Abstract:
A cold plate system including, in one embodiment, first and second flow paths extending from a common inlet to a common outlet, wherein the first and second flow paths enable two-phase coolant flow under pressure through micro-channels for cooling heat loads on the cold plate system, first and second orifices disposed in the first flow path on an inlet side of the first flow path, and a third orifice spaced from a fourth orifice, the third and fourth orifices disposed in the second flow path on an inlet side of the second flow path, wherein the first and second orifices in the first flow path and the third and fourth orifices in the second flow path minimize a difference in mass flow rate between the first and second flow paths when the first and second flow paths are exposed to different heat loads.

Description:
BACKGROUND 
     As is known in the art, many applications involving semiconductor devices require mechanisms to dissipate heat. For example, fans can be used to force air flow for enhancing heat dissipation. Heat sinks can increase surface area to transfer heat away from devices. Known systems can also include liquid cooling by circulating a fluid to dissipate heat. 
     Micro-channel cold plates utilizing phase-change heat removal have emerged as a viable technique for coping with increased dissipation density in semiconductor devices. However, the increased pressure loss associated with micro-channels necessitates shortening of flow paths and forces flow path parallelism to achieve optimal thermal and hydraulic performance. 
     A variety of complex, active component flow-balancing devices for two-phase flows are commonly used in the HVAC&amp;R (Heating, Ventilation, Air Conditioning &amp; Refrigeration) industry. However, such mechanisms include relatively large, complicated mechanical elements (springs, diaphragms, etc.) that are not suitable for reliable integration as part of a monolithic cold plate. 
     Conventional implementations of parallel micro-channel phase-change cooling schemes for spatially varying thermal loads have design specific flow arrangements, which limit applicability and increase complexity. U.S. Pat. No. 7,218,519 to Prasher et al., which is incorporated herein by reference, discloses micro-channel cold plates having channels designed with a priori knowledge of high and low heat load locations. Thus, the cold plates disclosed by Prasher are limited to particular board layouts with integrated circuits, such as microprocessors, in given locations.  FIGS. 1A and 1B  show implementations from Prasher having channels for low and high heat areas defined by the locations of microprocessors. Thus, Prasher discloses a cold plate that is limited to one particular board layout. 
     As is further known in the art, coolant flow misdistribution in phase change cooling systems with varying heat loads on parallel flow paths is the result of uneven vapor fraction at the flow path exit. When one flow path dissipates more heat than another path, the total volumetric flow of vapor in that path is greater on average, thereby incurring a larger pressure drop. Since parallel paths must have equivalent pressure drops, the flow rates for the flow paths dissipating less heat increase, while the flow rates for paths dissipating more heat decrease, until an equivalent pressure drop across all paths is established to restore equilibrium. The consequence is that the flow paths with larger heat loads can be starved of fluid flow, degrading thermal performance. Relatively small amounts of vapor can significantly impact pressure drops. The phenomenon of pressure drop multiplication due to vapor fraction is illustrated in  FIG. 2 , from Kojasoy, G. et al., “Two-Phase Pressure Drop in Multiple Thick and Thin Orifice Plates,” Experimental and Fluid Science, 1997, 15:347-358. 
     SUMMARY 
     The present invention provides exemplary embodiments of a cold plate having parallel flow paths with respective sets of orifices spaced and sized to minimize misdistribution of coolant in the presence of varying heat loads. With this arrangement, flow paths with larger heat loads are not starved of flow as in conventional micro-channel cold plates. While exemplary embodiments of the invention are shown and described in conjunction with particular applications, geometries and configurations, it is understood that invention is applicable to cooling in general, in which it is desirable to maximize the cooling performance of cold plates. 
     In one aspect of the invention, a cold plate system comprises first and second flow paths extending from a common inlet to a common outlet, the first and second flow paths being hydraulically parallel, wherein the first and second flow paths enable two-phase coolant flow under pressure through micro-channels for cooling heat loads on the cold plate system, a first orifice spaced from a second orifice, the first and second orifices disposed in the first flow path on an inlet side of the first flow path, and a third orifice spaced from a fourth orifice, the third and fourth orifices disposed in the second flow path on an inlet side of the second flow path, wherein the first and second orifices in the first flow path and the third and fourth orifices in the second flow path minimize a difference in mass flow rate between the first and second flow paths when the first and second flow paths are exposed to different heat loads. 
     The system can further include one or more of the following features: the cold plate system is passive, for a selected temperature and pressure, the first orifice is sized to not cause flashing of the liquid coolant and the second orifice is sized to flash subcooled liquid coolant to a two-phase flow, a further orifice in the first flow path, the second orifice is located at an inlet to a micro-channel, a slot for coupling the cold plate system to circuit card, the circuit card forms a part of a radar system, and the first and second orifices are spaced to minimize the difference in mass flow rate between the first and second flow paths when the first and second flow paths are exposed to different heat loads. 
     In another aspect of the invention, a cold plate system comprises: a first flow path, a first orifice disposed in the first flow path, the first orifice having a fluid inlet and a fluid outlet into a region, a second orifice disposed at an inlet of a first micro-channel coupled to the region, a third orifice disposed at an inlet of a second micro-channel coupled to the region, wherein the first flow path and the first and second micro-channels enable two-phase coolant flow under pressure through the first and second micro-channels to minimize a difference in mass flow rate between the first and second micro-channels when the first and second flow micro-channels are exposed to different heat loads. 
     The system can further include one or more of the following features: the first and second micro-channels are sized to generate flashing bubbles to maximize cooling for a selected pressure and temperature, the first and second micro-channels are hydraulically parallel, the system is passive, and a slot for coupling the cold plate system to circuit card. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing features of this invention, as well as the invention itself, may be more fully understood from the following description of the drawings in which: 
         FIGS. 1A and 1B  show prior art micro-channel cooling plates; 
         FIG. 2  is a prior art graphical representation of quality versus pressure drop; 
         FIG. 3  is a schematic representation a cold plate having parallel flow paths with respective sets of orifices in accordance with exemplary embodiments of the invention; 
         FIG. 3A  is a schematic representation of a further embodiment of a cold plate having parallel flow paths; 
         FIG. 4  is a schematic representation of a cold plate having parallel flow paths for cooling respective heat loads; 
         FIG. 5A  is a schematic representation of a flow path having a first orifice and a second orifice in accordance with exemplary embodiments of the invention; 
         FIG. 5B  is a schematic representation of an alternative flow path having a first orifice and a second orifice in accordance with exemplary embodiments of the invention; 
         FIGS. 6A and 6B  are schematic representations of exemplary flow path illustrating orifice spacing and sizing; 
         FIG. 7  is a schematic representation of a further flow path illustrating orifice spacing and sizing with additional detail of micro-channels; 
         FIG. 7A  is a schematic representation of a further flow path showing more than two orifices; 
         FIG. 8A  is a schematic representation of a circuit card assembly coupled to a micro-channel cold plate in accordance with exemplary embodiments of the invention; 
         FIG. 8B  is a further schematic representation of a circuit card assembly coupled to a micro-channel cold plate in accordance with exemplary embodiments of the invention 
         FIG. 9A  is a pictorial representation of a circuit card assembly that can form a part of an enclosure shown in  FIG. 9B  having micro-channel cold plates to cool the circuit card assemblies in accordance with exemplary embodiments of the invention; and 
         FIG. 10  is a schematic representation of a radar system having micro-channel cold plates to cool receive and transmit components of the system in accordance with exemplary embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Before describing exemplary embodiments of the invention in detail, some information is provided. It is known that for micro-channel cold plates an increase in vapor percent in a flow path due to a higher heat load results in an increased pressure drop, which reduces the mass flow rate of the coolant. Pressure and temperature determine a change in phase from liquid to vapor, i.e., the boiling point of a liquid. A subcooled liquid, also referred to as a compressed liquid, is a liquid at a temperature lower than the saturation temperature at a given pressure. Liquid flashing occurs when a saturated liquid stream undergoes a reduction in pressure to below the vapor pressure, creating vapor without external heat addition. Flashing can occur in response to pressure drops. Two-phase cooling in cold plates refers to a coolant in liquid and gas states. 
     In general, exemplary embodiments of the invention provide a cold plate with enhanced cooling performance by minimizing the reduction of coolant flow in parallel flow paths caused by differing vapor percent in the paths due to uneven spatial and/or temporal heat loading on the cold plate. Exemplary embodiments of the invention provide micro-channel cold plates having parallel paths and spaced orifices to facilitate balanced coolant flow in the hydraulically parallel paths in the presence of varying heat loads. 
     A first orifice is sized such that, under nominal balanced flow conditions, the pressure drop incurred will not cause flashing. A second orifice is sized to flash the subcooled liquid to a two-phase flow. If the heat load on a given parallel path decreases, the resultant increase in liquid flow through that path causes flashing through the first orifice, which results in a two-phase mixture being supplied to the second orifice. This causes an increase in the pressure drop at the first orifice and a significant increase in the pressure drop at the second orifice. The increased pressure losses through the orifices offsets the reductions in downstream pressure drop due to the reduced exit quality for preventing misdistribution of flow. The presence of the flashing orifice can provide the added benefit of improved thermal performance and increased heat flux dissipation capability. 
     As used herein, parallel paths or channels means parallel from a hydraulic standpoint. This may include any combination of parallel-series construction or fractal network to achieve an optimized design for a given application geometry and heat loading conditions. As used herein, micro-channel is defined as any channel with micron-scale dimensions (i.e., &lt;1 mm hydraulic diameter) where parallel hydraulic arrangement is typically required to dissipate heat over large areas. 
     The orifices passively effect robust flow distribution to parallel micro-channel heat sinks under varying spatial and/or temporal load. Cold plates can be fabricated to allow the orifices to be integrated into a large-scale, fully scalable monolithic cold plate. It is understood that the orifices can be relatively long or short and can be fabricated from suitable materials having a relatively high hardness. The particular number, spacing, material, and geometry of the orifices can vary to meet the needs of a particular application. 
       FIG. 3  shows first, second, and third flow paths MP 1 , MP 2 , MP 3 , all of which are hydraulically parallel. Each path MP 1 , MP 2 , MP 3  has a respective pressure drop ΔP 1 , ΔP 2 , ΔP 3 , heat load Q 1 , Q 2 , Q 3 , and mass flow rate mdot 1 , mdot 2 , mdot 3 . The outlet of each path has a respective vapor quality xout 1 , xout 2 , xout 3 . 
     The first path MP 1  includes a first orifice O 1   a  and a second orifice O 1   b  spaced a given distance from the first orifice. In the illustrated embodiment, the orifices O 1   a , O 1   b  are located on an inlet side of the flow path MP 1 . The second and third paths MP 2 , MP 3  similarly have respective first orifices O 2   a , O 3   a  and second orifices O 2   b , O 3   b . The first and second orifices O 1   a , O 1   b  of the first flow path MP 1  can be considered a first set of orifices, the first and second orifices O 2   a , O 2   b  of the second flow path can be considered a second set of orifices and so on. In general, these orifices can be located close to, or far from, the heat sources of interest, as required for a specific design. 
     If the first path heat load Q 1  is greater than the second and third path heat loads Q 2 , Q 3 , slight increases in the second and third path mass flow rates mdot 2 , mdot 3 , induce flashing across the first orifices, which results in increased pressure drops ΔP 2   a,b , ΔP 3   a,b  across the first and second orifices O 2   a,b , O 3   a,b  in the second and third paths MP 2 , MP 3 . This offsets the lower pressure drop ΔP 2 , 3  in the micro-channels due to reduced heating. 
     If the first path heat load Q 1  is less than the second and third heat path loads Q 2 , Q 3 , the orifices again bias coolant flow to optimize cooling. 
     EXAMPLE 
     Tin=5° C., Pin=0.375 MPa 
     The first orifice Ola, O 2   a , O 3   a  for the paths is sized to result in a pressure drop of 0.02 MPa (ΔP 1   a , ΔP 2   a , ΔP 3   a ) at a desired flow rate mdot 1 , mdot 2 , mdot 3 . The second orifice O 1   b , O 2   b , O 3   b  is sized to result in a pressure drop of 0.01 MPa (ΔP 1   b , ΔP 2   b , ΔP 3   b ) at a desired flow rate mdot. The quality x at the inlet of the heat loads Q 1 , Q 2 , Q 3 =˜0.02. The nominal heat load Q 1 , Q 2 , Q 3  for the paths is 100 W with pressure drops ΔP 1 , ΔP 2 , ΔP 3  equal to 0.03 MPa. 
     If the heat load Q 1  in the first path MP 1  becomes 0, while the other heat loads Q 2 , Q 3  remain at 100 W, the pressure drop ΔP 1  is &lt;0.01 MPa at the desired flow rate mdot 1 . 
     With the orifices, a minimal increase in flow rate mdot 1 , e.g., &lt;&lt;20%, which is orifice design specific, causes a quality x&gt;0 due to flashing prior to the second orifice O 1   b , in turn causing ΔP 1   b  to be &gt;&gt;ΔP 2   b  and ΔP 3   b . This offsets the reduction in the first path pressure drop ΔP 1  and decreases the second and third path mass flow rates mdot 2 , mdot 3  by &lt;&lt;10% to maintain acceptable heat transfer characteristics. 
     Without orifices, assuming laminar flow, this would result in an increase of between about 20% and 40% for the first path mass flow rate mdot 1  and a decrease of between about 10% and 20% for the second and third path mass flow rates mdot 2 , mdot 3 . As is known in the art, laminar flow, which is also known as streamline flow, occurs when a fluid flows in parallel layers without disruption between the layers, i.e., laminar flow is ‘smooth.’ 
     In an alternative embodiment shown in  FIG. 3A  only the first flow path includes an orifice O 1   a ′, O 1   b ′ for a cooling application in which a first heat load Q 1 ′ is expected to be to significantly greatly than a second heat load Q 2 ′ in the second path. In this configuration, the orifices O 1   a &#39;, O 1   b ′ can prevent flow starvation in the non-orificed path and/or large pressure-drop induced thermal gradients caused by having excessive mass flow. 
       FIG. 4  shows an exemplary cold plate  200  having first, second, and third micro-channel flow paths  202   a, b, c . A liquid coolant is supplied under pressure via an inlet  204  to the flow paths  202  and a liquid/vapor mixture is returned via an outlet  206  after cooling the heat loads Q 1 , Q 2 , Q 3 . The cold plate  200  is well suited for cooling circuit card assemblies and other units that generate heat. It is understood that any practical number of inlets and outlets can be used to meet the needs of a particular application. 
       FIG. 5A  is a cutaway partial side view of an exemplary cold plate  300  including a first orifice  302  and a second orifice  304  spaced from the first orifice. The first orifice  302  is placed proximate an inlet  306  of a first flow path  308  and the second orifice  304  is placed proximate the micro-channels  310 . 
     In an alternative embodiment shown in  FIG. 5B , a cold plate  300 ′ includes a first orifice  302 ′ and a second orifice  304 ′ placed relatively close together near an inlet  306 ′. 
     It is understood that orifice spacing and structure can be varied to meet the needs of a particular application. In general, small orifice spacings (e.g., S/D 1 &lt;˜10), as shown in  FIG. 6A , can fit within compact spaces. However, increased hydrodynamic interactions between orifices may complicate behavior. Large spacings (e.g., S/D 1 &gt;˜10), as shown in  FIG. 6B , may facilitate decoupling of orifice flow characteristics to improve predictability. 
     A variety of known fabrication processes can be used to achieve desired orifice geometries, including with D 2 , L ( FIG. 6A ) and leading edge geometry to achieve desired characteristics. Illustrative processes to provide orifices in the flow paths include drilled threaded or press-fit inserts with circular holes and machined, or lithographically patterned, rectangular slots. 
       FIG. 7  shows a further embodiment of a cold plate  400  in accordance with exemplary embodiments of the invention. A first orifice  402  in the flow path provides flow into a region  404  coupled to a series of micro-channels  406   a - n , each having a respective second orifice  408   a - n  located at the inlet of each micro-channel. Flashing bubbles  420  and bubbles  422  on the channel surface can be generated in use, as shown. 
     It is understood that an inventive cold plate can have any practical number of paths to meet the needs of a particular cooling application. It is further understood that inventive cold plates can be provided for a variety of pressures, temperatures, and heat loads. In alternative embodiments, more than two orifices in series (O 1 , O 2 , . . . ON), are used in a flow path as needed to achieve the same passive flow balancing effect, as shown in the exemplary embodiment of  FIG. 7A . 
     Exemplary embodiments of the invention provide flow balancing of cooling fluid to enable the application of high performance micro-channel phase-change cooling technology to large high power electronic assemblies with spatially and temporally varying thermal loads. Micro-channel cold plates provide low thermal resistance heat rejection, especially under high heat fluxes when using two-phase coolants. Inventive embodiments of micro-channel cold plates with a passive multi-orifice flow regulation scheme can be readily integrated into large-scale monolithic cold plate assemblies with “n” parallel micro-channel flow paths. The inventive flow regulation scheme significantly increases the robustness of highly parallel micro-channel-based phase change cooling in electronics assemblies with varying heat loads, such as active electronically steered array (AESA) radars. 
       FIG. 8A  shows an exemplary circuit card assembly CCA thermally coupled to a micro-channel cold plate MCP in accordance with exemplary embodiments of the invention. The circuit card assembly CCA can slide into a card slot to provide contact with the cold plate MCP during operation. 
     It is understood that the orientation of the circuit card assembly CCA with respect to the cold plate MCP can vary to provide a desired contact surface area between the circuit card assembly and the cold plate to meet the needs of a particular application. For example, a circuit card assembly top and/or bottom surface can be placed on cold plate(s), as shown in  FIG. 8B . 
       FIGS. 9A and 9B  show a circuit card assembly CCA that is removably insertable into an enclosure ENC for holding a number of circuit card assemblies, each of which can be thermally coupled to micro-channel cold plates within the enclosure. The enclosure can form part of a phased array radar, for example. In general, inventive embodiments of a micro-channel cold plate can form a part of a rack for a computer, storage, and the like, in which it is desirable to cool a circuit card assembly. 
       FIG. 10  shows an exemplary radar system having transmit and receive arrays. The transmit system includes a driver, beamformer, power amplifier module, and transmit array. The receive system includes a signal processor, a digital receiver, a beamformer, low noise amplifiers, and receive array. Each of the transmit and receive system components can include one or more micro-channel cold plates in accordance with the present invention. 
     Having described exemplary embodiments of the invention, it will now become apparent to one of ordinary skill in the art that other embodiments incorporating their concepts may also be used. The embodiments contained herein should not be limited to disclosed embodiments but rather should be limited only by the spirit and scope of the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.