Separated heat pipe and operation method thereof

Abstract

The invention discloses a separated heat pipe which comprises a semi-permeable membrane arrangement pipeline, an evaporator, a gas ascending pipe, a condenser and a liquid descending pipe which are sequentially connected in series to form a refrigerant loop. The separated heat pipe is provided with a refrigerant and an ionic liquid working medium pair; due to the fact that the semi-permeable membrane has selectivity on permeable substances, under the action of osmotic pressure of the ionic liquid with the large molecular weight, the refrigerant with the small molecular weight can overcome resistance and permeate into the ionic liquid through the semi-permeable membrane, and therefore driving force is provided for circulation of the refrigerant.

Description

[0001] This application is a divisional application of the patent application entitled "A Separated Heat Pipe and Its Operation Method". The application date of the original application is January 31, 2024, and the application number is 202410130097.1. Technical Field

[0002] The present application relates to the field of heat pipe heat dissipation, and in particular to a separate heat pipe and an operating method thereof. Background Art

[0003] With the rapid development of global information and communications technology in recent years, data centers, as crucial infrastructure in the digital economy, have seen a steady increase in both number and scale year after year, leading to a corresponding surge in energy consumption. Cooling systems play a crucial role in ensuring uninterrupted, stable, and efficient year-round operation of data centers. However, traditional vapor compression cooling systems account for approximately 40% of total data center energy consumption, leaving significant room for optimization and improvement. These systems are a key component in improving data center energy efficiency.

[0004] Split heat pipes are highly efficient heat transfer devices that utilize natural cooling sources. They boast high energy efficiency, long heat transfer distances, and a simple structure, making them widely applicable in data centers. However, their circulation driving force is entirely derived from the liquid level difference between the condenser and evaporator. In practice, when the height difference between the heat exchangers is insufficient or circulation resistance is high, the heat transfer efficiency of split heat pipes drops sharply.

[0005] To address the performance degradation of traditional gravity-fed split heat pipes due to insufficient circulation driving force, a common approach is to pressurize the liquid or gas phase of the heat pipe to create a mechanically driven, dynamic split heat pipe. However, this approach increases moving parts, increasing energy consumption and also carries the risk of cavitation or liquid hammer, leading to system instability and reduced reliability. Summary of the Invention

[0006] In view of the above-mentioned defects of the prior art, the technical problem to be solved by the present application is: how to increase the circulation driving force of the separate heat pipe without increasing the mechanical drive.

[0007] To solve the above technical problems, the present application provides a split heat pipe, which adopts the following technical solution: a split heat pipe, comprising an evaporator, a gas riser, a condenser, and a liquid downcomer; characterized in that the split heat pipe further comprises a semi-permeable membrane arrangement pipeline, wherein the semi-permeable membrane arrangement pipeline, the evaporator, the gas riser, the condenser, and the liquid downcomer are sequentially connected in series to form a refrigerant circuit, and the split heat pipe is provided with a refrigerant and an ionic liquid working medium pair;

[0008] The semi-permeable membrane setting pipeline is installed between the liquid downcomer and the evaporator, and can circulate the refrigerant. A semi-permeable membrane is arranged in the semi-permeable membrane setting pipeline; the semi-permeable membrane allows the refrigerant with a smaller molecular weight to pass through but does not allow the ionic liquid with a larger molecular weight to pass through;

[0009] The ionic liquid is filled in the evaporator, maintains a liquid state within the operating temperature range of the split heat pipe, does not react with the refrigerant, and has the ability to dissolve the refrigerant; the semi-permeable membrane cooperates with the refrigerant and ionic liquid working fluid pair, and utilizes the selectivity of the semi-permeable membrane for permeable substances. Under the osmotic pressure of the ionic liquid with a larger molecular weight in the evaporator, the refrigerant with a smaller molecular weight in the liquid downcomer can overcome the resistance and pass through the semi-permeable membrane to permeate the ionic liquid, thereby providing additional driving force for the refrigerant circulation.

[0010] The ionic liquid has an extremely low vapor pressure and is almost non-volatile; it has high thermal stability and a wide liquid range, and can maintain a liquid state within the operating temperature range of the separate heat pipe; it has high chemical stability and does not react with refrigerants; it exhibits good solubility in refrigerants such as [bmim]PF6 and [hmim]Tf2N; the refrigerant is safe and environmentally friendly, with a phase change temperature that matches the temperature of the cold and hot sources of the separate heat pipe, and heat transfer is completed through evaporation and condensation processes, such as R134a and CO2.

[0011] The semi-permeable membrane may be, for example, a reverse osmosis membrane, a nanofiltration membrane, or the like.

[0012] Furthermore, the evaporator and condenser adopt fin-tube heat exchangers, microchannel heat exchangers or plate heat exchangers.

[0013] Furthermore, the gas riser and liquid downcomer are made of metal, such as copper, aluminum or stainless steel.

[0014] According to the second aspect of the present application, the present application provides an operating method for the above-mentioned separate heat pipe: when the separate heat pipe is not in operation, the refrigerant continuously penetrates into the ionic liquid until the osmotic pressure and resistance are balanced and the system is stable; when the separate heat pipe is in operation, the refrigerant absorbs heat and evaporates in the evaporator, enters the condenser through the gas riser to condense, and then enters the liquid downcomer. Since the amount of refrigerant dissolved in the ionic liquid is reduced at this time and the osmotic pressure is greater than the resistance, the refrigerant in the liquid downcomer continues to penetrate into the ionic liquid through the semi-permeable membrane, providing the driving force for the refrigerant circulation.

[0015] Compared with the existing technology, the present application has at least the following beneficial technical effects: the present application utilizes the selectivity of the semi-permeable membrane for the permeable substance, installs the semi-permeable membrane between the liquid downcomer and the evaporator of the traditional gravity-type separated heat pipe, and fills the evaporator with ionic liquid. Under the osmotic pressure of the ionic liquid with a larger molecular weight, the refrigerant with a smaller molecular weight will overcome the resistance and pass through the semi-permeable membrane to penetrate into the ionic liquid, thereby providing the driving force required for the refrigerant circulation; this driving method can not only completely overcome the circulation resistance of the system and improve the heat exchange efficiency of the separated heat pipe, but also does not increase additional energy consumption, has high reliability and low initial investment, and has good application prospects in aspects such as cooling of data center computer rooms. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a schematic diagram of the device of the separated heat pipe of the present application;

[0017] Figure 2 It is a diagram of the working principle of the semi-permeable membrane in this application;

[0018] Figure 3 This is a schematic diagram of the device of a traditional gravity-type separated heat pipe;

[0019] Figure 4 This is a schematic diagram of a conventional liquid-driven separated heat pipe;

[0020] Figure 5 This is a schematic diagram of a conventional gas-phase driven separated heat pipe;

[0021] Reference numerals: 1 - semi-permeable membrane, 2 - evaporator, 3 - gas riser, 4 - condenser, 5 - liquid downcomer, 6 - refrigerant, 7 - ionic liquid, 8 - liquid pump, 9 - gas pump. DETAILED DESCRIPTION

[0022] The following describes preferred embodiments of the present application with reference to the drawings in the specification, so that the technical content of the present application is clearer and easier to understand. The present application can be embodied through many different forms of embodiments, and the scope of protection of the present application is not limited to the embodiments mentioned below. When the following description refers to the drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. The size and thickness of each component shown in the drawings are arbitrarily shown, and the present application does not limit the size and thickness of each component. In order to make the illustration clearer, the size of the components is appropriately exaggerated in some places in the drawings.

[0023] The schematic diagram of the traditional gravity-type separated heat pipe is as follows Figure 3However, when the height difference between the condenser and the evaporator is insufficient or the gas and liquid pipes are too long, resulting in excessive circulation resistance, the driving force provided by gravity alone is limited, resulting in a sharp decrease in the heat exchange efficiency of the split heat pipe. Pressurizing the liquid or gas phase to form a mechanically driven power-type split heat pipe can effectively solve the problem of insufficient circulation driving force. The schematic diagram of the traditional liquid-phase driven and gas-phase driven split heat pipe is shown in Figure 4 and 5 However, the addition of additional moving parts increases energy consumption and creates the risk of cavitation or liquid hammer, leading to system instability and reduced reliability.

[0024] The embodiment of the present application provides a separate heat pipe, such as Figure 1 As shown, it includes a semi-permeable membrane 1, an evaporator 2, a gas riser 3, a condenser 4 and a liquid downpipe 5, as well as a refrigerant 6 and an ionic liquid 7 working pair. The condenser 4 is set at a higher height than the evaporator 2. The liquid downpipe 5 and the gas riser 3 are connected between the condenser 4 and the evaporator 2. The inlet end of the liquid downpipe 5 is connected to the condenser 4, the inlet end of the gas riser 3 is connected to the evaporator 2, and the outlet end is connected to the condenser 4. The semi-permeable membrane 1 is fixed in a small section of the refrigerant liquid pipe and installed between the outlet end of the liquid downpipe 5 and the evaporator 2. The semi-permeable membrane is set close to the evaporator, allowing the refrigerant 6 with a smaller molecular weight to pass through but not allowing the ionic liquid 7 with a larger molecular weight to pass through. The working principle is as follows Figure 2As shown, the semi-permeable membrane 1 can be a reverse osmosis membrane, a nanofiltration membrane, etc.; the refrigerant 6 is safe and environmentally friendly, and its phase change temperature matches the temperature of the cold and hot sources of the separate heat pipe, and heat transfer is completed through evaporation and condensation processes, such as R134a, CO2, etc.; the ionic liquid 7 filled in the evaporator 2 has an extremely low vapor pressure and is almost non-volatile; it has high thermal stability and a wide liquid range, and can maintain a liquid state within the operating temperature range of the separate heat pipe; it has high chemical stability, does not react with the refrigerant 6, and exhibits good solubility for refrigerants, such as [bmim]PF6, [hmim]Tf2N, etc.; the semi-permeable membrane 1 and the refrigerant 6 / ionic liquid 7 working fluid pair cooperate with each other, and utilize the selectivity of the semi-permeable membrane 1 for permeable substances. Under the action of the osmotic pressure of the larger molecular weight ionic liquid 7 in the evaporator 2, the smaller molecular weight refrigerant 6 in the liquid downcomer 5 can overcome the resistance and pass through the semi-permeable membrane 1 to permeate the ionic liquid 7, thereby providing additional driving force for the refrigerant circulation. In the present invention, the ionic liquid is filled into the evaporator, and the semipermeable membrane is installed between the outlet end of the liquid downcomer and the evaporator, that is, the ionic liquid is completely stored in the evaporator and is not arranged in other pipelines. When the refrigerant penetrates the ionic liquid, due to the high temperature inside the evaporator, the semipermeable membrane is always in contact with the ionic liquid with a higher temperature. That is, the refrigerant is always in contact with the ionic liquid with a higher temperature, which can improve the evaporation efficiency of the refrigerant, thereby improving the permeation efficiency and driving force of the refrigerant. The present invention places the semipermeable membrane in close proximity to the evaporator. When the refrigerant penetrates the ionic liquid, due to the high temperature inside the evaporator, the temperature of the ionic liquid is also relatively high. Because the semipermeable membrane is placed in close proximity to the evaporator, it is known that the temperature of the ionic liquid in contact with the semipermeable membrane is also relatively high. The osmotic pressure P depends only on the molar concentration and temperature of the solute, and is independent of the type of solute. It can be seen that the temperature of the ionic liquid is proportional to the osmotic pressure. Therefore, it can be inferred that the semipermeable membrane is placed in close proximity to the evaporator in the present invention, so that the ionic liquid can be completely stored in the evaporator, thereby ensuring the concentration of the ionic liquid and improving the driving force.

[0025] The evaporator 2 and condenser 4 can be fin-tube heat exchangers, microchannel heat exchangers, or plate heat exchangers. The liquid downcomer 5, gas upcomer 3, and refrigerant liquid pipe are made of metal, such as copper, aluminum, or stainless steel. Connections between the refrigerant liquid pipe and the evaporator 2, between the evaporator 2 and the gas upcomer 3, between the gas upcomer 3 and the condenser 4, between the condenser 4 and the liquid downcomer 5, and between the liquid downcomer 5 and the refrigerant liquid pipe are welded, threaded, or flared.

[0026] Specifically, for the split heat pipe currently used in a standard 42U server cabinet, the evaporator uses a copper tube and aluminum fin evaporator with a 7mm inner diameter and a length of 1.8m, with 17 tubes per row, totaling four rows. The condenser uses a brazed plate heat exchanger. The gas riser has an inner diameter of 17mm and a length of 6m, while the liquid downcomer has an inner diameter of 14mm and a length of 6.5m (i.e., the gas riser has a larger inner diameter than the liquid downcomer). Both are copper tubes. The heat exchanger height difference is the vertical distance between the evaporator outlet and the condenser outlet. The system refrigerant is R134a, with a total charge of 4.5kg. The average indoor air temperature on the evaporation side is 35°C, and the average outdoor cooling water temperature on the condensation side is 16°C. Calculations show how the refrigerant mass flow rate, heat transfer capacity, and system driving force of a traditional gravity-type split heat pipe vary with the heat exchanger height difference under these conditions, as shown in Table 1. Among them, the present application sets the inner diameter of the gas riser to be larger than the inner diameter of the liquid downcomer. The larger inner diameter of the gas riser can reduce the resistance to steam flow and allow more steam to be transmitted from the evaporation section to the condensation section, which can not only improve the heat transfer capacity, but also increase the pressure in the condenser due to excessive steam transmission to the condenser, so that there is a pressure difference between the liquid downcomer and the gas riser. At this time, the liquid in the liquid downcomer penetrates toward the gas riser under the action of the pressure difference, further improving the penetration efficiency and driving force of the refrigerant. Moreover, the smaller inner diameter of the liquid downcomer can make the liquid fully exchange heat with the outside world (if the inner diameter is too large, the heat exchange effect between the refrigerant in the middle of the liquid downcomer and the outside world will be reduced), which can ensure that the refrigerant can be fully supercooled before entering the evaporator, thereby improving the heat exchange efficiency. That is, in this application document, an ionic liquid is filled into the evaporator, and a semi-permeable membrane is installed between the outlet end of the liquid downcomer and the evaporator. Combined with the method of adopting an inner diameter of the gas riser larger than the inner diameter of the liquid downcomer, the ionic liquid is completely stored in the evaporator and is not set in other pipelines. When the refrigerant penetrates into the ionic liquid, due to the high temperature inside the evaporator, the semi-permeable membrane is always in contact with the ionic liquid with a higher temperature, that is, the refrigerant is always in contact with the ionic liquid with a higher temperature, which can improve the evaporation efficiency of the refrigerant, thereby improving the penetration efficiency and driving force of the refrigerant. It can not only improve the heat transfer capacity, because excessive steam is transmitted to the condenser, it will increase the pressure in the condenser, so that there is a pressure difference between the liquid downcomer and the gas riser. Under the action of the pressure difference, the liquid in the liquid downcomer penetrates toward the gas riser, further improving the penetration efficiency and driving force of the refrigerant. In addition, the smaller inner diameter of the liquid downcomer allows the liquid to fully exchange heat with the outside world, ensuring that the refrigerant can be fully supercooled before entering the evaporator, thereby improving the heat exchange efficiency.

[0027] Table 1 Variation of refrigerant mass flow, heat transfer capacity and system driving force of traditional gravity-type separated heat pipe with the height difference of heat exchanger

[0028] Heat exchanger height difference (m) Refrigerant mass flow rate (kg / s) Heat exchange capacity (kW) System driving force (kPa) 0 0.024 4.397 11.84 0.1 0.027 4.950 12.50 0.2 0.029 5.527 13.16 0.3 0.032 5.864 13.82 0.4 0.033 6.361 14.47 0.5 0.036 6.815 15.13 0.6 0.038 7.008 15.79 0.7 0.040 7.089 16.45 0.8 0.042 7.095 17.11 0.9 0.043 7.101 17.76 1.0 0.044 7.107 18.42

[0029] As can be seen from Table 1, for the traditional gravity-type split heat pipe, as the height difference between the condenser and the evaporator gradually increases, the refrigerant mass flow rate increases nearly linearly, while the heat exchange rate shows a trend of first increasing and then remaining basically unchanged.

[0030] When the height difference between the heat exchangers increases from 0m to 0.6m, the heat transfer rate rapidly increases from 4.397kW to 6.815kW. However, as the height difference continues to increase to 1.0m, the heat transfer rate remains essentially unchanged. The system driving force at a height difference of 0.6m is 15.79kPa. To ensure heat transfer performance, a sufficient height difference must be reserved during the design and installation of split heat pipes.

[0031] When the height difference of the heat exchanger is 0.5m, the refrigerant mass flow rate and heat transfer capacity of the traditional gravity-type separated heat pipe vary with the length of the gas riser and liquid downcomer, as shown in Table 2.

[0032] Table 2 Variation of refrigerant mass flow rate and heat transfer capacity of traditional gravity-type separated heat pipe with the length of gas riser and liquid downcomer

[0033] Gas riser length (m) Liquid downcomer length (m) Refrigerant mass flow rate (kg / s) Heat exchange capacity (kW) 4 6.5 0.036 6.977 5 6.5 0.036 6.894 6 6.5 0.036 6.815 7 6.5 0.036 6.663 8 6.5 0.035 6.413 9 6.5 0.033 6.181 10 6.5 0.032 5.875 11 6.5 0.029 5.482 12 6.5 0.027 5.162 6 4 0.036 6.858 6 5 0.036 6.849 6 6 0.036 6.838 6 7 0.036 6.805 6 8 0.035 6.759 6 9 0.035 6.644 6 10 0.034 6.549 6 11 0.034 6.381 6 12 0.033 6.223

[0034] As can be seen from Table 2, for traditional gravity-type split heat pipes, increasing the length of the connecting pipe increases the flow resistance, resulting in a decrease in the refrigerant mass flow rate and a subsequent decrease in heat exchange. Furthermore, as the length of the connecting pipe increases, the rate of decrease in heat exchange also increases. As the length of the gas riser increases from 4m to 8m, the heat exchange rate decreases slightly from 6.977kW to 6.413kW, a decrease of 8.08%. As the length continues to increase to 12m, the heat exchange rate decreases rapidly to 5.162kW, a decrease of 19.51%. Therefore, when designing and installing split heat pipes, it is necessary to minimize the length of the connecting pipe to avoid excessive pipe resistance that affects heat exchange performance.

[0035] When the height difference of the heat exchanger is insufficient or the connecting pipeline is long, the heat transfer improvement effect of the air pump at different input powers on the separate heat pipe is shown in Table 3. The air pump uses the Embraco air suspension linear oil-free compressor.

[0036] Table 3 Effect of air pump on heat transfer of separated heat pipe under different input powers

[0037]

[0038] Table 3 shows that for split heat pipes with insufficient heat exchanger height differences or long gas riser lengths, the heat transfer rate initially increases and then remains essentially constant as the air pump input power increases. The air pump can provide additional driving force for split heat pipes with insufficient system driving force or high circulation resistance, overcoming the circulation resistance and thus improving heat transfer performance. However, the increased system energy consumption caused by the air pump operation should not be ignored.

[0039] For a separate heat pipe provided in an embodiment of the present application, the molecular weight of the system refrigerant R134a is 102.03; the ionic liquid in the evaporator is selected as [hmim]Tf2N, whose molecular weight is 447.40. According to the R134a / [hmim]Tf2N working fluid pair, Dow NF-4040 nanofiltration membrane is selected as the semi-permeable membrane. It can intercept organic matter with a molecular weight of more than 200, that is, it allows R134a to pass through but does not allow [hmim]Tf2N to pass through. The nanofiltration membrane has the advantages of durability, good treatment effect, and low operating cost. When the heat pipe is not in operation, the refrigerant continues to penetrate into the ionic liquid until the osmotic pressure and resistance are balanced and the system is stable. When the heat pipe is in operation, the refrigerant absorbs heat and evaporates in the evaporator, enters the condenser through the gas riser for condensation, and then enters the liquid downcomer. Since the refrigerant dissolved in the ionic liquid is reduced at this time, the osmotic pressure is greater than the resistance, and the refrigerant in the liquid downcomer continues to penetrate into the ionic liquid through the semi-permeable membrane, thereby completing the cycle. According to calculations, the circulation driving force provided by osmotic pressure in this device can reach at least 25kPa, which is equivalent to the driving force provided when the height difference between the condenser and evaporator of a traditional gravity-type separated heat pipe is 2m. It is also equivalent to the driving force provided when the air pump input power in a traditional gas-phase-driven separated heat pipe is 80W-100W. It can overcome the flow resistance caused by the increase of 8m in the length of the liquid pipe or gas pipe, greatly facilitates the structural size design of traditional gravity-type separated heat pipes, and broadens its application areas.

[0040] The above embodiments are only used to illustrate the technical solutions of the present application and are not intended to limit them. Although the present application has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present application can still be modified or replaced by equivalents. Any modifications or equivalent replacements that do not depart from the spirit and scope of the present application should be included in the scope of protection of the claims of the present application.

[0041] Those skilled in the art will readily conceive of other embodiments of the present application after considering the specification and practicing the contents disclosed herein. This application is intended to cover any variations, uses, or adaptations of the present application that follow the general principles of this application and include common knowledge or customary techniques in the art that are not disclosed in this application.

[0042] It will be understood that the present application is not limited to the exact construction that has been described above and shown in the drawings, and that various modifications and changes may be made without departing from the scope thereof.

Claims

1. A separate heat pipe, comprising: A semi-permeable membrane, an evaporator, a gas riser, a condenser, and a liquid downcomer; wherein the separate heat pipe further comprises a semi-permeable membrane arrangement pipeline, wherein the semi-permeable membrane arrangement pipeline, the evaporator, the gas riser, the condenser, and the liquid downcomer are sequentially connected in series to form a refrigerant circuit, and the separate heat pipe is provided with a refrigerant and an ionic liquid working medium pair; The semi-permeable membrane setting pipeline is installed between the liquid downcomer and the evaporator, and can circulate refrigerant. A semi-permeable membrane is arranged in the semi-permeable membrane setting pipeline; the semi-permeable membrane allows refrigerants with smaller molecular weights to pass through but does not allow ionic liquids with larger molecular weights to pass through; the semi-permeable membrane can pass molecules with a molecular weight of less than 200; The ionic liquid is filled in the evaporator, maintains a liquid state within the operating temperature range of the split heat pipe, does not react with the refrigerant, and has the ability to dissolve the refrigerant; the semi-permeable membrane cooperates with the refrigerant and ionic liquid working medium, and utilizes the selectivity of the semi-permeable membrane for permeable substances. Under the osmotic pressure of the ionic liquid with a larger molecular weight in the evaporator, the refrigerant with a smaller molecular weight in the liquid downcomer can overcome the resistance and pass through the semi-permeable membrane to permeate the ionic liquid, thereby providing additional driving force for the refrigerant circulation; The semi-permeable membrane is installed between the outlet end of the liquid downcomer and the evaporator, and the semi-permeable membrane is arranged close to the evaporator; The inner diameter of the gas ascending pipe is larger than the inner diameter of the liquid descending pipe.

2. A separate heat pipe according to claim 1, characterized in that: The ionic liquid is [bmim]PF6 or [hmim]Tf2N; the refrigerant is R134a or CO2.

3. The separate heat pipe according to claim 1, wherein: The evaporator and the condenser adopt fin-tube heat exchangers, microchannel heat exchangers or plate heat exchangers.

4. The split heat pipe according to claim 1, wherein: The semi-permeable membrane includes a reverse osmosis membrane and a nanofiltration membrane.

5. The method for operating a separate heat pipe according to claim 1, wherein When the separate heat pipe is not in operation, the refrigerant continues to penetrate into the ionic liquid until the osmotic pressure and resistance are balanced and the system is stable; when the separate heat pipe is in operation, the refrigerant absorbs heat and evaporates in the evaporator, enters the condenser through the gas riser to condense, and then enters the liquid downcomer. Since the amount of refrigerant dissolved in the ionic liquid decreases at this time and the osmotic pressure is greater than the resistance, the refrigerant in the liquid downcomer continues to penetrate into the ionic liquid through the semi-permeable membrane, providing the driving force for the refrigerant circulation.

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