COOLING APPARATUSES AND POWER ELECTRONICS MODULES COMPRISING THE SAME

Abstract

Cooling apparatuses and power electronics modules with cooling apparatuses are disclosed. In one embodiment, a cooling apparatus includes a heat transfer plate having a heat output surface and a periodic fractal pattern formed in the heat output surface. The periodic fractal pattern increases the surface area of the heat output surface and provides vapor bubble nucleation sites. An enclosure encloses the heat transfer plate and forms a fluid chamber between the enclosure and the heat transfer plate. A fluid source is fluidly coupled to the fluid chamber and provides cooling fluid to the fluid chamber. When the heat transfer plate is thermally coupled to the heat source, the heat source heats the transfer plate which vaporizes the cooling fluid in the fluid chamber thereby dissipating the heat of the heat source.

Description

TECHNICAL FIELD

The present specification generally relates to apparatuses for cooling heat generating devices and, more specifically, to cooling apparatuses and power electronics modules utilizing a heat transfer plate with a periodic fractal pattern on a heat output surface.

BACKGROUND

Power electronics are commonly utilized in a variety of commercial and industrial applications. In general, the amount of heat generated by a power electronics device increases with increased power output of the device. However, such power electronics may cease to function and/or malfunction when the devices overheat. Accordingly, the power output of such devices may be limited by temperature considerations.

Heat sinks are commonly used in conjunction with heat generating devices, such as power electronics devices, to dissipate heat emitted by the device. The heat sinks are thermally coupled to the heat generating devices such that heat from the device is conveyed to the heat sink and dissipated by various mechanisms. In some heat sinks a flow of cooling fluid may be used to receive heat generated by the heat generating device by convective thermal transfer. The flow of cooling fluid carries the heat away from the heat generating device. For example, the heat sink may utilize a jet of cooling fluid which is directed such that it impinges on a surface of the heat generating device. Another method for removing heat from a heat generating device is to couple the device to a finned heat sink made of a thermally conductive material, such as aluminum. Heat produced in the heat generating device is convectively transferred to the heat generating device which, in turn, radiates the heat away from the heat generating device.

However, as the power output of power electronics devices increases to meet the demands of newly developed electronic systems, conventional heat sinks may be unable to adequately remove this increased heat flux to effectively lower the temperature of the power electronics devices to acceptable temperature levels.

Accordingly, a need exists for alternative apparatuses for cooling heat generating devices, particularly power electronics devices.

SUMMARY

In one embodiment, a cooling apparatus for a heat source may include a heat transfer plate comprising a heat output surface and a periodic fractal pattern formed in the heat output surface. The periodic fractal pattern increases the surface area of the heat output surface and provides vapor bubble nucleation sites. An enclosure may enclose at least the heat output surface of the heat transfer plate such that the enclosure forms a fluid chamber between the enclosure and the heat output surface of the heat transfer plate. A fluid source may be fluidly coupled to the fluid chamber. The fluid source may provide cooling fluid to the fluid chamber. When the heat transfer plate is thermally coupled to the heat source, the heat source heats the transfer plate which vaporizes the cooling fluid in the fluid chamber thereby dissipating the heat of the heat source.

In another embodiment, a power electronics module may include a heat transfer plate having a heat input surface, a heat output surface, and a periodic fractal pattern formed in the heat output surface. The periodic fractal pattern increases the surface area of the heat output surface and provides vapor bubble nucleation sites. A power electronics device may be thermally coupled to the heat input surface of the heat transfer plate. An enclosure encloses at least the heat output surface of the heat transfer plate such that the enclosure forms a fluid chamber between the enclosure and the heat output surface of the heat transfer plate. A vapor condenser may be coupled to the fluid chamber. A fluid source may be fluidly coupled to the fluid chamber and provide cooling fluid to the fluid chamber. The power electronics device heats the heat transfer plate which vaporizes the cooling fluid in the fluid chamber thereby dissipating the heat of the power electronics device and the vapor condenser condenses cooling fluid vapor in the fluid chamber and returns the cooling fluid to the fluid source.

In yet another embodiment, a cooling apparatus for a power electronics module includes a heat transfer plate comprising a periodic fractal pattern comprising a plurality of fractal units, each fractal unit having a maximum depth from about 250 nm to about 500 nm. The periodic fractal pattern increases a surface area of the heat transfer plate and provides vapor bubble nucleation sites. An enclosure encloses at least the heat output surface of the heat transfer plate and forms a fluid chamber between the enclosure and the heat output surface of the heat transfer plate. A vapor condenser may be coupled to the fluid chamber. A fluid source may be fluidly coupled to the fluid chamber such that the fluid source provides cooling fluid to the fluid chamber. The power electronics device heats the heat transfer plate which vaporizes the cooling fluid in the fluid chamber thereby dissipating the heat of the power electronics device and the vapor condenser condenses cooling fluid vapor in the fluid chamber and returns the cooling fluid to the fluid source.

These and additional features provided by the embodiments described herein will be more fully understood in view of the following detailed description, in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments set forth in the drawings are illustrative and exemplary in nature and not intended to limit the subject matter defined by the claims. The following detailed description of the illustrative embodiments can be understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:

FIG. 1 schematically depicts a cross section of a cooling apparatus thermally coupled to a heat generating device in a power electronics module, according to one or more embodiments shown and described herein;

FIG. 2 schematically depicts a heat transfer plate of the cooling apparatus of FIG. 1 submerged in a pool of cooling fluid;

FIG. 3A schematically depicts a periodic fractal pattern formed in the heat output surface of a heat transfer plate, according to one or more embodiments shown and described herein;

FIG. 3B schematically depicts a fractal unit of the periodic fractal pattern of FIG. 3A;

FIG. 4A schematically depicts another embodiment of a periodic fractal pattern formed in the heat output surface of the heat transfer plate;

FIG. 4B schematically depicts a fractal unit of the periodic fractal pattern of FIG. 4A;

FIG. 5 schematically depicts a cross section of a cooling apparatus thermally coupled to a heat generating device in a power electronics module, according to one or more embodiments shown and described herein;

FIG. 6 graphically depicts the boiling curve for a saturated liquid (i.e., a cooling fluid) in terms of the heat flux from a substrate (y-axis) and the change in temperature of the substrate (x-axis); and

FIG. 7 graphically depicts theoretical pool boiling curves for heat transfer plates with different surface finishes.

DETAILED DESCRIPTION

FIG. 1 generally depicts one embodiment of a cooling apparatus for use with a power electronics module. The cooling apparatus may generally comprise a heat transfer plate having a periodic fractal pattern formed in a heat output surface, an enclosure, and a fluid source. The enclosure encloses at least the heat output surface of the heat transfer plate and forms a fluid chamber between the enclosure and the heat output surface. The fluid source supplies a flow of cooling fluid to the fluid chamber such that the cooling fluid cools the heat output surface. In some embodiments, the cooling apparatus may additionally include a vapor condenser coupled to the fluid chamber. Various embodiments of the cooling apparatus and the operation of the cooling apparatus will be described in more detail herein.

Referring to FIG. 1, one embodiment of a cooling apparatus 100 is schematically depicted. The cooling apparatus 100 generally comprises a heat transfer plate 102, an enclosure 104 and a fluid source 108. In the embodiments described herein, the cooling apparatus 100 may also comprise a vapor condenser 106. The heat transfer plate 102 is formed from a thermally conductive material which facilitates the conduction of thermal energy through the heat transfer plate from a heat input surface 132 to a heat output surface 130. In the embodiment shown in FIG. 1, the heat input surface 132 and the heat output surface 130 are generally parallel with one another. However, it should be understood that, in other embodiments, the heat input surface 132 and the heat output surface 130 may have other relative configurations. Suitable materials from which the heat transfer plate 102 may be formed include, without limitation, copper, aluminum, thermally enhanced composite materials, and polymer composite materials. The heat transfer plate 102 may be formed by a molding process, a machining process or similar processes to achieve the desired shape and configuration.

Referring now to FIGS. 2 and 3A-3B, the heat output surface 130 of the heat transfer plate 102 is formed with a periodic fractal pattern. One exemplary embodiment of a portion of a heat output surface 130a with an exemplary periodic fractal pattern 134a is schematically depicted in FIG. 3A. The periodic fractal pattern 134a is formed from a plurality of fractal units 136a (FIG. 3B) which are repeated on the heat output surface 130a.

In the embodiments described herein, each fractal unit 136a generally has a width from about 100 nm to about 500 nm and a length from about 100 nm to about 500 nm. Further, each fractal unit generally comprises a plurality of fractal sub-units 137a (one shown as shaded in FIG. 3B) which are interconnected to form the fractal unit 136a. Accordingly, it should be understood that each fractal unit 136a is formed from a plurality of interconnected fractal sub-units 137a to form a continuous fractal unit 136a. Moreover, each fractal unit 136a is repeated on the heat output surface 130a to form the periodic fractal pattern 134a. In some embodiments (not shown) the periodic fractal pattern 134a is formed by interconnecting individual fractal units 136a while, in other embodiments, the periodic fractal pattern is formed by arranging discrete fractal units in a regular pattern as illustrated in FIG. 3A.

As shown in FIGS. 3A and 3B, the periodic fractal pattern 134a consists of individual fractal units 136a which resemble snowflakes or ice crystals. FIGS. 4A and 4B depict another exemplary embodiment of a periodic fractal pattern 134b formed in a heat output surface 130b. In this embodiment, the periodic fractal pattern 134b is also formed from a plurality of fractal units 136b which resemble snowflakes or ice crystals, as described above. However, while FIGS. 3A-3B and 4A-4b depict fractal units that resemble snowflakes or ice crystals, it should be understood that other types of fractal units may also be used.

Referring again to FIGS. 3A and 3B, the periodic fractal patterns 134a described herein may be formed in the heat output surface 130a to a depth up to about 500 nm. For example, in some embodiments, the depth of the periodic fractal pattern 134a may be from about 250 nm to about 500 nm. The periodic fractal pattern 134a may be formed in the heat output surface 130a by laser ablation, laser etching, ion milling, photo-etching, photolithography or any other technique suitable for forming nano-scale structures in the surface of a component. For example, in one embodiment, the periodic fractal pattern 134a is formed using a step and flash imprint lithography technique.

In one embodiment, forming the periodic fractal pattern in the surface of the heat output surface entails removing material from within the periodic fractal pattern thereby creating a pocket or depression in the heat output surface. For example, referring to FIG. 3B, the material within each fractal sub-unit 137a may be removed from the heat output surface 130a such that the periodic fractal pattern is embedded in the heat output surface 130a.

Alternatively, the periodic fractal pattern formed in the heat transfer surface may be formed by removing material around each fractal unit such that the periodic fractal pattern projects from the heat output surface. Accordingly, it should be understood that the phrase “a periodic fractal pattern formed in the heat transfer surface” describes both periodic fractal patterns embedded in the heat surface and periodic fractal patterns projecting from the heat transfer surface.

Moreover, it should be understood that the periodic fractal pattern may have variations in height (i.e., surface topography) within each individual fractal unit. For example, discrete portions of each periodic fractal pattern may be at a first height while other portions may be at a second height.

While not wishing to be bound by theory, it is believed that forming the heat output surface 130a with periodic fractal patterns as described herein facilitates improved heat transfer between the heat transfer plate 102 and cooling fluid which contacts the heat transfer plate 102. For example, the periodic fractal pattern generally increases the surface area density of the heat output surface of heat transfer plates such that the interface area between the cooling fluid and the heat output surface is greater thereby improving the heat transfer between the cooling fluid and the heat output surface.

Further, it is also believed that the periodic fractal pattern increases the density of vapor bubble nucleation sites on the heat output surface and, as such, increases the cooling capability of the heat transfer plate. In general, it should be understood that the cooling capacity of the heat transfer plate and the cooling apparatus incorporating the heat transfer plate generally increase with an increase in the density of vapor bubble nucleation sites. The periodic fractal pattern may also improve the wettability of the heat output surface.

Moreover, the periodic fractal pattern may enhance boiling of the cooling fluid on the heat output surface of the heat transfer plate thereby improving the cooling efficiency of the heat transfer plate and the cooling apparatus. More particularly, the periodic fractal pattern promotes the formation of vapor bubbles as a result of both the increased surface area density and vapor bubble nucleation site density. As the vapor bubbles are released from the heat output surface and heat energy is carried away from the heat transfer plate, the area vacated by the bubble is filled with cooling fluid having a lower temperature, thereby drawing more heat energy from the heat transfer plate. Combined with the increased surface area density and increased number of vapor bubble nucleation sites, this effect may significantly improve the cooling efficiency of the heat transfer plate and an associated cooling apparatus. The aforementioned effect may be further enhanced when the heat transfer plate is used in conjunction with a flowing cooling fluid stream (as opposed to a stationary pool of cooling fluid) as the cooling fluid moving across the heat output surface cools the surface by convection and delays the onset of nucleate boiling, as will be described further herein.

Referring again to the cooling apparatus 100 depicted in FIG. 1, the heat transfer plate 102 may be enclosed by the enclosure 104. The enclosure 104 is generally formed from a material capable of withstanding high temperatures (i.e., temperatures greater than about 60° C.). Suitable materials from which the enclosure 104 may be formed include, without limitation, ceramics, metals, polymer materials suitable for high temperature applications, and/or composite materials suitable for high temperature applications.

The enclosure 104 is positioned around the heat transfer plate 102 to form a fluid chamber 122 between the enclosure 104 and the heat transfer plate 102. In one embodiment, at least the heat output surface 130 of the heat transfer plate 102 is enclosed by the enclosure 104, as depicted in FIG. 1. In this embodiment, the fluid chamber 122 is formed between the enclosure 104 and the heat output surface 130 of the heat transfer plate 102 which enables cooling fluid 110 to directly contact the heat output surface 130 of the heat transfer plate 102.

In the embodiments described herein, the cooling fluid source 108 is fluidly coupled to the fluid chamber 122 and is configured to deliver cooling fluid 110 to the fluid chamber 122 to facilitate cooling of the heat transfer plate 102. The cooling fluid source 108 may be any suitable source for supplying cooling fluid 110 to the fluid chamber 122. For example, in embodiments where the cooling apparatus 100 is utilized in a vehicle, the cooling fluid source 108 may be the radiator system of the vehicle and the cooling fluid 110 may be water, radiator fluid, or any other suitable cooling fluid. In other embodiments, the cooling fluid source 108 may be a stand-alone system which supplies cooling fluid 110 directly to the fluid chamber 122, as depicted in FIG. 1. In this embodiment, the cooling fluid source 108 is coupled to the fluid chamber 122 with a fluid inlet conduit 118 which extends through the enclosure 104. The fluid inlet conduit 118 is positioned such that relatively low temperature cooling fluid 110 enters the fluid chamber 122 and flows over the heat output surface 130 of the heat transfer plate 102, thereby facilitating the transfer of heat from the relatively high temperature heat transfer plate 102 to the relatively low temperature cooling fluid 110. In the embodiment of the cooling apparatus 100 depicted in FIG. 1, the cooling fluid source 108 is configured to supply cooling fluid 110 to the heat output surface 130 such that the cooling fluid 110 forms a pool 112 on the heat output surface 130 of the heat transfer plate 102.

In the embodiments of the cooling apparatus 100 described herein, the cooling apparatus 100 further comprises a vapor condenser 106. The vapor condenser 106 receives cooling fluid vapor 114 from the fluid chamber 122, condenses the cooling fluid vapor 114, and returns the condensed cooling fluid to the cooling fluid source 108. Accordingly, it should be understood that the vapor condenser 106 is coupled to both the fluid chamber 122 and the cooling fluid source 108. In the embodiment shown in FIG. 1, the vapor condenser 106 is positioned within the enclosure 104 in the fluid chamber 122. The vapor condenser 106 is coupled to the cooling fluid source 108 with a fluid outlet conduit 120 such that condensed cooling fluid 110 flows from the vapor condenser 106 to the cooling fluid source 108 through the fluid outlet conduit 120.

Still referring to FIG. 1, in the embodiments described herein, the cooling apparatus 100 is incorporated in a power electronics module 300 to facilitate cooling the power electronics module 300. For example, the power electronics module 300 may include at least one heat source 200. In this embodiment, the heat source 200 is at least one power electronics device 202. The power electronics device may be one or more semiconductor devices that may include, without limitation, IGBTs, RC-IGBTs, MOSFETs, power MOSFETs, diodes, transistors, and/or combinations thereof (e.g., power cards). As an example, the power electronics device or devices 202 may be used in an electrical system of a vehicle, such as in hybrid-electric or electric vehicles (e.g., as an inverter system). Such power electronics devices may generate significant heat flux when supplying the propulsion power to the vehicle. However, it should be understood that the embodiments of the cooling apparatuses and power electronics modules described herein may also be utilized in other applications and are not limited to vehicular applications.

The power electronics device 202 is thermally coupled to the heat input surface 132 of the heat transfer plate 102 to facilitate the transfer of heat from the power electronics device 202 to the cooling apparatus 100. In one embodiment (not shown), the power electronics device 202 is directly coupled to the heat input surface 132 of the heat transfer plate 102, such as when the power electronics device 202 is adhesively bonded directly to the heat transfer plate 102, mechanically coupled to the heat transfer plate such with mechanical fasteners, or even proximity coupled (i.e., by surface to surface contact) with the heat transfer plate 102.

However, in alternative embodiments, a thermally conductive substrate layer 204 is positioned between the power electronics device 202 and the heat transfer plate 102, as depicted in FIG. 1. The thermally conductive substrate layer 204 assists in transferring the heat generated by the power electronics device 202 to the cooling apparatus 100. Accordingly, it should be understood that the thermally conductive substrate layer 204 is formed from thermally conductive materials. For example, the thermally conductive substrate layer 204 may comprise a direct bonded aluminum substrate, a direct bonded copper substrate, or another similar thermally conductive substrate layer which is directly bonded to the heat transfer plate 102.

The power electronics device 202 may be coupled to the thermally conductive substrate layer 204 using any appropriate coupling method. For example, in one embodiment, a layer of bonding material (not shown) may be used to couple the power electronics device 202 to the thermally conductive substrate layer 204 and the cooling apparatus 100. By way of example and not limitation, the bond layer may comprise a solder layer, a nano-silver sinter layer, or a transient-liquid-phase bonding layer. Alternatively, the thermally conductive substrate layer 204 may be mechanically coupled to both the heat transfer plate 102 and the power electronics device 202 with mechanical fasteners such as screws, bolts, clips or the like.

While the cooling apparatus 100 is shown and described herein as being incorporated in a power electronics module 300, it should be understood that the cooling apparatus 100 may be used in other applications. By way of example and not limitation, the cooling apparatus 100 may be incorporated in laser devices or in any other device in which the dissipation of thermal energy is needed. The cooling apparatuses described herein are particularly well suited for cooling devices which generate a heat density of 100 W/cm² or more although they are equally suitable for cooling devices with lower heat densities.

The operation of the cooling apparatus 100 will now be described in detail with specific reference to the cooling apparatus 100 incorporated in a power electronics module 300 depicted in FIG. 1 and the cross section of the heat transfer plate 102 depicted in FIG. 2.

Cooling fluid 110 is supplied to the fluid chamber 122 of the cooling apparatus 100 through the inlet conduit 118. In this embodiment, the cooling fluid 110 is supplied to the fluid chamber 122 at a rate which is sufficient to create a pool 112 of cooling fluid 110 on the heat output surface 130 of the heat transfer plate 102. Accordingly, it should be understood that the cooling fluid 110 is supplied at a rate which is greater than the rate at which the cooling fluid 110 is vaporized on the heat output surface 130 of the heat transfer plate 102 such that the pool 112 of cooling fluid 110 is created and maintained on the heat transfer plate 102.

As noted hereinabove, the periodic fractal pattern formed in the heat output surface 130 of the heat transfer plate 102 increases the wettability of the heat output surface 130 with respect to the cooling fluid 110 thereby providing a uniform wetting of the heat output surface 130 by cooling fluid 110. The periodic fractal pattern increases the surface area density of the heat output surface 130 thereby providing more surface area on which cooling fluid vapor bubbles may nucleate. Moreover, the periodic fractal pattern also increases the density of vapor bubble nucleation sites on the heat output surface 130 thereby providing a greater number of vapor bubble nucleation sites per unit area of the heat transfer surface. The periodic fractal pattern also disrupts the liquid to vapor boundary thereby further lowering the nucleation energy required for bubble formation.

As the temperature of the power electronics device 202 increases, thermal energy 206 (i.e., heat) generated in the power electronics device 202 flows from the power electronics device 202, through the thermally conductive substrate layer 204 and into the heat input surface 132 of the heat transfer plate 102, thereby heating the heat transfer plate. As the temperature of the heat transfer plate 102 increases, thermal energy 206 is radiated from the heat output surface 130 and into the cooling fluid 110, thereby heating the cooling fluid 110. When sufficient thermal energy is radiated from the heat output surface 130, bubbles 116 of cooling fluid vapor 114 nucleate and grow until the bubble 116 rises to the surface of the pool 112 and the cooling fluid vapor 114 is released into the fluid chamber 122, carrying with it the thermal energy imparted to the cooling apparatus 100 by the power electronics device 202.

The cooling fluid vapor 114 rises upward, into the vapor condenser 106 where the cooling fluid vapor 114 is condensed back into liquid phase cooling fluid 110 which flows from the condenser unit into the cooling fluid source 108 through the outlet conduit 120. Thereafter, the cooling fluid 110 is re-circulated into the fluid chamber 122 where the process is repeated, such that the thermal energy 206 emitted by the power electronics device 202 is continuously removed from the power electronics device and the temperature of the power electronics module 300 is reduced.

While the embodiment of the cooling apparatus 100 and power electronics module 300 depicted in FIG. 1 transfer heat away from the power electronics device 202 by boiling a pool of cooling fluid 110 that collects on the heat output surface 130 of the heat transfer plate 102, it should be understood that other configurations are possible. For example, in an alternative embodiment, the cooling apparatus may utilize jets of cooling fluid to remove heat from the heat transfer plate, as in the embodiment of the cooling apparatus 150 depicted in FIG. 5.

Referring to FIG. 5, an alternative embodiment of the cooling apparatus 150 is schematically depicted coupled to a heat source 200, specifically a power electronics device 202, in a power electronics module 300. In this embodiment, the cooling apparatus 150 comprises a heat transfer plate 102, an enclosure 104, a cooling fluid source 108 and a vapor condenser 106, as described hereinabove with respect to FIG. 1. Also, the heat output surface 130 of the heat transfer plate 102 is formed with a periodic fractal pattern to improve heat transfer from the heat output surface 130 to the cooling fluid 110, as described above.

However, in this embodiment, the cooling fluid 110 is supplied to the fluid chamber 122 as at least one cooling fluid stream 164 emitted from a cooling fluid jet 162 and impinged against the heat output surface 130 of the heat transfer plate 102. Specifically, in this embodiment, the cooling fluid 110 is pumped from the cooling fluid source 108 into a fluid manifold 160 with a pump (not shown). The manifold 160 comprises at least one jet 162 (a plurality of jets 162 are schematically depicted in FIG. 5) positioned in the fluid chamber 122 of the cooling apparatus 150. In the embodiment shown in FIG. 5, the at least one jet 162 extends from the manifold 160, through the enclosure 104 and the vapor condenser 106. The at least one jet 162 is oriented to direct a pressurized cooling fluid stream 164 onto the heat output surface 130 of the heat transfer plate 102 such that the heat transferred from the power electronics device 202 to the heat transfer plate 102 can be dissipated by vaporization of the cooling fluid. In one embodiment, the at least one jet 162 is positioned to direct the cooling fluid stream 164 onto the center of the periodic fractal pattern, such as when the periodic fractal pattern is centered on the heat output surface 130 of the heat transfer plate 102.

In operation, cooling fluid 110 from the cooling fluid source 108 is pumped into the manifold 160 and into the jets 162 from the cooling fluid source 108. The jets 162 pressurize the cooling fluid 110 and emit a pressurized cooling fluid stream 164 which is directed onto the heat output surface 130 of the heat transfer plate 102. As noted hereinabove, the periodic fractal pattern formed in the heat output surface 130 of the heat transfer plate 102 improves the ability of the heat transfer plate 102 to transfer heat energy to the cooling fluid 110.

Specifically, as the temperature of the power electronics device 202 increases, thermal energy 206 generated in the power electronics device 202 flows from the power electronics device 202, through the thermally conductive substrate layer 204 and into the heat input surface 132 of the heat transfer plate 102, thereby heating the heat transfer plate. As the temperature of the heat transfer plate 102 increases, thermal energy is radiated from the heat output surface 130. Simultaneously, the pressurized cooling fluid stream 164 from the jets 162 are impinged against the heat output surface 130. The cooling fluid wets the heat input surface 132 assisted by the periodic fractal pattern, as described above. As the cooling fluid begins to vaporize, bubbles of cooling fluid nucleate on the heat output surface 130 and grow (i.e., boil) until sufficient heat energy is available to release the cooling fluid vapor 114 into the fluid chamber 122, carrying with it the thermal energy imparted to the cooling apparatus 150 by the power electronics device 202.

The cooling fluid vapor 114 rises upward, into the vapor condenser 106 where the cooling fluid vapor 114 is condensed back into liquid phase cooling fluid 110 which flows from the condenser unit into the fluid supply source through the outlet conduit 120. Thereafter, the cooling fluid 110 is re-circulated into the fluid chamber 122 where the process is repeated, such that the thermal energy 206 emitted by the power electronics device 202 is continuously removed from the power electronics device and the temperature of the power electronics module 300 is reduced.

Referring now to FIG. 6, a typical boiling curve 400 for a saturated fluid, such as a cooling fluid, is graphically depicted as a function of the change in temperature of the surface of the heat output surface (ΔT_sub). As shown in FIG. 6, the heat flux from the heat output surface rapidly increases at the onset 402 of nucleate boiling until a maximum heat flux 404 is reached prior to entering a transition boiling stage. In the transition boiling stage, the boiling of the cooling fluid is unstable and may rapidly progress until the all fluid is boiled from the surface thereby terminating cooling. Accordingly, while maximizing the heat flux from the heat output surface is desirable, it is tempered by the risk of unstable boiling and loss of cooling capacity and, in the worst scenario, overheating.

The cooling apparatuses described herein may assist in improving the cooling capacity of a heat transfer substrate without the necessity of reaching the maximum heat flux and the corresponding risk of unstable boiling. While not being bound by theory, it is believed that the periodic fractal pattern provides the heat output surface of the heat transfer plate with an increased surface area density and increased vapor bubble nucleation site density, as described above. Both of these factors may improve the exchange of heat between the heat output surface and the cooling fluid. Moreover, the use of a cooling fluid stream impinged against the heat output surface provides for convective cooling of the heat output surface which, in turn, delays the onset of nucleate boiling while still providing for two-phase heat transfer (i.e., from liquid phase to vapor phase). In particular, the cooling fluid streams impinged against the heat output surface convectively cool the heat output surface while, at the same time, cooling vapor bubbles are nucleated on the heat output surface thereby cooling the heat output surface by two-phase heat transfer. These two processes used in conjunction delay the onset of nucleate boiling while increasing the heat flux from the surface relative to single phase forced convection as shown in FIG. 6.

Referring to FIG. 7, theoretical boiling curves for heat transfer plates with a periodic, nano-scale fractal pattern (A), a randomly roughened surface (B), and a polished surface (C) are graphically depicted. While not wishing to be bound by theory, it is believed that the heat transfer plates with the nano-scale fractal patterns exhibit superior heat transfer characteristics than either a heat transfer plate with a randomly roughened surface or a heat transfer surface with a polished surface. Specifically, higher heat fluxes are obtained at lower wall superheats (i.e., the temperature of the heat output surface (T_S) minus the fluid saturation temperature (T_Sat)) with a heat transfer surface having the periodic nano-scale fractal patterned surface compared to either the roughened or polished heat output surfaces. This means that more power may be managed with the periodic nano-scale fractal patterned surface than with either the randomly roughened surface or the polished surface for an equivalent temperature gradient across a heated package.

It should now be understood that the cooling apparatuses described herein may provide for improved cooling of heat generating devices, such as power electronics modules and the like. In particular, the heat transfer plates comprising periodic fractal patterns as described herein may be used to increase the surface area density and the vapor bubble nucleation site density of the heat transfer plate thereby increasing the cooling capacity of the heat transfer plate and the cooling apparatus in which the heat transfer plate is utilized.

Moreover, it should be understood that the heat transfer plates comprising periodic fractal patterns may be utilized in pool-boiling configurations (e.g., as shown in FIG. 1) or in fluid jet configurations (e.g., as shown in FIG. 5). In either configuration it is believed that the cooling apparatuses described herein offer improved cooling capability over conventional fluid cooling devices.

It is noted that the terms “substantially” and “about” may be utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. These terms are also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.

While particular embodiments have been illustrated and described herein, it should be understood that various other changes and modifications may be made without departing from the spirit and scope of the claimed subject matter. Moreover, although various aspects of the claimed subject matter have been described herein, such aspects need not be utilized in combination. It is therefore intended that the appended claims cover all such changes and modifications that are within the scope of the claimed subject matter.

Claims

1. A cooling apparatus for a heat source, the cooling apparatus comprising:

a heat transfer plate comprising a heat output surface and a periodic fractal pattern formed in the heat output surface, the periodic fractal pattern increasing a surface area density of the heat output surface and providing vapor bubble nucleation sites;

an enclosure enclosing at least the heat output surface of the heat transfer plate, the enclosure forming a fluid chamber between the enclosure and the heat output surface of the heat transfer plate; and

a fluid source fluidly coupled to the fluid chamber, the fluid source providing cooling fluid to the fluid chamber, wherein, when the heat transfer plate is thermally coupled to the heat source, the heat source heats the transfer plate which vaporizes the cooling fluid in the fluid chamber thereby dissipating the heat of the heat source.

2. The cooling apparatus of claim 1, wherein the periodic fractal pattern comprises a plurality of fractal units, each fractal unit having a length from about 100 nm to about 500 nm and a width from about 100 nm to about 500 nm.

3. The cooling apparatus of claim 1, wherein the periodic fractal pattern has a depth less than or equal to about 500 nm.

4. The cooling apparatus of claim 2, wherein each of the plurality of fractal units comprises a plurality of fractal sub-units.

5. The cooling apparatus of claim 4, wherein the plurality of fractal units are interconnected.

6. The cooling apparatus of claim 4, wherein the plurality of fractal units are not interconnected.

7. The cooling apparatus of claim 1, further comprising a vapor condenser coupled to the fluid chamber and the fluid source, the vapor condenser condensing cooling fluid vapor in the fluid chamber and returning the cooling fluid to the fluid source.

8. The cooling apparatus of claim 1, further comprising a fluid manifold coupled to the fluid source, the fluid manifold comprising at least one fluid jet positioned to direct the cooling fluid into the fluid chamber, wherein the at least one fluid jet emits a cooling fluid stream onto the periodic fractal pattern of the heat transfer plate.

9. The cooling apparatus of claim 8, wherein the at least one fluid jet is a single fluid jet and the cooling fluid stream from the single fluid jet impacts the heat output surface at a center of the periodic fractal pattern.

10. The cooling apparatus of claim 1, wherein the cooling fluid is pooled in the fluid chamber on the heat output surface of the heat transfer plate.

11. The cooling apparatus of claim 1, wherein the heat source is a power electronics module coupled to a heat input surface of the heat transfer plate.

12. A power electronics module comprising:

a heat transfer plate comprising a heat input surface, a heat output surface, and a periodic fractal pattern formed in the heat output surface, the periodic fractal pattern increasing a surface area density of the heat output surface and providing vapor bubble nucleation sites;

a power electronics device thermally coupled to the heat input surface of the heat transfer plate;

an enclosure enclosing at least the heat output surface of the heat transfer plate, the enclosure forming a fluid chamber between the enclosure and the heat output surface of the heat transfer plate;

a vapor condenser coupled to the fluid chamber; and

a fluid source fluidly coupled to the fluid chamber, the fluid source providing cooling fluid to the fluid chamber, wherein, the power electronics device heats the heat transfer plate which vaporizes the cooling fluid in the fluid chamber thereby dissipating the heat of the power electronics device and the vapor condenser condenses cooling fluid vapor in the fluid chamber and returns the cooling fluid to the fluid source.

13. The power electronics module of claim 12, wherein the periodic fractal pattern comprises a plurality of fractal units, each fractal unit having a length from about 100 nm to about 500 nm, a width from about 100 nm to about 500 nm and a depth from about 250 nm to about 500 nm.

14. The power electronics module of claim 13, wherein the plurality of fractal units are interconnected.

15. The power electronics module of claim 13, wherein the plurality of fractal units are not interconnected.

16. The power electronics module of claim 12, further comprising a fluid manifold coupled to the fluid source, the fluid manifold comprising at least one fluid jet disposed in the fluid chamber, wherein the at least one fluid jet emits a cooling fluid stream onto the periodic fractal pattern of the heat transfer plate.

17. The power electronics module of claim 12, wherein the at least one fluid jet is a single fluid jet and the cooling fluid stream from the single fluid jet impacts the heat output surface at a center of the periodic fractal pattern.

18. The power electronics module of claim 12, wherein the cooling fluid is pooled in the fluid chamber on the heat output surface of the heat transfer plate.

19. A cooling apparatus for a power electronics module, the cooling apparatus comprising:

a heat transfer plate comprising a periodic fractal pattern, the periodic fractal pattern comprising a plurality of fractal units, each fractal unit having a depth less than or equal to about 500 nm, the periodic fractal pattern increasing a surface area of the heat transfer plate and providing vapor bubble nucleation sites;

an enclosure enclosing at least a heat output surface of the heat transfer plate, the enclosure forming a fluid chamber between the enclosure and the heat output surface of the heat transfer plate;

a vapor condenser coupled to the fluid chamber; and

a fluid source fluidly coupled to the fluid chamber, the fluid source providing cooling fluid to the fluid chamber, wherein a power electronics device of the power electronics module thermally coupled to the heat transfer plate heats the heat transfer plate which vaporizes the cooling fluid in the fluid chamber thereby dissipating the heat of the power electronics device and the vapor condenser condenses cooling fluid vapor in the fluid chamber and returns the cooling fluid to the fluid source.

20. The cooling apparatus of claim 19, wherein each fractal unit of the plurality of fractal units has a length from about 100 nm to about 500 nm and a width from about 100 nm to about 500 nm.