Loop Heat Pipes Loop heat pipe thermal solutions are completely passive, two-phase heat transfer devices that are bendable, flexible and routable. The. Looped heat pipes (LHPs) are two-phase heat transfer devices that employ the same capillary pumping of a working fluid as used in. A loop heat pipe (LHP) is a two-phase heat transfer device that uses capillary action to remove heat from a source and passively move it to a condenser or radiator.‎Construction · ‎Mechanism · ‎Motivation: Limitations of · ‎Applications.


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Loop Heat Pipe Design, Manufacturing and Testing - An Industrial Perspective

A heater, sized to provide uniform loop heat pipe, is attached onto the heat input surface of the evaporator; and, a series of thermocouples are attached to the evaporator body along its length.

The heater power is increased stepwise as the thermocouples are monitored for uniformity.


A non-uniform temperature profile indicates a hydraulic coupling flaw between the primary and secondary wicks.

Secondary Wick Loop heat pipe Testing. There are times when the solar heating load is quite high on one side of the satellite, while the other side sees a much colder environment. LHPs can be designed so that the LHP rejects the waste heat to the cold side condenser while the hot side condenser is inactive, and filled with superheated vapor.

As the hot and cold sides switch, a parallel condenser flow balancer insures that the LHP always passively rejects heat to the coldest condenser. With dual condensers, the vapor flowing out of the pump loop heat pipe enters a tee that branches into the two, parallel condenser loops.

The loops are connected again through a second tee which collects liquid from the condensers and returns it to the pump body.

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If the solar load is particularly high on one of the two LHP condensers, it is possible that the loop heat pipe of that condenser will be greater than the LHP operating temperature. In this case, the warm condenser segment will be filled with relatively static superheated ammonia vapor.

Loop heat pipe

If this vapor were allowed to mix freely with the sub-cooled liquid returning from the other condenser segment, the superheated vapor would condense into the sub-cooled liquid and this would result in canceling the loop heat pipe which would likely cause an uncontrolled rise in the loop loop heat pipe.

A parallel condenser flow balancer, shown in Figure 12, is installed to prevent the superheated vapor from one condenser segment from mixing with the sub-cooled liquid returning from the other segment.

The flow balancer works by establishing a liquid — vapor interface on a porous membrane.


There are three requirements for the parallel condenser flow balancer: Use capillary forces to prevent superheated vapor from entering the liquid return line loop heat pipe. Minimize the liquid pressure drop from the active condenser 3. Supply liquid from the active condenser to the inactive condenser through a capillary link, to replenish liquid evaporated from the screen on the inactive side.

The parallel flow balancer in Loop heat pipe 12 uses composite screen membranes to meet the three requirements. The pore radius of the screen was small enough to hold-off vapor penetration and the membrane loop heat pipe thin enough such that the pressure drop from liquid flow through the membrane was small.

The screen capillary link supplies liquid to the screen adjacent to the vapor. A fourth consideration is preventing conduction from the hot side vapor from canceling the subcooling in the liquid return line.

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The tee length must be long enough that conduction though the tubing walls and the stagnant liquid slug in the non-flowing side of the tee would be negligible.

This distance is on the order of 2. After the secondary flow balancer has been tested, it is installed in the LHP liquid return line. Loop heat pipe leak checking, the LHP is ready for processing and loop heat pipe testing.

Loop Heat Pipe Design | Flexible Heating Pipe Tech | Two Phase Thermal Management

Historically, some loop heat pipes have been difficult to start at low power levels. This generally occurs when the evaporator grooves are flooded with liquid and the loop heat pipe attached to the evaporator is large.

In some cases, a low power input may simply warm-up the pump assembly very slowly and never create vapor to set up the pressure difference required to initiate circulation. Loop heat pipe heaters are installed on the LHP evaporator to prevent this problem.

These heaters apply a large heat flux to a small portion of the evaporator grooves, initiating a vapor bubble that clears the grooves.

During startup testing, the LHP is cooled down in a manner that maximizes the chances for the grooves to fill with liquid, and then restarted with the startup heaters.

In some spacecraft applications, the electronics to be cooled operate intermittently. During the portion of the orbit when the electronics payload is turned off, it is loop heat pipe for the loop to stop transferring power to maintain the temperature on the loop heat pipe deck and minimize the magnitude of the temperature swing of the electronics through an orbit.

Typically, a working LHP will continue to transfer power from the evaporator to the condenser, until the evaporator and condenser temperatures are nearly equal.