The present invention relates to a heat sink/exchanger for a cooling apparatus adapted to transfer heat from a component which generates thermal energy (mainly for electronic devices, such as the CPU, CPU or larger devices in various possible configurations) to a heat transfer fluid (refrigerating fluid), as well as an operating method of the entire cooling system.

The heat source could be a device which generates high thermal power densities, and therefore not limited to a microchip (CPU, CPU, LED, etc.), an integrated circuit or a battery or any component which produces high thermal power densities up to 1000 W/ciri and more.

4s a result, the present invention is suitable for a wide variety of applications, from electronic cooling devices, electrical devices, mechanical cooling devices to chemical cooling reactions and/or related devices and processes .

A better understanding of the invention will be achieved by means of the following detailed description and with reference to the figures, which illustrate a preferred embodiment of the invention by way of non-1 imiting example and of some variants thereof.

In the drawings:

figure 1 shows a two-phase flow cooling system of known type;

figure 2 is a plan view of the base plate of an evaporator according to the present invention ;

figure 3 is a plan view of an exchanger- evaporator according to the present invention , together with two sections taken along respective plotting planes A-A and B-B;

figure 4 is a diagram of the geometric parameters of the finned surface of the evaporator ;

figure 5 shows a first variant of the evaporator , which has an elongated plate with channels parallel to the longest side;

figure 6 is a section taken along a plotting plane A-A of the elongated plate in fig. 5;

figure 7 shows a second variant of the evaporator , which has an elongated plate with channels parallel to the shorter side;

figure 8 is a section taken along a plotting plane A-A of the elongated plate shown in fig. 7.

Hereinafter, for the sake of simplicity of presentation , reference will be made to an example of embodiment with a two-phase fluid, i.e. with a liquid phase and vapor phase, but the inventive concept underlying the invention is also applicable without modification to multi -phase fluids, in which incondensable gases and/or solids, such as ( but not limited to) nanoparticles , may be present in addition to the liquid phase and the gaseous phase of the evaporated fluid.

The two-phase cooling system with flow boiling according to the present invention exploits the physical phenomena of convection , conduction , evaporation and radiation and substanti ally consists of a closed hydraulic circuit apparatus of a heat transfer refrigerating fluid compri si ng :

-means for pumping a refrigerating fluid which, in some stretches of the circuit, is two-phase ;

-a heat sink-evaporator capable of transferring heat by conduction from a component to be cooled to the refrigerating fluid,

-a condenser-radiator, capable of condensing the vapor which develops on the evaporator while simultaneously di ssi pati ng the heat given by the refrigerating fluid both by conduction , convection and radiation and by condensation into the environment or into a cold sink or into another fluid;

-an optional heat exchanger (recuperator), which transfers heat from the fluid exiting the evaporator to the fluid entering the evaporator itself;

-a tank which acts as expansion vessel or a tank and expansion vessel connected to each other;

-sensors and an electronic control system of the pump, of the air flow to the condenser and of the pressure.

According to a particular feature of the invention, the hydraulic circuit is a closed loop in which the refrigerating fluid, which is present in two or multi phase state is ci rculated by a pump. According to the invention , this refrigerating fluid initially starts from the pump totally in liquid phase, when it reaches the element to be cooled, it passes partially from the liquid state to the vapor state with an optimal percentage definable by the system parameters and controllable by an electronic system which regulates the pump flow rate, the speed of the fans which cool the condenser and the system pressure, in order to keep the evaporation and the temperature of the element to be controlled at the desired level.

Evaporation phenomena therefore occur on the heat sink which, for this reason, is also indicated as evaporator , in which heat is transferred from the heat source to the fluid by means of a series of physical phenomena: convection , heat conduction , evaporation , radiation.

According to a further particular feature of the invention, if the vapor content ( vapor quality) is controlled, heat dissipation is maximized because it also extracts the latent heat of vaporization, thus obtaining results that the current technique does not allow. However, in order to do this, it is important not only to manage the evaporation control and the successive condensation by means of sensors arranged in an appropriate position in the circuit which send their signals to an electronic control board which controls: the pump flow rate, the condenser fan speed, as well as the pressure regulation system, but it is also important to provide an evaporator capable of subtracting as much heat as possible from the component to be cooled.

For this purpose, it is necessary to manage the evaporator/component interface to be cooled, but also to use the best materials for the making of the evaporator and its geometry on fluid side.

A typical two- phase cooling system is shown in Figure 1, in which a closed loop system with a pump is provided to circulate a working fluid. Downstream of the pump, the fluid flows towards an evaporator , which is the heat exchanger in which the l iquid evaporates , whereby absorbing the heat of the component to be cooled (e lectronic device: CPU, CPU, ICBT, inverter, etc. ) .

According to the present invention, the two-phase or multi -phase mixture of fluid coming out of the evaporator may pass through a heat exchanger (l iquid- liquid, indicated l ike a “recuperator” in the diagram in Figure 1) and give either part or a l l of the heat accumulated to the fluid entering the evaporator , whereby achieving a sort of preheating. Such a recuperator may be present in some appl ications . At the outlet of the recuperator , the two-phase or multi phase fluid thus flows to a condenser , in which the vapor condenses again in l iquid phase and the heat is transferred from the fluid to the environment or to the heat sink . The circuit contains, as mentioned, a storage tank-accumulator which ensures expansion to al low variations in fluid density and thus changes in volume due to evaporation and condensation . This storage tank controls the system pressure and therefore the fluid saturation temperature . This temperature is settable to maintain the electronic components at their operating temperatures . In a computer, for example, CPU and CPU should operate in a temperature range from 25 ^° C to 60 ^° C and should not exceed 85-100 ^° C.

Advantageous ly, an innovative heat control system, such as the one described herein, can remove high heat densities with low mass flow rate of the refrigerating fluid and of mai ntai ni ng the surface of the electronic component i sothermal . In addition to providing the above benefits, the system according to the present invention is character! zed by smaller volumes and masses compared to those of known cooling technologies : as a result, it requires less energy for its operation . In addition to these advantages , there is also reduced system noise.

Compared to the solutions known in the art, diagrammatical ly shown in figure 1, the present invention contains original and innovative elements represented by the evaporator , as well as the control and regulation system of the flow rate, pressure and air flow on the condenser .

Hereafter , this invention will be described in greater detail, with parti cul ar reference to the innovative heat transfer method related to the described heat sink, with reference to figures 2 to 7, which show only by way of example a preferred , non binding embodiment (and some of its variants) .

The heat sink-evaporator comprises a housing consi sting of:

a base (Figs. 2 and 3) and of

a cover

which together define obligatory passages for the refrigerating fluid.

These obligatory passages can also be defined by the presence of additional elements, such as baffles, conveyors or reducers. The base is a thermal conductive material plate, with a lower surface in contact with the device to be cooled and an upper surface on which a plurality of preferably ( but not exclusively ) mutually parallel fins are present which define a corresponding plurality of channels. According to the invention , each of said channels is delimited by two adjacent fins and by the surface of the base itself present therebetween . The upper side of each channel may be closed by the bottom surface of the cover or it can be opened, in which case the cover is placed at a given distance from the free end of the upper part of the fins. In the example shown, the cover is a plate which can contain manifolds (mainly the inlet and the outlet) and possibly other channels.

Between the base and the cover there is a system of seals to ensure the hydraulic tightness of the system.

It is also possible to provide that between the cover and the base there is a spacer element which may contain the sealing system or not to ensure the hydraulic sealing of the heat sink, respectively with the base and with the cover.

Another particul ar feature of the invention is that the channels (also named micro-channels) present on the upper surface of the exchanger may all have the same length or lengths which are not uniform.

Furthermore , the height of the fins which define the channels can also be either uniform or uneven. In case of non-uniform height, the fins may have decreasing heights in linear, quadratic , cubic, etc. manner from an inlet or outlet side to the opposite side. Alternatively, they may have decreasing heights from the center toward the ends or vice versa.

Furthermore , the fins may have a discontinuous surface, i.e. with openings or holes which connect a channel with at least one adjacent channel. The upper free end, or edge, of the fins can also be di s conti nuous .

Finally, the body of the fins may be orthogonal to the surface of the base or have a given inclination with respect thereto.

Figure 4 shows the geometrical parameters “a”, “b” and “c” of the fins which define the channels (which hereafter will also be named “micro-channels”) .

In the preferred embodiment described by way of non-limiting example, the values of these three parameters are:

a: the thickness of a fin: 0.1 mm 1.0 mm

b: distance between two fins: 0.1 mm 2.0 mm c: height of a fin: 0.1 mm 5.0 mm

Another particular feature of the invention is in that the refrigerating fluid flow may be either parallel or transverse to the axis of the channels, or oblique thereto if the channels are arranged according to an angle with respect to the flow axis.

The wetted surface may contain a pair of peri pheral channels A and B (Fig. 2) in relation to the finned surface, which act as inlet/outlet plenum (manifolds) or conveyors according to the fluid flow di recti on . These channels, which mainly function as channels to allow easy removal of vapor from the heat transfer surface, may a lso be obtained in the cover.

The surface of the micro-channe ls may be pol ished or sandblasted or may be rough with a Ra roughness parameter preferably within the range : 0. 01 micron 500 microns .

This surface may have micro-structures consisting of, by way of non- l imiting example:

sintered powder with particle size in the range:

0.05 pm 1000 pm;

specia l paint or coating: 3M© coating system;

paint or coating:

chemica l finishes or chemica l surface treatments .

The lower surface of the base can be flat or assume a concave or convex curvature .

The radius of curvature may be comprised within the range: 3000-7000 mm

The lower surface finish can be rough or smoothed with a roughness described by the Ra parameter in the range : 0.05 pm 500 pm.

The base may have any plan shape, such as ( but not l imited to) : square, circular, rectangular, hexagona l , octagona l , etc.

For quadrangular bases, dimensions may vary from 10 mm x 10 mm to 120 mm x 120 mm and more .

For circular symmetry bases, the diameter can vary from 10 mm to 120 mm and more .

The base could a lso be inscribed in a circle of a few mi l l imeters in diameter up to 120 mm and more, according to the appl ications .

For rectangular shapes, the ratio of the longest side to the shortest side is preferably 1 to 7 and more.

It is worth noting that in addition to these geometric configurations of single evaporator construction solutions , modular solutions of two or more evaporators can be provided to cool large surfaces and/or surfaces with complex geometry, or more surfaces at the same time. In such modular configurations, the single evaporators can be arranged on the surfaces to be cooled according to matrices of various shapes (e.g. 1x2, 4x4, 3x5 etc.) and the hydraulic connections , between the same evaporators , can be in series or in parallel or series/parallel combi nations .

The channels delimited by the fins can have the longi tudi nal axis parallel or inclined with respect to one of the sides.

Furthermore , different geometries are possible: such as V-shaped channels, wavy, fretted, etc.

The following is worth noting with regard to the operation of the heat and hydraulic exchange of the heat sink described above.

The flow direction may be parallel to the longi tudi nal axis of the micro-channels ( upward vertical arrow in Fig. 2) or transverse to the longi tudi nal axis of the micro-channels ( left horizontal arrow in Fig. 2).

The fluid flow rate can vary over a wide range, up to 1000 kg/h and more.

The average speed of the fluid in the central section of the heat exchanger can vary from 0 to 30 m/s and more. The heat transfer from the base to the refrigerating fluid takes place at least according to the heat exchange methods described below in a non- exhaustive manner.

The refrigerating fluid is conveyed into the heat sink casing through an inlet manifold, from which it either flows along the side of base plate with micro- multi channels and flows towards the outlet, or flows towards the center of the base plate and is divided into two flows towards the two ends continuing to the outlet.

In some preferred configurations , such as those shown in Figures 2, 5 and 7, the fluid is pushed through an inlet pipe obtained from the thickness of the cover. The flow direction is orthogonal , from top to bottom ( or vice versa) , to the plate itself. In other configurations , the manifold can be integrated into the finned plate in a direction paral lel to the base of the plate or orthogonal . In such a case, the plate has a preferably extended configuration to accommodate the manifold.

The geometry of the manifold may be triangular, rectangular or may have other shapes, even complex, according to the application or the overal l dimensions which are avai lable or to optimize the distribution of flow rates in the individual channels . The inner surface of the manifold can be optimized to reduce pressure losses, to improve flow distribution in the micro-channel s , or to have sharp edges to promote the formation of vapor (e. g. by generating cavitation or turbulent motions) . Furthermore, there may be a section shrinkage at the inlet of the manifold to either reduce or eliminate the instability of the two- phase fluid in the heat exchanger .

Finally, the refrigerating fluid flows out of the heat sink casing through an outlet manifold, to which the same considerations concerning the shape and direction of flow as described for the inlet manifold apply.

The fluid can enter the heat exchanger in subcooled conditions , with the average temperature of the fluid lower than saturation temperature , or it can enter in saturation conditions with an average temperature equal to about the saturation temperature (it can also have a slightly lower or higher temperature) . The inlet fluid is generally in liquid state, but it can also be in two-phase conditions with presence of vapor or gaseous phase due to air or other incondensable gases.

During the flow to the outlet of the heat exchanger , the fluid passes from the liquid phase to the vapor phase in the micro-channels. In some cases, the fluid may change phase in the side manifolds (A or B) and/or the inlet manifold.

/4s a result, the main heat transfer takes place between the base of the heat exchanger and the refrigerating fluid flowing inside the exchanger itself. Furthermore , if the surface of the component to be cooled is smaller than the surface of the base of the evaporator , there is an additional heat transfer between the fluid and the surface of the heat exchanger not coupled to the component to be cooled. Such a heat exchange takes place as soon as the fluid temperature is h igher than the ambient temperature in which the exchanger is immersed.

In this regard, it is worth noting that the cover and any spacer between the cover and the finned base may be made of transparent materia l (Pyrex, glass , sapph ire, polycarbonate , acryl ic or other) or plastic, meta l , ceramic, carbon fiber or glass or other

conductivity, such materia ls may either contribute or not to the transfer of heat to the outside environment.

Di fferent heat exchange methods occur during the flow of the fluid in two-phase or multi -phase condi tions (l iquid-vapor mixture and/or i ncondensable gases and/or nanoparticles and/or a combi nation thereof) :

convection ,

therma l conduction ,

phase change,

radiation .

Convection can be either forced or natura l . However, the latter mechanism has a minima l influence compared to other heat transfer methods . Convection is due both to the motion of the l iquid-vapor mixture, of the vapor or gas bubbles and to the motion of the individua l l iquid, vapor and gaseous phases or to the motion of any other phases present in the fluid (e . g. in the case of the presence of nanoparticles) . These motions generate turbulences which increase the transfer of heat in the various phases , main ly the liquid phase.

Thermal conduction is present when there are temperature differences between the various portions of fluid, mainly in the liquid phase. Such differences can be generated by the mixing due to the sudden change of direction of the fluid particles and to the turbulence and turbulence phenomena induced by the presence of the vapor phase or by the other phases ( gaseous and/or solid) .

Boi ling ( phase change) is the main mechanism of heat transfer.

Evaporation occurs both by nucleation , i . e. by the formation of vapor bubbles on the surface of the channels, and by evaporation of the liquid fi lm trapped between the vapor phase and the wal l of the channel .

The orientation of the finned plate is defined by the direction of the normal axis at the bottom of the plate. There may be various orientations with respect to the force of gravity: horizontal with fins facing upwards, vertical with fluid from bottom to top, horizontal with fins facing downwards, vertical with fluid from top to bottom.

In addition to these main orientations, other possible orientation may also be provided.

In addition to these orientations with respect to the force of gravity, there are also orientations with respect to accelerations to which the plate is subjected , e.g. when the cooling system is mounted on a moving vehicle. Such accelerations generate forces of inertia which are added to the force of gravity. The term “refrigerating fluid” is used to indicate a heat transfer fluid consisting of any fluid able to evaporate and condense under the operating conditions ( temperatures and pressures) of the cooling systems for the various applications.

A variety of cooling system fluids can be selected based on cost, desired optimization type, operating pressure, boiling point, safety regulations , target temperature range (typically 30-85 ^° C for an electronic device or even higher for military devices or other special appl ications , but also low ambient temperatures down to -273 ^° C) of the heat source, material compatibility, environmental compatibility, stability, etc.

Typical pure fluids which can be used as refrigerating fluid for the cooling system are: water, dielectric fluids, alcohols (ethanol, methanol, iso propanol, etc.), refrigerating fluids (R245fa, R136a, etc.), natural refrigerating fluids such as ammonia, carbon dioxide (C0 ₂ ) , 3M™ Novec™ (7000, 7100, 649, etc.), other fluids of 3M (e.g. FC 72) or other brands, as well as fluids such as acetone, metals in liquid form, cryogenic fluids (e.g. liquid helium), etc.

Furthermore , mixtures consisting of various fluids in addition to those mentioned above may also be used for cooling systems. Such mixtures can have relative concentrations which vary according to the specific application and the type of optimization desired (target temperatures of the component to be cooled, for example). It also means mixtures with incondensable gases ( e.g . air) and/or with nanoparticles , i.e. with the presence of solid or colloidal particles in various concentrations .

It is also worth noting that, according to the present invention, the base plate of the heat sink/evaporator is preferably made of highly thermally conductive material s , such as copper, aluminum, graphite, silver, etc. according to the application for which it is intended, but also of composite materials contai ni ng carbon fiber, ceramics, graphite, etc., as well as metal alloys. 4s mentioned, transparent materials ( Pyrex , glass, sapphire, polycarbonate , acrylic, etc.) or plastics or mixtures of these materials with metallic particles may also be used to improve thermal conductivity.

Various materials such as those described for the base plate can also be used for the cover.

The base plate of the heat sink/evaporator can also be manufactured using metal sintering techniques ( such as: copper and molybdenum , copper and graphite, aluminum and graphite, etc. and other material s ; or by chip forming techniques with CNC machines, 3D printing techniques of the additive manufacturi ng type (SLS, Selective Layer Sintering or DMLS, Direct Metal Laser Sintering) or other mechanical processes , such as for example skiving, brazing, extrusion rolling, stamping, etc.

In order to maximize the heat exchange towards the refrigerating fluid, the evaporator/liquid interface surface on which these fins are made can be treated chemi cally and/or mechanically. In the preferred embodiment described above, the two- phase fluid cooling system described above further comprises: a low flow pump, an appropriate expansion tank, an optional heat exchanger ( recuperator ) between the two-phase or multi -phase fluid leaving the evaporator and the fluid entering the evaporator itself, a condenser (1 iquid/ai r heat exchanger) with attached ventilation fans for heat dissipation , an electronic control system including sensors, actuators connected to an electronic control unit, all connected with pipes with lengths and diameters configured to achieve the results of maxi mi zi ng heat dissipation .

According to the present invention , it is also possible to provide a variant ( not shown), in which the inlet of the refrigerating fluid is:

either tangential or oblique to the upper surface or to the base of the fins,

or

from above, perpendicular to the finned base, in the center of the base or along lines or surfaces on the top surface of the heat si nk/evaporator .

KEY:

A-B peri pheral channels in relation to the finned surface, which act as inlets, outlets or conveyors according to the fluid flow direction

AL cooling fins

BA finned base heat sink-evaporator

C heat sink-evaporator cover

D optional spacer between cover and base I refrigerating fluid inlet pipe/manifold U refrigerating fluid outlet pipe/manifold