SLM Additive Manufacturing of Oscillating Heat Pipe

Abstract

This study employed metal additive manufacturing technology to fabricate oscillating heat pipes using SUS316L as the material and conducted related printing parameter experiments and thermal performance tests. The initial experimentation involved testing the relative density and size error of the metal additive manufacturing process. Density measurement was performed using the Archimedes method, and further X-ray CT scanning was utilized to observe the internal structure and compactness. The outcomes indicate that suitable laser parameters yield favorable results in producing oscillating heat pipes, achieving good compactness and minimal dimensional error with proper parameter adjustments. Following relevant pre-processing and post-processing on the oscillating heat pipe, leakage experiments were conducted to ensure experimental accuracy. The oscillating heat pipe had dimensions of 120 mm in length and 51 mm in width, with five turns of 2 mm × 2 mm cross-sectional channels inside. Interval design was employed to address inter-channel thermal interaction commonly encountered in flat heat pipes for comparison. Methanol was selected as the working fluid to investigate the oscillating characteristics and thermal performance under different input powers (20 W, 30 W, 40 W, 60 W, 80 W). The results indicated that the inter-channel spacing can significantly decrease the lateral thermal interaction and enhance the oscillation effect during the operation of the oscillating heat pipe, resulting in improved thermal performance. The experiments demonstrated that at 20 W, the equivalent thermal conductivity of the heat pipes with and without inter-channel spacing was 2428 and 1743 (W/mK), respectively, and at 80 W, it was 2663 and 2511 (W/mK), respectively. These results indicate that reducing thermal interaction can significantly improve the oscillation effect, leading to higher equivalent thermal conductivity at low power.

1. Introduction

With the advancement of science and technology, there is an increasing need for high-density heat dissipation in various industries, including aerospace, electronics, and sustainable energy. As the world’s attention shifts towards sustainable energy applications, the need for heat dissipation in various sectors such as solar energy [1], waste heat recovery [2], electronic servers [3], high reliability, and sustainable use [4], and aerospace development [5] has grown. The oscillating heat pipe is widely regarded as having significant potential for use due to its ability to provide efficient heat transfer with a simple structure.

Additive manufacturing (AM) is defined by the American Society for Testing and Materials (ASTM) as “a process of joining materials to make parts from 3D model data. Usually, the material is joined layer upon layer, as opposed to subtractive and formative manufacturing methods.” Unlike traditional manufacturing processes, AM technology does not require the use of cutting tools or specific processing techniques to manufacture complex 3D parts. Instead, it directly manufactures these parts by slicing and supporting the design of 3D CAD files. This eliminates the need for extensive geometry and feature analysis, streamlining the manufacturing process. Due to its ability to produce highly customized parts and reduce material waste, many scholars consider AM a third industrial revolution technology [6].

AM technology has a promising development outlook, and various academic and industrial sectors have begun to invest in research and production technology development. In the biomedical field, for example, there is an effort to manufacture bones that can be implanted into the body using materials that are suitable for this purpose [7], as well as the printing of blood vessels and tissues using biological proteins as raw materials [8]. Particularly for topological structures that are difficult to process using traditional manufacturing methods [9], AM technology can be utilized to produce lightweight support structures and bones that possess both sufficient strength and quality [10]. The aerospace industry is currently considered to be one of the most promising sectors in collaboration with AM technology, as this technology can manufacture parts with complex geometries [11], reduce the loss of precious metal materials [12], and perform composite material printing [13].

In the area of heat dissipation, AM technology offers unique design capabilities and fewer geometric constraints [14], making it an important tool for industrial applications [15], particularly for complex heat pipe structures [16]. However, AM still faces several challenges, such as poor surface roughness, larger tolerances, the requirement for support structures in design, complicated post-processing for finished products, and the strength of final products. Further research is needed to address these issues [17,18,19].

Powder-bed fusion (PBF) is an AM technology that employs a high-density, focused energy source to sinter or melt the material powder into the desired shape. This technology enables the creation of parts with intricate designs and high precision. Due to the high-intensity energy required to melt the material, PBF necessitates a stable bottom plate and support materials for heat dissipation between layers to maintain stability during printing. Additionally, inert gas is utilized to prevent material reactions during high-temperature melting. PBF technology encompasses three categories, namely Selected Laser Sintering (SLS), Selective Laser Melting (SLM), and Electron Beam Melting (EBM), each with its own set of advantages and disadvantages that can impact the material selection and printing quality [20]. SLS boasts a low processing temperature and does not require support structures, but the selection of available materials is limited. SLM can fully melt the material and has a wider range of materials available for printing, but it necessitates support structures due to the high working temperature. EBM can print harder-to-print metals and has lower residual stress and support structure requirements, but manufacturing requirements and cost are higher, and the produced surface may be relatively rough. The PBF method selected depends on the specific application and the properties of the materials to be printed.

The complexity and numerous variables associated with AM technology necessitate significant resources and research to establish a foundation. However, AM’s ability to create intricate geometries and reduce material waste makes it highly valuable for research purposes [21]. Although the SLM manufacturing process may seem straightforward, involving powder laying, laser scanning, and platform lowering cycles, various influencing factors must be considered to ensure the production of high-quality and structurally sound workpieces. The following are several commonly encountered basic properties.

The SLM process may seem straightforward, involving steps such as powder application, laser scanning, and positioning of the bottom plate. However, achieving high-quality and structurally sound workpieces requires considering various factors affecting the outcome. Density is a critical property that demands close attention in AM [22,23], as insufficient density can lead to reduced mechanical properties and poor airtightness [24]. Additionally, excessive energy can cause rapid powder melting and subsequent splashing, leading to an inadequate density of printed parts. Careful adjustment of laser parameters and powder quality is, therefore, necessary to ensure successful results.

SLM technology has the advantage of being able to completely melt the powder using high-intensity energy, allowing for the printing of common materials such as stainless steel, aluminum, titanium, and their alloys with a density of over 99% [25,26]. However, the performance of the machines and the quality of the powders used can vary between different brands and manufacturing processes, as well as under different background conditions. Therefore, it is necessary to conduct testing and adjust the conditions of the machines in use in order to determine the optimal printing parameters, which can require significant effort.

The oscillating heat pipe (OHP) relies on the pressure difference of the working fluid in its vaporization state to achieve oscillation, as it lacks a capillary structure inside to facilitate heat transfer cycles. In the realm of electronic heat dissipation, heat sources with flat heat output surfaces, such as CPUs, are commonplace. The OHP is typically composed of a bent circular metal pipe embedded into a metal block or evaporator for conduction. However, these devices tend to exhibit poor efficiency due to the contact thermal resistance. Traditional methods, such as milling and lapping, are employed to create a flat plate heat dissipation surface. Flow channels are made and welded separately, rendering it difficult to achieve higher efficiency. To circumvent contact loss that results from bonding and welding processes, it is imperative to develop an improved design and production structure to enhance heat dissipation capacity.

The ability of AM to create non-conformal shapes and features has generated significant interest among researchers [27], as it enables the construction of designs that are challenging to achieve with traditional technologies [28]. To design an oscillatory heat pipe with superior performance, it is crucial to conduct fundamental theoretical research and integrate it with fluid flow visualization to determine design parameters. Analyzing fluid parameters can aid in selecting and improving the properties of the working fluid, channel design, and other components, ultimately enhancing performance [29].

As electronic components become increasingly thinner and high-performing, cooling systems need to efficiently extract waste heat from the equipment. Researchers have focused on developing thin and high-performance heat sinks that align with this trend. The OHP offers advantages over other heat transfer devices in terms of its operating characteristics and the ability to produce various free-form designs using AM technology. This flexibility is particularly suitable for heat dissipation in aerospace equipment and components. With the development of AM, it is possible to realize the ideal characteristics of printing with optimal output for aerospace use.

The study of oscillating heat pipes usually focuses on various factors, such as fill ratio, working fluid, startup conditions, channel size, fluid/material properties, geometry, and operational conditions [30,31,32,33]. However, the development of such pipes has been limited by traditional manufacturing techniques. The emergence of additive manufacturing (AM) technology offers possibilities to improve the performance of oscillating heat pipes. For example, polycarbonate can be employed to fabricate serpentine channels for visualization experiments [34]. Another approach is to use a thin polymer board as a substrate and print heat pipe channels onto its surface to decrease the overall thermal resistance of the polymer OHP [35], and special flow channels can be designed [36]. Studies have also explored the effect of dividing simulation pipes on fluid distribution [37].

In 2015, Thompson et al. [38] successfully printed multilayer OHP using AM technology with Ti-6Al-4V material. The higher surface roughness of AM technology was found to enhance the evaporation phenomenon and improve the performance. An effective thermal conductivity of 110 W/mK was achieved by filling 67% water as the working fluid. Compared to solid Ti-6Al-4V, it shows a performance increase of over 400%. Thompson et al. conducted a further investigation in 2017 on different working fluids and heating methods. The study found that all OHPs could operate effectively, demonstrating the advantageous use of AM technology’s rough surface as an aid to OHP operation [39]. Chang et al. [40] utilized AlSi10Mg to produce OHP and maintained the LED temperature below 60 °C by filling 40–70% acetone and conducting measurement experiments. Han et al. [41] utilized polymers to create flexible OHP that can be bent between 0 and 90 degrees and operated successfully, but the thermal resistance increased with the bending angle.

Xu et al. [42] conducted a comparison of the performance of flat plate OHPs manufactured using additive manufacturing (AM) and CNC machining for waste heat recovery systems. The study found that while flat plate OHPs have good application and economic benefits in waste heat recovery, CNC machined OHPs exhibit better effective thermal conductivity compared to those made by AM. However, as the heat source temperature increased, the improvement in the performance of AMOHPs was greater than that of the CNC-machined OHPs. The authors suggested that the rough surface resulting from AM technology increased the effect of thin film evaporation, leading to higher performance. Additionally, the authors compared the thermal conductivity of the two manufacturing methods in OHPs with no working fluid and found that CNC machined OHPs demonstrated better empty tube performance. The authors hypothesized that this difference may be due to the lower density of the metal material produced by AM technology and the overall performance deviation caused by dimensional errors.

Heat pipes, which belong to the category of phase-change devices, have always been a popular choice for enhancing heat transfer efficiency. However, traditional flat-plate heat pipes, commonly used for flat heat sources, exhibit significant thermal interaction between channels due to their interconnected design [43], resulting in a reduced temperature difference between different channels. This unfavorable condition hinders the circulation of the operating fluid in oscillating heat pipes. In recent years, the successful development of additive manufacturing has led to its consideration for use in the internal structure of heat management devices [10,44,45,46,47,48,49,50]. The advantage of additive manufacturing lies in its ability to improve the problem of creating special structures that are difficult to achieve with traditional manufacturing methods, as well as its ability to produce designs with complex geometric shapes. Additionally, its characteristic of being a single-piece fabrication can reduce the contact thermal resistance caused by connecting various components or reduce the occurrence of low quality due to process flaws.

In the context of sustainability, one of the major challenges in improving the efficiency of renewable energy technologies, such as solar and geothermal, is the difficulty in heat transfer. Heat pipes, which are phase-change devices, have been widely recognized as a reliable means of increasing heat transfer efficiency. With the advent of AM technology, it has become possible to produce heat pipes with custom designs that better meet the requirements of specific applications. Moreover, AM can overcome the limitations of traditional manufacturing methods, which cannot create complex structures. Due to its integrated characteristics, it can minimize contact thermal resistance and further enhance overall efficiency.

The main novelty of this research lies in the pursuit of producing optimized OHPs with the required density and dimensional accuracy for specialized structures using 3D printing technology. However, there is a lack of current research focused on the manufacturing process of 3D printing and heat pipes. Many studies instead focus on unique designs and experimental results. 3D printing technology is a novel approach compared to traditional manufacturing methods, and researchers often need to acquire costly manufacturing machines and conduct extensive research to become familiar with the advantages and disadvantages of 3D printing. To address this issue, this study presents a series of manufacturing processes utilizing SLM technology.

The study utilized the SLM technique, which offers a wide range of materials, excellent density, and the ability to adjust laser parameters during production to fine-tune the properties of the OHPs. The primary goal of this investigation is to create two distinct types of OHPs and experimentally verify their suitability for additive manufacturing. Successful results from this study have the potential to advance the development of renewable energy technologies that are more efficient and environmentally friendly.

2. Heat Pipe Design and Additive Manufacturing Experiment

2.1. Heat Pipe Design

Figure 1 illustrates two distinct designs of OHP. The first design features a 1 mm space between the channels, thereby avoiding thermal interaction. The second design follows the conventional flat OHP. Both heat pipes have the same dimensions, measuring 120 mm in length (excluding the welded filling tube) and 51 mm in width. The evaporation and condensation ends measure 40 mm each, with a final product thickness of approximately 4.2 mm.

Table 1. OHP size parameters.

SUS316L FPOHP
Length	Width	Thickness	Hydraulic Diameter	Turn
120 mm	51 mm	4.2 mm	2 mm	5

outlines the size settings inputted into the printing software. The internal flow channel is a square channel with a side length of 2 mm, and there is a 1 mm gap between the flow channels. The channel’s turn number is 5. SUS316L was used as the material during printing.

2.2. Testing of Additive Manufacturing

The equipment utilized in this experimental AM system, namely the Dongtai AMP-160, can be subdivided into a nitrogen manufacturing system, powder processing, software, and printing control.

Table 2. AMP-160 Technical data.

Laser Power	$300 \mathsf{W}$
Building volume	$\varnothing 160 \mathrm{mm}\times 160 \mathrm{mm}$
Focus diameter	$50 \mathsf{\mu }\mathrm{m}$
Building volume	$1~10{ \mathrm{cm}}^3/\mathrm{h}$
Scanning speed	up to $6600 \mathrm{mm}/\mathrm{s}$
Layer thickness	$20~100 \mathsf{\mu }\mathrm{m}$
Size accuracy	$\mathrm{about} 100 \mathsf{\mu }\mathrm{m}$

specifies the working parameters of the AMP-160 machine.

When considering the application of AM technology, it is crucial to take into account three key factors: input, process, and output. Input refers to the configuration of laser parameters, materials, scanning strategies, and other prerequisites. These parameters give rise to various physical and chemical processes, which are then followed by post-processing to produce the final product. Finally, the input must be optimized and modified based on the output. In the following section, we will discuss some important considerations for the fabrication of OHP using AM technology.

Air tightness is a crucial characteristic of OHPs. Any leaks in the system can result in reduced performance and reliability, thus the OHP must have a sufficiently high density to prevent any leaks. Furthermore, size deviation can have a significant impact on the design of the heat pipe. Therefore, it is necessary to determine whether a certain level of size deviation can be maintained while achieving good density in order to explore and attempt various different designs.

Insufficient density can often be determined through measurements and cross-sectional observations. Figure 2 shows the scanning of samples using a scanning electron microscope (SEM). The cross-section reveals that due to missing printing, the sample has stomas and abnormal melting, which would lead to insufficient metal density and result in leakage.

This study conducts a theoretical analysis of the manufacturing process of AM and laser parameters, encompassing powder screening, experiment parameters, and post-processing. Amongst the various laser input parameters being adjusted, the most extensively discussed parameter is the “Volumetric Energy Density, $E_v$” represented by Equation (1):

$$E_v=\frac{P}{V}\times \frac{1}{H}\times \frac{1}{t}$$

(1)

This formula incorporates various parameters such as laser power (P, W), scanning velocity (V, mm/s), hatch distance (H, μm), and layer thickness (t, μm). By adjusting these parameters to obtain varying volumetric energy densities, the structure and state of the printed sample can be modified. Increasing the energy density enhances the melting of the powder, leading to an improved density of the final product. However, excessively high energy density may result in printing defects. Therefore, different energy densities can alter the unit energy received by the powder, consequently affecting the relevant characteristics of the final molded part.

The printing parameters experiment utilized Equation (1), with the hatch distance and layer thickness fixed at 0.1 mm and 0.03 mm, respectively. Twenty-four groups with varying laser power and scanning velocity were utilized to obtain samples with volumetric energy densities ranging from 40 to 170 $\mathrm{J}/{\mathrm{mm}}^3$. The size deviation and relative density were then calculated using Equations (2) and (3), respectively. Size deviation refers to the difference between the designed and actual size, while relative density is calculated by dividing the OHP density by the density of 316 L metal, which is equal to 7.98 $\mathrm{g}/{\mathrm{cm}}^3$.

$$\mathrm{Relative} \mathrm{density}=\frac{\mathrm{sample} \mathrm{density}}{316 \mathrm{L} \mathrm{metal} \mathrm{density}}\times 100\%$$

(2)

$$\mathrm{Size} \mathrm{deviation}=\mathrm{actual} \mathrm{size}-\mathrm{design} \mathrm{size}$$

(3)

The samples were removed using EDM cutting, as shown in Figure 3. The dimensions of the samples were measured, and their density was determined using the Archimedes method. Subsequently, the surface was polished and SEM imaging was used to observe the samples microscopically.

3. Fabrication and Heat Transfer Experiments of Oscillating Heat Pipes

The final product of the AMOHP is shown in Figure 4, which can be classified into two categories: with or without thermal interaction. X-ray CT can be utilized to inspect the internal structure of the completed product.

The experimental design, illustrated in Figure 5, involves controlling the heating power at the evaporator end using the power output from the power supply, while the condenser end is cooled by a stable temperature water cooling system provided by the constant temperature water tank. The inlet and outlet temperatures of the water cooling system are measured to calculate the cooling capacity. After conducting a series of tests, the heat pipe is filled using a filling system, and temperature data is collected using a thermocouple and data logger for performance calculations.

Regarding OHP design, please refer to Figure 1 and Figure 4. Both samples use methanol as the working fluid with a filling rate of 55%. The heating power is gradually increased from 20 W to 80 W, and the oscillation phenomenon is observed by measuring the cold end surface temperature (T₁~T₅) and the heater surface temperature (T_H) of the heat pipe.

The AM of heat pipes involves printing, cutting, and post-processing, which presents challenges for achieving accurate product size control. In particular, EDM and grinding are necessary to adjust the thickness, which can result in significant size deviation that affects the heat pipe’s characteristics. Therefore, it is necessary to continually refine the accuracy and make necessary adjustments in future experiments.

Table 3. Experimental parameters for heat transfer.

Experimental Parameters	Conditions
Condensed water temperature	$30 °C$
Filling rate	55%
Heating power	20 W, 30 W, 40 W, 60 W, 80 W
Placement angle	${90}^{\mathrm{o}}$
Working fluid	Methanol

shows the experimental parameters, and

Table 4. AMOHP size.

	Interval OHP		No Interval OHP
	Design	Real	Design	Real
Length	120.0 mm	120.1 mm	120.0 mm	119.8 mm
Width	51.0 mm	51.1 mm	51.0 mm	51.1 mm
Thickness	4.0 mm	4.21 mm	4.0 mm	4.11 mm

presents the actual dimensions of the two heat pipes used in this study. The experimental conditions included a condensed water temperature of 30 °C, a filling rate of 55%, heating power levels of 20 W, 30 W, 40 W, 60 W, and 80 W, a placement angle of 90°, and methanol as the working fluid. The two heat pipe designs tested were with and without a channel separation.

4. Results and Discussion

4.1. Laser Parameters Experiment

Figure 6 depicts the test results of the relative density for samples produced using four different laser powers. The findings show a positive correlation between $E_v$ and relative density, with most samples achieving over 98% relative density when $E_v$ is greater than 80. It is also evident that the samples produced using laser powers of 100 W and 80 W have lower relative densities, which can be observed in the SEM images of the samples with additional parameters (P = 220, V = 900) and (P = 80, V = 328), respectively. Figure 7 summarizes the relative density of samples with $E_v$ around 80, indicating that relative density decreases with decreasing laser power due to incomplete melting caused by low laser power. Therefore, even when $E_v$ is the same, the laser power should not be too low. Nonetheless, from the SEM image of the sample with additional parameters (P = 100, V = 250) in Figure 6, it can be observed that this phenomenon becomes less significant as $E_v$ increases.

It is also essential to consider the impact of size deviation during printing. Figure 8 shows that the size deviation increases with the increase of $E_v$, as excessive energy causes more significant thermal expansion at the boundaries. Finally, taking into account the relative density, size deviation, and printing speed, the selected printing parameters are presented in

Table 5. AM OHP printing parameters.

P	V	t	H	${\mathit{E}}_{\mathit{v}}$
220 W	900 mm/s	$0.03 \mathrm{mm}$	$0.1 \mathrm{mm}$	81.48 $\mathrm{J}/{\mathrm{mm}}^3$

.

Although high $E_v$ can result in high relative density, and it can also cause defects due to overmelting or poor design. Figure 9a,b show the detachment of the part from the build platform and distortion of the overall shape, indicating a failed print. In such cases, it is necessary to investigate whether there are design issues or adjust the parameters and support structures. Figure 9c shows the same part printed after improvements were made, and no noticeable defects are present.

4.2. OHP Internal Structure Scan

Figure 10 shows the results of internal structure observation using CT. Additively manufactured prints typically have poor surface properties, making it challenging to perform surface treatment on the internal surface. However, rough surfaces may serve as nucleation points during evaporation, potentially enhancing the heat pipe’s overall performance. This aspect requires further investigation to determine whether surface roughness can improve oscillating heat pipe performance. The cross-sections of the two heat pipes were observed in Figure 11 to determine their size and structure. It can be observed that the channels, walls, and spacing essentially conform to the original design, indicating that AM can produce samples that meet user requirements.

However, upon closer examination of the channel cross-sections, it was discovered that the top and bottom edges of the channels differed significantly from the two side edges. The actual dimensions ranged from approximately 1.87 to 1.94 mm. Since the top edge of the OHP has no support during printing, the material expands excessively in this area, resulting in reduced surface quality. To address this issue and investigate whether the surface roughness of AM could enhance OHP performance, OHPs with rhombus-shaped channels were designed and prototyped (Figure 12). The rhombus channels provide better heat transfer paths during printing compared to rectangular channels, resulting in improved surface quality.

The density of the heat pipe wall is a crucial factor in ensuring the smooth operation of the heat pipe, as only a sealed cavity can maintain the vacuum. Thus, it is necessary to conduct a leak test on the heat pipe after processing. The test is typically performed using both positive and negative pressure tests

4.3. Leak Test

4.3.1. Positive Pressure Test

In order to verify the absence of leaks in the PHP and ensure the stability of the connecting pipeline, a positive pressure test can be performed by filling the PHP with 5 bar of air and submerging it in water to check for significant bubble formation. If a minor leak is detected, it can be sealed with vacuum glue. However, if there is significant leakage, as shown in Figure 13, it may be necessary to adjust the laser parameters and reprint the OHP.

4.3.2. Negative Pressure Test

To replicate the working conditions of the OHP in actual environments, a vacuum pump was utilized to remove air from the experimental pipeline, which was then sealed using a valve. Digital pressure gauges were used to measure and monitor the vacuum levels. The pressure levels were continuously monitored over an extended period to verify the OHP’s ability to maintain the designated pipeline vacuum.

To obtain precise information regarding the leakage status of the OHP, we performed several tests on the connecting pipelines, sealing all input tubes with vacuum glue. To create a vacuum environment, we first utilized a vacuum pump, and then clamped the hose connecting the pump and OHP with hemostatic forceps. After maintaining this test configuration for a predetermined duration, we were able to evaluate the extent of the pipeline’s leakage. The results of these tests are presented in

Table 6. Vacuum pipe leak testing.

Time	Initial Vacuum	Final Vacuum	Average Leak Rate
135 min	0.235 Torr	3.6 Torr	0.0248 Torr/min
360 min	0.235 Torr	8.8 Torr	0.0239 Torr/min
900 min	0.235 Torr	13.4 Torr	0.0145 Torr/min
1440 min	0.235 Torr	20.7 Torr	0.0142 Torr/min
1750 min	0.235 Torr	25.0 Torr	0.0141 Torr/min

.

Based on the test results, it was found that the leakage rate decreased gradually as the internal vacuum level increased. Therefore, it is important to conduct appropriate testing scenarios for the PHP experiment. After 905 min, which is approximately 15 h, the average leakage rate decreased progressively and approached a leakage rate of 0.014 Torr per minute.

Table 7. OHP leak test (including vacuum line).

	Initial Vacuum	Final Vacuum	Time	Average Leak Rate
Interval OHP	0.2 Torr	23.4 Torr	24 h	0.016 Torr/min
No interval OHP	0.2 Torr	24.8 Torr	24 h	0.017 Torr/min

presents the results of the leak tests conducted on the two heat pipes. The average leak rates of the two heat pipes were found to be approximately 0.016 and 0.017 Torr per minute, respectively. These values are slightly higher than that of the pipeline, which is a reasonable observation. Although it is not possible to calculate the exact leakage rate of the OHP from these values, we can conclude that the heat pipe’s leakage is within an acceptable range, as indicated by the numerical values. These results meet the requirements of the experiment.

4.4. Heat Transfer Experiment

Figure 14 and Figure 15 present the experimental results, which indicate that OHPs without thermal interaction exhibit a more pronounced and vigorous oscillation range, even at low heating power. This is due to the separation between the flow channels, which allows for the formation of bubbles and liquid blocks that oscillate between the two ends, thereby enhancing the overall performance of the heat pipe. In contrast, OHP with thermal interaction tends to balance the temperature and pressure differences between the internal pipes due to lateral heat transfer, making it challenging to generate an oscillating flow state. It is noteworthy that the oscillation range of the OHP exhibits a significant difference with varying intervals. Even at 20 W heating power, the startup state of the without thermal interaction OHP is evident, with temperatures of the cold end and heater oscillating in an up-and-down fashion. During the slow-rise state, temperature oscillations indicate a more vigorous nature of the internal pressure change, causing the bubble and liquid block to oscillate between the two ends, thus facilitating the working fluid’s evaporation and condensation and ultimately leading to enhanced overall performance. In the case of thermal interaction OHP, some oscillations occur at 40 W, while other temperatures start oscillating at 80 W. This is primarily attributed to the tendency of lateral heat transfer between the internal pipes of the solid block OHP, which tends to balance the temperature, eventually leading to a gradual balancing of pressure difference, making it challenging to generate an oscillating flow state.

To calculate the thermal resistance, the average of the condenser temperature can be taken, and Equation (4) can be applied. As shown in Figure 16, the working fluid experiences substantial oscillations at low power, resulting in the interval channel OHP demonstrating better thermal resistance. However, as the input power increases, the oscillation effect of the non-interval channel OHP becomes more significant, leading to improved heat transfer efficiency.

$$R=\frac{{\mathrm{T}}_{\mathrm{H}}-{\mathrm{T}}_{\mathrm{c},\mathrm{avg}}}{Q}$$

(4)

As per Equation (5), the thermal resistance of a heat pipe decreases with a larger conduction area and shorter conduction distance. However, due to the small contact area and long conduction distance, the thermal resistance of the interval channel OHP is marginally higher than that of the non-interval channel OHP. Nonetheless, the interval channel OHP exhibits better performance than the non-interval channel OHP at low power. Therefore, we conclude that designing the interval PHP can effectively enhance heat transfer performance.

$$R=\frac{L}{k\times A}=\frac{1}{h\times A}$$

(5)

Figure 17 shows the utilization of the effective thermal conductivity (calculated using Equation (6)) to remove the influence of dimensional errors. By removing the size factor, the impact of oscillating flow on the heat transfer performance becomes more evident.

$$k=\frac{Q\times L}{A\times \left({\mathrm{T}}_{\mathrm{H}}-{\mathrm{T}}_{\mathrm{c},\mathrm{avg}}\right)}$$

(6)

Table 8. Comparison of similar study.

	Present Study		Ref. [36]
	Without Thermal Interaction	With Thermal Interaction
Manufacturing	Additive manufacturing		Tradition
Dimension (L $\times$ W $\times$ H)	120 $\times$ 51 $\times$ 4.2 (mm)		200 $\times$ 50 $\times$ 3.5 (mm)
Channel size	2 $\times$2 (mm)		2 $\times$ 2 (mm)
Number of turns	5		6
Material	Stainless steel 316 L		Stainless steel 316 L
Working fluid	Methanol		Methanol
Input power	20~80 W		20~100 W
Thermal resistance	$0.4~0.35 °C/\mathrm{W}$	$0.47~0.34 °C/\mathrm{W}$	$1.3~0.3 °C/\mathrm{W}$

presents a comparison with another similar study. It can be observed that our OHP has better thermal resistance even at low wattage. The two papers have similar dimensions, designs, materials, and working fluids. The final thermal resistance values of 80 W to 100 W are consistent with the expected results. The inter-pipe spacing avoids thermal interaction, and the slightly rough evaporation surface enhances the operation at low wattage. This enables the AMOHP to have good thermal resistance values even at low wattage. This demonstrates the uniqueness and feasibility of applying additive manufacturing technology to the manufacture of heat pipes.

5. Conclusions

This study utilizes additive manufacturing (AM) to produce a SUS316L oscillatory heat pipe (OHP) with a length of 120 mm, a width of 51 mm, and a 2 mm square flow channel. Methanol is used as the working fluid to examine the oscillation characteristics and thermal resistance at various input power levels (20 W, 30 W, 40 W, 60 W, 80 W). The design and production stages of AM necessitate intricate experiments, and various factors can influence the final state of the end product. The primary conclusions of this study are as follows:

1.

When utilizing AM to fabricate OHP, it is critical to consider the equipment’s limitations and the application field’s requirements to ensure product quality. This includes studying the printing parameters for the production target, paying attention to structure relative density and size deviation, and using special design to address the technology’s limitations. It is essential to make necessary adjustments accordingly to achieve optimal results.

2.

The use of volumetric energy density ($E_v)$ as a manufacturing parameter is a simple and efficient method. While it may overlook some variables, it leads to the production of reliable parts.

3.

The post-processing defects and internal structural defects may cause larger errors in the thickness of parts compared to length and width errors.

4.

Incorporating spacing into the channel design of the OHP can effectively reduce the heat conduction between the channels, thereby increasing the oscillation range of the working fluid and improving the performance of the OHP. The experiment demonstrated that at 20 W power, the OHP with spacing exhibited an equivalent thermal conductivity of 2428 (W/mK), while the one without spacing showed a value of 1743 (W/mK). At 80 W, the values were 2663 and 2511 (W/mK), respectively. Thus, it is evident that OHP without thermal interaction can significantly enhance the oscillation effect and produce a superior equivalent thermal conductivity at low power.

5.

AM technology enables the exploration of new ideas and facilitates the creation of complex designs that were not possible with traditional manufacturing methods. The integrated one-body manufacturing method offers numerous possibilities for creating specialized designs and expanding the potential thermal applications.

6.

We conducted a comparison with similar research studies, and our results showed distinct advantages compared to the other studies. These advantages can be attributed to the use of inter-pipe spacing, which prevents thermal interaction, and the slightly rough evaporation surface that enhances the operation at low wattage. Our findings demonstrate the feasibility of applying AM technology to the production of heat pipes.

AM is a versatile technology that offers flexibility in the creation of complex designs. However, it is essential to consider various constraints and factors when utilizing this technology when fabricating OHP. Basic theory regarding the flow channel can be studied to optimize evaporation capacity and pressure distribution. Additionally, the direct printing of the condenser onto the OHP can be explored as a means to reduce contact thermal resistance. OHPs are simple and efficient heat transfer devices that can benefit from the unique capabilities of AM technology.