Keywords

1 Introduction

Heat pipe is an efficient heat transfer component that uses working medium to transfer heat at different parts of the phase change. The heat pipe principle was first proposed by Gaugler [1] in 1944. In 1965, Cotter [2] first proposed a relatively complete theory of thermal management. Because of their superiority, Heat pipes are also used for cooling abyssal sea reactors and abyssal sea reactors. However, the heat transfer capacity of the heat pipe is limited by its own heat transfer limit. When the heat transfer limit of the heat pipe occurs, the heat of the reactor core cannot be exported in time, which leads to a reactor safety accident. The capillary limit is due to the fact that the capillary indenter produced by the evaporation and condensation sections of the heat pipe is not enough to overcome the pressure drop caused by the return of the working medium. The working fluid cannot flow back to the evaporation section normally. It will cause the evaporation section to dry up, and cause the wall temperature of the evaporation section to rise rapidly, and even burnout the heat pipe wall. Since the heat pipes are all operating at high temperatures in the heat pipe cooling reactor. The heat pipes must use high temperature heat pipes. The working medium of high-temperature heat pipes is generally lithium, sodium, potassium and other alkali metals, so high-temperature heat pipes are often referred to as alkali metal heat pipes. Because of the high viscosity and density of alkali metals, a greater driving force is required in the reflux of the wick. Therefore, exploring the capillary force of the high-temperature heat pipe wick can guide the selection of the operating conditions of the heat pipe, so as to avoid the occurrence of the capillary limit of the high-temperature heat pipe and ensure the safety of the operation of the space reactor.

The most commonly used methods for testing capillary characteristics in the wick are the bubble method [3] and the capillary rising method. Shufeng Huang [4] tested the capillary characteristics of a new type of stainless steel fiber-powder composite wick with the capillary rise method and compared it with a composite wick with a single structure and other structures. Guanghan Huang [5] used an infrared camera to set up a capillary rise rate test device that measured the capillary characteristics of the axial channel wick of the alkaline corrosion treatment. Yong Tang [6] used an infrared camera combined with capillary rise method to measure the capillary performance of the sintered groove composite wick, sintered wick and groove wick with ethanol as the working medium. The results show that the sintered powder wick has better capillary characteristics than the single structure wick. Heng Tang [7] measured the capillary characteristics of a new micro-V-shaped channel wick using the capillary ascending method and acetone as the working medium. Daxiang Deng [8] used a new infrared thermal imaging method to test the capillary characteristics of the sintered groove wick with ethanol as the working medium. Daxiang Deng [9] also tested the capillary characteristics of the micro-V-channel wick with ethanol and acetone for the working medium. Li [10] calculated the capillary characteristics of different porosity wicks in acetone using the mass change curve assessment recorded by the electronic balance.

Most of the above scholars are experiments on the capillary characteristics of sintered wicks, groove wicks and composite wicks with ethanol and acetone as working medium. Due to the reactive chemical properties of the alkali metal working medium, there are fewer test experiments for the capillary core performance of alkali metal heat pipes. In this paper, 304 stainless steel is used as the material of the wire mesh wick, sodium is used as the working medium, and the capillary characteristics of the wire mesh wick is studied by the capillary rising method, and the capillary characteristics of the sodium heat pipe are preliminarily explored. The sodium film in the screen was photographed, and the spread of liquid sodium at different temperatures on the stainless steel wire mesh was obtained, and the law of the capillary ability of the wire mesh wick changed with temperature was verified.

2 Experimental Methods and Principles

2.1 Experiment on Capillary Ability of the Wire Mesh Wick

The capillary ability experiment of the wick is measured by the capillary rise method and the quality change process of the wick is recorded with an electronic balance. Figure 1 shows the entire experimental setup and schematic. The experimental principle is that when the wire mesh wick sample is extended into a stainless steel test tube containing liquid sodium, the liquid sodium will be sucked into the wick due to the influence of capillary force and can reach a certain height. When the rise of sodium in the wire mesh wick reaches stability, the following relationship is satisfied:

$$ h = \frac{2\sigma \cos \theta }{{\rho gr_{p} }} = \frac{2\sigma }{{\rho gr_{eff} }} $$
(1)

wherein θ is the contact angle between sodium and the wire mesh wick, the σ is the surface tension of liquid sodium, rp is the liquid sodium density, reff is the effective capillary radius of the wick, and rp is the pore radius. The experimental equipment includes glove box, well-type heating furnace, electronic balance and so on. The glove box provides an inert gas environment with its own dehydration and deaeration functions, which can avoid the contamination of the sodium working medium by oxygen and water vapor during the experiment. Both the well furnace and the electronic balance are placed inside the glove box in the argon atmosphere. The electronic balance has a maximum range of 220 g, an accuracy of 0.2 mg, and can be connected to the computer segment to output real-time quality changes. The test tube contains a K-type thermocouple for internal liquid sodium temperature measurement, and the XSR21A series paperless recorder is used to implement the output temperature data, and the paperless recorder error is 0.2%· F, which F is the set range.

2.2 Sodium Membrane Spreading Observation Experiment

The spreading observation experiment of liquid sodium on the surface of stainless steel wire mesh is based on a heating stage and a microscope, and the experimental equipment diagram is shown in Fig. 2. The hot stage contains platinum electric heating material and crucibles inside and can be heated at a maximum heating temperature of up to 1500 ℃. Cold water circulation devices and inert gas runners are provided around the hot stage. Cooling circulation units are used to cool hot stage materials, and inert gases can reduce air pollution to sodium during experiments. The microscope has a maximum magnification of 1000X and can observe samples in the micron range.

Fig. 1.
figure 1

Capillary ability test equipment diagram

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Fig. 2.
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Hot stage experimental equipment diagram

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During the experiment, argon gas is continuously pumped in the crucible of the hot stage, and then the sodium block is placed inside the crucible of the hot stage, and then the cleaned stainless steel wire mesh is covered on the surface of the sodium block. Close the hot stage cover and place under the microscope and select multiples of the microscope objective, focusing until the mesh structure is clearly visible. Then turn on the chiller and turn on the heating to photograph the screen at the set temperature.

3 Analysis of Experimental Processes and Results

The experiment adopted 800 mesh 304 stainless steel wire mesh wick as the material, and it was fixed by three-layer rolling. The wick was cleaned in absolute ethanol and acetone successively using an ultrasonic cleaner. Samples of the cleaned wick was sent to the inside of the glove box through the glove box transition chamber. Removed the sodium from the kerosene in the glove box and cut off the surface oxide layer of sodium with a knife. Weigh 120 g of sodium on a balance and place in a stainless steel test tube. The balance indication was adjusted and zeroed, and the wick was weighed and fixed with a hook under the balance, resulting in an initial mass of 44.847 g. The tube clamping height was adjusted by the motor, and the initial liquid sodium infiltration depth was calculated to be about 1.2 cm. The experimental heating temperature program was set to rise first and then fall. When the water content and oxygen content in the gloves dropped below 0.5 ppm and 1 ppm, respectively, and the balance indication was no longer significantly changed, the well furnace was opened and the experiment began. The experimental results of the 800 mesh 3-layer wire mesh aspiration core are shown in Fig. 3.

Fig. 3.
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800 mesh wick experimental result diagram

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As can be seen from the Fig. 3, the mass of the wick remains essentially unchanged when heated to 300 ℃ at the initial temperature. At 300 ℃, the stepper motor is driven to move the wick sample downward so that it comes into contact with the liquid sodium, so there is a turning point of mass degradation at 300 ℃. In the temperature range of 300–400 ℃, the mass of the wick increases only slightly, and is accompanied by small mass fluctuations, and the analysis may be due to the combined effect of sodium flow and sodium evaporation. After the temperature reaches 400 ℃, the mass of the wick as shown in Fig. 3 increases significantly and basically reaches the initial level. When heated from 400 ℃ to 450 ℃, the mass of the wick changes are small and there are no significant mass fluctuations. When the liquid sodium temperature reaches 450 ℃, the evaporation of metallic sodium is significantly enhanced, which directly leads to significant fluctuations in the quality of the wick in the range of temperature 450 ℃ to 650 ℃. When heated to around 520 ℃ at 450 ℃, the mass of the wick increases slightly. Bader [11] shows that the contact angle of sodium on the 304 stainless steel wire varies with temperature as shown in Fig. 4, which shows that sodium has a wetting transition point near about 400 ℃. Therefore, the quality of the wick changes significantly at 400 ℃ in the experiment, because the contact angle of liquid sodium on the surface of stainless steel is suddenly reduced, so that the capillary ability of the wick is enhanced. The experimental results are basically consistent with the results of the contact angle change given in the literature. And after 500 ℃, the liquid sodium and stainless steel have basically been completely wet, the mass change of the wick is not as obvious as when it is 400 ℃, and the violent sodium evaporation phenomenon makes the quality of the wick fluctuate greatly.

Fig. 4.
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The contact angle between liquid sodium and stainless steel changes with temperature

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However, during heating from 520 ℃ to 650 ℃, the quality of the wick decreases slightly. However, in the cooling stage after 650 ℃, due to the increase in surface tension of liquid sodium, the capillary effect is enhanced, the suction effect of the wick on sodium is enhanced, the quality of the wick continues to rise, and the quality of the wick reaches stability after the cooling reaches 400 ℃. Figure 5 shows the results of the wick, which shows that the sample has been basically completely infiltrated at the end of the experiment, and the highest infiltration height of sodium measured with a ruler is about 15 cm, and the final wick weighing mass has reached 49.378 g.

Fig. 5.
figure 5

wire mesh wick sodium aspiration experimental result diagram.

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The results of the wire mesh wick sodium liquid film observation experiment are shown in the following figure. Figure 6 shows the microstructure diagram of the wick in the initial state, and the surface of the wire mesh shows a silvery-white luster. At a heating temperature of 400 ℃, the wire mesh surface loses its original luster, producing yellow and green corrosive products. One of the corrosion products contains a Cr2O3 oxide layer on the surface of stainless steel that reacts with sodium to form a NaCrO2 ternary oxide. At this time, a clear metallic luster appears inside the wire mesh, and liquid sodium begins to be gradually sucked onto the surface of the wire mesh wick Fig. 7.

Fig. 6.
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Mesh structure diagram in the initial state

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When the heating temperature reaches 500 ℃, liquid sodium is sucked onto the surface of the wire mesh due to capillary force, and the experimental results are shown in Fig. 8. When the heating temperature reaches 550 ℃, the objective lens is contaminated with sodium vapor due to the evaporation of sodium, resulting in a decrease in the brightness of the picture as shown in Fig. 9. At 600 ℃, due to the evaporation of sodium, the reason for the reduction of the sodium film inside the wire mesh and the reason why the sodium vapor covers the lens cannot be observed significantly metallic. The observation experiment of sodium film spreading in the wire mesh is a good verification of the results of the increase of the capillary capacity of the 800 mesh wire mesh wick with the increase of temperature, and the increase of sodium suction of the wick leads to the increase of its own quality. During the experiment, a momentary temperature transition point was captured, it was about 410 ℃. Since the argon gas flow has been passed through during the experimental process, the temperature on the surface of the wire mesh will be slightly lower than the temperature of the heating wire, so the temperature transition point should be before 410 ℃.

Fig. 7.
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Spread of sodium film in the wick at 400 ℃

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Fig. 8.
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Spread of sodium film in the wick at 500 ℃

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Fig. 9.
figure 9

Spread of sodium film in the wick at 600 ℃

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4 Conclusion

In the capillary ability experiment of the wick, when heated from 300 ℃ to 450 ℃, the quality of the wick increased to a certain extent, and the increase of sodium vapor after 450 ℃ made the quality of the suction core fluctuate significantly. In the temperature range of 520–650 ℃, the quality of the wick is slightly reduced. The magnitude of the fluctuations also increases as the temperature increases. And in the process of cooling down by 650 ℃ for the first time, with the increase of the surface tension of liquid sodium, the wick has a significant increase in mass. Before and after the experiment, the sodium infiltration height of the 800-mesh wick increased from 1.2 cm to 15 cm, and the mass increased from 44.847 g to 49.378 g. The observation experiment of the spread of sodium film in the wire mesh proved that when near 410 ℃, there is a transition point of liquid sodium and stainless steel wetting, and the capillary ability increases more obviously, and more sodium is sucked on the suction core, which is mutually verified with a strong growth when the quality of the wick is about 400 ℃ in the capillary experiment. And at 500 ℃, liquid sodium has good wetting properties on the surface of stainless steel wire mesh, and liquid sodium has been better spread on the surface of the wire mesh wick.