1Heat pipes and ThermosyphonsCold 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 inside the system is continuously changing phase from liquid to gas.

The fluid is evaporating at the hot end, thereby absorbing heat from the component.

At the cold end, the fluid is condensed and the heat is dissipated 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

2Heat 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

1015

10

273

Fal

ling

tube

5 hole with d_f=1.5 mm

Evaporator

Ris

ing

tube

3Example of a Thermosyphon cooling three components in series

Areas in a thermosyphon

Component, 1 cm2Evaporator, front, 2.2 cm2Evaporator, inside, 3.5 cm2Condenser, inside, 108 cm2Condenser, 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 a Thermosyphon system that has three evaporators that each cool one component. The total heat dissipation is 170 W.

Component Contact resistance

Evaporation

Saturation temperature

CondensationContact 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.STA 10v * 23c

P (W)

Tem

p.di

ffere

nce

(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

10 m

m

d=1.5 mm

Tc, d=0.8 mm

d=2.5 mm d=3.5 mm

4Cooling 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 transfer mechanism

5Thermosyphons

Rahmatollah Khodabandeh

Heat Transfer Coefficient

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

General accepted that the convective boiling increases along a tube with increasing vapor fraction and mass flux. Increasing convective boiling reduces the wall superheat and suppresses the nucleate boiling. When heat transfer increases with heat flux with almost constant vapor fraction and mass flux, the nucleate boiling dominates the flow boiling process. Due to the fact that the mechanism of convective and nucleate boiling can coexist, a good procedure for calculating flow boiling must have both elements.

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

( ) ( )nnbnLnbncb

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 pool boiling 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 inan advanced closed two-phase flow thermosyphon loop.

Liu-Winterton correlation( ) ( )[ ]

( )( )( ) ( )

( )

( )[ ]( )( ) ( ) 4.0l8.0lll

116.0l

1.0

35.0

g

ll

67.05.055.0r

12.0rpool

5.02pool

2ltp

PrRedk

023.0h

ReE055.01s

1Prx1E

qMp10logp55h

hshEh

=

+=

-

rr

+=

-=

+=

-

--

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 due

to the high heat transfer coefficient in condensation and the large condensing area.

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

6 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.

Consideration 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.

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

mAV

w

dwf

dL

wfp

r

m

prpr

pr

u

r

=D

===

=

=

=D

-

Fig. shows ratio of viscosity to density and heat of vaporization vs. Saturated pressure, we find that the the general trend is decreasing pressure 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)

Fig

ure

of

mer

it (

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 boiling correlation is plotted versus saturated pressure for different fluids: (for saturated temp. between 0-60 C)

As can been seen heat transfer coefficient generally increases with increasing pressure and decreasing the molecular weights.

05000

100001500020000

250003000035000

4000045000

0 5 10 15 20 25 30 35 40

Ps (bar)

h-C

oope

r (W

/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 with 3-4 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

Reduced pressure

CH

F (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

7FC fluids In immersion boiling FC fluids have been used FC fluids generally have poor heat transfer

properties: -Low thermal conductivity -Low specific heat -Low heat of vaporization -Low surface tension -Low critical heat flux -Large temperature overshoot at boiling

incipience

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 =

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 temperature difference 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

02468

1012141618202224

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 transfercoeff. 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 withIsobutane 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 will be reduced to less than a third by increasing the reduced pressure from 0.02 to 0.3.

0123456789

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

8Effect of heat flux on heat transfer coefficientFigur 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 0.6-0.8Presented data follows h=f (q0.57)

y = 0.8761x0.5755

R2 = 0.9984

0

5

10

15

20

25

0 40 80 120 160 200 240 280

q (kW/m)h

(kW

/m.

K)

R600a

h=f (qn)

h=f (q0.57)

Comparison between Coopers correlation andexperimental resultsThe 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 transfercoeff. calculated by Coopers correlation is in good agreement with the experimental resultsFor the most points the deviation is less than 25 percent.

0

10000

20000

30000

40000

50000

0 10000 20000 30000 40000 50000

h-exp (W/mK)

h-C

oo

per

(W

/m

K)

Q=10 W

Q=30 W

Q=50 W

Q=70 W

Q=90 W

Q=110 W

25%

25%

Comparison between Liu-Wintertons correlation andexperimental resultsThe Fig. shows heat transfer coeff., comparison between Liu-Wintertonscorrelation versus experimental results for smooth tube surfaces at different reduced pressure.

As can be seen the heat transfercoeff. calculated by Liu-Wintertonscorrelation is in good agreement with the experimental resultsFor the most points the deviation is less than 25 percent.

0

10000

20000

30000

40000

50000

0 10000 20000 30000 40000 50000

h-exp (W/mK)

h-L

W(W

/m

K)

10 W

30 W

50 W

70 W

90 W

110 W

25%

25%

Influence of diameterTesting 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-e

xp. (

kW/m

K

) d=6 mm

d=3.5 mm

d=2.5 mm

d=1.9 mm

1.5 mm

d=1.1mm

Conclusions

Heat transfer coefficients and CHF can be expected to Increase with increasing reduced pressure and with decreasing molecular weight

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

The pressure level has a significant effect on heat transfer coefficient. h=f (prm) m=0.317 h=f (qn) where n=0.57

9Conclusion

Heat transfer coefficient can be improved by using the threaded surfaces.

Heat transfer coefficient at a given heat flux is more than three times larger at the reduced pressure 0.3 than 0.02 on threaded surfaces.

The experimental heat transfer coefficients are in relatively good agreement with Coopers Pool boiling and Liu-Wintertons correlations.

Conclusion The effects of pressure, mass flow, vapor quality, and enhanced 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 improved by using the higher pressure.

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.

Classification and application of thermosyphon system.

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 syatems removing heat from prescribed sourceand transporting heat and mass over a specific path and rejecting the heat or mass to a prescribed sink.

The most common industrial thermosyphon applications include:

gas turbin blade cooling electrical machine rotor cooling transformer cooling nuclear reactor cooling steam tubes for bakers oven cooling for internal combustion engines electronic cooling.

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 thermosyphonNua=C1Raam(a/L)C2, Nua=(ha)/ka: based on radius

10

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.

ThermosyphonSimple thermosyphon

loopAdvanced thermosyphon

loop

Evaporator

Condenser

ThermosyphonThermosyphon pipe

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 condensation2. 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 lower in general compared to the thermosyphon with the same tube diameter.

Closed advanced two-phase thermosyphon loop

Thermosyphon cooling offers passive circulation and the ability to dissipate high heat fluxes with low temperature differences between evaporator wall and coolant when implemented with surface enhancement.

An advanced two-phase loop has the possibility of reducing the total cross section area of connecting tubes and better possibility of close contact between the component and the refrigerant channels than a thermosyphon pipe or a heat pipe.

11

Operation condition of an advanced two-phase thermosyphon loop

The net driving head caused by the difference in density between 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 the system.

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 for

the friction factor can used:

ll d

LGp

r

=D2

Re16

l

l dLG

pr

=D -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

rx

=D

2

x

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 drop2. Friction pressure drop3. Gravitational pressure drop 4. Contraction pressure drop5. Enlargement pressure drop6. Pressure drop due to the bends7. 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.

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 correlation2. CESNEF-2 correlation3. Friedel correlation4. Homogeneous flow model correlation

In the homogeneous model, the analysis for single-phase flow is valid for homogeneous density and viscosity. The homogeneous density is given by:

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

Cicchitti et al. Beattie- Whalley

Mc Adams et al.

Dukler et al.

Lgh

xxrrr-+= 11

( ) Lgh xx mmm 1 -+=

bmbbmm )5.21)(1( gLh ++-=

Lgh

xxmmm-

+=11

( )L

hL

g

hgh

xx

rrm

r

rmm

1 -+=

g

hxrr

b

=

Gravitational pressure drop The gravitational or head pressure change at the riser The momentum equation gives: Where a 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 model2. Zivi model [1963]3. Turner& Wallis two-cylinder model [1965]4. Lockhart-Martinelli correlation [1949]5. Thom correlation [1964]6. Baroczy correlation [1963]

rmRG Hgp , r=D

Lgm rarar )1( -+=

A

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

uL=ug, , where S is the slip ratio.

-+

=

L

g

L

g

xx

u

u

r

ra

)1(1

1

L

g

u

uS =

-+

=

L

gh

xx

r

ra

)1(1

1

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 -=D

Experimental setup

Not to scale

939

974

Condenser

8

95

1015

10

Evaporator

Dow

ncom

er

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

ID=6.1 mm

Abs.pressuretransduc

er

1160

1862

55150

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 on CHF:The Fig shows temperature difference between inside wall temperature and refrigerant for three evaporators, vs CHF.For pr =0.2 the CHF is 690 W which correspond to 230 W/cm front area of the component which correspond to 650 kW/m heat flux for smooth channels. As can be seen, the saturation pressure strongly affected the temp. diff. With increased pressure the temp. diff. decreases in the total range of 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)

0.0350.10.2

pr=0.2

pr=0.1pr=0.035

smooth channel

Effect of mass flow on CHF

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

As can be seen simulations at CHF shows that mass flow increases with increasing reduced pressure.This is believed to be the explanation for the higher CHF.

Higher pressure gives higher mass flow on CHF, which facilitates the deposition and replenishment of liquid 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_

do

t (k

g/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.

00.10.20.30.40.50.60.70.80.9

1

0 100 200 300 400 500 600 700

Qcri (W)

x

pr=0.035

pr=0.1

pr=0.2

smooth channel

14

Effect of enhanced surface on CHF

Generally at enhanced surfaces increases the heat transfer.In this study threaded surfaces have been used to investigate the effect of surface structure on CHF.

The picture shows the CHF versus reduced pressure for both surfaces.

However the CHF is independent on surface condition.The fact that the surface condition is unimportant for CHF were reported by other researcher.

0

100

200300

400

500600

700

0 0.05 0.1 0.15 0.2 0.25

pr

Qcr

i (W

)

threaded

smooth

Comparison between Kutateladzes correlation andexperimental results

The Fig. shows CHF, comparison betweenKutateladzes pool boiling correlation versus experimental results for smooth tube surfaces.

Deviation is less than 15 percent.

0

100

200

300

400

500

600

700

0 100 200 300 400 500 600 700

Q_cri_exp. (W)

Q_c

ri_p

b. (

W)

15%

-15%