HORIZONTAL TWO-PHASE LOOP THERMOSYPHON WITH CAPILLARY STRUCTURES BACKGROUND OF THE INVENTION Two-phase loop thermosyphons are devices that use gravity to maintain the two-phase fluid circulation when a thermosyphon is operating. Each loop thermosyphon has an evaporator, where vaporization occurs when it is heated, a vapor tube (or line) where the vapor flows to the condenser, a cooled condenser, where condensation takes place, and a liquid return line (transport lines). Sometimes, a capillary structure is used in the evaporator to reduce its thermal resistance. The entire evaporator and the capillary structure are flooded with liquid that boils when the evaporator is heated. This means that there is a liquid pool in the evaporator. The thermosyphon condenser is always above the evaporator in the gravity field, since the gravity is used to supply the heat loaded zone in the evaporator with liquid. Typical loop thermosyphons cannot operate with the evaporator at the same level with the condenser in the gravity field.

It is necessary for some electronics applications (for example, server boards) to have loop thermosyphons operating in strictly horizontal orientation with the evaporator at the same level with the condenser and horizontal transport lines. While the fluid flow through the transport lines is maintained due to the difference between the liquid levels in the evaporator and condenser, the present invention relates to an apparatus which allows operation of loop thermosyphons in horizontal orientation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 displays an embodiment of the two-phase loop thermosyphon in accordance with the present invention.

FIG. 2 displays the thermosyphon of FIG. 1 with the addition of multiple small diameter channels.

FIG. 3 displays the thermosyphon of FIG. 2 with the addition of a capillary-porous barrier.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the present invention, an additional extension of the capillary-porous structure in the evaporator is attached to the inner walls of the evaporator and extends from the liquid pool level to the top portion of the heated zone. This additional porous extension supplies liquid to the hot zone against the gravity field due to the capillary pressure developed by the evaporating liquid-vapor menisci, enabling operation of the loop thermosyphon in horizontal orientation (the heated zone at the same horizontal level with the condenser).

The evaporator contains multiple vertical channels in the porous structure extending from the liquid pool level to the top portion of the heated zone with the predetermined effective diameter of the size of larger vapor bubbles forming in the boiling pool of working liquid in the evaporator. Since the channels are of small diameter, the vapor bubbles expanding due to heating push liquid"slugs"up the channel. These liquid slugs supply liquid to the top portion of the heated capillary structure, which therefore can be at the same horizontal level with the condenser.

A continuous capillary-porous barrier is added to the evaporator, separating the liquid pool from the evaporator outlets at the vapor line side. The porous barrier is in mechanical contact with the capillary structure on the inner surface of the heated zone of the evaporator, however it is not in direct contact with the heated wall. This capillary-porous barrier with small pores maintains a pressure difference between the liquid pool side (lower pressure) and the vapor side (higher pressure) due to the capillary pressure produced by the liquid-vapor menisci in the pores. The pressure difference helps to pump vapor through the vapor line, which increases the maximum allowable heat flow rate in the loop thermosyphon (increased maximum heat transfer capability).

This porous barrier principle (the pressure difference across a porous barrier) is presently used in loop heat pipes. A difference between the barrier in a loop heat pipe of the prior art and the loop thermosyphon with a porous barrier of the present invention is that the porous barrier in the loop heat pipe is in direct contact with the heated wall, unlike the barrier in the loop thermosyphon, which is only in contact with the capillary structure. Evaporation takes place at the interface between the porous barrier and the heated wall of the loop heat pipe evaporator. For the loop thermosyphon, evaporation takes place on the open surface of the capillary structure, and there should be no evaporation from the porous barrier. Another difference between the loop thermosyphon and a loop heat pipe is that the liquid return to the loop thermosyphon evaporator is due to the gravity field (liquid head). This gives additional advantages to the loop thermosyphon compared to a loop heat pipe: (1) the condenser volume of the loop thermosyphon is not limited, while it is very limited in a loop heat pipe, (2) there is no need for a bulky liquid reservoir at the evaporator, which makes the loop thermosyphon more compact, and (3) flat configuration of the evaporator is straightforward for the loop thermosyphon, while it is a technical challenge yet to be resolved for a loop heat pipe due to the contact between the barrier and the heated wall in a loop heat pipe.

In a further preferred embodiment of the present invention, a unit would include two pieces that are placed together and welded along the perimeter to produce a complete unit. Each stamped shell is 50% of the unit. This includes the evaporator, condenser and the fluid transport lines. The evaporator wick could be sintered or a drop-in design, (such as felt metal or a pre- sintered wick). A variant could utilize a drop-in check valve/restriction in the liquid line, as opposed to the evaporator wick.

The posts made out of the porous material would be part of the sintered wick, and since the sintering process anneals the copper envelope material, the posts help to support the walls of the evaporator chamber during the evacuation process.

As shown in FIG. 1, the two-phase loop thermosyphon 10 contains both evaporator 11 and condenser 12 sections. The capillary porous structure 13 in evaporator 11 is attached to the inner walls of the evaporator 11 and extends from the liquid pool level 14 to the top portion of the heated or hot zone 17. This additional capillary porous extension/structure 13 supplies liquid to the hot zone 17 against the gravity field due to capillary pressure developed by the evaporating liquid-vapor menisci, enabling operation of the loop thermosyphon 10 in a horizontal orientation, i. e., the heated zone 17 at the same horizontal level with the condenser 12.

The evaporator 11 includes multiple vertical channels 20 in the capillary porous structure 13 extending from the liquid pool level 14 to the top portion of the heated zone, as shown in FIG.

2, with the predetermined effective diameter of the size of larger vapor bubbles forming in the boiling pool of working liquid 14 in the evaporator. Since the channels 20 are relatively small in diameter (e. g., 3mm), the vapor bubbles expanding due to heating push liquid slugs up the channel 20. The liquid slugs supply liquid to the top portion of the heated capillary structure 13, which may be at the same horizontal level with the condenser 12.

As shown in FIG. 3, a continuous capillary-porous barrier 21 is added to the evaporator 11, separating the liquid pool 16 from the evaporator 11 outlets at the vapor flow line 18. The porous barrier 21 is in mechanical contact with the capillary structure 13 on the inner surface of the heated zone of the evaporator 11 ; however, the porous barrier 21 is not in direct contact with the heated wall 19. The capillary porous barrier 21 contains small pores with liquid vapor menisci, which maintain a pressure difference between the liquid pool side 15 (lower pressure) and the vapor side 18 (higher pressure).

While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of this invention will be obvious to those skilled in the art. The appended claims and this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the present invention.