1. Field of the Invention
A closed-looped heat exchanger assembly for cooling an electronic device.
2. Description of the Prior Art
All electronic devices generate waste heat. Dissipation of that heat is necessary for the optimum and reliable operation of electronics. With increasing power density of micro-electronic devices, dissipation of the heat load becomes a critical factor in overall system design. Available approaches for electronics cooling include air-cooling, liquid-cooling, and refrigeration.
As an example, for cooling of computer microprocessors, forced air-cooling of finned heat sinks attached to the processor package has been sufficient for several years. However, the relentless increase in processor speed and corresponding heat dissipation has made air-cooling impractical from the standpoint of heat sink size and the noise associated with forced air convection, e.g., fans. Liquid cooling can meet or exceed the power dissipation demands for present and future computer systems. Approaches include forced liquid flow through heat sinks attached to electronic device packages, and the use of thermosiphons that incorporate a closed liquid/vapor flow system.
Heat pipes are attractive since they do not require any mechanical pump, either external or internal, thus reducing size, cost, and noise. They also can accommodate high heat loads since they use the large latent heat of vaporization of the working fluid, and thus “pin” the device temperature to the vapor saturation temperature. However, the maximum power dissipation is limited by the rate of condensing the vapor and returning liquid to the vaporization area to repeat the cycle. The liquid return rate usually uses gravity, wicking or capillary structures that are limited by orientation and flow rate.
Examples of pulsating heat exchangers are included in an article entitled “Passive Oscillatory Heat Transport Systems” by Weislogel, and an article entitled “Thermal Modeling of Unlooped and Looped Pulsating Heat Pipes” by Shafrii et al.
The Shafrii et al. article discloses a method for determining the movement of a liquid plug in an open heat pipe and in a closed heat pipe. Shafrii discloses a first liquid plug having a first sum of forces acting thereon and a second liquid plug having a second sum of forces acting thereon but fails to disclose a given relationship between the first and second sums of forces.
The Weislogel article discloses four pulsating systems: the Tamburini's T-System, the Akachi's Pulsating Heat Pipe, the Weislogel's Pulse Thermal System, and the Cargille's Thermal Transport Oscillator.
The T-System includes a condenser, an evaporator, and a reservoir. A vapor conduit interconnects the evaporator and condenser. A filling conduit interconnects the evaporator and the condenser. The reservoir is disposed along the filling conduit between the evaporator and condenser. A first valve is disposed along the filling conduit between the evaporator and the reservoir and a second valve is disposed along the filling conduit between the condenser and the reservoir for causing the circulation of the refrigerant.
The Pulsating Heat Pipe teaches an evaporator and a condenser and a closed looped conduit winding back and forth between the evaporator and the condenser. A plurality of liquid plugs each having a sum of forces acting thereon are enclosed in the conduit. The heat pipe relies on capillary forces in order to oscillate each liquid plug back and forth between the evaporator and condenser.
The Pulse Thermal System includes a plurality of evaporators in series with a plurality of condensers broken up by two way valves and check valves in order to produce a one way self pumping acting. The Thermal Transport Oscillator discloses an evaporator connected to a three way valve. Two condensers are connected in parallel between the evaporator and the three way valve. The Thermal Transport Oscillator produces a self pumping action via the three way valve which alternates between the two condensers.
Although the prior art discloses self pumping heat exchanger systems, there remains a need for a self pumping system that does not rely on capillary forces or on check valves, and the like, in order to produce the one way self pumping action.
The invention provides for a closed looped heat exchanger comprising a refrigerant for undergoing liquid-to-vapor cyclical transformations and an evaporator attached to the electronic device for evaporating the refrigerant by transferring heat from the electronic device to the refrigerant. A vapor conduit having a first cross sectional area defines a flow path for the refrigerant exiting the evaporator. A reservoir holds the refrigerant and has an inlet and an outlet. A filling conduit having a second cross sectional area defines a flow path for refrigerant entering the evaporator. A first liquid plug of the refrigerant is disposed in the vapor conduit and has a first sum of forces acting thereon. A second liquid plug of the refrigerant is disposed in the filling conduit and has a second sum of forces acting thereon. The first sum of resistive forces acting on the first liquid plug is less than the second sum of resistive forces acting on the second liquid plug for creating a head pressure during expansion of the refrigerant causing the refrigerant to expand more into the vapor conduit than into the filling conduit during vaporization of the refrigerant. The refrigerant in the evaporator evaporates and expands into the filling conduit and into the vapor conduit and thereafter condenses in the filling conduit and the vapor conduit causing additional refrigerant to enter the evaporator from the filling conduit. The additional refrigerant causes a cyclical expansion of the refrigerant into the vapor conduit to a larger extent than into the filling conduit for continuously circulating the refrigerant in the same direction through the assembly.
The present invention entails a cooling system that advances the pulsating heat pipe principle by incorporating the ability to self pump coolant vapor and hot coolant liquid at the same time. Because the system does not require a pump its weight, size, and cost are reduced. Furthermore, conduits having a larger diameter can be used for conveying the working fluid to and from the evaporator because the system does not rely on capillary forces. This can result in higher heat dissipation within a simplified package.
Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:
Referring to the Figures, wherein like numerals indicate corresponding parts throughout the several views, a closed-looped heat exchanger assembly 20 is generally shown for cooling an electronic device 22.
A refrigerant 24 is disposed in the assembly 20 for undergoing liquid-to-vapor cyclical transformations. As shown in
The assembly 20 further includes a reservoir 30 for holding the refrigerant 24. The reservoir 30 includes at least one inlet 32 and at least one outlet 34. The refrigerant 24 defines a surface 36 in the reservoir 30. The inlet 32 of the reservoir 30 is disposed above the surface 36 of the refrigerant 24 and the outlet 34 of the reservoir 30 is disposed below the surface 36 of the refrigerant 24.
The evaporator 26 is also disposed below the surface 36 of the refrigerant 24 in the reservoir 30. Although, the assembly 20 can operate with the evaporator 26 at or above the surface 36 of the refrigerant 24, it is preferable to have the evaporator 26 below the surface 36 to allow the force of gravity to assist the circulation of the refrigerant 24.
A vapor conduit 38 defines a flow path for refrigerant 24 exiting the evaporator 26. The vapor conduit 38 has a first cross sectional area Avc and is made of a material having a first high thermal conductivity. The first cross sectional area Avc of the vapor conduit 38 is circular but can be of various other shapes such as a ring, as shown in
A filling conduit 40 defines a flow path for the refrigerant 24 entering the evaporator 26. The filling conduit 40 has a second cross sectional area Afc and is made of a material having a second high thermal conductivity. The second cross sectional area Afc is circular but can be of various other shapes, including a plurality of shapes as shown in
As shown in
In the embodiments shown in
In the embodiment shown in
In the embodiments shown in
In
In
In two embodiments, respectively shown in
The embodiment in
A first liquid plug 54 of the refrigerant 24 is disposed in the vapor conduit 38 and has a first sum of forces Fvc acting thereon. A second liquid plug 56 of the refrigerant 24 is disposed in the filling conduit 40 and has a second sum of forces Ffc acting thereon.
As illustrated in
The assembly 20 is distinguished by the first sum of forces Fvc acting on the first liquid plug 54 being less than the second sum of forces Ffc acting on the second liquid plug 56 for creating a head pressure during expansion of the refrigerant 24. The forces Ffc, Fvc cause the refrigerant 24 to expand more into the vapor conduit 38 than into the filling conduit 40 during vaporization of the refrigerant 24 causing continuous circulation of the refrigerant 24 in the same direction.
The first sum of flow restrictive forces Fvc acting on the first liquid plug 54 can be determined using the following equations one through five:
F
vc
=[F
g]vc+[Fτ]vc+[Fσ]vc+[Fe]vc (1)
[Fg]vc=ρvcghvc (2)
[Fτ]vc=πrvclvcCfvcvvc2 (3)
[F
σ]vc=2σvcAvc/rvc cos θvc (4)
[Fe]vc=0.5ρvcAvcKvcvvc2 (5)
wherein ρvc is the density of the first liquid plug 54, hvc is the height of the reservoir 30 above the first liquid plug 54, g is the gravitational constant, rvc is the radius of the vapor conduit 38, and lvc is the length of the first liquid plug 54 in the vapor conduit 38. Cfvc is the coefficient of friction between the refrigerant 24 and the vapor conduit 38, vvc is the velocity of the first liquid plug 54 into the vapor conduit 38, and σvc is the surface 36 tension of the first liquid plug 54 in the vapor conduit 38. Avc is the first cross sectional area of the vapor conduit 38, cos θvc is a geometric factor of the first liquid plug 54 found experimentally, and Kvc is the expansion loss coefficient for the inlet 32 of the reservoir 30.
The second sum of flow restrictive forces Ffc acting on the second liquid plug 56 can be determined using the following equations six through ten:
F
fc
=[F
g]fc+[Fτ]fc+[Fσ]fc+[Fe]fc (6)
[Fg]fc=ρfcghfc (7)
[Fτ]fc=πrfclfcCffcvfc2 (8)
[F
σ]fc=2σfcAfc/rfc cos θfc (9)
[Fe]fc=0.5ρfcAfcKfcvfc2 (10)
wherein ρfc is the density of the second liquid plug 56, hfc is the height of the reservoir 30 above the second liquid plug 56, rfc is the radius of the filling conduit 40, and lfc is the length of the second liquid plug 56 of the filling conduit 40. Cffc is the coefficient of friction between the refrigerant 24 and the filling conduit 40, vfc is the velocity of the second liquid plug 56 into the filling conduit 40, and σfc is the surface 36 tension of the second liquid plug 56 in the filling conduit 40. Afc is the second cross sectional area, cos θfc is a geometric factor of the second liquid plug 56 found experimentally, and Kfc is the expansion loss coefficient for the outlet 34 of the reservoir 30.
Preferably, the first cross sectional area Avc of the vapor conduit 38 is greater than the second cross sectional area Afc of the filling conduit 40, as shown in
In operation, heat is transferred from the electronic device 22 to the evaporator 26 causing the refrigerant 24 to boil. The refrigerant 24 evaporates in the evaporator 26 forming the first liquid plug 54 in the vapor conduit 38 and the second liquid plug 56 in the filling conduit 40 as the vapor expands into the filling conduit 40 and vapor conduit 38. The expanding vapor volume pushes the first liquid plug 54 in the vapor conduit 38 and the second liquid plug 56 in the filling conduit 40. The refrigerant 24 expands into the filling conduit 40 and into the vapor conduit 38. The first sum of forces Fvc acts on the first liquid plug 54 less than the second sum of forces Ffc acting on the second plug during the expanding stage. After the refrigerant 24 enters the filling conduit 40 and the vapor conduit 38, the heat is transferred from the refrigerant 24 to the respective conduits 38, 40 and thereafter dissipated causing the refrigerant 24 to condense and contract back into the evaporator 26. Contraction of the refrigerant 24 causes additional refrigerant 24 from the filling conduit 40 to enter the evaporator 26. The additional refrigerant 24 evaporates upon making contact with the hot surfaces 36 of the evaporator 26 and once again expands causing a cyclical expansion of the refrigerant 24. Because the first flow restrictive forces Fvc acting on the first liquid plug 54 in the vapor conduit 38 are less than the second flow restrictive forces Ffc acting on the second liquid plug 56 in the filling conduit 40, the refrigerant 24 expands into the vapor conduit 38 to a larger extent than into the filling conduit 40 causing a continuous circulation of the refrigerant 24 in the same direction through the assembly 20, thereby eliminating the need for a mechanical pump or valves.
While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.