Cooling electronics via two-phase tangential jet impingement in a semi-toroidal channel
A two-fluid-phase cooling device for absorbing high thermal flux from electronics devices and other thermally dissipating devices. It consists of a thermally conductive plate with thermally dissipating elements on one face and a semi-toroidal cavity in the opposite face with the cavity's axis perpendicular to the face of the plate, a liquid refrigerant supply tube ending in a thermodynamic cycle's refrigeration expansion valve that directs jets of liquid to impact the conical surface in the center region of the semi-toroidal cavity in a direction along the cavity's axis and tangent to the conical surface, a second plate with a semi-toroidal protrusion extending into the semi-toroidal cavity to form a thin, semi-toroidal channel between the two plates, and a seal between the liquid supply tube and the second semi-toroidal plate. In operation liquid refrigerant jets strike the conical surface generally tangential to the surface and flow at high velocity in a thin film on the surface of the semi-toroidal cavity from its center radially to the outer edge of the toroidal channel, absorbing heat and boiling as it does so. The high radial acceleration forces caused by the liquid film moving at high velocity on the cavity's concave surface force the liquid film against the surface and create a pressure gradient that biases evaporation toward the liquid/vapor interface. The vapor moves parallel to the liquid flow radially outwards between the liquid film and the surface of the semi-toroidal protrusion at very high velocity, causing extreme turbulence in the liquid film and highly augmented heat transfer between the heated plate and the liquid film, while the liquid film nevertheless remains intact and forced against the heated surface by radial acceleration and carried to a distance significantly greater than in conventional jet impingement systems. The device may also be composed of wedge-shaped sections of the semi-toroidal plates. It may further have two expansion valves in series in the liquid supply line, the first generating a small amount of vapor (increase in quality) so the resulting increase in flow volume greatly increases the velocity through the second expansion valve toward the heated surface to further enhance heat transfer.
This application claims the benefit of provisional patent application Ser. No. 60/621,894, filed 2004 Oct. 22 by the present inventors.
FEDERALLY SPONSORED RESEARCHNot Applicable
SEQUENCE LISTING OF PROGRAMNot Applicable
BACKGROUND OF THE INVENTION1. Field of Invention
This invention relates to cooling electronics, specifically to spray-cooling of two-phase fluid on a heated surface contained within a conventional refrigeration loop.
2. Prior Art
The problem addressed in this invention is removal of high thermal dissipation flux from electronic devices such as amplifier gate arrays, laser diodes, etc.
Heat flux from electronics is now in the range of 100 to 1,000 Watts per square centimeter (W/cm2). Thermal literature refers to this as the high-flux range, and ultra-high flux being from 103 to 105 W/cm2 and describes a number of ways to remove the heat. If the heated surface is in the interior of an electronics package it can be removed only by circulation of a fluid against the heated surface.
Fluids commonly available for this are air, water and fluorochemicals (generally called “refrigerants”, although they may be used in high temperature applications), and the means of circulation can be natural convection, single-phase forced (mechanically pumped) convection, and boiling (2-phase pumped flow). The heat transfer coefficient Watts per centimeter-squared and degree centigrade (W/cm2-C) defines the rate of heat removal from a surface for a given temperature difference between the surface and the cooling liquid, and is highly dependant on the type of fluid and the means of circulation. Air is a poor choice for any type of circulation because of its low mass and low thermal conductivity. Water will have a coefficient about an order of magnitude greater than a refrigerant. Natural convection with water reaches only about 0.1 W/cm2-C, so this process cannot be considered for use with a refrigerant for high flux needs. In single-phase forced convection flow refrigerants reach about 1 W/cm2-C and water 10 W/cm2-C, and in boiling heat transfer refrigerants reach about 10 W/cm2-C and water over 100 W/cm2-C. In single-phase flow, water would require a temperature difference of 100C to carry away 1 kW/cm2, limiting the practical approach in most cases to boiling heat transfer. A further, key advantage of phase change flow is that only a modest increase in heated surface temperature results in a large increase in heat flux, and in certain situations such as freezing environments only a refrigerant can be used in the two-phase system.
There are several phase change cooling schemes available: micro- and mini-channel cooling, jet impingement cooling and spray cooling. In all of these the upper limit of heat transfer is set by critical heat flux (CHF) which is the point at which liquid cannot reach the heated surface fast enough to prevent dryout of the surface. Micro-channel and mini-channel refer to flow devices having hydraulic diameters of 10 to several hundred micro-meters, and one to a few millimeters, respectively. Typically the channels are rectangular grooves cut in a metal plate on which the thermally dissipating element is mounted. High heat transfer coefficients, inversely proportional to the Reynolds Number, are achieved by the thinness of the liquid channel in laminar flow. Drawbacks include the limitations of the minimum size of the hydraulic diameter necessary to avoid flow clogging, and high streamwise pressure drops that can cause flow choking as the fluid suddenly evaporates. This latter problem limits the size of the cooling device. In addition, there will be thermal resistance to the flow of heat through the fins to the heated baseplate. Typical values for heat transfer coefficient with refrigerant fluids are 3 to 5 W/cm2-C. Conventional Jet impingement cooling (
There are three other relevant two-phase phenomenon that must be listed. The first is flow in a curved channel where the concave surface is heated. Here the g-forces generated by the flow velocity on the curved heated surface tend to force bubbles to move away from the heated surface and so prevent the bubbles from blocking access of liquid to the surface. Another flow regime of interest is annular flow in a pipe (
The following prior-art patents describe specific attempts to solve he problem of high thermal flux removal.
Chu (U.S. Pat. No. 6,519,151) discloses a jet impingement thermal control device consisting of a nozzle that directs a fluid to strike perpendicular to, and at the bottom center of, a (bowl-shaped) concave conic-sectioned heated surface, so the liquid flows radially outwards along the surface of the bowl and exits the apparatus in a direction generally opposite to the incoming jet (
Rini et al. (U.S. Pat. No. 6,571,569) shows a design of an evaporative cooling system wherein the refrigeration expansion valve (nozzle) directs fluid directly against the flat plate having the heat dissipating elements on its opposite side. This approach suffers from the same problems described above in spray cooling. This patent further describes a means for a mechanical pump to force a high velocity vapor steam into the stream of liquid refrigerant to increase its velocity and cooling effectiveness. This approach adds to the weight and complexity of the cooling system.
Remsburg (U.S. Pat. Nos. 5,864,466 and 6,064,572) shows a conic-sectioned plate in a heat exchange apparatus. However, the function of the curved piece is to create a themosyphon action to direct liquid flow against a heated flat plate. The flow is then convectional to that heat transfer coefficients will be very low. Searight (U.S. Pat. No. 4,108,242) shows a means to inject fluid jets into a cylindrical cavity to induce swirling flow in general flow along the axis of the cavity. Here the heated surface has a single axis of curvature so the flow is not accelerated by motion along the curved surface nor is a thin flow film created. Lynch (U.S. Pat. No. 4,140,302) shows a water-cooled blast-furnace tuyeres nozzle having a number of liquid jets at high speed directed against the contoured inner surface of the nozzle. The jet impinges the surface at low angle to avoid momentum loss, but the curved surface shown is only to direct flow against a heated surface that is flat. Further, in this design the water passages are filled with liquid, so this arrangement does not produce a thin film liquid flow nor does the single-axis curved surface provide an acceleration of flow. Bemisderfer (U.S. Pat. No. 5,056,586) shows a spray system whereby the liquid is directed against cusp-shaped surfaces to increase turbulence. This does not produce a thin film nor accelerate flow. Tilton (20030172669) shows transverse thin-film evaporative spray cooling. The spray nozzle directs droplets down a narrow channel on whose side(s) are electronic devices to be cooled. This does not create a continuous liquid film, nor does it provide uniform cooling of the devices.
Niggeman (U.S. Pat. No. 4,643,250) shows a heat exchanger whereby a conical surface is used as a means to separate cryogenic liquid from vapor phase, and then to condense the vapor phase in a heat exchanger wherein the liquid phase is the heat sink. This is not possible since the two phases will be at the same temperature at the entrance to the apparatus.
OBJECTS AND ADVANTAGESSeveral objects and advantages of the present invention are:
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- to provide a means to direct a coolant fluid jet against a heated surface without loss of velocity or momentum after contact with the surface;
- to provide a means to enable the coolant fluid jet to form into a thin, high-velocity liquid film of consistent thickness on the heated surface to create a high value of convection heat transfer coefficient and significant increase in critical flux;
- to provide a means to maintain velocity of a cooling liquid film over a heated surface to a distance significantly longer than conventional jet impingement devices;
- to increase the effectiveness of the coolant fluid jet beyond what is available from the thermodynamic refrigeration system's expansion valve jet but without addition of a mechanical system;
- to create an equivalent of circular pipe annular flow over the heated surface to increase the heat flux removal rate.
In accordance with the present invention a coolant fluid jet directed against a doubly-curved, semi toroidal surface located in a conductive plate on whose the opposite face are thermally dissipating electronic devices.
BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
The present invention is designed to use a two-phase cooling fluid to remove high heat flux from electronics systems over a surface area that is relatively large compared with state-of-art cooling systems. Electronic system designers are now seeking cooling system for thermal fluxes greater than 1 kW/cm2 over areas of tens of square centimeters. Thermal research shows the highest heat removal rate is achieved by a two-phase fluid system wherein heat dissipating devices mounted on a conductive plate evaporate a liquid directed against the opposite side of the plate. The highest flux rates are achieved with water. However, in some cases, e.g., when the system must be dormant in freezing temperatures, it is necessary to use a volatile fluid referred to as a refrigerant (although the operating temperature of the system may be above that normally thought of as refrigeration).
Claims
1. A cooling device comprising: a thermally conductive solid such as a plate with thermally dissipating elements attached to one face and a semi-toroidal cavity in the opposite face, the cavity formed by cutting in the material face, and about an axis perpendicular to the face, a groove of generally semi-circular shape of radius r1, with the center of radius r1 at a radius C from the axis of the circle to the center of the groove, so that setting the radius r1 equal to radius C causes the semi-toroidal cavity to form an apex point that lies both on the center line of the cavity and in the plane of the plate in which the cavity is cut, so the surface is, to a distance from its centerline to the radius C, convex in a direction circumferential to the axis of the cavity and concave in a direction radial from the axis of the cavity, and past the radius C is concave both in circumferential direction and radial direction; and a second plate having a semi-toroidal protrusion in the shape of a circular ridge of semi-circular cross-section of radius r2<r1, with the center of radius r2 located at radius C from the axis of the circular protrusion, and the protrusion located concentric with and extending into the semi-toroidal cavity so a semi-toroidal channel exists between the cavity wall surface and the protrusion wall surface; and a nozzle on the centerline of the circular protrusion and parallel to its axis, passing through and sealed to the plate containing the protrusion, and containing several orifices that direct streams of a volatile fluid in a direction generally parallel to the axis of the semi-toroidal cavity to strike the upper surface of the cavity immediately below the centerline apex of its surface and in a direction generally tangent to the cavity surface, and flows outward in the semi-toroidal channel and exits the channel in a direction generally opposite that of the fluid streams directed from the nozzle.
2. The cooling device of claim 1 wherein the nozzle acts as the expansion valve in a thermodynamic cooling cycle, so that a volatile compressed liquid at subcooled temperature forced through the nozzle drops to a saturated temperature and pressure causing a fraction of the liquid to flash to vapor so a mix of cooled liquid and vapor enters the semi-toroidal channel.
3. The cooling device of claim 1, wherein the fluid striking the upper surface of the cavity directly below its centerline apex in a direction generally tangent to the surface does so virtually without loss of momentum and velocity.
4. The cooling device of claim 1 where the velocity of the liquid on the cavity surface immediately below the impact point has a relatively small component in a direction perpendicular to the axis of the cavity, so the liquid film has negligible axial velocity and so thins rapidly in this region giving the film time to coalesce into an even thickness in the circumferential dimension about the axis of the cavity by flowing in the convex direction of the surface under the Coanda effect.
5. The cooling device of claim 1 wherein the volatile liquid film flowing in the semi-toroidal channel experiences very high centripetal acceleration that forces the film against the concave cavity wall of the channel, so the liquid and vapor phases of the fluid are separated with liquid against the cavity wall and vapor between the liquid film and the semi-toroidal protrusion wall surface above it, creating a high pressure gradient in the liquid film with the minimum value equal to the saturation pressure at the liquid/vapor interface, so that when the liquid is heated by the thermally dissipating elements the boiling is biased toward the vapor/liquid interface that is at saturation pressure, and any bubbles forming on the cavity surface are immediately pushed to the liquid/vapor interface thereby increasing the efficiency of the heat transfer process.
6. The cooling device of claim 1 wherein the rate of thinning of the liquid film as it expands outwards in the channel is increased by the evaporation of liquid from the film, so the heat transfer coefficient between cavity wall and liquid film increases more rapidly and generally in inverse proportion to the thinness of the liquid film.
7. The cooling device of claim 1 wherein the vapor formed by the expansion valve process combines with the vapor formed by the energy input from the dissipating elements to expand radially outwards in the semi-toroidal channel at a great velocity relative to the velocity of the liquid film moving in the same direction, so that the vapor creates extreme turbulence in the liquid film to increase the heat transfer from the cavity wall to the liquid film, and the vapor pushes the liquid film to overcome the flow friction between wall and film so the film increases its velocity, over that rate of increase naturally afforded by the effect of centripetal acceleration on the thinning film, even when the film becomes extremely thin, and maintains this velocity to provide cooling over an area much larger than state of art jet impingement, while the centripetal acceleration forces on the liquid film prevent the film from being broken up by the vapor into mist flow that would decrease the heat transfer rate as occurs in annular pipe flow.
9. The cooling device of claim 1 wherein the expansion valve is formed by extending the refrigerant liquid supply tube over the apex of the semi-toroidal cavity until a narrow annulus is created between tube and cavity surface, at which point the tube expands orthogonally to create irreversible flow conditions in the fluid and then blends into the semi-toroidal protruding surface
10. The cooling device of claim 1 wherein the semi-toroidal protruding surface is removed so the vapor is not channeled above the semi-toroidal cavity, but the vapor emitted from the nozzle flows above and in a general direction tangential to the liquid film flowing on the surface of the semi-toroidal cavity so that some increased turbulence is created in the liquid film on the surface of the semi-toroidal cavity and heat transfer is enhanced.
11. The cooling device of claim 1 wherein the liquid refrigerant supply tube has an orifice upstream of the nozzle, acting to create irreversible flow to cause a small amount of liquid refrigerant to flash to vapor so that the volume flow rate through nozzle directed at the semi-toroidal cavity is greatly increases to increase the heat transfer of the system.
12. A cooling device wherein a flow channel with a rectangular cross-section is located below a surface on which is mounted a generally elongated thermally dissipating device, with the entrance and exits of the channel in directions generally orthogonal to the plane on which the dissipating devices are located and the connecting channel curved in a convex direction toward the dissipating elements, the wall of the channel in opposition to the dissipating devices being initially convex in the cross-sectional view at the entrance, changing to flat and then concave with increasing distance along the channel; and a nozzle sealed around the entrance to the channel and directing a jet of volatile liquid into entrance of the channel.
13. The cooling device of claim 12 wherein the a jet of volatile liquid strikes the convex surface at the entrance to the channel in a direction generally tangential to the surface, spreads out in the convex direction of the surface to form a film of even thickness and maintains an even thickness by means of the gradual change of the cross-sectional shape of the surface from convex to concave, the liquid film being held against the wall by the centripetal force caused by the concave shape of the shape in the flow direction.
14. A cooling device wherein a number of semi-toroidal channels and nozzles are located in a face of a conductive plate to increase the effective heat-gathering area of the apparatus.
15. The cooling device of claim 14 wherein cooling channels are composed of a number of alternately placed wedge-shaped sections of the semi-toroidal surface so that heat may be evenly absorbed from a large rectangular surface, and the dividing walls between the alternately placed channels act as structural support ribs in the plate to allow the thickness of the plate between the thermally dissipating elements and the cavity surfaces to be minimized to enhance heat transfer.
Type: Application
Filed: Feb 21, 2006
Publication Date: Jul 27, 2006
Inventors: Triem Hoang (Clifton, VA), Michael Brown (Columbia, MD)
Application Number: 11/256,007
International Classification: F25D 23/12 (20060101); F28D 5/00 (20060101);