A heat pipe cooling system which employs a sensible heat sink is discussed. With this type of system, incident aerodynamic heat is transported via a heat pipe from the stagnation region to the heat sink and absorbed by raising the temperature of the heat sink material. The use of a sensible heat sink can be advantageous for situations where the total mission heat load is limited, as it is during re-entry, and a suitable radiation sink is not available

Silverstein, Calvin C.

English

NASA Technical Reports Server

N89- 12615
HEAT PIPE COOLING SYSTEM WITH SENSIBLE HEAT SINK*
Calvin C. Silverstein
CCS Associates
INTRODUCTION
During hypersonic flight, the aerodynamic heat flux at uncooled stagnation
regions can produce temperatures beyond the operational capability of virtually
all structural materials. In such instances, the use of ablative materials may
limit the temperature rise to acceptable levels. However, ablation can produce
undesirable changes in the size and shape of affected stagnation regions.
If there is a suitable heat sink available, an alternate thermal protection
approach is to utilize a heat pipe cooling system to transport heat from regions
of high aerodynamic heating to the heat sink for ultimate disposal. The heat
pipe approach is attractive because, in contrast to pumped cooling loops, heat
transport is virtually isothermal and separate pumps and power supplies are not
required.
Earlier studies have indicated the feasibility of heat pipe cooling for
the leading edges of hypersonic aircraft wings (refs. 1,2,3). With this concept,
a radiation heat sink is employed on the upper wing surface, where the aerodyn-
amic heating rate is relatively low. A continuous heat pipe structure extends
around the wing surface, from the regions of high aerodynamic heating to the
radiation heat sink on the upper surface. Aerodynamic heat incident on the lead-
ing edge is then transported through the heat pipe structure to the upper sur-
face, and dissipated by radiation to the environment.
This paper deals with a heat pipe cooling system which employs a sensible
heat sink. With this type of system, incident aerodynamic heat is transported
via a heat pipe from the stagnation region to the heat sink, and absorbed by
raising the temperature of the heat sink material. The use of a sensible heat
sink can be advantageous for situations where the total mission heat load is
limited, as during re-entry, and a suitable radiation heat sink is not available.
BASIC CONCEPT
The basic principle involved is illustrated in figure 1 for nose tip cool-
ing. A cylindrical heat pipe occupies the central region of the nose cone.
The hemispherical forward end of the heat pipe protrudes into the air stream,
and serves as the nose tip. The heat pipe is surrounded by an appropriate heat
* Based on work performed under Contract 33-0878, Sandia National Laboratories,
Albuquerque, NM.
409
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sink, which in turn is surrounded by ablative or temperature-resistant material.
An optional heat shield limits heat transfer between the heat sink and the outer
layer.
Aerodynamic heat incident on the nose tip is transported through the heat
pipe and deposited in the adjacent heat sink. The cooling system thermal capa-
city is sufficient to absorb the total mission heat load on the nose tip, while
limiting peak temperatures to levels consistent with the operational temperature
capability of the heat pipe and heat sink materials. Heat incident on the later-
al nose cone surface is generally of a much lower intensity than that experienced
over the nose tip, and can be dissipated through the conventional ablative pro-
cess or through sensible heating of a temperature-resistant layer.
With this cooling system, temperatures can be limited to levels permitting
a metallic nose tip of fixed size and shape, even under exposure to stagnation
heat fluxes on the order of thousands of w/cm 2.
DESIGN CONSIDERATIONS
A preliminary investigation of a heat pipe nose tip cooling system of the
type described has been carried out for a re-entry vehicle (ref. 4).
COOLING SYSTEM GEOMETRY
The geometry of the heat pipe cooling system is shown in figure 2. In gen-
eral, the axial cooled length X must exceed the nose radius R in order to main-
a
tain a nonzero ablator thickness between the cone surface _nd the heat sink.
The heat sink may be mounted outside the heat pipe in a conical configuration,
as shown in figures 1 and 2, in a cylindrical configuration, or as a combination
of the two. Alternatively, the heat sink may be located inside the heat pipe
by thickening the wick by an appropriate amount.
The conical heat sink configuration makes more effective use of the avail-
able volume within the nose cone than the cylindrical or internal configurations.
The overall length required to accommodate the heat pipe cooling system can then
be minimized.
To further reduce the heat load and hence cooling system size and weight,
the heat pipe radius R at the start of the heat sink section can be made smaller
than the nose radius _ . The axial cooled length X is then smaller than if
n a
R were equal to R .
c n
MATERIALS
Since sensible heat is the heat absorbing mechanism, the cooling system
should be characterized by a high thermal capacity (i.e., heat absorption capa-
bility) per unit weight and volume. This requirement leads to the selection
of heat pipe and heat sink materials with high mass and volumetric specific heats,
and with high operational temperature capability. In addition, the heat pipe
envelope should have high thermal conductivity to minimize the temperature grad-
410
ient and thermal stress in the stagnation region, as well as good strength at
anticipated operating temperatures. Finally, the various heat pipe and heat
sink materials must be chemically compatible.
For the application under consideration, lithium was selected as the heat
pipe fluid and as the primary heat sink material. TZM molybdenum was selected
as the heat pipe containment and wick material, and for containment of the lith-
ium heat sink liquid. These materials satisfy cooling system operational re-
quirements quite well. Additionally, lithium has the highest heat transport
capability of the various heat pipe fluids, and its operational temperature range
is quite compatible with thermal capacity requirements.
OPERATING TEMPERATURE
As previously stated, the highest feasible operating temperature is advis-
able for the heat pipe cooling system, in order to minimize system volume and
weight. TZM is compatible with lithium to at least 1920°K (3000°F), and is pro-
bably adequate from the strength standpoint to at least 1810°K (2800°F). The
principal limitation on temperature arises from the need to protect TZM from
oxidation at temperatures above around 920°K (1200°F).
Data on the lifetime of TZM coated with molybdenum disilicide was obtained
from reference 5. At the end-of-mission temperature of 1590°K (2400°F) which
was selected as the basis for the design studies, a lifetime of 150-170 hr in
still air is indicated. The actual coating lifetime will be determined by such
additional factors as the influence of the pressure and velocity field over the
nose cone, temperature gradients through the coating, and cyclic temperature
operation.
COOLING SYSTEM DESIGN
A preliminary design of the heat pipe cooling system is shown in figure 3.
The cooling system was designed for a peak cold wall heat flux of 3400 w/cm 2
(3000 Btu/ft2-sec), and a total heat load of 6640 kw-sec (6300 Btu). The system
length is 61 cm (24 in.), and the half-angle of the lithium heat sink is 3.91 deg.
The exposed surface of the TZM heat pipe is coated with molybdenum disilicide
to prevent oxidation. The total cooling system weight is about 3.6 kg (8 ib).
The cooled surface extends beyond the nose tip to a small portion of the
lateral nose cone surface, for a total axial cooled length of 3.28 cm (1.29 in.).
This extension beyond the nose tip increases the heat load on the heat pipe cool-
ing system, but is necessary to assure that the ablative layer will have a mini-
mum thickness of 0.38 cm (0.15 in.) at its juncture with the forward end of the
heat pipe.
The interior of the heat pipe is lined with a wire screen wick structure.
The wick consists of two layers of different mesh TZM screen, of variable thick-
ness. The wick is thinnest at the stagnation point, to minimize the temperature
drop as heat flows through the wick thickness to vaporize the lithium in the
wick pores at the inner wick surface. In the cooled region, the wick varies
411
from a thickness of 0.046 cm (0.018 in.) at the stagnation point to 0.091 cm
(0.036 in.) at the beginning of the heat sink section. The outer layer, which
serves as the lithium flow channel, is fabricated from i00 mesh screen. The
inner layer, which provides the necessary capillary pressure for circulation
of the lithium heat pipe fluid, is constructed from 400 mesh screen.
The wick is considerably thicker in the heat sink section of the heat pipe,
with a total thickness of 0.254 cm (0.i00 in.). Here, the outer liquid flow
channel is fabricated from 50 mesh screen, and is covered by a 400 mesh capillary
pumping layer. The thicker wick also makes a significant contribution to cool-
ing system thermal capacity.
Heat pipe heat transport limits were evaluated at the most severe heating
condition, which occurs when the cold wall stagnation heat flux has peaked at
3400 w/cm 2 (3000 Btu/ft2-sec). At this time the hot heat pipe vapor has advanced
about 36 cm (14 in.) into the heat sink section of the heat pipe. The calcula-
tions show that heat transport will not be constrained by limits imposed by:
the attainment of sonic velocity in the heat pipe vapor, entrainment of the heat
pipe liquid by its adjacent vapor, boiling of lithium in the heat pipe wick,
or the maximum capillary pressure available in the wick pore structure.
During re-entry, the cooling system temperature reaches a peak of 1670°K
(2550°F) at the stagnation point. The end-of-flight temperature is 2400°F.
The internal cooling system pressure reaches a maximum value of 128,900 N/m 2
(18.7 psia), which is the vapor pressure of lithium at its maximum re-entry tem-
perature of 1640°K (2500°F).
CONCLUSIONS
Heat pipe cooling, in conjunction with an appropriate sensible heat sink,
constitutes a simple, effective technique for the thermal protection of stagna-
tion regions without compromising the size and shape of such regions. The tech-
nique is applicable to missions of limited duration for which moderate total
heat loads are experienced.
412
REFERENCES
1 ,
2 °
3,
4.
5o
Silverstein, C.C., "A Feasibility Study of Heat-Pipe-Cooled Leading Edges
for Hypersonic Cruise Aircraft," NASA CR-1857, NASA Langley Research Center,
Hampton, VA, Nov. 1971.
Peeples, M.E., et al, "Thermostructural Applications of Heat Pipes," NASA
Contractor Report 159096, NASA Langley Research Center, Hampton, VA, June
1979.
Camarda, C.J. & Masek, R.V., "Design, Analysis, and Tests of a Shuttle-Type
Heat-Pipe-Cooled Leading Edge," J. Spacecraft & Rockets, Vol. 18, No. i,
Jan.-Feb. 1981, pp. 71-78.
Silverstein, C.C., "Preliminary Study of Nose Tip Heat Pipe," P.O. 33-0878,
Sandia National Laboratories, Albuquerque, NM, March 1987.
Tuominen,o S.M. & Dahl, J.M., "Cyclic Oxidation Testing of Molybdenum Pro-
tected by Silicide Coatings," J. Less Common Metals, Vol. 81, 1981, pp. 249-
260.
ABLATIVE
MATERIAL
HEAT SINK--
HEAT PIPE
HEAT SHIELD
Figure I. Basic Heat Pipe Cooling Concept for Nose Tip
413
ABLATOR
HEAT SINK
R
n
HEAT
N
PIPE
X
a
Lp
L
C
R = nose radius
n
Rc = heat pipe radius in heat sink section
Ymin = minimum ablator thickness
= nose cone half angle
B = heat sink half angle
Xa = axial cooled length
Lc = heat sink length
L = cooling system lengthP
Figure 2. Geometry of Heat Pipe Cooling System
414
4_
_=_
STRUCTURAL & WICK MATERIAL: TZM DIMENSIONS IN CM. r-- HEAT PIPE
HEAT PIPE & HEAT SINK FLUID: LITHIUM I
WICK POROSITY: 0.65 / F--LITHIUM RESERVOIR _
TZ T,O iI _!D _'_-T
__ L
I/3 SCALE
_--0.030
oo
60.96
VIEW D-D
0.076
I
ABLATOR
RESERVOIR
i__ HEAT PIPE WALL
WICK
VAPOR SPACE
FUL____LSCAL____EE
VAPOR SPACE
0.015, 400 MESH WICK
0,076, 100 MESH WICK,
TAPERS TO O.030AT
STAGNATION POINT
0.051 HEAT PIPE WALL
VAPOR SPACE
0.025, 400 MESH WICK
0.229, 50 MESH WICK
0.127 HEAT PIPE WALL
0.127 RESERVOIR WALL
LITHIUM RESERVOIR
Figure 3. Preliminary Design of Reference Sensible Heat Pipe Cooling System

Session IX
THERMAL CONTROL
COATINGS AND MEASUREMENTS
PR]_CI_)]NG PAGI_ BLANK NOT FILM]_)



Heat pipe cooling system with sensible heat sink

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