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Pool boiling investigation on gradient metal foams with double layers
Accepted Manuscript Pool boiling investigation on gradient metal foams with double layers Z.G. Xu, J. Qin PII: DOI: Reference:
S1359-4311(17)31386-8 https://doi.org/10.1016/j.applthermaleng.2017.12.040 ATE 11561
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
28 February 2017 21 October 2017 9 December 2017
Please cite this article as: Z.G. Xu, J. Qin, Pool boiling investigation on gradient metal foams with double layers, Applied Thermal Engineering (2017), doi: https://doi.org/10.1016/j.applthermaleng.2017.12.040
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Pool boiling investigation on gradient metal foams with double layers Z.G. Xu*, J. Qin School of Mechanical Engineering, Shanghai Jiao Tong University, 200240, Shanghai, China *E-mail : zhiguoxu@ sjtu.edu.cn Highlights:
1.
Gradient metal foams’ pool boiling is experimentally studied.
2.
Adding nanoparticles deeply influence surfactant contact angle.
3.
Nanoparticle and bubble growth affects fiber temperature.
4.
Surfactant heavily changes GMF pool boiling heat transfer.
Abstract Fiber temperature, bubble growth and boiling curves for gradient metal foams (GMFs) with double layers have been experimentally investigated during pool boiling under atmospheric pressure in deionized water, surfactant solutions and surfactant-nanoliquids. The surfactants are Triton X-100 (TX-100) and polyvinyl pyrrolidone (PVP). The surfactant-nanoliquids are prepared by adding alumina nanoparticles into TX-100 or PVP solutions. GMFs are made by uniform copper foam and nickel foam. The pore densities are 20 PPI and 5PPI. The porosity is fixed as 0.98. The results show that PVP worsens GMF pool boiling heat transfer in most experimental heat flux region while TX-100 improves it in a certain heat flux region. Adding 10 nm alumina nanoparticles into TX-100 solutions increases the static contact angle on copper and nickel surfaces, while adding 10 nm alumina nanoparticles into PVP solutions decreases the static contact angle. Fiber temperature is affected by bubble growth and heat flux. Neither TX-100 nor PVP alumina-particle nanoliquid improves GMF pool boiling heat transfer.
Key words: Gradient metal foam; Fiber temperature; Bubble growth; Contact angle; Surfactant-nanoliquid
1
NOMENCLATURE Cu
copper
Db
bubble departure diameter, m
f
bubble frequency, s-1
g
gravitational acceleration, m s 2
GMF
gradient metal foam
h
heat transfer coefficient, W m K
k
thermal conductivity, W m K
Ni
nickel
PPI
pores per inch
ppm
parts per million
PVP
polyvinyl pyrrolidone
q
heat flux, W m
T
temperature, K
TX-100
Triton X-100
UMF
uniform metal foam
wt
weight
2
-1
-2
2
1
1
z
position along vertical direction on the heating surface, m
GREEK SYMBOLS
porosity
pore density, PPI
θ
static contact angles for droplets on copper or nickel surfaces
mass density, kg m-3
surface tension of liquid-vapor interface, N m1
SUBSCRIPTS l
liquid
s
solid
v
vapor
w
wall
1. Introduction Increasing heat flux heavily affects the working life of electronic devices, and high-efficient heat exchanger must be developed in the future. The enhanced surfaces, including the specially treated surface, rough surface and extended surface, are the promising candidates to improve boiling heat transfer performance. Among them, the extended surface achieved by depositing spherical particles on the heating surface[1], etching or machining fins [2], sintering porous coating [3] is more efficient.
Compared with traditional surface, extended surface increases nucleation sites, surface area and transfers more heat at a less wall temperature. As a special extended surface, porous structure considerably enhances heat transfer and can be applied for high-heat-flux tubes and shells. The experimental results of Liu et al. [4] showed that, the heat transfer performance of porous tubes is better than the smooth tubes and T-shaped finned tubes. Jiang et al. [5] found an optimal film thickness for micro-liquid film heat transfer which has the minimum thermal resistance for porous channels. Tang et al. [6] found that the bi-continuous structures of copper nanoporous surface facilitate the onset of boiling and significantly enhance the nucleate boiling heat transfer. Sarangi et al. [7] found that the sintered coating which has the best pool boiling heat transfer performance 3
provides a 95% decrease in wall superheat, albeit at a critical heat flux that is 33% lower than the polished surface. Tang et al. [8] found that the porous interconnected microchannel nets exhibit a lower wall-superheat at the onset of nucleate boiling and a higher nucleate boiling heat transfer coefficient than the solid interconnected microchannel net. The results of Jun et al.[9] showed the critical heat flux and a maximum nucleate boiling heat transfer coefficient of the high-temperature thermally conductive microporous coating surface are 2 and 8 times higher than those of a plain copper surface, respectively. More recently, Deng et al. [10] found that porous structures with reentrant cavities by sintered copper powder enhance the pool boiling heat transfer significantly at small to moderate heat fluxes compared to the solid structures. Bai et al. [11] achieved a maximum heat flux of 416 W·cm-2 on a heating area of 0.78 cm2 without the occurrence of any dryout via an innovative artery porous structure.
Open-celled uniform metal foam (UMF) has been extensively investigated in phase-change heat transfer field because of the advantages of large specific surface area and strong disturbing liquid ability [12-13]. Pool boiling heat transfer performance of UMF is affected not only by the liquid property but also by the morphology parameters, such as pore density and porosity. Xu et al. [14] found that the pool boiling heat transfer performance of low-pore-density uniform copper foam is better at the low wall superheat while high-pore-density uniform copper foam is better at high wall superheat. Ji et al. [15] found that boiling heat transfer performance of horizontal tubes sintered with copper foams significantly outstrips smooth tubes at relatively low heat flux. The experimental results of Zhu et al. [16] showed that uniform copper foam increases boiling heat transfer coefficient 4.5 times of smooth surfaces in the refrigerant/oil mixture. The present authors [17-18] found that escaping bubbles in the UMFs meet resistance of the replenishing liquid and metal skeleton during pool boiling. Especially when the UMF is thick or dense, the resistance is particularly obvious, which slows down the bubbles and worsens boiling heat transfer. Pool boiling heat transfer performance of some UMF samples is even worse than the copper plate. The method of cutting grooves in UMFs improves boiling heat transfer, but destroys original structures. Gradient metal foams (GMFs) developed by the present authors [19-20] not only have the UMF advantages of large surface area and strong disturbing liquid ability, but also provide reasonable space for the escaping bubbles by the extended interconnected pores. The present authors [19-20] have investigated macroscopic pool boiling heat transfer performance of GMFs in saturated liquid, such as pool boiling curves. To further understand GMF pool boiling heat transfer mechanisms, fiber temperature, boiling patterns, pool boiling curves and contact angles on metallic
surfaces
are
investigated
for
deionized
water,
surfactant
solutions
and
surfactant-nanoliquids in the present study.
2. Experimental setup and procedures The experimental setup systems (Fig. 1) include a heating system, a cooling system, and a data 4
acquisition system. The heating system consists of the copper block heater with maximum power of 5 kW, two auxiliary heaters with maximum power of 1 kW and three corresponding voltage regulators. Two single-phase heating rods are sealed in the main copper block. The upper heating surface size of the main copper block heater is 25 mm (length) × 25 mm (width). Other surfaces of the main heater are wrapped by glass fiber with very good thermal insulation performance to prevent heat loss to the surroundings.
Two UMFs are sintered together by Ag-Cu alloy sheets in a high-temperature muffle furnace to form a GMF (Fig. 2). In the present study, the two gradient copper & nickel foams with the copper plate of 25 mm (length) × 25 mm (width), GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni and GMF 20PPI-4mm-Ni & 5PPI-4mm-Cu are selected as the investigation objects. To reduce contact thermal resistance, GMF samples are welded on the main heater upper surface. Before welding, the surfaces of copper plate and main heater are roughened by sandpapers. Lead-free solder is melt by a soldering iron and cooled down to be solidified on a GMF sample copper substrate. Lead-free solder is placed on the heating surface and the main heater is switched on to increase surface temperature up to the melting point 180 oC. Then the GMF sample is pressed on the main heater upper surface until the temperature returns to the room temperature. The thickness of lead-free solder layer between the main heater upper surface and GMF copper substrate is approximately 0.03 mm, which is less than 1.5% of copper base plate thickness (2 mm), and thermal contact resistance can be negligible. During the pool boiling tests in the liquid, the temperature on the heater copper block surface is lower than 130 °C and the lead-free solder layer cannot be melted.
To accurately calculate the heat flux on the heating surface, three T-type thermocouples (T1, T2 and T3) with the maximum uncertainty of 0.16 K [19] are inserted into the small holes with 5 mm depth and 0.4 mm diameter along the main heater side wall. The distance between the two neighboring thermocouples is 10 mm. To measure the wall temperature of the GMF, the three T-type thermocouples are inserted small holes with 3 mm depth and 0.4 mm diameter on the copper substrate side wall. The distance between the two neighboring thermocouples is 6 mm. To measure the fiber temperature of the GMF, one T-type thermocouple (Tinner fiber) is welded on the central fiber on the upper surface of GMF by lead-free solder, another T-type thermocouple (Touter fiber)
is welded on the corner fiber. When the upper fiber temperature increases up to lead-free
solder’s melting point 180 oC by the electric iron, lead-free solder is placed on the fiber together with the T-type thermocouples. After the temperature returns to the room temperature, the T-type thermocouples are welded on the upper fibers. The location sketch of inner and outer fiber is shown in Fig. 1(b). The distance between the two thermocouples is about 12 mm. To calculate bulk liquid temperature, another three T-type thermocouples Tp1, Tp2, Tp3 are vertically fixed on a plastic bar placed in the chamber and their neighboring distance is about 15 mm, as shown in Fig 1(a). The liquid temperature can be calculated by the mean value of the three T-type thermocouple 5
signals.
Before the experiment, the two auxiliary heaters are switched on to improve bulk liquid temperature up to 100 °C. Then boiling liquid is slowly cooled down to the room temperature after the heaters are switched off. The above procedure is repeated three times to discharge the dissolved gas. During the experiment, the two auxiliary heaters are switched on to keep the water in the chamber saturated. The main copper heater is switched on to increase the heat flux up to 8×103 W·m-2. When the temperature variation range is less than 0.3 K per 20 minutes, thermocouple signals are collected by Keithley data acquisition system, boiling patterns are captured by NXA7-S1 high-speed camera with the speed of 1000 frames per second (Fig. 3(b)). To obtain the pool boiling curves, the above procedure is repeated when heat flux is increased by the step of 2×104-6×104 W·m-2.
The liquid volume is 7 liters. For the additive experiments, the weights of surfactants or nanoparticles are measured by the high-precision electronic balance (Fig. 3(a)). In the present study, the nonionic surfactants Triton X-100 (TX-100) and polyvinyl pyrrolidone (PVP) are used as the solutes and mixed by the ultrasonic stirrer with deionized water and poured into the chamber. In the present study, DSA30 with the accuracy of ±0.1° is used to measure the contact angel of the liquid on the copper and nickel surface, as shown in Fig.3(c).
Heat transfer can be considered one-dimensional thermal conduction in the vertical direction. Based on the temperature data of T-type thermocouples, heat flux on heating surface can be calculated as:
qw kCu
dT dz
(1)
where kCu is the copper thermal conductivity( 398 W·m-2·K-1), dT dz is the temperature gradient of upper part of the main heater. The maximum uncertainty of is 8.9% by performing the standard uncertainty analysis [19].
3. Results and discussion 3.1 Pool boiling in deionized water Fig. 4 presents pool boiling curves of GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni and GMF 20PPI-4mm-Ni & 5PPI-4mm-Cu in deionized water. It can be found that, the former’s pool boiling heat transfer performance is better than the latter. With the same morphology, the nucleation sites in the lower copper foam layer are activated more quantitatively than the lower nickel foam layer because of higher thermal conductivity, which means more bubbles generating inside the former GMF. The static contact angle 72.4o (as shown in Fig. 5(a)) for deionized water droplet on a copper surface is higher than the static contact angle 67.7 o (as shown in Fig. 5(b)) for 6
deionized water droplet on a nickel surface. As indicated in Eq. (2) and Eq. (3) [21], more contact angle means bigger departure bubble and lower bubble frequency. Thus, there is more time for the bubbles in the lower 20PPI copper foam of the GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni to absorb heat from metal skeleton than the lower 20 PPI nickel foam of the GMF 20PPI-4mm-Ni & 5PPI-4mm-Cu. On the other hand, higher bubble frequency on nickel surface means more number bubbles escaped form the upper 5 PPI nickel foam of the GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni.
Db 0.0208 g ( l v ) 0.314g( l v ) f Db l
0.5
(2)
0.5
(3)
Fig. 4(a) shows that, for the GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni, when the heat flux increases from 202479 W·m-2 to 264462 W·m-2, the wall superheat sharply increases from 6.5 K to 11.2 K. More nucleation sites on metal skeletons will be activated with increasing heat flux. However, if too many bubbles generate inside the lower copper foam layer, the limited space inside the upper nickel foam layer may remarkably slow down the rising bubble and bubbles will coalescent inside the upper foam layer to decrease the heat transfer coefficient. The pool boiling curves of GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni and GMF 20PPI-4mm-Cu & 5PPI-6mm-Ni [20] are shown in Fig. 4(b). It is found that the latter’s pool boiling heat transfer is considerably better than the former due to more surface area. Fig. 6(a) indicates that the temperature difference between inner fiber and outer fiber in the upper surface of GMF 20 PPI-4mm-Cu & 5 PPI-4 mm-Ni increases with increasing heat flux in deionized water. On the one hand, heat flux in the heating wall transfers to the upper surface fibers by conduction, on the other hand, the rising bubbles inside the GMF exchange heat with the upper surface fibers by collision, cutting, etc. The temperature difference between inner wall and outer wall increases with increasing heat flux (Fig. 6(a)). More bubbles generating inside the central region of GMF escape (as shown in Fig. 6(b)) and more heat transfers from bubbles to upper inner fiber. More seriously, the inner fiber temperature outstrips the outer wall temperature at the heat flux of 202479 W·m-2 (Fig. 6(a)). 3.2 Pool boiling in surfactant solutions Fig. 7(a) shows that, for GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni, pool boiling heat transfer is enhanced in 100 ppm and 400 ppm TX-100 solutions when q 2.5 10 W·m-2, but worsened in 5
the 800 ppm TX-100 solution. Fig. 8 shows that TX-100 decreases the static contact angles of deionized water on the copper and nickel surface compared to Fig. 5 (a). As indicated in Eq. (2) and Eq.(3) [21], the less contact angle means the smaller departure bubbles and the higher bubble 7
frequency, and thus enhance boiling heat transfer. However, adding TX-100 into deionized water increases liquid viscosity and reduces escaping velocity of bubbles.
Fig. 7(b) shows that, pool boiling heat transfer of GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni is worsened in PVP solutions. Fig. 8 shows that, PVP sharply increases the static contact angle for deionized water on the copper surface and nickel surface at 100 ppm concentration. As indicated in Eq. (2) and Eq. (3) [21], more contact angle means bigger departure bubbles and lower bubble frequency. Moreover, adding PVP into deionized water increases liquid viscosity and reduces escaping velocity of bubbles. Although PVP reduces the contact angle for deionized water on nickel surface by small degrees at 400 ppm concentration, the positive effect by reducing bubble size and increasing bubble frequency is outstripped by the negative effect of increasing liquid viscosity with increasing concentration. Fig. 7(b) also shows that, for the pool boiling curves of GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni in PVP solutions, there is a transition point when
q 2.64462 105 W·m-2. Fig. 9 shows that the inner fiber temperature outstrips the outer wall temperature in 100 ppm TX-100 solution when q 1.033 10 W·m-2. However, when the heat flux increases up to 5
202479 W·m-2, the phenomenon disappears. The bubbles become smaller with increasing heat flux and thus escape more easily from the interconnected pores in GMF 20 PPI-4mm-Cu & 5 PPI-4 mm-Ni to the bulk liquid in the chamber, as is shown in Fig. 9(b). Fig. 9(c) shows that, for GMF 20 PPI-4mm-Cu & 5 PPI-4 mm-Ni in 400 ppm TX-100 solutions, the inner fiber temperature also outstrips the outer wall temperature. Fig. 9 also show that, the temperature difference between inner fiber and outer fiber decreases with increasing heat flux because of uniform bubbles escaping from the foam surface of GMF 20 PPI-4mm-Cu & 5 PPI-4 mm-Ni.
Fig. 10 show that, the temperature difference between inner fiber and outer fiber decreases with increasing PVP concentrations mainly caused by uniform bubbles escaping from the foam surface, as is shown in Fig. 10(b) and (d). Compared with TX-100, heat flux and concentration have mild effect on the temperature difference between inner wall and outer wall in PVP solutions.
3.3 Pool boiling in surfactant nanoliquids In the present study, 10 nm alumina particles are mixed into surfactant solutions by the ultrasonic stirrer to prepare surfactant-nanoliquids. Fig. 11(a)&(b) show that, TX-100 alumina-particle nanoliquids worsen pool boiling heat transfer of GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni. Adding alumina nanoparticles into surfactant solutions increases the effective thermal conductivity to enhance heat transfer. However, adding alumina nanoparticles into surfactant solutions increases liquid viscosity and the strong Brown movement of nanoparticles reduce bubble escaping velocity. Compared with Fig. 8, Fig. 12 shows that, adding 10 nm alumina nanoparticles into TX-100 8
solutions increases the static contact angles on copper and nickel surfaces. Thus, bigger departure bubbles and lower bubble frequency (as indicated in Eq. (2) and Eq. (3) [21]) worsens pool boiling heat transfer. Pool boiling curves of GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni in deionized water, 400 ppm TX-100, and 400 ppm TX-100& 0.01% alumina-particle nanoliquid are shown in Fig. 11(c). It can be found that adding alumina nanoparticles into TX-100 solution heavily decreases the pool boiling heat transfer due to increasing contact angle, as shown in Fig. 8(b)&(f) and Fig. 12(a)&(c).
However, compared with Fig. 8, Fig. 12 shows that, adding 10 nm alumina nanoparticles with PVP solutions decreases the static contact angles on copper and nickel surfaces and thus improves pool boiling heat transfer. The positive effect is outstriped by the negative effect caused by increasing liquid viscosity and the strong Brown movement by adding nanopartiles. Therefore, PVP alumina-particle nanoliquids worsens pool boiling heat transfer of GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni , as shown in Fig. 11.
Fig. 11 also shows that, for the pool boiling curve of GMF
20PPI-4mm-Cu & 5PPI-4mm-Ni in 0.01% alumina-particle & 400 ppm PVP nanoliquid, there is an obvious transition point when q 1.4876 10 W·m-2. When PVP concentration is fixed as 5
800 ppm, pool boiling heat transfer of GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni decreases with increasing alumina-particle concentration. Fig. 13 shows that, the temperature difference between inner fiber and outer fiber changes mildly with TX-100 concentrations, because the strong Brown movement of nanoparticles results in uniform liquid temperature field.
4 Conclusions Fiber temperature, bubble growth and pool boiling curves of GMFs have been experimentally investigated during pool boiling under atmospheric pressure in deionized water, surfactant solutions and surfactant-nanoliquids. The main conclusions are as follows: (1) In deonized water, pool boiling heat transfer performance of GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni is better than GMF 20PPI-4mm-Ni & 5PPI-4mm-Cu. The inner fiber temperature on the surface of GMF 20 PPI-4mm-Cu & 5 PPI-4 mm-Ni outstrips the outer wall temperature at the heat flux of 202479 W·m-2. (2) Pool boiling heat transfer of GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni is worsened by PVP while enhanced by TX-100 in a certain heat flux region. The inner fiber temperature outstrips the outer wall temperature in 100 ppm TX-100 solution at the heat flux of 103300 W·m-2, but the phenomenon disappears when the heat flux increases up to 202479 W·m-2. (3) Neither TX-100 nor PVP alumina-particle nanoliquids improves pool boiling heat transfer of GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni. The outer fiber temperature outstrips the inner fiber temperature in 100 ppm and 400 ppm TX-100 nanoliquids at the heat flux of 103300 W·m-2.
Acknowledgments 9
This work is supported by the National Natural Science Foundation of China (Grant No. 51576126) and Shanghai Natural Science Foundation (15ZR1423400).
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Figure captions Fig. 1 Sketch of the experimental facility Fig. 2 Gradient copper-nickel foam Fig. 3 The instruments used in the present study Fig. 4 Pool boiling curves in deionized water Fig. 5 Static contact angles for deionized water droplets on metallic surfaces Fig. 6 Solid temperature and bubble growth in deionized water Fig. 7 Pool boiling curves in surfactant solutions Fig. 8 Static contact angles for surfactant solution droplets on metallic surfaces Fig. 9 Solid temperature and bubble growth in TX-100 solutions Fig. 10 Solid temperature and bubble growth in PVP solutions Fig. 11 Pool boiling curves in surfactant nanoliquids Fig. 12 Static contact angles for surfactant nanoliquid droplets on metallic surfaces Fig. 13 Solid temperature in surfactant nanoliquids
12
(a) The chamber
(b) The main heating facility
(c) Upper surface thermocouples’ location
Fig.1. Sketch of the experimental facility. All dimensions are in mm.
13
Fig.2. Gradient copper-nickel foam
(a) The high-precision electronic balance
(b) The high-speed cameral
(c) Contact angle measuring device Fig.3. The instruments used in the present study
14
GMF 20 PPI-4 mm-Cu & 5 PPI-4 mm-Ni
5
5x10
5
4x10
5
2x10
5
1x10
GMF 20 PPI-4 mm-Ni & 5 PPI-4 mm-Cu
0 4
6
8
10
12
14
16
18
Tsat/K
(a)
GMF 20 PPI-4 mm-Cu & 5 PPI-6 mm-Ni [20]
5
5x10
5
-2
4x10
q/W·m
q/W·m
-2
5
3x10
5
3x10
5
2x10
5
1x10
GMF 20 PPI-4 mm-Cu & 5 PPI-4 mm-Ni
0 0
2
4
6
8
Tsat/K
10
12
14
(b)
Fig.4. Pool boiling curves in deonized water
15
20
(a) Deionized water droplet on a copper surface (b) Deionized water droplet on a nickel surface Fig.5. Static contact angles for deionized water droplets on metallic surfaces.
Tinner wall
106
110 Tinner wall
Touter wall
108
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni q= 202479 W ·m-2 In deionized water
℃
Temperature/
Temperature/
℃
104 GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q=103300 W ·m In deionized water
102
Tinner fiber
106
Tinner fiber
104 Touter wall
102
Touter fiber
100
100
Touter fiber 0
20
40
60 Time/s
80
100
120
0
20
40
60 Time/s
80
q=103300 W•m-2 q=202479 W•m-2 (a) Fiber and wall temperatures
Fig.6.
(b) Boiling patterns Solid temperature and bubble growth in deionized water
16
100
120
GMF-20 PPI-4 mm-Cu & 5 PPI-4 mm-Ni 5
4x10
5
q/W·m
-2
3x10
5
2x10
In deoinized water In 100 ppm TX-100 solution In 400 ppm TX-100 solution In 800 ppm TX-100 solution
5
1x10
0 4
6
8
10
12
14
16
Tsat/K
(a) Copper-nickel foam in TX-100 solutions
5
deoinized water 100 ppm PVP solution 400 ppm PVP solution 800 ppm PVP solution
4x10
5
-2
q/W·m )
3x10
5
2x10
5
1x10
GMF 20 PPI-4 mm-Cu & 5 PPI-4 mm-Ni
0 4
6
8
10
12
14
16
Tsat/K
(b) Copper-nickel foam in PVP solutions Fig.7. Pool boiling curves in surfactant solutions
(a) 100 ppm TX-100 droplet on a copper surface (b) 400 ppm TX-100 droplet on a copper surface
17
(c) 100 ppm PVP droplet on a copper surface
(d) 400 ppm PVP droplet on a copper surface
(e) 100 ppm TX-100 droplet on a nickel surface (f) 400 ppm TX-100 droplet on a nickel surface
(g) 100 ppm PVP droplet on a nickel surface (h) 400 ppm PVP droplet on a nickel surface Fig.8. Static contact angles for surfactant solution droplets on metallic surfaces. 112
108
Tinner wall
Tinner wall
110
106
104
108 ℃
Temperature/
Temperature/
℃
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q= 103300 W ·m In 100ppm TX-100 solution Tinner fiber
102
Touter wall
104 Tinner fiber
100
100 20
Touter wall
106
102
Touter fiber 0
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q= 202479 W·m In 100ppm TX-100 solution
40
60 Time/s
80
100
120
Touter fiber 0
20
40
60 Time/s
80
q=103300 W•m-2 q=202479 W•m-2 (a) Fiber and wall temperatures (c=100 ppm) 18
100
120
q=103300 W•m-2 q=202479 W•m-2 (b) Boiling patterns (c=100 ppm) 108
112
Tinner wall
108
Touter wall
102
Tinner fiber
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q= 202479 W·m In 400ppm TX-100 solution Tinner fiber
104
Touter fiber
100
Touter fiber 20
106
102
100 0
Touter wall
℃
Temperature/
℃
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q=103300 W ·m In 400ppm TX-100 solution
104
40
60 Time/s
80
100
120
0
20
40
60 Time/s
80
100
120
q=103300 W•m-2 q=202479 W•m-2 (c) Fiber and wall temperatures (c=400 ppm)
q=103300 W•m-2 q=202479 W•m-2 (d) Boiling patterns (c=400 ppm) Fig.9. Solid temperature and bubble growth in TX-100 solutions
112
116
Tinner wall
114
110
℃
℃
104 102
Temperature/
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q=103300 W ·m In 100ppm PVP solution
106
Tinner fiber Touter fiber
100 0
20
40
60 Time/s
80
100
Tinner wall
112
Touter wall
108 Temperature/
Temperature/
Tinner wall
110
106
110
106 104
Tinner fiber
102
Touter fiber
100
120
Touter wall GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q= 202479 W ·m In 100ppm PVP solution
108
0
20
40
60 Time/s
80
q=103300 W•m-2 q=202479 W•m-2 (a) Fiber and wall temperatures (c=100 ppm)
19
100
120
q=103300 W•m-2 q=202479 W•m-2 (b) Boiling patterns (c=100 ppm) 112 114 110
112
Touter wall Tinner wall
104 Tinner fiber 102
Tinner wall
℃
Temperature/
106
Touter wall
110
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q=103300 W ·m In 400ppm PVP solution
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q= 202479 W ·m In 400ppm PVP solution
108 106 104
Tinner fiber
Touter fiber
Touter fiber
102 0
20
40
60
80
100
120
0
20
40
Time/s
60 Time/s
80
100
q=103300 W•m-2 q=202479 W•m-2 (c) Fiber and wall temperatures (c=400 ppm)
q=103300 W•m-2 q=202479 W•m-2 (d) Boiling patterns (c=400 ppm) Fig.10. Solid temperature and bubble growth in PVP solutions
In 100ppm TX-100&0.01% 10nm alumina-particle nanoliquid
2.0x10
5
1.6x10
5
1.2x10
5
8.0x10
4
4.0x10
4
In deionized water
In 800ppm TX-100&0.01% wt 10nm alumina-particle nanoliquid
-2
100
q/W·m
Temperature/
℃
108
In 400ppm TX-100&0.01% wt 10nm alumina-particle nanoliquid In 800ppm TX-100&0.1% wt 10nm alumina-particle nanoliquid GMF 20 PPI-4 mm-Cu & 5 PPI-4 mm-Ni
4
6
8
10
12
14
16
18
20
22
Tsat/K
(a) Copper-nickel foam in TX-100& alumina-particle nanoliquids 20
120
In 400ppm PVP& 0.01% wt 10nm alumina-particle nanoliquid 5
1.6x10
5
1.2x10
5
8.0x10
4
4.0x10
4
q/W·m
-2
2.0x10
In deionized water
In 800ppm PVP&0.01% wt 10nm alumina-particle nanoliquid In 800ppm PVP&0.1% wt 10nm alumina-particle nanoliquid
In 100ppm PVP&0.01% wt 10nm alumina-particle nanoliquid
GMF 20 PPI-4 mm-Cu & 5 PPI-4 mm-Ni
4
6
8
10
12
14
16
18
Tsat/K
(b) Copper-nickel foam in PVP& alumina-particle nanoliquids
In deionized water 5
1.6x10
5
1.2x10
5
8.0x10
4
4.0x10
4
q/W·m
-2
2.0x10
In 400 ppm TX-100 solution
In 400ppm TX-100&0.01% wt 10nm alumina-particle nanoliquid
GMF-20 PPI-4 mm-Cu & 5 PPI-4 mm-Ni
4
6
8
10
12
14
16
18
Tsat/K
(c) Copper-nickel foam in deionized water, 400 ppm TX-100, and 400 ppm TX-100& alumina-particle nanoliquid Fig.11. Pool boiling curves in surfactant nanoliquids
21
(a) 400ppm TX-100 & 0.01%wt alumina-particle nanoliquid droplet on a copper surface
(b) 400 ppm PVP & 0.01%wt alumina-particle nanoliquid droplet on a copper surface
(c) 400 ppm TX-100 & 0.01%wt alumina-particle nanoliquid droplet on a nickel surface
(d) 400ppm PVP & 0.01% wt alumina-particle nanoliquid droplet on a nickel surface. Fig.12. Static contact angles for surfactant nanoliquid droplets on metallic surfaces. 22
118
112
116
Touter wall
110
Tinner wall
114
Touter wall
112
Tinner wall
106
℃
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q=103300 W ·m In 100ppm TX-100 & 0.01% wt 10nm alumina-particle nanoliquid
Temperature/
Temperature/
℃
108
104 Tinner fiber 102
Touter fiber
100 0
20
40
60 Time/s
80
100
110
106 104
Tinner fiber
102
Touter fiber
100
120
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q= 202479 W ·m In 100ppm TX-100 & 0.01% wt 10nm alumina-particle nanoliquid
108
0
20
40
60 Time/s
80
100
120
(a) In 100ppm TX-100 & 0.01% wt 10nm alumina-particle nanoliquid 114
Touter wall
114
108
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q= 103300 W ·m In 400ppm TX-100 & 0.01% wt 10nm alumina-particle nanoliquid
106 104
℃
Temperature/
℃
116
Tinner wall
110 Temperature/
118
Touter wall
112
Tinner fiber Touter fiber
102
Tinner wall
112 110
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q= 202479 Wm In 400ppm TX-100 & 0.01% wt 10nm alumina-particle nanoliquid
108 106
Tinner fiber
104
Touter fiber
102
100 0
20
40
60 Time/s
80
100
100
120
0
20
40
60
80
100
120
Time/s
(b) In 400ppm TX-100 & 0.01% wt 10nm alumina-particle nanoliquid Fig.13. Solid temperature in surfactant nanoliquids
Table 1
Gradient metal foams used in the present study (ε=0.98)
GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni GMF 20PPI-4mm-Ni & 5PPI-4mm-Cu Table 2
Pore density gradient
Thickness gradient
Material gradient
20 PPI & 5 PPI
4 mm & 4 mm
Cu & Ni
20 PPI & 5 PPI
4 mm &4 mm
Ni & Cu
Physico-chemical properties of water, PVP and TX-100 at room temperature [22-23] Ionic nature
water
Form
Molecular weight
Specific gravity
Viscosity, cps
liquid
18
1
0.90275
PVP
nonionic
powder
40000 (average)
Triton X-100
nonionic
liquid
624(average)
23
Cloud point
0.925 (100ppm) 1.065
240
65℃
S1359-4311(17)31386-8 https://doi.org/10.1016/j.applthermaleng.2017.12.040 ATE 11561
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
28 February 2017 21 October 2017 9 December 2017
Please cite this article as: Z.G. Xu, J. Qin, Pool boiling investigation on gradient metal foams with double layers, Applied Thermal Engineering (2017), doi: https://doi.org/10.1016/j.applthermaleng.2017.12.040
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Pool boiling investigation on gradient metal foams with double layers Z.G. Xu*, J. Qin School of Mechanical Engineering, Shanghai Jiao Tong University, 200240, Shanghai, China *E-mail : zhiguoxu@ sjtu.edu.cn Highlights:
1.
Gradient metal foams’ pool boiling is experimentally studied.
2.
Adding nanoparticles deeply influence surfactant contact angle.
3.
Nanoparticle and bubble growth affects fiber temperature.
4.
Surfactant heavily changes GMF pool boiling heat transfer.
Abstract Fiber temperature, bubble growth and boiling curves for gradient metal foams (GMFs) with double layers have been experimentally investigated during pool boiling under atmospheric pressure in deionized water, surfactant solutions and surfactant-nanoliquids. The surfactants are Triton X-100 (TX-100) and polyvinyl pyrrolidone (PVP). The surfactant-nanoliquids are prepared by adding alumina nanoparticles into TX-100 or PVP solutions. GMFs are made by uniform copper foam and nickel foam. The pore densities are 20 PPI and 5PPI. The porosity is fixed as 0.98. The results show that PVP worsens GMF pool boiling heat transfer in most experimental heat flux region while TX-100 improves it in a certain heat flux region. Adding 10 nm alumina nanoparticles into TX-100 solutions increases the static contact angle on copper and nickel surfaces, while adding 10 nm alumina nanoparticles into PVP solutions decreases the static contact angle. Fiber temperature is affected by bubble growth and heat flux. Neither TX-100 nor PVP alumina-particle nanoliquid improves GMF pool boiling heat transfer.
Key words: Gradient metal foam; Fiber temperature; Bubble growth; Contact angle; Surfactant-nanoliquid
1
NOMENCLATURE Cu
copper
Db
bubble departure diameter, m
f
bubble frequency, s-1
g
gravitational acceleration, m s 2
GMF
gradient metal foam
h
heat transfer coefficient, W m K
k
thermal conductivity, W m K
Ni
nickel
PPI
pores per inch
ppm
parts per million
PVP
polyvinyl pyrrolidone
q
heat flux, W m
T
temperature, K
TX-100
Triton X-100
UMF
uniform metal foam
wt
weight
2
-1
-2
2
1
1
z
position along vertical direction on the heating surface, m
GREEK SYMBOLS
porosity
pore density, PPI
θ
static contact angles for droplets on copper or nickel surfaces
mass density, kg m-3
surface tension of liquid-vapor interface, N m1
SUBSCRIPTS l
liquid
s
solid
v
vapor
w
wall
1. Introduction Increasing heat flux heavily affects the working life of electronic devices, and high-efficient heat exchanger must be developed in the future. The enhanced surfaces, including the specially treated surface, rough surface and extended surface, are the promising candidates to improve boiling heat transfer performance. Among them, the extended surface achieved by depositing spherical particles on the heating surface[1], etching or machining fins [2], sintering porous coating [3] is more efficient.
Compared with traditional surface, extended surface increases nucleation sites, surface area and transfers more heat at a less wall temperature. As a special extended surface, porous structure considerably enhances heat transfer and can be applied for high-heat-flux tubes and shells. The experimental results of Liu et al. [4] showed that, the heat transfer performance of porous tubes is better than the smooth tubes and T-shaped finned tubes. Jiang et al. [5] found an optimal film thickness for micro-liquid film heat transfer which has the minimum thermal resistance for porous channels. Tang et al. [6] found that the bi-continuous structures of copper nanoporous surface facilitate the onset of boiling and significantly enhance the nucleate boiling heat transfer. Sarangi et al. [7] found that the sintered coating which has the best pool boiling heat transfer performance 3
provides a 95% decrease in wall superheat, albeit at a critical heat flux that is 33% lower than the polished surface. Tang et al. [8] found that the porous interconnected microchannel nets exhibit a lower wall-superheat at the onset of nucleate boiling and a higher nucleate boiling heat transfer coefficient than the solid interconnected microchannel net. The results of Jun et al.[9] showed the critical heat flux and a maximum nucleate boiling heat transfer coefficient of the high-temperature thermally conductive microporous coating surface are 2 and 8 times higher than those of a plain copper surface, respectively. More recently, Deng et al. [10] found that porous structures with reentrant cavities by sintered copper powder enhance the pool boiling heat transfer significantly at small to moderate heat fluxes compared to the solid structures. Bai et al. [11] achieved a maximum heat flux of 416 W·cm-2 on a heating area of 0.78 cm2 without the occurrence of any dryout via an innovative artery porous structure.
Open-celled uniform metal foam (UMF) has been extensively investigated in phase-change heat transfer field because of the advantages of large specific surface area and strong disturbing liquid ability [12-13]. Pool boiling heat transfer performance of UMF is affected not only by the liquid property but also by the morphology parameters, such as pore density and porosity. Xu et al. [14] found that the pool boiling heat transfer performance of low-pore-density uniform copper foam is better at the low wall superheat while high-pore-density uniform copper foam is better at high wall superheat. Ji et al. [15] found that boiling heat transfer performance of horizontal tubes sintered with copper foams significantly outstrips smooth tubes at relatively low heat flux. The experimental results of Zhu et al. [16] showed that uniform copper foam increases boiling heat transfer coefficient 4.5 times of smooth surfaces in the refrigerant/oil mixture. The present authors [17-18] found that escaping bubbles in the UMFs meet resistance of the replenishing liquid and metal skeleton during pool boiling. Especially when the UMF is thick or dense, the resistance is particularly obvious, which slows down the bubbles and worsens boiling heat transfer. Pool boiling heat transfer performance of some UMF samples is even worse than the copper plate. The method of cutting grooves in UMFs improves boiling heat transfer, but destroys original structures. Gradient metal foams (GMFs) developed by the present authors [19-20] not only have the UMF advantages of large surface area and strong disturbing liquid ability, but also provide reasonable space for the escaping bubbles by the extended interconnected pores. The present authors [19-20] have investigated macroscopic pool boiling heat transfer performance of GMFs in saturated liquid, such as pool boiling curves. To further understand GMF pool boiling heat transfer mechanisms, fiber temperature, boiling patterns, pool boiling curves and contact angles on metallic
surfaces
are
investigated
for
deionized
water,
surfactant
solutions
and
surfactant-nanoliquids in the present study.
2. Experimental setup and procedures The experimental setup systems (Fig. 1) include a heating system, a cooling system, and a data 4
acquisition system. The heating system consists of the copper block heater with maximum power of 5 kW, two auxiliary heaters with maximum power of 1 kW and three corresponding voltage regulators. Two single-phase heating rods are sealed in the main copper block. The upper heating surface size of the main copper block heater is 25 mm (length) × 25 mm (width). Other surfaces of the main heater are wrapped by glass fiber with very good thermal insulation performance to prevent heat loss to the surroundings.
Two UMFs are sintered together by Ag-Cu alloy sheets in a high-temperature muffle furnace to form a GMF (Fig. 2). In the present study, the two gradient copper & nickel foams with the copper plate of 25 mm (length) × 25 mm (width), GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni and GMF 20PPI-4mm-Ni & 5PPI-4mm-Cu are selected as the investigation objects. To reduce contact thermal resistance, GMF samples are welded on the main heater upper surface. Before welding, the surfaces of copper plate and main heater are roughened by sandpapers. Lead-free solder is melt by a soldering iron and cooled down to be solidified on a GMF sample copper substrate. Lead-free solder is placed on the heating surface and the main heater is switched on to increase surface temperature up to the melting point 180 oC. Then the GMF sample is pressed on the main heater upper surface until the temperature returns to the room temperature. The thickness of lead-free solder layer between the main heater upper surface and GMF copper substrate is approximately 0.03 mm, which is less than 1.5% of copper base plate thickness (2 mm), and thermal contact resistance can be negligible. During the pool boiling tests in the liquid, the temperature on the heater copper block surface is lower than 130 °C and the lead-free solder layer cannot be melted.
To accurately calculate the heat flux on the heating surface, three T-type thermocouples (T1, T2 and T3) with the maximum uncertainty of 0.16 K [19] are inserted into the small holes with 5 mm depth and 0.4 mm diameter along the main heater side wall. The distance between the two neighboring thermocouples is 10 mm. To measure the wall temperature of the GMF, the three T-type thermocouples are inserted small holes with 3 mm depth and 0.4 mm diameter on the copper substrate side wall. The distance between the two neighboring thermocouples is 6 mm. To measure the fiber temperature of the GMF, one T-type thermocouple (Tinner fiber) is welded on the central fiber on the upper surface of GMF by lead-free solder, another T-type thermocouple (Touter fiber)
is welded on the corner fiber. When the upper fiber temperature increases up to lead-free
solder’s melting point 180 oC by the electric iron, lead-free solder is placed on the fiber together with the T-type thermocouples. After the temperature returns to the room temperature, the T-type thermocouples are welded on the upper fibers. The location sketch of inner and outer fiber is shown in Fig. 1(b). The distance between the two thermocouples is about 12 mm. To calculate bulk liquid temperature, another three T-type thermocouples Tp1, Tp2, Tp3 are vertically fixed on a plastic bar placed in the chamber and their neighboring distance is about 15 mm, as shown in Fig 1(a). The liquid temperature can be calculated by the mean value of the three T-type thermocouple 5
signals.
Before the experiment, the two auxiliary heaters are switched on to improve bulk liquid temperature up to 100 °C. Then boiling liquid is slowly cooled down to the room temperature after the heaters are switched off. The above procedure is repeated three times to discharge the dissolved gas. During the experiment, the two auxiliary heaters are switched on to keep the water in the chamber saturated. The main copper heater is switched on to increase the heat flux up to 8×103 W·m-2. When the temperature variation range is less than 0.3 K per 20 minutes, thermocouple signals are collected by Keithley data acquisition system, boiling patterns are captured by NXA7-S1 high-speed camera with the speed of 1000 frames per second (Fig. 3(b)). To obtain the pool boiling curves, the above procedure is repeated when heat flux is increased by the step of 2×104-6×104 W·m-2.
The liquid volume is 7 liters. For the additive experiments, the weights of surfactants or nanoparticles are measured by the high-precision electronic balance (Fig. 3(a)). In the present study, the nonionic surfactants Triton X-100 (TX-100) and polyvinyl pyrrolidone (PVP) are used as the solutes and mixed by the ultrasonic stirrer with deionized water and poured into the chamber. In the present study, DSA30 with the accuracy of ±0.1° is used to measure the contact angel of the liquid on the copper and nickel surface, as shown in Fig.3(c).
Heat transfer can be considered one-dimensional thermal conduction in the vertical direction. Based on the temperature data of T-type thermocouples, heat flux on heating surface can be calculated as:
qw kCu
dT dz
(1)
where kCu is the copper thermal conductivity( 398 W·m-2·K-1), dT dz is the temperature gradient of upper part of the main heater. The maximum uncertainty of is 8.9% by performing the standard uncertainty analysis [19].
3. Results and discussion 3.1 Pool boiling in deionized water Fig. 4 presents pool boiling curves of GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni and GMF 20PPI-4mm-Ni & 5PPI-4mm-Cu in deionized water. It can be found that, the former’s pool boiling heat transfer performance is better than the latter. With the same morphology, the nucleation sites in the lower copper foam layer are activated more quantitatively than the lower nickel foam layer because of higher thermal conductivity, which means more bubbles generating inside the former GMF. The static contact angle 72.4o (as shown in Fig. 5(a)) for deionized water droplet on a copper surface is higher than the static contact angle 67.7 o (as shown in Fig. 5(b)) for 6
deionized water droplet on a nickel surface. As indicated in Eq. (2) and Eq. (3) [21], more contact angle means bigger departure bubble and lower bubble frequency. Thus, there is more time for the bubbles in the lower 20PPI copper foam of the GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni to absorb heat from metal skeleton than the lower 20 PPI nickel foam of the GMF 20PPI-4mm-Ni & 5PPI-4mm-Cu. On the other hand, higher bubble frequency on nickel surface means more number bubbles escaped form the upper 5 PPI nickel foam of the GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni.
Db 0.0208 g ( l v ) 0.314g( l v ) f Db l
0.5
(2)
0.5
(3)
Fig. 4(a) shows that, for the GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni, when the heat flux increases from 202479 W·m-2 to 264462 W·m-2, the wall superheat sharply increases from 6.5 K to 11.2 K. More nucleation sites on metal skeletons will be activated with increasing heat flux. However, if too many bubbles generate inside the lower copper foam layer, the limited space inside the upper nickel foam layer may remarkably slow down the rising bubble and bubbles will coalescent inside the upper foam layer to decrease the heat transfer coefficient. The pool boiling curves of GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni and GMF 20PPI-4mm-Cu & 5PPI-6mm-Ni [20] are shown in Fig. 4(b). It is found that the latter’s pool boiling heat transfer is considerably better than the former due to more surface area. Fig. 6(a) indicates that the temperature difference between inner fiber and outer fiber in the upper surface of GMF 20 PPI-4mm-Cu & 5 PPI-4 mm-Ni increases with increasing heat flux in deionized water. On the one hand, heat flux in the heating wall transfers to the upper surface fibers by conduction, on the other hand, the rising bubbles inside the GMF exchange heat with the upper surface fibers by collision, cutting, etc. The temperature difference between inner wall and outer wall increases with increasing heat flux (Fig. 6(a)). More bubbles generating inside the central region of GMF escape (as shown in Fig. 6(b)) and more heat transfers from bubbles to upper inner fiber. More seriously, the inner fiber temperature outstrips the outer wall temperature at the heat flux of 202479 W·m-2 (Fig. 6(a)). 3.2 Pool boiling in surfactant solutions Fig. 7(a) shows that, for GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni, pool boiling heat transfer is enhanced in 100 ppm and 400 ppm TX-100 solutions when q 2.5 10 W·m-2, but worsened in 5
the 800 ppm TX-100 solution. Fig. 8 shows that TX-100 decreases the static contact angles of deionized water on the copper and nickel surface compared to Fig. 5 (a). As indicated in Eq. (2) and Eq.(3) [21], the less contact angle means the smaller departure bubbles and the higher bubble 7
frequency, and thus enhance boiling heat transfer. However, adding TX-100 into deionized water increases liquid viscosity and reduces escaping velocity of bubbles.
Fig. 7(b) shows that, pool boiling heat transfer of GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni is worsened in PVP solutions. Fig. 8 shows that, PVP sharply increases the static contact angle for deionized water on the copper surface and nickel surface at 100 ppm concentration. As indicated in Eq. (2) and Eq. (3) [21], more contact angle means bigger departure bubbles and lower bubble frequency. Moreover, adding PVP into deionized water increases liquid viscosity and reduces escaping velocity of bubbles. Although PVP reduces the contact angle for deionized water on nickel surface by small degrees at 400 ppm concentration, the positive effect by reducing bubble size and increasing bubble frequency is outstripped by the negative effect of increasing liquid viscosity with increasing concentration. Fig. 7(b) also shows that, for the pool boiling curves of GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni in PVP solutions, there is a transition point when
q 2.64462 105 W·m-2. Fig. 9 shows that the inner fiber temperature outstrips the outer wall temperature in 100 ppm TX-100 solution when q 1.033 10 W·m-2. However, when the heat flux increases up to 5
202479 W·m-2, the phenomenon disappears. The bubbles become smaller with increasing heat flux and thus escape more easily from the interconnected pores in GMF 20 PPI-4mm-Cu & 5 PPI-4 mm-Ni to the bulk liquid in the chamber, as is shown in Fig. 9(b). Fig. 9(c) shows that, for GMF 20 PPI-4mm-Cu & 5 PPI-4 mm-Ni in 400 ppm TX-100 solutions, the inner fiber temperature also outstrips the outer wall temperature. Fig. 9 also show that, the temperature difference between inner fiber and outer fiber decreases with increasing heat flux because of uniform bubbles escaping from the foam surface of GMF 20 PPI-4mm-Cu & 5 PPI-4 mm-Ni.
Fig. 10 show that, the temperature difference between inner fiber and outer fiber decreases with increasing PVP concentrations mainly caused by uniform bubbles escaping from the foam surface, as is shown in Fig. 10(b) and (d). Compared with TX-100, heat flux and concentration have mild effect on the temperature difference between inner wall and outer wall in PVP solutions.
3.3 Pool boiling in surfactant nanoliquids In the present study, 10 nm alumina particles are mixed into surfactant solutions by the ultrasonic stirrer to prepare surfactant-nanoliquids. Fig. 11(a)&(b) show that, TX-100 alumina-particle nanoliquids worsen pool boiling heat transfer of GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni. Adding alumina nanoparticles into surfactant solutions increases the effective thermal conductivity to enhance heat transfer. However, adding alumina nanoparticles into surfactant solutions increases liquid viscosity and the strong Brown movement of nanoparticles reduce bubble escaping velocity. Compared with Fig. 8, Fig. 12 shows that, adding 10 nm alumina nanoparticles into TX-100 8
solutions increases the static contact angles on copper and nickel surfaces. Thus, bigger departure bubbles and lower bubble frequency (as indicated in Eq. (2) and Eq. (3) [21]) worsens pool boiling heat transfer. Pool boiling curves of GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni in deionized water, 400 ppm TX-100, and 400 ppm TX-100& 0.01% alumina-particle nanoliquid are shown in Fig. 11(c). It can be found that adding alumina nanoparticles into TX-100 solution heavily decreases the pool boiling heat transfer due to increasing contact angle, as shown in Fig. 8(b)&(f) and Fig. 12(a)&(c).
However, compared with Fig. 8, Fig. 12 shows that, adding 10 nm alumina nanoparticles with PVP solutions decreases the static contact angles on copper and nickel surfaces and thus improves pool boiling heat transfer. The positive effect is outstriped by the negative effect caused by increasing liquid viscosity and the strong Brown movement by adding nanopartiles. Therefore, PVP alumina-particle nanoliquids worsens pool boiling heat transfer of GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni , as shown in Fig. 11.
Fig. 11 also shows that, for the pool boiling curve of GMF
20PPI-4mm-Cu & 5PPI-4mm-Ni in 0.01% alumina-particle & 400 ppm PVP nanoliquid, there is an obvious transition point when q 1.4876 10 W·m-2. When PVP concentration is fixed as 5
800 ppm, pool boiling heat transfer of GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni decreases with increasing alumina-particle concentration. Fig. 13 shows that, the temperature difference between inner fiber and outer fiber changes mildly with TX-100 concentrations, because the strong Brown movement of nanoparticles results in uniform liquid temperature field.
4 Conclusions Fiber temperature, bubble growth and pool boiling curves of GMFs have been experimentally investigated during pool boiling under atmospheric pressure in deionized water, surfactant solutions and surfactant-nanoliquids. The main conclusions are as follows: (1) In deonized water, pool boiling heat transfer performance of GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni is better than GMF 20PPI-4mm-Ni & 5PPI-4mm-Cu. The inner fiber temperature on the surface of GMF 20 PPI-4mm-Cu & 5 PPI-4 mm-Ni outstrips the outer wall temperature at the heat flux of 202479 W·m-2. (2) Pool boiling heat transfer of GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni is worsened by PVP while enhanced by TX-100 in a certain heat flux region. The inner fiber temperature outstrips the outer wall temperature in 100 ppm TX-100 solution at the heat flux of 103300 W·m-2, but the phenomenon disappears when the heat flux increases up to 202479 W·m-2. (3) Neither TX-100 nor PVP alumina-particle nanoliquids improves pool boiling heat transfer of GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni. The outer fiber temperature outstrips the inner fiber temperature in 100 ppm and 400 ppm TX-100 nanoliquids at the heat flux of 103300 W·m-2.
Acknowledgments 9
This work is supported by the National Natural Science Foundation of China (Grant No. 51576126) and Shanghai Natural Science Foundation (15ZR1423400).
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[14] J. Xu, X. Ji, W. Zhang, Pool boiling heat transfer of ultra-light copper foam with open cells, International Journal of Multiphase Flow, 34 (2008) 1008-1022. [15] W. T. Ji, Z. G. Qu, Z. Y. Li, Pool boiling heat transfer of R134a on single horizontal tube surfaces sintered with open-celled copper foam , International Journal of Thermal Sciences, 50(2011) 2248-2255. [16] Y. Zhu, H. Hu, G. Ding, Heat transfer enhancement by metal foam during nucleate pool boiling of refrigerant/oil mixture at a wide range of oil concentration, HVAC&R RESEARCH, 18 (2012) 377-389. [17] Z. G. Xu, Z. G. Qu, C. Y. Zhao, W. Q. Tao, Pool boiling heat transfer on open-celled metallic foam sintered surface under saturation condition, International Journal of Heat and Mass Transfer, 54(2011) 3856-3867. [18] Z. G. Xu, Z. G. Qu, C. Y. Zhao, W. Q. Tao, Experimental study of pool boiling heat transfer on metallic foam surface with U-shaped and V-shaped grooves, Journal of Enhanced Heat Transfer, 41 (2012) 44-55 [19] Z. G. Xu, C. Y. Zhao, Experimental study on pool boiling heat transfer in gradient metal foams, International Journal of Heat and Mass Transfer, 85 (2015) 824-829. [20] Z. G. Xu, C. Y. Zhao, Enhanced boiling heat transfer by gradient porous metals in saturated pure water and surfactant solutions, Applied Thermal Engineering, 100 (2016) 68-77. [21] R.I. Elghanam , M.M.E. Fawal , R. A.Aziz ,M.H. Skr , A. H. Khalifa, Experimental study of nucleate boiling heat transfer enhancement by using surfactant, Ain Shams Engineering Journal, 2 (2011) 195–209. [22] R. M. Manglik, V. M. Wasekar, J. Zhang, Dynamic and equilibrium surface tension of aqueous surfactant and polymeric solutions, Experimetnal Thermal and Fluid Science, 25 (2001) 55-64. [23] S. V. Ravikumar, J. M. Jha, A.M. Tiara, S. K. Pal, S. Chakraborty, Experimental investigation of air-atomized spray with aqueous polymer additive for high heat flux applications, International Journal of Heat and Mass Transfer 72 (2014) 362-377.
11
Figure captions Fig. 1 Sketch of the experimental facility Fig. 2 Gradient copper-nickel foam Fig. 3 The instruments used in the present study Fig. 4 Pool boiling curves in deionized water Fig. 5 Static contact angles for deionized water droplets on metallic surfaces Fig. 6 Solid temperature and bubble growth in deionized water Fig. 7 Pool boiling curves in surfactant solutions Fig. 8 Static contact angles for surfactant solution droplets on metallic surfaces Fig. 9 Solid temperature and bubble growth in TX-100 solutions Fig. 10 Solid temperature and bubble growth in PVP solutions Fig. 11 Pool boiling curves in surfactant nanoliquids Fig. 12 Static contact angles for surfactant nanoliquid droplets on metallic surfaces Fig. 13 Solid temperature in surfactant nanoliquids
12
(a) The chamber
(b) The main heating facility
(c) Upper surface thermocouples’ location
Fig.1. Sketch of the experimental facility. All dimensions are in mm.
13
Fig.2. Gradient copper-nickel foam
(a) The high-precision electronic balance
(b) The high-speed cameral
(c) Contact angle measuring device Fig.3. The instruments used in the present study
14
GMF 20 PPI-4 mm-Cu & 5 PPI-4 mm-Ni
5
5x10
5
4x10
5
2x10
5
1x10
GMF 20 PPI-4 mm-Ni & 5 PPI-4 mm-Cu
0 4
6
8
10
12
14
16
18
Tsat/K
(a)
GMF 20 PPI-4 mm-Cu & 5 PPI-6 mm-Ni [20]
5
5x10
5
-2
4x10
q/W·m
q/W·m
-2
5
3x10
5
3x10
5
2x10
5
1x10
GMF 20 PPI-4 mm-Cu & 5 PPI-4 mm-Ni
0 0
2
4
6
8
Tsat/K
10
12
14
(b)
Fig.4. Pool boiling curves in deonized water
15
20
(a) Deionized water droplet on a copper surface (b) Deionized water droplet on a nickel surface Fig.5. Static contact angles for deionized water droplets on metallic surfaces.
Tinner wall
106
110 Tinner wall
Touter wall
108
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni q= 202479 W ·m-2 In deionized water
℃
Temperature/
Temperature/
℃
104 GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q=103300 W ·m In deionized water
102
Tinner fiber
106
Tinner fiber
104 Touter wall
102
Touter fiber
100
100
Touter fiber 0
20
40
60 Time/s
80
100
120
0
20
40
60 Time/s
80
q=103300 W•m-2 q=202479 W•m-2 (a) Fiber and wall temperatures
Fig.6.
(b) Boiling patterns Solid temperature and bubble growth in deionized water
16
100
120
GMF-20 PPI-4 mm-Cu & 5 PPI-4 mm-Ni 5
4x10
5
q/W·m
-2
3x10
5
2x10
In deoinized water In 100 ppm TX-100 solution In 400 ppm TX-100 solution In 800 ppm TX-100 solution
5
1x10
0 4
6
8
10
12
14
16
Tsat/K
(a) Copper-nickel foam in TX-100 solutions
5
deoinized water 100 ppm PVP solution 400 ppm PVP solution 800 ppm PVP solution
4x10
5
-2
q/W·m )
3x10
5
2x10
5
1x10
GMF 20 PPI-4 mm-Cu & 5 PPI-4 mm-Ni
0 4
6
8
10
12
14
16
Tsat/K
(b) Copper-nickel foam in PVP solutions Fig.7. Pool boiling curves in surfactant solutions
(a) 100 ppm TX-100 droplet on a copper surface (b) 400 ppm TX-100 droplet on a copper surface
17
(c) 100 ppm PVP droplet on a copper surface
(d) 400 ppm PVP droplet on a copper surface
(e) 100 ppm TX-100 droplet on a nickel surface (f) 400 ppm TX-100 droplet on a nickel surface
(g) 100 ppm PVP droplet on a nickel surface (h) 400 ppm PVP droplet on a nickel surface Fig.8. Static contact angles for surfactant solution droplets on metallic surfaces. 112
108
Tinner wall
Tinner wall
110
106
104
108 ℃
Temperature/
Temperature/
℃
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q= 103300 W ·m In 100ppm TX-100 solution Tinner fiber
102
Touter wall
104 Tinner fiber
100
100 20
Touter wall
106
102
Touter fiber 0
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q= 202479 W·m In 100ppm TX-100 solution
40
60 Time/s
80
100
120
Touter fiber 0
20
40
60 Time/s
80
q=103300 W•m-2 q=202479 W•m-2 (a) Fiber and wall temperatures (c=100 ppm) 18
100
120
q=103300 W•m-2 q=202479 W•m-2 (b) Boiling patterns (c=100 ppm) 108
112
Tinner wall
108
Touter wall
102
Tinner fiber
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q= 202479 W·m In 400ppm TX-100 solution Tinner fiber
104
Touter fiber
100
Touter fiber 20
106
102
100 0
Touter wall
℃
Temperature/
℃
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q=103300 W ·m In 400ppm TX-100 solution
104
40
60 Time/s
80
100
120
0
20
40
60 Time/s
80
100
120
q=103300 W•m-2 q=202479 W•m-2 (c) Fiber and wall temperatures (c=400 ppm)
q=103300 W•m-2 q=202479 W•m-2 (d) Boiling patterns (c=400 ppm) Fig.9. Solid temperature and bubble growth in TX-100 solutions
112
116
Tinner wall
114
110
℃
℃
104 102
Temperature/
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q=103300 W ·m In 100ppm PVP solution
106
Tinner fiber Touter fiber
100 0
20
40
60 Time/s
80
100
Tinner wall
112
Touter wall
108 Temperature/
Temperature/
Tinner wall
110
106
110
106 104
Tinner fiber
102
Touter fiber
100
120
Touter wall GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q= 202479 W ·m In 100ppm PVP solution
108
0
20
40
60 Time/s
80
q=103300 W•m-2 q=202479 W•m-2 (a) Fiber and wall temperatures (c=100 ppm)
19
100
120
q=103300 W•m-2 q=202479 W•m-2 (b) Boiling patterns (c=100 ppm) 112 114 110
112
Touter wall Tinner wall
104 Tinner fiber 102
Tinner wall
℃
Temperature/
106
Touter wall
110
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q=103300 W ·m In 400ppm PVP solution
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q= 202479 W ·m In 400ppm PVP solution
108 106 104
Tinner fiber
Touter fiber
Touter fiber
102 0
20
40
60
80
100
120
0
20
40
Time/s
60 Time/s
80
100
q=103300 W•m-2 q=202479 W•m-2 (c) Fiber and wall temperatures (c=400 ppm)
q=103300 W•m-2 q=202479 W•m-2 (d) Boiling patterns (c=400 ppm) Fig.10. Solid temperature and bubble growth in PVP solutions
In 100ppm TX-100&0.01% 10nm alumina-particle nanoliquid
2.0x10
5
1.6x10
5
1.2x10
5
8.0x10
4
4.0x10
4
In deionized water
In 800ppm TX-100&0.01% wt 10nm alumina-particle nanoliquid
-2
100
q/W·m
Temperature/
℃
108
In 400ppm TX-100&0.01% wt 10nm alumina-particle nanoliquid In 800ppm TX-100&0.1% wt 10nm alumina-particle nanoliquid GMF 20 PPI-4 mm-Cu & 5 PPI-4 mm-Ni
4
6
8
10
12
14
16
18
20
22
Tsat/K
(a) Copper-nickel foam in TX-100& alumina-particle nanoliquids 20
120
In 400ppm PVP& 0.01% wt 10nm alumina-particle nanoliquid 5
1.6x10
5
1.2x10
5
8.0x10
4
4.0x10
4
q/W·m
-2
2.0x10
In deionized water
In 800ppm PVP&0.01% wt 10nm alumina-particle nanoliquid In 800ppm PVP&0.1% wt 10nm alumina-particle nanoliquid
In 100ppm PVP&0.01% wt 10nm alumina-particle nanoliquid
GMF 20 PPI-4 mm-Cu & 5 PPI-4 mm-Ni
4
6
8
10
12
14
16
18
Tsat/K
(b) Copper-nickel foam in PVP& alumina-particle nanoliquids
In deionized water 5
1.6x10
5
1.2x10
5
8.0x10
4
4.0x10
4
q/W·m
-2
2.0x10
In 400 ppm TX-100 solution
In 400ppm TX-100&0.01% wt 10nm alumina-particle nanoliquid
GMF-20 PPI-4 mm-Cu & 5 PPI-4 mm-Ni
4
6
8
10
12
14
16
18
Tsat/K
(c) Copper-nickel foam in deionized water, 400 ppm TX-100, and 400 ppm TX-100& alumina-particle nanoliquid Fig.11. Pool boiling curves in surfactant nanoliquids
21
(a) 400ppm TX-100 & 0.01%wt alumina-particle nanoliquid droplet on a copper surface
(b) 400 ppm PVP & 0.01%wt alumina-particle nanoliquid droplet on a copper surface
(c) 400 ppm TX-100 & 0.01%wt alumina-particle nanoliquid droplet on a nickel surface
(d) 400ppm PVP & 0.01% wt alumina-particle nanoliquid droplet on a nickel surface. Fig.12. Static contact angles for surfactant nanoliquid droplets on metallic surfaces. 22
118
112
116
Touter wall
110
Tinner wall
114
Touter wall
112
Tinner wall
106
℃
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q=103300 W ·m In 100ppm TX-100 & 0.01% wt 10nm alumina-particle nanoliquid
Temperature/
Temperature/
℃
108
104 Tinner fiber 102
Touter fiber
100 0
20
40
60 Time/s
80
100
110
106 104
Tinner fiber
102
Touter fiber
100
120
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q= 202479 W ·m In 100ppm TX-100 & 0.01% wt 10nm alumina-particle nanoliquid
108
0
20
40
60 Time/s
80
100
120
(a) In 100ppm TX-100 & 0.01% wt 10nm alumina-particle nanoliquid 114
Touter wall
114
108
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q= 103300 W ·m In 400ppm TX-100 & 0.01% wt 10nm alumina-particle nanoliquid
106 104
℃
Temperature/
℃
116
Tinner wall
110 Temperature/
118
Touter wall
112
Tinner fiber Touter fiber
102
Tinner wall
112 110
GMF 20PPI-4mm-Cu&5PPI-4mm-Ni -2 q= 202479 Wm In 400ppm TX-100 & 0.01% wt 10nm alumina-particle nanoliquid
108 106
Tinner fiber
104
Touter fiber
102
100 0
20
40
60 Time/s
80
100
100
120
0
20
40
60
80
100
120
Time/s
(b) In 400ppm TX-100 & 0.01% wt 10nm alumina-particle nanoliquid Fig.13. Solid temperature in surfactant nanoliquids
Table 1
Gradient metal foams used in the present study (ε=0.98)
GMF 20PPI-4mm-Cu & 5PPI-4mm-Ni GMF 20PPI-4mm-Ni & 5PPI-4mm-Cu Table 2
Pore density gradient
Thickness gradient
Material gradient
20 PPI & 5 PPI
4 mm & 4 mm
Cu & Ni
20 PPI & 5 PPI
4 mm &4 mm
Ni & Cu
Physico-chemical properties of water, PVP and TX-100 at room temperature [22-23] Ionic nature
water
Form
Molecular weight
Specific gravity
Viscosity, cps
liquid
18
1
0.90275
PVP
nonionic
powder
40000 (average)
Triton X-100
nonionic
liquid
624(average)
23
Cloud point
0.925 (100ppm) 1.065
240
65℃