Unidirectional heat pipe

A heat pipe with limited heat transfer capabilities in one direction. The heat pipe, which transfers heat in one direction in normal fashion, also transfers heat in the reverse direction, but only up to a prescribed point, beyond which the reverse heat flow cuts off. It operates because of the use of a limited liquid filling and at least one artery which is closed at the normal evaporator end and open ended at the normal condenser end.

SUMMARY OF THE INVENTION 
This invention deals generally with heat transfer and more specifically 
with a heat pipe designed to limit heat transfer in one direction while 
permitting free heat transfer in the other direction. 
Unidirectional heat transfer is a frequent goal in many applications. Its 
major benefit is the ability to heat or cool a device without the risk of 
the intended heat transfer path transferring heat in the opposite 
direction to negate the design goals. A simple example of such an 
application is that of heating a greenhouse with sunlight passing through 
glass. When the sun goes down it is desirable to prevent loss of the heat 
back through the glass. 
In more sophisticated applications, such as in space vehicles, a similar 
phenomenum can occur when cooling electronic devices by transferring heat 
to the shaded side of the vehicle. In such an application it is desirable 
to assure that, if the normally shaded side of the vehicle turns to the 
sun, the electronic devices, while not being cooled, are also not 
overheated by the sun's heat and therefore damaged. 
Heat pipes are used for both heating and cooling in industrial and space 
applications because they are so effective in transferring heat, but this 
very effectiveness raises the danger that, particularly during a 
malfunction of the equipment, a heat pipe may transfer heat to a device 
being cooled, or cool a device which demands heat. 
The present invention deals with just that problem. It results in a heat 
pipe which transfer heat normally in a forward direction, and, within 
limits prescribed by its design, also transfers only limited heat in the 
reverse direction. Moreover, when the prescribed limit of reverse heat 
flow is surpassed, the reverse heat flow stops entirely. 
This operation is accomplished by, first, using a limited supply of liquid 
in the heat pipe and, second, specially designing the liquid arteries 
within the heat pipe so that the arteries are not sealed off at the normal 
condenser end. 
Since the capillary pumping capability of an artery is limited by the size 
of its largest opening, an open-ended artery has very minimal capillary 
pumping ability. It is this phenomenum which is used to control the 
reverse heat transfer capability of the present invention. 
It is well understood in the heat pipe art that on limitation on the power 
a heat pipe can transfer is caused by the "drying out" of the evaporator 
of the heat pipe. This situation occurs when heat is being applied to the 
evaporator at such a rate that the heat pipe liquid transport system is 
incapable of returning liquid fast enough from the condenser to the 
evaporator. In effect the liquid is evaporated from the heat input side, 
and the liquid return system does not operate well enough to assure that 
the vapor is condensed and returned to the evaporator, so the evaporator 
has no more liquid to evaporate and the heat transfer action ceases. 
The simplest example of such a malfunction might be a gravity return heat 
pipe, one in which the evaporator is below the condenser so that the 
liquid simply runs down the inside of the casing from the condenser to the 
evaporator. If such a heat pipe is tilted to place the evaporator above 
the condenser, the liquid return mechanism no longer operates, the 
evaporator drys out and the heat transfer action stops. 
In the present invention a different mechanism is used, but the reverse 
heat transfer is similarily limited by lack of liquid delivery to the 
reverse evaporator. The mechanism used to limit liquid delivery is the 
open-ended capillary artery previously mentioned, and the key to operation 
is that the artery end is kept sealed by the heat transfer liquid itself 
when the heat pipe is operating in its normal direction, but is opened up 
by liquid evaporation when heat transfer in the reverse direction 
surpasses a predetermined limit. When the artery becomes open-ended, 
liquid is no longer returned to the reverse direction evaporator, the 
normal condenser, and the reverse heat transfer stops. 
The present invention therefore furnishes a unidirectional heat pipe with a 
very simple mechanical structure which operates reliably largely because 
no additional mechanical devices are added to the heat pipe.

DETAILED DESCRIPTION OF THE INVENTION 
The FIGURE shows the preferred embodiment of the invention in an axial 
cross section view in which heat pipe 10 is constructed with sealed 
evacuated casing 12 and internal wick structure 14 within which are 
located arteries 16 and 18. 
The structure of heat pipe 10 is extremely simple, and its novelty arises 
from the fact that arteries 16 and 18 are constructed to be sealed off at 
normal evaporator end 20 of heat pipe 10 while they are constructed as 
open ended at normal condenser end 22 of heat pipe 10. 
Additionally, heat transfer liquid 24 is placed into heat pipe 10 in a 
limited quantity so that in normal use with heat being applied to normal 
evaporator 20, and considering the total amount of liquid retained within 
wick 14 and arteries 16 and 18, sufficient liquid will accumulate in 
liquid retainer 26 at condenser end 22 to seal off at least one of the 
arteries. In the preferred embodiment of the invention liquid retainer 26 
is simply the lowest portion of tilted heat pipe 10. The liquid 
accumulation can not, however, be excessive, since the quantity of liquid 
normally accumulated is what determines the limit of heat transfer in the 
reverse direction. 
The present invention uses a particularly simple system for changing 
arteries 16 and 18 from closed to open-ended arteries. The mechanism used 
is heat pipe liquid 24 itself. Heat pipe 10 is designed and its liquid 
fill 24 measured so that, during normal operation and for limited reverse 
heat flow, heat transfer liquid 24 accumulates in liquid retainer 26 at 
normal condenser 22 in such quantities that it floods and closes off the 
ends of at least one artery. With the normal evaporator end of an artery 
originally constructed as closed off, and the normal condenser end closed 
off by the accumulated liquid, the artery functions in its prescribed 
manner and moves liquid from normal condenser 22 to the normal evaporator 
20. 
However, when the heat input is changed to normal condenser 22 to make it 
the reverse evaporator, the limitations of the present invention become 
effective. As liquid 24 is evaporated from reverse evaporator 22 and 
condensed at normal evaporator 20, which is then the reverse condenser, 
accumulated liquid 24 at reverse evaporator 22 is depleted until, at the 
prescribed design point, the liquid no longer seals off the ends of any 
arteries. At this point of operation the liquid flow to the reverse 
evaporator through the arteries stops, and the drying out process 
accelerates dramatically. Heat transfer from reverse evaporator 22 then 
quickly terminates as reverse evaporator 22 completely drys out. 
By the simple combination of arteries which are mechanically unsealed at 
the normal condenser and regulation of the liquid quantity in the heat 
pipe to provide for only sufficient liquid to flood and seal off the open 
arteries, the present invention provides a heat pipe which dramatically 
limits reverse heat transfer. 
It is to be understood that the form of this invention as shown is merely a 
preferred embodiment. Various changes may be made in the function and 
arrangement of parts; equivalent means may be substituted for those 
illustrated and described; and certain features may be used independently 
from others without departing from the spirit and scope of the invention 
as defined in the following claims. 
For example, liquid retainer 26 need not make use of gravity, but could use 
centrifugal force or other means to accumulate sufficient liquid to flood 
the open ends of the arteries. In a gravity free environment the force 
resulting from vapor movement alone is sufficient to sweep liquid to the 
condenser region and hold it there. Moreover, the arteries could be 
constructed of screen material formed into cylinders.