Immersion cooling of electronic systems by controlled injection of air bubbles

Abstract

The present invention relates to a method and equipment for the immersion cooling of an electronic component in a dielectric liquid contained in a chamber (1), characterized in that said electronic component (11) is thermally coupled with a heat sink (16) having fins (17) optimized to extract the heat from the electronic components in order to transfer it to the liquid, the entire assembly being immersed in the dielectric liquid, and in that air bubbles are injected from the lower part of the chamber towards the heat sink (16).

Description

Cooling of electronic systems in immersion by controlled injection of air bubbles Field of invention

[0001] The present invention relates to the field of cooling electronic circuits using a cooling liquid (in English "immersion cooling").

[0002] Cooling computer systems is becoming a major issue for many applications in telecommunications, data management, and IT security. Indeed, the power densities to be dissipated are increasing due to the miniaturization of computer components and the increase in computing power. Furthermore, the acceptable temperature limit for a computer processor chip remains constant, around 85°C. Conventional cooling by air circulation is therefore reaching its limits for many applications requiring significant resources and, consequently, requires a high electrical load dedicated to cooling. Cooling by immersion in a dielectric liquid is an alternative to air cooling, which is more expensive but more effective.

[0003] The development of an electrically non-conductive or dielectric fluid enables the use of immersion cooling, in which computer and other electronic components can be immersed to transfer heat directly from the component to the liquid. Immersion cooling can be used to reduce the total energy required to cool computer components and can also reduce the amount of space and equipment required for adequate cooling.

[0004] Immersion cooling involves immersing all computer components (motherboard, graphics card, processor, RAM, etc.) in a dielectric fluid, i.e. an electrically insulating fluid. This technology allows for better cooling of components than air cooling due to the greater heat capacity of liquids. The evacuated flow densities are around 20 to 50 times greater than with conventional air cooling. On the other hand, these fluids are viscous and in order to reduce the energy consumption of cooling systems, the power linked to the movement of the fluid must be as low as possible. State of the art

[0005] Patent US20140321053A1 describes an apparatus and method for regulating the temperature of electronic or optical devices immersed in a liquid using a bubble introduction system and metal tubes for gas flow. It comprises a reservoir designed to contain a volume of liquid, with metal walls for improved thermal conduction.

[0006] A heat transfer fluid is used to immerse electronic or optical devices. The fluid can be water or a dielectric fluid such as R134a. Located at the bottom of the tank, injectors are configured to inject gas bubbles into the fluid. These rising bubbles mix the fluid and improve heat transfer. The injectors are designed to produce large-diameter bubbles (3 to 8 mm), which allows for more efficient mixing of the fluid and the breakdown of the fluid boundary layers upon contact with the surfaces of the immersed device.

[0007] The heat transfer system includes an external structure formed by thermally conductive fins attached to the outer walls of the tank and piezoelectric fans to force air to circulate along the fins, thus improving heat dissipation. It also includes an internal structure formed by metal tubes passing through the tank and immersed in the liquid. These tubes carry a flow of gas that can be heated or cooled, allowing the temperature of the liquid to be regulated.

[0008] A pump forces the gas flow through the metal tubes. In some configurations, a hermetically sealed chamber at the top of the tank and a controllable membrane can be used to recycle the gas from bubbles bursting on the liquid surface, forming a closed system.

[0009] Electronic Control:

[0010] A controller can be used to monitor and stabilize the liquid temperature. This controller adjusts the operation of the external and internal heat transfer systems based on temperature measurements.

[0011] Electronic or optical devices are immersed in the liquid contained in the tank.

[0012] During operation of these devices, gas bubbles are injected into the lower part of the tank. The gas bubbles rise through the liquid, mixing it and disrupting the thermal boundary layers, which improves heat transfer between the liquid and the devices. The temperature of the liquid is regulated by circulating gas through the metal tubes immersed in the liquid and / or by circulating air along the external thermally conductive fins. Temperature control can include heating or cooling operations as needed, and is managed by the electronic controller. This prior patent application proposes a solution for regulating the temperature of immersed electronic or optical devices, by combining the introduction of gas bubbles and heat transfer systems.

[0013] European patent EP3229103 describes a cooling system for electronic devices that uses a liquid immersion cooling method. The system includes a cooling tank having an open space defined by a bottom wall and side walls. Internal partitions divide the open space into multiple aligned storage sections, each capable of holding an electronic device. Each storage section has an inlet and an outlet opening for the coolant. The inlet opening is located at the bottom or on a side surface of each section, while the outlet opening is located near the liquid level. Inlet and outlet pipes pass through the bottom wall of the tank and extend to near the liquid level.The inlet pipes are equipped with several nozzles along their length to inject coolant into the storage sections. The system uses a pump to force the coolant to circulate and a heat exchanger to cool the heated liquid before it returns to the reservoir.

[0014] Patent application WO2020100816A1 describes another example of a device for cooling electronic circuits immersed in a coolant, with a bubble-emitting porous tubular body immersed in the coolant. The components are mounted in the chamber to be bathed by the coolant. A gas-in-liquid discharge mechanism, for example a structure powered by a pump, is connected to a tubular body or a porous body having small holes to form bubbles. The action of the bubbles causes circulation of the fluid in the chamber, which circulates the fluid around the electronic components and around the chamber. Cooling fins mounted outside the chamber dissipate the heat generated by the components. The bubble-guiding tubes guide the bubbles upward, and the fluid confined in the tubes between the bubbles is displaced (pushed) upward. Disadvantages of the prior art

[0015] Most of the prior art solutions provide for the formation of bubbles by local vaporization of the coolant, which is not satisfactory because it induces local hot spots.

[0016] The solutions presented in the prior art present a multiplicity of elements placed in the tank, which solution presents a technical problem resulting from the thermal losses caused by the different heat exchange interfaces and a great complexity in manufacturing and assembly.

[0017] Application WO2020100816A1 proposes a solution for cooling computer systems (Computers, Servers) by liquid immersion with air injection by a bubbler. The bubbles move vertically in the cooling liquid, erratically and only exceptionally interact with the surfaces of the electronic circuit immersed in the dielectric cooling liquid. Solution provided by the invention

[0018] In order to overcome these drawbacks, the present invention relates to a method having the characteristics set out in claim 1.

[0019] It relates to a method for cooling electronic components by connecting them to a heat sink optimized to extract heat from the electronic components and transfer it to the liquid. The assembly is then immersed in a bath of dielectric liquid, where the electronic components can be positioned vertically, inclined, or horizontally, and in that bubbles are further introduced from the lower part of said chamber towards said heat sink.

[0020] Preferably, the control of the production of bubbles is done in a varied manner, either continuously or intermittently. In continuous mode, the air supply is constant. For intermittency, i.e. non-continuously, alternating periods of generation of air bubbles with periods where the generation of air bubbles is interrupted.

[0021] In other words, the bubble injection cycle alternates sequences of bubble introduction for N seconds and periods of stopping the introduction of bubbles for PxN seconds, N being between 0.3 and 0.8 and P being between 2 and 6.

[0022] The invention also relates to equipment for cooling an electronic component by immersion in a dielectric liquid contained in a chamber, characterized in that it further comprises a heat sink thermally linked to said electronic component immersed in the dielectric liquid, said heat sink is designed to have a shape optimized for the heat exchange process, having fins arranged in vertical planes, and in that said chamber has a bottom provided with at least one bubbler in the lower part of said chamber, vertical to said heat sink.

[0023] Preferably, said bubbler is powered by a pump controlled for sequential operation.

[0024] According to a variant, the equipment according to the invention comprises a cooling circuit for the dielectric liquid comprising:a first liquid-liquid exchanger immersed in the chamber and in the dielectric liquid contained in this chambera heat exchanger, in the example described a water-air exchangera circulation pump, ensuring the circulation of a heat transfer liquid in this cooling circuit.Detailed description of a non-limiting example of embodiment

[0025] The present invention will be better understood on reading the following description, concerning a non-limiting example of embodiment illustrated by the appended drawings where:

[0026] represents a schematic front view of an alternative embodiment

[0027] represents a schematic top view of an alternative embodiment

[0028] represents the time evolution curve of air bubble injection over 30s

[0029] represents the time evolution curve of the processor temperature with and without the introduction of air bubbles

[0030] represents the time evolution curve of the convective exchange coefficient between the processor and the fluid General principle of the invention

[0031] The following description illustrates a non-limiting example of a system for cooling computer systems (computers, servers) by liquid immersion with air injection. This objective is particularly in line with the objective of reducing the electricity consumption dedicated to cooling computer components in companies.

[0032] The invention mainly differs from the prior art by the introduction of air bubbles into the bottom of the chamber for immersing the electronic components in the dielectric liquid which rise to the surface by the effect of Archimedes' thrust, the air inside the bubbles having a much lower density than the liquid in which the bubbles form.

[0033] These bubbles rise to the surface and interact with electronic component fins (11) with a heat sink (16) having fins (17) of a heat sink (16) thermally coupled with the electronic component (11). These fins (17) are arranged to present an exchange surface on the path of the air bubbles generated in the lower part of the chamber, in order to extract the heat from the electronic components to transfer it to the dielectric liquid in which the assembly is immersed.

[0034] These bubbles should not be confused or equated with vaporization bubbles of the liquid phase in contact with hot spots as in the prior art. Thanks to the buoyancy effect of the bubbles and the wetting effect when the bubbles pass through the heat sink and come into contact with the solid surface of the fins (17), they contribute to effectively breaking the thermal boundary layer, thus significantly increasing the efficiency of the heat exchange.

[0035] To reduce the number of thermal interfaces ensuring the heat transfer between the liquid, the cooling fins and the electronic components, and to improve the heat exchanges, the invention provides for: directly thermally coupling the electronic component with a heat sink having fins, immersing the assembly in a dielectric liquid, introducing bubbles from the lower part towards the heat sink. These fins (17) can be arranged vertically or slightly inclined to be flush with the bubbles rising to the surface. They can also have a curved or irregular exchange surface to increase the interactions with the bubbles and promote the disturbance of the boundary layer.

[0036] The bubbles rise from the bottom, moving through the heat sink and heat exchanger. This movement causes the liquid to follow behind the bubble. This phenomenon creates a flow that disrupts the thermal boundary layer, thus improving heat exchange. In the areas where the bubble passes, the surface temperature of the heat sink is significantly lower than in other areas. The bubble also creates a low-pressure region behind it, which forces the surrounding liquid to move, generating a convection current in the bubble's wake.

[0037] By arranging multiple vertical fins (17) in parallel, and distributing the bubble injection points to ensure that the bubbles pass evenly to all areas of the heat sink or significantly increases the overall heat exchange compared to the single fin column configuration. Description of an example of architecture

[0038] Figures 1 and 2 represent front and top views respectively of electronic circuit cooling equipment according to the invention.

[0039] It comprises a chamber (10) optionally associated with a cooling circuit for the dielectric liquid comprising: a first liquid-liquid exchanger (15) immersed in the chamber (10) and in the dielectric liquid contained in this chamber (10) a heat exchanger (20), in the example described a water-air exchanger a circulation pump (25), ensuring the circulation of a heat transfer liquid in this cooling circuit.

[0040] The chamber (10) consists of a tank containing a dielectric liquid (2) as well as the first liquid-liquid heat exchanger (15).

[0041] The electronic component(s) (11) are associated with a heat sink (16) formed in the example described by fins (17) extending a base (12) thermally associated with the electronic component (11), for example via a thermal paste.

[0042] The electronic component(s) (11) and the heat sink (16) are immersed in the dielectric liquid (2).

[0043] The dielectric liquid (2) is a thermally conductive fluid, but electrically substantially non-conductive or substantially dielectric. It is necessary for the immersion fluid to have two very specific physical properties: The fluid must be dielectric over the operating voltage range of the components that will be immersed in it, that is to say it must be insulating in order to prevent short circuits. The fluid must have suitable thermo-physical properties to be a good heat transfer fluid (high thermal capacity, good thermal conductivity, etc.).

[0044] Examples of such fluids include demineralized water, fluorocarbon fluids and mineral or synthetic oils, and more particularly technical fluids of the EC-110 type TM of EngineeredFluids or SHELL™ S3X or S5X, although the disclosed invention is not limited to any particular dielectric fluid.

[0045] For example, the thermophysical properties of EC-110 fluid TM at 20°C are as follows:

[0046] To avoid using harmful dielectric fluids such as fluorocarbon liquids, the invention aims to improve the cooling efficiency by the formation of bubbles injected from the bottom (12) to circulate vertically on the surface of the heat sink (16) in order to disturb the formation of the thermal boundary layer by turbulence produced near the heat exchange surfaces.

[0047] Depending on the nature and especially the viscosity of the dielectric liquid, the tank comprises a circulation pump injecting the dielectric liquid through nozzles arranged on the bottom (12), preferably below the bubble injection points. For example, the dielectric liquid (2) is injected through the bottom (12) having one or more inlets (13) with a diameter of approximately 4 mm, with a speed of approximately 5 cm / s.

[0048] The introduction of bubbles is done through an inlet (13) of 2mm with a speed of 1m / s for 0.5s every 2s, according to a sequence illustrated by the. The speed of the liquid at the entrance of the domain is 5cm / s while air is injected periodically with a speed of 1m / s. The temperature of the two fluids is 300K.

[0049] The introduction of gas bubbles is done by a bubbler (18) powered by an air compressor. The bubbler (18) is made up for example of porous bars or perforated membranes ensuring the formation of bubbles on the wall of a sheared flow. Sequencing of bubble introduction

[0050] Lamontre shows the temporal evolution of the average value of the processor temperature. Without the introduction of air bubbles (curve 5), we first observe a slight cooling then a rise in temperature until reaching approximately 54°C while the presence of air (curve 6) allows a rapid cooling of 10°C in 15s then slower reaching 13°C after 30s. The weak cooling observed initially without injection is probably due to the establishment of the thermal boundary layer. Indeed, in the first moments the hot processor is in direct contact with the colder liquid which creates a significant heat flow leaving the CPU and which is not compensated by the heat generation of the latter. On the other hand, as the fluid heats up and the thermal boundary layer is established, the heat transfer between the processor and the fluid decreases and becomes weaker than the internal heat generation and therefore the CPU heats up.To ensure this, the convective exchange coefficient, defined by equation 1, is represented as a function of time on the.

[0051] (1)

[0052] Or , T CPU and T f represent the heat flux density at the processor surface, the processor surface temperature and the average fluid temperature respectively.

[0053] Without injection, the convective exchange coefficient curve is characteristic of the establishment of the thermal boundary layer around the processor to reach a value of approximately 30W / m²K after 30s. On the other hand, with the introduction of bubbles, significant fluctuations are noted but above all, it is clearly visible that the thermal boundary layer does not have time to establish itself and is constantly destroyed. Consequently, the average convective exchange coefficient stagnates around approximately 400W / m²K, a gain of a factor of 13 compared to cooling without injection.

[0054] Sequential bubble injection generates turbulence and prevents the formation of a thermal boundary layer around the processor, thus improving its cooling.

Claims

Method for cooling an electronic component by immersion in a dielectric liquid contained in a chamber (1) characterized in that air bubbles are introduced from the lower part of said chamber towards said heat sink (16), in that said electronic component (11) is thermally coupled with a heat sink (16) having fins (17) arranged to present an exchange surface on the path, under the effect of Archimedes' thrust, of the air bubbles generated in the lower part of the chamber. Method for cooling an electronic component according to claim 1, characterized in that the generation of air bubbles is controlled intermittently. Method for cooling an electronic component according to claim 2, characterized in that the air bubble injection cycle alternates sequences of introducing air bubbles for N seconds and periods of stopping the introduction of air bubbles for PxN seconds, N being between 0.3 and 0.8 and P being between 2 and 6. Equipment for cooling an electronic component by immersion in a dielectric liquid contained in a chamber, characterized in that it further comprises a heat sink (16) thermally linked to said electronic component immersed in the dielectric liquid, said heat sink (16) having fins (17) arranged to present an exchange surface on the path, under the effect of Archimedes' thrust, of the air bubbles generated in the lower part of the chamber, and in that said chamber (1) has a bottom provided with at least one bubbler (18) in the lower part of said chamber, vertically above said heat sink (16). Electronic component cooling equipment according to claim 4 characterized in that said bubbler (18) is supplied by a pump controlled for sequential operation. Electronic component cooling equipment according to claim 4 characterized in that it comprises a dielectric liquid cooling circuit comprising: a first liquid-liquid exchanger (15) immersed in the chamber (10) and in the dielectric liquid contained in this chamber (10) a heat exchanger (20), a circulation pump (25), ensuring the circulation of a heat transfer liquid in this cooling circuit.

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