Patent Description:
Specifically, the present invention relates to a modular evaporator for a Loop Heat Pipe (LHP) system.

LHP systems are highly efficient passive heat transfer devices based on liquid/vapor phase change in a closed and hermetically sealed loop. Fluid circulation inside the loop is produced by capillary effect. LHP systems are used as cooling devices in the thermal control of space systems.

A LHP system <NUM> is schematically shown in <FIG>, and typically comprises an evaporator assembly <NUM> and a condenser <NUM>. An outlet port <NUM> of the evaporator assembly <NUM> is connected to an inlet port <NUM> of the condenser <NUM> by a vapor line <NUM>; an outlet port <NUM> of the condenser <NUM> is connected to an inlet port <NUM> of the evaporator assembly <NUM> by a liquid line <NUM>.

The evaporator assembly <NUM> (<FIG>) includes a compensation chamber <NUM> and a capillary pump <NUM> connected in series.

The capillary pump <NUM> is usually provided with a saddle <NUM> for attachment to a heat source (not shown) from which a heat flow <NUM> is extracted by conduction.

The capillary pump <NUM> includes a first micro-porous medium or primary wick <NUM> that surrounds, and sucks liquid from, a second porous medium or secondary wick <NUM> that extends throughout the compensation chamber <NUM> and the capillary pump <NUM>. Secondary wick <NUM> is hollow and houses an inner tubing <NUM>.

The inlet port <NUM> of the evaporator assembly <NUM> is connected to the liquid line <NUM> and communicates with the inner tubing <NUM> of the secondary wick <NUM>.

The capillary pump <NUM> includes a plurality of circumferential (not shown in the Figures) and axial vapor channels <NUM> located on a perimeter surface of the primary wick <NUM> constituting a vapor-liquid interface where evaporative heat transfer takes place. Vapor flows from channels <NUM> to the outlet port <NUM>, and hence to the condenser <NUM> through vapor line <NUM>.

Vapor is condensed in the condenser <NUM>, thus releasing the heat absorbed in the evaporator assembly <NUM>. The primary wick <NUM> forces the liquid to return back from the condenser <NUM> to the evaporator assembly <NUM> via the liquid line <NUM> by capillary action. The returned liquid passes though the inner tubing <NUM> located inside the compensation chamber <NUM>, cooling down the liquid in the compensation chamber <NUM>. The compensation chamber <NUM><NUM> is used as a volume buffer compensator for accumulation of liquid excess during LHP operation due to temperature variations and for ensuring LHP start-up from non-operational conditions.

LHP system are simple and reliable, but suffer limitations that limit their use.

One of such limitations is the limited pumping capability of the secondary wick <NUM>, which is not as powerful as the primary wick <NUM> and can pump the fluid against gravity only if the elevation difference is not more than <NUM>-<NUM>, depending on the surface tension properties of the working fluid.

Another important drawback of the limited pumping capability of the secondary wick <NUM> is the impossibility to create an evaporator design with multiple capillary pumps, which can cover large area for heat collecting, e. because the distance from the compensation chamber to the furthest capillary pump can be a restrictive factor for the LHP system performance. In this case, in fact, the elongated secondary wick can be unable to provide the necessary flow rate of liquid from the compensation chamber to the furthest capillary pump, which would lead to the primary wick dryout and, consequently, to failure in LHP operation.

Thus, the limited pumping capability of the secondary wick in the known designs of capillary pump-compensation chamber assemblies poses restrictions on the orientation of the evaporator assembly in a gravitational field and does not allow to construct long multiple evaporator assemblies.

This is clearly shown in <FIG>, where the evaporator assembly is shown in an anti-gravity position with the capillary pump over the compensation chamber. The more is the distance between liquid level in the compensation chamber <NUM> and the primary wick <NUM> (indicated as h1), the less is the pumping capability of the secondary wick <NUM>. When such a distance reaches a threshold level, the LHP system will stop working.

Another issue is that heat is extracted only by conduction through the small footprint of the evaporator saddle <NUM>, which makes this solution unfit for extracting heath from large areas and/or a gas flow.

<CIT> discloses an evaporator assembly according to the preamble of claim <NUM>.

An object of the present invention is to provide an improved modular evaporator assembly which overcomes the problems of the prior art LHPs specified above, and specifically allows to significantly extend the limits of LHP operation in any orientation in a gravity field.

A further object of the present invention is to create evaporator units configured to cover large areas and to grab heat from a gas flow.

The above object is attained by an evaporator assembly according to claim <NUM>.

The parallel arrangement of compensation chamber(s) and capillary pump(s) removes the limitations in evaporator size and layout due to insufficient pumping capability of the secondary wick. Moreover, it minimizes the orientation constraints in a gravity field.

Preferably, the evaporator assembly includes a plurality of compensation chambers and a plurality of capillary pumps extending along respective axes that are distinct and parallel to one another.

By combining a plurality of compensation chambers and capillary pumps in different arrangements, it is possible to obtain an evaporator assembly in the form of a large "cold plate" that can be used to extract heat from an extended area and/or multiple sources.

According to an embodiment, the evaporator assembly includes a plurality of parallel branches, each branch including at least two compensation chambers or capillary pumps.

This further enhances the ability of the assembly to cover extended areas.

According to further embodiments of the invention, the evaporator assembly includes a first manifold connected to a liquid inlet port and to first ends of the compensation chambers, and a second manifold connecting respective second ends of the compensation chambers and first ends of the capillary pumps.

This arrangement simplifies connection and allows easy integration of compensation chambers and capillary pumps into a "cold plate" arrangement.

Optionally, where the system includes a plurality of branches each having two or more compensation chambers or capillary pumps, a third manifold connecting intermediate nodes of the branches can be used. This enhances a balanced liquid feed and improves the performance in transients.

According to a preferred embodiments, the compensation chambers are disposed at the opposite ends of the evaporator, and capillary pumps are arranged between the compensation chambers. Grouping the capillary pumps in a middle area of the evaporator assembly simplifies connection of the vapor outlet ports of the compensation chambers, and allows the capillary pumps to be integrated into a finned heat exchanger. This enables heat collection from a gas flow by convection.

The present invention also relates to an LHP system including at least one evaporator assembly as previously defined and at least one condenser.

A modular LHP system according to the present invention can be provided as a system that can be finally assembled and filled as a part of the integration activities of the spacecraft, as opposed to standard LHP systems that are supplied as a single-piece filled and sealed circuit, the integration of which is tricky on satellites and substantially impossible across a pressurized shell.

For a better understanding of the present invention, a plurality of preferred embodiments are described herein by way of non-limiting examples and with reference to the accompanying drawings, wherein:.

A LHP system <NUM> according to the present invention, in its simplest form, is schematically shown in <FIG>.

As known in the art, system <NUM> comprises an evaporator assembly <NUM> and a condenser <NUM>. An outlet port <NUM> of the evaporator assembly <NUM> is connected to an inlet port <NUM> of condenser <NUM> by a vapor line <NUM>; an outlet port <NUM> of condenser <NUM> is connected to an inlet port <NUM> of evaporator assembly <NUM> by a liquid line <NUM>. Evaporator assembly <NUM> (<FIG>) includes a compensation chamber <NUM> and a capillary pump <NUM>.

According to the present invention, and as clearly shown in <FIG>, compensation chamber <NUM> and capillary pump <NUM> of evaporator assembly <NUM> are arranged parallel to one another along respective distinct axes A, B. The expression "arranged parallel to one another" as used herein refers to the spatial relationship between compensation chamber <NUM> and capillary pump <NUM>, rather than their fluidic connection.

According to the claimed invention, compensation chamber <NUM> and capillary pump <NUM> are also parallel-connected hydraulically, as a pair of manifolds <NUM>, <NUM> connect the axial ends of compensation chamber <NUM> with respective ends of capillary pump <NUM>.

Capillary pump <NUM> includes a first micro-porous medium or primary wick <NUM> that surrounds, directly contacts, and sucks liquid from, a second porous medium or secondary wick <NUM> extending throughout the evaporator <NUM>, including compensation chamber <NUM> and manifolds <NUM>, <NUM>. Primary wick <NUM> has a typical pore size of <NUM>-<NUM>.

For example, primary wick <NUM> may be made of sintered metal ceramic, preferably from the mixture of stainless steel powder and fiber.

Secondary wick <NUM> is flexible and more permeable than primary wick <NUM>, and has a typical pore size around <NUM>-<NUM>. Secondary wick <NUM> serves the purpose of supplying the primary wick <NUM> with liquid in case of transients due to rapid changes in power or condenser temperature.

For example, secondary wick can be made of stainless steel mesh or braided stainless steel fiber. Inlet port <NUM> is located at a midpoint of compensation chamber <NUM> and is connected to liquid feed inner tubing <NUM> extending through secondary wick <NUM> from inlet port <NUM> to capillary pump <NUM>.

Specifically, liquid feed inner tubing <NUM> (also known as "bayonet tubing") extends within secondary wick <NUM> through compensation chamber <NUM> from inlet port <NUM> to opposite ends thereof, pass through manifolds <NUM>, <NUM> and enters opposite ends of capillary pump <NUM> through secondary wick <NUM>. Therefore, capillary pump <NUM> communicates with inlet port directly through liquid feed inner tubing <NUM> and with compensation chamber <NUM> only indirectly through secondary wick <NUM>.

Capillary pump <NUM> includes a plurality of circumferential (not shown in the Figures) and axial vapor channels <NUM> located on a perimeter surface of primary wick <NUM> constituting a vapor-liquid interface where evaporative heat transfer takes place. Vapor flows from channels <NUM> to outlet port <NUM>, and hence to condenser <NUM> through vapor line <NUM>.

Vapor is condensed in the condenser <NUM>, thus releasing the heat absorbed in the evaporator assembly <NUM>. Primary wick <NUM> forces the liquid to return back from the condenser <NUM> to the evaporator assembly <NUM> via the liquid line <NUM> by capillary action. The returned liquid passes though inner tubing <NUM>-<NUM> located inside the compensation chamber <NUM>, cooling down the liquid in the compensation chamber <NUM>. The compensation chamber <NUM> is used as a volume buffer compensator for accumulation of liquid excess during LHP operation due to temperature variations and for ensuring LHP start-up from non-operational conditions.

For proper operation, the compensation chamber <NUM> always has two phases inside, liquid and vapor, which are separated by a vapor-liquid interface. The compensation chamber <NUM> plays a critical role during transients such as rapid input power drop down or rise up and/or quick variations of the condenser temperature.

As shown in <FIG>, where the evaporator assembly is shown in an antigravity position with capillary pump <NUM> above compensation chamber <NUM>, the distance between the liquid level in compensation chamber <NUM> and primary wick <NUM> is h2, which can be kept much smaller than h1 in <FIG> if compensation chambers of the same volume are used. This shows that the pumping capability of secondary wick <NUM> is no longer a critical factor.

Evaporator assembly <NUM> of <FIG> can be seen as an elementary module, starting from which larger and more complex evaporator assemblies can be developed, as shown in <FIG>. The same numerals are used to reference parts that are similar or corresponding to parts of the embodiment of <FIG>.

In the embodiment of <FIG>, evaporator assembly <NUM> is composed of two compensation chambers <NUM> connected hydraulically in parallel by manifolds <NUM>, <NUM>. Evaporator assembly <NUM> also includes a capillary pump <NUM> arranged parallel to and in between the two compensation chambers <NUM>. Manifold <NUM> is connected to liquid line <NUM> at respective first ends of compensation chambers <NUM>, manifold <NUM> is connected to second ends of the compensation chambers <NUM> and to capillary pump <NUM>. Vapor outlet <NUM> at an opposite end of capillary pump <NUM> is connected to vapor line <NUM>.

The secondary wick <NUM> extending along compensation chambers <NUM>, capillary pump <NUM> and manifolds <NUM>, <NUM> is shown schematically by dotted lines.

<FIG> show different, more complex embodiments of the invention. Primary and secondary wicks <NUM>, <NUM> not shown, and only steady--state flow is indicated schematically.

The embodiment of <FIG> is very similar to that of <FIG>, but includes two capillary pumps <NUM> arranged parallel to one another and in between the two compensation chambers <NUM>. Manifold <NUM> connects liquid line <NUM> to first ends of compensation chambers <NUM>, manifold <NUM> connects second ends of compensation chambers <NUM> with first ends of capillary pumps <NUM>. Vapor outlet ports <NUM> at second ends of capillary pumps <NUM> are connected to vapor line <NUM>.

The embodiment of <FIG> is very similar to that of <FIG>, and further includes a third compensation chamber <NUM> disposed in between the two capillary pumps <NUM>. As described for the embodiment of <FIG>, manifold <NUM> connects liquid line <NUM> to first ends of compensation chambers <NUM>, manifold <NUM> connects second ends of compensation chambers <NUM> with first ends of capillary pumps <NUM>. Vapor outlet ports <NUM> at second ends of capillary pump <NUM> are connected to vapor line <NUM>.

<FIG> discloses an embodiment with a plurality of compensation branches <NUM> and three capillary pump branches <NUM> arranged parallel to one another Rather than having a single compensation chamber <NUM> or a single capillary pump <NUM> for each branch, the LHP system <NUM> of <FIG> includes two compensation chambers <NUM> in each compensation branch <NUM> and two capillary pumps <NUM> in each capillary pump branch <NUM>.

It is clear that this modular approach can be further extended by combining a number of elementary modules to form an evaporator assembly <NUM> of any dimensions.

<FIG> discloses an example of a physical embodiment of an evaporator assembly <NUM> according to the present invention.

Evaporator assembly <NUM> of <FIG> includes four compensation branches <NUM> each including two compensation chambers <NUM>, and five pump branches each including two capillary pumps <NUM>. All of the branches <NUM>, <NUM> are arranged parallel to one another. Compensation branches <NUM> are located at the sides of the assembly, two on each side; pump branches <NUM> are in the middle.

Manifolds <NUM>, <NUM> connect respective first ends and second ends of the branches <NUM>, <NUM>. An additional manifold <NUM> connects intermediate nodes of the branches <NUM>, <NUM>, in between each pair of compensation chambers <NUM> and capillary pumps <NUM>. Manifold <NUM> is connectable to liquid line <NUM>. Vapor outlet ports <NUM> of each of the capillary pumps <NUM> are connected to a vapor manifold <NUM>, which is in turn connectable to vapor line <NUM>).

The parallel arrangement of the capillary pump branches <NUM> allows the latter to be incorporated as a tube bundle in a finned heat exchanger <NUM> having a pack of parallel fins <NUM>, of which only a few are schematically depicted in <FIG>. Capillary pump branches <NUM> run across fins <NUM>, in conductive contact therewith. <FIG> shows heat exchanger <NUM> in cross section and a gas flow <NUM> from which heat exchanger <NUM> may extract heat.

Claim 1:
An evaporator assembly (<NUM>) including at least one compensation chamber (<NUM>) and at least one capillary pump (<NUM>) comprising a primary wick (<NUM>), the evaporator assembly (<NUM>) including a secondary wick (<NUM>) extending through the at least one compensation chamber (<NUM>) and the at least one capillary pump (<NUM>) and directly contacting the primary wick (<NUM>) thereof, the primary wick (<NUM>) surrounding the secondary wick (<NUM>), characterized in that the at least one compensation chamber (<NUM>) and the at least one capillary pump (<NUM>) are arranged parallel to one another along respective distinct axes (A, B), and in that the compensation chamber (<NUM>) and the capillary pump (<NUM>) are hydraulically connected in parallel at both their axial ends through respective manifolds (<NUM>, <NUM>), the secondary wick (<NUM>) extending through said manifolds (<NUM>, <NUM>).