TRANSIENT CHARACTERISTICS OF A GROOVED WATER HEAT PIPE

WITH VARIABLE HEAT LOAD

long Hoon lang

Sverdrup Technology, Inc.

Lewis Research Center Group

Brook Park, Ohio 44142

ABSTRACT

The transient characteristics of a grooved

water heat pipe have been studied by using

variable heat load. First, the effects of the

property variations of the working fluid with

temperature were investigated by operating the

water heat pipe at several different

temperatures. The experimental results show

that, even for the same heat input profile and

heat pipe configuration, the heat pipe

transports more heat at higher temperature

within the tested temperature range. Adequate

liquid return to the evaporator due to

decreasing viscosity of the working fluid

permits continuous vaporization of water

without dry-out. Second, rewetting of the

evaporator was studied after the evaporator

had experienced dry-out. To rewet the

evaporator, the elevation of the condenser end

was the most effective way. Without elevating

the condenser end, rewetting is not straightforward even with power turned off unless the

heat pipe is kept at isothermal condition for

sufficiently long time.

NOMENCLATURE

: diameter of heat pipe, m

: latent heat of the vaporization,

J/Kg

: total length of heat pipe, m

: liquid transport factor, kW/cm 2

: viscosity, N-s/m 2

density, Kg/m 3

: surface tension, N/m

INTRODUCTION

The heat pipe is an effective device to

transport a large amount of energy with a

small temperature drop by using phase change

of the working fluid in a closed system. Due

to this extremely high thermal conductance at

relatively light weight, the heat pipe has

received much attention for the space

applications to meet the demand for an

effective thermal management device. However,

the heat pipe has several operational

restrictions such as the capillary limit,

sonic limit, entrainment limit, and boiling

limit. When heat pipes are operated within

these limits, the inherent advantages of using

heat pipes can be obtained.

For non-metallic heat pipes, the capillary

limit is a major restriction so performance of

the heat pipe depends largely on the capillary

pumping force. When the heat input at the

evaporator section is greater than allowed by

the capillary limit, the evaporator section

may experience dry-out which, if not relieved,

will lead to burnout. So, defining the dryout conditions and finding effective methods

to rewet the evaporator section are very

important to avoid any hazardous conditions

during heat pipe operation. To achieve this

goal, the experimental test was conducted by

using the water heat pipe which is safer than

the liquid metal heat pipe and is a prime

candidate for manned applications, with

operating temperatures up to 500 K.

Water heat pipes have been studied by

several authors. First, Neal (1967) used a

water heat pipe with stainless steel shell and

screen wick of 2 foot length for a long term

test of reliability and for a startup test.

Also, Deverall et al. (1970) investigated to

achieve a successful startup from the frozen

state by using a water heat pipe with

stainless steel shell and a screen wick. A

conventional water heat pipe and a concentric

annular water heat pipe with a grooved wick

were tested to examine the increase of heat

transport capacity per unit length and the

effect of tilt angle by Faghri and Thomas

(1988). All parts of the pipe were made of

copper. The temperature difference of 12 C

between the evaporator end cap and the

adiabatic section was used as reference to

define dry-out at the evaporator. Jang et al.
(1990) tested the Copper-water heat pipe with

axial grooves to study transient behavior.

The viscosity change of the working fluid

significantly affects the capillary force and

the capillary limit is major restriction for

the non-metallic heat pipe. The effect of

property variations versus temperature was

studied. Then, dry-out and rewetting was

experimentally investigated to define

conditions at dry-out and provide effective

way to rewet the evaporator.

EXPERIMENTAL SETUP

To investigate transient characteristics

of the heat pipe at low temperature, a water

heat pipe with axial grooved wick was used as

shown in Figure I. The total length of this

_tr_ _ _.. _L. 4_a_'_'r ,_1

(lwd;in| _ )111_,.| _4

±,..,. ,.,. ,: t. .......... .I..... /

.,..tololololop o o o o o o I o o oN,

-T--" ..... I I'

I_k,'ntlhl4eH e.m,t_,,'_ " I

__ I r;bl Pipr I_',Slelk = I m _'_-I

I.m,l_l..In_..O _@

O L*m,',.,,_ d o_

Figure i. Schematic of a water heat pipe.

heat pipe is 1 m, and the lengths of the

evaporator and condenser sections are 28.6 cm

and 20 cm, respectively. The rest of the

length is the adiabatic section. The outer

diameter of the heat pipe is 5 cm and the

inner diameter is 4.66 cm. The heat pipe shell

is made of copper and the wick structure

consists of 120 axial grooves cut on the inner

surface of the shell. The depth and width of

groove are .5 mm. Water is the working fluid.

All processes of fabrication were described in

a paper by Faghri and Thomas (1988). Figure 1

shows schematic diagram of the grooved water

heat pipe and Table 1 shows summary of heat

pipe design parameters.

Table i. Summary of physical dimension of the

heat pipe.

I'loat Pipe Shell Grooves

Material Copper

Total length i00.0 cm

Evaporator 28.6 cm

Condenser 20.0 cm

Adiabatic 51.4 cm

Outer diameter 5.0 cm

Inner diameter 4.66 cm

Pipe thickness 0.17 cm

Number 120

Width 0.5 mm

Depth 0.5 mm

If the working heat pipe configuration is

to be maintained, it is difficult to

instrument the inside of the heat pipe to

monitor vapor and liquid flow. This is because

performance of the heat pipe depends mainly on

the internal flow, and instrumentation at the

inside of the heat pipe would lead to internal

flow disturbances which would affect operation

of the heat pipe. So, the temperatures at the

surface of the evaporator, adiabatic, and

condenser sections, and the evaporator and

condenser end caps were measured. Also,

temperatures measured at the evaporator and

condenser surfaces may be influenced by

heaters on the evaporator section and cooling

coil on the condenser section. Therefore,

temperatures at the adiabatic section may

represent heat pipe temperatures. The wick

structure with the working fluid is located

inside heat pipe shell. However, there is no

wick structure attached on the evaporator and

condenser end caps. One side of these thin

caps is insulated and another side is in

contact with the vapor space in the heat pipe.

So, thermocouples mounted at the center of

insulated surfaces of the evaporator and

condenser end caps measure temperatures

closest to the vapor temperature in the heat

pipe.

To measure the surface temperature, a

total of 16 copper-constantan thermocouples

were installed on the outer surface of heat

pipe shell as shown in Figure I. Two

thermocouples were mounted at the center of

the evaporator and condenser end caps. Also,

another two thermocouples were used to measure

coolant inlet and outlet temperatures. A

Fluke 2280A data logger was used to record

temperature. Five flexible heaters with a

rectangular shape were tightly wrapped on the

surface of the evaporator and connected to two

Variacs. The length and width of each heater

are 15.95 cm and 5.72 cm, respectively. Also,

each heater can provide 250 watts with 115

volts. An electrical switch was installed

between a heater and a Variac output so each

heater can be operated independently. Also,

voltmeters and ammeters were installed to get

exact electrical power output from the Variac.

The cooling coil was tightly wrapped around

the condenser section to provide controlled

condenser cooling. The cooling coil made of

.25 inch outer diameter soft copper tube was

held in place by stainless steel clamps at

each end.

Since it is desired to have the liquid

condensed at the condenser section return to

the evaporator by using solely the capillary

force, the effect of a gravity field is

significant on the heat pipe performance. So,

to minimize this effect, the water heat pipe

was placed on the optical bench to level

accurately between the evaporator and

condenser. Then, insulation material made

from ceramic fibers was wrapped on the entire

heat pipe in 1.5 inch thickness. The thermal

conductivity of this insulation material is

.03 W/m-C. A schematic diagram of

experimental test setup is shown in Figure 2.

Cooling water was pumped to the cooling coil

from the coolant bath which keeps constant

water temperature. Flow meter was installed
at the inlet of the cooling coil to measure

cooling water flow rates. Table 2 showsall

heat pipe test cases.

VIM_ I

I

I1, " ,

Fig. 2 Schematic diagram of the water heat

pipe experimental setup.

Table 2. Summary of heat pipe test.

Case No. Initial Temp. (X)

of the Heat Pipe

2S7

313

333

350

313

333

350

313

313

Heat Input (w)

50

50

50

50

150

200

200

110

150

Time Variation

of Heat Input

Coherent

Constant

Constant

Constant

Stepped

(see Fig. 6)

Stepped

(eee Fig. 6)

Stepped

(see Fig. 6)

Stepped

(see Fig. 8)

Stepped

(see Fig. 9)

RESULTS AND DISCUSSIONS

Effects of Property V_iations

The effects of the property variations of

the working fluid with temperature were

investigated by operating the water heat pipe

at several different temperatures. The

initial heat pipe temperatures of 297 K, 313

K, 333 K, and 350 K were used with 50 watts

uniform heat input at the evaporator section.

Except for the 297 K initial temperature, to

operate the heat pipe at the elevated

temperatures, the heat pipe was slowly heated

until the temperatures at the surface and end

caps reached the desired initial temperatures.

During the heating period, instead of turning

the heater on, the coolant inlet temperature

was increased from ambient temperature to the

desired initial temperature to add heat to the

condenser section. This way can avoid partial

dry-out at the evaporator section while the

heat pipe reaches the desired initial

temperature. Since the heat pipe was

insulated and the coolant inlet temperature

was fixed at the initial temperature, the heat

pipe finally reached the steady state at the

desired initial temperature with uniform

temperature distribution. Once this condition

was obtained and then 50 watts heat input was

applied on the evaporator while the coolant

inlet temperature was kept constant. The

coolant inlet temperature was the same as the

initial temperature.

lYansient Temperature Variations

50. Watts Uniform Heat Input

Cooling at the Condenser

Surface Tamp

o = 0.0 rain

a = 5.0 min

+ = 10 min

= 15 sin

o = Z00 sin

v = 40.0 sin

a = 600 sin

. = 80.0 sin

• = 100 sin

• = 120 rnin.

Evaporator Adiabatic Section

O0 02 04 06

Heat Pipe Axial Length, m

Condenser

08 10

Fig. 3 Transient surface temperature

distributions for case I.

Figure 3 shows transient surface

temperature distributions for an initial

temperature 297 K. Up to i0 minutes,

temperature at the evaporator end cap was

close to that at the adiabatic section, and

then the former rose faster than the latter.

Temperature at the evaporator end cap may be

considered to be close to that of vapor space

at the evaporator end. So, a higher

temperature at the evaporator end cap than

that at the adiabatic section implies that the

vapor near the evaporator end cap may be

super-heated. Figure 4 shows temperature

distributions along the axial direction at

steady state conditions for four different

operating temperatures. Note that the

i

=iJ

-L

Temperature at Steady State

50 Watts Uniform Heat Input

Cooling at the Condenser

Surface Tamp

o = Case I

= Case 2

÷ = Case 3

x = Case 4

a r t Adiabatic Section

oo 62 6, 6,

Heat Pipe Axial Length, m

[ Condenser

08 ] 0

Fig. 4 Temperature distributions at steady

state for various operation conditions

(cases i, 2, 3, and 4).
temperature distribution becomesmore uniform

as the operating temperature increased. The

temperature distribution for the initial

temperature of 350 K shows that the heat pipe

was operating at near isothermal condition.

Temperatures under heaters were slightly

higher than those at the adiabatic section due

to influencing of the heaters. Temperature

distributions at the adiabatic section for all

four cases are very uniform.

To understand the effects of the property

variations, the liquid transport factor of the

working fluid was calculated for different

temperatures based on following equation:

N =D_H (i)

This liquid transport factor describes the

capillary pumping ability of the working

fluid. For the water heat pipe, unlike liquid

metal heat pipes, the capillary pumping limit

may be the first limit encountered. So, the

highest performance of the water heat pipe may

be obtained at operation temperature which

gives the greatest value of liquid transport

factors. Figure 5 shows thermo-physical

properties (Incropera and DeWitt, 1981) and

liquid transport factor variations, calculated

by using Equation (i), versus temperatures for

water. Up to temperature about 460 K, the

liquid transport factor is increased mainly

due to decrease in the viscosity and then

decreased because the surface tension,

density, and heat of vaporization decrease

more rapidly than the viscosity. This implies

that for given heat input the heat pipe can

approach more isothermal condition until

liquid transport factor reaches the maximum.

Thermophysica| Properties of Satura_d Water

Melting _mperature = _3 15 K

Boiling _mpe_ture = _315 K

_..° _o Critical_mperature=6473K _ ._

_1,o t

.-. =_ _° ._=

#

_0 _00 0 340 0 380 0 420 0 460 0 _0 0 _00 _0 0 _0 0 BJO 0

_mperature, K

Fig. 5 Thermophysical properties and liquid

transport factor for water.

Another test was conducted with stepped

heat input to investigate the effects of the

property variations with temperature. At

uniform initial temperature of 350 K (case 7),

the stepped heat inputs of 50, i00, 150, and

200 watts at 20 minute intervals were applied

on the evaporator. As shown in Figure 6, up

to 150 watts the heat pipe was near isothermal

condition and then with 200 watts heat input

i

Condenser end elevated

o = Temp =t Evaporator End Cap by lib degree angle

O0

o

Time, rntn

Fig. 6 Transient temperature variations at

the evaporator and condenser end caps

and the adiabatic section with stepped

heat input (cases 5, 6, and 7).

temperature at the evaporator end cap rapidly

increased while temperatures at the condenser

end cap and adiabatic section approached

steady state condition. So, the condenser end

was elevated by 1.18 degree angle while heat

input at the evaporator was continued. Then,

temperature at the evaporator end cap rapidly

decreased and temperatures at the adiabatic

section and condenser end cap increased. The

water heat pipe again reached an isothermal

condition. This means that the part of the

evaporator section had been dry-out. When the

condenser end was elevated, adequate liquid

returned from the condenser to the evaporator

section to rewet the evaporator. Consequently

more vaporization occurred at the evaporator

section so more heat was transported from the

evaporator to the condenser section.

For initial temperature of 313 K (case 5),

the stepped heat inputs of 50, I00, and 150

watts at 20 minute intervals were added at the

evaporator. However, temperature at the

evaporator end cap started to deviate from

that at the adiabatic section with 100 watts

heat input. Then, with 150 watts heat input

that temperature rapidly increased. So heat

input was terminated. Also, a greater

temperature gradient between the evaporator

end and the condenser end was observed.

For an initial temperature of 333 K (case

6), the same heat input profile as that for

350 K was used. With 150 watts heat input

some temperature gradient between the

evaporator end cap and adiabatic section was

observed. When the heat input was increased

to 200 watts, the temperature at the

evaporator end cap rapidly increased while

temperatures at the condenser end cap and

adiabatic section slowly increased. Again

this implies that the vapor in vapor space at

near evaporator end cap is super-heated. So,

power input was turned off. As shown in

Figure 6, the water heat pipe tested at the
highest temperaturewasclearly working at the

mostisothermal condition andalso transported

more heat. Figure 7 shows temperature

distributions along the entire heat pipe for

the initial temperature of 350 K.

_l- % % __-_-c--i_N]mu4 I Y'i-._0e

Fig. 7 Transient surface temperature

distributions for the initial

temperature of 350 K (case 7).

Dry-out and Rewettino

As shown in Figure 6, the rapid increase

of the temperature at the evaporator end cap

means that part of the evaporator section is

experiencing dry-out. Using this phenomenon,

dry-out and rewetting of the water heat pipe

was studied experimentally (case 8). First,

a uniform heat input of 50 watts was added to

the evaporator section until the heat pipe

reached steady state condition. Then, heat

input was increased by l0 watts at 5 minute

intervals so the maximum heat input reached

ii0 watts. Figure 8 shows heat input profile,

and temperatures at the evaporator and

condenser end caps, and the adiabatic section

ii I: Condenser end elevated

by I 18 degree angle

i F: O0 300 600 900 1200 1_)00 180.0 2100 :P400 LP"/O.O

Time, rain

Fig. 8 Dry-out and rewetting of a water

heat pipe with variable heat load

and tilting (case 8).

.2

versus time. During this period, temperature

at the condenser end cap is lower than those

at the evaporator end cap and adiabatic

section. Up to heat input of 80 watts,

temperature at the evaporator end cap is close

to that at the adiabatic section. However,

more heat input lead to rapid increase of

temperature at the evaporator end cap while

temperatures at other locations stayed steady.

This indicates that the vaporization of the

working fluid at the evaporator was not enough

to cool the evaporator section. Then, the

condenser section end was elevated to rewet

the evaporator so temperature at the evaporator end cap rapidly decreased and

temperatures at the adiabatic section and

condenser end cap increased at the same time.

This temperature change shows that the part of

the evaporator section had been dryed out.

The experimental results shown in Figure 8

indicate that about 20 K temperature

difference between at the evaporator end cap

and adiabatic section existed.

After the heat pipe reached the steady

state with the elevation of the condenser end,

the heat pipe was leveled again while the same

heat input was being added. The temperature

at the evaporator end cap rose rapidly as

before, indicating that part of the evaporator

section was undergoing dry-out again. From

this one may conclude that the gravity field

resulting from elevating the condenser end

aided the capillary force to return the liquid

from the condenser to the evaporator.

In an attempt to rewet the evaporator

section, heat input was reduced in a stepwise manner to the initial heat input rate of

50 watts as shown in Figure 8, but the

temperature at the evaporator end cap did not

drop to its previous value. This indicates

that even at a lower heat input rate part of

the evaporator section was still in a dry-out

condition. Next, power was completely turned

off until the heat pipe reached the initial

condition, and then 50 watts heat input was

re-applied in the same manner as at the

beginning of the test. However, the

temperature distribution was quite different

from that obtained previously. The

temperature at the evaporator end cap

increased faster than other temperatures.

From this one may conclude that the evaporator

section was not successfully rewetted.

Figure 9 shows another heat input profile

and temperature variation versus time for a

dry-out and attempted rewetting test (case 9)

without elevating the condenser section. The

initial temperature was 313 K. After the heat

pipe reached a steady state condition with a

uniform heat input rate of 50 watts, the heat

rate was increased by l0 watt increments at i0

minute intervals to a maximum of 150 watts.

When the heat input rate was i00 watts, the

temperature at the evaporator end cap started

to increase faster than did all other

temperatures on the heat pipe. As the heat

input rate was further increased, the

temperature at the evaporator end cap

continued to increase at a fast rate while

temperatures at the adiabatic section and the

condenser end cap increased more slowly. When

the temperature difference between at the
o o = Temp at Evaporator End Cap

g. A --Temp at AdiabaticSection

o = Temp. at Condenser End Cap )

_" • = Heat Input Profile J

_o

E_

o

"o e,

o

o

o

O0 200 400 GO0 80Q tO00 |200 1400 I_O 1800

Time. rain

Fig. 9 Dry-out and rewetting of a water

heat pipe with variable heat load

(case 9).

evaporator end cap and the adiabatic section

was 25 K, instead of reducing heat input stepwise as before, heat input was suddenly

terminated for 40 minutes in an attempt to

rewet the evaporator section without elevating

the condenser end, as shown in Figure 9.

Then, a uniform heat input of 50 watts was

applied at the evaporator section. However,

temperature at the evaporator section

increased faster than did all other

temperatures, indicating a persisting dry-out

condition. Figure I0 is a 3-D plot showing

_me, rain

00 • jO. "_

O

Figure I0. Transient surface temperature

distribution for case 9.

temperature distribution along the heat pipe

length versus time. Figures 8 shows that

elevation of the condenser end is an effective

way to rewet the evaporator, assuming of

course that a gravity field exists. On the

other hand, reducing the heat input at the

evaporator did not results in successful

rewetting of the evaporator as shown in

Figures 8 and 9.

CONCLUSIONS

A grooved water heat pipe was tested to

study the effects on heat pipe performance of

the property variations of the working fluid

with temperature. Also studies were dry-out

and rewetting conditions of the evaporator

section. The test results show that, even for

the same heat input profile and heat pipe

configuration, the heat pipe operated at a

more isothermal condition and transported more

heat at higher operating temperature within

tested temperature range. Dry-out at the

evaporator section was characterized by rapid

temperature increase at the evaporator end

cap, while temperatures at the adiabatic

section and the condenser end cap stayed

steady. When the evaporator had experienced

dry-out, elevation of the condenser end was

the most effective way to rewet the evaporator

if a gravity field exists. Without elevating

the condenser end, rewetting was not achieved

even with power turned off for 40.0 minutes.

ACKNOWLEDGEMENT

The author is indebted to Dr. A. Faghri

for providing a water heat pipe to test.

Also, author thanks A. J. Juhasz, W. Lucas,

and F. Calfo for helpful comments and

assistance of experimental setup.

REFERENCES

Deverall, J. E.; Kemme, J. E.; and

Florschuetsz, L. W.: 1970, "Sonic Limitations

and Startup Problems of Heat Pipes," LA-4518.

Faghri, A.; and Thomas, S.: 1988,

"Performance Characteristics of a Concentric

Annular Heat Pipe Part I. Experimental

Prediction and Analysis of the Capillary

Limit," Proceedings of 1988 ASME National Heat

Transfer Conference, HTD-96, Vol. I, pp. 379-

387.

Incropera, F. P.; and DeWitt, D. P.: 1981,

Fundamentals of Heat Pipe, ist edition, John

Wiley & Sons, New York, pp. 782 - 783.

Jang, J. H.; Lucas, W. P.; Baker, K. W.; and

Juhasz, A. J.: 1990, "Preliminary Report on

Copper-Water Heat Pipe Tests at NASA Lewis

Research Center," Proceedings of the Seventh

Symposium on Space Nuclear Power Systems,

January 7 - 10, New Mexico.

Neal, L. G.: 1967, "An Analytical and

Experimental Study of Heat Pipes," TRW RP.

99900-6114-R000.
NationalAeronautics and

Space Adminislration

1. Report No. 2. Government Accession No.

NASA CR- 185280

4. Title and Subtitle

Transient Characteristics of a Grooved Water Heat Pipe With Variable

Heat Load

7. Author(s)

Jong Hoon Jang

9. Performing Organization Name and Address

Sverdrup Technology, Inc.

Lewis Research Center Group

2001 Aerospace Parkway

Brook Park, Ohio 44142

12. Sponsoring Agency Name and Address

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

3, Recipient's Catalog No.

5. Report Date

July 1990

6. Performing Organization Code

8. Performing Organization Report No.

None (E-5668)

Report Documentation Page

10. Work Unit No.

590-13-21

11. Contract or Grant No.

NAS3-24105 and NAS3-25266

13. Type of Report and Period Covered

Contractor Report

Final

14. Sponsoring Agency Code

15. Supplementary Notes

Project Manager, James E. Calogeras, Power Technology Division, NASA Lewis Research Center.

Prepared for the 1990 Winter Annual Meeting of the American Society of Mechanical Engineers, Dallas,

Texas, November 25-30, 1990.

16. Abstract

The transient characteristics of a grooved water heat pipe have been studied by using variable heat load. First,

the effects of the property variations of the working fluid with temperature were investigated by operating the

water heat pipe at several different temperatures. The experimental results show that, even for the same heat

input profile and heat pipe configuration, the heat pipe transports more heat at higher temperature within the

tested temperature range. Adequate liquid return to the evaporator due to decreasing viscosity of the working

fluid permits continuous vaporization of water without dry-out. Second, rewetting of the evaporator was studied

after the evaporator had experienced dry-out. To rewet the evaporator, the elevation of the condenser end was the

most effective way. Without elevating the condenser end, rewetting is not straight-forward even with power

turned off unless the heat pipe is kept at isothermal condition for sufficiently long time.

17. Key Words (Suggested by Author(s))

Transient; Water heat pipe;

Variable heat load; Dry-out;

Rewetting

18. Distribution Statement

Unclassified- Unlimited

Subject Category 34

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of pages

Unclassified Unclassified 8

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