1 s2.0 S129007292300580X main

Available online 26 October 2023

1290-0729/© 2023 Elsevier Masson SAS. All rights reserved.

Kangning Xiong

^a,b

, Yuhao Luo

^b

, Yixian Hu

^b

, Shuangfeng Wang

^b,*

aState Key Laboratory of Green Building, School of Building Services Science and Engineering, Xi’an University of Architecture and Technology, Xi’an, 710055, PR China

bKey Laboratory of Heat Transfer Enhancement and Energy Conservation of Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, PR China

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

loop heat pipe High-efficiency condenser Flat evaporator Numerical analysis Heat transfer characteristics

A B S T R A C T

Loop heat pipe (LHP) is a high efficient and passive heat transfer device with long heat transfer distance and high cooling capacity. In this study, an advanced thermal management system based on a novel flat evaporator LHP with high-efficiency condenser was proposed to obtain high heat transfer capacity and low energy consumption.

Firstly, the cooling efficiency of two cooling heat sink units was evaluated by numerical analysis, and the optimization heat sink unit was applied in the LHP condenser. Then, the heat transfer characteristics of the flat evaporator LHP with high-efficiency condenser were researched under different charging ratios, fan voltages and LHP test directions. Finally, the energy consumption of the LHP cooling system was evaluated. The numerical results showed that the novel louver folded fin heat sink unit has a better cooling efficiency than that of the conventional straight fin heat sink unit. And the experimental results indicated that the flat evaporator LHP with high-efficiency condenser exhibited better heat transfer characteristics at the input power range from 50 W to 450 W when the charging ratio of LHP was 33.1%. At the maximum input power of 550 W, the junction tem- perature of the flat evaporator LHP with high-efficiency condenser was 87.93 ^◦C at the fan voltage of 12 V, and the junction temperature of this LHP at positive gravity direction of 30^◦was lower than that of other LHP test directions. Additionally, the energy consumption of the efficient air-cooled LHP system was lower than one- eighteenth of that of the water-cooled LHP system.

1. Introduction

In recent years, the thermal management of electronic components and devices have become a major challenge due to high thermal design power and power density [1,2]. For instance, the thermal design power of the AMD EPYC 7763 processor and the intel Core i9-10990XE pro- cessor have been up to 280 W and 380 W, respectively. In addition, the actual power consumption of Core i9-10990XE will exceed 1000 W when the electronic chip operates in the full Core 5.0 GHz [3]. Mean- while, for large-scale electronic devices cooling, energy savings should also be considered while meeting cooling requirements. For example, based on the energy statistics, the electric energy consumption of the worldwide data center industry was account for about 1.3 % of the world [4]. For a typical data center, the electric power consumption of cooling system accounts for about 38 % of the total electric power consumption [5]. Therefore, the suitable thermal management technology should be proposed and developed to meet the cooling requirement and save the

electric power consumption. According to the cooling methods, the cooling systems of electronic devices mainly have two types: air-cooled system and liquid-cooled system. At present, the liquid-cooled system has demonstrated excellent cooling performance, which can remove effectively more than 100 W/cm² power densities for electronic chips [6]. However, in comparison to the air-cooled system, the liquid-cooled system requires a large number of auxiliary equipment, such as pump, compressor, flow valve, filter, cooling tower and pipeline, which will lead to high equipment costs, maintenance costs and management costs [6]. Additionally, some low-toxic, volatile and flammable dielectric fluids will result in potential security risks and environmental problem [7]. The air-cooled system, as a common cooling technology, has been widely applied in cooling electronic chips. Among air-cooled system, the air-cooled heat pipe system is a passive, indirect two-phase cooling technology, which has many markedly merits, such as high effective thermal conductivity, low energy consumption, low cost, low thermal resistance, high reliability and non-pollution [8]. According to the operating principle and structure, the heat pipe can be divided into

* Corresponding author.

E-mail address: sfwang@scut.edu.cn (S. Wang).

https://doi.org/10.1016/j.ijthermalsci.2023.108719

Received 18 January 2023; Received in revised form 5 September 2023; Accepted 12 October 2023

conventional heat pipe, micro-heat pipe, pulsating heat pipe (PHP), sorption heat pipe (SHP), rotating heat pipe (RHP), loop heat pipe (LHP), and so on [9–11]. Compared with other types of heat pipe, the LHP has some outstanding advantages, including strong anti-gravity ability, long distance heat transport, high heat transfer efficiency, good thermal equilibrium and so on [12–14]. LHP is a long heat transfer distance, high cooling capacity phase change heat transfer equipment, which applying the capillary force of capillary wick to circulate the working medium, and the working medium transfers heat from the evaporator to the condenser in the cycle process [15–17]. At present, many researchers have reported the application of air cooling LHP technology in the thermal management challenge of electronic chips.

Maydanik et al. [18] provided a stainless steel-ammonia LHP with a cylindrical evaporator measuring 8 mm (OD) ×59 mm (L) and equipped an effective heating area 40 ×40 mm² for cooling the 1U server chip.

Their research conclusions showed that the LHP can effectively remove the heat power of 130 W by the forced air convection at the highest heat source temperature of 70 ^◦C. And the corresponding minimum total heat resistance was 0.33 ^◦C/W. Li et al. [19] presented and investigated a copper-water LHP with a plane, square evaporator measuring 30 ×30 × 15 mm³ for thermal management of electronic cooling. Their research results showed that the junction temperature of heating copper block can be controlled at around 85 ^◦C when the input power was 350W. In this study, the thermal properties of the LHP were researched in the direction of positive gravity (+90^◦). However, the thermal behavior of LHP in the conventional horizontal orientation has not been reported.

Liu et al. [20] demonstrated and studied a copper-acetone LHP with a miniature plane type evaporator sizing 40 ×30 ×14.5 mm³ and also equipped with a 4 mm thick stainless steel mesh wick of 82 layers 500 grids. Their research results displayed that the junction temperature of heating copper block and the LHP heat resistance were 52 ^◦C and 0.45 ^◦C/W at the input power of 60 W, respectively. Xue et al. [21]

developed and researched an aluminum-ammonia LHP with a plane type evaporator sizing 54 ×44 ×12 mm³ and outfitted a ceramic heat source with 40 ×40 mm² for cooling chip. And the volume of condenser fins for the air-cooled LHP was 150 ×80 ×23 mm³, with 0.5 mm thickness for each fin. Their research conclusions presented that at the highest input power of 513W (32.06 W/cm²), the surface temperature of ceramic electrical heater was controlled at 72.3 ^◦C, and the LHP heat resistance and the evaporator heat resistance were 0.058 ^◦C/W and 0.011 ^◦C/W, respectively. Li et al. [22] devised a new plane type evaporator LHP with a multi-porosity gradient sintered superhydrophilic wick structure, without the reservoir to reduce the size of the evaporator as much as possible. A series of experimental studies found that under fan voltage of

24 V, the junction temperature of heating copper block was no more than 85 ^◦C when the input power reached 150 W (24 W/cm²).

Based on the previous studies of air-cooled LHP, it can be demon- strated that the air-cooled LHP are hard to limit the temperature of electronic chips in a reasonable temperature range (normally <85 ^◦C) when the power density was larger than 37 W/cm² [23]. Compared to liquid cooling technology, the air-cooled LHP have some advantages such as fewer auxiliary equipment, high safety, non-pollution and low cost [6–8]. If the cooling capacity of the air-cooled LHP can be further improved, it will have a broader prospect in electronic chip cooling.

Therefore, in this work, a high-capacity air-cooled LHP with a new condenser and flat evaporator was proposed for the first time to cool the high power density electronic chips. Firstly, the cooling efficiency of two cooling heat sink units is evaluated by numerical simulation study, and the optimization heat sink unit is applied in the design of LHP condenser. Then, the impact of charging ratios, fan voltages and LHP test directions on the heat transfer characteristics of the air-cooled LHP with a new condenser and flat evaporator are investigated in detail. Finally, the energy consumption analysis of the air-cooled LHP system is pre- sented compared with the water-cooled LHP system. In summary, the comprehensive study demonstrates that the novel air-cooled LHP can become a promising, low-cost and high-efficiency technology for future high power density electronic chip cooling.

2. Numerical analysis of heat sink 2.1. Model geometry

In order to reduce the computational burden, some simplified geo- metric units are often applied in numerical studies. In this work, two cooling heat sink units, conventional straight fin heat sink (Design 1) and novel louver folded fin heat sink (Design 2), were used for numerical study and comparison of cooling efficiency in order to design the effi- cient LHP condenser, as described in Fig. 1. The conventional straight fin heat sink (Design 1) and novel louver folded fin heat sink (Design 2) were made of aluminum alloy. For the Design 2, the louver folded fin was welded to the middle of straight fins, and the detailed structure of the partially louver folded fin is presented in Fig. 2. Seen from Fig. 2, the folded fin unit was provided with louvers at angles of 45^◦and − 45^◦to enhance the disturbance of the cooling air. In the length direction, every ten louvers with an angle of 45^◦and − 45^◦were arranged alternately, and a total of 140 louvers from a folded fin. Additionally, the main geometric dimensions of the conventional straight fin heat sink (Design 1) and novel louver folded fin heat sink (Design 2) are given in Table 1.

Nomenclature

cp constant-pressure specific heat J/(kg⋅K) D diameter m

H height m ID inner diameter m L length m

OD outer diameter m P power W

p pressure Pa Q heat load W

R thermal resistance ^◦C/W T temperature ^◦C t time s

u, v, w x, y, z velocity components m/s V volume m³

W width m

x, y, z Cartesian coordinates

Subscripts

a ambient

c condenser

cc compensation chamber e evaporator

j junction

l liquid

sys system tot total

v vapor

Greek symbols α charging ratio % ρ density kg/m³ μ viscosity kg/(m⋅s)

λ thermal conductivity W/(m⋅K)

And the relative parameters in the model are written in Table 2. In order to facilitate the numerical simulation process, the following assumptions were considered:

● The outer surface of the Design 1 and Design 2 was thermally insulated.

● The cooling air flowed only in the length direction, and the diffusion in the height direction was neglected.

● The louver of folded fin was neglected to simplify the settings of the grid.

2.2. Model equations

The governing equations for the heat sink unit design were presented as bellows:

Continuity equation:

∂u

∂x+∂ν

∂y+∂w

∂z=0 (1)

where u, v, w are the velocity components of the fluid in the directions of x, y, and z, respectively.

Momentum equations:

∂(ρu)

∂t +∂(ρuu)

∂x +∂(ρνu)

∂y +∂(ρwu)

∂z =μ (∂²u

∂x²+∂²u

∂y²+∂²u

∂z² )

− ∂p

∂x (2)

∂⁽ρv)

∂t +∂⁽ρuv)

∂x +∂⁽ρνv)

∂y +∂⁽ρwv)

∂z =μ (∂²v

∂x²+∂²v

∂y²+∂²v

∂z² )

− ∂p

∂y (3)

∂(ρw)

∂t +∂(ρuw)

∂x +∂(ρνw)

∂y +∂(ρww)

∂z =μ (∂²w

∂x²+∂²w

∂y²+∂²w

∂z² )

− ∂p

∂z (4)

where ρ is density, μ is viscosity, and p is pressure.

Energy equations:

∂(ρT)

∂t +∂(ρuT)

∂x +∂(ρνT)

∂y +∂(ρwT)

∂z =λ cp

(∂²T

∂x²+∂²T

∂y²+∂²T

∂z² )

+ST (5) where T is temperature, λ is the fluid thermal conductivity, cp is constant-pressure specific heat, and ST is the term of viscous dissipation.

The thermal fluid inlet, thermal fluid outlet, and side of heat sink unit boundary condition can be defined as below:

Thermal fluid inlet:v=w=0,T=Tin (6) Thermal fluid outlet:∂T

∂x=∂u

∂x=∂v

∂x=∂w

∂x=0 (7)

Side of heat sink unit:v=w=0,∂u

∂z=∂v

∂z=∂T

∂z=0 (8)

Fig. 1. Schematic view of heat sink unit design.

Fig. 2. Schematic diagram of the partially folded fin.

Table 1

The main geometric dimensions of heat sink unit.

Type Zone Length

(mm) Width/

Diameter (mm)

Height (mm)

Cooling heat sink unit

Design

1 Overall

dimension 180 10 40

Straight fin 180 1 30

Thermal

liquid channel 180 6 –

Design

2 Overall

dimension 180 10 40

Folded fin 180 8 30

Thermal

liquid channel 180 6 –

Table 2

Relative parameters applied in the model.

Parameter Value

Fluid water

Thermal fluid rate 4 L/min

Thermal fluid temperature 40, 50, 60, 70, 80 ^◦C

Cooling air temperature 20 ^◦C

Cooling air rate 8.76 m/s

Flow directions of the thermal fluid and cooling air Counter-flow

In which Tin represents the inlet temperature of thermal liquid.

2.3. Grid independence

The grid independence verification is necessary in order to guarantee the reliability of the numerical simulation results. In this work, the in- fluence of grid number on the computational results of Design 1 and Design 2 were studied, and the corresponding results were presented in Table 3. As presented in Table 3, the relative deviation of the thermal fluid outlet temperatures for Design 1 was only 0.20 % when the grid numbers were 655920 cells and 5100354 cells, respectively. Similarly, the relative deviation of the thermal fluid outlet temperatures for Design 2 was only 0.04 % at the grid numbers of 728280 cells and 6513120 cells, respectively. Therefore, the numerical simulation studies of Design 1 and Design 2 in next work were carried out at the grid numbers of 5100354 cells and 6513120 cells, respectively.

3. Experimental apparatus and procedure 3.1. Description of the novel air-cooled LHP

The air-cooled LHP with a high-efficiency condenser and a new flat evaporator is proposed and shown in Fig. 3, which was mainly composed of a flat evaporator, a vapor line, a condensation line, an air-cooled condenser and a liquid line. Based on the numerical analysis results in Section 2, the novel louver folded fin heat sink unit is used in the design of the air-cooled condenser of LHP, and the corresponding air-cooled condenser structure is illustrated in Fig. 4. It can be observed from Fig. 4 (a) that the novel air-cooled condenser was mainly consisted of condenser cover plate, condenser body and fans. The condenser cover plate and condenser body were provided with the condensation line bayonet, respectively. And the condensation line bayonet, a typical serpentine arrangement, was presented in Fig. 4 (b). The condenser body was provided with 15 channels with measuring 8 mm ×30 mm (W ×H).

And the folded fins were provided with some louvers to increase the heat dissipation area, which were welded to the channel wall of condenser body. Additionally, three fans with measuring 40 mm ×40 mm ×28 mm (L ×W ×H) were fixed on the bracket of condensation cover plate by using M4 bolts. The structure of the new flat evaporator in the thickness direction was displayed in Fig. 5. As illustrated from Fig. 5, the capillary wick with sintered 106–150 μm copper powder was designed with the double rows vapor channels in order to enhance the evapora- tion area of working fluid. At the same time of enhancing evaporation, it was necessary to ensure that the working medium in the capillary wick can be quickly replenished. Therefore, 1 mm thick copper powder (<38 μm) was sintered on the inside surface of the reservoir, which was used as another method for working medium replenishment. Additionally, the 1 mm thick sintered copper powder can make stable operation of the novel air-cooled LHP in more LHP test directions. To increase the strength of the reservoir wall, two SS304 rings were wrapped in the 1 mm thick sintered copper powder.

The principal design specifications of the novel air-cooled LHP is presented in Table 4. At present, the distilled water has some advan- tages, such as high latent heat value, great thermal stability, non-toxic and cheap, which has been widely used as the working medium of heat pipes. In this work, the distilled water was injected into the new air- cooled LHP as the working medium, and the charging ratio can be expressed as below:

α= V Vtol

×100% (9)

here α defines the charging ratio of distilled water; V is the volume of the distilled water; V_tot represents the entire inside volume of LHP.

Table 3

Grid independence of heat sink unit.

Type Number of grids Inlet

temperature (^◦C)

Outlet

temperature (^◦C) Relative deviation (%)

Design

1 12993 80 78.90 –

34398 80 78.68 0.28

96096 80 78.42 0.33

655920 80 78.23 0.24

5100354 80 78.07 0.20

Design

2 19624 80 78.29 –

37391 80 77.84 0.57

82264 80 77.61 0.30

728280 80 77.47 0.18

6513120 80 77.44 0.04

Fig. 3. An illustration of a novel air-cooled LHP.

Fig. 4.The devised novel air-cooled condenser structure.

3.2. Testing methods

In this study, the new air-cooled LHP system experimental device is presented in Fig. 6. The heat sources simulator was a rectangular copper block with an effective heating area of 30 mm ×30 mm, and assembled with four calefaction sticks, the highest total input power of which was 750 W. Additionally, the junction temperature of heat sources simulator (T_j) can be gained and recorded by using a k-type thermo-couple wire welded in the effective heating area of the rectangular copper block, as demonstrated in Fig. 6(a). Fig. 6(b) displays the locations of the ten k- type thermo-couple wires in air-cooled LHP module. And the twelfth k- type thermo-couple wires were used to record the testing environment temperature Ta. The core diameter of all thermocouples was only 0.1 mm to improve the accuracy of the measurement, and its uncertainty was ±0.3 ^◦C. The novel air-cooled LHP module test system is illustrated in Fig. 6(c). Base on Fig. 6(c), the new air-cooled LHP module test system was mainly composed of the new air-cooled LHP, a constant temperature and humidity equipment, an Agilent 34970A data collector, a personal computer and the DC power system. The DC power I and II were used as the input power of the cooled fans and the heat sources simulator, respectively. The maximum uncertainty of the DC power II was esti- mated to be ±0.5%. The Agilent 34970A data collector with the mea- surement error of ±0.1 ^◦C was employed to collect and present the test point temperature at a time acquisition step of 3s. All thermal perfor- mance tests were performed at the environment temperature of 20 ± 1 ^◦C by applying the constant temperature and humidity equipment (BTH-150C, China). Additionally, the junction temperature was main- tained within 100 ^◦C for all thermal tests.

3.3. Data reduction

As significant indicators for evaluating the heat transfer capability of the new air-cooled LHP, the LHP heat resistance RLHP and the entire LHP system heat resistance Rsys in this work were calculated by Eqs. (10) and (11), respectively.

RLHP=Te− Tc

Q (10)

Rsys=Tj− Ta

Q (11)

In which Tc denotes the mean temperature of air-cooled condenser cover plate,‾Tc =(Tc1 +Tc2 +Tc3)/3; Q is the input power.

In this work, the operating temperature of the air-cooled LHP was obtained at the input power range from 50 W to 750 W. And the oper- ating temperature error was the largest and did not exceed 0.8 ^◦C when the input power is 50W.

The uncertainty of RLHP can be defined as follows [24,25]:

ΔRLHP

RLHP

=

̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅̅

(ΔQ Q

)₂ +

(Δ(ΔT) ΔTec

)₂

√

(12)

where T denotes the temperature; ΔTec represents the temperature dif- ference between evaporator and the condenser. Meanwhile, the calcu- lation equation of uncertainty of Rsys was similar to that of RLHP. For instance, at a horizontal orientation, fan voltage of 10 V, the un- certainties of RLHP and Rsys were estimated to be ±1.93 % and 2.05 % at highest input power of 550 W, respectively.

4. Results and discussion 4.1. Optimization of heat sink

Structural optimization is one of the most critical means to enhance the cooling performance of heat sink. In this work, a novel louver folded fin heat sink unit (Design 2) was designed and analyzed numerically in comparison to the conventional straight fin heat sink unit (Design 1).

The temperature contours of Design 1 and Design 2 at inlet water tem- peratures of 40 ^◦C, 50 ^◦C, 60 ^◦C, 70 ^◦C and 80 ^◦C are exhibited in Fig. 7. It can be observed from this figure that the heat sink temperatures of Design 1 and Design 2 were increased gradually with the increasing of inlet water temperature from 40 ^◦C to 80 ^◦C. And the heat sink tem- peratures constantly decreased in the direction of thermal fluid flow. In addition, it can be markedly noticed that the temperature distribution of Design 2 was lower than that of Design 1 at same inlet water Fig. 5. Structural schematic of new flat evaporator in the thickness direction.

Table 4

The primary design specifications of the new air-cooled LHP.

Components Parameters Value

Evaporator Length ×width ×thickness 92 mm ×58.9 mm ×15 mm

Material Copper

Heating wall Thickness 0.8 mm

Wick Length ×width ×thickness 45 mm ×57.3 mm ×13.4 mm

Material Copper

Vapor channel Length ×D 35 mm ×1.5 mm

Reservoir Length ×width ×thickness 38 mm ×55.3 mm ×13.4 mm Vapor line Length ×ID/OD 300 mm ×5/6 mm Condenser line Length ×ID/OD 545.5 mm ×5/6 mm Liquid line Length ×ID/OD 552.6 mm ×3/4 mm Charging line Length ×ID/OD 20 mm ×3/4 mm Vacuum line Length ×ID/OD 20 mm ×3/4 mm Condenser Length ×width ×thickness 180 mm ×136 mm ×40 mm

Material Aluminum alloy

Flow channel Width ×height 8 mm ×6 mm Effective length 416 mm

Fig. 6. The experimental device of the new air-cooled LHP system.

temperature. In the height direction, the temperature difference be- tween the bottom and top of Design 2 is larger than that of Design 1, which indicated that the Design 2 has more efficient cooling capacity than the Design 1. Therefore, the Design 2 can remove more heat at the same height of heat sink. The inlet water temperature dependence of the temperature difference between inlet water temperature and outlet water temperature (Tinlet - Toutlet) for the Design 1 and Design 2 is demonstrated in Fig. 8. As noticed from Fig. 8, the temperature differ- ence (Tinlet - Toutlet) of Design 2 was larger than that of Design 1 at the inlet water temperature range from 40 ^◦C to 80 ^◦C, which also illustrated

that at same inlet water temperature Design 2 has lower outlet water temperature and better cooling efficiency. Additionally, the out water temperature difference between Design 2 and Design 1 was gradually improved with the increasing of inlet water temperature from 40 ^◦C to 80 ^◦C. At the inlet water temperature of 40 ^◦C, 50 ^◦C, 60 ^◦C, 70 ^◦C and 80 ^◦C, the outlet water temperature (temperature difference (Tinlet - Toutlet)) of Design 2 were 39.15 ^◦C (0.85 ^◦C), 48.72 ^◦C (1.28 ^◦C), 58.30 ^◦C (1.7 ^◦C), 67.81 ^◦C (2.19 ^◦C) and 77.44 ^◦C (2.56 ^◦C), respectively.

Similarly, the corresponding outlet water temperature(temperature difference (Tinlet - Toutlet)) of Design 1 were 39.36 ^◦C (0.64 ^◦C), 49.04 ^◦C Fig. 7. Temperature contour of Design 1 and Design 2 at different inlet water temperatures.

(0.96 ^◦C), 58.72 ^◦C (1.28 ^◦C), 68.39 ^◦C (1.61 ^◦C) and 78.07 ^◦C (1.93 ^◦C), respectively. Based on the results of Figs. 7 and 8, the novel louver folded fin heat sink unit has a significant heat dissipation advantage.

4.2. Heat transfer characteristics of new air-cooled LHP 4.2.1. Effect of LHP charging ratio

The charging ratio of working medium for the LHP is a significant internal factor impacting the heat transfer capacity of LHP. The large

charging ratio of working fluid can decrease the thermal performance of LHP. More importantly, the LHP will not startup successfully when the charging ratio of working medium is lower than the minimum circula- tion of the LHP. Therefore, the research on the optimized charging ratio of the LHP is very necessary in order to enhance the thermal perfor- mance of LHP. Based on the previous research results of novel water- cooled LHP [24], the thermal performance of the new air-cooled LHP with charging ratios of 33.1 % and 46.1 % was further studied, including the startup characteristics, operating characteristics, and comparison of thermal properties between the new water-cooled LHP and the new air-cooled LHP.

For a LHP module cooling system, the startup characteristics of the LHP are important for the thermal performance evaluation. And the successful startup of the LHP is always a prerequisite for practical application [26]. In this paper, the startup characteristics of the new air-cooled LHP with charging ratios of 33.1 % and 46.1 % are demon- strated in Figs. 9 and 10, respectively. These tests were executed at the LHP horizontal position, fan voltage of 10 V and the input power of 20 W, 30 W, 40 W and 50 W. According to the description from Fig. 9, the new air-cooled LHP can successfully started and run at the input power of 20 W, 30 W, 40 W and 50 W, which mainly through three stages of temperature rise stage, start-up stage and stable running stage. When the input power was used to heat the copper block, the junction temperature of copper block Tj first raised. In the meantime, the evaporator wall temperature Te was raised immediately. Next, the evaporation of working medium in the capillary wick and the heat leak from capillary wick and evaporator wall lead to the increase of evaporator outlet temperature Tvl and reservoir temperature Tr, respectively. With the increase of working medium evaporation in the vapor channels, the pressure difference between the two ends of the capillary wick gradually increased, and the pressure difference drove the circulation of the Fig. 8. Inlet water temperature dependence of the Tinlet-Toutlet for the Design 1

and Design 2.

Fig. 9. Startup process under different input power for a 33.1 % charging ratio.

working medium in the LHP. Finally, the temperatures, working me- dium circulation and pressure drop of LHP gradually reached a steady state. Moreover, all startup processes of the novel air-cooled LHP with 33.1 % charging ratio at the input power of 20 W, 30 W, 40 W and 50 W can reach a stable running stage before 2500s, and the corresponding steady-state temperatures were 40.87 ^◦C, 43.07 ^◦C, 44.56 ^◦C and 46.19 ^◦C, respectively. In comparison to the results of Fig. 9, the startup characteristics of the air-cooled LHP with 46.1 % charging ratio (Fig. 10) presented a pseudo-startup phenomenon at input power of 20 W and 30 W. And the startup processes of the air-cooled LHP at input power of 20 W and 30 W displayed the regular oscillation phenomenon when the test time was more than 4750 s and 2150 s, respectively. Based on the results of Fig. 10(a–b), the regular oscillation amplitude of the condenser inlet

temperature T_v2 was the most obvious compared with other tempera- tures of the LHP, and corresponding oscillation amplitude was more than 25 ^◦C. And highest oscillating temperature of junction tempera- tures T_j at input power of 20 W and 30 W were no more than 55.60 ^◦C and 59.89 ^◦C. These results explained that the working medium circu- lation of the air-cooled LHP with 46.1 % charging ratio at input power of 20 W and 30 W was intermittent, and intermittent time of working fluid circulation was reduced with the enhancing of input power from 20 W to 30 W. Under the input power of 40 W and 50 W, the startup processes of the novel air-cooled LHP with 46.1 % charging ratio can reach a rela- tively stable running stage when the test time was more than 3000 s and 1250 s, respectively. And the LHP temperature at the relatively stable running stage also showed small fluctuations, which main reason may be Fig. 10.Startup process under different input power for a 46.1 % charging ratio.

Fig. 11.LHP operating characteristics vs input power under different charging ratios: (a) 33.1 %; (b) 46.1 %.

the effect of heat leakage for the reservoir and fan frequency fluctuation.

The mean steady-state junction temperatures T_j for the novel air-cooled LHP with 46.1 % charging ratio were 56.91 ^◦C and 58.25 ^◦C corre- sponding to 40 W and 50 W, respectively.

Fig. 11 displays the steady-state operating characteristics of the novel air-cooled LHP with charging ratios of 33.1 % and 46.1 % at different input power, including the different testing point temperatures Tj, Te, Tr, Tv1, and condenser cover plate average temperature‾Tc. As shown in Fig. 11(a), the pattern of the curve Tr =f (Q) displayed a near flattened LHP operating curve, which included a variable conductance mode (VCM) and a constant conductance mode (CCM). When the operating power did not surpassed 250 W, the temperature Tr of the novel air-cooled LHP with 33.1 % charging ratio was operating in VCM stage, which mainly reason may be the impact of heat leak for the reservoir and working medium circulation. However, the temperature T_r exhibited a linear increasing trend when the input power was increasing from 300 W to 550 W. In the whole input power range, the temperature T_j, T_e, and‾T_c was also shown the near linear increasing trend with the enhancing of input power. At the maximum input power of 550 W, the steady-state temperature Tj and Te were 92.09 ^◦C and 82.21 ^◦C, respectively. As demonstrated from Fig. 11(b), the pattern of the curve Tr =f (Q) for the air-cooled LHP with 46.1 % charging ratio exhibited a near U-shaped LHP operating curve. And the temperature Tr presented a near decreasing trend with the enhancing of input power from 50 W to 250 W, while it was constantly gone up as input power was higher than 300 W. The main reason was that the low circulation rate and quantity of working fluid under low heat loads were difficult to weaken the influ- ence of heat leakage for the reservoir. And the circulation rate and quantity of working medium were continuously rose with the improving of input power, which will result in the decrease of reservoir tempera- ture. However, when the increased input power reached a higher value, the effect of the circulation rate and quantity of working medium will not increase due to the limitation of pipe diameter and charging ratio.

Therefore, the reservoir temperature T_r was constantly gone up when the input power is more than 300 W. At the maximum input power of 550 W, the steady-state temperature Tj and Te of the air-cooled LHP with 46.1% charging ratio were 90.93 ^◦C and 77.20 ^◦C, respectively. In comparison to the results of Fig. 11(a) and (b), the temperature Te, Tr

and Tv1 of the air-cooled LHP with 46.1% charging ratio presented a more obvious U-shaped curve. The reason for this phenomenon was that the higher charging ratio will result in higher steady-state operating temperature at low input power.

The operating characteristics of the air-cooled LHP and the water- cooled LHP at different charging ratios are illustrated in Fig. 12. Based

on the results of Fig. 12, the thermal performance of the air-cooled LHP with the 33.1 % charging ratio was better than that of the 46.1 % air- cooled LHP in the input power range of 50–450W. And the junction temperature difference of air-cooled LHP between 33.1 % charging ratio and 46.1 % charging ratio constantly reduced with the enhancing of the input power from 50 W to 450 W. The reason for this result was that the working medium of reservoir for the air-cooled LHP with a high charging ratio will absorb more leakage heat, which will result in a higher junction temperature. However, the increase circulation rate and quantity of working medium will decrease the influence of heat leakage with the enhancing of input power. At the maximum of input power of 550 W, the junction temperature of the air-cooled LHP with 33.1 % and 46.1 % charging ratio were 92.09 ^◦C and 90.93 ^◦C, respectively. Simi- larly, the heat transfer performance of the water-cooled LHP with the 33.1 % charging ratio was also better than that of the 46.1 % water- cooled LHP when the input power did not exceed 500 W. However, the junction temperature of the water-cooled LHP with a 46.1 % charging ratio was lower than that of the water-cooled LHP with a 33.1

% charging ratio in the input power range from 500 W to 750 W. In addition, the thermal characteristics of the water-cooled LHP at the same filling ratio were better than that of the air-cooled LHP. And the junction temperature difference between water-cooled LHP and air- cooled LHP gradually increased with the increase of the input power.

The principal reason can be that the most of the condensing line of the air-cooled condenser and water-cooled condenser were in sub-cooled stage at low input power. In comparison to the water-cooled condenser, the sub-cooled area of air-cooled condenser was signifi- cantly reduced with the enhancing of heat load due to low condensing efficiency. Many factors, including cooling requirements, economic costs and spatial layout, should be considered for the use of the type of condenser. Based on some literature reports, the temperature of elec- tronic chip should not exceed 85 ^◦C to guarantee the stable and safe operation of the electronic chip. Therefore, the air-cooled LHP with 33.1

% and 46.1 % charging ratio can meet the cooling requirements of 450 W and 500 W electronic chips, respectively. Similarly, the water-cooled LHP with 33.1 % and 46.1 % charging ratio can efficiently dissipate the input power of 600 W and 700 W, respectively. Compared with the water-cooled LHP, the air-cooled LHP has some advantages in the acceptable range of electronic chip temperature such as less auxiliary equipment, save energy consumption and low cost. Comprehensive consideration of the cooling requirements and cooling costs of electronic chips, the air-cooled LHP with 33.1 % charging ratio was a better cooling solution when the thermal design power of electronic chips was less than 500 W. On the contrary, only the water-cooled LHP with 46.1 % charging ratio can meet the heat dissipation requirements of the elec- tronic chips when the thermal design power of the electronic chip was 500–700 W.

4.2.2. Effect of fan voltage

The fan voltage of air-cooled condenser is a significant factor that influences the cooling efficiency of the air-cooled LHP. Hence, the input power dependence of the junction temperature Tj under different fan voltages and a horizontal position of the air-cooled LHP was presented in Fig. 13. It is worth noting that the junction temperature of the air-cooled LHP under the same heat load was constantly reduced with the improving of fan voltage from 6 V to 12 V. When the input power did not exceed 200 W, the fan voltage of condenser has insignificant effect on the heat transfer performance of the air-cooled LHP. The chief reason can be that the heat transferred of the working medium evaporation is relatively less at low input power. And the air-cooled condenser was sufficient for cooling requirements at low fan voltages. However, the heat transferred of the working medium evaporation was constantly improved with the enhancing of input power, which will result in the effect of fan voltage to gradually become obvious. At the maximum input power of 550 W, the junction temperatures of the air-cooled LHP were 102.5 ^◦C, 94.14 ^◦C, 92.09 ^◦C and 87.93 ^◦C corresponding to the fan Fig. 12. Comparison of operating characteristics between air-cooled LHP and

water-cooled LHP.

voltage of 6 V, 8 V, 10 V and 12 V. The relationship between the RLHP, R_sys and input power at different fan voltages were exhibited in Fig. 14 (a) and (b), separately. The result of Fig. 14 was displayed that the RLHP

was obviously lower than the Rsys. And the RLHP and Rsys were illustrated a trend of declining obviously at first and then tending to a constant value with the increasing of input power. At the input power range from 50 W to 350 W, the RLHP at fan voltage of 6 V was lower than that at other fan voltages. When the input power was more than 400 W, the minimum R_LHP value of 0.028 ^◦C/W was gained at the input power of 500 W and fan voltage of 12 V. At the entire input power range from 50 to 550 W, the Rsys of 0.123 ^◦C/W at fan voltage of 12 V was lower than that at other fan voltages. At the largest input power of 550 W, the R_sys values of 0.154 ^◦C/W, 0.136 ^◦C/W, 0.131 ^◦C/W and 0.123 ^◦C/W were achieved at the fan voltage of 6 V, 8 V, 10 V and 12 V, respectively.

4.2.3. Effect of LHP test direction

The study of LHP test directions is crucial to the thermal performance of the LHP, which can effectively determine the range of the working position of the LHP and avoid the operation of the LHP in the non- working range. Seven different air-cooled LHP test directions are illus- trated in Fig. 15. When the position of evaporator is lower than that of condenser, the LHP test directions 1–3 are regarded as the positive gravity directions. On the contrary, the LHP test direction 5 is consid- ered as the anti-gravity test direction. The LHP test direction 4 is the horizontal position without gravity assistance. And the LHP test di- rections 8 and 9 are the horizontal side directions, which may be

wick, and only relies on sintering copper powder on the inside wall of the reservoir for the transfer of the working medium. For the anti-gravity test direction 5, the junction temperature Tj and Tj, Direction i - Tj, Direction 4

of the air-cooled LHP at the test direction 5 is higher than that of the other LHP test directions at the low input power (≤100 W). This is because under anti-gravity test direction 5 and low input power, low vapor evaporation makes it more difficult to push the working medium from the pipeline into the reservoir. However, with the increasing of input power, the influence of the anti-gravity test direction 5 on the thermal performance of the air-cooled LHP gradually decreases due to the increasing vapor evaporation capacity and low anti-gravity angle (only 5^◦). On the other hand, the circulation rate of the working medium increases at the relatively high input power. And the working medium of the reservoir was more conducive to transfer to the capillary wick at the anti-gravity test directions. In the entire input power range from 50 W to 750 W, the thermal performance of the air-cooled LHP at the test di- rections 6, 7 is generally worse than that of the horizontal position due to the influence of gravity. At the largest input power of 550 W, the junction temperature of the air-cooled LHP at LHP test directions 1–7 are arranged from small to large as follows: LHP direction 3 <LHP direction 5 <LHP direction 4 <LHP direction 7 <LHP direction 2 <LHP di- rection 6 <LHP direction 1, and corresponding junction temperature values were 87.27 ^◦C, 87.63 ^◦C, 87.93 ^◦C, 89.36 ^◦C, 91.36 ^◦C, 93.81 ^◦C and 94.50 ^◦C. Fig. 17(a) and (b) display the relationship between ther- mal resistance (RLHP and Rsys) and input power at seven different LHP test directions and the fan voltage of 12 V. It is found here that the RLHP

and Rsys shown a tendency of increasing first and then approaching a constant value at all LHP test directions. The effect of LHP test directions on thermal resistance (RLHP and Rsys) was significant when the input power was less than 300W. However, when the input power was more than 300 W, the effect of LHP test directions on thermal resistance (RLHP

and Rsys) gradually decreased. For the LHP test directions 1–7, the minimum RLHP values of 0.037 ^◦C/W, 0.038 ^◦C/W, 0.025 ^◦C/W, Fig. 13.Junction temperature vs input power at different fan voltages.

Fig. 14.The input power dependences of (a) RLHP and (b) Rsys at different fan voltages.

0.028 ^◦C/W, 0.024 ^◦C/W, 0.031 ^◦C/W and 0.029 ^◦C/W were reached at the input power of 400 W, 550 W, 550 W, 500 W, 550 W, 500 W and 550 W, respectively. And the minimum Rsys values of 0.135 ^◦C/W, 0.131 ^◦C/

W, 0.121 ^◦C/W, 0.123 ^◦C/W, 0.124 ^◦C/W, 0.135 ^◦C/W and 0.128 ^◦C/W were reached at the input power of 500 W, 550 W, 550 W, 550 W, 550 W, 550 W and 550 W, respectively.

4.3. Comparison of thermal characteristics

At present, many researchers were devoted to the research of LHP for cooling high power density electronic chips. Compared with the research of water-cooled LHP, the research of air-cooled LHP is less due to low heat transfer capacity. However, as a traditional and cheap cooling technology, some researchers still insist on strengthening the Fig. 15.Seven test directions of the new air-cooled LHP.

Fig. 16.(a) Junction temperature Tj and (b) Tj, Direction i – Tj, Direction 4 as a function of input power at different LHP test directions.

Fig. 17.(a) RLHP and (b) Rsys as a function of input power at different LHP test directions.

for cooling high power density electronic chip.

4.4. Energy consumption analysis

In order to save energy and reduce cost, it is necessary to assess the energy consumption of the LHP cooling system. The energy consump- tion of water-cooled LHP system mainly comes from compressors, pump and fan. And the power of compressors and pump was selected by the volume flow velocity and refrigerating capacity, respectively. For the air-cooled LHP system, the energy consumption came only from the cooling fans. The coefficient of performance (COP) of the air-cooled LHP system and water-cooled LHP system can be expressed in Eqs. (12) and (13), respectively, as follows:

COPair−cooled= Q

Pfan (13)

COPwater−cooled= Q

Pcompressor+Ppump+Pfan (14)

where Pfan, Pcompressor, and Ppump are the power of fan, compressor and pump, respectively.

The COP value of the air-cooled LHP system and water-cooled LHP system at different operating heat loads for the air cooling condition of fan voltage 12 V, and the water cooling condition of temperature of 20 ^◦C and volume flow speed of 4 L/min are displayed in Fig. 19. It was obviously found that the COP value of the air-cooled LHP system was much larger than that of water-cooled LHP system in the entire input power range from 50 to 550 W, which indirectly indicated the air-cooled LHP system was more efficient and energy-saving than the water-cooled LHP system. At the highest input power of 550 W, the COP value of the air-cooled LHP system and water-cooled LHP system were 20.37 and

Fig. 18. Comparison of thermal characteristics between this study and other literatures: (a) Junction temperature vs input power; (b) Junction temperature vs power density. { , the data from references [27,28]; , the data from reference [29]; , the data from reference [22]; , the data from reference [30]; , the data from reference [31]; , the data from reference [21]; , the data in this work}.

Fig. 19.COP vs input power for different LHP cooling systems.

Fig. 20.Total energy consumption in a 5-year period for different LHP cool- ing systems.

1.08, respectively. In addition, the energy consumption of air-cooled LHP system and water-cooled LHP system in a 5-year period is shown in Fig. 20. Contrary to the COP value, the energy consumption of the air- cooled LHP system was much lower than that of water-cooled LHP system in the entire input power range from 50 to 550 W. When the operating time reached 5 years, the energy consumption of the air- cooled LHP system and water-cooled LHP system were 1182.60 kW h and 22212.29 kW h, respectively. And the energy consumption of the air-cooled LHP system was lower than one-eighteenth of that of the water-cooled LHP system. In general, the energy consumption of the air- cooled LHP system is lower than that of the water-cooled LHP system on the premise of the cooling requirements.

5. Conclusions

In this paper, a novel air-cooled flat evaporator LHP with a high- efficiency condenser was developed for cooling the high power den- sity chips. Firstly, the cooling capacity of two cooling heat sink units was analyzed numerically to guide the design of LHP condenser. Then, the effect of charging ratios, fan voltages and LHP test directions on the heat transfer performance of the air-cooled LHP were studied in depth. In the end, the energy consumption of the novel LHP for different cooling system was evaluated. Main conclusions were shown as follows:

(1) The novel louver folded fin heat sink unit (Design 2) has a better cooling efficiency than that of the conventional straight fin heat sink unit (Design 1).

(2) The new air-cooled LHP with a 33.1 % charging ratio can be successfully started up under input power of 20 W, and presented a lower junction temperature at input power range from 50 W to 450 W.

(3) The junction temperature of the air-cooled LHP was constantly reduced with the enhancing of fan voltage from 6 V to 12 V. At the input power of 550 W, the junction temperature of 87.93 ^◦C was achieved at the fan voltage of 12 V. And the RLHP, Rsys values were 0.028 ^◦C/W and 0.123 ^◦C/W, separately.

(4) The LHP test directions have an evident influence on the heat transfer performance of the new air-cooled LHP. Under the input power of 550 W, the junction temperature of the air-cooled LHP at positive gravity direction of 30^◦was lower than that of other LHP test directions.

(5) The energy consumption of the air-cooled LHP system was lower than one-eighteenth of that of the water-cooled LHP system.

CRediT authorship contribution statement

Kangning Xiong: Conceptualization, Methodology, Investigation, Data curation, Writing – original draft, Writing – review & editing.

Yuhao Luo: Data curation. Yixian Hu: Writing – review & editing.

Shuangfeng Wang: Writing – review & editing, Supervision, Project administration.

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.

Data availability

Data will be made available on request.

Acknowledgment

This work is supported by the National Natural Science Foundation of China (Granted No.52176156).

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