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Fabrication and experimental investigation of the bionic vapor chamber
Applied Thermal Engineering 168 (2020) 114889
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Fabrication and experimental investigation of the bionic vapor chamber ⁎
T
Yuanqiang Luo, Wangyu Liu , Guangwen Huang School of Mechanical and Automotive Engineering, South China University of Technology, 381 Wushan Road, Tianhe District, Guangzhou 510641, People’s Republic of China
H I GH L IG H T S
of bionic vapor chamber was designed and manufactured. • AThenewwicktypestructure the fractal network microchannels and copper foam. • The porosity gradientcombined of the copper foams around the fractal microchannels was added. • The fractal network microchannels processed by laser engraving and mould coining. • K of the bionic vapor chamber waswere higher than most existing vapor chambers. • eff
A R T I C LE I N FO
A B S T R A C T
Keywords: Heat dissipation Bionic vapor chamber Fractal network microchannels Copper foam Porosity gradient Laser engraving
With the rapid development of photoelectric products, their miniaturization and high integration have intensified the problem of heat dissipation. In this paper, a new type of bionic vapor chamber is designed and manufactured. The wick structure for the condenser combines the fractal network microchannels and copper foam, which are used to imitate the macroscopic leaf vein network and microscopic mesophyll tissue, respectively. Furthermore, the permeability gradient distribution between the leaf veins and mesophyll tissue is added, presented as the porosity gradient distribution of the copper foams around the fractal network microchannels. Then the fractal network microchannels on the wick are processed by laser engraving and mould coining, respectively. The wick structure for the evaporator is homogeneous copper foam. The minimum value of the maximum temperature difference on the upper surface of the condenser of the bionic vapor chamber is only 1.21 K and that of the overall thermal resistance of the bionic vapor chamber is only 0.094 K/W. Compared with the existing novel vapor chambers, when using deionized water as the working fluid, the maximum throughplane effective thermal conductivity and minimum in-plane effective thermal conductivity are 9.52 W/m·K and 10500 W/m·K, respectively, which are higher than most existing vapor chambers.
1. Introduction With the rapid development of photoelectric products, their miniaturization and high integration have intensified the problem of heat dissipation. Vapor chamber is a special type of heat pipe [1–3] that is a particularly effective heat spreader for electronics. It is also referred as flat heat pipe and is widely applied due to its uniform temperature distribution, large condensation area, light weight, geometric flexibility and extremely high thermal conductivities [4–8]. The role of wick in vapor chamber is to direct condensed liquid from the condenser to the evaporator of the vapor chamber. Typically, the porous wick structure can consist of the following types: sintered, groove and mesh [9]. Powder sintering wick structure and mesh sintering wick structure are generally formed through graphite mould sintering, while the groove ⁎
wick structure is generally formed by directly removing the materials on the groove part through machining, or by increasing the materials on the non-groove part through powder sintering. In the past five years, researchers at home and abroad constantly tried to design various novel vapor chamber structures and then explore the most suitable processing method to form the structures and manufacture the vapor chambers, in order to obtain better flow and heat transfer performances to meet higher working condition requirements. Ji et al. [10] designed a novel integrated flat heat pipe (IFHP), which consisted of an evaporator and a condenser with multiple channels fabricated in the fin heat sink. A layer of compressed copper foam was sintered on the inner surface of the evaporator bottom plate, and many copper foam bars were inserted into the channels, both of which formed a porous network wick. Compared with the conventional flat heat pipe
Corresponding author. E-mail address: [email protected] (W. Liu).
https://doi.org/10.1016/j.applthermaleng.2019.114889 Received 22 July 2019; Received in revised form 24 December 2019; Accepted 29 December 2019 Available online 30 December 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
Applied Thermal Engineering 168 (2020) 114889
Y. Luo, et al.
Nomenclature
Greek symbols
K D L l W w n m V ΔT R Keff Q T Leff A xi y E
θ ρ ε
fractal level effective diameter, mm length of the microchannel, mm length ratio width of the microchannel, mm width difference, mm quantity of the support columns filling amount, g volume, mm3 temperature difference, K thermal resistance, K/W effective thermal conductivity, W/m·K heating power, W measured temperature, K effective length, m cross sectional area, m2 independent variable given function measurement error
fractal angle, ° density of water, g/mm3 porosity
Subscripts steam support fractal c e max eff
steam chamber support columns fractal network microchannels wick for the condenser wick for the evaporator maximum effective
Superscripts th in
(CFHP), the IFHP presented good fin temperature uniformity and heat performance. Shaeri et al. [11] fabricated a novel vapor chamber to assess the feasibility of combining hydrophobic and hydrophilic wettabilities in the evaporator to optimize thermal performance. The proposed vapor chamber included a separate layer of hydrophilic sintered copper powder wick that was pressed in intimate contact with a hydrophobic evaporator substrate with a water contact angle around 140°. Compared with those of a baseline vapor chamber that was fabricated by sintering hydrophilic copper particles on a hydrophilic copper evaporator substrate, the thermal resistance of the proposed vapor chamber was lower. Mizuta et al. [12] developed a type of flat laminate vapor chamber called FGHP (Fine Grid Heat Pipe), and its thermal performance was investigated. Fine etching technique enabled formation of microstructures on laminate parts. Without regarding the heat input, the FGHP showed more uniform temperature distribution than the copper heat spreader. Ju et al. [13] developed an innovative vapor chamber concept incorporating hybrid wicks of two integrated structures: a low thermal resistance spreading layer and a dedicated liquid supply structure. The spreading layer was comprised of a monolayer of sintered Cu particles to minimize the thermal resistance in the evaporator. Three different liquid supply structures were studied: vertical columnar arteries, converging lateral arteries, and bi-porous structures. Deng et al. [14] developed a type of composite porous vapor chambers (CPVCs) with uniform radial grooves in the evaporator, providing radial multi-artery channels for the fast liquid backflow with low
through-plane in-plane
hydraulic resistance. Compared with a pure copper plate, the CPVCs were found to maintain good temperature uniformity. They were able to operate efficiently to high heat fluxes without notable performance degradation, and presented smaller thermal resistances. Patankar et al. [15] considered the unique transport limitations and thermal requirements encountered in mobile applications, and developed a methodology for the design of vapor chambers to yield improved condenserside temperature uniformity at ultra-thin form factors, which focused on manipulating the condenser-side wick to improve lateral heat spreading. A biporous condenser-side wick design was proposed that facilitated a thicker vapor core, and thereby reduced the condenser surface peak-to-mean temperature difference relative to a monolithic wick structure. Natural selection has evolved over billions of years, giving us endless inspiration for bionics design. With regard to plant bionic vapor chambers, Ji et al. [16,17] verified the synergy of various length scales to activate various functions. The strategy guided multiscale design to realize an enhancement of capillary pressure, a well management of flow resistance and an ultra-thin liquid thickness on condenser surface. The porous wick consisted of a particle sub-layer and 3D mastoid process array inspired by the uniform conical structures on a leaf of Calathea zebrine. The tips of mastoid process directly contacted the condenser wall. The liquid suction and heat transfer experiments indicated that nano-roughness increased the vapor-liquid interface area to have 3–4 times of evaporation heat transfer coefficients compared
Fig. 1. The manufacturing process of the bionic vapor chamber. 2
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support columns, wick for the evaporator, evaporator and liquid filling tube. The structural design and manufacturing process of each part is shown in Fig. 1. Among them, design of the wick structure for the condenser, design of the configuration of the support columns and processing of the fractal network microchannels on the wick on the upper shell plate are the key processes.
with smooth particle surface, and increased the wettability to capture liquid from condenser, having ~18 times of condensation heat transfer coefficients to those without nano-roughness. Whereas, the processing of nano-roughness structure is complicated, which is not conducive for batch production. Moreover, the temperature uniformity of the vapor chamber was not taken into account. Seeing the similarity between vapor chamber and plant leaf, as they both have flat structure and cool themselves with phase change, our research team [5,18–22] has designed a novel structure based on the leaf vein system to form the wick of a vapor chamber. In this novel wick fractal network microchannels and the micro fin-pins were used to simulate the leaf vein network and mesophyll tissue respectively. The theoretical, experimental and numerical analysis all showed excellent fluid and thermal performances of the bio-inspired vapor chamber. Whereas, the imitation of plant leaf was just its macroscopic fractal network structure, and the micro finpins used to imitate the mesophyll tissue showed an overall structure of porous media merely under macroscale. In addition, the microchannels formed through chemical etching were unable to be deep and narrow simultaneously. In this paper, a new type of bionic vapor chamber will be designed and manufactured. Using the experimental setup established in this paper, the heat transfer performances of the bionic vapor chamber samples with different structures will be investigated and compared.
2.1. Design of the wick structure for the condenser From our further research on the multiscale structure of the leaf vein [23,24], we found that first- and second-order veins have high axial conductance and relatively small radial permeability, thus allowing water to reach distal areas of the leaf with only a small loss of water potential. Higher order veins tend to be more hydraulically resistant and permit greater radial leakage. This design allows for a relatively equitable distribution of water potential and thus reflects the capacity of the venation to provide a relatively homogeneous water supply across the leaf lamina, with only the leaf margins being hydraulically disadvantaged relative to the rest of the leaf [25–27]. Similarly, this natural optimized structure can be applied on the design of the wick structure for the condenser of vapor chamber. Due to the excellent flow characteristic of fractal structure, i.e. higher permeability and larger capillary suction than parallel structure [5], the fractal network microchannels used to imitate the macroscopic leaf vein network was retained in the present design. At the same time, the micro fin-pins (showed an overall structure of porous media merely under macroscale) used to imitate the microscopic mesophyll tissue were modified to the real porous media (copper foam) structure, adding
2. Design and fabrication of the bionic vapor chamber The bionic vapor chamber designed and manufactured in this paper is composed of the following parts: condenser, wick for the condenser,
Fig. 2. Four designed wick structures for the condenser with different fractal levels and fractal angles: (a) k = 5, θ = 30°; (b) k = 5, θ = 45°; (c) k = 7, θ = 30°; (d) k = 7, θ = 45°. 3
Applied Thermal Engineering 168 (2020) 114889
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fractal angles, which increases the easy repeatability of batch manufacturing and reduces the manufacturing quantity of the graphite mould for the subsequent sintering, thus greatly reducing the manufacturing cost.
the permeability gradient distribution between the leaf veins and mesophyll tissue, presented as the porosity gradient distribution of the porous media around the fractal network microchannels. As our research team [5,18,21] has done fairly sufficient research about the influence of various design parameters of the fractal network microchannel structure on its flow and heat transfer performances, only four different fractal network microchannel structures were chosen in this paper to further investigate the influence of different porous wick structures on the performance of the bionic vapor chamber. From the observation of leaf vein system of three ordinary species of plants with heat-resistance in our former research [23], the visible leaf vein levels are 5–7 levels generally. Therefore, two fractal levels (k = 5, 7) were selected for the fractal network microchannels of the porous wick structures for the condenser. In addition, from the measurement of the angles between the midrib and secondary veins on leaves of the three selected plants [23], those angles of Ficus virens Ait. var. sublanceolata (Miq.) Corner and Hamelia patens with better heat and mass transfer characteristics were 41.9°–65.3° and 36.1°–53.7°, respectively, while that of Plumeria rubra L. cv. Acutifolia with worse heat and mass transfer characteristics was 51.5°–89.5°. Therefore, two fractal angles (θ = 30°, 45°) were selected for the fractal network microchannels of the porous wick structures for the condenser. The effective diameter D of each wick is 70 mm. The length of the level 0 microchannel L0 is 12/11 mm and the length ratio defined as l = Lk+1/Lk (k = 0, 1, …, 5) is 0.7, as our former research [5] has found that when 0 < l < 1, the flow resistance of the fractal network microchannels was smaller and the permeability was larger than when l = 1. The width of the level 0 microchannel W0 is 0.35 mm and the width ratio is replaced with the width difference defined as w = Wk+1 − Wk (k = 0, 1, …, 5) in this work to ensure that it is easy to manufacture. As the wick structure was designed for the condenser, where the fluid flows from the centre to the circumference of the wick, w was set to be positive (0.05 mm). From the above, four designed wick structures for the condenser are shown in Fig. 2, and the detailed dimensions of the microchannel of each level are shown in Table 1. For porosity gradient design, the porous media zones around the microchannels of each two levels, i.e. levels 0–1, 2–3 and 4 (k = 5), are defined as porous_1, porous_2 and porous_3, respectively, as shown in Fig. 3. Furthermore, it should be noted that a multiscale simulation of the designed wick structure for the condenser has been carried out to predict its performances [28]. The simulation results had verified the superiority of the porosity gradient design of the wick structure, which had superior fluid and thermal performances with a relatively uniform pressure distribution. The temperature of the gradient porous wick structure could reach balanced state faster and its temperature uniformity was better.
2.3. Processing of the fractal network microchannels on the wick on the upper shell plate As can be seen from Fig. 2 and Table 1, with the increase of k of the fractal network microchannels, the width W of the microchannel of each level decreases gradually, achieving a good balance between the capillary force and permeability. Whereas, it gives rise to great difficulty on the processing of the microchannels, which thus becomes the most critical step in the manufacturing process of the bionic vapor chamber. The microchannels formed previously by our research team [19–22] through chemical etching were unable to be deep and narrow simultaneously. The final processed depth of the microchannels was only 0.16–0.24 mm, which greatly limited the performance of the bionic vapor chamber. In this paper, copper foam was selected as the porous media in the wick structure of the bionic vapor chamber. Taking advantage of the easy processing characteristic of copper foam, two different processing methods, i.e. laser engraving (Fig. 5c and d) and mould coining (Fig. 5e and f) were used to process the fractal network microchannels on the copper foam of the wick sintered on the upper shell plate (Fig. 5a) respectively, to finally form the complete wick structure for the condenser. For porosity gradient design, the copper foam of the gradient porous wick of the upper shell plate after processing and the gradient porous wick on the upper shell plate processed by laser engraving and after annealing treatment (n = 33, k = 5, θ = 30°) are shown in Fig. 6. For working fluid filling, a specific filling amount was set on the high-precision metering pump (Canada HIBAR 1SC-10 M, ± 0.5%). Then the filling needle was inserted into the liquid filling tube and started filling. For evacuation, the liquid filling tube was placed in the bleeding point of the automatic vacuumizer and clamped by the fixture. After evacuating for 300 s, the equipment will automatically flatten the end of the liquid filling tube to form a temporary cold welding sealing and the vacuum level inside the cavity was less than 50 Pa. To ensure the designed filling amount as far as possible, the mass difference between the bionic vapor chamber before and after evacuation was measured and compensated to the set filling amount. Finally, after the whole manufacturing process (Fig. 1), the finished product of the bionic vapor chamber is shown in Fig. 5g. The external diameter is 80 mm and the total thickness is 4.5 mm. As shown in Fig. 7, the surface morphology of the partial fractal network microchannels of the wick on the upper shell plate processed by two different methods (k = 7, θ = 30°) were observed and compared under the 3D microscope with super depth of field (with the magnification times of 100 × ). The depth of the microchannel of each level was measured and shown in Table 2 (with the designed depth of 1 mm). From Fig. 7a–d and Table 2, the surface morphology of the fractal network microchannels of the wick on the upper shell plate processed by laser engraving and after annealing treatment (k = 7,
2.2. Design of the configuration of the support columns Fig. 4 shows the configuration of the support columns when the quantity n is 41 and the positional relation between them and different wick structures (blue). When the quantity is 33, the 8 support columns marked (red) are removed. There are three different sizes of support columns, the one in the center is Φ1.5 mm, the 8 outermost ones are Φ3 mm and the remaining 32/24 support columns are Φ2 mm. Except for the interference between the 8 outermost support columns and a few microchannels on the wick structures with 7 fractal levels, the rest of the support columns were all arranged within the porous media regions between the microchannels. As the fractal network microchannel structure itself has the characteristic of preventing the overall blockage, the partial blockage of a few microchannels interfered with the support columns will not cause too many damages to the overall microchannel structure of the wick. In particular, there is no interference between the support columns and the two wick structures with 5 fractal levels. Hence the configuration of the support columns is of common use to the four wick structures for the condenser with different fractal levels and
Table 1 The detailed dimensions of the microchannel of each level of the wick structures for the condenser with different fractal levels. k
1
2
3
4
5
6
5
L1 (mm) W2 (mm)
12 0.35
8.4 0.30
5.88 0.25
4.116 0.20
2.8812 0.15
/ /
/ /
7
L1 (mm) W2 (mm)
11 0.35
7.7 0.30
5.39 0.25
3.773 0.20
2.6411 0.15
1.84877 0.10
1.294139 0.05
1 2
4
0
The length of the microchannel of each level. The width of the microchannel of each level.
Applied Thermal Engineering 168 (2020) 114889
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disadvantages. Generally speaking, the forming effect of laser engraving is better than that of mould coining in processing the fractal network microchannels on the copper foam of the wick sintered on the upper shell plate. In order to obtain the best forming effect and machining precision, we can use laser engraving to process the microchannels that basically reach the designed depth, and then use mould coining to process them again, so as to deepen the depth of the microchannels of high levels. In this way, the collapse of the structures at both sides of the microchannels due to extrusion can be avoided. Moreover, the depth at every position of the whole fractal network microchannels can be ensured to basically reach the designed depth. Whereas, the processing cost and time will increase correspondingly.
Fig. 3. The 1/24 model and porosity gradient design of the wick structure for the condenser (k = 5, θ = 30°).
θ = 30°) are smoother. The forming effect is better and the depths of the microchannels of the same level at different positions are relatively uniform. Whereas, with the increase of k of the fractal network microchannels, the width W of the microchannel of each level decreases gradually, thus the engraving effect becomes poor and the depth becomes shallow gradually. From Fig. 7e–h and Table 2, the surface morphology of the fractal network microchannels of the wick on the upper shell plate processed by mould coining (k = 7, θ = 30°) are relatively less smooth. As during coining, the mould processed the concave microchannels on copper foam through extrusion instead of cutting the material directly, the structures at both sides of the microchannels collapsed to the center of the microchannels, which worsened the forming effect. To sum up, the two processing methods have their advantages and
3. Experimental setup and samples for the performance test 3.1. Experimental setup Using the experimental setup established in this part (Fig. 8), the heat transfer performances of the bionic vapor chamber samples with different structures were investigated and compared, mainly included the temperature uniformity on the upper surface of the condenser and the overall thermal resistance. As shown in Fig. 8, the entire experimental setup could be divided into four parts: the heating module, cooling module, temperature data acquisition module and locking module. The heating area of the copper block was 20 × 20 mm2. The temperature of the cooling water was 60 °C and its rate of flow was 80 L/h. The thermocouples were T type (Omega,
Fig. 4. The configuration of the support columns when the quantity n is 41 and the positional relation between them and different wick structures (blue): (a) k = 5, θ = 30°; (b) k = 5, θ = 45°; (c) k = 7, θ = 30°; (d) k = 7, θ = 45°. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 5
Applied Thermal Engineering 168 (2020) 114889
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Fig. 5. Two different methods for the processing of the fractal network microchannels on the wick on the upper shell plate (n = 41): (a) the copper foam of the wick sintered on the upper shell plate; (b) the copper foam of the wick sintered on the bottom shell plate; (c) the wick on the upper shell plate processed by laser engraving (k = 5, θ = 30°); (d) the wick on the upper shell plate after annealing treatment (k = 5, θ = 30°); (e) the coining mould formed by 3D printing (k = 7, θ = 30°); (f) the wick on the upper shell plate processed by mould coining (k = 7, θ = 30°); (g) the finished product of the bionic vapor chamber.
−200 ~ 260 °C, ± 0.5 °C) and the data acquisition card was NI 9213 + cDAQ 9171.
calculated as Eq. (1).
m = 63% × ρ (Vsteam − −Vsupport + Vfractal + εc1 Vc1 + εc 2 Vc 2 + εc 3 Vc3 + εe Ve )
3.2. Samples
(1)
where ρ is the density of water. Vsteam, Vsupport and Vfractal are the volumes of the steam chamber, support columns and fractal network microchannels, respectively. εc1, εc2, εc3 and Vc1, Vc2, Vc3 are the porosities and volumes of the porous_1, porous_2, porous_3 zones of the wick for the condenser, respectively. εe and Ve are the porosity and volume of the wick for the evaporator, respectively.
The labels and corresponding structure parameters of the bionic vapor chamber samples are shown in Table 3. There were two samples for each structure, e.g. 41S-5L-30-LE#1 and 41S-5L-30-LE#2. Taking the sample 33S-5L-30-LE as a benchmark (the porosity of the copper foam of the wick of the upper shell plate was 50% homogeneously), for porosity gradient design (33S-5L-30-LE-G), the porosities of the three different layers of copper foams of the wick of the upper shell plate from inside to outside (Fig. 6a) were 45%, 50% and 55%, respectively. On the contrary, for porosity anti-gradient design (33S-5L-30-LE-A), that were 55%, 50% and 45%, respectively. The wick structure for the evaporator (Fig. 5b) was copper foam with the porosity of 95% homogeneously for all samples. The working fluid was deionized water and the filling ratio was set to be 63%, as our former research [19] has found that the vapor chamber with fractal network microchannels and micro fin-pins has the lowest thermal resistance under this filling ratio. For each sample with different internal structures/volumes, the filling amount m was
3.3. Data reduction and uncertainty analysis The temperatures of the center point on the bottom surface of the evaporator and the upper surface of the condenser of the bionic vapor chamber samples were measured respectively under different heating powers (60 W, 80 W, 100 W, 120 W and 140 W). For the two samples of the same structure, only the one with better performance was selected. According to Eqs. (2)–(4), the maximum temperature difference ΔTmax on the upper surface of the condenser, the overall thermal resistance R and the effective thermal conductivity Keff of the bionic vapor chamber
Fig. 6. (a) The copper foam of the gradient porous wick of the upper shell plate after processing; (b) the gradient porous wick on the upper shell plate processed by laser engraving and after annealing treatment (n = 33, k = 5, θ = 30°). 6
Applied Thermal Engineering 168 (2020) 114889
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Fig. 7. The surface morphology of the partial fractal network microchannels of the wick on the upper shell plate processed by two different methods (k = 7, θ = 30°). Left: (a) level 0; (b) levels 0–1; (c) levels 3–4; (d) levels 5–6 microchannels processed by laser engraving. Right: (e) level 0; (f) levels 0–1; (g) levels 3–4; (h) levels 5–6 microchannels processed by mould coining.
7
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Table 2 Comparison between the depths of the fractal network microchannels of each level of the wick on the upper shell plate processed by two different methods (k = 7, θ = 30°). k
0
1
2
3
4
5
6
laser engraving (mm) mould coining (mm)
0.984 0.625
1.061 0.920
1.049 0.924
1.042 1.093
1.105 1.215
0.723 0.986
0.751 0.823
Table 3 The labels and corresponding structure parameters of the bionic vapor chamber samples.
samples with different structures could be calculated respectively.
ΔTmax = max{T4, T5, T6, T7, T8} − min{T4, T5, T6, T7, T8}
R=
T0 −
K eff =
1 5
(2)
Label
n
k
θ (°)
Processing method
41S-5L-30-LE 41S-5L-45-LE 41S-7L-30-LE 41S-7L-30-MC 41S-7L-45-LE 33S-5L-30-LE 33S-5L-45-LE 33S-7L-30-LE 33S-7L-45-LE
41 41 41 41 41 33 33 33 33
5 5 7 7 7 5 5 7 7
30 45 30 30 45 30 45 30 45
laser engraving laser engraving laser engraving mould coining laser engraving laser engraving laser engraving laser engraving laser engraving
8
∑i = 4 Ti
Q
(3)
between the evaporator and condenser (T0 − in K eff ,
8
∑i = 4 Ti ). While for the in-
Leff and A in Eq. (4) are equal plane effective thermal conductivity to the distance between thermocouples T6 and T4/T5/T7/T8 (28 mm) (Fig. 9) and the radial cross sectional area of the bionic vapor chamber, respectively, ΔT is equal to the temperature difference on the upper surface of the condenser. The uncertainty of the testing system was mainly caused by the heating module and temperature data acquisition module. The maximum measurement error of the heating module was related to the adjustable DC power supply with a 0.5% current accuracy and a 0.1% voltage accuracy. The measurement accuracy of the temperature data acquisition module was 0.02 °C and the T type thermocouples were calibrated with a measurement accuracy of 0.5 °C. The relative measurement errors of Q, R and Keff could be calculated by the method proposed by Kline and McClintock [29]:
QLeff A ·ΔT
1 5
(4)
where T0 is the measured temperature of the center point on the bottom surface of the evaporator, through a thermocouple T0 adhered between the vapor chamber and the heating copper block (Fig. 8). A thin layer of heat-conducting silicone grease with the thermal conductivity of 2.90 W/m·K was smeared between the vapor chamber and the heating copper block. Combining with the appropriate clamping force applied by the locking module, the air gap could be eliminated and the thermal contact resistance could be reduced. T6 is the measured temperature of the center point on the upper surface of the condenser, T4, T5, T7 and T8 are the measured temperatures of the four points distributing equidistantly on a circle with a diameter of 56 mm on the upper surface of the condenser, respectively (Fig. 8). Q is the heating power, Leff, A and ΔT are the effective length, cross sectional area and temperature difference along the heat transfer direction, respectively. Fig. 9 shows the heat transfer within the bionic vapor chamber. As the condenser and evaporator are arranged on two different surfaces, there are both throughplane heat transfer and in-plane heat transfer. Thus, for the throughth plane effective thermal conductivity K eff , Leff and A in Eq. (4) are equal to the thickness and axial cross sectional area of the bionic vapor chamber, respectively, ΔT is equal to the temperature difference
n
E (y ) = y
∂y
∑i = 1 ( ∂x Ex i )2 i
y
(5)
where y is a given function of the independent variable xi, E(y) is the measurement error of y and Exi is the maximum measurement error of xi. Accordingly, the maximum relative measurement uncertainties of Q and R were 0.1% and 5.88%, respectively. Keff only depended on R for a specific bionic vapor chamber, so they had the same uncertainty.
Fig. 8. Schematic diagram of the experimental setup for the performance test of the bionic vapor chambers. 8
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Fig. 9. Schematic diagram of the heat transfer within the bionic vapor chamber.
4. Results and discussion
effect is better and the depths of the microchannels of the same level at different positions are relatively uniform. These diminish ΔTmax and thus performing better temperature uniformity, which also confirms the conclusion obtained from Fig. 7. In terms of different n, ΔTmax of the samples with n = 41 are basically smaller than that of the corresponding samples with n = 33, namely the temperature uniformity of the former are better. This may be attributed to that the larger number of support columns in this designed configuration (Fig. 4) can provide a more uniform distribution, leading to a more uniform flow of the gaseous working medium. These diminish ΔTmax and thus performing better temperature uniformity. When Q = 140 W, sample 33S-7L-45-LE has the largest ΔTmax as 2.99 K, performing the worst temperature uniformity, while sample 41S-5L-30-LE has the smallest ΔTmax as 1.21 K, performing the best temperature uniformity. Fig. 11 shows the overall thermal resistance R of the bionic vapor
4.1. Comparisons among the bionic vapor chamber samples with different fractal levels and fractal angles Fig. 10 shows the maximum temperature difference ΔTmax on the upper surface of the condenser of the bionic vapor chamber samples with different fractal levels (k = 5, 7) and fractal angles (θ = 30°, 45°). ΔTmax increases with the increase of the heating power Q and the rates of increase of the samples with θ = 45° are faster than that of the samples with θ = 30°. In terms of different k, when Q is small, ΔTmax of the samples with k = 5 are basically greater than that of the samples with k = 7, namely the temperature uniformity of the former are worse. This may be attributed to that the capillary force predominates in the working fluid flowing under this condition. The higher fractal level can provide larger capillary force with narrower width of the microchannels of the last level. These facilitate the working fluid flowing and diminish ΔTmax, thus performing better temperature uniformity. Whereas, when Q becomes larger, the situation is just the opposite, the temperature uniformity of the samples with k = 5 are better than that of the samples with k = 7. This may be attributed to that as Q increases, the capillary force becomes limited and the permeability predominates in the working fluid flowing under this condition. The lower fractal level can provide larger permeability with wider width of the microchannels of the last level. These facilitate the working fluid flowing and diminish ΔTmax, thus performing better temperature uniformity. In terms of different θ, when Q is small, ΔTmax of the samples with θ = 30° are basically greater than that of the samples with θ = 45°, namely the temperature uniformity of the former are worse, which is also consistent with the previous research conclusion obtained by our research team (Q = 20 W and 30 W) [22]. Whereas, when Q becomes larger, the situation is just the opposite, the temperature uniformity of the samples with θ = 30° are better than that of the samples with θ = 45°. This may be attributed to that as Q increases, the rapid circulation of working fluid becomes more crucial. The smaller fractal angle can provide larger density of the fractal network microchannels. These diminish ΔTmax and thus performing better temperature uniformity. In terms of different processing methods of the fractal network microchannels on the wick on the upper shell plate (41S-7L-30-LE and 41S-7L-30-MC shown in Fig. 10a), ΔTmax of the sample processed by laser engraving (LE) is always smaller than that of the sample processed by mould coining (MC), namely the temperature uniformity of the former is better. This may be attributed to that the surface morphology of the fractal network microchannels of the wick on the upper shell plate processed by laser engraving and after annealing treatment are smoother. The forming
Fig. 10. The maximum temperature difference ΔTmax on the upper surface of the condenser of the bionic vapor chamber samples with different fractal levels (k = 5, 7) and fractal angles (θ = 30°, 45°): (a) n = 41; (b) n = 33. 9
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k = 5 are basically the same as that of the samples with k = 7, indicating that the influence of k on R is not obvious. In terms of different θ, R of the samples with θ = 30° are always greater than that of the samples with θ = 45°, which is also consistent with the previous research conclusion obtained by our research team [21]. In terms of different processing methods of the fractal network microchannels on the wick on the upper shell plate (41S-7L-30-LE and 41S-7L-30-MC shown in Fig. 11a), R of the sample processed by laser engraving (LE) is much greater than that of the sample processed by mould coining (MC). This may be attributed to that there are still some residual oxides which have not been fully reduced on the surface of the fractal network microchannels of the wick on the upper shell plate processed by laser engraving, leading to the increase of the conduction thermal resistance, thus improving R. On the other hand, the structures at both sides of the fractal network microchannels of the wick on the upper shell plate processed by mould coining collapsed to the center of the microchannels, resulting in the reduction of the conduction thermal resistance. Meanwhile, the increased volume of the steam chamber leads to the reduction of the thermal resistance in the flow process of the gaseous working medium, thus diminishing R. In terms of different n, R of the samples with n = 41 are always greater than that of the corresponding samples with n = 33. This may be attributed to that the larger number of support columns greatly reduce the volume of the steam chamber, blocking the flow of the gaseous working medium, leading to the increase of the thermal resistance in its flow process, thus improving R. When Q = 140 W, sample 41S-7L-30-LE has the largest R as 0.26 K/W, while sample 33S-7L-45-LE has the smallest R as 0.094 K/W.
Fig. 11. The overall thermal resistance R of the bionic vapor chamber samples with different fractal levels (k = 5, 7) and fractal angles (θ = 30°, 45°): (a) n = 41; (b) n = 33.
4.2. Comparisons among the bionic vapor chamber samples with different porosity designs Fig. 12a shows the maximum temperature difference ΔTmax on the upper surface of the condenser of the bionic vapor chamber samples with different porosity designs (homogeneous, gradient and anti-gradient). ΔTmax increases with the increase of the heating power Q and the rate of increase of the sample with porosity anti-gradient design (33S5L-30-LE-A) is faster than that of the samples with porosity gradient design (33S-5L-30-LE-G) and porosity homogeneous design (33S-5L-30LE). On the whole, ΔTmax of the sample 33S-5L-30-LE-A is always the largest, performing the worst temperature uniformity. Whereas, although ΔTmax of the sample 33S-5L-30-LE-G is smaller, it is basically greater than that of the sample 33S-5L-30-LE, which is not consistent with the previous research conclusion obtained from our simulation analysis [28], i.e. the temperature uniformity of the wick structure with porosity gradient design is better. By observing the processed gradient porous wick on the upper shell plate (Fig. 6b), as there were non-negligible gaps in some places when the three layers of copper foams with different porosities from inside to outside were spliced together, the incomplete wick structure would seriously affect the performance of the bionic vapor chamber, especially the temperature uniformity. Therefore, it is necessary to further improve the processing method of the copper foams and the way of splicing the copper foams with different porosities together, in order to achieve the theoretically good temperature uniformity of the gradient porous wick structure. When Q = 140 W, ΔTmax of the samples 33S-5L-30-LE, 33S-5L-30-LE-G and 33S-5L-30-LE-A are 1.44 K, 2.04 K and 2.8 K, respectively. Fig. 12b shows the overall thermal resistance R of the bionic vapor chamber samples with different porosity designs (homogeneous, gradient and anti-gradient). R decreases with the increase of Q and the rate of decrease of the sample with porosity anti-gradient design (33S-5L30-LE-A) is faster than that of the samples with porosity gradient design (33S-5L-30-LE-G) and porosity homogeneous design (33S-5L-30-LE). On the whole, R of the sample 33S-5L-30-LE-A is always the largest, followed by that of the sample 33S-5L-30-LE, while R of the sample 33S-5L-30-LE-G is always the smallest, which validates the superiority of the gradient porous wick structure designed. When Q = 140 W, R of
Fig. 12. (a) The maximum temperature difference ΔTmax on the upper surface of the condenser and (b) the overall thermal resistance R of the bionic vapor chamber samples with different porosity designs (homogeneous, gradient and anti-gradient).
chamber samples with different fractal levels (k = 5, 7) and fractal angles (θ = 30°, 45°). When n = 41 (Fig. 11a), the variation of R with the increase of Q is small (except for 41S-7L-45-LE). Whereas, when n = 33 (Fig. 11b), R decreases with the increase of Q and the rates of decrease of the samples with θ = 45° are much faster than that of the samples with θ = 30°. In terms of different k, R of the samples with 10
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Table 4 Comparison of the effective thermal conductivity Keff among the bionic vapor chamber and the existing vapor chambers. Dimension (mm)
Wick structure
Working fluid
th (W/m·K) K eff
in (W/m·K) K eff
The present work Our research team [21] Ji et al. [10]
Φ80 × 4.5 Φ80 × 7 Φ100 × 8
Deionized water Deionized water Acetone
9.52 5.57 8.49
10,500 3887 /
Mizuta et al. [12] Deng et al. [14] Zhou [30] Li [31]
50 75 74 74
Fractal network microchannels and copper foam Fractal network microchannels and micro fin-pins Copper foam bars inserted into the multiple channels fabricated in the fin heat sink Fine etching copper laminate Sintered copper powder Copper foam Copper foam
Deionized water Alcohol Deionized water Deionized water
10 7.11 6.48 3.00
/ 6222 16,718 8013
× × × ×
50 75 74 74
× × × ×
2 6 5 2.5
engraving was better than that of mould coining in this case. The wick structure for the evaporator was homogeneous copper foam. Finally, using the experimental setup established in this paper, the heat transfer performances of the bionic vapor chamber samples with different structures were investigated and compared. The minimum value of the maximum temperature difference on the upper surface of the condenser of the bionic vapor chamber was only 1.21 K and that of the overall thermal resistance of the bionic vapor chamber was only 0.094 K/W. Compared with the existing novel vapor chambers, when using deionized water as the working fluid, the maximum through-plane effective thermal conductivity and minimum in-plane effective thermal conductivity of the bionic vapor chamber were 9.52 W/m·K and 10500 W/ m·K, respectively, which were higher than most existing vapor chambers.
the samples 33S-5L-30-LE, 33S-5L-30-LE-G and 33S-5L-30-LE-A are 0.16 K/W, 0.11 K/W and 0.17 K/W, respectively. 4.3. Comparison among the bionic vapor chamber and the existing vapor chambers Table 4 shows the comparison of the effective thermal conductivity Keff among the bionic vapor chamber and the existing vapor chambers. Compared with the existing novel vapor chambers, when using deionized water as the working fluid, the maximum through-plane effective th thermal conductivity K eff and minimum in-plane effective thermal in conductivity K eff of the bionic vapor chamber designed in this paper by using the fractal network microchannels combined with copper foam as the wick structure are 9.52 W/m·K and 10500 W/m·K, respectively, th is just slightly which are higher than most existing vapor chambers. K eff lower than that of the vapor chamber proposed by Mizuta et al. [12], which used fine etching copper laminate as the wick structure. However, the multilayer structure may intensify the risk of air leakage in is just lower than that of the vapor during manufacturing. While K eff th . In particular, chamber proposed by Zhou [30], but with a higher K eff compared with the novel vapor chamber designed previously by our research team [21], the fractal network microchannels used to imitate the macroscopic leaf vein network was retained in the present design. At the same time, the micro fin-pins (showed an overall structure of porous media merely under macroscale) used to imitate the microscopic mesophyll tissue were modified to the real porous media (copper foam) structure, adding the permeability gradient distribution between the leaf veins and mesophyll tissue, presented as the porosity gradient distribution of the porous media around the fractal network microchannels, which better imitating the multiscale heat-fluid-structure of in th heat-resistant plant leaves, improving K eff and K eff by 71.0% and 170.1%, respectively. In addition, the fractal network microchannels on the copper foam of the wick sintered on the upper shell plate of the bionic vapor chamber were processed by laser engraving in this paper, the final processed depth of which can be up to 1 mm or more, much deeper than that of the microchannels formed previously by our research team through chemical etching (0.16–0.24 mm). Thus, more working fluid can be stored and the heat transfer performance of the bionic vapor chamber under high heat flux can be improved well.
CRediT authorship contribution statement Yuanqiang Luo: Conceptualization, Methodology, Validation, Investigation, Writing - original draft. Wangyu Liu: Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition. Guangwen Huang: Methodology, Investigation. 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. Acknowledgements The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (NOs. 51375169 & 11572128 & 11972161) and the third batch of introduced innovative research team of Dongguan, Guangdong Province, China (NO. 2017360004004). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2019.114889.
5. Conclusions
References
In this paper, a new type of bionic vapor chamber was designed and manufactured. The wick structure for the condenser combined the fractal network microchannels and copper foam, which were used to imitate the macroscopic leaf vein network and microscopic mesophyll tissue, respectively. Furthermore, the permeability gradient distribution between the leaf veins and mesophyll tissue was added, presented as the porosity gradient distribution of the copper foams around the fractal network microchannels. Then the fractal network microchannels on the wick on the upper shell plate were processed by laser engraving and mould coining, respectively. After observing and comparing the surface morphology, it indicated that the forming effect of laser
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imitating the transporting principle of plant leaf, Int. J. Heat Mass Transf. 71 (2014) 79–90. Y. Peng, W. Liu, B. Liu, J. Liu, K. Huang, L. Wang, W. Chen, The performance of the novel vapor chamber based on the leaf vein system, Int. J. Heat Mass Transf. 86 (2015) 656–666. W.-Y. Liu, Y.-Q. Luo, W. Chen, Y. Peng, Investigation on thermal resistance of a novel evaporator wick structure, Appl. Therm. Eng. 91 (2015) 731–738. W. Liu, Y. Peng, T. Luo, Y. Luo, K. Huang, The performance of the vapor chamber based on the plant leaf, Int. J. Heat Mass Transf. 98 (2016) 746–757. W.-Y. Liu, L. Wang, Y.-Q. Luo, Experimental investigation into leaf-vein-like fractal structure applied to evaporation of vapor chamber, J. South China Univ. Technol. (Natural Science Edition) 45 (2017) 118–122, 128. W. Liu, Y. Luo, L. Wang, T. Luo, Y. Peng, L. Wu, Water transport in leaf vein systems and the flow velocity measurement with a new method, J. Plant Physiol. 204 (2016) 74–84. Y. Luo, W. Liu, L. Wang, W. Xie, Heat and mass transfer characteristics of leaf-veininspired microchannels with wall thickening patterns, Int. J. Heat Mass Transf. 101 (2016) 1273–1282. M.A. Zwieniecki, P. Melcher, C.K. Boyce, L. Sack, N. Holbrook, Hydraulic architecture of leaf venation in Laurus nobilis L. Plant, Cell Environ. 25 (2002) 1445–1450. S. Salleo, F. Raimondo, P. Trifilo, A. Nardini, Axial-to-radial water permeability of leaf major veins: a possible determinant of the impact of vein embolism on leaf hydraulics? Plant, Cell Environ. 26 (2003) 1749–1758. H. Cochard, A. Nardini, L. Coll, Hydraulic architecture of leaf blades: where is the main resistance? Plant, Cell Environ. 27 (2004) 1257–1267. Y. Luo, W. Liu, J. Gou, Multiscale simulation of a novel leaf-vein-inspired gradient porous wick structure, J. Bionic Eng. 16 (2019) 828–841. S. Kline, F. McClintock, Describing uncertainties in single-sample experiments, Mech. Eng. 75 (1953) 3–8. W. Zhou, Fabricated process and heat transfer performance analysis of copper vapor chamber, in: South China University of Technology, Guangzhou, 2015. Z. Li, Manufacturing process and thermal performance analysis of ultra-thin vapor chamber, in: South China University of Technology, Guangzhou, 2016.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Fabrication and experimental investigation of the bionic vapor chamber ⁎
T
Yuanqiang Luo, Wangyu Liu , Guangwen Huang School of Mechanical and Automotive Engineering, South China University of Technology, 381 Wushan Road, Tianhe District, Guangzhou 510641, People’s Republic of China
H I GH L IG H T S
of bionic vapor chamber was designed and manufactured. • AThenewwicktypestructure the fractal network microchannels and copper foam. • The porosity gradientcombined of the copper foams around the fractal microchannels was added. • The fractal network microchannels processed by laser engraving and mould coining. • K of the bionic vapor chamber waswere higher than most existing vapor chambers. • eff
A R T I C LE I N FO
A B S T R A C T
Keywords: Heat dissipation Bionic vapor chamber Fractal network microchannels Copper foam Porosity gradient Laser engraving
With the rapid development of photoelectric products, their miniaturization and high integration have intensified the problem of heat dissipation. In this paper, a new type of bionic vapor chamber is designed and manufactured. The wick structure for the condenser combines the fractal network microchannels and copper foam, which are used to imitate the macroscopic leaf vein network and microscopic mesophyll tissue, respectively. Furthermore, the permeability gradient distribution between the leaf veins and mesophyll tissue is added, presented as the porosity gradient distribution of the copper foams around the fractal network microchannels. Then the fractal network microchannels on the wick are processed by laser engraving and mould coining, respectively. The wick structure for the evaporator is homogeneous copper foam. The minimum value of the maximum temperature difference on the upper surface of the condenser of the bionic vapor chamber is only 1.21 K and that of the overall thermal resistance of the bionic vapor chamber is only 0.094 K/W. Compared with the existing novel vapor chambers, when using deionized water as the working fluid, the maximum throughplane effective thermal conductivity and minimum in-plane effective thermal conductivity are 9.52 W/m·K and 10500 W/m·K, respectively, which are higher than most existing vapor chambers.
1. Introduction With the rapid development of photoelectric products, their miniaturization and high integration have intensified the problem of heat dissipation. Vapor chamber is a special type of heat pipe [1–3] that is a particularly effective heat spreader for electronics. It is also referred as flat heat pipe and is widely applied due to its uniform temperature distribution, large condensation area, light weight, geometric flexibility and extremely high thermal conductivities [4–8]. The role of wick in vapor chamber is to direct condensed liquid from the condenser to the evaporator of the vapor chamber. Typically, the porous wick structure can consist of the following types: sintered, groove and mesh [9]. Powder sintering wick structure and mesh sintering wick structure are generally formed through graphite mould sintering, while the groove ⁎
wick structure is generally formed by directly removing the materials on the groove part through machining, or by increasing the materials on the non-groove part through powder sintering. In the past five years, researchers at home and abroad constantly tried to design various novel vapor chamber structures and then explore the most suitable processing method to form the structures and manufacture the vapor chambers, in order to obtain better flow and heat transfer performances to meet higher working condition requirements. Ji et al. [10] designed a novel integrated flat heat pipe (IFHP), which consisted of an evaporator and a condenser with multiple channels fabricated in the fin heat sink. A layer of compressed copper foam was sintered on the inner surface of the evaporator bottom plate, and many copper foam bars were inserted into the channels, both of which formed a porous network wick. Compared with the conventional flat heat pipe
Corresponding author. E-mail address: [email protected] (W. Liu).
https://doi.org/10.1016/j.applthermaleng.2019.114889 Received 22 July 2019; Received in revised form 24 December 2019; Accepted 29 December 2019 Available online 30 December 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
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Nomenclature
Greek symbols
K D L l W w n m V ΔT R Keff Q T Leff A xi y E
θ ρ ε
fractal level effective diameter, mm length of the microchannel, mm length ratio width of the microchannel, mm width difference, mm quantity of the support columns filling amount, g volume, mm3 temperature difference, K thermal resistance, K/W effective thermal conductivity, W/m·K heating power, W measured temperature, K effective length, m cross sectional area, m2 independent variable given function measurement error
fractal angle, ° density of water, g/mm3 porosity
Subscripts steam support fractal c e max eff
steam chamber support columns fractal network microchannels wick for the condenser wick for the evaporator maximum effective
Superscripts th in
(CFHP), the IFHP presented good fin temperature uniformity and heat performance. Shaeri et al. [11] fabricated a novel vapor chamber to assess the feasibility of combining hydrophobic and hydrophilic wettabilities in the evaporator to optimize thermal performance. The proposed vapor chamber included a separate layer of hydrophilic sintered copper powder wick that was pressed in intimate contact with a hydrophobic evaporator substrate with a water contact angle around 140°. Compared with those of a baseline vapor chamber that was fabricated by sintering hydrophilic copper particles on a hydrophilic copper evaporator substrate, the thermal resistance of the proposed vapor chamber was lower. Mizuta et al. [12] developed a type of flat laminate vapor chamber called FGHP (Fine Grid Heat Pipe), and its thermal performance was investigated. Fine etching technique enabled formation of microstructures on laminate parts. Without regarding the heat input, the FGHP showed more uniform temperature distribution than the copper heat spreader. Ju et al. [13] developed an innovative vapor chamber concept incorporating hybrid wicks of two integrated structures: a low thermal resistance spreading layer and a dedicated liquid supply structure. The spreading layer was comprised of a monolayer of sintered Cu particles to minimize the thermal resistance in the evaporator. Three different liquid supply structures were studied: vertical columnar arteries, converging lateral arteries, and bi-porous structures. Deng et al. [14] developed a type of composite porous vapor chambers (CPVCs) with uniform radial grooves in the evaporator, providing radial multi-artery channels for the fast liquid backflow with low
through-plane in-plane
hydraulic resistance. Compared with a pure copper plate, the CPVCs were found to maintain good temperature uniformity. They were able to operate efficiently to high heat fluxes without notable performance degradation, and presented smaller thermal resistances. Patankar et al. [15] considered the unique transport limitations and thermal requirements encountered in mobile applications, and developed a methodology for the design of vapor chambers to yield improved condenserside temperature uniformity at ultra-thin form factors, which focused on manipulating the condenser-side wick to improve lateral heat spreading. A biporous condenser-side wick design was proposed that facilitated a thicker vapor core, and thereby reduced the condenser surface peak-to-mean temperature difference relative to a monolithic wick structure. Natural selection has evolved over billions of years, giving us endless inspiration for bionics design. With regard to plant bionic vapor chambers, Ji et al. [16,17] verified the synergy of various length scales to activate various functions. The strategy guided multiscale design to realize an enhancement of capillary pressure, a well management of flow resistance and an ultra-thin liquid thickness on condenser surface. The porous wick consisted of a particle sub-layer and 3D mastoid process array inspired by the uniform conical structures on a leaf of Calathea zebrine. The tips of mastoid process directly contacted the condenser wall. The liquid suction and heat transfer experiments indicated that nano-roughness increased the vapor-liquid interface area to have 3–4 times of evaporation heat transfer coefficients compared
Fig. 1. The manufacturing process of the bionic vapor chamber. 2
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support columns, wick for the evaporator, evaporator and liquid filling tube. The structural design and manufacturing process of each part is shown in Fig. 1. Among them, design of the wick structure for the condenser, design of the configuration of the support columns and processing of the fractal network microchannels on the wick on the upper shell plate are the key processes.
with smooth particle surface, and increased the wettability to capture liquid from condenser, having ~18 times of condensation heat transfer coefficients to those without nano-roughness. Whereas, the processing of nano-roughness structure is complicated, which is not conducive for batch production. Moreover, the temperature uniformity of the vapor chamber was not taken into account. Seeing the similarity between vapor chamber and plant leaf, as they both have flat structure and cool themselves with phase change, our research team [5,18–22] has designed a novel structure based on the leaf vein system to form the wick of a vapor chamber. In this novel wick fractal network microchannels and the micro fin-pins were used to simulate the leaf vein network and mesophyll tissue respectively. The theoretical, experimental and numerical analysis all showed excellent fluid and thermal performances of the bio-inspired vapor chamber. Whereas, the imitation of plant leaf was just its macroscopic fractal network structure, and the micro finpins used to imitate the mesophyll tissue showed an overall structure of porous media merely under macroscale. In addition, the microchannels formed through chemical etching were unable to be deep and narrow simultaneously. In this paper, a new type of bionic vapor chamber will be designed and manufactured. Using the experimental setup established in this paper, the heat transfer performances of the bionic vapor chamber samples with different structures will be investigated and compared.
2.1. Design of the wick structure for the condenser From our further research on the multiscale structure of the leaf vein [23,24], we found that first- and second-order veins have high axial conductance and relatively small radial permeability, thus allowing water to reach distal areas of the leaf with only a small loss of water potential. Higher order veins tend to be more hydraulically resistant and permit greater radial leakage. This design allows for a relatively equitable distribution of water potential and thus reflects the capacity of the venation to provide a relatively homogeneous water supply across the leaf lamina, with only the leaf margins being hydraulically disadvantaged relative to the rest of the leaf [25–27]. Similarly, this natural optimized structure can be applied on the design of the wick structure for the condenser of vapor chamber. Due to the excellent flow characteristic of fractal structure, i.e. higher permeability and larger capillary suction than parallel structure [5], the fractal network microchannels used to imitate the macroscopic leaf vein network was retained in the present design. At the same time, the micro fin-pins (showed an overall structure of porous media merely under macroscale) used to imitate the microscopic mesophyll tissue were modified to the real porous media (copper foam) structure, adding
2. Design and fabrication of the bionic vapor chamber The bionic vapor chamber designed and manufactured in this paper is composed of the following parts: condenser, wick for the condenser,
Fig. 2. Four designed wick structures for the condenser with different fractal levels and fractal angles: (a) k = 5, θ = 30°; (b) k = 5, θ = 45°; (c) k = 7, θ = 30°; (d) k = 7, θ = 45°. 3
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fractal angles, which increases the easy repeatability of batch manufacturing and reduces the manufacturing quantity of the graphite mould for the subsequent sintering, thus greatly reducing the manufacturing cost.
the permeability gradient distribution between the leaf veins and mesophyll tissue, presented as the porosity gradient distribution of the porous media around the fractal network microchannels. As our research team [5,18,21] has done fairly sufficient research about the influence of various design parameters of the fractal network microchannel structure on its flow and heat transfer performances, only four different fractal network microchannel structures were chosen in this paper to further investigate the influence of different porous wick structures on the performance of the bionic vapor chamber. From the observation of leaf vein system of three ordinary species of plants with heat-resistance in our former research [23], the visible leaf vein levels are 5–7 levels generally. Therefore, two fractal levels (k = 5, 7) were selected for the fractal network microchannels of the porous wick structures for the condenser. In addition, from the measurement of the angles between the midrib and secondary veins on leaves of the three selected plants [23], those angles of Ficus virens Ait. var. sublanceolata (Miq.) Corner and Hamelia patens with better heat and mass transfer characteristics were 41.9°–65.3° and 36.1°–53.7°, respectively, while that of Plumeria rubra L. cv. Acutifolia with worse heat and mass transfer characteristics was 51.5°–89.5°. Therefore, two fractal angles (θ = 30°, 45°) were selected for the fractal network microchannels of the porous wick structures for the condenser. The effective diameter D of each wick is 70 mm. The length of the level 0 microchannel L0 is 12/11 mm and the length ratio defined as l = Lk+1/Lk (k = 0, 1, …, 5) is 0.7, as our former research [5] has found that when 0 < l < 1, the flow resistance of the fractal network microchannels was smaller and the permeability was larger than when l = 1. The width of the level 0 microchannel W0 is 0.35 mm and the width ratio is replaced with the width difference defined as w = Wk+1 − Wk (k = 0, 1, …, 5) in this work to ensure that it is easy to manufacture. As the wick structure was designed for the condenser, where the fluid flows from the centre to the circumference of the wick, w was set to be positive (0.05 mm). From the above, four designed wick structures for the condenser are shown in Fig. 2, and the detailed dimensions of the microchannel of each level are shown in Table 1. For porosity gradient design, the porous media zones around the microchannels of each two levels, i.e. levels 0–1, 2–3 and 4 (k = 5), are defined as porous_1, porous_2 and porous_3, respectively, as shown in Fig. 3. Furthermore, it should be noted that a multiscale simulation of the designed wick structure for the condenser has been carried out to predict its performances [28]. The simulation results had verified the superiority of the porosity gradient design of the wick structure, which had superior fluid and thermal performances with a relatively uniform pressure distribution. The temperature of the gradient porous wick structure could reach balanced state faster and its temperature uniformity was better.
2.3. Processing of the fractal network microchannels on the wick on the upper shell plate As can be seen from Fig. 2 and Table 1, with the increase of k of the fractal network microchannels, the width W of the microchannel of each level decreases gradually, achieving a good balance between the capillary force and permeability. Whereas, it gives rise to great difficulty on the processing of the microchannels, which thus becomes the most critical step in the manufacturing process of the bionic vapor chamber. The microchannels formed previously by our research team [19–22] through chemical etching were unable to be deep and narrow simultaneously. The final processed depth of the microchannels was only 0.16–0.24 mm, which greatly limited the performance of the bionic vapor chamber. In this paper, copper foam was selected as the porous media in the wick structure of the bionic vapor chamber. Taking advantage of the easy processing characteristic of copper foam, two different processing methods, i.e. laser engraving (Fig. 5c and d) and mould coining (Fig. 5e and f) were used to process the fractal network microchannels on the copper foam of the wick sintered on the upper shell plate (Fig. 5a) respectively, to finally form the complete wick structure for the condenser. For porosity gradient design, the copper foam of the gradient porous wick of the upper shell plate after processing and the gradient porous wick on the upper shell plate processed by laser engraving and after annealing treatment (n = 33, k = 5, θ = 30°) are shown in Fig. 6. For working fluid filling, a specific filling amount was set on the high-precision metering pump (Canada HIBAR 1SC-10 M, ± 0.5%). Then the filling needle was inserted into the liquid filling tube and started filling. For evacuation, the liquid filling tube was placed in the bleeding point of the automatic vacuumizer and clamped by the fixture. After evacuating for 300 s, the equipment will automatically flatten the end of the liquid filling tube to form a temporary cold welding sealing and the vacuum level inside the cavity was less than 50 Pa. To ensure the designed filling amount as far as possible, the mass difference between the bionic vapor chamber before and after evacuation was measured and compensated to the set filling amount. Finally, after the whole manufacturing process (Fig. 1), the finished product of the bionic vapor chamber is shown in Fig. 5g. The external diameter is 80 mm and the total thickness is 4.5 mm. As shown in Fig. 7, the surface morphology of the partial fractal network microchannels of the wick on the upper shell plate processed by two different methods (k = 7, θ = 30°) were observed and compared under the 3D microscope with super depth of field (with the magnification times of 100 × ). The depth of the microchannel of each level was measured and shown in Table 2 (with the designed depth of 1 mm). From Fig. 7a–d and Table 2, the surface morphology of the fractal network microchannels of the wick on the upper shell plate processed by laser engraving and after annealing treatment (k = 7,
2.2. Design of the configuration of the support columns Fig. 4 shows the configuration of the support columns when the quantity n is 41 and the positional relation between them and different wick structures (blue). When the quantity is 33, the 8 support columns marked (red) are removed. There are three different sizes of support columns, the one in the center is Φ1.5 mm, the 8 outermost ones are Φ3 mm and the remaining 32/24 support columns are Φ2 mm. Except for the interference between the 8 outermost support columns and a few microchannels on the wick structures with 7 fractal levels, the rest of the support columns were all arranged within the porous media regions between the microchannels. As the fractal network microchannel structure itself has the characteristic of preventing the overall blockage, the partial blockage of a few microchannels interfered with the support columns will not cause too many damages to the overall microchannel structure of the wick. In particular, there is no interference between the support columns and the two wick structures with 5 fractal levels. Hence the configuration of the support columns is of common use to the four wick structures for the condenser with different fractal levels and
Table 1 The detailed dimensions of the microchannel of each level of the wick structures for the condenser with different fractal levels. k
1
2
3
4
5
6
5
L1 (mm) W2 (mm)
12 0.35
8.4 0.30
5.88 0.25
4.116 0.20
2.8812 0.15
/ /
/ /
7
L1 (mm) W2 (mm)
11 0.35
7.7 0.30
5.39 0.25
3.773 0.20
2.6411 0.15
1.84877 0.10
1.294139 0.05
1 2
4
0
The length of the microchannel of each level. The width of the microchannel of each level.
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disadvantages. Generally speaking, the forming effect of laser engraving is better than that of mould coining in processing the fractal network microchannels on the copper foam of the wick sintered on the upper shell plate. In order to obtain the best forming effect and machining precision, we can use laser engraving to process the microchannels that basically reach the designed depth, and then use mould coining to process them again, so as to deepen the depth of the microchannels of high levels. In this way, the collapse of the structures at both sides of the microchannels due to extrusion can be avoided. Moreover, the depth at every position of the whole fractal network microchannels can be ensured to basically reach the designed depth. Whereas, the processing cost and time will increase correspondingly.
Fig. 3. The 1/24 model and porosity gradient design of the wick structure for the condenser (k = 5, θ = 30°).
θ = 30°) are smoother. The forming effect is better and the depths of the microchannels of the same level at different positions are relatively uniform. Whereas, with the increase of k of the fractal network microchannels, the width W of the microchannel of each level decreases gradually, thus the engraving effect becomes poor and the depth becomes shallow gradually. From Fig. 7e–h and Table 2, the surface morphology of the fractal network microchannels of the wick on the upper shell plate processed by mould coining (k = 7, θ = 30°) are relatively less smooth. As during coining, the mould processed the concave microchannels on copper foam through extrusion instead of cutting the material directly, the structures at both sides of the microchannels collapsed to the center of the microchannels, which worsened the forming effect. To sum up, the two processing methods have their advantages and
3. Experimental setup and samples for the performance test 3.1. Experimental setup Using the experimental setup established in this part (Fig. 8), the heat transfer performances of the bionic vapor chamber samples with different structures were investigated and compared, mainly included the temperature uniformity on the upper surface of the condenser and the overall thermal resistance. As shown in Fig. 8, the entire experimental setup could be divided into four parts: the heating module, cooling module, temperature data acquisition module and locking module. The heating area of the copper block was 20 × 20 mm2. The temperature of the cooling water was 60 °C and its rate of flow was 80 L/h. The thermocouples were T type (Omega,
Fig. 4. The configuration of the support columns when the quantity n is 41 and the positional relation between them and different wick structures (blue): (a) k = 5, θ = 30°; (b) k = 5, θ = 45°; (c) k = 7, θ = 30°; (d) k = 7, θ = 45°. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 5
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Fig. 5. Two different methods for the processing of the fractal network microchannels on the wick on the upper shell plate (n = 41): (a) the copper foam of the wick sintered on the upper shell plate; (b) the copper foam of the wick sintered on the bottom shell plate; (c) the wick on the upper shell plate processed by laser engraving (k = 5, θ = 30°); (d) the wick on the upper shell plate after annealing treatment (k = 5, θ = 30°); (e) the coining mould formed by 3D printing (k = 7, θ = 30°); (f) the wick on the upper shell plate processed by mould coining (k = 7, θ = 30°); (g) the finished product of the bionic vapor chamber.
−200 ~ 260 °C, ± 0.5 °C) and the data acquisition card was NI 9213 + cDAQ 9171.
calculated as Eq. (1).
m = 63% × ρ (Vsteam − −Vsupport + Vfractal + εc1 Vc1 + εc 2 Vc 2 + εc 3 Vc3 + εe Ve )
3.2. Samples
(1)
where ρ is the density of water. Vsteam, Vsupport and Vfractal are the volumes of the steam chamber, support columns and fractal network microchannels, respectively. εc1, εc2, εc3 and Vc1, Vc2, Vc3 are the porosities and volumes of the porous_1, porous_2, porous_3 zones of the wick for the condenser, respectively. εe and Ve are the porosity and volume of the wick for the evaporator, respectively.
The labels and corresponding structure parameters of the bionic vapor chamber samples are shown in Table 3. There were two samples for each structure, e.g. 41S-5L-30-LE#1 and 41S-5L-30-LE#2. Taking the sample 33S-5L-30-LE as a benchmark (the porosity of the copper foam of the wick of the upper shell plate was 50% homogeneously), for porosity gradient design (33S-5L-30-LE-G), the porosities of the three different layers of copper foams of the wick of the upper shell plate from inside to outside (Fig. 6a) were 45%, 50% and 55%, respectively. On the contrary, for porosity anti-gradient design (33S-5L-30-LE-A), that were 55%, 50% and 45%, respectively. The wick structure for the evaporator (Fig. 5b) was copper foam with the porosity of 95% homogeneously for all samples. The working fluid was deionized water and the filling ratio was set to be 63%, as our former research [19] has found that the vapor chamber with fractal network microchannels and micro fin-pins has the lowest thermal resistance under this filling ratio. For each sample with different internal structures/volumes, the filling amount m was
3.3. Data reduction and uncertainty analysis The temperatures of the center point on the bottom surface of the evaporator and the upper surface of the condenser of the bionic vapor chamber samples were measured respectively under different heating powers (60 W, 80 W, 100 W, 120 W and 140 W). For the two samples of the same structure, only the one with better performance was selected. According to Eqs. (2)–(4), the maximum temperature difference ΔTmax on the upper surface of the condenser, the overall thermal resistance R and the effective thermal conductivity Keff of the bionic vapor chamber
Fig. 6. (a) The copper foam of the gradient porous wick of the upper shell plate after processing; (b) the gradient porous wick on the upper shell plate processed by laser engraving and after annealing treatment (n = 33, k = 5, θ = 30°). 6
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Fig. 7. The surface morphology of the partial fractal network microchannels of the wick on the upper shell plate processed by two different methods (k = 7, θ = 30°). Left: (a) level 0; (b) levels 0–1; (c) levels 3–4; (d) levels 5–6 microchannels processed by laser engraving. Right: (e) level 0; (f) levels 0–1; (g) levels 3–4; (h) levels 5–6 microchannels processed by mould coining.
7
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Table 2 Comparison between the depths of the fractal network microchannels of each level of the wick on the upper shell plate processed by two different methods (k = 7, θ = 30°). k
0
1
2
3
4
5
6
laser engraving (mm) mould coining (mm)
0.984 0.625
1.061 0.920
1.049 0.924
1.042 1.093
1.105 1.215
0.723 0.986
0.751 0.823
Table 3 The labels and corresponding structure parameters of the bionic vapor chamber samples.
samples with different structures could be calculated respectively.
ΔTmax = max{T4, T5, T6, T7, T8} − min{T4, T5, T6, T7, T8}
R=
T0 −
K eff =
1 5
(2)
Label
n
k
θ (°)
Processing method
41S-5L-30-LE 41S-5L-45-LE 41S-7L-30-LE 41S-7L-30-MC 41S-7L-45-LE 33S-5L-30-LE 33S-5L-45-LE 33S-7L-30-LE 33S-7L-45-LE
41 41 41 41 41 33 33 33 33
5 5 7 7 7 5 5 7 7
30 45 30 30 45 30 45 30 45
laser engraving laser engraving laser engraving mould coining laser engraving laser engraving laser engraving laser engraving laser engraving
8
∑i = 4 Ti
Q
(3)
between the evaporator and condenser (T0 − in K eff ,
8
∑i = 4 Ti ). While for the in-
Leff and A in Eq. (4) are equal plane effective thermal conductivity to the distance between thermocouples T6 and T4/T5/T7/T8 (28 mm) (Fig. 9) and the radial cross sectional area of the bionic vapor chamber, respectively, ΔT is equal to the temperature difference on the upper surface of the condenser. The uncertainty of the testing system was mainly caused by the heating module and temperature data acquisition module. The maximum measurement error of the heating module was related to the adjustable DC power supply with a 0.5% current accuracy and a 0.1% voltage accuracy. The measurement accuracy of the temperature data acquisition module was 0.02 °C and the T type thermocouples were calibrated with a measurement accuracy of 0.5 °C. The relative measurement errors of Q, R and Keff could be calculated by the method proposed by Kline and McClintock [29]:
QLeff A ·ΔT
1 5
(4)
where T0 is the measured temperature of the center point on the bottom surface of the evaporator, through a thermocouple T0 adhered between the vapor chamber and the heating copper block (Fig. 8). A thin layer of heat-conducting silicone grease with the thermal conductivity of 2.90 W/m·K was smeared between the vapor chamber and the heating copper block. Combining with the appropriate clamping force applied by the locking module, the air gap could be eliminated and the thermal contact resistance could be reduced. T6 is the measured temperature of the center point on the upper surface of the condenser, T4, T5, T7 and T8 are the measured temperatures of the four points distributing equidistantly on a circle with a diameter of 56 mm on the upper surface of the condenser, respectively (Fig. 8). Q is the heating power, Leff, A and ΔT are the effective length, cross sectional area and temperature difference along the heat transfer direction, respectively. Fig. 9 shows the heat transfer within the bionic vapor chamber. As the condenser and evaporator are arranged on two different surfaces, there are both throughplane heat transfer and in-plane heat transfer. Thus, for the throughth plane effective thermal conductivity K eff , Leff and A in Eq. (4) are equal to the thickness and axial cross sectional area of the bionic vapor chamber, respectively, ΔT is equal to the temperature difference
n
E (y ) = y
∂y
∑i = 1 ( ∂x Ex i )2 i
y
(5)
where y is a given function of the independent variable xi, E(y) is the measurement error of y and Exi is the maximum measurement error of xi. Accordingly, the maximum relative measurement uncertainties of Q and R were 0.1% and 5.88%, respectively. Keff only depended on R for a specific bionic vapor chamber, so they had the same uncertainty.
Fig. 8. Schematic diagram of the experimental setup for the performance test of the bionic vapor chambers. 8
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Fig. 9. Schematic diagram of the heat transfer within the bionic vapor chamber.
4. Results and discussion
effect is better and the depths of the microchannels of the same level at different positions are relatively uniform. These diminish ΔTmax and thus performing better temperature uniformity, which also confirms the conclusion obtained from Fig. 7. In terms of different n, ΔTmax of the samples with n = 41 are basically smaller than that of the corresponding samples with n = 33, namely the temperature uniformity of the former are better. This may be attributed to that the larger number of support columns in this designed configuration (Fig. 4) can provide a more uniform distribution, leading to a more uniform flow of the gaseous working medium. These diminish ΔTmax and thus performing better temperature uniformity. When Q = 140 W, sample 33S-7L-45-LE has the largest ΔTmax as 2.99 K, performing the worst temperature uniformity, while sample 41S-5L-30-LE has the smallest ΔTmax as 1.21 K, performing the best temperature uniformity. Fig. 11 shows the overall thermal resistance R of the bionic vapor
4.1. Comparisons among the bionic vapor chamber samples with different fractal levels and fractal angles Fig. 10 shows the maximum temperature difference ΔTmax on the upper surface of the condenser of the bionic vapor chamber samples with different fractal levels (k = 5, 7) and fractal angles (θ = 30°, 45°). ΔTmax increases with the increase of the heating power Q and the rates of increase of the samples with θ = 45° are faster than that of the samples with θ = 30°. In terms of different k, when Q is small, ΔTmax of the samples with k = 5 are basically greater than that of the samples with k = 7, namely the temperature uniformity of the former are worse. This may be attributed to that the capillary force predominates in the working fluid flowing under this condition. The higher fractal level can provide larger capillary force with narrower width of the microchannels of the last level. These facilitate the working fluid flowing and diminish ΔTmax, thus performing better temperature uniformity. Whereas, when Q becomes larger, the situation is just the opposite, the temperature uniformity of the samples with k = 5 are better than that of the samples with k = 7. This may be attributed to that as Q increases, the capillary force becomes limited and the permeability predominates in the working fluid flowing under this condition. The lower fractal level can provide larger permeability with wider width of the microchannels of the last level. These facilitate the working fluid flowing and diminish ΔTmax, thus performing better temperature uniformity. In terms of different θ, when Q is small, ΔTmax of the samples with θ = 30° are basically greater than that of the samples with θ = 45°, namely the temperature uniformity of the former are worse, which is also consistent with the previous research conclusion obtained by our research team (Q = 20 W and 30 W) [22]. Whereas, when Q becomes larger, the situation is just the opposite, the temperature uniformity of the samples with θ = 30° are better than that of the samples with θ = 45°. This may be attributed to that as Q increases, the rapid circulation of working fluid becomes more crucial. The smaller fractal angle can provide larger density of the fractal network microchannels. These diminish ΔTmax and thus performing better temperature uniformity. In terms of different processing methods of the fractal network microchannels on the wick on the upper shell plate (41S-7L-30-LE and 41S-7L-30-MC shown in Fig. 10a), ΔTmax of the sample processed by laser engraving (LE) is always smaller than that of the sample processed by mould coining (MC), namely the temperature uniformity of the former is better. This may be attributed to that the surface morphology of the fractal network microchannels of the wick on the upper shell plate processed by laser engraving and after annealing treatment are smoother. The forming
Fig. 10. The maximum temperature difference ΔTmax on the upper surface of the condenser of the bionic vapor chamber samples with different fractal levels (k = 5, 7) and fractal angles (θ = 30°, 45°): (a) n = 41; (b) n = 33. 9
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k = 5 are basically the same as that of the samples with k = 7, indicating that the influence of k on R is not obvious. In terms of different θ, R of the samples with θ = 30° are always greater than that of the samples with θ = 45°, which is also consistent with the previous research conclusion obtained by our research team [21]. In terms of different processing methods of the fractal network microchannels on the wick on the upper shell plate (41S-7L-30-LE and 41S-7L-30-MC shown in Fig. 11a), R of the sample processed by laser engraving (LE) is much greater than that of the sample processed by mould coining (MC). This may be attributed to that there are still some residual oxides which have not been fully reduced on the surface of the fractal network microchannels of the wick on the upper shell plate processed by laser engraving, leading to the increase of the conduction thermal resistance, thus improving R. On the other hand, the structures at both sides of the fractal network microchannels of the wick on the upper shell plate processed by mould coining collapsed to the center of the microchannels, resulting in the reduction of the conduction thermal resistance. Meanwhile, the increased volume of the steam chamber leads to the reduction of the thermal resistance in the flow process of the gaseous working medium, thus diminishing R. In terms of different n, R of the samples with n = 41 are always greater than that of the corresponding samples with n = 33. This may be attributed to that the larger number of support columns greatly reduce the volume of the steam chamber, blocking the flow of the gaseous working medium, leading to the increase of the thermal resistance in its flow process, thus improving R. When Q = 140 W, sample 41S-7L-30-LE has the largest R as 0.26 K/W, while sample 33S-7L-45-LE has the smallest R as 0.094 K/W.
Fig. 11. The overall thermal resistance R of the bionic vapor chamber samples with different fractal levels (k = 5, 7) and fractal angles (θ = 30°, 45°): (a) n = 41; (b) n = 33.
4.2. Comparisons among the bionic vapor chamber samples with different porosity designs Fig. 12a shows the maximum temperature difference ΔTmax on the upper surface of the condenser of the bionic vapor chamber samples with different porosity designs (homogeneous, gradient and anti-gradient). ΔTmax increases with the increase of the heating power Q and the rate of increase of the sample with porosity anti-gradient design (33S5L-30-LE-A) is faster than that of the samples with porosity gradient design (33S-5L-30-LE-G) and porosity homogeneous design (33S-5L-30LE). On the whole, ΔTmax of the sample 33S-5L-30-LE-A is always the largest, performing the worst temperature uniformity. Whereas, although ΔTmax of the sample 33S-5L-30-LE-G is smaller, it is basically greater than that of the sample 33S-5L-30-LE, which is not consistent with the previous research conclusion obtained from our simulation analysis [28], i.e. the temperature uniformity of the wick structure with porosity gradient design is better. By observing the processed gradient porous wick on the upper shell plate (Fig. 6b), as there were non-negligible gaps in some places when the three layers of copper foams with different porosities from inside to outside were spliced together, the incomplete wick structure would seriously affect the performance of the bionic vapor chamber, especially the temperature uniformity. Therefore, it is necessary to further improve the processing method of the copper foams and the way of splicing the copper foams with different porosities together, in order to achieve the theoretically good temperature uniformity of the gradient porous wick structure. When Q = 140 W, ΔTmax of the samples 33S-5L-30-LE, 33S-5L-30-LE-G and 33S-5L-30-LE-A are 1.44 K, 2.04 K and 2.8 K, respectively. Fig. 12b shows the overall thermal resistance R of the bionic vapor chamber samples with different porosity designs (homogeneous, gradient and anti-gradient). R decreases with the increase of Q and the rate of decrease of the sample with porosity anti-gradient design (33S-5L30-LE-A) is faster than that of the samples with porosity gradient design (33S-5L-30-LE-G) and porosity homogeneous design (33S-5L-30-LE). On the whole, R of the sample 33S-5L-30-LE-A is always the largest, followed by that of the sample 33S-5L-30-LE, while R of the sample 33S-5L-30-LE-G is always the smallest, which validates the superiority of the gradient porous wick structure designed. When Q = 140 W, R of
Fig. 12. (a) The maximum temperature difference ΔTmax on the upper surface of the condenser and (b) the overall thermal resistance R of the bionic vapor chamber samples with different porosity designs (homogeneous, gradient and anti-gradient).
chamber samples with different fractal levels (k = 5, 7) and fractal angles (θ = 30°, 45°). When n = 41 (Fig. 11a), the variation of R with the increase of Q is small (except for 41S-7L-45-LE). Whereas, when n = 33 (Fig. 11b), R decreases with the increase of Q and the rates of decrease of the samples with θ = 45° are much faster than that of the samples with θ = 30°. In terms of different k, R of the samples with 10
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Table 4 Comparison of the effective thermal conductivity Keff among the bionic vapor chamber and the existing vapor chambers. Dimension (mm)
Wick structure
Working fluid
th (W/m·K) K eff
in (W/m·K) K eff
The present work Our research team [21] Ji et al. [10]
Φ80 × 4.5 Φ80 × 7 Φ100 × 8
Deionized water Deionized water Acetone
9.52 5.57 8.49
10,500 3887 /
Mizuta et al. [12] Deng et al. [14] Zhou [30] Li [31]
50 75 74 74
Fractal network microchannels and copper foam Fractal network microchannels and micro fin-pins Copper foam bars inserted into the multiple channels fabricated in the fin heat sink Fine etching copper laminate Sintered copper powder Copper foam Copper foam
Deionized water Alcohol Deionized water Deionized water
10 7.11 6.48 3.00
/ 6222 16,718 8013
× × × ×
50 75 74 74
× × × ×
2 6 5 2.5
engraving was better than that of mould coining in this case. The wick structure for the evaporator was homogeneous copper foam. Finally, using the experimental setup established in this paper, the heat transfer performances of the bionic vapor chamber samples with different structures were investigated and compared. The minimum value of the maximum temperature difference on the upper surface of the condenser of the bionic vapor chamber was only 1.21 K and that of the overall thermal resistance of the bionic vapor chamber was only 0.094 K/W. Compared with the existing novel vapor chambers, when using deionized water as the working fluid, the maximum through-plane effective thermal conductivity and minimum in-plane effective thermal conductivity of the bionic vapor chamber were 9.52 W/m·K and 10500 W/ m·K, respectively, which were higher than most existing vapor chambers.
the samples 33S-5L-30-LE, 33S-5L-30-LE-G and 33S-5L-30-LE-A are 0.16 K/W, 0.11 K/W and 0.17 K/W, respectively. 4.3. Comparison among the bionic vapor chamber and the existing vapor chambers Table 4 shows the comparison of the effective thermal conductivity Keff among the bionic vapor chamber and the existing vapor chambers. Compared with the existing novel vapor chambers, when using deionized water as the working fluid, the maximum through-plane effective th thermal conductivity K eff and minimum in-plane effective thermal in conductivity K eff of the bionic vapor chamber designed in this paper by using the fractal network microchannels combined with copper foam as the wick structure are 9.52 W/m·K and 10500 W/m·K, respectively, th is just slightly which are higher than most existing vapor chambers. K eff lower than that of the vapor chamber proposed by Mizuta et al. [12], which used fine etching copper laminate as the wick structure. However, the multilayer structure may intensify the risk of air leakage in is just lower than that of the vapor during manufacturing. While K eff th . In particular, chamber proposed by Zhou [30], but with a higher K eff compared with the novel vapor chamber designed previously by our research team [21], the fractal network microchannels used to imitate the macroscopic leaf vein network was retained in the present design. At the same time, the micro fin-pins (showed an overall structure of porous media merely under macroscale) used to imitate the microscopic mesophyll tissue were modified to the real porous media (copper foam) structure, adding the permeability gradient distribution between the leaf veins and mesophyll tissue, presented as the porosity gradient distribution of the porous media around the fractal network microchannels, which better imitating the multiscale heat-fluid-structure of in th heat-resistant plant leaves, improving K eff and K eff by 71.0% and 170.1%, respectively. In addition, the fractal network microchannels on the copper foam of the wick sintered on the upper shell plate of the bionic vapor chamber were processed by laser engraving in this paper, the final processed depth of which can be up to 1 mm or more, much deeper than that of the microchannels formed previously by our research team through chemical etching (0.16–0.24 mm). Thus, more working fluid can be stored and the heat transfer performance of the bionic vapor chamber under high heat flux can be improved well.
CRediT authorship contribution statement Yuanqiang Luo: Conceptualization, Methodology, Validation, Investigation, Writing - original draft. Wangyu Liu: Resources, Writing - review & editing, Supervision, Project administration, Funding acquisition. Guangwen Huang: Methodology, Investigation. 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. Acknowledgements The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (NOs. 51375169 & 11572128 & 11972161) and the third batch of introduced innovative research team of Dongguan, Guangdong Province, China (NO. 2017360004004). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2019.114889.
5. Conclusions
References
In this paper, a new type of bionic vapor chamber was designed and manufactured. The wick structure for the condenser combined the fractal network microchannels and copper foam, which were used to imitate the macroscopic leaf vein network and microscopic mesophyll tissue, respectively. Furthermore, the permeability gradient distribution between the leaf veins and mesophyll tissue was added, presented as the porosity gradient distribution of the copper foams around the fractal network microchannels. Then the fractal network microchannels on the wick on the upper shell plate were processed by laser engraving and mould coining, respectively. After observing and comparing the surface morphology, it indicated that the forming effect of laser
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