Transcript
Calhoun: The NPS Institutional Archive Theses and Dissertations
Thesis Collection
1975-12
Investigation of gravitational effects on a variable conductance heat pipe utilizing liquid crystal thermography Batts, William Henry Monterey, California. Naval Postgraduate School http://hdl.handle.net/10945/20913
INVESTIGATION OF GRAVITATIONAL EFFECTS ON A VARIABLE CONDUCTANCE HEAT PIPE UTILIZING LIQUID CRYSTAL THERMOGRAPHY
Wl 11 iam Henry Batts
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INVESTIGATION OF GRAVITATIONAL EFFECTS ON A VARIABLE CONDUCTANCE HEAT PIPE
UTILIZING LIQUID CRYSTAL THERMOGRAPHY by
William Henry Batts, Jr. December 19 75 Thesis Advisor: "^
Approved
Matthew Kelleherl
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for public release; distribution unlimited.
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Investigation of Gravitational Effects on a Variable Conductance Heat Pipe Utilizin Liquid Crystal Thermography AUTHORf*;
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PEpiOD COVERED
Master's Thesis; December 1975
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William Henry Batts, Jr.
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Naval Postgraduate School Monterey, California 93940 U.
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Naval Postgraduate School Monterey, California 93940
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ABSTRACT
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Observations were made of the operation of a gas loaded, variable conductance heat pipe two inches in diameter and sixty inches long. The heat pipe was operated in the horizontal and vertical positions while input power was varied from twenty five to one hundred fifty watts. Liquid crystal thermographic techniques were used to observe the temperature gradients existing when non-condensible gases both heavier and lighter than the DD
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OBSOLETE SECURITY CLASSIFICATION OF THIS PAOS
(TThan
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working fluid had been introduced. Methanol was used as the working fluid; krypton and helium were the non-condensible gases Isothermal maps, photographs of liquid crystal displays, and summarized temperature data for the various operating conditions are presented.
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SECURITY CLASSIFICATION OF THIS PAGEf»T>»n Date Erfrtd)
Investigation of Gravitational Effects on a Variable Conductaace Heat Pipe
Utilizing Liquid Crystal Thermography by
William Henry Batts Jr. Commander, United States Navy B.S., United States Naval Academy, 1959 ,
Submitted in partial fulfillment of the requirements for the degree of
MASTER OF SCIENCE IN MECHANICAL ENGINEERING from the
NAVAL POSTGRADUATE SCHOOL DecemJDer 1975
c./
DUDLEY KNOX UERAR* NAVAL POSTGPaoUATE SCHOOL MONTEREY. CALIFORNIA 83940
ABSTRACT Observations were made of the operation of a gas loaded, variable conductance heat pipe two inches in diameter and sixty inches long.
The heat pipe was operated in the horizontal and
vertical positions while input power was varied from twenty five to one hundred fifty watts.
Liquid crystal thermographic
techniques were used to observe the temperature gradients
existing when non-condensible gases both heavier and lighter than the working fluid had been introduced. as the working fluid;
gases.
Methanol was used
krypton and helium were the non-condensible
Isothermal maps, photographs of liquid crystal displays,
and summarized temperature data for the various operating con-
ditions are presented.
TABLE OF CONTENTS
I.
INTRODUCTION
II.
OBJECTIVE
9
14
III. EXPERIMENTAL APPARATUS
15
IV.
LIQUID CRYSTAL APPLICATION
17
V.
EXPERIMENTAL PROCEDURE
19
VI.
EXPERIMENTAL RESULTS
23
VII. SUMMJ^RY
.
44
APPENDIX A
SUMMARY OF DATA
45
APPENDIX B
CALCULATION OF NET ABSORBED P0V7ER
49
APPENDIX C
CALCULATION OF GAS LOAD
51
LIST OF REFERENCES
52
INITIAL DISTRIBUTION LIST
53
LIST OF TABLES
Table
Page
I.
Heat Pipe Loading
20
II.
Liquid Crystal Temperature Correlation
21
6
.
LIST OF FIGURES Page
Figure 1.
Evaporator Minus Ambient Temperature vs. Absorbed Power
11
2.
Flat Front Theory Model of a Gas Loaded Heat Pipe
3.
Surface Minus Ambient Temperature vs. Condenser Length - Horizontal and Vertical - .0116 Ibm Krypton
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
—
12
24
Surface Minus Ambient Temperature vs. Condenser Length - Horizontal and Vertical ".000637 Ibm Helium.
25
Surface Minus Ambient Temperature vs. Condenser Length - Horizontal and Vertical - Methanol Only
26
Liquid Crystal Isotherms Ibm Krypton
-
Liquid Crystal Isotherms Ibm Krypton
-
Liquid Crystal Isotherms Ibm Krypton
-
Liquid Crystal Isotherms Krypton
-
Liquid Crystal Isotherms Krypton
-
Liquid Crystal Isotherms Krypton
-
Liquid Crystal Isotherms Ibm Helium
-
Liquid Crystal Isotherms Ibm Helium
-
Liquid Crystal Isotherms Ibm Helium
-
Liquid Crystal Isotherms Ibm Helium
-
Liquid Crystal Isotherms Ibm Helium
-
Horizontal
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Horizontal
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.0116 31
Horizontal
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.0163 32
Vertical
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Vertical
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.0116 Ibm 34
Vertical
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.0163 Ibm 35
Horizontal
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Horizontal
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.000637 37
Horizontal
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.00139 38
Vertical
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Vertical
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.000637 40
17.
18.
19.
20.
21.
Liquid Crystal Isotherms Ibm Helium
-
Vertical
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Photograph of Liquid Crystals Krypton Load
-
Photograph of Liquid Crystals Helium Load
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Photograph of Liquid Crystals Krypton Load
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Photograph of Liquid Crystals Helium Load
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Horizontal
-
42
Horizontal
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42
Vertical
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Vertical
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43
I.
INTRODUCTION
The heat pipe is a very efficient heat transfer device. In its simplest form it is a closed cylinder containing a wick
and a condensing/evaporating fluid.
There are no moving parts.
The wick can be any of several materials and configurations; the essential requirement is that it be capable of moving the
working fluid by capillary pumping action.
Many geometric
configurations of the basic pipe are possible. A thorough presentation on the theory and design of heat pipes can be found in Marcus
[Ref.
1].
This report is parti-
cularly concerned with variable conductance heat pipes and includes an extensive bibliography on the subject. In operation, heat from a source is absorbed by the pipe,
evaporating the working fluid.
The vapor travels along the
pipe to the condenser section where heat is rejected to a sink and the vapor is condensed.
The condensate is returned to the
evaporator section through the wick by capillary pumping.
Due
to the phase changes of the working fluid occurring along its
length, the basic heat pipe operates at essentially isothermal
conditions. One drawback to this simple system is that, given a con-
stant sink temperature, the heat transfer rate can be increased
only by raising the operating temperature of the heat pipe. This is not desirable if the goal is to maintain a constant
source temperature.
The gas-loaded variable conductance heat pipe provides a A non-condensible gas is
partial solution to this problem.
introduced along with the working fluid.
During operation the
working fluid behaves as described above; however, the noncondensible gas migrates to the condenser end where it forms a plug.
This effectively reduces the condenser length and,
therefore, the condenser heat transfer area.
As the heat trans-
fer rate into the evaporator increases, the temperature and
partial pressure of the working fluid increase, compressing the plug of non-condensible gas.
The effective condenser heat
transfer area is increased, thus modifying and reducing the
system temperature rise.
This concept of variable conductance
is shown graphically in Fig.
which is
a
partial plot of data
The addition of the helium gas caused
presented in Appendix A. a
1,
decrease in the slope of the curve; the slope is a measure
of the conductance.
The simplest model for examining the behavior of a gas-
loaded heat pipe uses the flat front theory proposed by Bienert [Ref.
2
J.
assumptions
This model [Ref.
1
1:
(see Fig.
2)
is based on the following
steady state operation,
a
flat inter-
face between vapor and gas regions, no axial conduction, heat
transfer per unit area of condenser proportional to the temperature difference betv/een vapor and sink, uniform pressure
distribution, and ideal behavior of the vapor-gas mixture.
Although this model allows some vapor in the gas section (at sink temperature conditions) gas in the vapor section.
,
it is assumed that there is no
Experimental results have shown
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11
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concerning the direction of temperature gradients is retained. Figures 18 through 21 are photographs showing color bands produced by the liquid crystals.
These particular photographs
were taken at gas loads and power levels slightly different from those for which numerical data is presented.
Earlier
photographs were not usable because of difficulties encountered with photographic lighting techniques.
There is no essential
difference, however, in the type of color variation displayed. The orientation of the isotherms for all of the horizontal
settings shows clearly that the non-condensible gas did not form a plug at the condenser end of the heat pipe.
Although
there was an axial temperature gradient along the pipe, there
was a marked temperature gradient perpendicular to the axis.
While no quantitative judgements can be made regarding the
relative effects of conduction and diffusion face,
across the inter-
it seems clear that there was a significant difference
in gas concentration from, the top to the bottom of the pipe.
When the gas was the heavier krypton, the lower temperatures and higher gas concentrations were along the bottom of the pipe.
With the lighter helium gas present, the lower tempera-
tures and higher gas concentrations were at the top of the pipe. This leads to a conclusion that the gradient perpendicular to the pipe was the result of gravitational effects.
The temperatures measured by the thermocouples along the top of the pipe provided little indication of the concentra-
tion gradients which were present.
There was very little
difference in the nature of the temperature variation between
27
the horizontal readings for the two different gases.
At
higher power levels, the temperature variations had more
resemblance to those of the conventional heat pipe than to the gas-loaded variable conductance heat pipe described by the diffuse front theory.
The results with the apparatus in the vertical position
were quite different for the two gases.
The helium-loaded
pipe data showed an axial temperature variation much like the classic diffuse front theory predicts.
In the case of
the krypton- loaded pipe, temperature decreased continually
from adiabatic section to condenser end.
In this position
the temperature information provided by thermocouples and
liquid crystals was quite compatible.
It should be noted
here that the color bands observed with the pipe in the
vertical position when krypton was the non-condensible gas
were not stationary.
They changed continually with a low
amplitude, wave-like motion indicative of a measure of
instability. This data again suggests the influence of gravitational forces.
The lighter helium gas readily rose to the upper
end of the pipe forming a plug which effectively blocked
diffusion of the methanol vapor.
Krypton, the heavier gas,
was subjected to stronger gravitational forces opposing the
momentum im.parted by the upward flow of methanol. appeared to be
a
The result
gradually increasing gas concentration,
though some methanol vapor did penetrate to the end of the condenser.
28
The use of liquid crystals proved an effective technique for thermographic mapping.
late in the
One difficulty was encountered
experimental procedure.
The color presentation
became much less vivid over the section of the pipe approximately twelve inches from the adiabatic section.
The only
apparent difference was that this section operated at
a
generally higher temperature than other portions of the condenser.
Thus it seems possible that operation at high temp-
erature may have a deleterious effect on encapsulated liquid crystals.
29
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48
o Pi
*
APPENDIX B CALCULATION OF NET ABSORBED POWER The net power absorbed by the heat pipe is assumed to be
approximately equal to the power dissipated in the heater minus the power lost through the insulation.
The calculation of the
power lost makes use of the temperature difference between
thermocouples #94 and #95 positioned at different radial locations in the insulation. q
(net)
q
= q
(htr)
(htr)
-
q
(lost)
= V^ V2 / R
volts
V,
= voltage drop across heater,
V-j
= voltage drop across series resister, volts
R
= series resistance
(2.0236 ohms)
dT q
(lost)
=
Cln "^2^^\ 27TKL
dT = T94
-
T^5
= radius to thermocouple 94
(25/16 inches)
r^ = radius to thermocouple 95
(25/16 inches)
r,
K
= thermal conductivity of insulation
02 ( ^'^'^
S^^ hr ft OF^
)
of insulation (1.1 ft)
L
= length
C
= conversion constant
49
(3.412
BTU ^^^t-hr
^
Sample calculation for first data point listed in Appendix A: V, = 19.58 volts
V2 =
2.64 volts
dT = 15.4 °F q
q
4-> met; ,
(net)
(19.58)
(15.4) (2) (3.14) (.02) (1.1) (3.412) In (35.25)
(2.64)
2.0236
= 23.7 watts
Appendix A contains the results for other conditions.
50
APPENDIX C CALCULATION OF GAS LOAD The amount of gas introduced into the pipe was calcu-
lated from data obtained at isothermal, ambient conditions. (P.
t
^ =
-
P^ m
)
V M
Tt
P,
= total pressure,
P
= methanol partial pressure, osia
V
= pipe volume
M
=
molecular weight of gas
T
=
pipe temperature,
R
=
gas constant
t
psia
(155 cu.
(154 5
in.)
R
~
— 5—R
lb-mole
)
Sample calculation for first }:rypton load; P
= 14.08 psia
T
=
P
m
536°R
=2.35 psia ^
M
=
m
=
m
=
83.8 (14.08 - 2.35) (1545)
.0163 Ibm
(536)
(165) (83.8) (12 in/ft)
krypton
Other results are listed in Table
51
I.
(saturation data)
.
LIST OF REFERENCES
1.
Marcus, B.D., Theory and Design of Variable Conductance Heat Pipes NASA Contract Report 2018, April 1972. ,
2.
Bienert, W., Heat Pipes for Temperature Control presented at Intersociety Energy Conversion Engineering Conference, Washington, D.C., 1969.
3
Investigation of a Variable Conductance Heat Naydan T P Pipe M.S. Thesis, Naval Postgraduate School, 1975.
,
,
.
.
,
,
4.
Cooper, T.E., Field, R.J., and Meyer, J.F., "Liquid Crystal Thermography and Its Application to the Study of Convective Heat Transfer," Journal of Heat Transfer, Trans. ASME v. 97, Series C, No. 3, p. 442-450, August 1975. ,
5.
Naval Postgraduate School Report NPS-59CG73061A, An Analytical Model for Predicting the Daily Tem.perature C ycle of Container Stored Ordnance by T.E. Cooper and A. H. Wirzburger, p. 12, 18 June 1973. ,
52
INITIAL DISTRIBUTION LIST No. Copies 1.
Defense Documentation Center Cameron Station Alexandria, Virginia 22314
2
2.
Library, Code 0212 Naval Postgraduate School Monterey, California 93940
2
3.
Department Chairman, Code 59 Department of Mechanical Engineering Naval Postgraduate School Monterey, California 93940
2
4.
Assoc. Professor M. D. Kelleher, Code 59Kk. Department of Mechanical Engineering Naval Postgraduate School Monterey, California 93940
1
5.
CDR William H. Batts, Jr., USN 228 N. Blake Road Norfolk, Virginia 23505
1
53
1
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Batts Investigation of "fpvi tationa effects on a variable conductance heat pipe uti 1Izing iquid crysta thermography. 1
1
109
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