Variable thermal conductance reflux heat pipe

A thermal energy transfer device having controllably variable thermal conductance comprises a reflux heat pipe having a capillary wick therein, an evaporator and a condenser zone, a circulating supply of working fluid which is controlled by a bendable section in the heat pipe body and an external device to control the bend in the heat pipe body, thereby controlling the amount of liquid returning to the evaporator. As returning liquid condensate is trapped in the center of the deformed tube, less liquid is available for evaporation in the evaporator. This results in a low liquid level that saturates only a portion of the evaporator wick. This results in higher thermal impedance. If the heat pipe body is deformed so that all liquid is trapped in the deformed portion, no liquid is available in the evaporator and there is a high thermal impedance.

BACKGROUND OF THE INVENTION 
This invention relates to devices that transfer heat from a heat source to 
a heat sink by means of a closed internal evaporation-condensation cycle. 
Heat pipes have long been known in the art. A typical heat pipe utilizes a 
closed vessel, containing a wick and working fluid. As heat is applied to 
one end of the heat pipe called the evaporator, the liquid vaporizes, and 
as vapor pressure builds, vapor is driven to the cooling area, called the 
condenser, where the vapor condenses. The resulting liquid condensate then 
returns by capillary action or by gravity to the evaporator to be used to 
repeat the cycle. The resultant structure is characterized by high thermal 
conductance and very low temperature drop. 
The interior of the heat pipe normally contains a wick extending throughout 
its entire length. However, certain designs that rely on a gravity liquid 
return system may require a wick only in the evaporator to uniformly 
distribute the liquid. The free space inside the structure is the vapor 
passage and must be kept clear for efficient flow of vapor from the 
evaporator to the condenser. 
In many heat pipe applications it is advantageous to control the thermal 
impedance of the device. For example, the heat coming from a solar 
collector may be required in the winter but not in the summer. Therefore, 
in the summer, the controllably variable conductance heat pipe would be 
turned off. When the heating season arrives, energy from the solar 
collector will be needed and the variable conductance heat pipe can be 
turned on. Another application of a variable conductance heat pipe is in 
the utilization of energy escaping in the flue stack from a home furnace. 
The variable conductance heat pipe can conduct the heat (waste heat) to 
the sidewalk and remove ice in the winter. It is disadvantageous however, 
to allow the heat pipe to run continuously since this does remove heat 
from the flue gas exhaust and may allow flue gas cooling below the dew 
point resulting in condensate accumulation. The variable conductance heat 
pipe can be actuated only when necessary so that furnace damage due to 
condensates from the flue gas is minimized. 
SUMMARY OF THE INVENTION 
This invention relates to a reflux heat pipe having a deformable zone and a 
circulating supply of working fluids whose circulation is controlled by 
bending or deforming the heat pipe to provide a trap in the deformable 
portion of the heat pipe. The inner section of the hot or evaporating end 
of the heat pipe is composed of various wick slab structures that end at 
different depths in the liquid puddle in the evaporator. When the 
evaporator is full of liquid all wicks are saturated because the liquid 
surface contacts them. When the heat pipe body is in the "on" or 
undeformed position, liquid is allowed to return freely to the evaporator 
puddle and heat is transferred readily from the evaporator to the 
condenser. This is called the "on" mode. Should higher thermal impedance 
be desired in this heat transfer apparatus the heat pipe body can be 
deformed slightly resulting in a partial liquid trap. Since this liquid 
trap prevents a portion of the total evaporator liquid from returning, the 
evaporator puddle reduces in depth and does not contact all the wick slabs 
in the evaporator. This results in desaturation and loss of liquid to 
portions due to evaporation. Since there is less liquid available 
circumferencially to wet the evaporator there is a higher thermal 
impedance at the same heat flux. This is called the "variable conductance" 
mode of operation. Should no heat transfer be desired, the heat pipe body 
can be deformed to its maximum to trap all the liquid in the middle of the 
deformed heat pipe body. No liquid returns to the evaporator and the 
evaporation condensation is effectively interrupted. Accordingly, no heat 
transfer takes place. This is the third mode of operation called the "off" 
mode.

DESCRIPTION OF PREFERRED EMBODIMENT 
Referring now to the drawings, there is depicted, for purposes of 
illustrative disclosure, a preferred embodiment of the thermal energy 
transfer device of the invention. Portions of the drawings have been cut 
away to expose inner surfaces. 
As shown, the reflux heat pipe device 20 includes an uninterrupted pipe 24 
which incorporates a deformable adiabatic zone 26. The heat pipe 24, 
serving as the main channel for liquid return flow and vapor flow, is 
closed at each end. 
A pipe deforming assembly 28 includes a lever 30 and a pair of spaced arms 
34 and 36 which bridge the pipe 24 in its deformable center or adiabatic 
zone 26. The arms 34 and 36 are mounted 40 on a pivotal shaft 44 so that 
they will contact and stressingly bear upon opposed upper 46 and lower 48 
surfaces of the deformable zone 26 to effect a distortion or inflection in 
the zone 26 in a vertical plane when the shaft 44 is rotated. 
In the normal "on" mode of operation the pipe 24 contains a liquid puddle 
50 which contacts an array of slab wicks 52 bonded to an internal surface 
56 in an evaporator zone 60 of the device. When heat is applied at the 
evaporator zone 60k vapor is liberated from the wicks 52 and is driven by 
a pressure gradiant to a condenser zone 64 displaced upwardly from the 
evaporator zone 60. Vapor is condensed to liquid and returns by gravity to 
the liquid puddle 50. 
The degree of deformation of the deformable section 26 is selectively 
adjustable and is shown on an indicator 70 correlated with three different 
operation modes, A,B, or C. With the deformation lever 30 positioned so 
that the indicator 70 points to A or the "on" mode normal heat transfer 
results (FIG. 1). 
In FIG. 2 the deformation apparatus indicator is in the B position 
correlated with the variable conductance mode. The adiabatic section 26 is 
thus deformed to define a liquid trap 74 in the deformable zone 26 and 
liquid 76 collects. Accordingly, there is a liquid deficiency in the 
evaporator zone 60 which exposes one of the wick slabs 52 resulting in 
desaturation of that slab, since no liquid can be wicked. This reduction 
in wetted wicking area results in higher thermal impedance since heat now 
must be transferred from a smaller portion of the evaporator. 
In FIG. 3, the deforming apparatus is moved into position C which results 
in a greater degree of deformation of the adiabatic section 26 and a 
larger amount of liquid is trapped. In the case illustrated, all the 
liquid is trapped in the adiabatic section 26 and there is no liquid 
puddle in the evaporator. The result is cessation of the liquid 
evaporation-condensation cycle. This represents the "off" mode of the 
variable conductance heat pipe.