surfaces.

However the CHF is independenton surface condition.

The fact that the surface conditionis unimportant for CHF werereported by other researcher.

0

100

200

300

400

500

600

700

0 0.05 0. 1 0. 15 0. 2 0.25

pr

Qcri(W)

threaded

smooth

Comparison between

Kutateladzes

correlation and

experimental results

The Fig. shows CHF,

comparison between

Kutateladzes pool boiling

correlation versus

experimental results for

smooth tube surfaces.

Deviation is less than 15

percent.

0

100

200

300

400

500600

700

0 100 200 300 400 500 600 700

Q_cri_exp. (W)

Q_

cri_

pb.

(W)

15%

-15%

Old Exam Problem 2003-03-07A thermosyphon can be quite complex to model. In this assignment

we will investigate the behavior of a simplified thermosyphon. The

difference in height between the condenser and the evaporator is 15

cm. The tube diameter is 5 mm and the downcomer tube length is

16 cm. The heat exchanger area in the condenser and the evaporator

is 40 cm and 4 cm respectively. The total pressure drop in the

rising tube can be calculated using pRiser = 6.21x, where pRiseris in kPa, x is the change in vapor quality in the evaporator. Therefrigerant is R134a for which the latent heat of vaporization,

hfg = 163 kJ/kg, the liquid density,L=1146 kg/m, and dynamicviscosity, L=1.7810-4 Pas. The temperature of the evaporatorwalls is 50 C, the boiling heat transfer coefficient is 20.000

W/(mK), and the heat dissipation is 60 W. Calculate the mass flow

rate, , the change in vapor quality, x, and the saturationtemperature of the refrigerant (6 credits).

m&

thermo syphons

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