Patent Publication Number: US-9903664-B2

Title: Jet impingement cooling apparatuses having non-uniform jet orifice sizes

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of and claims priority to U.S. patent application Ser. No. 13/847,186, filed on Mar. 19, 2013, entitled “Jet Impingement Cooling Apparatuses Having Non-Uniform Jet Orifice Sizes,” and issued as U.S. Pat. No. 8,981,556, which is incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The present specification generally relates to cooling apparatuses for cooling heat generating devices and, more particularly, to jet impingement cooling apparatuses having non-uniform jet orifice sizes. 
     BACKGROUND 
     Heat generating devices, such as power semiconductor devices, may be coupled to a heat spreader to remove heat and lower the maximum operating temperature of the heat generating device. In some applications, cooling fluid may be used to receive heat generated by the heat generating device by convective thermal transfer, and remove such heat from the heat generating device. For example, jet impingement may be used to cool a heat generating device by directing impingement jets of coolant fluid onto the heat generating device or a target surface that is thermally coupled to the heat generating device. Additionally, jet impingement may also be combined with two-phase cooling, where the heat generating device is cooled by the phase change of the coolant fluid from a liquid to a vapor. 
     However, impingement jets that are positioned in close proximity to a fluid outlet may experience lower pressure within the impingement chamber, and be diverted toward the fluid outlet by a suction force and not impinge the target surface. The thermal properties of the cooling apparatus may be inhibited when the impingement jets do not impinge the target surface. 
     Accordingly, a need exists for alternative jet impingement cooling apparatuses wherein each impingement jet impinges a target surface. 
     SUMMARY 
     In one embodiment, a cooling apparatus includes at least one fluid inlet channel, at least one fluid outlet channel, a target surface, and a jet orifice surface that is offset from the target surface. The jet orifice surface includes an array of jet orifices fluidly coupled to the at least one fluid inlet channel, wherein each individual jet orifice of the array of jet orifices has an area corresponding to a distance of the individual jet orifice to the at least one fluid outlet channel such that individual jet orifices closer to the at least one fluid outlet have an area that is smaller than individual jet orifices further from the at least one fluid outlet. In addition, the area of the individual jet orifices of the array of jet orifices increases radially from a central region of the array of jet orifices 
     In another embodiment, a cooling apparatus includes at least one fluid inlet channel, at least one fluid outlet channel, a target surface, and a jet orifice surface that is offset from the target surface. The jet orifice surface includes an array of jet orifices fluidly coupled to the at least one fluid inlet channel. The target surface and the jet orifice surface at least in part define an impingement chamber having at least one region of relatively high pressure and at least one region of relatively low pressure having a pressure that is lower than a pressure at the region of relatively high pressure when the coolant fluid flows through the array of jet orifices. Each individual jet orifice of the array of jet orifices has an area such that individual jet orifices proximate the at least one region of relatively low pressure have an area that is smaller than individual jet orifices proximate the at least one region of relatively high pressure. In addition, the area of each individual jet orifice of the array of jet orifices increases radially from a central region of the array of jet orifices. 
     In yet another embodiment, a cooling apparatus includes at least one fluid inlet channel, at least one fluid outlet channel, a target surface, and a jet orifice surface that is offset from the target surface. The jet orifice surface includes an array of jet orifices fluidly coupled to the at least one fluid inlet channel, wherein each individual jet orifice of the array of jet orifices has an area corresponding to a distance of the individual jet orifice to the at least one fluid outlet channel such that the area of the individual jet orifices decreases radially from a central region of the array of jet orifices. The cooling apparatus further includes an insulation assembly thermally coupled to the target surface. 
     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 sectional view of a jet impingement, two-phase cooling apparatus according to one or more embodiments described and illustrated herein; 
         FIG. 2  graphically depicts simulated pressure contours at a target surface impinged with two impingement jet arrays and the effect of a fluid outlet; 
         FIG. 3A  schematically depicts a jet orifice surface having an array of jet orifices according to one or more embodiments described and illustrated herein; 
         FIG. 3B  schematically depicts a jet orifice surface having a 5×5 array of jet orifices according to one or more embodiments described and illustrated herein; 
         FIG. 3C  schematically depicts a jet orifice surface having a 3×3 array of jet orifices according to one or more embodiments described and illustrated herein; 
         FIG. 3D  schematically depicts a jet orifice surface having an array of jet orifices wherein the diameter of the array of jet orifices radially increases from a center according to one or more embodiments described and illustrated herein; 
         FIG. 4  schematically depicts a jet orifice surface having an array of jet orifices including asymmetrically sized jet orifices to compensate for the pressure variations depicted in  FIG. 2  according to one or more embodiments described and illustrated herein; 
         FIG. 5  schematically depicts a cross sectional view of a cooling apparatus having sloped vapor outlet channels according to one or more embodiments described and illustrated herein; 
         FIG. 6  schematically depicts an exploded view of a cooling apparatus according to one or more embodiments described and illustrated herein; 
         FIG. 7  schematically depicts a bottom view of the inlet-outlet manifold of the cooling apparatus depicted in  FIG. 6  according to one or more embodiments described and illustrated herein; 
         FIG. 8A  schematically depicts a perspective view of a jet orifice plate of the cooling apparatus depicted in  FIG. 6  according to one or more embodiments described and illustrated herein; 
         FIG. 8B  schematically depicts a bottom view of the jet orifice plate depicted in  FIG. 8A ; 
         FIG. 9A  schematically depicts a top view of the jet plate manifold of the cooling apparatus depicted in  FIG. 6  according to one or more embodiments described and illustrated herein; 
         FIG. 9B  schematically depicts a bottom view of the jet plate manifold depicted in  FIG. 9A ; 
         FIG. 9C  schematically depicts a side view of the jet plate manifold depicted in  FIG. 9A ; 
         FIG. 10A  depicts a top view of the vapor manifold of the cooling apparatus depicted in  FIG. 6  according to one or more embodiments described and illustrated herein; 
         FIG. 10B  schematically depicts a bottom view of the vapor manifold depicted in  FIG. 10A ; 
         FIG. 10C  schematically depicts a side view of the vapor manifold depicted in  FIG. 10A ; 
         FIG. 11  schematically depicts a cross sectional, partially transparent perspective view of an assembled cooling apparatus according to one or more embodiments described and illustrated herein; and 
         FIG. 12  schematically depicts a fluid domain of coolant fluid flowing within the cooling apparatus depicted in  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure are directed to jet impingement cooling apparatuses that may be utilized to cool heat generating devices, such as semiconductor devices. In the embodiments described herein, jet impingement is provided by directing jets of coolant fluid at an impingement region of a target surface, which may be a heat generating device or a thermally conductive target plate coupled to the heat generating device. Heat is transferred to the coolant fluid as it impinges the target surface. In addition to jet impingement, some embodiments may provide two-phase cooling, wherein the coolant fluid changes phase from a fluid to a vapor, thereby further removing heat flux from the heat generating device. Accordingly, some embodiments are directed to submerged two-phase, jet impingement cooling devices. Embodiments described herein may provide for impingement jets having substantially equal fluid velocity at the target surface despite variations in pressure within the cooling apparatus. As described in more detail below, regions near the fluid outlet(s) of the cooling apparatus may have lower pressure, and impingement jets located proximate to the fluid outlet(s) may lose velocity near the target surface and therefore not impinge the target surface, or non-orthogonally impinge the target surface. It is noted that pressure variation within the cooling apparatus may also be caused by non-uniform boiling of coolant fluid therein. Embodiments described herein have jet orifices with varying sizes (i.e., areas) such that each impingement jet impinges the target surface at substantially the same fluid velocity. For example, jet orifices closer to the fluid outlet(s) may have an area (e.g., a circular hole having a diameter) that is smaller that is an area of jet orifices further from the fluid outlet(s). 
     Optionally, embodiments may also guide vapor that is formed at the heat source (e.g., at the semiconductor device) due to the boiling of the coolant fluid away from the heat source to prevent the build-up of pressure within the cooling apparatus. More particularly, pitched vapor outlet channels may optionally be oriented to take advantage of the buoyancy of vapor bubbles to guide them away from the heat source. Accordingly, the pitched (i.e., sloped) vapor outlet channels resolve the inherent pressure build-up associated with the incomplete evacuation of vapor from the cooling apparatus which causes an increase in the saturation temperature of the coolant fluid and diminishes the effectiveness of heat transfer. Various embodiments of cooling apparatuses having jet orifices of varying size to provide for uniform fluid velocity at the target surface are described in detail below. 
     Referring now to  FIG. 1 , an example jet impingement cooling apparatus  10  is schematically depicted in cross section. The cooling apparatus  10  generally comprises a fluid inlet  12  that is fluidly coupled to a fluid inlet channel  13 , and several fluid outlet channels  14  that are fluidly coupled to one or more fluid outlets  15 . In some embodiments, the fluid outlet channels  14  may converge to a single fluid outlet  15 , and/or exit one or more sides of the cooling apparatus  10  rather than the top as depicted in  FIG. 1 . The fluid inlet  12  and the fluid outlets  15  may be fluidly coupled to fluid lines (not shown) that are fluidly coupled to a coolant fluid reservoir (not shown). The coolant fluid may be any appropriate liquid, such as deionized water or radiator fluid, for example. The fluid inlet  12  and the fluid outlets  15  may be configured as couplings, such as male or female fluid couplings, for connecting fluid lines to the fluid inlet  12  and the fluid outlets  15 . The fluid inlet channel  13  terminates at a jet orifice surface  26  having an array of jet orifices  25 . Coolant fluid  30  flows through the fluid inlet channel  13  and the array of jet orifices  25 . The coolant fluid  30  exits the jet orifices  25  as impingement jets  32  that impinge a thermally conductive target plate acting as a target surface  50  that is thermally coupled to a heat generating device, such as a semiconductor device  80 . The impingement jets  32  may be substantially normal with respect to the target surface  50  in embodiments. Semiconductor devices  80  may include, but are not limited to, insulated gate bipolar transistors (IGBT), metal-oxide-semiconductor field effect transistors (MOSFET), power diodes, power bipolar transistors, and power thyristor devices. As an example and not a limitation, the semiconductor device  80  may be included in a power electronic module as a component in an inverter and/or converter circuit used to electrically power high load devices, such as electric motors in electrified vehicles (e.g., hybrid vehicles, plug in hybrid electric vehicles, plug in electric vehicles, and the like). In alternative embodiments, the impingement jets  32  directly impinge the heat generating device rather than an intermediate target surface  50 . As described in more detail below, the target surface  50  may further include surface features, such as thermally conductive fins, posts, and the like, to further enable heat transfer from the heat generating device to the coolant fluid. 
     After impinging the target surface  50 , which may be configured as a plate of thermally conductive material such as copper or aluminum, for example, the coolant fluid  30  flows away from an impingement region  23  within an impingement chamber  24  defined by the target surface  50  and the jet orifice surface  26 . Some of the coolant fluid  30  may change phases from a liquid to a vapor due to the high temperature heat generating device being cooled. This phase change will cause vapor bubbles to form near the impingement region  23  and the heat generating device. 
     Pressure within the impingement chamber  24  may vary. For example, regions of low pressure may be present within the impingement chamber  24  at locations close to one or more fluid outlet channels  14  or fluid outlets  15 , while regions of high pressure may be present within the impingement chamber  24  at locations further away from the one or more fluid outlet channels  14  or fluid outlets  15 . Pressure variation may also be caused by non-uniform boiling of the coolant fluid within the impingement chamber  24 . Referring now to  FIG. 2 , simulated pressure contours at a target surface  50 ′ resulting from two adjacent arrays of impingement jets are graphically depicted. It should be understood that  FIG. 2  is provided for illustrative purposes only, and not intended to limit the embodiments described herein. The adjacent arrays of impingement jets impinge the target surface  50 ′ at first and second impingement regions  23 A,  23 B. In the simulation, impingement locations  33  of individual impingement jets are positioned between surface enhancement features configured as posts  92  extending about 1.0 mm from the surface of the target surface  50 ′. As described in more detail below, surface features, such as the posts  92  shown in  FIG. 2 , may be provided to enhance thermal transfer as well as guide coolant fluid flow on the target surface  50 ′. In embodiments, the target surface  50  may be substantially flat with no surface enhancement features. 
     The impingement locations  33  have the highest pressure on the simulated target surface  50 ′. A region of relatively high pressure  17  surrounds the first and second impingement regions  23 A,  23 B and is present throughout a majority of the impingement chamber  24 . A fluid outlet channel  14  is located proximate a bottom, left corner of the first impingement region  23 A such that coolant fluid, after impinging the target surface  50 ′ flows toward and out of the fluid outlet channel  14 . However, the presence of the fluid outlet channel  14  produces a region of relatively low pressure  18  (i.e., lower pressure than the region of relatively high pressure) near the bottom, left corner of the first impingement region  23 A that causes a suction effect that pulls the impingement jets of the array of impingement jets toward the fluid outlet channel  14 . The suction effect may slow down and prevent the impingement jets that are closest to the fluid outlet channel  14  from impinging the target surface  50 ′, which may adversely affect the thermal performance of the cooling apparatus. As shown in  FIG. 2 , the pressure at the impingement locations  33  at the bottom, left corner of the first impingement region  11 A is lower and less pronounced than other impingement locations  33  further away from the region of relatively low pressure  18 . This indicates that either some of the impingement jets do not impinge the target surface  50 ′, or impinge the target surface  50 ′ with a reduced fluid velocity and/or at a non-orthogonal angle as compared with impingement jets located further away from the region of relatively low pressure  18  (e.g., near the region of relatively high pressure). 
     The non-impingement of impingement jets reduces the thermal performance of the cooling apparatus  10 . For example, non-impingement of neighboring impingement jets may cause cross-flow, which may prevent flow of coolant fluid across the target surface  50 ′. Further, if the impingement jet does not impinge upon the target surface  50 ′, then the critical heat flux or maximum heat flux reached at the stagnation point will be lower than the impingement jet that has a higher velocity at the stagnation point. 
     Embodiments of the present disclosure ensure that each impingement jet of the array of impingement jets strike the target surface at a desired velocity by using jet orifices that are non-uniform in size (i.e., area) while still providing the same flow rate as an array of jet orifices having uniform size. Generally, the fluid velocity of an impingement jet may be increased by decreasing the size of the respective jet orifice. By increasing the fluid velocity of impingement jets close to a fluid outlet channel, such impingement jets may strike the target surface. Accordingly, jet orifices closer to a fluid outlet channel (or fluid outlet) may be smaller in size than jet orifices further away from a fluid outlet channel. Jet orifice spacing and size may be selected based on the size of the heated area to be cooled and the location of the fluid out channel(s). 
       FIGS. 3A-3C  schematically depict three exemplary jet orifice surfaces  26 ,  26 ′,  26 ″, each having an array of jet orifices (individually as numbered  25 A,  25 B,  25 C depending on the size). Although the jet orifices are depicted as circular in  FIGS. 3A-3C , embodiments are not limited thereto. For example, the jet orifices may take on other shapes, such as cross-shaped, lobed, helical, and the like. In each of the illustrated jet orifice surfaces  26 ,  26 ′,  26 ″, the size (i.e., area) of the jet orifices  25 B,  25 C decreases radially from a large, central jet orifice  25 A that is located at a central region of the array of jet orifices. Accordingly, the jet orifice surfaces  26 ,  26 ′,  26 ″ depicted in  FIGS. 3A-3C  may be suited for a cooling apparatus having four fluid outlet channels (or a continuous fluid outlet channel) about a perimeter of the impingement chamber  24  (see  FIGS. 6-12  for an exemplary embodiment). Because the central orifice  25 A is furthest from the fluid outlets, it is the largest orifice. In the embodiments illustrated in  FIGS. 3A-3C , the jet orifices  25 A,  25 B,  25 C are evenly spaced in an array. 
     Referring specifically to  FIG. 3A , a jet orifice surface  26  includes an array of jet orifices  25 A,  25 B,  25 C of different sizes. The jet orifice surface  26  may define an impingement chamber (e.g., impingement chamber  24  depicted in  FIG. 1 ) having four fluid outlet channels  14  (either discrete fluid outlet channels or a single, continuous fluid outlet channel). Accordingly, the jet orifices  25 C close to the perimeter of the jet orifice surface  26  are closer to a fluid outlet channel  14 , and therefore in a region of relatively low pressure within the impingement chamber  24 . Impingement jets exiting these jet orifices, if uniformly sized, may not successfully impinge the target surface  50 . However, the jet orifices  25 A,  25 B,  25 C depicted in  FIG. 3A  are non-uniformly sized such that the array of impingement jets orthogonally impinge the target surface  50 . Central jet orifice  25 A has a diameter d 1  that is greater than the diameter d 2  of jet orifices  25 B, and the diameter d 2  is greater than a diameter d 3  of the smallest, outermost jet orifices  25 C. In the arrangement of jet orifices depicted in  FIG. 3A , secondary jet orifices  25 B form a ring around central jet orifice  25 A, while the smallest, outermost jet orifices  25 C (tertiary jet orifices) form a second ring around jet orifices  25 B. 
     The jet orifices  25 A,  25 B,  25 C may be sized such that each impingement jet impinges the target surface  50 . The smaller, outermost jet orifices  25 C produce impingement jets that have a higher velocity upon exiting the jet orifice surface  26  than the larger jet orifices  25 A,  25 B. The higher velocity may ensure that impingement jets closest to the fluid outlet channels  14  impinge the target surface  50  despite the suction effect present in the regions of relatively low pressure at the perimeter of the cooling apparatus  10 . In some embodiments, the jet orifices  25 A,  25 B,  25 C are sized such that each impingement jet impinges the target surface  50  at substantially the same velocity. In other embodiments, the jet orifices  25 A,  25 B,  25 C may be sized such that the velocity of the individual impingement jets are not substantially the same but different according to a desired impingement velocity pattern. For example, the jet orifices may be designed such that the outermost impingement jets impinge the target surface at a greater (or lesser) velocity than the inner impingement jets. The velocity and impingement velocity pattern may vary depending on the application. Accordingly, the smaller, outermost jet orifices  25 C may mitigate the effects seen by a jet orifice surface having uniform jet orifice sizes as described above, thereby improving thermal performance. 
     In some embodiments, the smaller, outermost jet orifices  25 C may be sized to produce impingement jets that have a fluid velocity that is double that of the fluid velocity of the impingement jet of the central jet orifice  25 A upon exiting the jet orifice surface  26 . However, it should be understood that embodiments may have a velocity ratio different than two times between the fluid velocity of the impingement jets exiting the outermost jet orifices  25 C and the impingement jet(s) exiting the central jet orifice  25 A. As an example and not a limitation, the central jet orifice  25 A may have a diameter of about 1.35 mm, the secondary jet orifices  25 B may have a diameter of about 0.9 mm, and the tertiary jet orifices  25 C may have a diameter of about 0.6 mm. These diameters provide for about the same flow area as a uniform orifice design having jet orifices with a 0.75 mm diameter. It should be understood that other dimensions may be utilized, and these dimensions are used only as an example. The number, size, and spacing of the jet orifices may depend on many factors, such as the size of the heated area, the number of heated areas, the heat flux present at the heated area, and the distance from the jet orifice surface  26  to the target surface  50 . As described above, the smaller sized jet orifices  25 C may ensure that impingement jets close to the fluid outlets impinge the target surface  50 . 
     Similarly,  FIG. 3B  schematically depicts a jet orifice surface  26 ′ with a 5×5 array of jet orifices  25 A,  25 B,  25 C having a diameter that radially decreases from a large, central jet orifice  25 A (d 1 &gt;d 2 &gt;d 3 ). The secondary jet orifices  25 B form a ring around the central jet orifice  25 A, while the tertiary jet orifices  25 C form a ring around the secondary jet orifices  25 B.  FIG. 3C  schematically depicts a jet orifice surface  26 ″ having a 3×3 array of jet orifices  25 A,  25 B. A central jet orifice  25 A having a diameter d 1  is surrounded by eight secondary jet orifices  25 B having a diameter d 2  that is less than diameter d 1 , in the illustrated embodiment. 
     Embodiments are not limited to jet orifice surfaces having an array of jet orifices  25 C,  25 B,  25 C wherein the diameter of the jet orifices radially decreases from a center.  FIG. 3D  depicts an embodiment wherein the jet orifices  25 A- 25 D increase radially from a central region. The smallest jet orifices  25 C having a diameter d 3  are located at a center of the array of jet orifices, while intermediate jet orifices  25 B having a diameter d 2  surround the smallest primary jet orifices  25 C, and large jet orifices  25 A having a diameter d 1  surround the intermediate jet orifices (d 1 &gt;d 2 &gt;d 3 ). Accordingly, the diameter of the jet orifices  25 A- 25 C increases radially from the center. Such an arrangement may be desirable in cooling apparatuses wherein the fluid outlet is centrally located with respect to the target surface  50 . 
     It should be understood that other configurations are also possible. For example, in embodiments wherein there is only one fluid outlet channel, such as depicted in  FIG. 2 , the diameter of the jet orifices may not radially decrease. Referring now to  FIG. 4 , an exemplary jet orifice surface  26 ′″ having an array of jet orifices  25 A,  25 B,  25 C sized to ensure that the impingement jets impinge the target surface  50 ′ with the placement of the fluid outlet channel  14  depicted in  FIG. 2  is schematically illustrated. The size of the jet orifices  25 A,  25 B,  25 C decrease asymmetrically from the upper, right corner of the jet orifice surface  26 ′″. The small jet orifices  25 C having a diameter d 3  are disposed proximate the region of relatively low pressure  18  and closest to the fluid outlet channel  14  depicted in  FIG. 2 . Secondary jet orifices  25 B having a diameter d 2  are adjacent to the small jet orifices  25 C, while large jet orifices  25 A having a diameter d 1  are adjacent to the secondary jet orifices  25 B and furthest from the fluid outlet channel  14  (wherein d 1 &gt;d 2 &gt;d 3 ). Diameter d 3  may be such that the impingement jets produced by the small jet orifices  25 C impinge the target surface despite being within the region of relatively low pressure  18  depicted in  FIG. 2 . It should be understood that embodiments are not limited to the configuration depicted in  FIG. 4 , and that other configurations are possible. For example, the jet orifice surface  26 ′″ depicted in  FIG. 4  may have jet orifices of only two sizes (e.g., small jet orifices  25 C and large jet orifices  25 A). 
     The jet orifice surfaces having non-uniform jet orifice sizes described above may be implemented into any jet impingement cooling apparatus. Referring now to  FIG. 5 , an example cooling apparatus  10 ′ having sloped vapor channels is schematically depicted in cross section. As described in more detail below, the sloped vapor channels may aid in efficiently removing the vapor bubbles from cooling apparatus  10 ′. The cooling apparatus  10 ′ generally comprises a fluid inlet  12 ′ that is fluidly coupled to a fluid inlet channel  13 ′, and several fluid outlet channels  14 ′ that are fluidly coupled to one or more fluid outlets  15 ′. In some embodiments, the fluid outlet channels  14  may converge to a single fluid outlet. The fluid inlet  12 ′ and the fluid outlets  15 ′ may be fluidly coupled to fluid lines (not shown) that are fluidly coupled to a coolant fluid reservoir (not shown). As described above with reference to  FIG. 1 , the fluid inlet  12 ′ and the fluid outlets  15 ′ may be configured as couplings, such as male or female fluid couplings, for connecting fluid lines to the fluid inlet  12 ′ and the fluid outlets  15 ′. The fluid inlet channel  13 ′ terminates at a jet orifice surface  26 ′ having an array of jet orifices  25 ′ with non-uniform sizes as described above. Coolant fluid  30 ′ flows through the fluid inlet channel  13 ′ and the jet orifices  25 ′. The coolant fluid  30 ′ exits the jet orifices  25 ′ as impingement jets  32 ′ that impinge a thermally conductive target surface  50 ″ that is thermally coupled to a heat generating device, such as a semiconductor device  80 ′. 
     After impinging the target surface  50 ″, the coolant fluid  30 ′ flows away from an impingement region  23 ′ within an impingement chamber  24 ′ defined by the target surface  50 ″ and the jet orifice surface  26 ′. Some of the coolant fluid  30 ′ changes phases from a liquid to a vapor due to the high temperature heat generating device being cooled. This phase change will cause vapor bubbles to form near the impingement region  23 ′ and the heat generating device. Collection of vapor bubbles within the impingement chamber  24 ′ causes the pressure within the cooling apparatus to increase, which further causes an increase in the saturation temperature of the coolant fluid that diminishes the effectiveness of heat transfer. 
     Body forces from the bulk fluid motion of the coolant fluid alone may not sufficiently remove all of the vapor formed within the impingement chamber  24 ′. Buoyant forces, arising from the lower density of the vapor relative to its surrounding liquid medium, can counteract the body force, thereby leading to the pooling of vapor at the top of the cooling chamber. Because vapor is a gas and is compressive, pressure gradually increases over time as vapor collects, causing an increase in the saturation temperature of the coolant fluid. 
     The example cooling apparatus  10 ′ further includes several sloped vapor outlet channels  27 ′ that are fluidly coupled to the impingement chamber  24 ′. The sloped vapor outlet channels  27 ′ take advantage of the buoyancy of the vapor bubbles to guide them away from the impingement region  23 ′. Accordingly, the geometry of the sloped vapor outlet channels  27 ′ accounts for, and utilizes, both body and buoyant forces to drive the vapor away from the impingement region  23 ′ and the heat generating device. Thus, pressure and the saturation temperature of the coolant fluid  30 ′ should remain constant regardless of heat input and vapor generation within the impingement chamber  24 ′ of the cooling apparatus  10 ′. In the illustrated embodiment, the sloped vapor outlet channels  27 ′ transition to vertical vapor outlet channels  28 ′ through which the coolant flows and exits the cooling apparatus  100 ′. 
     Referring now to  FIG. 6 , an example cooling apparatus  100  is depicted in an exploded view. It should be understood that the cooling apparatus  100  depicted in  FIGS. 6-12  is provided for illustrative purposes only, and that the jet orifice surfaces with non-uniform orifice sizes may be implemented in cooling apparatuses other than that depicted in  FIGS. 6-12 . Generally, the cooling apparatus comprises an inlet-outlet manifold  110 , a jet plate manifold  150  coupled to the inlet-outlet manifold  110 , a jet orifice plate  120  disposed within the jet plate manifold  150 , a vapor manifold  170  coupled to the jet plate manifold  150 , and a target surface  180  disposed within an insulation assembly  190  that is coupled to the vapor manifold  170 . Several gaskets may be provided between the various components to prevent fluid from escaping the cooling apparatus  100 . For example, a jet plate gasket  130  may be positioned between a flange portion  122  of the jet orifice plate  120  and a seat  131  (see  FIG. 11 ) of the jet plate manifold  150 , a jet plate manifold gasket  140  may be positioned between the inlet-outlet manifold  110  and the jet plate manifold  150 , and a vapor manifold gasket  160  may be positioned between the jet plate manifold  150  and the vapor manifold  170 . 
       FIG. 7  is a bottom view of the inlet-outlet manifold  110 . Referring to  FIGS. 6 and 7 , the inlet-outlet manifold  110  comprises a fluid inlet  102  at a first surface  101  that is fluidly coupled to an inlet manifold channel  103  within a bulk of the inlet-outlet manifold  110 . The inlet manifold channel  103  opens at a second surface  105  of the inlet-outlet manifold  110 . In the illustrated body, the inlet manifold channel  103  widens to a larger opening  107  at the second surface  105 . A fluid outlet  104  is also present on the first surface  101 . The fluid inlet  102  and the fluid outlet  104  may be fluidly coupled to input and output fluid lines, respectively. Coolant fluid is provided to the cooling apparatus  100  through the fluid inlet  102 , and is removed from the cooling apparatus  100  through the fluid outlet  104 . 
     Referring to  FIG. 7 , the second surface  105  of the inlet-outlet manifold  110  also comprises four slot-shaped outlet openings  117  along a perimeter of the inlet-outlet manifold  110 . Briefly referring to  FIG. 11 , each slot-shaped outlet opening  117  is fluidly coupled to an internal outlet manifold channel  114   a - 114   d  that is fluidly coupled to the fluid outlet. It is noted that outlet manifold channel  114   d  is not visible in  FIG. 7 . The outlet manifold channels  114   a - 114   d  are disposed around the inlet manifold channel  103 . As described in more detail below, the outlet manifold channels  114   a - 114   d  slope upwardly toward the fluid outlet  104  near the first surface  101  of the inlet-outlet manifold  110 . Referring once again to  FIG. 7 , the inlet-outlet manifold  110  further comprises a plurality of through-holes configured to receive a plurality of fasteners (e.g., screws) to maintain the various components of the cooling apparatus  100  in an assembled configuration. It should be understood that, in alternative embodiments, the various components may be coupled together by bonding layers (e.g., solder layers) rather than by mechanical fasteners. 
       FIG. 8A  schematically depicts a perspective view of an example jet orifice plate  120 , while  FIG. 8B  schematically depicts a bottom view of the jet orifice plate  120  depicted in  FIG. 8A . Generally, the illustrated jet orifice plate  120  comprises a flange portion  122 , and a narrow portion  124  extending from the flange portion  122 . A jet orifice surface  126  is provided on an underside surface of the narrow portion  124 . A jet channel extends  123  through the flange portion  122  and the narrow portion  124 , and is fluidly coupled to the inlet manifold to receive input coolant fluid. The jet orifice surface  126  comprises an array of jet orifices  125  through which coolant fluid flows as impingement jets, as described above. In the illustrated embodiment, the jet orifice surface has an array of jet orifices with non-uniform jet orifice sizes similar to that depicted in  FIG. 3B . It should be understood that other jet orifice array configurations are also possible. The jet orifice plate  120  is configured to be disposed within the jet plate manifold  150 . In some embodiments, the flange portion  122  further includes through-holes configured to receive fasteners that couple the jet orifice plate  120  to the jet plate manifold  150 , or one or more through-holes serving as a pressure relief. 
       FIG. 9A  is a top view of the jet plate manifold  150  depicted in  FIG. 6 , while  FIGS. 9B and 9C  are bottom and side views of the jet plate manifold  150 , respectively. Referring now to  FIGS. 6 and 9A-9C , the jet plate manifold  150  generally comprises an upper portion  152  defined by four walls  158 , and a tapered portion  153  extending from the upper portion  152 . Four slot channels  157  are provided within the four walls  158  of the upper portion. The slot channels  157  extend from a first surface  151  to a second surface  159  of the jet plate manifold  150 , and are positioned such that they are fluidly coupled to the slot-shaped outlet openings  117  and outlet manifold channels  114   a - 114   d  of the inlet-outlet manifold  110  when the jet plate manifold  150  is coupled to the inlet-outlet manifold  110 . The jet plate manifold  150  may further comprise a plurality of through-holes  155  for receiving fasteners. 
     The first surface  151  (i.e., the upper surface) has a first opening  154  that extends to a depth D, and then narrows to a second opening  156 , thereby defining a seat  131  ( FIG. 9C ). The exemplary tapered portion  153  comprises four walls that taper downwardly away from the upper portion  152 . As described in more detail below, the tapered portion  153  defines sloped vapor outlet channels  176  (i.e., fluid outlet channels) through which coolant fluid flows. 
     The first opening  154  and the second opening  156  define a jet plate manifold channel  161  extending from the first surface  151  to the second surface  159 . The jet orifice plate  120  is disposed within the jet plate manifold channel  161  of the jet plate manifold  150 . As shown in  FIG. 11 , which is a cross-sectional view of an exemplary cooling apparatus  100 , the flange portion  122  of the jet orifice plate  120  is positioned on the seat  131  defined by the transition between the first opening  154  and the second opening  156 . In some embodiments, a jet plate gasket  130  may be positioned between the flange portion  122  and the seat  131 . The narrow portion  124  of the jet orifice plate  120  is disposed within the second opening  156  and may extend beyond the tapered portion  153 , as depicted in  FIG. 11 . 
     Referring now to  FIGS. 6, 10A-10C and 11 , the vapor manifold  170  comprises tapered walls  177  that taper from a first surface  171  toward a second surface  173 . The tapered walls  177  define a first opening at  174 A at the first surface  171  of the vapor manifold  170 . The tapered walls  177  terminate at straight walls  178  that extend to the second surface  173 , thereby defining a second opening  174 B. The first and second openings  174 A,  174 B define a vapor manifold opening  172  into which the tapered portion  153  of the jet plate manifold  150  and the narrow portion  124  of the jet orifice plate  120  are disposed. As described in more detail below and depicted in  FIG. 11 , the tapered walls  177  cooperate with the tapered portion  153  of the jet plate manifold  150  to define the sloped vapor outlet channels  176  that act as fluid outlet channels that surround the jet orifice surface  126 . The exemplary vapor manifold  170  further comprises through-holes  175 , which may be threaded to receive fasteners to couple various components of the cooling apparatus  100  together. 
     The vapor manifold  170  is coupled to a thermally conductive target surface  180  disposed within an insulation assembly  190 , as shown in  FIGS. 6 and 11 . The target surface  180  also comprises through-holes  185  for receiving fasteners. The target surface  180  may be fabricated from a thermally conductive material, such as copper or aluminum, for example. In the illustrated embodiment, the target surface  180  comprises a plurality of surface fins  182  that orthogonally extend from the target surface  180 . The surface fins  182  are arranged to be spaced between rows (or columns) of jet orifices  125 , as shown in  FIG. 11 . The surface fins  182  increase the surface area in contact with the coolant fluid, thereby increasing heat transfer. Further, the surface fins  182  assist in directing the coolant fluid within a impingement chamber  179  defined by the target surface  180 , the jet orifice plate  120  and the tapered portion  153  of the jet plate manifold. 
     The insulation assembly  190  is configured to receive the target surface  180 . In the illustrated embodiment, the insulation assembly  190  includes a recessed area  194  into which the target surface  180  is disposed. The illustrated insulation assembly  190  further includes a device recess  192  that is configured to accept a heat generating device  197 , such as a semiconductor device (see  FIG. 11 ). The target surface  180  is thermally coupled to the heat generating device  197 . The insulation assembly  190  may further include a notch  193  that allows electrical connections to pass from the heat generating device  197  out of the cooling apparatus  100 . The insulation assembly  190  may also include through-holes or blind bores for receiving fasteners to maintain the various components in an assembled state. 
     The insulation assembly  190  may be fabricated from any non-electrically conductive material capable of withstanding the high operating temperatures of the heat generating device  197 . Exemplary materials include, but are not limited to, solidified polymers (e.g., polyether ether ketone (“PEEK”)), ceramic materials (e.g., aluminum nitride), and the like. 
     Referring specifically now to  FIG. 11 , a cross-sectional, partially transparent view of an assembled cooling apparatus  100  is schematically depicted. A heat generating device  197  is positioned in a device recess  192  of the insulation assembly  190 . The target surface  180  is positioned within the recessed area  194 . A second surface  173  of the vapor manifold  170  is coupled to the insulation assembly  190  and the target surface  180 . 
     The jet plate manifold  150  is coupled to the first surface  171  of the vapor manifold  170 . In some embodiments, a vapor manifold gasket  160  is positioned between the jet plate manifold  150  and the vapor manifold  170  to prevent coolant fluid from leaking between the two components. The jet plate manifold  150  is arranged with respect to the vapor manifold  170  such that the tapered portion  153  is disposed within the vapor manifold opening  172 . The tapered portion  153  of the jet plate manifold  150  is offset with respect to the tapered walls  177  of the vapor manifold  170  such that the tapered portion  153  and the tapered walls  177  define a plurality of sloped vapor outlet channels  176 . The sloped vapor outlet channels  176  slope outwardly away from an impingement region (i.e., a region at the surface fins  182 ) and upwardly toward the fluid inlet  102  (i.e., opposing gravity). The sloped vapor outlet channels  176  are aligned with, and fluidly coupled to, the slot channels  157 . As described above, the sloped vapor outlet channels  176  take advantage of the buoyancy of the vapor bubbles to guide them away from the impingement region. 
     The jet orifice plate  120  is positioned within the jet plate manifold channel  161  such that the jet orifice surface  126  contacts, or nearly contacts, the surface fins  182  of the target surface  180 . It is noted that, in some embodiments, the target surface  180  does not include surface fins  182 . Additionally, the surface fins  182  may have a geometric configuration that is different from that depicted in  FIGS. 6 and 11 . In the illustrated embodiment, the rows of jet orifices  125  are aligned with respect to the surface fins  182  such that the impingement jets exiting the jet orifices  125  are between adjacent surface fins  182 . 
     The narrow portion  124  of the jet orifice plate  120 , the tapered portion  153  of the jet plate manifold  150 , and the target surface  180  define an impingement chamber  179  into which the coolant fluid flows after impinging the target surface  180 , as described in more detail below. 
     The inlet-outlet manifold  110  is coupled to the jet plate manifold  150 . In some embodiments, a jet plate manifold gasket  140  is positioned between the inlet-outlet manifold  110  and the jet plate manifold  150  to prevent coolant fluid from escaping the cooling apparatus  100 . The slot-shaped outlet openings  117  of the inlet-outlet manifold are aligned with the slot channels  157  of the jet plate manifold  150 , thereby fluidly coupling the outlet manifold channels  114   a - 114   d  of the inlet-outlet manifold  110  to the slot channels  157 . 
     The outlet manifold channels  114   a - 114   d  are fluidly coupled to the fluid outlet  104 . In the illustrated embodiment, the outlet manifold channel  114   c  that is closest to the fluid outlet  104  has the largest height, and the outlet manifold channel  114   a  opposite from the fluid outlet  104  has the smallest height. Each of the outlet manifold channels  114   a - 114   d  slope upwardly toward the fluid outlet  104 . It is noted that outlet manifold channel  114   d  is not visible in  FIG. 11 , and that outlet manifold channel  114   d  is symmetrically similar to outlet manifold channel  114   b . The outlet manifold channels  114   a - 114   d  surround the inlet manifold channel  103  near the perimeter of the inlet-outlet manifold  110 . 
     Referring now to  FIGS. 11 and 12 , coolant fluid flowing through the cooling apparatus  100  will now be described.  FIG. 12  schematically depicts a cross section of the fluid domain  200  of coolant fluid (in the form of liquid and vapor) flowing through the cooling apparatus  100 . Coolant fluid enters the fluid inlet  102  and the inlet manifold channel  103  as indicated by arrow  133   a , and fluid region  202  of  FIG. 12 . The coolant fluid may originate from a coolant fluid reservoir. The coolant fluid flows from the inlet-outlet manifold  110  into the jet channel  123 , as indicated by arrows  133   b  and  133   c , as well as fluid region  223  of  FIG. 12 . The coolant fluid then flows through the jet orifices  125  as an array of impingement jets between adjacent surface fins  182  (represented generically by fluid region  272 ). Because of the non-uniform sizes of the jet orifices  125  of the jet orifice surface  126 , each impingement jet impinges the target surface  180 . In embodiments, the jet orifices  125  are sized such that the impingement jets impinge the target surface  180  at substantially the same velocity. In other embodiments, the jet orifices  125  are sized such that the impingement jets impinge the target surface  180  at different velocities according to a desired impingement velocity pattern. The coolant fluid flows between and around the surface fins  182  toward a perimeter of the impingement chamber  179  as indicated by arrows  133   d  (fluid region  274  of  FIG. 12 ). Due to the high operating temperature of the heat generating device  197 , some of the coolant fluid changes from a liquid to a vapor. Accordingly, vapor bubbles form within the impingement chamber. Body forces and buoyant forces direct the coolant fluid (both liquid and vapor bubbles) into the sloped vapor outlet channels  176  as indicated by arrows  133   e  and  133   f  (fluid region  276  of  FIG. 12 ). It is noted that coolant fluid flows through all four sloped vapor outlet channels  176 , and that only two arrows (arrows  133   e ,  133   f ) are depicted for ease of illustration. The sloped vapor outlet channels  176  take advantage of the buoyancy of the vapor bubbles to guide them away from the surface fins  182  and the impingement region. 
     The coolant fluid then flows from the sloped vapor outlet channels  176  into the slot channels  157  of the jet plate manifold  150  as indicated by arrows  133   g  and  133   f , wherein it flows upward and into the outlet manifold channels  114   a - 114   d  of the inlet-outlet manifold  110 . Referring to  FIG. 12 , the coolant fluid flowing within the slot channels  157  and the outlet channels  114   a - 114   d  are combined into individual flow regions  290   a - 290   c  for ease of illustration. It is noted that a fourth fluid region  290   d  is not depicted in  FIG. 12 , and is symmetrically similar to fluid region  290   b.    
     A top portion of each of the fluid regions  290   a - 290   d  slope upwardly toward the fluid outlet. Fluid region  290   a  is the shortest of the four fluid regions  290   a - 290   d , and slopes upwardly toward fluid region  290   b  and  290   d  (not shown). Fluid region  290   c  is the tallest of the four fluid regions  290   a - 290   d  and is closest to the fluid outlet, which is indicated by fluid region  204  in  FIG. 12 . A bridge channel indicated by bridge fluid region  292  fluidly couples fluid region  290   a  to fluid region  292   b . In other words, outlet manifold channels  114   a  and  114   b  are fluidly coupled by an internal bridge channel  118   a  ( FIG. 11 ). Similarly, outlet manifold channels  114   a  and  114   d  are fluidly coupled by a bridge channel (not shown) that is symmetrically similar to internal bridge channel  118   a . Coolant fluid flowing through internal bridge channel  118   a  is indicated by arrow  133   h  in  FIG. 11 . 
     Coolant fluid flowing up within outlet manifold channels  114   b  and  114   d  through slot channels  157 , as well as coolant fluid entering from outlet manifold channel  114   a , flows upwardly toward outlet manifold channel  114   c  as indicated by arrow  133   i . Accordingly,  FIG. 12  depicts the fluid region  290   b  that slopes upwardly toward fluid region  290   c . Outlet manifold channel  114   b  is fluidly coupled to outlet manifold channel  114   c  by an internal bridge channel  118   b . Thus, coolant fluid flows from outlet manifold channel  114   b  into outlet manifold channel  114   c  through the internal bridge channel  118   b  as indicated by arrow  133   j .  FIG. 12  depicts a bridge fluid region  294  that fluidly couples fluid region  290   b  to fluid region  290   c . It is noted that outlet manifold channel  114   d  (not shown) is also fluidly coupled to outlet manifold channel  114   c  by an internal bridge channel that is symmetrically similar to internal bridge channel  118   b.    
     Coolant fluid flowing from the slot channel  157  aligned with outlet manifold channel  114   c , as well as coolant fluid entering outlet manifold channel from the other outlet manifold channels  114   b - 114   d , flow upwardly toward the fluid outlet  104  as indicated by  133   k . The coolant fluid then exits the cooling apparatus through the fluid outlet  104 . Coolant fluid within the fluid outlet  104  is depicted as fluid region  204  in the fluid domain  200  of  FIG. 12 . 
     It should now be understood that embodiments described herein are directed to jet impingement cooling apparatuses having non-uniformly sized jet orifices for enhanced thermal performance. The non-uniformly sized jet orifices are sized to ensure that each impingement jet of an array of impingement jets directly impinge the target surface being cooled despite regions of low pressure within the impingement chamber due to one or more fluid outlets. Direct impingement of each impingement jet may increase heat transfer by convection and minimize cross flow of coolant fluid across the target surface. 
     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.