Numerical case study of packed sphere wicked heat pipe using
Al
2
O
3
and CuO based water nanofluid
Taoufik Brahim
n, Abdelmajid Jemni
Université de Monastir, École Nationale d′Ingénieurs de Monastir, Laboratoire d′Études des Systèmes Thermique et Énergétique LESTE, Tunisia
a r t i c l e i n f o
Article history: Received 16 March 2016 Received in revised form 18 July 2016
Accepted 12 September 2016 Available online 16 September 2016 Keywords: Heat pipe Nanofluid Numerical model Thermal resistance Particle size
a b s t r a c t
Nanofluids have been receiving increased attention worldwide and various research groups have tried to engage it within heat pipes systems to study the subsequent thermal enhancement. The observations based on the reviewed literature showed that the theo-retical investigations on nanofluids in heat pipes are very few and hence validating the experimental findings is difficult.
In this paper, a two-dimensional numerical model is developed to study the thermal performance of a packed sphere heat pipe utilizing nanofluids due to the recent devel-opment of nanofluids and porous media structures. Two of the most common nano-particles, namely Al2O3and CuO based water are considered. The substantial change in the heat pipe thermal resistance, liquid pressure, axial and radial velocities profiles in pre-sence of suspended nanoparticles within the base fluid are discussed. The effect of particle size on the thermal performance of the heat pipe is also investigated. Results showed that Al2O3 nanofluid slightly outperformed CuO nanofluid heat pipes especially with de-creasing nanofluid particle diameter. It was found that reduction of 68% on thermal re-sistance can be obtained for 9% CuO nanofluid concentration level which trend to reach 20% without significant shear stress effect. Nanofluid based heat pipe is able to dissipate up to 26% more heat without experiencing an increase in the wall temperature and an optimal nanofluid concentration level and diameter are found to better heat pipe per-formance.
& 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
Heat pipe cooling technology is currently spreading out for many applications: e.g., electronic cooling, solar energy, air conditioning systems, space applications, telecommunications, food industries, geothermal systems, building etc.
A limiting factor for the heat transfer capability of a heat pipe is related to the working fluid transport properties. Indeed, the lower thermal conductivity of these working fluids limits the thermal performance enhancement of the heat pipes. In order to overcome this limitation, the thermo-physical properties of the fluid must be improved. An innovative way to enhance liquid thermal conductivity is the dispersion of highly conductive solid nanoparticles within the base fluid. Thus, the mixture will have a higher overall thermal conductivity since the thermal conductivity of solid materials is higher than fluids.
Nowadays, nanofluids have gained international attention. Nanofluids are produced by suspending nanoparticles with
Contents lists available atScienceDirect
journal homepage:www.elsevier.com/locate/csite
Case Studies in Thermal Engineering
http://dx.doi.org/10.1016/j.csite.2016.09.002
2214-157X/& 2016 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
nCorresponding author.
E-mail addresses:[email protected](T. Brahim),[email protected](A. Jemni). Case Studies in Thermal Engineering 8 (2016) 311–321
average sizes below 100 nm in traditional fluids such as water and ethylene glycol, providing new working fluids that can be used in heat pipes.
The research on application of nanofluids in heat pipes was firstly published in 2003[1].
Recently, many researchers have presented the heat transfer characteristics of heat pipe using nanofluids. Most of the research works are carried out experimentally to focus on finding out the key factors affecting the reliable application of nanofluids in the heat pipes. The type, size of heat pipes and operating conditions of heat pipes, the kind of the base fluids, the material and size of nanoparticles all varied in very wide ranges among these experiments.
Different nanoparticles have been utilized within the heat pipe working fluid[2–9]. The improved thermal performance
is observed through a reduction in thermal resistance[10,11], a drop in the temperature gradient along the heat pipe[12,13]
and an increase in the heat pipe efficiency[14]. Wang et al.[15]investigated the operation characteristics of a cylindrical miniature grooved heat pipe using aqueous CuO nanofluid as the working fluid. Result show that the total heat resistance and the maximum heat removal capacity of a heat pipe containing CuO nanofluids decreased by 50% and increased by 40% respectively, compared with a heat pipe containing water. Results also revealed that the start-up time of a heat pipe using CuO nanofluid was significantly reduced.
The thermal performance of a cylindrical heat pipe containing different nanofluids as Al2O3, CuO and TiO2was studied by
Shafai et al.[16]. The effect of nanofluid in their analytical study applied by using the mathematical model of effective heat
conductivity; introduced by Yu and Choi [17,18]. Results reveal that nanoparticles in the liquid enhanced the thermal
performance of the heat pipe by reducing the thermal resistance and enhancing the maximum heat load. Smaller nano-particles have a more pronounced effect on the temperature gradient along the heat pipe. Also they have proven that there is an optimum mass concentration which restricts the amount of removed heat load.
The effect of Copper particles with average particle size of 80–90 nm on the thermal performance of a heat pipe operated
with nanoparticle screen mesh wick was studied by Soloman et al.[19,20]. A copper heat pipe of outer diameter 19.5 mm
and length 400 mm respectively is investigated at three different heat inputs. Their experimental results indicate that the total resistance of heat pipe is decreased by 19%, 15% and 14% at heat inputs as 100 W, 150 W and 200 W respectively..
Reji Kumar et al.[21]experimentally investigated effects of using 0.1% mass concentration Al2O3-DI (De-ionized) water
nanofluid on the heat transfer performance of heat pipe. The heat pipe was tested for various angle of inclination, different fill ratio and for different heat inputs. The results were recorded for the thermal performances of the cylindrical heat pipe under various operating parameters. The temperature gradient along the heat pipe axis increases with increase in heat input and the temperature difference across the condenser and evaporator section is larger.
Alizad et al.[22]used flat-shaped heat pipes with CuO, Al2O3, and TiO2nanofluids to investigate their transient behavior
and operational start-up characteristics. Their results show that a higher concentration of nanoparticles increased the thermal performance of both flat-plate and disk- shaped heat pipes.
Hung et al.[23]studied the enhancement of the thermal performance of a heat pipe charged with Al2O3Water-based
nanofluid with three concentrations as 0.5%, 1.0%, and 3.0% by weight in heat pipes. The heat pipes in this study are a straight copper tube with an outer diameter of 9.52 mm and different lengths of 300 mm, 450 mm and 600 mm respec-tively. Their experimental results indicate that at heating power of 40 W, the optimal thermal performance for Al2O3/Water
nanofluid heat pipes measuring 300 mm, 450 mm and 600 mm was 22.7%, 56.3% and 35.1% respectively, better than that of Nomenclature
A surface area [m2]
Cp specific heat [J/kg K]
hfg latent heat of vaporization [J/kg]
K wick permeability [m2]
k thermal conductivity [W/m K]
keff effective thermal conductivity [W/m K]
klayer nanolayer thermal conductivity [W/m K]
L length [m]
P pressure [Pa]
Q heat input [W]
r cylindrical coordinate [m]
rp nanoparticles radius [m]
r0 heat pipe's outer radius [m]
rw heat pipe's wick radius [m]
ri heat pipe's vapor core radius [m]
u axial velocity component [m]
v radial velocity component [m/s]
w nanolayer thickness [nm] z axial coordinate [m] Greeks φ nanoparticle concentration ε wick porosity μ dynamic viscosity [N s/m2] ρ density ⎡⎣⎢kg⎤⎦⎥ m3 Subscripts eff effective in input l liquid nf nanofluid pe particle s solid sat saturation 0 reference
pipes using distilled water as working fluid. They also stated that the thermal performance of the heat pipe decreases with nanoparticle volume concentration at the concentrations higher than the optimum. It is due to the fact that the high concentrations of nanoparticles lead to high water absorption, which in turn facilitates forming a coating layer through the sedimentation of nanoparticles on the surface of the evaporation section.
Moraveji et al.[24]experimentally investigated the effect of using 35 nm Al2O3/water nanofluid on the thermal efficiency
enhancement of a heat pipe. The heat pipe was made of a straight copper tube with an outer length of 8 and 190 mm and a 1 mm wick-thickness sintered circular heat pipe. They highlighted the influences of the nanoparticle concentration level on the temperature difference between the evaporator and condenser under various input powers. They reported that by using nanofluid as a working fluid, the heat pipe can be operated under a larger heat load and observed that the thermal
re-sistance of the heat pipe decreased with volume fraction for different concentration of Al2O3. They explained one of the
main reasons for reducing the heat pipe thermal resistance as follows. The suspended nanoparticles tend to bombard the vapor bubble during bubble formation to create a smaller bubble nucleation size with a lower thermal resistance.
Senthilkumar et al. [25]discussed about the thermal efficiency enhancement of the heat pipe different inclinations,
based on the ratio of cooling capacity rate of condenser fluid at the condenser section and the supplied power at the evaporator section regarding 40 nm-Cu/DI-water nanofluid as the working fluid (40 ml). Experimental results showed that the thermal efficiency of the heat pipe enhances about 10% using copper nanofluid and the heat pipe thermal efficiency
increases with increase in inclination of the heat pipe up to 30° for DI water and 45° for copper nanofluid.
An analytical model based on the startup model presented in Zhu and Vafai[26]for investigating the impact of using
different types and concentrations of nanofluids on reducing the size and thermal resistance of the flat-shaped heat pipes under a given imposed load. Their model indicated a clear effect of the nanoparticle material on the decrease of the thermal resistance. The results obtained in this work were in agreement with the experimental results.
In recent years, nanofluids have attracted more attention for cooling and heating in various geometries because of re-markable increase in effective thermal conductivity of base fluid. The uses of nanofluids are usually considered in regular and optimization considerations[27–30].
Hajmohammadi et al.[31]carried out a numerical study on laminar boundary layer flow of nanofluid (Cu-water and
Ag-water) over a permeable flat plate with convective boundary condition. They investigated for first time the effect of type of nanofluid, concentration of nanoparticles and permeability of flow on skin friction and heat transfer coefficients. Their results indicated that the convective heat transfer rate is augmented by increasing nanoparticles concentration for case of injection and impermeable surface while in the case of suction, adding Cu and Ag particles reduces the convection heat
transfer coefficient at the surface. A new technique is proposed also by Hajmohammadi et al.[32]to enhance the heat
transfer from a discretely heated pipe to a developing laminar fluid flow. The proposed technique consists in heating the fluid with stepwise distributed heat flux, namely by placing insulated segments between the heated segments. Applying this technique, the effective length of the thermal entrance region is enlarged and minimizing therefore of the hot spot (peak) temperature of the pipe wall.
The observations based on the reviewed literature showed that the theoretical investigations on nanofluids in heat pipes are very few and hence most of the research works are carried out experimentally to focus on finding out the key factors affecting the reliable application of nanofluids in the heat pipes. The type, size of heat pipes and operating conditions of heat pipes, the kind of the base fluids, the material and size of nanoparticles all varied in very wide ranges among these experiments.
The focus of the present work is to model numerically the influence of a nanofluid on the thermal performance of a heat pipe under various heat inputs and with different nanoparticle sizes. Two of the most common nanoparticles namely CuO and Al2O3are considered. Following the analytical model done by Shafai et al.[16]and the numerical code developed in the
previous works[33,34]. The affect of the nanofluid volume fractions on the thermal resistance, velocity and pressure dis-tribution within a sintered copper cylindrical heat pipe is investigated.
2. Numerical model
The schematic of the heat pipe used is given inFig. 1. The porous media is assumed to be isotropic and homogeneous,
saturated with the fluid which have constant physical properties at local thermal equilibrium with the wick structure. The flow in the porous media is described by Darcy-Brinkman-Forchheimer model which accounts for the boundary and inertial effects. Due to the complex geometric configuration in the porous media, the flow becomes very complicated and some kind of averaging is necessary to make the problem tractable mathematically.
Following the analysis given by Vafai and Tien[35], a volume averaged method is used. The governing equations for the
liquid flow and heat transfer in the wick structure are: Conservation of mass: ∂ ∂ ( ) + ∂ ∂ = ( ) r r rv u z 1 0 1 Momentum equation in the radial direction:
ρ ∂ μ εμ ε ρ ∂ − + ∂ ∂ =− ∂^ ∂ + ∂ ∂ + ∂ ∂ − + ∂ ∂ − − ( ) ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ v v r v r u v z p r v r r v r v r v z Kv C K v V 1 2 nf nf nf F nf 2 2 2 2 2 2
Momentum equation in the axial direction:
ρ ∂ μ εμ ε ρ ∂ + ∂ ∂ =− ∂^ ∂ + ∂ ∂ + ∂ ∂ + ∂ ∂ − − ( ) ⎜ ⎟ ⎛ ⎝ ⎞⎠ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ v u r u u z p r u r r u r u z Ku C K u V 1 3 nf nf nf F nf 2 2 2 2 2 Energy equation: α ∂ ∂ + ∂ ∂ = ∂ ∂ + ∂ ∂ + ∂ ∂ ( ) ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ v T r u T z T r r T r T z 1 4 nf 2 2 2 2 αnf=ερ k C eff nf Pnf
is the effective thermal diffusivity in the porous media. Where u, and v are the axial and radial velocity
components, respectively. K is the effective pore diameter,CF is the Ergun's constant which took approximately equal to
0.55.keff is the effective thermal conductivity of the saturated wick composed of copper packed sphere[36].
(
)
(
)
(
)
(
)
(
)
(
)
ε ε = + − − − + + − + ( ) ⎡ ⎣ ⎤⎦ ⎡ ⎣ ⎤⎦ k k k k k k k k k k 2 2 1 2 1 5 eff nf nf s nf s nf s nf sThe subscript“s” accounts for the properties of the solid material of the wick and subscript “nf” for the properties of the working fluid. Boundary conditions considered are:
= = ∂ ∂ = ∂ ∂ = ( ) v u k T r k T ratr r 0; 6 eff nf s s w = = ( ) = ( ) = ( ) u 0,vl v z Ti ; nf TsatP atri rv 7
where the injection velocityv zi( )is the velocity which couples the vapor and the porous regions at the interface and defined as: λ ρ + ∂ ∂ = ( ) Q T r v h 8 eff nf v i fg
At both heat pipe ends:
= = ∂ ∂ = = = ( ) v u T r at z z L 0; 0 0, 9 nf
The effective nanofluid density and the heat capacity depend on the volume fractionφof the nanoparticles used and
given by Santra et al.[37]for both alumina and copper nanofluids.
(
)
ρnf = 1−φ ρf +φρs (10)
(
φ)
φ= − + ( )
CPnf 1 CPf CPs 11
The dynamic viscosity of the nanofluid is given by Brinkman[38]as follows:
(
)
μ μ φ = − ( ) 1 12 Pnf f 2.5The nanofluids tested are composed of solid sphere nanoparticles of rp¼50 nm radius mixed with water. The effective
thermal conductivity of copper nanofluid has been calculated using the correlation given by[16,17].
(
)
(
)
(
)
(
)
(
)
(
)
β φ β φ = + + − + + − − + ( ) k k k k k k k k k k 2 2 1 2 2 1 13 nf pe l pe l pe l pe l l 3 3(
)
(
) (
)
(
)
(
) (
)
α β α α α β α = − + + + − − + + + ( ) ⎡ ⎣⎢ ⎤⎦⎥ ⎡ ⎣⎢ ⎤⎦⎥ k k 2 1 1 1 2 1 1 1 2 14 pe p 3 3 α=klayerkp is the ration of nano-layer and the solid particles thermal conductivity and
β=rt
pis the ration of the thickness of nanolayer and the radius of the particle itself. Where = ⎜ρ ⎟
⎛ ⎝ ⎞ ⎠ t M N 1 3 4 1/3 f A is the thickness of
adsorption layer given Langmuir formula[39].M is the molecular weight andNAthe Avagadro's number.
kpand klayer are the thermal conductivities of the nanoparticle and nanolayer, respectively.
For vapor region we consider that ε = 1 and K→∞.
The boundary conditions for vapor region are as following: At both heat pipe ends:
= = ∂ ∂ = = = ( ) v u T r at z z L 0; 0 0, 15 v = = ( ) = ( ) = ( ) u 0,vv v zi ; Tv Tsat P at ri rv 16
The temperature at the vapor-liquid interface of the evaporator and condenser is calculated using Clausius-Clapeyron equation: = − ( ) − ⎛ ⎝ ⎜⎜ ⎞ ⎠ ⎟⎟ T T R h ln P P 1 17 sat V fg v 0 0 1
In the wall region only by heat conduction equation is considered: ∂ ∂ ∂ ∂ + ∂ ∂ = ( ) ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ r r r T r T z 1 0 18 S 2s 2
With the following boundary conditions:
λ ∂ λ ∂ = ∂ ∂ = ( ) T r T r at r r 19 eff l s s w λ∂ ∂ = = = ( ) T r 0 at z 0, z L 20 s s
In other hand, at the surface of the outer wall, a constant heat flux is imposed and extracted in/out the evaporator and the condenser regions.
λ − ∂ ∂ =± ( ) T r Q 21 s s in
3. Results and discussion
The simulations were done for a reference heat pipe pressure ofP 1.0197 100= × −5N/m2and reference saturation
tem-perature T0¼373 K. Tube's radius for wall, wick and vapor region are respectively: r0¼0.021 m, rw¼0.021 m, ri¼0.021 m.
Total heat pipe length, evaporator and adiabatic lengths are respectively: L=1 m, Le=0.6 m, La=0.2 m. Wick permeability, porosity and thermal conductivity are respectively: K=0.1424 10× −10m ,2 ε=0.46, ks=387.6 , dp=40 nm
W
mK , Q¼600 W.
Fig. 2shows the mean axial and liquid velocities changes with reference to different nanoparticle concentrations for both
CuO and Al2O3nanofluids. A slower liquid flow through the wick structure is observed when increasing the nanoparticle
concentration within the working fluid due to the increase in fluid density by adding more nanoparticles. For 8% particle concentration the axial liquid velocity decreases by 30.8% for CuO nanofluid and by 20% for Al2O3nanofluid. The observation
made is that, the liquid velocity is linear proportional to the nanofluid density and slower axial liquid velocity results to the reduction in radial velocities in both evaporator and condenser regions.
Fig. 3displays the effect of nanoparticles on liquid pressure distributions. With respect to the liquid pressure, it can be seen that the pressure gradient decreases as the concentration of the nanoparticles increases. Density effect prevails leading to a smaller pressure drop. However, when increasing nanofluid concentration level upon a certain value, the viscosity overcomes the density effect resulting in a larger liquid drop and shear stress. This behavior is due to the opposite roles played by density and viscosity, both growing with particle concentration in the liquid. This can clearly observed for Al2O3
nanofluid when the concentration increases beyond 5% as revealed in the analytical model of Shafai et al.[15]. For CuO
nanofluid the concentration level trend to reach a specific value which is 20% without significant shear stress effect. In the wick region, Van der Waals forces among nanoparticles made them to conglomerate or cluster, stick to the wick and deposit, partially. The consequence of such phenomena is the variation of nanofluid concentration during the time which may result in some irregularities. The choice of the wick structure can have an effect on heat transfer within the heat pipe.
Fig. 3(b) and (d) illustrate the temperature changes with reference to different nanoparticle concentration. By increasing the nanoparticle accumulation a lower temperature difference between evaporator and condenser can be observed owing to
Fig. 4. Effect of nanoparticle concentration on the thermal performance of a heat pipe under various heat input and for different particle diameters: dp¼10 nm and dp¼20 nm.
the less heat flux accumulation on the wall and then higher heat flux passes through wick layer as the fluid thermal conductivity increases. The figure clearly highlights the positive impact of increasing nanoparticles concentrations on the thermal performance of the heat pipe. Indeed, for a 4% particle concentration, the temperature gradient is reduced by about 5% CuO and Al2O3nanofluid as mentioned in[15]. The effective thermal conductivity of the system is increased by shrinkage
the particle size owing to the less heat flux accumulation on the wall since the local temperature rise in evaporator section is reduced. Additionally the increase in adiabatic and condenser sections temperature is due to the higher amount of heat which is transferred. Also the formation of thin porous coating layer produced by the nanoparticles in the evaporation
Fig. 5. The effect of different nanoparticle concentration levels on the heat pipe thermal resistance. (a) dp¼10 nm; (b) dp¼40 nm.
region improves the surface wet ability by reducing the contact angle and increasing the surface roughness, which in turn increases the critical heat flux and it significantly reduces the thermal resistance of the heat pipe using nanofluids.
Fig. 4shows the effect of particle concentration levels on the thermal performance of the heat pipe. The figure shows that increasing the particle concentration decreases the temperature difference between the evaporator and condenser regions. Moreover, the temperature difference decreases with a reduction in particle diameter. It can be seen that for a constant temperature difference between condenser and evaporator the use of a nanofluid as the working fluid allows the heat pipe to operate under a larger heat load. A nanofluid based heat pipe is able to dissipate up to 26% more heat without experi-encing an increase in the wall temperature.
Fig. 5shows the influence of different nanoparticle concentration levels for two particle sizes on the heat pipe thermal resistance defined as⎡⎣Q/
(
Te̅ − ̅Tc)
⎤⎦−1
. It can be seen that increasing the nanoparticle concentration decreases the heat pipe thermal resistance and provides a better performance because of the higher amount of transmitted heat flux along adiabatic and condenser sections. For example a 75% reduction in the thermal resistance ratio is obtained for a 4% Al2O3nanoparticle
concentration level at nanoparticle size of 10 nm. The figure reveals that Al2O3 nanofluid heat pipe perform better CuO
nanofluid heat pipes as particles concentration level increases.
Fig. 6 clearly highlights that by decreasing the particle size within the fluid, the heat pipe thermal resistance ratio reduces and no significant thermal ratio difference showed between the two nanofluids. The reason for this reduction can be explained by the higher velocity of smaller particles which enhances the likelihood of fragments collision. The higher rate of impact among the particles causes a quasi-convection state to be formed between the bulk fluid molecules and nano-particles and hence the overall thermal conductivity of the fluid was improved.
Fig. 7displays the effect of nanoparticles concentration level on vapor pressure distributions. The vapor pressure drop increases when particle concentration increases. This is due not only to the changes in the physical properties of vapor. In fact, the operating temperature decreases with increasing volume fraction, vapor density decreases which causes higher
Fig. 7. The effect of nanoparticle concentration levels on the vapor pressure distribution.
vapor speed and pressure drop. Increasing volume Concentration of nanoparticles increases the pressure drop of Nanofluids.
Fig. 8reveals the effect of particle size on the vapor pressure distribution for the volume fraction of 4%. Results show that vapor pressure increases as the particle diameter decreases due the decreasing of the temperature gradient as mentioned in
Fig. 4. However, an optimal diameter can be obtained and then maximum heat load can be achieved for the heat pipe. Indeed, diameter of 20 nm can be chosen since increasing particle size greater than this diameter has no significant effect on the vapor pressure drop. Increasing size of nano-particles led to decreasing in heat transfer because area per unit volume decreases.
4. Conclusion
The recent development of nanofluids, or fluids consisting of a conventional heat transfer base with nanometer-sized oxide or metallic particles suspended within, offers the exciting possibility of increased heat transfer rates over conventional systems.
In this paper, the thermal performance of a cylindrical heat pipe was numerically simulated in presence of water based nanofluid using a two dimensional proper numerical model. A packed sphere wick is used for the heat pipe porous structure and the effect of nanofluid concentration levels on the on heat pipe wall temperature distribution, thermal resistance; liquid pressure and liquid velocities were explored. Results showed the positive influence of nanofluid utilizing as a heat pipe working fluid on the system thermal performance. It was found that reduction of 68% on thermal resistance can be obtained for 9% CuO nanofluid concentration level. For CuO nanofluid the concentration level trend to reach a specific value which is 20% without significant shear stress effect. An optimal diameter can be obtained and then maximum heat load can be achieved for the heat pipe. Indeed, diameter of 20 nm can be chosen since increasing particle size greater than this diameter has no significant effect on the vapor pressure drop.
Both liquid axial and radial velocities in the wick structure decreases as the particle concentration level increases. Results also reveal that an optimal nanofluid concentration level and particle size can be obtained in producing the maximum heat transfer. Therefore, it is expected that the thermal performance of heat pipe will be enhanced. The heat transfer enhancement of a heat pipe using nanofluids is not only due to the thermo physical properties of nanofluids but also due to the formation of thin porous coating layer produced by the nanoparticles in the evaporation region. The coating layer formed by the nano-particles improves the surface wet ability by reducing the contact angle and increasing the surface roughness, which in turn increases the critical heat flux and it significantly reduces the thermal resistance of the heat pipe using nanofluids.
References
[1] H.T. Chien, C.Y. Tsai, P.H. Chen, P.Y. Chen, Improvement on thermal performance of a disk-shaped miniature heat pipe with nanofluid, in: Proceedings of the Fifth International Conference on Electronic Packaging Technology, IEEE, Shanghai, China, 2003, pp. 389–391.
[2] Y. Chen, W. Wei, S. Kang, C. Yu, Effect of nanofluid on flat heat pipe thermal performance, in: Proceedings of the 24th IEEE SEMI-THERM Symposium, pp. 16–19.
[3]Y. Lin, S. Kang, H. Chen, Effect of silver nano-fluid on pulsating heat pipe thermal performance, Appl. Therm. Eng. 28 (2008) 1312–1317.
[4]X.F. Yang, Z.H. Liu, J. Zhao, Heat transfer performance of a horizontal microgrooved heat pipe using CuO nanofluid, J. Micromech. Microeng. 18 (2008) 035038.
[5]Z. Liu, J. Xiong, R. Bao, Boiling heat transfer characteristics of nanofluids in a flat heat pipe evaporator with micro-grooved heating surface, Int. J. Multiph. Flow 33 (2007) 1284–1295.
[6]H.B. Ma, C. Wilson, Q. Yu, K. Park, U.S. Choi, M. Tirumala, An experimental investigation of heat transport capability in a nanofluid oscillating heat pipe, J. Heat Transf. 128 (2006) 1213–1216.
[7]P. Naphon, P. Assadamongkol, T. Borirak, Experimental investigation of titanium nanofluids on the heat pipe thermal efficiency, Int. Commun. Heat Mass Transf. 35 (2008) 1316–1319.
[8] R.R. Riehl, Analysis of loop heat pipe behavior using nanofluid, in: Proceedings of the Heat Powered Cycles International Conference (HPC), New Castle, UK, Paper 06102, 2006.
[9]C.Y. Tsai, H.T. Chien, P.P. Ding, B. Chan, T.Y. Luh, P.H. Chen, Effect of structural character of gold nanoparticles in nanofluid on heat pipe thermal performance, Mater. Lett. 58 (2004) 1461–1465.
[10]S.W. Kang, W.C. Wei, S.H. Tsai, C.C. Huang, Experimental investigation of nanofluids on sintered heat pipe thermal performance, Appl. Therm. Eng. 29 (2009) 973–979.
[11] B. Gavtash, K. Hussain, M. Layeghi, S.S. Lafmejani, Numerical simulation of the effects of nanofluid on a heat pipe thermal performance, Int. J. Mech. Aerosp. Eng. 6 (2012) 445–451.
[12]S.W. Kang, W.C. Wei, S.H. Tsai, S.Y. Yang, Experimental investigation of silver nano-fluid on heat pipe thermal performance, Appl. Therm. Eng. 26 (2006) 2377–2382.
[13]H.B. Ma, C. Wilson, B. Borgmeyer, K. Park, Q. Yu, S.U.S. Choi, M. Tirumala, Effect of nanofluid on the heat transport capability in an oscillating heat pipe, Appl. Phys. Lett. 88 (2006) 143116.
[14]P. Naphon, D. Thongkum, P. Assadamongkol, Heat pipe efficiency enhancement with refrigerant–nanoparticles mixtures, Energy Convers. Manag. 50 (2009) 772–776.
[15]G.S. Wang, B. Song, Z.H. Liu, Operation characteristics of cylindrical miniature grooved heat pipe using aqueous CuO nanofluids, Exp. Therm. Fluid Sci. 34 (2010) 1415–1421.
[16]M. Shafai, V. Bianco, K. Vafai, O. manca, An investigation of the thermal performance of cylindrical heat pipes using nanofluids, Int. J. Heat Mass Transf. 53 (2010) 376–383.
[17]S.K. Das, S.U.S. Choi, W. Yu, T. Pradeep, Nanofluids Science and Technology, John Wiley & Sons, Hoboken, 2008.
[18]W. Yu, S.U.S. Choi, The role of interfacial layers in the enhanced thermal conductivity of nanofluids: a renovative Maxwell model, J. Nanopart. Res. 5 (2003) 167–171.
[20] A. Brusly Solomon, K. Ramachandran, L. Godson Asirvatham, B.C. Pillai, Numerical analysis of a screen mesh wick heat pipe with Cu-water, Int. J. Heat Mass Transf. 75 (2014) 523–533.
[21] R. Reji Kumar et al., Heat Transfer Performance in Heat Pipe Using Al2O3-DI Water Nanofluid, IJMME, February 2014.
[22] K. Alizad, K. Vafai, M.M. Shafahi, Thermal performance and operational attributes of the startup characteristics of flat-shaped heat pipes using nanofluids, Int. J. Heat Mass Transf. 55 (2012) 140–155.
[23] Yi-Hsuan Hung, Tun-Ping Teng, Bo-Gu Lin, Evaluation of thermal performance of a heat pipe using alumina nanofluids, Exp. Therm. Fluid Sci. 44 (2013) 504–511.
[24] M.K. Moraveji, S. Razvarz, Experimental investigation of aluminum oxide nanofluid on heat pipe thermal performance, Int. Commun. Heat Mass Transf. 39 (2012) 1444–1448.
[25] R. Senthilkumar, S. Vaidyanathan, B. Sivaraman, Effect of inclination angle in heat pipe performance using copper nanofluid, Procedia Eng. 38 (2012) 3715–3721.
[26] N. Zhu, K. Vafai, Analytical modeling of the startup characteristics of asymmetrical flat-plate and disk shaped heat pipes, Int. J. Heat Mass Transf. 41 (1998). 2619 2637.l.
[27] M.R. Hajmohammadi, A. Campo, S.S. Nourazar, A.M. Ostad, Improvement of forced convection cooling due to the attachment of heat sources to a conducting thick plate, J. Heat Transf., 135, 12, p. 124504.
[28] M.R. Hajmohammadi, S. Poozesh, M. Rahmani, A. Campo, Heat transfer improvement due to the imposition of non-uniform wall heating for in-tube laminar forced convection, Appl. Therm. Eng., 61, 2, pp. 268–277.
[29] H. Najafi, B. Najafi, P. Hoseinpoori, Energy and cost optimization of a plate and fin heat exchanger using genetic algorithm, Appl. Therm. Eng., 31, 10, pp. 1839–1847.
[30] T.H. Ko, K. Ting, Optimal Reynolds number for the fully developed laminar forced convection in a helical coiled tube, Energy 31 (2006) 2142–2152. [31]M.R. Hajmohammadi, et al., Effects of Cu and Ag nano-particles on flow and heat transfer from permeable surfaces, Adv. Powder Technol. 26 (2015)
193–199.
[32] M.R. Hajmohammadi, S.S. Nourazar, A. Campo, S. Poozesh, Optimal discrete distribution of heat flux elements for in-tube laminar forced convection, Int. J. Heat Fluid Flow, 40, pp. 89–96.
[33] T. Brahim, A. Jemni, Effect of the heat pipe adiabatic region, J. Heat Transf. (ASME)http://dx.doi.org/10.1115/1.4025132.
[34] T. Brahim, A. Jemni, Heat pipe simulation under critical conditions, Front. Heat Pipes, 3, 033003, vol. 3, No. 3. DOI:http://dx.doi.org/10.5098/fhp.v3.3. 300.
[35] K. Vafai, C.L. Tien, Boundary and inertia effects on flow and heat transfer in porous media, Int. J. Heat Mass Transf. 24 (1981) 195–203. [36] Amir Faghri, Heat Pipe Science and Technology, Taylor & Francis Publishing, Oxon, 1995.
[37]A.K. Santra, S. Sen, N. Chakraborty, Study of heat transfer due to laminar flow of copper water nanofluid through two isothermally heat parallel plates, Int. J. Therm. Sci. 48 (2009) 391–400.
[38] H.C. Brinkman, The viscosity of concentrated suspensions and solutions, J. Chem. Phys. 20 (1952) 571–581.
