Patent Publication Number: US-2023156963-A1

Title: Two-Phase Flow Active and Passive Multi-Level Data Center Cabinet Cooling Device and Method

Description:
TECHNICAL FIELD 
     The invention relates to a cooling device and method, in particular to a two-phase flow active and passive multi-level data center cabinet cooling device and method, and belongs to the technical field of equipment room heat dissipation. 
     BACKGROUND 
     With the rapid development of cloud computing services, the scale of a data center has gradually increased, and the blade server in the data center cabinet generates more and more heat; in addition, the number of transistors integrated in an electronic device, especially a CPU, has increased dramatically. In order to ensure a normal operation of electronic equipment, the heat needs to be efficiently transmitted out. Therefore, the cooling system of the data center cabinet needs to be updated to meet higher heat dissipation requirements. In addition, the high heat dissipation requirements of the data center cabinet server will also increase the PUE (Power Usage Effectiveness) value. The PUE value refers to the ratio of all the energy consumed by the data center to the energy consumed by the IT load, and has become a relatively common international measurement index for a power usage efficiency of the data center. For the purpose of energy saving, the cooling system of the data center cabinet needs to minimize energy consumption on the premise of meeting heat dissipation requirements. 
     Currently, there are mainly three data center cooling schemes: air cooling, single-phase liquid cooling, and a combination of these two schemes. Some schemes involve the use of heat pipes. Air cooling is mainly aimed at the heat dissipation of the equipment room of the data center. The air-cooling scheme is mature and consumes less energy, but it is difficult to meet the heat dissipation requirements inside the cabinet. The heat dissipation method combining air cooling and single-phase liquid cooling is also widely used, taking into account the heat dissipation of the equipment room and the heat dissipation of the cabinet, but the two heat dissipation systems need to be separately supplied with energy, thus the energy consumption is high. The single-phase liquid cooling scheme is used less, and due to the lack of equipment room-level air cooling, the heat dissipation effect of the single-phase liquid cooling scheme is not as good as that of the scheme combining air cooling with single-phase liquid cooling. There are also many schemes that apply the existing heat pipe technology and gas-liquid two-phase flow. However, the heat pipe only functions as a heat dissipation component, and the gas-liquid two-phase flow is not the main heat dissipation means for the data center system. 
     According to the current heat dissipation schemes for data center cabinets, it can be found that the following specific problems exist. Firstly, there is no efficient and energy-saving cooling scheme for cabinet cooling. Two-phase flow cooling can solve this problem, but there is currently no well-formed scheme to support two-phase flow cooling in data centers. Secondly, the level of cooling schemes for data centers is not clear enough at present, and there is no efficient and easy-to-integrate cooling scheme that integrates chip-level cooling, component-level cooling, and system-level cooling. The existing schemes rarely involve chip-level cooling, are not specific and feasible enough, and are not compatible with component-level cooling and system-level cooling. Thirdly, the current cooling schemes mostly involve the conversion of cooling modes so as to achieve the goal of reducing energy consumption by changing the cooling mode according to the cooling requirements, but the cooling mode conversion between air cooling and single-phase liquid cooling is difficult to provide a scheme that combines high cooling performance with energy saving. 
     The patent with issued number of CN 111479441 B provides a data center cooling system with pump-driven two-phase flow circulation, involves the conversion of two-phase cooling and cooling mode, as well as the heat exchange with the external environment through air cooling, and has efficient cooling performance. However, this patent relates to a gas-liquid separator and a spray device, and has a complex structure and numerous cooling modes, also meaning an increase in energy consumption. In addition, this patent uses two-phase cooling, but there is no application mode conversion of the heat collection module described in this patent, the cooling means at the direct heat source are not improved, and the specific mode conversion effect may be unobvious. 
     Based on the above description, the current data center cabinets urgently need a cooling system that combines high cooling performance with energy saving, and two-phase flow cooling is an excellent choice worth considering. Moreover, considering that the data center load varies greatly, there is a need for a cooling system that can change the cooling mode according to cooling requirements so as to minimize energy consumption. Of course, this system also requires a high degree of automatic control to reduce the difficulty of application. 
     SUMMARY OF THE INVENTION 
     Technical problem: aiming to solve the problems including difficult integration of cooling schemes for data center cabinets, low energy efficiency, and low degree of automation, the invention proposes a two-phase flow active and passive data center cabinet cooling device and method; the cooling mode is adjusted according to the different working conditions of data center cabinets, so as to achieve low energy consumption and automatic control of cooling. 
     Technical solutions of the present invention are described below. 
     A two-phase flow active and passive multi-level data center cabinet cooling device is disclosed. In the cooling device, a data center cabinet array comprises a plurality of data center cabinets, each data center cabinet comprises a plurality of blade server motherboards, and a rack-level loop thermosiphon of each blade server motherboard is cascaded and connected to a thermosiphon cooling medium inlet pipeline and thermosiphon cooling medium outlet pipeline; the thermosiphon cooling medium inlet pipeline and the thermosiphon cooling medium outlet pipeline of each cabinet are cascaded and connected with other components through a liquid pipeline; a condensate inlet pipeline and a condensate outlet pipeline of a cabinet condensing unit located on each rack-level loop thermosiphon are cascaded; both the liquid reservoir at the cooling medium inlet and the liquid reservoir at the cooling medium outlet have a gas space, and are interconnected via a gas manifold between the liquid reservoirs and the gas manifold of the cabinet and connected to the gas chamber with a very large gas space and the top of the rack-level loop thermosiphon; the liquid reservoir at the cooling medium inlet is located at a higher position than the rack-level loop thermosiphon, while the liquid reservoir at the cooling medium outlet is located at a lower position than the rack-level loop thermosiphon. 
     Detail connection method is described below. 
     The liquid outlet (b) of the liquid reservoir at the cooling fluid inlet is connected to the thermosiphon cooling medium inlet pipeline of the data center cabinet, the thermosiphon cooling medium outlet pipeline is connected to a cooling medium circulating waste heat recovery device and the liquid reservoir at the cooling medium outlet via the liquid pipeline, and is connected to the liquid inlet (f) of the liquid pump circulating the cooling medium, and the liquid outlet (g) of the liquid pump circulating the cooling medium is connected to the liquid inlet (a) of the liquid reservoir at the cooling medium inlet via the liquid pipeline. 
     The condensate outlet pipeline of the cabinet condensing unit is connected to the condensate water cycle waste heat recovery device through a liquid pipeline, and then connected to the liquid inlet (n) of the liquid pump of the condensate system through the liquid pipeline, and the liquid outlet (m) of the liquid pump of the condensate system is connected to the condensate inlet pipeline of the cabinet condensing unit. 
     The gas outlet (c) of the liquid reservoir of the cooling medium inlet is connected to the gas inlet (d) of the liquid reservoir at the cooling medium outlet, and the gas outlet (e) of the liquid reservoir of the cooling medium outlet is connected to the gas chamber and the top pipeline (p) of the thermosiphon of the rack-level loop. 
     Wherein the data center cabinet comprises a plurality of blade server motherboards, and the cooling device of the blade server motherboard comprises a rack-level loop thermosiphon and a blade server condensing unit; the inlet and outlet of each rack-level loop thermosiphon are respectively connected to the thermosiphon cooling medium inlet pipeline and the thermosiphon cooling medium outlet pipeline; the inlet and outlet of each blade server condensing unit are respectively connected to the condensate inlet pipeline and the condensate outlet pipeline. 
     Further, the connecting pipeline between the blade server condensing unit and the condensate outlet pipeline is provided with a condensing unit temperature sensor and a condensate outlet temperature control valve respectively, and the condensing unit temperature sensor is closer to the blade server condensing unit than the condensate outlet temperature control valve; the condensate inlet temperature control valve is located at the connecting pipeline of the blade server condensing unit and the condensate inlet pipeline; the condensing unit temperature sensor, the condensate inlet temperature control valve and the condensate outlet temperature control valve are connected through a condensing unit wire. 
     Wherein the connecting pipe line of the rack-level loop thermosiphon and the thermosiphon cooling medium inlet pipeline is provided with a cooling medium inlet temperature control valve; a cooling medium outlet temperature control valve is located at the connecting pipeline of the rack-level loop thermosiphon and the thermosiphon cooling medium outlet pipeline; a gas valve is located at the connecting pipeline of the rack-level loop thermosiphon and the gas manifold of the cabinet; a first CPU temperature sensor and a second CPU temperature sensor are respectively located on a first CPU of the blade server and a second CPU of the blade server on the blade server motherboard; the cooling medium inlet temperature control valve, the cooling medium outlet temperature control valve, the gas valve, the first CPU temperature sensor, and the second CPU temperature sensor are connected by a thermosiphon wire. 
     Wherein the first CPU of the blade server, the second CPU of the blade server and a thermal interface material are integrated, and a ring structure wrapping a thermosiphon evaporation section pipeline in the thermal interface material wraps the thermosiphon evaporation section pipeline; 4 thermosiphon evaporation section pipelines are connected in parallel and connected with the rack-level loop thermosiphon; the thermal interface material has a fractal tree-like flow channel, two fluid inlets such as a thermal interface material working medium left inlet and a thermal interface material working medium right inlet, and two fluid outlets such as a thermal interface material working medium left outlet and a thermal interface material working medium right outlet; the thermal interface material working medium left inlet and the thermal interface material working medium right inlet are connected to the pipeline of the rack-level loop thermosiphon on the left side of the thermosiphon evaporation section pipeline via a thermal interface material working medium inlet pipeline; the thermal interface material working medium left outlet and the thermal interface material working medium right outlet are connected to the pipeline of the rack-level loop thermosiphon on the right side of the thermosiphon evaporation section pipeline via a thermal interface material working medium outlet pipeline. 
     Wherein the thermal interface material contains two upper and lower layers of fractal tree-like flow channels, and the two layers of fractal tree-like flow channels are connected by a connecting pipeline of the upper and lower layers of the fractal tree-like flow channels located at the tail end of the fractal tree-like flow channel; the fractal tree-like flow channel contains multiple micro-cells, the ring structure that wraps the thermosiphon evaporation section pipeline is etched with multiple flow channels in the ring structure of the thermal interface material, and the micro-cells are connected with the flow channels in the ring structure of the thermal interface material one by one. 
     Wherein the liquid reservoir at the cooling medium inlet, the rack-level loop thermosiphon, the cooling medium circulating waste heat recovery device, the liquid reservoir at the cooling medium outlet, and the liquid pump circulating the cooling medium form a closed circulation structure; the cooling medium works at low air pressure, and distilled water is used as the working medium; the closed circulation structure is evacuated and then filled with a small amount of nitrogen, so that the system works at 0.1 atmospheric pressure finally after injecting the working medium; the cabinet condensing unit, the condensate circulating waste heat recovery device, and the liquid pump of the condensate system form a closed circulation structure; the closed circulation structure uses water as the cooling medium and works at normal pressure; the condensate temperature is monitored by the condensing unit temperature sensor; the CPU junction temperature is monitored by the first CPU temperature sensor and the second CPU temperature sensor. 
     Wherein when the condensate temperature monitored by the condensing unit temperature sensor is lower than the high temperature threshold of the condensate, the condensate inlet temperature control valve and the condensate outlet temperature control valve are both closed, and the liquid pump of the condensate system is closed; when the condensate temperature monitored by the condensing unit temperature sensor is higher than the high temperature threshold of the condensate, the condensing unit temperature sensor sends a signal to open the condensate inlet temperature control valve and the condensate outlet temperature control valve and start the liquid pump of the condensate system to continuously inject condensate into the cabinet condensing unit; when the condensate temperature monitored by the condensing unit temperature sensor is lower than the low temperature threshold of the condensate, and the condensate inlet temperature control valve, the condensate outlet temperature control valve and the liquid pump of the condensate system are all in the open state, the condensing unit temperature sensor sends a signal, the liquid pump of the condensate system is closed, and then the condensate inlet temperature control valve and the condensate outlet temperature control valve are also closed one after another. 
     Wherein when the CPU junction temperature monitored by the first CPU temperature sensor and the second CPU temperature sensor is lower than the CPU high temperature threshold, the cooling medium inlet temperature control valve, the cooling medium outlet temperature control valve and the gas valve are all closed, and the rack-level loop thermosiphon performs passive two-phase flow cooling; when the CPU junction temperature monitored by the first CPU temperature sensor or the second CPU temperature sensor is higher than the CPU high temperature threshold, the sensor that exceeds the threshold temperature sends a signal, the cooling medium inlet temperature control valve and the cooling medium outlet temperature control valve are opened, the gas valve is kept closed, the working medium in the liquid reservoir at a high position and the cooling medium inlet flows to the rack-level loop thermosiphon, and to the liquid reservoir at a low position and the cooling medium outlet through a pipeline, and the rack-level loop thermosiphon begins to perform active two-phase flow cooling; when the CPU junction temperature monitored by the first CPU temperature sensor and the second CPU temperature sensor is lower than the CPU low load threshold temperature, and the cooling medium inlet temperature control valve and the cooling medium outlet temperature control valve are opened, the first CPU temperature sensor and the second CPU temperature sensor send a signal, the cooling medium inlet temperature control valve is closed, the gas valve is opened, the working medium in the top pipeline of the rack-level loop thermosiphon flows to the low-position liquid reservoir at the cooling medium outlet, then the cooling medium outlet temperature control valve and the gas valve are closed, and the rack-level loop thermosiphon returns to the passive two-phase flow cooling mode. 
     Wherein the working medium in the pipeline on the left side of the rack-level loop thermosiphon flows into the fractal tree-like flow channel in the thermal interface material through the thermal interface material working medium inlet pipeline, and flows out to the pipeline on the right side of the rack-level loop thermosiphon via the thermal interface material working medium outlet pipeline; the micro-cell and the flow channel in the ring structure of the thermal interface material form a micro thermosiphon. 
     Beneficial effects: the invention has the following advantages. 
     Firstly, the invention provides a complete set of implementation scheme for two-phase flow cooling and has a better cooling effect than traditional liquid cooling and air cooling. 
     Secondly, the invention designs a multi-level cooling structure and an automatic control scheme, solving the problem that the current data center cooling system has a low degree of integration and is difficult to realize automatic control. 
     Thirdly, according to the load of the data center cabinet, the invention designs two different cooling modes such as active and passive two-phase flow cooling modes, and the cooling system only needs to provide the electric energy to drive the liquid pump and the sensor, ensuring that the PUE of the data center is as low as possible on the basis of ensuring a good cooling effect. 
     Fourthly, the invention combines and improves the micro-channel cooling technology, so that the heat generated by a chip enters the cooling system more efficiently. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is the schematic diagram of the system structure of the invention. 
         FIG.  2    is the internal schematic diagram of the cabinet cooling system. 
         FIG.  3    is the top structure diagram of the cooling system of a single blade server. 
         FIG.  4    is the structure diagram of the heat source of the cooling system of a single blade server. 
         FIG.  5    is the schematic diagram of a thermal interface material. 
         FIG.  6    shows the fractal tree-like flow channel of the thermal interface material. 
         FIG.  7    shows the flow channel in the ring structure. 
         FIG.  8    is the detailed drawing of the fractal tree-like flow channel. 
     
    
    
     The drawings include:
     a data center cabinet array  1 ,   data center cabinets  2 ,   a cabinet condensing unit  3 ,   the liquid pump  4  circulating the cooling medium,   the liquid pump  5  of the condensate system,   the liquid reservoir  6  at the cooling medium inlet,   the liquid reservoir  7  at the cooling medium outlet,   condensate circulating waste heat recovery device  8 - 1 ,   the cooling medium circulating waste heat recovery device  8 - 2 ,   gas manifold  9 - 1  between the liquid reservoirs,   gas manifold  9 - 2  of the cabinet,   the gas chamber  10 ,   the rack-level loop thermosiphon  11 ,   a thermosiphon cooling medium inlet pipeline  12 ,   the thermosiphon cooling medium outlet pipeline  13 ,   the condensate inlet pipeline  14 ,   the condensate outlet pipeline  15 ,   cabinet shell  16 ,   blade server motherboard  17 ,   the blade server condensing unit  18 ,   a cooling medium inlet temperature control valve  19 ,   the cooling medium outlet temperature control valve  20 ,   the gas valve  21 ,   the condensing unit temperature sensor  22 ,   the condensate inlet temperature control valve  23 ,   the condensate outlet temperature control valve  24 ,   a condensing unit wire  25 - 1 ,   the thermosiphon wire  25 - 2 ,   the first CPU  26 - 1  of the blade server,   the second CPU  26 - 2  of the blade server,   a first CPU temperature sensor  27 - 1 ,   a second CPU temperature sensor  27 - 2 ,   the thermosiphon evaporation section pipeline  28 ,   the thermal interface material  29 ,   a thermal interface material working medium inlet pipeline  30 ,   a thermal interface material working medium outlet pipeline  31 ,   the ring structure  32  that wraps the thermosiphon evaporation section pipeline   a thermal interface material working medium left inlet  33 - 1 ,   a thermal interface material working medium right inlet  33 - 2 ,   the thermal interface material working medium left outlet  34 - 1 ,   the thermal interface material working medium right outlet  34 - 2 ,   the fractal tree-like flow channel  35 ,   micro-cells  36 ,   the flow channels  37  in the ring structure of the thermal interface material,   a connecting pipeline ( 38 ) of the upper and lower layers of the fractal tree-like flow channels,   the liquid inlet (a) of the liquid reservoir at the cooling medium inlet,   the liquid outlet (b) of the liquid reservoir at the cooling medium inlet,   the gas outlet (c) of the liquid reservoir at the cooling medium inlet,   the gas inlet (d) of the liquid reservoir at the cooling medium outlet,   the gas outlet (e) of the liquid reservoir at the cooling medium outlet,   the liquid inlet (f) of the liquid pump circulating the cooling medium,   the liquid outlet (g) of the liquid pump circulating the cooling medium,   the liquid outlet (m) of the liquid pump of the condensate system,   the liquid inlet (n) of the liquid pump of the condensate system, and   the top pipeline (p) of the thermosiphon of the rack-level loop.   

     DETAILED EMBODIMENTS 
     The invention A two-phase flow active and passive multi-level data center cabinet cooling device, wherein in the cooling device, a data center cabinet array  1  comprises a plurality of data center cabinets  2 , each data center cabinet  2  comprises a plurality of blade server motherboards  17 , and a rack-level loop thermosiphon  11  of each blade server motherboard  17  is cascaded and connected to a thermosiphon cooling medium inlet pipeline  12  and a thermosiphon cooling medium outlet pipeline  13 ; the thermosiphon cooling medium inlet pipeline  12  and the thermosiphon cooling medium outlet pipeline  13  of each cabinet are cascaded and connected with other components through a liquid pipeline; a condensate inlet pipeline  14  and a condensate outlet pipeline  15  of a cabinet condensing unit  3  located on each rack-level loop thermosiphon  11  are cascaded; both the liquid reservoir  6  at the cooling medium inlet and the liquid reservoir  7  at the cooling medium outlet have a gas space, and are interconnected via a gas manifold  9 - 1  between the liquid reservoirs and the gas manifold  9 - 2  of the cabinet and connected to the gas chamber  10  with a very large gas space and the top of the rack-level loop thermosiphon  11 ; the liquid reservoir  6  at the cooling medium inlet is located at a higher position than the rack-level loop thermosiphon  11 , while the liquid reservoir  7  at the cooling medium outlet is located at a lower position than the rack-level loop thermosiphon  11 . 
     The specific connection method is the following. 
     the liquid outlet b of the liquid reservoir at the cooling fluid inlet is connected to the thermosiphon cooling medium inlet pipeline  12  of the data center cabinet  2 , the thermosiphon cooling medium outlet pipeline  13  is connected to a cooling medium circulating waste heat recovery device  8 - 2  and the liquid reservoir  7  at the cooling medium outlet via the liquid pipeline, and is connected to the liquid inlet f of the liquid pump circulating the cooling medium, and the liquid outlet g of the liquid pump circulating the cooling medium is connected to the liquid inlet a of the liquid reservoir at the cooling medium inlet via the liquid pipeline. 
     The condensate outlet pipeline  15  of the cabinet condensing unit  3  is connected to the condensate circulating waste heat recovery device  8 - 1  through the liquid pipeline, and then is connected to the liquid inlet n of the liquid pump of the condensate system through the liquid pipeline, and the liquid outlet m of the liquid pump of the condensate system is connected to the condensate inlet pipeline  14  of the cabinet condensing unit  3 . 
     The gas outlet c of the liquid reservoir at the cooling medium inlet is connected to the gas inlet d of the liquid reservoir at the cooling medium outlet, and the gas outlet e of the liquid reservoir at the cooling medium outlet is connected to the gas chamber  10  and the top pipeline p of the thermosiphon of the rack-level loop 
     The invention uses a multi-level cooling structure, including chip level, component level and system level. 
     Chip-level cooling scheme: The dual CPU chip of a blade server is integrated with a thermal interface material (TIM); the thermal interface material uses diamond with high thermal conductivity as the construction material; and a fluid channel is etched inside the diamond, and the channel body uses a fractal tree structure, so that the heat can be evenly distributed in the thermal interface material layer; in addition, the thermal interface material adds numerous micro-cells at the tail end of the fractal tree structure so as to form a micro-loop thermosiphon with the channel of the ring structure above; the ring channel is etched in the ring structure in the upper part of the thermal interface material, and the structure wraps the evaporation section of the rack-level loop thermosiphon; the ring structure is bonded with the loop thermosiphon using thermally conductive silicone grease, so that the heat can be transferred to the fluid working medium more efficiently; the channel inlet in the thermal interface material is connected to the inlet flow channel of the evaporation section of the loop thermosiphon through a conduit, and the outlet is also connected to the outlet of the evaporation section of the thermosiphon. 
     Component-level cooling scheme: The main body of component-level cooling is the rack-level loop thermosiphon, and also includes a condensing unit, a temperature sensor and a temperature control valve. As described in the chip-level cooling scheme, the evaporation section of the rack-level loop thermosiphon is bonded with the thermal interface material, and heat dissipation is achieved by directly cooling the thermal interface material; the condensing section of the loop thermosiphon is immersed in the condensing unit, and the low-temperature water in the condensing unit is used as a cooling medium, so that the temperature of the working medium at the inlet of the evaporation section of the thermosiphon is reduced, and then the thermosiphon has a better continuous heat dissipation effect; the temperature sensor obtains the CPU chip junction temperature and the condensate outlet temperature in a working state, and the temperature control valve automatically controls the working mode of the thermosiphon and the working mode of the condensing unit according to the temperature change. 
     System-level cooling scheme: The system-level cooling scheme is connected to multiple rack-level loop thermosiphons in a cabinet in parallel, and the cooling system between multiple cabinets in the equipment room can also be connected in parallel to form the main body of the cooling system. In addition, the components included are a liquid pump, a liquid reservoir, a waste heat recovery device, a gas chamber, a corresponding gas manifold and valve. The liquid pump drives the working medium and the condensing system to circulate; the liquid reservoir stores liquid and reserves a gas space, and the gas space is connected to the gas chamber, so that the phase change of the liquid working medium will not cause a drastic change in the pressure; the waste heat recovery device recovers the waste heat in the cooling working medium and the condensate, so that the temperature of the working medium and condensate at the inlet is maintained at a relatively low temperature; the gas manifold is connected to the gas part of the two liquid reservoirs and the top of the rack-level loop thermosiphon, so that the air pressure of each part of the system is balanced, and the liquid flows under the action of gravity when the gas valve is opened. Moreover, the condensing unit of each rack-level loop thermosiphon is connected in parallel, the waste heat is recovered by the waste heat recovery device, and the liquid is pumped by the liquid pump to the inlet end. 
     The system is composed of two closed circulations, including a cooling medium circulation and a condensate circulation. The main cooling medium works at low air pressure, and distilled water is used as the working medium; the closed system is evacuated and then filled with a small amount of nitrogen, so that the system works at 0.1 atmospheric pressure finally. The condensate circulation structure uses water as the cooling medium and works at normal pressure. 
     As the heat load of the data center changes, the system can use two working modes such as active and passive two-phase flow modes. 
     Passive two-phase flow: When the data center cabinet runs at low load, all temperature control valves and gas valves are closed, and the system performs a passive two-phase flow cooling mode relying on the rack-level loop thermosiphon. The liquid working fluid undergoes a phase change in the evaporation section of the thermosiphon and the thermal interface material, so that a two-phase flow appears at the outlet of the evaporation section, and its density is lower than that of the single-phase flow at the inlet; then the pressure at the inlet end is higher than that at the outlet end, the working medium flows under this pressure difference, and the thermosiphon works in the passive two-phase flow cooling mode. 
     Active two-phase flow: The temperature sensor obtains the chip junction temperature of the blade server CPU; when the data center cabinet runs at high load and the chip junction temperature exceeds the dangerous threshold, the temperature control valves at the inlet and outlet of the thermosiphon are opened, and the liquid working medium in the liquid reservoir at a high position flows to the thermosiphon due to the action of gravity; after passing through the loop, the liquid working medium flows out from the outlet valve, and flows into the liquid reservoir at a low position through the waste heat recovery device. In this case, the system works in an active mobile phase change cooling mode. It should be noted that when the liquid in the liquid reservoir at the low position is accumulated to a certain extent, the liquid pump is started to pump the liquid to the liquid reservoir at the high position. 
     When the system works in a low load state, the overall cooling device uses the passive two-phase flow cooling mode; when the system changes from the low load state to a high load state, the overall cooling device changes to the active two-phase flow cooling mode; when the system changes from the high load state to the low load state, the overall cooling device returns to the passive two-phase flow cooling mode. 
     The liquid circulation of the condensate system is independent of the above-mentioned working mode conversion process, and the temperature sensor obtains the temperature at the outlet of the condensing unit; when the temperature exceeds the threshold, the temperature control valves at the inlet and outlet of the condenser are opened, and the liquid pump is started to inject condensate into the corresponding condensing unit; when the temperature at the outlet is lower than a certain threshold, the liquid pump is closed, then the two temperature control valves are closed, and the condensing unit exchanges heat with the top pipe of the thermosiphon again in the case of being enclosed. 
     The cooling system and the control method thereof have the following characteristics:
         1. A multi-level cooling structure; clear levels including chip level, component level and system level; easy integration.   2. Adoption of a two-phase flow cooling mode, achieving a higher heat transfer coefficient and heat dissipation effect.   3. The cooling system adjusts the working mode according to different heat dissipation requirements of the data center to maintain a large energy efficiency ratio.   4. The use of temperature sensors and temperature control valves to achieve system mode conversion and a high degree of automatic control.   5. The gas chamber maintains a stable air pressure in the closed system, and gas can assist in the automatic control of the system.   6. The use of micro-flow channels and micro-cells combined with improvement to achieve a more significant heat dissipation effect at the chip.       

     According to the drawings, the invention is further described as follows. 
     As shown in  FIG.  1   , the liquid pump  5  of the condensate system, the waste heat recovery device  8 - 1  and the cabinet condensing unit  3  form the condensate system; the rack-level loop thermosiphon  11  in the cabinet array  1  of the data center equipment room, the liquid pump  4  for circulating the cooling working medium, the liquid reservoir  6  at the cooling working medium inlet, the liquid reservoir  7  at the cooling working medium outlet, the cooling working medium circulating waste heat recovery device  8 - 2 , the gas manifold  9 - 1  between the liquid reservoirs, the gas manifold ( 9 - 2 ) of the cabinet, and the gas chamber  10  form the cabinet cooling device. The cabinet cooling device forms a closed loop and works at 0.1 atmospheric pressure, where the liquid working medium is distilled water (the boiling point of water at 0.1 atmospheric pressure is about 46° C., meeting the working conditions of two-phase flow). When the heat load of the data center is small, the rack-level loop thermosiphon  11  is in passive cooling mode, the cabinet cooling device does not work, there is no energy consumption, and only the condensate system works. When the heat load of the data center is large, the rack-level loop thermosiphon  11  is in an active cooling mode and becomes a fluid pipeline in the active cooling mode, and the liquid reservoir  6  at the cooling medium inlet is located at a high position and injects the liquid working medium into the rack-level loop thermosiphon  11 , while the outlet working medium enters the cooling working medium circulating waste heat recovery device  8 - 2  to recover the waste heat, and then enters the liquid reservoir  7  at the cooling working medium outlet. When the liquid working medium in the liquid reservoir  6  at the cooling medium inlet is too little, the liquid pump  5  of the condensate system is started, and the liquid in the liquid reservoir  7  at the cooling medium outlet is pumped to the liquid reservoir  6  at the cooling medium inlet. It is worth noting that the gas parts of the two liquid reservoirs are connected to each other through the gas manifold  9 - 1  between the liquid reservoirs, and are connected to the gas chamber  10  and the condensing section of the rack-level loop thermosiphon  11  through the gas manifold  9 - 2  of the cabinet, ensuring that the air pressure of the two liquid reservoirs is balanced and also achieving favorable liquid exchange between the two; moreover, due to the large space of the gas chamber  10 , the air pressure will not change drastically. 
     As shown in  FIGS.  2  and  3 ,  18    is a blade server condensing unit, which cools the top of the rack-level loop thermosiphon  11  and maintains the working medium at the inlet of the evaporation section of the rack-level loop thermosiphon  11  at a low temperature; the temperature control valve  19  at the inlet of the cooling medium controls the inlet of the rack-level loop thermosiphon, and the temperature control valve  20  at the outlet of the cooling medium controls the outlet of the rack-level loop thermosiphon  11 , while the gas valve  21  controls the connection between the rack-level loop thermosiphon  11  and the gas manifold  9 - 2  of the cabinet in  FIG.  2   . When the rack-level loop thermosiphon  11  is in the passive cooling mode, the temperature control valve  19  at the cooling medium inlet, the temperature control valve  20  at the cooling medium outlet, and the gas valve  21  are all closed; when the rack-level loop thermosiphon  11  enters the active cooling mode, the temperature control valve  19  at the cooling medium inlet and the temperature control valve  20  at the cooling medium outlet are opened; in this case, since the gas part of the liquid reservoir  6  at the inlet of the cooling medium and the gas part of the liquid reservoir  7  at the outlet of the cooling medium in  FIG.  1    are connected to the rack-level loop thermosiphon  11 , the air pressure is balanced; moreover, the working medium in the liquid reservoir  6  at the cooling medium inlet automatically flows into the rack-level loop thermosiphon  11  due to gravity, and the rack-level loop thermosiphon  11  enters the active two-phase flow cooling mode; when the rack-level loop thermosiphon  11  enters the passive cooling mode again, the temperature control valve  19  at the inlet of the cooling medium is closed, and the gas valve  21  is opened; in this case, the gas manifold  9 - 2  of the cabinet is connected to the top pipeline of the rack-level loop thermosiphon  11  and the gas part of the liquid reservoir  7  at the outlet of the cooling medium, and the air pressure is balanced; moreover, the working medium in the top pipeline of the rack-level loop thermosiphon  11  will flow to the liquid reservoir  7  at the outlet of the cooling medium due to gravity so as to complete the cooling mode conversion, then the temperature control valve  20  at the outlet of the cooling medium and the gas valve  21  are closed, and the system enters the passive cooling mode. The condensing unit temperature sensor  22  monitors the outlet temperature of the condensing unit, and the condensate inlet temperature control valve  23  and the condensate outlet temperature control valve  24  respectively control the inlet and outlet of the condensing unit branch. When the condensing unit temperature sensor  22  monitors that the temperature rises to a certain threshold, the condensing unit temperature sensor  22  sends a signal to the condensate inlet temperature control valve  23  and the condensate outlet temperature control valve  24  through a condensing unit wire  25 - 1 , the condensate inlet temperature control valve  23  and the condensate outlet temperature control valve  24  are opened, and the liquid pump  4  circulating the cooling medium in  FIG.  4    is started to inject condensate into the blade server condensing unit  18 . When the temperature drops to a certain threshold and the condensate inlet temperature control valve  23  and the condensate outlet temperature control valve  24  are opened, the liquid pump  4  circulating the cooling medium is closed, the condensate inlet temperature control valve  23  and the condensate outlet temperature control valve  24  are then closed, and the blade server condensing unit  14  performs heat exchange with the top pipeline of the rack-level loop thermosiphon  11  again in the case of being enclosed. 
     As shown in  FIG.  4    and  FIGS.  5 ,  26 - 1  and  26 - 2    are the first CPU of the blade server and the second CPU of the blade server, respectively, and are also the heat source of the cooling system; the first CPU temperature sensor  27 - 1  and the second CPU temperature sensor  27 - 2  respectively monitor the chip junction temperature of the first CPU  26 - 1  of the blade server and the second CPU  26 - 2  of the blade server, and are connected with the temperature control valve  19  at the cooling medium inlet, the temperature control valve  20  at the cooling medium outlet and the gas valve  21  in  FIG.  3    through a wire  26 - 2 . When the junction temperature of one of the CPU chips exceeds the dangerous threshold, the first CPU temperature sensor  27 - 1  and the second CPU temperature sensor  27 - 2  send a signal to the temperature control valve  19  at the cooling medium inlet, the temperature control valve  20  at the working medium outlet and the gas valve  21  through the thermosiphon wire  25 - 2  so as to control the rack-level loop thermosiphon  11  to enter the active two-phase flow cooling mode. When the junction temperature of both CPU chips is lower than a certain threshold, and the temperature control valve  19  at the cooling medium inlet and the temperature control valve  20  at the cooling medium outlet are opened, the first CPU temperature sensor  27 - 1  and the second CPU temperature sensor  27 - 2  send a signal through the thermosiphon wire  25 - 2  to the temperature control valve  19  at the cooling medium inlet, the temperature control valve  20  at the cooling medium outlet and the gas valve  21 , so that the rack-level loop thermosiphon  11  enters the passive two-phase cooling mode again. The specific control mode is described above.  28  is the pipeline of the evaporation section of the thermosiphon; in order to increase the heat exchange area, the evaporation section is divided into four pipelines.  29  is the thermal interface material, which is integrated with the first CPU  26 - 1  of the blade server and the second CPU  26 - 2  of the blade server, and directly exchanges heat with the chip. In addition, the thermal interface material is diamond, and there is a micro-flow channel etched inside the material. The micro-flow channel is connected to the rack-level loop thermosiphon  11  through the thermal interface material working medium inlet pipeline  30  and the thermal interface material working medium outlet pipeline  31 , and exchanges heat with the thermosiphon. Moreover, the ring structure of the thermal interface material  29  wraps the thermosiphon evaporation section pipeline  28 , so that the heat is more evenly conducted to the rack-level loop thermosiphon  11 . The four ring structures  32  wrapping the evaporation section of the thermosiphon respectively wrap the pipeline  28  of the evaporation section of the loop thermosiphon shown in  FIG.  4   , and the two are bonded with thermally conductive silicone grease to make the thermal resistance smaller. The inside of the ring structure  32  wrapping the evaporation section of the thermosiphon is also etched with a flow channel, so that the heat is evenly conducted on the thermal interface material. 
     As shown in  FIG.  6   ,  FIG.  7    and  FIG.  8   , the internal flow channel of the thermal interface material uses the upper and lower double-layer fractal tree-like flow channel  35 . The flow channel shown in the figure is the upper-layer flow channel. The thermal interface material working medium left inlet  33 - 1  and the thermal interface material working medium right inlet  33 - 2  are connected to the lower-layer fractal tree structure, while the thermal interface material working medium left outlet  34 - 1  and the thermal interface material working medium right outlet  34 - 2  are connected to the upper-layer fractal tree structure. The upper and lower layers of flow channels have the same structure except for different inlet and outlet directions. The two layers of flow channels are connected by the upper and lower layers of connecting pipelines  38  of the fractal tree-shaped flow channel in  FIG.  8   . The fractal tree-like flow channel  35  is divided into two independent fractal trees. The left side of the left fractal tree and the right side of the right fractal tree are directly above the first CPU  26 - 1  of the blade server and the second CPU  26 - 2  of the blade server, respectively. In order to uniformly conduct heat in the thermal interface material, the two fractal trees are respectively extended to the central area of the thermal interface material  29 . The thermal interface material  29  also contains a large number of micro-cell structures, namely the micro-cell  36 , which increases the mass of the fractal tree working medium and is also connected to the flow channel  37  in the ring structure of the thermal interface material in  FIG.  7   . The thickness of the flow channel  37  in the ring structure of the thermal interface material is very small, the internal working medium is in a thin film state, and the evaporation efficiency is high, making for heat exchange.