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Applied Thermal Engineering 236 (2024) 121857

Available online 29 October 2023

Research Paper

Experimental investigation on the optimization of different filling ratios for large-size flat plate heat pipe array

Qinli Xue, Guodong Xia

^*

, Wenbin Zhou

Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China

A R T I C L E I N F O Keywords:

Flat plate heat pipe array Filling ratio

Asymmetric filling Single channel Experiment

A B S T R A C T

To meet the increased heat dissipation demands of 5G base stations, a flat plate heat pipe array (FPHPA) of 500

×200 ×3 mm³comprising of 19 independent channels is developed. To investigate the thermal performance of the FPHPA under different filling ratios (FR) and heat powers (30–300 W), a multi-channel independent filling system is designed to conduct two experimental cases: uniform or asymmetric filling of all channels. Results indicate that 30 %–70 % FR exhibits better thermal performance and can replace the aluminum plate over a broader heat power range. Furthermore, separate tests are conducted to evaluate the heat transfer and start-up performance of the FPHPA. The optimal filling ratio interval of single channel at different heat fluxes is derived through asymmetric filling. Subsequently, a mixture model is proposed and compared with the uniform filling ratio case, resulting in optimized heat transfer performance. The optimized FPHPA provides better heat transfer performance, effectively reducing the area averaged wall temperature and ensuring the normal operation of the 5G base station.

1. Introduction

With the rapid development of 5G communication networks, the high level of integration of telecom equipment has raised the performance requirements and challenges for heat dissipation processing technology [1]. To meet the key performance needs of 5G networks, which include high power, high frequency band and high speed, 5G base stations have undergo substantial changes compared to 4G. The use of Mass MIMO (Multiple-Input Multiple-Output) technology has significantly increased the connectivity of 5G base stations, resulting in power consumption that is approximately 2.5–3 times higher than that of 4G [2,3]. The increased power consumption leads to higher heat genera- tion, if heat dissipation is not prompt, it can cause the internal temperature of the base station to exceed its rated temperature (e.g., the critical temperature for multiple chips in the base station is required to be within 80 ^◦C), which would significantly impact network stability and equipment lifespan. Moreover, since base stations are ordinarily installed on rooftop racks and elevated locations in the field, reducing the size and weight is crucial for easier installation. However, this reduction in size and weight also makes it increasingly challenging to dissipate heat from 5G base stations [4].

As shown in Fig. 1, from a dustproof and maintenance-free perspective, 5G base stations are typically enclosed devices that employ natural heat dissipation. When the heat is emitted from the components, it is initially absorbed by the internal devices, causing an increase in device temperature. This temperature difference facilitates heat transfer from high temperature areas to low temperature areas.

Consequently, the heat from the base station is first transferred to the shell, and then conducted from the shell to the surrounding air [5,6].

Currently, the prevalent heat dissipation solution for 4G base stations involves thermally conductive silicone，cast aluminum or semi-solid die-cast aluminum. For 5G base stations, a new heat dissipation mod- ule solution, combining fins and flat plate heat pipe/VC with thermal interface materials, will be adopted.

Extensive research has been conducted on fins [14–21] and thermal interface material [22–25]. However, there have been relatively fewer reports on flat heat pipes specifically designed for base stations. The flat plate heat pipe utilizes the principle of internal phase change heat transfer, resulting in a higher heat transfer coefficient compared to single-phase cooling. Furthermore, it can be matched with different forms of fins, making it ideal for heat dissipation with base stations.

Experimental studies on large size flat heat pipes are counted in Table 1.

Although copper is often used as a material for heat pipes due to its high

* Corresponding author.

E-mail address: [email protected] (G. Xia).

Contents lists available at ScienceDirect

Applied Thermal Engineering

journal homepage: www.elsevier.com/locate/apthermeng

https://doi.org/10.1016/j.applthermaleng.2023.121857

Received 17 July 2023; Received in revised form 28 September 2023; Accepted 23 October 2023

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Applied Thermal Engineering 236 (2024) 121857

thermal conductivity, its high density and thermal expansion in- compatibility make it unsuitable for base stations. On the other hand, aluminum with its lower density and better ductility, is currently considered the most suitable material for base station enclosures.

Noteworthy studies utilizing aluminum as the material include the following: Tang et al. [7] developed a size of 150 ×150 ×2.8 mm³ aluminum with a vacuum cavity with struts inside for LED heat dissipation. Zhong et al. [10] designed an aluminum flat heat pipe array sized 220 ×200 ×1.5 mm³for solar energy collection. Li et al. [13]

studied an aluminum flat heat pipe (420 ×210 ×6.3 mm³) with a stainless steel wick for the base station. Due to the vertical placement of 5G base stations, the working fluid can complete the return flow under gravity, wickless construction could save processing costs and reduces weight. And the structure of vacuum cavity is not suitable for large size, therefore the flat plate heat pipe array (FPHPA) is a preferable option.

The FPHPA is classified as a thermosiphon, since there is no wick in the tubes, the return of work fluid from the condenser to the evaporator does not rely on the capillary force generated by the wick, but on the gravity of the fluid itself. Due to its wickless structure, the heat transfer performance of FPHPA largely is largely dependent on filling ratio, Lips et al. [26] conducted experiments to measure the wall temperature fields in the FPHP under different filling ratios and heat fluxes, and showed that both filling ratio and thickness of the vapor space have a significant effect on the thermal performance. Zhang et al. [27–29]

designed a closed-space visualization experiment to investigate the relationship between coexisting boiling and condensation in phase change heat transfer. The results show that the boiling and condensation processes are closely linked and interact with each other, and that there is an optimal filling ratio. Aly et al. [30] developed a helically-micro- groove heat pipe with nanofluid under different filling ratios (20 %–

80 %) and heat input (45–65 W). They revealed that increasing the filling ratio resulted in increased heat transfer coefficients of evaporation and condensation while reducing thermal resistance. Wang et al.

[31] comprehensively investigated the effects of filling ratios (10 %–60

%), inclination angle, and input heat power (10–40 W) on FPHPA. They found that the optimal filling ratio differed for each input power and that 20 % filling ratio worked best at 15 W. In addition, several Nomenclature

c specific heat capacity (J/kg⋅K) h heat transfer coefficient (W/(m²•K)) m mass flow rate of cooling water (kg/s) q heat flux (W/m²)

A area (mm²)

k thermal conductivity(W/m⋅K) L length (mm)

W width (mm)

T Thickness(mm) Q heat power (W)

R thermal resistance (K/W) T temperature (^◦C) U voltage (V) I current (A)

Acronyms

AAU Active antenna unit BBU Baseband unit

FPHPA Flat plat heat pipe array FR Filling ratios

MIMO Multiple-Input Multiple-Output Greek letters

η ratio

Δ difference Subscripts

c condenser section e evaporator section sig single channel

Fig. 1. Comparison of 4G and 5G base station.

Table 1

Experimental studies of large FHP or vapor chamber.

Author Dimensions

(L*W*T mm) Shell/wick material Wick structure Working fluid Cooling method Performances

Tang et al. [7] 150*150*2.8 aluminum/aluminum microgrooves acetone fins& natural convection 130 W; 0.83 ^◦C/W

Feng et al. [8] 180*180*3 copper/copper microchannels water liquid cooling best 35 %FR;

920 W/(m•K)

Reyes et al. [9] 190*140*15 aluminum alloy none Novec™ 7100 fins & forced convection 25 W

Zhong et al. [10] 220*200*1.5 aluminum/aluminum microchannels Novec™ 7100 natural convection 40 W; 0.22 ^◦C/W

Hsieh et al.[11] 300*300*100 copper & glass none water fins & natural convection 140 W 0.2 ^◦C/W

Deng et al. [12] 400*100*1.1–2.3 aluminum none HFO-1233zd natural convection 50 W 1.15 ^◦C/W

Li et al. [13] 420*210*6.3 aluminum/stainless stee capillary acetone fins& natural convection 150 W 0.67 ^◦C/W Q. Xue et al.

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theoretical studies existed on filling ratios, such as Cerza et al. [32], who analytically and experimentally investigated the mechanism of bubble growth in thin, falling, superheated, laminar water film. El-Genk et al.

[33] developed the heart transfer correlations for liquid film and sorted the heat transfer regimes into three patterns. Jiao et al. [34] developed a theoretical model to investigate the effect of filling ratio on a two-phase closed thermosyphon, suggesting a range of filling ratio intervals that

can keep the heat pipe stable and effective. Zhang et al. [35] determined the lower and upper limits of the recommended volume-to-filling ratio by using the criteria of local drying, flooding limit and boiling limit.

Numerous experimental and theoretical studies have established that filling ratio and heat input have a significant impact on the heat transfer performance of FPHPA. However, previous studies have only examined cases where the filling ratio is uniform across all channels, and no Fig. 2. Structure size and physical drawing of FPHPA.

Fig. 3. Schematic of the experimental system.

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analysis of non-uniform filling ratios has been reported. The FPHPA consists of independent channels, and the heat transfer performance of each channel determines the overall heat transfer capability. This is typically evaluated using an experimental setup where thermocouples are positioned on the outer wall of the device to calculate the equivalent thermal resistance. In recent years, a more advanced approach has emerged, which involves the use of infrared imaging to measure the temperatures of the solid and fluid components. Cattani et al. [36]

conducted infrared measurements on the outer wall of a sapphire channel in a single-ring PHP. Another inverse technique was utilized by

Pagliarini et al. [37,38] to evaluate the wall-to-fluid heat fluxes on the metal opaque branch of a running multi-turn PHP. However, the use of high-precision and high-resolution infrared cameras can be expensive, and it requires the application of high emissivity coatings on the device surface, making it more challenging to implement [39]. In this paper, we propose a straightforward and practical method to investigate the optimal fluid filling rate range for a single channel. In this paper, we propose a simple and feasible method to explore the optimal fluid filling rate interval under a single channel by arranging thermocouples on the wall surface to measure the heat flux in the direction of the heat source Fig. 4.Photograph of the experimental system.

Fig. 5.Dimensions and installation diagram of heating equipment.

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diffusion, based on which we propose a hybrid filling model to optimize the thermal performance of FPHPA for a 5G base station.

2. Experimental system and measurement 2.1. Design and fabrication

A large size aluminum flat plate heat pipe array measuring 500(L) × 200(W) ×3(H) mm³and weighting 682.53 g, is composed of 19 independent channels, as shown in Fig. 2(a). Each channel measures 490(L)

×5(W) ×1.5(H) mm³contains 5 mm wide ribs in the middle layer of the FPHPA. Instead of using casting or chemical etching, the FPHPA uses a whole 6061 aluminum alloy plate for CNC machining. The process is as follows:

(1) Two aluminum plates of 500 ×200 mm²were cut out as upper cover and bottom plate with thicknesses of 0.75 mm and 2.25 mm respectively. These plates were de-oiled, decontaminated, and polished in order.

(2) The bottom cavity was slotted to a depth of 1.5 mm by CNC, cleaned with organic solvent in the ultrasonic cleaner, and then dried in a 60 ^◦C oven, as shown in Fig. 2(b).

(3) The upper cover and bottom cavity are welded by vacuum brazing, and then reduced in a gas mixture of 95 % nitrogen and 5 % hydrogen under a high temperature of 620 ^◦C, to remove the oxide on the surface. The final FPHPA is shown in Fig. 2(c).

2.2. Experimental system

The experimental system mainly consists of the FPHPA, heating system, cooling system, filling system and data acquisition unit, as shown in Figs. 3-4.

2.2.1. Heating system

The primary thermal power consumption of a 5G base station involves a point heat source simulated chip and a surface heat source simulated MIMO. Although the MIMO consumes relatively less power (10–50 W), it is large. In the experiment, a PI heating film is used, which has specific dimensions shown in Fig. 5(a). The heating film measures 200 ×200 mm²and is internally equipped with a serpentine electric heating wire arrangement. It is also externally wrapped with 2 mm thick insulation cotton. In the center of the film, a hole measuring 12 ×12 mm²is created to accommodate the placement of a point heat source simulated chip.

The heat flux of the simulated chip is large (15–250 W/cm²), and in order to be able to accurately simulate, a copper casting heating rod as well as a machined cut is shown in Fig. 5(b). The final heating surface is 10 ×10 mm². The heat flux is calculated using one-dimension Fourier’s law, and by adjusting the power supply, it can reach up to 300 W/cm². During the experiment, the total heat power is the sum of the two parts of the PI heating film and the copper heating block. The specific distribution from 30 W to 300 W is shown in Table 2. In the arrangement illustrated in Fig. 5(c), the evaporator’s surface is connected to the heated surface of the heater. To minimize thermal contact resistance between the bottom surface of the evaporator and the top surface of the heater, a thermally compounded interface material (with a thermal conductivity of 6 W/ (m⋅K)) is placed on the contact surface and secured by bolts.

2.2.2. Filling system

To achieve individual filling of each channel, a filling cap is manufactured as shown in Fig. 6(a). Each filling cap has a 6.35 mm tube diameter with a two-way stop valve on, and the interface is coated with super vacuum sealant (Agilent Torr Seal AB 95300). Fig. 6(b) shows the finished diagram of the filling system and Fig. 6(c) shows the operating system. The inflation cap is connected to the vacuum pump, and valve Table 2

Distribution of total input power from 30 W to 300 W.

Total heat power/W 30 60 90 120 150 180 210 240 270 300

PI heating film/W 15 30 45 60 75 90 90 90 90 90

Copper block heater/W 15 30 45 60 75 90 120 150 180 210

Fig. 6. Independent channel filling cap and vacuum filling system.

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V5 and V7 are opened. Next, the mechanical pump and the molecular pump are activated, pumping each channel to a pressure below 10 Pa in order to remove any non-condensable gas (NCG). Subsequently, the vacuum pump is deactivated, leaving only the mechanical pump running to maintain a pressure of 10 kPa in each single channel. Valve V5 and V7 are then closed. Following a 24-hour waiting period, the pressure change in each channel is examined. Results indicate that FPHPA exhibits exceptional air tightness, as the pressure variation remains within a 5 % limit. Finally, the high-pressure filler is connected and valve V5 is opened to inject the working fluid into the channel. The channel filling ratio can be defined using Eq. (1):

η=Vl

Vc

×100% (1)

where V_lis the volume of injected liquid and V_cis the total volume of a single channel.

The maintenance of a vacuum throughout the experiment is crucial for its success. To ensure the equipment’s airtightness, it was tested by filling it with deionized water. After repeated testing, the experimental process using resistance gauge (KYKY FF-620) to measure the pressure.

The operating pressure range is measured to be 10.3~16.7 kPa, using the pressure sensor (Games-2200BAA). Although the temperature of the working fluid gradually changes with the heating power, both the pressure and temperature inside the tube exhibit the same trend. The saturation pressure, deduced from the working fluid temperature and the actual test pressure in the range of 50 Pa, confirms the FPHPA’s suitability for the subsequent experiments. Therefore, the FPHPA can maintain excellent gas tightness in future experiments.

Considering the base station is in the outdoor need for anti-freeze, the working fluid is mixed with deionized water and ethanol (75:25 v/v), and the initial test pressure in each channel is set to 10 kPa.

2.3. Experimental procedures

The experimental procedure is conducted as follows: The total heat power consists of the PI heating film and the copper block heater, ranging from 30 W to 300 W. All thermocouples (T-type) are connected to the datalogger (Agilent 34970A). The temperature acquisition interval is set to 10 s. In order to maintain a constant temperature, the cooling water temperature in the water bath was kept at 20 ±0.5℃. Addi- tionally, the circulation pump flow ratio is set at 0.6 L/min. The locations of the various measurement points can be seen in Fig. 7. To measure the wall temperature of the cold plate, 4 thermocouples (T201- 204) are utilized, as shown in Fig. 7(a). The inlet and outlet water

temperatures of the cold plate were measured by Tin and Tout, respectively. While another 4 thermocouples (T301-304) are used to measure the temperature of the copper block heater, as shown in Fig. 7 (b). To investigate the overall and single channel thermal performance of FPHPA, two cases of experiments are carried out.

In case 1, the measurement points (T101-110) are arranged in the longitudinal direction to measure the temperature of the evaporator, adiabatic section and condenser respectively, as shown in Fig. 7(c). The temperatures measured at symmetrical positions on the left and right sides are averaged to obtain Te1, Te2, Ta, Tc1, Tc2. Subsequently, all channels are set to the same filling ratio, ranging from 30 % to 70 %, as shown in Fig. 8(a). The heat power is varied from 30 W to 300 W in increments of 30 W, and data from each measurement point is recorded to investigate the overall start-up performance, the average wall temperature, and the heat transfer limit of the FPHPA.

In case 2, the wall temperature measurement is rearranged to investigate the optimal filling interval of a single channel under different heat fluxes, as shown in Fig. 7(d). Thermocouples are positioned at a height of 100 mm for each channel, symmetrically distributed from left to right. The left channels are marked as #1–9 with corresponding temperature T1-9. For comparison, the right-side channels are labeled as

#1

′

-9

′

with temperature T1′-9′. During the experiment, the heat power is maintained at 150 W, with the PI heating film providing 75 W and the copper block heater supplying 75 W. At this heating power, the left channels #1–9 are filled with 30 %-70 % FR, respectively, and the right channels #1

′

-9

′

remain unfilled, as shown in Fig. 8(b). When the quasi- steady state is reached, the heat fluxes on a single channel are calculated from the data recorded at T1

′

-9

′

, allowing for investigation of the heat transfer performance of a single channel under different heat fluxes based on the recorded data at T1-9.

2.4. Data acquisition

The input heat power Qin can be calculated by:

Qin=Q1+Q2 (2)

where Q1 is power of PI heater, and Q2 is the power of copper block heater, which can be calculated by:

Q1=U⋅I (3)

where U and I is the voltage and current of the PI heater.

Q2= − A0kcu

(T301+T302)/2− (T303+T304)/2

Δy (4)

Fig. 7. The arrangement of the measuring points.

Q. Xue et al.

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where A₀is the area of final heating surface (10 ×10 mm²), k_cuis the thermal conductivity of copper, Δy is distance (4 mm) between measurement points in copper block heater.

The average temperature of the evaporation (T_e), condensation (T_c), adiabatic (Ta) and water cooling (Tw) can be calculated from the above measured point temperatures:

Te=1

2(Te1+Te2) =1 4

∑¹⁰⁴

101

Ti (5)

Tc=1

2(Tc1+Tc2) =1 4

∑¹¹⁰

107

Ti (6)

Ta=1

2(T105+T106) (7)

Tw=1 4

∑²⁰⁴

201

Ti (8)

The thermal resistance of the whole FPHPA (R) is defined as follows:

R=Te− Tc

Qin (9)

Effective thermal conductivity keff is defined as follows:

keff=Leff

AtR (10)

where Leff is the distance between the centers of the evaporation and condensation, At is the cross-sectional area of FPHPA.

Cooling capacity of water-cooling system (Qc) can be calculated as:

Fig. 8. Filling ratio set in different channels of FPHPA.

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Qc=m˙watercwater(ToutTin) (11)

where c_wateris the specific heat of cooling water, m˙wateris mass flow of water.

Heat transfer coefficient of water cooling (hw) can be calculated as:

hw= Qc

Ac(Tc− Tw) (12)

where Ac denotes contact area of water-cooling plate.

Heat transfer coefficient of evaporation for single channel (he-sig) is defined as follows:

he−sig= qsig

Tx− Ta (13)

where Tx denotes temperature of the corresponding channel of left side in case 2 (T1-9), qsig denotes the heat flux reaching a single channel, which can be calculated by:

qsig=T_x′− T_x+1′

Δx (14)

where Tx’ denotes temperature of the corresponding channel of right side in case 2 (T_1-9′), Δx is center distance (10 mm) between adjacent channels.

2.5. Experimental uncertainty

To ensure the credibility of the experiment, an uncertainty analysis of the collected data is required. The T-type thermocouple used in the experiments was calibrated to an accuracy of 0.1 K. The uncertainty of PI heater Q1 is less than ±0.5 W. Vernier calipers was used for distance measurement in the experiment, the accuracy of the vernier calipers is 0.01 mm. Uncertainty of 0.5 % for measured area. The uncertainties of copper block heater Q2, thermal resistance, heat transfer coefficient and heat flux were calculated as:

ΔQ2

Q2

=

̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅

(Δ(ΔT) ΔT

)₂ +

(Δ(Δy) Δy

)₂ +

(ΔA0

A0

)₂

√

≈4.76% (15)

ΔR R =

̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅

(Δ(ΔT) ΔT

)₂ +

(ΔQin

Qin

)₂

√

≈6.38% (16)

Δkeff

keff

≈ΔR

R (17)

ΔQc

Qc

=

̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅

(Δ(ΔT) ΔT

)₂ +

⎛

⎝Δm˙water

m˙water

⎞

⎠

2

√√

√ ≈3.23% (18)

Δhw

hw

=

̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅

(Δ(ΔT) ΔT

)₂ +

(Δ(ΔQc) ΔQc

)₂ +

(ΔAc

Ac

)₂

√

≈5.34% (19)

Δqsig

qsig

=

̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅

(Δ(ΔT) ΔT

)₂ +

(Δ(Δx) Δx

)₂

√

≈2.54% (20)

Δhe−sig

h_e−_sig =

̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅

(Δ(ΔT) ΔT

)₂ +

( Δ(

Δqsig

)

Δq_sig )₂

√√

√√ ≈3.27% (21)

3. Results and discussion

The effect of heat power and filling ratio on the thermal performance of FPHPA is examined below by analyzing the variation of wall temperature, thermal resistance, and heat transfer coefficient.

3.1. Heat transfer performance of FPHPA

The thermal performance of FPHPA without charging the working fluid (FR =0) was evaluated as shown in Fig. 9(a). The initial heat power was set to 30 W and increased in increments of 30 W until the chip’s junction temperature exceeded the maximum allowable temperature by 80℃. The junction temperature surpassed 80℃ at 90 W of heat power, indicating that the heat dissipation limit of the pure aluminum plate is 60 W. Fig. 9(b) shows the wall temperature variations of the FPHPA charged with 50 %FR, with the heat power ranging from 30 W to 300 W in increments of 30 W. In the initial stage, as the temperature of the evaporator and adiabatic section rises rapidly, heat transfer dominates at this stage. After reaching the peak, the temperature drops slightly, indicating the start of the heat pipe. Throughout the experiments, we observed that both evaporator and adiabatic sections exhibited a step increase and subsequent continuous stabilization at different heat powers, while the temperature step in the condenser was not obvious and showed a continuous increase. Besides, the temperature gradient interval gradually decreases, particularly after reaching 240 W. When the heating power exceeded 150 W, the FPHPA took longer to stabilize, and its temperature uniformity decreased. At the same time, the junction temperature exceeds 80 ^◦C, exceeding the upper limit of the equipment temperature requirement. Specifically, at a heat power of 210 W, the Fig. 9. The wall temperature variations of FPHPA charged with 0FR and 50%FR.

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Fig. 10.The variation of thermal resistance and convective heat transfer coefficient under different heat power.

Fig. 11.The wall temperature variations of FPHPA charged with30%-70%FR.

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temperature difference between the different measuring points in the evaporator significantly increased, while the temperature of the cooling water inlet and outlet were not significant. These observations suggest that the FPHPA reached its heat transfer limit.

As shown Fig. 10(a), the thermal resistance decreased with an increase in heat power, reaching a minimum value of 0.0938 K/W. This decrease in thermal resistance can be attributed to the forced convection and resulting condensation of the cooling water. The working fluid undergoes a phase change in the evaporator and the gaseous fluid is cooled in the condenser. In the case of low heat power, a liquid film forms on the pipe wall and flows back to the evaporator under gravity, resulting in maximum convective heat transfer coefficient hc, as shown in Fig. 10(b). However, with the increase in heat power, there is an acceleration of liquid evaporation and strengthening of mutual oscilla- tion flow. Consequently, more vapors enter the condenser and forms a liquid film on the wall. The presence of this film hinders condensation heat transfer and leads to a decrease in the condensation heat transfer coefficient. Additionally, as the average wall temperature of the condenser continues to rise, the difference with the average wall temperature of the evaporator decreases. This decrease in temperature difference further reduces the thermal resistance as the heating power increases. When the heat power exceeds 150 W, the condensate reflux rate on the condenser wall decreases, leading to a further increase in the thickness of the liquid film on the wall. As a result, the condensation heat transfer coefficient remains relatively constant. At this point, the gas–liquid mixing in the channel, the temperature gradient is gradually reduced. The evaporator wall temperature continues to rise and the FPHPA reaches its the heat transfer limit.

3.2. Effect of filling ratio on FPHPA

To investigate the effect of filling ratio on the thermal performance of FPHPA, a uniform fill ratio of 30 % to 70 % was employed in all channels. Fig. 11 illustrates the variation of the measurement point temperature with different filling ratios. Compared to the previous test results of 50 %FR, the FPHPA can be activated regardless of whether the filling ratio is lower (30 %–40 %) or higher (60 %–70 %). Although the filling ratio has little effect on the starting time and performance for this multi-channel parallel heat pipe, it does significantly impact the wall temperature. Reducing the filling ratio can decrease the starting quasi- steady-state temperature of the evaporator, when the heat power is

30 W, the maximum temperature of 30 %FR is about 40 ^◦C, and 50 ^◦C for 50 %, compared to higher, which is 55–60 ^◦C for 60 %FR and 70 %FR.

Fig. 9.(a) shows that when the filling ratio is 0, the temperature can reach 60 ^◦C at 30 W, indicating that the high filling ratio failed to start in the initial state. In contrast, the lower liquid filling ratio is due to the thinner liquid layer thickness and the weaker natural convection between the working mass and the heating wall. Natural convection has less effect on the thickness of the superheated liquid layer near the wall, and the superheated liquid layer is more easily established, so evaporator and boiling occur earlier. As the heat power increases, the wall temperature rises at a slower ratio during natural convection at a larger liquid filling ratio, while the overall temperature rises slowly during the nucleation boiling stage due to the higher heat transfer coefficient.

However, due to the high filling ratio the temperature is too high at the beginning stage, and although it rises more slowly afterwards, it still quickly reaches the maximum required temperature of the chip (80 ^◦C).

When the heat power is low, the heat transfer in the heat pipe is mainly determined by the heat conduction and the liquid film evaporator. A lower filling ratio promotes a faster phase change heat transfer, which in turn reduces the evaporator temperature and overall thermal resistance. With an increasing heat power, phase change heat transfer plays a major role, and the differences in thermal resistance between various filling ratios become less significant. As the heat power continues to increase, no liquid film is formed in the evaporator, and the liquid filling amount can only satisfy the flow of vapor and liquid film, which leads to the dry out limit, resulting in the increase of thermal resistance at low filling ratio, as shown in Fig. 12.

3.3. Heat transfer performance of single channel

To investigate the thermal performance of the single channel, the thermocouples were rearranged at a horizontal position at a height of 100 mm，with 50 %FR filling only one side of the channel. Setting the heat power of 150 W, when the FPHPA reaches quasi-steady state, the measured point temperature of each channel is shown in Fig. 13(a). On the right side, the temperature of the measurement point will be higher than that of the symmetric position on the left side due to the absence of liquid filling. There will be a temperature gradient, indicating that the corresponding heat flux on the different channels are not the same. The heat transfer mainly occurred by conduction in the right channel due to the absence of liquid. Phase change of the working fluid in the left Fig. 12.The thermal resistance of FPHPA charged with 30%-70%FR at the different heat powers.

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channel generated a temperature difference, which was used to measure the heat transfer performance of the single channel, as demonstrated in Fig. 13(b). Given the symmetrical structure of FPHPA, the heat flow of each channel can be calculated by the change of temperature gradient in the unfilled part on the right side. The temperature difference was then utilized to estimate the equivalent convective heat transfer coefficient of each channel, and the results are shown in Fig. 13(c). The heat transfer coefficient of a single channel increases as the heat flux increases, and when the heat flux reaches 15,500 W/m²(channel 2–6#), the heat transfer coefficient is the highest. The heat flux continues to increase,

the heat transfer coefficient decreases anyway, which indicates that the heat transfer limit of the channel has been reached at this time and the heat transfer begins to deteriorate.

The thermal performance of the heat pipe is significantly affected by the filling ratio and heat power. According to the distribution of liquid film and liquid pool in single channel, liquid film in evaporator may dry out or liquid pool exceeds the evaporator. The filling ratios corresponding to the transitions are called CFR and EFR, respectively. If the filling ratio is less than CFR, a local film dry out could occur, reducing the critical heat flux. However, with a filling ratio greater than EFR, the Fig. 13. The heat transfer performance of single channel with asymmetric filling ratio.

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liquid will be carried up to the condenser section and subcooled. This process leads to periodic burst boiling and, eventually, heat transmission deterioration. The optimal performance of FPHPA is achieved by keep- ing the liquid-filling interval within reasonable limits for different channels. The heat transfer coefficients for each channel were determined by varying the filling ratio of the left region between 30 % and 70

%, as illustrated in Fig. 13(d). Similarly, to the previous 50 % FR, a high heat transfer coefficient interval was observed without using the channel filling ratio at different heat fluxes, as depicted in Fig. 13(e). The optimal interval of filling ratios at different heat fluxes was recorded, with the starting and ending values identified as CFR and EFR, respectively, and plotted in Fig. 15(f).

Fig. 14. The heat transfer performance of the FPHPA with optimized Mixture01and Mixture02.

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3.4. Optimization of filling ratio model

Although the filling interval range is relatively large, the optimal filling ratio at different heat flows can be selected based on the results of previous experiments, the filling ratios at the maximum heat transfer coefficient for different channels are shown in Fig. 14 (a). This form of filling also exhibits a regular pattern, with the filling ratios gradually decreasing from the center to the sides, consistent with the decreasing heat flow along the X-axis. The FPHPA is set up according to the above filling ratios, with both sides being filled and symmetrical, which named Mixture01 as shown in Fig. 14(c).

Using the same test criteria as before, the heat power was set to start at 30 W, increasing in 30 W intervals until 300 W. As shown in Fig. 14 (b), the overall thermal resistance of Mixture01 is calculated for different heat powers and compared with the previous overall liquid filling ratio of 50 %. In the stage of 30 W-90 W, the thermal resistance changes similarly, after 120 W the thermal resistance of the Mixtue01 is lower, but after 210 W the thermal resistance tends to be the same again.

As Mixture01 was optimized for 150 W heat power, its heat transfer performance is superior in the 90 W-180 W range. However, as the thermal power increases to 210 W, the filling ratio of some channels has reached the heat transfer limit, resulting in the overall heat transfer performance not being optimized. To prevent local channels from drying out due to low filling ratio, the Mixture02 was established for heating power above 200 W, shown in in Fig. 14(d).

As shown in Fig. 14(e), the two mixture types are compared with 50

% FR at different heat powers. The thermal resistance of Mixture02 is close to 50 %FR in 0–60 W, and lower than 50 %FR in 90–180 W but higher than Mixture01. After 180 W, the thermal resistance is the lowest among the three, showing better heat transfer characteristics, indicating that increasing the filling ratio of the hybrid type can adapt to better power. The comparison of the maximum heat transfer capacity at different filling ratios revealed an improvement in both Mixture01 and Mixture02 compared to uniform filling, as depicted in Fig. 14(f).

3.5. Comparison to other studies

In this study, in order to gain a better understanding of the heat transfer characteristics of the FPHPA, heat pipes of similar materials and operational conditions were compared.

Wang et al. [31] conducted a study on a flat plate micro heat pipe measuring 150 mm in length, 26 mm in width, and 3 mm in thickness.

They found that the heat pipe had a maximum effective thermal

conductivity value of 8539 W/(m⋅K) when charged with a 20 % FR at 90 W. Narcy et al. [40] built a flat confined thermosyphon measuring 180 mm in length, 120 mm in width, and 5.5 mm in thickness. The experimental results showed that the effectiveness thermal conductivity value of the thermosiphon, using a 40 % water filling ratio, was 6023 W/

(m⋅K) at 300 W heat power when operating vertically. Manova et al.

[41] conducted research on an ultra-thin multiport mini channel thermosyphon measuring 200 mm in length, 20 mm in width, and 2 mm in thickness. They found that the thermosyphon reached the maximum effective thermal conductivity value of 2557 W/ (m⋅K) with 50 %FR at 50 W heat power. Deng et al. [42] proposes a heat pipe using a pressure welding process with dimensions of 400 mm in length, 100 mm in width, and 0–2.3 mm in thickness. Thermal conductivity with a peak of 16019 W/(m⋅K) and an average of 12616 W/(m⋅K) ranging from 0 to 90 W.

Shown in Fig. 15, compared to those flat heat pipes (thermosyphons), the Mixture01 and Mixture02 investigated in present study all show obvious higher overall effectivity thermal conductivity when heat power over 90 W. The current power of 5G base stations generally exceeds 100 W, so the FPHPA proposed in this paper has great potential for base station.

4. Conclusion

In this work，a flat plate heat pipe array was developed for 5G base station, whose size is 500 ×200 ×3 mm³. The thermal characteristics of FPHPA were examined under water-cooled conditions with different filling ratios and heat power. The study yielded the following conclusions:

(1) The FPHPA exhibit lower area surface temperature and thermal resistance compared to aluminum plate. Thermal performance ranging from 30 % to 70 % FR prove superior and can replace aluminum sheet over a wider range of thermal loads.

(2) The single channel with the optima filling ratio interval at different heat powers is the main reason for the different thermal performance limits of FPHPA. The heat transfer limit of low filling ratio is due to the local drying of the evaporator, which increases the evaporator thermal resistance. Accumulation of condensate in the condenser and weakened heat transfer performance caused heat transfer limit at high filling ratios.

(3) The effective filling intervals (CFR and EFR) of the single channel at different heat fluxes were obtained by asymmetric filling ratio experiments, and the asymmetric filling types of Mixture01 and Mixture02 were proposed based on this result.

(4) Both Mixture01 and Mixture02 have improved heat transfer compared to 50 % FR. Mixture01 is suitable for base stations with power within 200 W, and the lowest thermal resistance is 0.114 K/W at 180 W. When the power of the base station exceeds 200 W, Mixture02 is more suitable, with a minimum thermal resistance of 0.080 K/W at 300 W.

In conclusion, the FPHPA present better thermal performance than aluminum plate, which can effectively reduce the surface temperature of the area and ensure the normal operation of the base station. Further- more, aluminum FPHPAs have straightforward manufacturing with excellent weight and high structural strength. This paper provides an alternative solution for thermal management of 5G base stations and other similar high-power electronic devices.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 15. Comparison of the effective thermal conductivity of the current study with those in other literature.

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Applied Thermal Engineering 236 (2024) 121857 Data availability

Data will be made available on request.

Acknowledgment

This study was supported by National Natural Science Foundation of China (No. 51976002).

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