Abstract:
A two-phase heat transfer device that is capillary-driven or gravity-driven, has a two-phase working fluid contained in a closed general circuit, including an evaporator, a condenser, a reservoir having an inner volume with a liquid portion and a gas portion, a first vapor communication circuit, and a second liquid phase communication circuit. The reservoir comprises a plurality of floating bodies separating the liquid portion from the gas portion, by means of which the heat exchanges between the liquid portion and the gas portion are slowed down, which allows to diminish the effect of movement of the liquid portion or of an influx of cold or warm liquid into the reservoir.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to two-phase heat transfer devices, in particular mechanically passive devices with a two-phase fluid loop that are capillary-driven or gravity-driven. 
         [0003]    2. Description of the Related Art 
         [0004]    It is known from document FR-A-2949642 that such devices are used as cooling means for electrotechnical power converters. 
         [0005]    These devices can be temperature-controlled by controlling the pressure in the reservoir. This control can be active (electric heating resistor) or entirely passive (introduction of an auxiliary gas into the reservoir). 
         [0006]    However, it has appeared that the startup phases were particularly subject to problems for high thermal power levels, drying-out of the capillary wick can occur resulting in a startup failure. 
         [0007]    Moreover, if the device is subjected to accelerations, a phenomenon known as ‘cold shock’ may occur in the reservoir which suddenly lowers the pressure and degrades performance. 
         [0008]    Furthermore, strong fluctuations in the thermal load can lead to operational instability of the two-phase loop. 
         [0009]    There therefore appeared a need to increase on the one hand the startup reliability and on the other hand the operational reliability of such loops under conditions of thermal loads presenting strong fluctuations. 
       BRIEF SUMMARY 
       [0010]    To this end, the object of the invention is a capillary-driven heat transfer device, suitable for extracting heat from a heat source and for restoring this heat to a cold source by means of a two-phase working fluid contained in a closed general circuit, comprising:
       at least one evaporator ( 1 ), having an inlet and an outlet,   at least one condenser ( 2 ),   a reservoir ( 3 ) having an inner volume with a liquid portion and a gas portion, and at least one inlet and/or outlet port ( 31 ;  31   a,    31   b ),   a first communication circuit ( 4 ), for fluid mainly in the vapor phase, connecting the outlet of the evaporator to an inlet of the condenser,   a second communication circuit ( 5 ), for fluid mainly in the liquid phase, connecting an outlet of the condenser to the reservoir and to the inlet of the evaporator,       
 
         [0016]    wherein the reservoir ( 3 ) comprises a plurality of floating bodies separating the liquid portion from the gas portion, by means of which the heat exchanges between the liquid portion and the gas portion are slowed down. 
         [0017]    Thanks to these arrangements, the floating bodies form a thermal barrier which slows down the heat exchanges between the liquid portion and the gaseous portion, which has a number of beneficial effects explained below. 
         [0018]    Indeed, the two-phase loop can be subjected to strong fluctuations in thermal load, and therefore the flow of liquid entering the liquid part can have on the one hand a variable and even chaotic flow rate and on the other hand a temperature significantly different from that of the reservoir. 
         [0019]    First of all it is possible to diminish the effect of an influx of cold liquid into the reservoir. Such an influx of cold liquid can lead to a cold shock effect, namely a sharp decrease in the temperature of the upper surface of the liquid in the reservoir which results in a pressure drop and a sudden increase in load losses that are detrimental to the operation of the loop. Thanks to the presence of the floating bodies, the temperature fluctuation of the surface of the liquid phase will be much slower. This effect is particularly noticeable if the reservoir is pressure controlled thermally by means of a heating cartridge. 
         [0020]    In other circumstances, for example in the case of pressurization by the intentional introduction into the reservoir of an inert auxiliary gas, the influx of liquid can be at a temperature substantially higher than the average temperature of the reservoir which can lead to a hot shock effect, namely a rapid increase in the temperature of the upper surface of the liquid in the reservoir which results in an increase in pressure and temperature, with operating conditions potentially close to the maximum temperature and pressure limits. There too, the presence of the floating bodies acts as a thermal barrier to slow down the fluctuations in the gas-liquid surface temperature (temperature of the upper surface of the liquid in the reservoir). 
         [0021]    And finally, significant fluid movement can occur in the reservoir when the device is subjected to accelerations, for example if it is on board a transport vehicle. This can lead to a mixing effect in the reservoir, which can result in rapid and undesirable fluctuations of the temperature of the surface of the liquid in contact with the gaseous phase in the reservoir, and a resulting operational instability of the two-phase loop. Here again, the floating bodies form a barrier which dampens the mixing effect in the liquid. 
         [0022]    In various embodiments of the invention, one or more of the following arrangements can furthermore optionally be applied:
       the plurality of floating bodies may form a plurality of superimposed layers; This provides a good efficacy of the thermal barrier while still allowing liquid-gas exchanges;   said floating bodies are connected to each other by a flexible structure; This prevents one or more floating bodies from remaining stuck on a wall or from becoming separated from the other floating bodies;   the reservoir comprises a lower grid and an upper grid, arranged respectively away from the bottom wall and away from the top wall of the reservoir, causing these two grids to prevent the floating bodies from crossing them and forming in this way lower and upper limits for the movement of the floating bodies inside the reservoir; This prevents one or more floating bodies from getting stuck on the bottom wall or on the top wall;   all of said floating bodies have substantially the same shape; which represents an easy solution for industrialization, as only one type of element is used which is counted or weighed to determine the necessary quantity;   said floating bodies are made of a material chosen among stainless steel, Teflon, borosilicate, carbon, ceramics; whereby a chemically neutral material is chosen which does not age unfavorably over time;   said floating bodies can have a spherical shape, preferably with a diameter that is between 0.5 and 10 mm; which facilitates the spatial rearrangement of the floating bodies in the event of a change in the geometry of the liquid surface;   the volume occupied by the plurality of said floating bodies is between 3% and 12% of the total volume of the reservoir; whereby a sufficient and optimum thickness of floating bodies is obtained, in particular for a reservoir which has usual dimensions for this type of application;   the reservoir comprises an input stream deflector near the inlet port; This prevents the input stream from having a direct influence on the layer of floating bodies;   the evaporator comprises a microporous mass suitable for performing the capillary pumping of fluid in the liquid phase; so that a capillary-driven solution is used without calling upon the effect of gravity for the pumping function; as a result of which the device can be used in a microgravity environment;   the device can additionally comprise a non-return member in the form of a float fitted between the inner volume of the reservoir and the microporous mass of the evaporator, and arranged to prevent the liquid present in the evaporator from moving towards the inner volume of the reservoir; which improves the reliability of the startup of the two-phase loop;   the device being mainly subject to gravity, the evaporator can be placed below the condenser and the reservoir, so that gravity is used to move the liquid towards the evaporator; this represents an alternative to capillary pumping.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]    Other aspects, aims and advantages of the invention will become apparent upon reading the following description of several embodiments of the invention, given as non-limiting examples, with reference to the accompanying drawings in which: 
           [0035]      FIG. 1  is a general view of a device according to an embodiment of the invention, 
           [0036]      FIG. 2  shows in more detail the reservoir of the device of  FIG. 1 , 
           [0037]      FIG. 3  shows a variant of the reservoir of  FIG. 2 , 
           [0038]      FIG. 4  shows a second embodiment of the device of the invention, in which mainly gravity is used to perform the pumping of the two-phase loop. 
       
    
    
       [0039]    In the different figures, the same references refer to identical or similar items. 
       DETAILED DESCRIPTION 
       [0040]      FIG. 1  shows a capillary-driven heat transfer device, with a two-phase fluid loop. The device comprises an evaporator  1 , having an inlet  1   a  and an outlet  1   b , and a microporous mass  10  suitable for performing capillary pumping. For this purpose, the microporous mass  10  surrounds a blind central longitudinal recess  15  communicating with the inlet  1   a  in order to receive working fluid  9  in a liquid state from a reservoir  3 . 
         [0041]    The evaporator  1  is thermally coupled with a heat source  11 , such as for example an assembly comprising electronic power components or any other component generating heat, by Joule effect for example, or by any other method. 
         [0042]    Under the effect of the contribution of calories at the contact  16  with the microporous mass filled with liquid, fluid passes from the liquid state to the vapor state and is evacuated through the transfer chamber  17  and through a first communication circuit  4  which conveys said vapor to a condenser  2  which has an inlet  2   a  and an outlet  2   b.    
         [0043]    In the evaporator  1 , the cavities freed by the evacuated vapor are filled by liquid drawn in by the microporous mass  10  from the aforementioned central recess  15 ; this is the capillary pumping phenomenon as is well known per se. 
         [0044]    Inside of said condenser  2 , heat is released by the fluid in the vapor phase to a cold source  12 , which causes a cooling of the vapor fluid and its change of phase to the liquid phase, that is to say its condensation. 
         [0045]    At the condenser  2 , the temperature of the working fluid  9  is lowered below its liquid-vapor equilibrium temperature, which is also known as ‘subcooling,’ so that the fluid cannot revert to the vapor state without a significant heat input. 
         [0046]    The vapor pressure pushes the liquid in the direction of the outlet  2   b  of the condenser  2  which opens onto a second communication circuit  5 , which is also connected to the reservoir  3 . Said second communication circuit  5  can comprise two separate portions  5   a , 5   b  as will be seen below. 
         [0047]    The second communication circuit  5  comprises working fluid mainly in the liquid phase, whereas the first communication circuit  4  comprises fluid mainly in the vapor phase. 
         [0048]    The reservoir  3  has at least one inlet and/or outlet port  31 , here in the case of  FIG. 1  a separate inlet port  31   a  and a separate outlet port  31   b,  and the reservoir  3  has an inner volume  30 , filled with heat transfer fluid  9  in two-phase form. The working fluid  9  can be for example ammonia or any other suitable fluid, however preferentially methanol can be chosen. The working fluid  9  is in a two-phase form in the reservoir  3  partly in the liquid phase  6  and partly in the vapor phase  7 . In an environment where gravity is exerted (vertically according to Z), the gaseous phase part  7  is situated above the liquid phase part  6  and a liquid-vapor interface  19  separates the two phases (upper surface of the liquid in the reservoir). 
         [0049]    It is the temperature of this separation surface  19  which determines the pressure in the loop, this pressure corresponds to the saturation pressure of the fluid at the temperature prevailing at the separation surface  19 , according to the bijective relationship linking Psat and Tsat in conditions of saturation. 
         [0050]    It should be noted that, in cases where the heat transfer device is located in a moving vehicle, the reservoir can be subjected to a wide range of inertial forces. Moreover, the reservoir can find itself tilted with regard to its normal reference position, as illustrated in  FIG. 3 . 
         [0051]    Advantageously according to the invention, a plurality of floating bodies  8  are arranged inside the reservoir. These floating bodies  8  position themselves naturally in the upper part of the liquid portion, globally at the interface between the liquid portion  6  and the gas portion  7 . 
         [0052]    Their buoyancy in relation to the liquid of the working fluid must be sufficient, and consequently their density must be substantially less than the density of the liquid. According to the chosen material, these floating bodies  8  can therefore be full or hollow, the hollow internal volume being adapted in relation to the desired buoyancy. 
         [0053]    Preferably a material will be chosen that is chemically inert with regard to the working fluid. Preferably can be envisaged stainless steel, Teflon, borosilicate, carbon, ceramics, or other materials. 
         [0054]    These floating bodies are designed to resist without bursting a relatively low external pressure for example up to a few hundredths of atmosphere in the case of the use of a low pressure fluid such as methanol at −50° C. or during the initial vacuum creation phase at the filling of the system. 
         [0055]    Moreover, these floating bodies are designed to support the maximum pressure possibly prevailing in the two-phase loop at a maximum temperature and thermal load, that can go up to around 10 bars. 
         [0056]    Advantageously, a sufficient number of floating bodies  8  are provided so that they form a plurality of superimposed layers, as is shown in particular in  FIGS. 2 and 3 , as a result of which the floating bodies can reconfigure themselves spatially according to the surface and the geometry of the liquid-gas interface. For example, when the reservoir is tilted ( FIG. 3 ), the surface of the liquid-gas interface is higher and consequently, the floating bodies reconfigure themselves spatially to occupy all of this surface but with a thickness that is slightly inferior. 
         [0057]    In other words, the set of floating bodies  8  forms a variable-geometry thermal barrier. 
         [0058]    According to the application and the shape of the reservoir, a number of shapes can be chosen for the floating bodies. Advantageously, a spherical shape can be chosen, whereby the plurality of floating bodies will form a carpet of floating beads. 
         [0059]    For example, a diameter can be chosen for the beads (balls) that is between 0.5 mm and 10 mm, or preferably between 2 mm and 5 mm. 
         [0060]    According to an advantageous aspect, all of the floating bodies can substantially have the same shape, spherical or otherwise. In this case only one type is used which is counted or weighed to obtain the necessary quantity for insertion into the reservoir. 
         [0061]    Alternatively, when for example the reservoir can be in the form of a cylinder of a certain length and with an axis A, the floating bodies can be chosen as round rods with a small diameter extending parallel to the axis A over the whole length of the cylinder, and arranged next to each other. 
         [0062]    The reservoir  3  serves as an expansion tank for the working fluid of the two-phase loop and for pressure regulation if required. Consequently, the liquid portion in the reservoir alternates between a non-null minimum volume in the reservoir and a maximum volume which does not however occupy the entire inner volume  30  of the reservoir  3 . 
         [0063]    In order to prevent the floating bodies  8  from sticking to the walls, in particular the top wall  32  and the bottom wall  33  of the reservoir, horizontal grids can be provided, namely an upper grid  82 , and a lower grid  83 , their position being compatible with the minimum volume and the maximum volume of the liquid portion in the reservoir as mentioned above. 
         [0064]    The mesh of the grids is sufficiently small to prevent the floating bodies from crossing the grids. Care is taken to arrange the lower grid  83  a little below the minimum volume of liquid and the upper grid  82  a little above the maximum volume of liquid in the reservoir. 
         [0065]    For systems subjected to violent or periodical longitudinal accelerations, vertical grids  13  (see  FIG. 4 ) can prevent rapid movement of the liquid which could disturb the efficacy of the floating bodies. The free surface is therefore compartmented to obtain this anti-sloshing function. 
         [0066]    The volume occupied by the plurality of said floating bodies can advantageously be between 3% and 12% of the total volume of the reservoir, in order to obtain a sufficient and optimum thickness of floating bodies, in particular for a reservoir having usual dimensions for this type of application, namely three dimensions that are relatively close to each other. 
         [0067]    According to an optional feature, the floating bodies can be wrapped in a flexible structure for example of the net  35  or gauze type as illustrated in  FIG. 3 . In this case, at least one extremity of the envelope containing the floating bodies is attached to a side of the reservoir at an attachment point  40 . 
         [0068]    It is possible to have more than one attachment point, provided that the flexible structure is stretchable or presents a certain length of slack. The floating bodies are captive inside the net, which prevents one or more floating bodies from becoming separated from the group. 
         [0069]    Moreover, advantageously according to the invention, the reservoir comprises an input stream deflector  38  near the inlet port  31   a  or near the inlet/outlet port  31  according to the configuration of the second conduit. 
         [0070]    This input stream deflector  38  prevents a rapid arrival of liquid into the reservoir from creating a current in the liquid phase directed directly towards the liquid-gas interface. This deflector can take the form of a U section oriented downwards, or of a bowl or of any other shape creating a sufficient deviation of the trajectory of the liquid entering vertically towards the top. 
         [0071]    At the inlet port  31   a  and/or inlet/outlet port  31  of the reservoir, a filter  34  can optionally be provided, illustrated in  FIG. 4 , to prevent one or more floating bodies from entering the liquid conduit leading to the evaporator. 
         [0072]    The filter  34  can be for example a steel-wool type structure, or a sponge-type structure or a macroporous structure. 
         [0073]    There can further be provided a filler port  39  closed after initial filling with working fluid. 
         [0074]    The first and second fluid communication circuits  4 , 5  are preferably tubular conduits, but they could be other types of conduits or fluid communication channels (conduits that are rectangular, flexible, etc.). 
         [0075]    Similarly, the second fluid communication circuit  5  can be in the form of two separate independent conduits  5   a,    5   b  (see  FIG. 1 ) or a single conduit with a ‘T’ coupling  5   c  (see  FIG. 4 ). 
         [0076]    These conduit configurations remain relevant when a number of evaporators and/or a number of condensers are connected in parallel. 
         [0077]    In all cases, the second fluid communication circuit  5  connects the outlet of the condenser  2   b  to the inlet of the evaporator  1   a , either indirectly by going through the reservoir (in the case of two independent conduits) or directly (in the case of a single conduit with a ‘T’ coupling). 
         [0078]    The device can additionally comprise a non-return member  60  in the form of a float arranged between the inner volume of the reservoir and the evaporator. This non-return member forms a valve aimed at preventing liquid present in the evaporator from moving to the inner volume of the reservoir, in particular at the moment of a sharp startup. 
         [0079]    Advantageously according to the invention, the device does not include any mechanical pump even though the invention does not exclude the presence of an auxiliary mechanical pump.