Capillary Pumped Loop (CPL)

 
and Loop Heat Pipe (LHP)

 

Condenser

 
Capillary Pumped Loops and Loop Heat Pipes are two-phase

thermal control devices that work passively by means of capillary,

forces, removing heat from a source and transporting it to a condenser

(or radiator) to dissipate the heat. There are many applications for

Evaporator

 
CPL and LHP, such as satellite thermal control (batteries, structures),

home heating systems, HVAC systems capability improvement

(heat exchangers, compressors), computer thermal control (chip), etc.

 

Capillary forces are generated in the evaporator, driving

the working fluid, which works at its pure state, responsible

for the heat transport. The working fluid is evaporated and as the CPL and LHP are natural thermal diodes,

the vapor can only flow towards the condenser by the vapor line. Upon reaching the condenser, the working

fluid condenses back to liquid, returning to the evaporator by the liquid line. In the case of LHP, coupled to the

capillary evaporator, there is a two-phase reservoir

(also

 
called compensation chamber), which is responsible for

establishing the loops operation temperature and the fluid

inventory. In the case of a CPL, the reservoir is placed outside the

Condenser

 
loop, connected to the liquid line, where its temperature is controlled

at which the entire loop will operate. The fluid inventory in the loop is

dependent on the heat applied in the capillary evaporator: when low

heat is applied, more fluid is present in the lines; when high heat is

applied, less fluid is present in the lines, being held in the

compensation chamber or reservoir.

Evaporator

 
 


The most common working fluids used in LHPs are anhydrous

ammonia and propylene, but other options have been used such

as acetone and methanol, which represents less hazard during manipulation and reduced distillation costs.

On the same way, cryogenic LHPs have also been under investigation for heat loads management of up

to 30 W using N2 and CO2 as working fluids.

 

 
A test bed was built to test several LHPs at the same time,

where a data acquisition system connected to

a computer, a cooling bath (with operating range from 30 to

100 C, with a flow rate of 9 liters/min) and

DC power supplies can be used during the tests (see figure).

The test bed allows testing LHPs from

horizontal orientation to any angle up to compete

vertical orientation. All tests are performed in a controlled

environment with a room temperature ranging from 18 to

20 C and relative humidity around 45%.

 

Depending on the operation temperature, a LHP will require different working fluids. For temperatures ranging

from 80C to 200 C, ammonia, propylene, acetone and methanol can be applied. For

 
temperatures bellow 100C, cryogenic working fluids (CO2, N2, O2)

Condenser

 
should be used. For elevated temperatures (above 800 C),

liquid metals should be used.

 

Certain applications require that the LHP operates on a

Evaporator

 
reversible basis, i.e., the evaporator will have to operate as

the condenser when required and vice versa. This is a special

operation mode for LHPs, and in this case, the use of a

so-called Reversible Loop Heat Pipe (RLHP) is required.

This special configuration presents the same characteristics in both evaporator and condenser, which the same

porous structure and compensation chamber size.

 

Another special kind of loop heat pipe is the Ramified Loop Heat Pipe (ramLHP) which is characterized by the

 
presence of more than one heat source (capillary evaporator)

and more than one heat sink (condenser). Usually, the number

of capillary evaporators, which are placed in parallel,

should be the same as the number of condensers. Of course,

there is not a rule for such a configuration as the research

on this special kind of LHP is still undergoing and much need

to be done yet.

 
 


Researches have been also focused on computer and electronics

cooling using miniature LHPs. This special type of LHP presents

very small heat application area (resulting in high heat flux),

reduced size of the set capillary evaporator/compensation chamber

and transport line with small diameters (less than 1.5 mm).

Currently, the development has been made using mini LHPs

with flat evaporators built in stainless steel with water as working fluid and

fine pore size wick structure (with pores less than 3 microns of radius).

Ceramics materials have also been applied to this specific device

with outstanding heat management results.

 

 

Laboratory Life Tests Update

Life tests in laboratory conditions are proving the capability of using acetone as an alternative working fluid as all

LHPs currently being tested have presented reliable results with no influence of indication of non-condensable

Gases (NCGs). During all the tests with the power cycles applied to the systems alternatively, the LHPs have

shown outstanding thermal behavior as they properly accomplished the tests and also were in agreement to the

maximum heat load level applied to the capillary evaporator as they were designed for (up to 100 W) with

temperatures ranging up to 85 C. This information is important to evaluate the designing tool that has been

used to determine the geometric characteristics of any LHP built in the laboratory.

Accelerated life tests in laboratory conditions and thermal-vacuum chamber are scheduled to be performed

in the near future to better determine the operational limits of the LHPs. So far, both regular the LHPs have

passed all performance tests and have also repeated the entire test program for various sink temperatures.

The LHPs were also tested for various inclinations of the capillary evaporator (evaporator above and below the

condenser), which results are specially important when considering their operation in ground applications.

Another set of capillary evaporator/compensation chamber has been under test, where a new design of

grooves has been applied. Upon keeping the same active length, it has been possible to increase the

contact angle between the wick and evaporator housing by up to 20%. This has resulted in lower

capillary evaporator temperatures and thus reduced thermal resistances. Outstanding thermal

performance has been achieved with this new configuration (among other implemented in the evaporator

design) even using a less efficient working fluid, such as acetone.

The Reversible LHP has also accomplished the preliminary tests which has shown to properly operate

with either evaporator/condenser showing acceptable results for heat loads up to 150 W when

using acetone as the working fluid. The Reversible LHP has also been tested with power cycles ranging

from 2 to 100 W and temperatures up to 85 C, performing continuous operation along time.

The evaporator and condenser are often switched so their performance can be checked.

The Ramified LHP has presented outstanding performance when both evaporators are operating and when

one is switched to the other, showing continuous operation for heat loads up to 200 W using acetone as

the working fluid. Performance tests are still undergoing to check the upper limits of this special type of LHP.

Mini-LHP performance has shown very good heat management capability when reduced area for heat dissipation

is available (around 3 cm2). Tests with water have presented reliable operation and great potential in applying this

device for future computer and electronics cooling.

 

Main Objectives

The LHP technology has been under development at the Thermal Laboratory at INPE/DMC towards its application in space thermal control,

as well as ground applications. The objective is to disseminate the technology to apply it not only in space vehicles but also in ground

applications such as avionics thermal control, structures, electronics, motor/gearheads assemblies, refrigeration systems, etc.

Researches on life tests, materials compatibility, porous structures manufacturing and geometric characteristics influence have been

undergoing both experimentally and theoretically.