Cooling of immersed electronic systems by controlled injection of air bubbles

Abstract

The present invention relates to a method and equipment for cooling an electronic component by immersion in a dielectric liquid contained in a chamber (1), characterized in that said electronic component (11) is thermally coupled to a heat sink (16) having fins (17) arranged in vertical planes, the assembly being immersed in said dielectric liquid, and in that bubbles are further injected from the lower part of said chamber towards said heat sink (16). Abstract figure: Figure 2

Description

Title of the invention: 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 even computer security. Indeed, the power densities to be dissipated are increasing due to the miniaturization of computer components and the increase in computing power. On the other hand, 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 constitutes 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 components 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 consists of immersing all computer components (motherboard, graphics card, processor, RAM, etc.) in a dielectric fluid, i.e. electrically insulating. This technology allows for better cooling of the components than air cooling due to the greater heat capacity of the 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] Immersion cooling for electrical systems has been known for almost 150 years. Indeed, at the end of the 19th century, the first transformers Electrical components were immersed in oil to cool them. General interest in immersion cooling for computer systems grew in the mid-2000s when the heat dissipated by computer processors approached 1 kW / m2. There are currently two immersion cooling technologies: with or without phase change. In the first case, the dielectric fluid remains in the liquid state, while in the second, the liquid, whose boiling point is low (between 40 and 70°C), evaporates and is then condensed to form a cycle. Immersion cooling without phase change is limited to convective exchange coefficients of the order of 2 kW / m2.K, while phase change allows for better cooling but can lead to hydrodynamic instabilities.On the other hand, the fluids used in the latter case are per- and polyfluoroalkyl compounds whose use risks being severely limited by 2025 following the submission of a restriction proposal by the European Chemicals Agency.

[0006] Dielectric liquid cooling, also called liquid cooling, generally relies on circulating a dielectric liquid through a cooling system to dissipate heat from electronic components.

[0007] In dielectric liquid cooling systems, heat is transferred from the electronic component to the dielectric liquid by convection and then dissipated to the outside of the system using a radiator and a fan. This process generally takes place without the addition of air bubbles.

[0008] However, it is important to note that dielectric liquid cooling already offers superior efficiency compared to traditional air cooling in electronic systems. It typically uses solutions such as radiators and pumps to ensure efficient heat dissipation, which can lead in some cases to excess power consumption.

[0009] Patent application US20220151097A1 from GREEN REVOLUTION COOLING provides a coolant distribution unit, a coolant manifold, a supply and return line, and one or more immersion cooling grids. The coolant distribution unit may be configured to adjust a temperature and pump a fluid used as a coolant. The coolant manifold may redistribute the fluid. The immersion cooling grids may be disposed between the coolant distribution unit and the coolant manifold. Each immersion cooling grid may be coupled to the coolant manifold via an inlet conduit to receive fluid from the coolant manifold and an outlet conduit to return the fluid to the manifold. of coolant.

[0010] European patent EP2430893B1 from Iceotope describes a cooling module comprising a housing, a heat transfer device having a conduction surface, the housing and the conduction surface together defining a volume in which a first coolant can be placed, the heat transfer device further defining a channel for receiving a second coolant, the conduction surface separating the volume and the channel to allow conduction of heat between the volume and the channel through the conduction surface; and electronic components located in the volume. At least a portion of the conduction surface or the housing is shaped to the shape of the electronic components.

[0011] Multiple solutions for cooling electronic components by immersion in a circulating dielectric liquid are known in the state of the art.

[0012] Patent US6992888B1 describes a cooling module formed by a cold plate on which semiconductors are mounted and closed by a cover to form a chamber. The cooling of the semiconductors is ensured by a dielectric liquid, for example a mixture of fluorocarbons with different boiling points, circulating in the chamber. The liquid has a boiling point slightly lower than the temperatures of the "hot" points, in order to form small bubbles during operation, which improves the convection flow.

[0013] This document specifies that adjusting the boiling point of the thermally conductive liquid occupying the cooling cavity has the advantage of producing bubbles which "agitate" the liquid and generate a convection movement of the liquid from the hot spots to the surfaces of the cold plate exposed to the liquid. This convection movement is added to the heat movement provided by the dielectric liquid. The dielectric liquid uses the formation of small bubbles to improve convection in order to improve the heat transfer compared to that provided by the thermal conduction of the liquid taken alone.

[0014] Patent application WO2020100816A1 describes another example of a device for cooling electronic circuits immersed in a coolant, with a porous tubular body for emitting bubbles immersed in the coolant. The components are mounted in the chamber to be bathed by the coolant. A mechanism for discharging gas into a liquid, 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 a circulation of the fluid in the chamber, which allows the fluid to circulate around the electronic components and around the chamber. Cooling fins mounted outside the chamber dissipate the heat generated by the components. The bubble guide tubes guide the bubbles upward, and the fluid confined in the tubes between the bubbles is displaced (pushed) upward.

[0015] Patent application US20120247737A1 describes a cooling chamber for electronic components formed by a conductive chamber containing a low boiling point fluid.

[0016] US4050507A describes another example of transferring heat from the walls of an electronic unit such as a semiconductor chip or wafer by determining the required heat transfer characteristics of the units; drilling holes of a predetermined size and location in the walls of the unit with a high energy beam in accordance with the required heat transfer characteristics of the unit; immersing the unit in a suitable dielectric coolant such that nucleate boiling begins at approximately the desired wall temperature to achieve the required heat transfer from the unit, thereby providing a uniform wall temperature.Another aspect is the inclusion of a heating means located at or near the bottom of the heat transfer wall so that bubble generation can be started there, which bubbles wash liquid from downstream drilled holes or natural nucleation sites, thereby starting nucleate boiling when the required temperature is reached.

[0017] Disadvantages of the prior art

[0018] Most of the solutions of the prior art provide for the formation of bubbles by local vaporization of the coolant, which is not satisfactory because it induces local hot spots.

[0019] 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.

[0020] Solution provided by the invention

[0021] In order to overcome these drawbacks, the present invention relates, in its most general sense, to a method for cooling an electronic component by immersion in a dielectric liquid contained in a chamber, characterized in that said electronic component is thermally coupled with a heat sink having fins arranged in vertical planes, the assembly being immersed in said dielectric liquid, and in that bubbles are further injected from the lower part of said chamber towards said heat sink.

[0022] Preferably, the generation of bubbles is controlled intermittently.

[0023] Advantageously, the bubble injection cycle alternates injection sequences of bubbles for N seconds and periods of stopping the injection of bubbles for PxN seconds, N being between 0.3 and 0.8 and P being between 2 and 6.

[0024] 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 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.

[0025] Preferably, said bubbler is powered by a pump controlled for sequential operation.

[0026] 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 chamber - a heat exchanger, in the example described a water-air exchanger - a circulation pump, ensuring the circulation of a heat transfer liquid in this cooling circuit. Detailed description of a non-limiting example of embodiment

[0027] 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:

[0028] [Fig-1] [Fig.l] represents a schematic front view of a variant of the sheave lization

[0029] [Fig.2] [Fig.2] represents a schematic top view of a variant of the sheave lization

[0030] [Fig.3] [Fig.3] represents the time evolution curve of bubble injection of air for 30s

[0031] [Fig.4] [Fig.4] represents the time evolution curve of the temperature of the processor with and without air bubble injection

[0032] [Fig.5] [Fig.5] represents the time evolution curve of the coefficient convective exchange between the processor and the fluid

[0033] General principle of the invention

[0034] 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 part of the objective of reducing the electrical consumption dedicated to cooling computer components in companies.

[0035] The invention mainly concerns the method of injecting bubbles, which does not result not vaporization of the liquid phase in contact with hot points as in the prior art, but the injection of air bubbles into the bottom of the immersion chamber of the electronic components in the dielectric liquid, preferably vertically above cooling fins thermally associated with the electronic circuits, and preferably with a sequential release of bubbles.

[0036] By associating the electronic circuit with a heat sink, the assembly being immersed in the dielectric liquid, the effect of the bubbles which break the formation of the thermal boundary layer is significantly improved, thus improving the efficiency of the thermal exchanges with the dielectric fluid.

[0037] 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) is constituted by 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 vertical 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 which 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 fluids techniques such as EngineeredFluids' EC-110™ or SHELL™ S3X or S5X, although the disclosed invention is not limited to any particular dielectric fluid.

[0045] As an example, the thermophysical properties of EC-110™ fluid at 20°C are as follows: [Tables 1] Vdumique mass p Cspadiê thei'mique Viscosity. Conductivity màs&squeCp dynamtqu&p (kg-W) $ SK MO 21ÔÔ 0.008 0.136

[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 injection of bubbles is done through an inlet (13) of 2mm with a speed of Im / s for 0.5s every 2s, according to a sequence illustrated by [Fig.3]. The speed of the liquid at the entrance of the domain is 5cm / s while air is injected periodically with a speed of Im / s. The temperature of the two fluids is 300K.

[0049] The injection of gas bubbles is carried out by a bubbler (18) supplied by an air compressor. The bubbler (18) is constituted for example by porous bars or perforated membranes ensuring the formation of bubbles on the wall of a sheared flow.

[0050]

[0051] Sequencing of bubble injection

[0052] [Fig.3] shows the temporal evolution of the average value of the processor temperature. Without injection 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. In 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 in [Fig.4].

[0053] (1) V CPiri- f)

[0054] where P, TCpu and Tf represent respectively the heat flux density at the surface of the processor, the surface temperature of the processor and the average temperature of the fluid.

[0055] 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 bubble injection, 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.

[0056] Sequential injection of bubbles allows turbulence to be generated and prevents the formation of the thermal boundary layer around the processor. This way, the latter's cooling is much better.

Claims

Claims

1. Method for cooling an electronic component by immersion 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) arranged in vertical planes, the assembly being immersed in said dielectric liquid, and in that bubbles are further injected from the lower part of said chamber towards said heat sink (16).

2. Method for cooling an electronic component according to claim 1, characterized in that the generation of bubbles is controlled intermittently.

3. Method for cooling an electronic component according to claim 2 characterized in that the bubble injection cycle alternates bubble injection sequences for N seconds and periods of stopping the bubble injection for PxN seconds, N being between 0.3 and 0.8 and P being between 2 and 6.

4. 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 in vertical planes, and in that said chamber (1) has a bottom provided with at least one bubbler (18) in the lower part of said chamber, vertical to said heat sink (16).

5. Electronic component cooling equipment according to claim 4 characterized in that said bubbler (18) is powered by a pump controlled for sequential operation.

6. 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 fluid in this cooling circuit.

Images