FR2850453A1 - Heat exchanger and temperaure controller for spacecraft, e.g. satellite, has one or more axial grooves in inner wall of duct divided by separator for inccreased heat transfer - Google Patents

Abstract

The heat exchanger/temperature controller (4) consists of a metal, e.g. aluminium, duct in which a heat-carrying fluid such as ammonia in liquid form is able to circulate in one direction through axial grooves (5) in its inner wall (9), with its vapour phase generated by evaporation in heat dissipation zone flowing in the opposite direction, generally along the centre of the duct. One or more of the grooves (5) has a projection (7) in its base dividing it into two sub-grooves (6). In addition, one or more of the sub-grooves can be divided again into secondary sub-grooves.

Description

HEAT TRANSFER HEAT AND / OR

  The present invention relates to a heat transfer and / or temperature homogenization heat pipe, suitable for the evacuation of the power dissipated by equipment on board a space vehicle. and in particular from a satellite. The invention also relates to a device for heat transfer and / or temperature homogenization.

  During the operation in orbit of a spacecraft, a certain number of on-board electrical and electronic equipment dissipate a quantity of heat, a quantity indexed on the intrinsic efficiency of this equipment and which may prove to be significant on certain high-power equipment . In order to maintain the thermal environment of this equipment within temperature ranges compatible with their operation and their performance, it is necessary to provide a heat transfer device for collecting, transporting and then discharging this heat to space. Such a device can also have the function of homogenizing the temperature over a certain area.

  A number of systems are currently known to effect this heat transfer. These systems include a device for transporting and distributing heat as well as a device for transferring heat by radiation. The principle of the heat transport device is based on the use of a fluid circulating between the hot zone where the thermal power is dissipated, and a cooler zone where this thermal power absorbed by the fluid is evacuated to the outside environment. by radiation by one or more fixed or deployable radiators. The operating principle of the devices is therefore based on their transport and exchange capacity as well as, for two-phase systems, on the evaporation / condensation properties of the fluid.

  The heat pipe is one of the different types of known two-phase heat transport devices. This system comprises a metal tube, often rigid (aluminum for example), in which a heat-transfer fluid circulates under saturation conditions (generally ammonia 5 in most space applications), and uses the phase change properties liquid-vapor as well as the capillarity properties of liquids. Thus, a heat pipe is a closed two-phase system in which the vapor created at the level of the hot zone (called evaporation zone) is sucked towards a colder zone (where the pressure is lower) and condenses there on the metal wall of the tube. The thermal power is absorbed by the heat transfer fluid in the evaporator zone in the form of latent heat of evaporation and retransmitted to the cold zone by releasing the latent heat by condensation.

  The liquid phase of the fluid used slides along the metal wall of the tube in the opposite direction to the flow of the vapor phase of the fluid, which remains confined to the center of the tube. This return of the fluid along the wall is provided by a capillary structure (wick, axial grooves) connecting the two ends of the tube and which plays both the role of capillary pump and separator of the two liquid-vapor phases.

  Axial grooved heat pipes conventionally transport heat from dissipative equipment to radiators with a transport capacity ranging from 100 W.m to 700 W.m in flight conditions and a vapor temperature of 201C (performance measured without industrial margins).

  However, future generations of telecommunications satellites will evolve towards increasingly significant dissipation requirements and, moreover, the distances of heat transport between dissipative equipment and radiators will increase.

  The need for heat pipes therefore evolves towards products with high transport capacities and long lengths of heat pipes while retaining a reasonable linear mass (profile mass per length of heat pipe).

  It is shown in Figures la and lb two heat pipes 1 and 1 'of the prior art according to a view in cross section. Thus, one of the heat pipes with 5 axial grooves commonly used for high transport capacities is of the type illustrated in FIG. 1 a with a diameter of 22 mm, demonstrating a theoretical transport capacity of 700 Wm in flight conditions and 500 Wm in ground conditions (4mm "tilt", this last anglo-Saxon term commonly used by those skilled in the art corresponding to 10 the measured inclination of the heat pipe in ground conditions; it measures the difference in height between the two ends of the heat pipe. line length of this section is 620 g / m. This first type of heat pipes (fig. la) comprises an envelope 2, and is characterized by grooves 3 of trapezoidal section.

  Such a heat pipe is, as its technical characteristics demonstrate, limited in transport capacity and its linear density is significant. In addition, it is characterized by a thermal defect highlighted in the laboratory: its radial thermal conductance is low. The high transport capacity levels appear with conditions of strong gradients in the section. It becomes impossible to use it at 20 maximum power levels so as not to exceed the threshold temperatures of the equipment. The use of the heat pipe of FIG. 1a is then limited to the industrial value of 340 W.m in flight conditions for a vapor temperature of 201C.

  Other high-performance heat pipe profiles, such as that illustrated in FIG. 2a, have been developed with grooves 3 'with drops in order to meet the need for high transport capacities. Heat pipes in development, using such profiles, have shown variable performances. Operating anomalies have been highlighted on the ground: operating instabilities, variability of performance over time, etc. Thus, this concept of grooves is less robust than the rectangular and trapezoidal grooves. In addition, its manufacture is more complex.

  The objective of the present invention consists in improving the transport capacities of the prior art in order to reach values between 500 Wm and 5 1 kW.m in flight and to obtain a satisfactory mass balance (i.e., a linear mass less than this offered by the prior art).

  To this end, the subject of the invention is therefore a heat transfer heat pipe and / or temperature homogenization, part of the length of said heat pipe being intended to be arranged near a heat dissipation source d '' a spacecraft so as to collect the heat that the source is capable of releasing, a liquid phase of a heat transfer fluid circulating in operation through a plurality of so-called main axial grooves formed in the internal wall of the heat pipe and in the opposite direction from the flow of a vapor phase generated by evaporation of the fluid at the heat dissipation zone, characterized in that at least one of said main grooves has a projection at its base decomposing said groove into two secondary sub-grooves by its base.

  Said grooves are of axial, radial types or having any longitudinal development.

  Thus, the new concept of the invention with double or multiple nested grooves makes it possible to increase the performances in terms of transport capacity without degradation of the thermal conductances to the evaporator of the heat transfer device. This performance gain is obtained without significant increase in the linear mass of the profile.

  This improvement on the device makes it possible to respond to greater dissipation needs on future platforms (15 to 20kW) while keeping a favorable mass balance.

  According to one embodiment, at least one of the two sub-grooves has a second projection to decompose its own base into two sub-grooves.

  According to one embodiment, said groove comprises several 5 levels of secondary sub-grooves, each level corresponding to the decomposition of the upper level, it being understood that the highest level in this tree structure comprises the main groove.

  The concept of multiple grooves is based on the use of several networks of grooves of different sizes. While, on a heat pipe with 10 axial grooves, conventional of the prior art, a single network of grooves ensures the transport of heat, the invention proposes main grooves broken down into several smaller grooves, themselves capable of making the object of decompositions. Thus the internal structure of the heat pipe can be composed of several levels of grooves.

  The large grooves optimized to minimize losses of liquid loads ensure most of the capillary transport. The small grooves at lower levels make it possible to extend the transport length of the heat pipe when the menisci of the liquid / vapor interface withdraw in the small grooves and to ensure good thermal conductance in the evaporation zone while making it possible to keep a large number of attachment points for liquid menisci.

  Other characteristics and advantages of the present invention will appear on reading the following description of the embodiments of the invention, given by way of illustration and in no way limitative.

  In the following figures: - Figures 1a and 1b schematically represent heat pipes of the prior art, according to a view in cross section, - Figure 2 shows a heat pipe according to an embodiment of the invention, - Figures 3a to 3c represent the circulation of the liquid fluid throughout the length of the heat pipe, FIGS. 4a to 4c illustrate three embodiments of main grooves with sub-grooves according to the invention.

  In all these figures, the elements fulfilling identical functions bear the same reference numbers. FIG. 2 represents a cross-section view of a heat pipe 4 according to an embodiment of the invention. This heat pipe 4 comprises an extruded structure 2 and a capillary structure 5, 5 ′, 6 and 7 intended to convey a heat transfer fluid according to a mode of transport illustrated in FIGS. 3a, 3b and 3c.

  The principle of using a heat pipe is widely extended and known to those skilled in the art. This connects an evaporator zone to a condenser zone. The evaporator zone is in the vicinity of a heat source, i.e. the heat sink equipment on board a satellite, and the condenser zone is located at a heat dissipation area located at a panel of a fixed or deployable radiator.

  The following description also refers to Figures 3a, 3b and 3c.

  In FIG. 3a, the fluid 8, in the liquid state symbolized by hatching, withdraws from the large groove 5 either on evaporation of part of the liquid or on the progression of the liquid flow from the condenser to the evaporator. The steam generated by the double arrows in solid lines in Figure 3a, moves towards the condenser zone through the center of the heat pipe and in the opposite direction of the liquid. The dotted arrows represent the preferential flow paths in the solid capillary structure 5 ′ and 7.

  In FIG. 3b, the vapor is then condensed into liquid within the condenser zone, as represented by the solid arrows in FIG. 3b, and returns to the evaporator zone via the capillary structures 5 and 6.

  The heat pipe 4 has an internal wall 9 which has a capillary structure 5. It has on its internal wall a plurality 9 of grooves over the entire circumference of the section and these extending from one end to the other of the length of the heat pipe. More particularly, in relation to FIG. 2, the heat pipe 4 comprises two levels of grooves, namely a level of large grooves 5, called main, and a level of smaller grooves 6, 10 called secondary. Thus, it is noted that each main groove 5 is broken down by means of a projection 7 of different possible geometries (rectangular, trapezoidal, ...), according to this embodiment, into two secondary grooves, formed from the center of the base of the main groove.

  In FIG. 3c, in the so-called capillary return direction corresponding to the states Et towards E6, the menisci 10 formed hollow up to the state E2.

  This recession of the menisci 10 results from a balance of pressures at the liquid-vapor interface. The capillary pressure generated by the curvature of the liquid-vapor interface in the main groove 5 must counterbalance the pressure drops in the liquid flow and the vapor flow on either side of the liquid-vapor interface.

  One arrives here at one of the fundamental advantages of the invention. The invention takes advantage of the known principle that the increase in capillary pressure in the groove is proportional to the surface tension of the fluid 8 and inversely proportional to the equivalent radius of the menisci 10. Thus, thanks to the introduction of small grooves leading to the formation of menisci with radii smaller than those of the prior art, the capillary pressure which acts as the driving force of the fluid 8 is amplified. In state E2, the capillary pressure is maximum for the attachment of the menisci 10 in the main groove 5. The presence of the secondary grooves allows the menisci to widen further by withdrawing into the secondary grooves (state E3 to state E4 ). The capillary radius of the menisci decreasing, the capillary pumping is prolonged. The state E4 to the state E6 corresponds to the conventional functioning of a groove of the prior art: the menisci widen until reaching the minimum radius of curvature in the secondary grooves (state E5). The capillary pressure is then maximum: the capillary operating limit is reached. In this state E5, the increase in power to be transported causes the drying of the secondary grooves (state E6).

  The invention makes it possible to use the capillary potential of two sizes of grooves: the large grooves thanks to their large liquid surface make it possible to limit the losses of liquid charge, the small grooves make it possible to increase the term of maximum capillary pressure.

  Furthermore, the thermal conductance at the evaporator is optimized by the presence of the small grooves. These small grooves take advantage of the known principle according to which the thermal paths to the evaporator connect the base of the grooves to the points of attachment of the menisci (FIG. 3a). The breakdown of large grooves into small grooves, allows to have a large number of meniscus attachment points. In addition, the thermal path between the heat source and the liquid-vapor interface is shortened in the case of small grooves.

  In addition, the thermal transport length is also increased compared to a heat pipe which would only have a network of small grooves 6. Thus, the performance in terms of transport capacity is increased by 35% (estimation of a model hydrodynamics) in flight conditions for a heat pipe with double grooves having a linear mass of 20% less than the current heat pipe with a diameter of 22 mm.

  In short, the inventive and astute contribution of the invention is to have known how to combine two networks of grooves, so as to be able to confer on the new heat pipe according to the invention the respective advantages of each of them.

  Figures 4a to 4c illustrate the spirit of the invention according to three different embodiments.

  While FIG. 4a represents a main groove 5 with two sub-grooves 5 such as the previously described embodiments, FIG. 4b represents a main groove 5 with four sub-grooves 5 produced by the formation of three projections 7. FIG. 4c represents, for its part, a main groove 5 comprising a main projection 71 at its base breaking down the main groove into two sub-grooves 51. Each sub-groove 51 has a projection at its base breaking it down itself into two sub -drive 52.

  Of course, the present invention is not limited to the embodiments which have just been described.

  Finally, any means can be replaced by equivalent means without departing from the scope of the invention.

  Thus, the embodiments described have revealed the concept of multiple grooves with trapezoid-shaped grooves. The concept can be applied to grooves of different shapes, such as 20 rectangular, triangular, circular or drop-shaped ...

  Depending on the power requirements, the concept of double grooves can be adapted to all diameters of heat pipes. The levels of transport capacity calculated in flight conditions are greater than 700W.m for heat pipe diameters greater than 17mm at steam temperature of 200C. 25 The double-grooved heat pipe covers the needs of large transport capacities over long distances.

  In addition, new extrusion techniques make it easy to realize the concept of a double-grooved heat pipe.

Claims (5)

  1. Heat transfer and / or temperature homogenization heat pipe (4) for a thermal control device of a spacecraft, part of the length of said heat pipe being intended to be arranged near a source of heat dissipation of the spacecraft so as to collect the heat that the source is capable of releasing, a liquid phase of a coolant (8) circulating in operation through a plurality of main axial grooves (5) formed in the internal wall (9) of the heat pipe and in reverse direction of the flow of a vapor phase generated by evaporation of the fluid at the heat dissipation zone, characterized in that at least one of said grooves (5) has at least a projection (7) at its base breaking down said main groove into at least two secondary sub-grooves (6) by its base.   2. A heat pipe according to claim 1, characterized in that at least one of the two sub-grooves has a second projection to decompose its own base into two sub-grooves.   3. Heat pipe according to claim I or 2, characterized in that said groove comprises several levels of secondary sub-grooves, each level corresponding to the decomposition of the upper level, it being understood that the highest level in this tree structure corresponds to the groove main.   4. A thermal control device for a space vehicle, characterized in that it comprises a heat pipe (4) according to one of the preceding claims.   5. Space vehicle, characterized in that it comprises a thermal control device according to claim 4.