Two Phase Flow Phase Change and Numerical Modeling Part 14 pdf

Two Phase Flow, Phase Change and Numerical Modeling The effect of heat flux ratio qH1 qH2 on the mass flux G versus qHfor the steady-state conditions is presented in Fig.. 21 shows t

Two Phase Flow, Phase Change and Numerical Modeling

The effect of heat flux ratio qH1 qH2 on the mass flux G versus qHfor the steady-state conditions is presented in Fig 18 The mass flux increases with increasing of heat flux ratio

q q .

New Variants to Theoretical Investigations of Thermosyphon Loop 381

Fig 16 Mass flux G as a function of qH with LC2P as a parameter (2H2C)

(L=0.2 [m], D=0.002 [m], H=0.07 [m], B=0.03 [m], LH1= LH2=LC1= LC2=0.02 [m],

LH1P= LH2P =LC1P= 0.005 [m] )

Fig 17 Mass flux G as a function of qH with parameter B ( width of the loop) (2H2C)

(D=0.002 [m], H=0.07 [m], LH1= LH2=LC1= LC2=0.02 [m], LH1P= LH2P =LC1P= LC2P =0.005 [m]) The effect of heat flux ratio qC1 qC2 on the mass flux G versus qHfor the steady-state conditions is presented in Fig 19 The mass flux increases with increasing of heat flux ratio

q q .

Two Phase Flow, Phase Change and Numerical Modeling

New Variants to Theoretical Investigations of Thermosyphon Loop 383 minipump promotes natural circulation In the equation of motion of the thermosyphon loop with natural circulation, the pressure term of integration around the loop is zero dp

4

8 0 7

6P 7P PK

H

C

S S

S S

S S S

S S

Two Phase Flow, Phase Change and Numerical Modeling

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diabatic region (Tran et al 2000) The working fluid was distilled water A miniature pump curve from (Blanchard et al., 2004) was included in the calculation The Fig 21 shows the mass flux G decreases with increasing heat flux qH for minichannels with minipump (HHCV+P) for the steady-state condition

Fig 21 Distributions of the mass flux G versus heat flux q , for the steady-state conditions Hfor minichannels with minipump (HHCV+P) (L=0.2 [m], D=0.002 [m], H=0.09 [m], B=0.01 [m], LH=LC=0.008 [m], LHP= LCP =0.0001 [m], LPK=0.0001 [m] )

5 Conclusions

The presented new variants (HHVCHV, 2H2C, HHCV+P) and the previous variants (HHCH, HVCV, HHCV) described in the chapter (Bieliński & Mikielewicz, 2011) can be analyzed using the conservation equations of mass, momentum and energy based on the generalized model of the thermosyphon loop This study shows that the new effective numerical method proposed for solving the problem of the onset of motion in a fluid from the rest can be applied for the following variants: (HHVCHV+ψ90o) and (HHCH)

The results of this study indicate that the properties of the variants associated with the generalized model of thermosyphon loop depend strongly on their specific technical conditions For this reasons, the theoretical analysis of the presented variants can be applied, for example, to support the development of an alternative cooling technology for electronic systems The progress in electronic equipment is due to the increased power levels and miniaturization of devices The traditional cooling techniques are not able to cool effectively

at high heat fluxes The application of mini-loops can be successful by employing complex geometries, in order to maximize the heat transferred by the systems under condition of single- and two phase flows

The obtained results show that the one-dimensional two-phase separate flow model can be used to describe heat transfer and fluid flow in the thermosyphon loop for minichannels The evaluation of the thermosyphon loop with minichannels can be done in calculations using correlations such as the El-Hajal correlation (El-Hajal et al., 2003) for void fraction, the

New Variants to Theoretical Investigations of Thermosyphon Loop 385 Zhang-Webb correlation (Zhang & Webb, 2001) for the friction pressure drop of two-phase flow in adiabatic region, the Tran correlation (Tran et al., 2000) for the friction pressure drop

of two-phase flow in diabatic region and the Mikielewicz correlation (Mikielewicz et al., 2007) for the heat transfer coefficient in evaporator and condenser

Two flow regimes such as GDR- gravity dominant regime and FDR – friction dominant regime can be clearly identified (Fig 8) The distribution of the mass flux against the heat flux approaches a maximum and then slowly decreases for minichannels The effect of geometrical and thermal parameters on the mass flux distributions was obtained numerically for the steady-state conditions as presented in Figs 11-19 The mass flux strongly increases with the following parameters: (a) increasing of the internal tube diameter, (b) increasing length of the vertical section H, (c) decreasing length of the

precooled section L C2P The mass flux decreases with the parameters, such as (d) increasing

length of the cooled section L C2 , (e) increasing length of the horizontal section B, (f) decreasing of the heat flux ratio: qH1 qH2and qC1 qC2 If the mass flow rate is not high enough to circulate the necessary fluid to transport heat from evaporator to condenser, the minipump can be used to promotes natural circulation For the steady-state condition as is demonstrated in Fig 21, the mass flux G decreases with increasing heat flux qH for minichannels with minipump (HHCV+P)

Each variant of thermosyphon loop requires an individual analysis of the effect of geometrical and thermal parameters on the mass flux Two of the reasons are that the variants include the heated and cooled sections in different places on the loop and may have different quantity of heaters and coolers

In future the transient analysis should be developed in order to characterize the dynamic behaviour of single- and two phase flow for different combination of boundary conditions Attempts should be made to verify the presented variants based on numerical calculations for the theoretical model of thermosyphon loops with experimental data

6 References

Bieliński, H.; Mikielewicz, J (1995) Natural Convection of Thermal Diode., Archives of

Thermodynamics, Vol 16, No 3-4

Bieliński, H.; Mikielewicz, J (2001) New solutions of thermal diode with natural laminar

circulation., Archives of Thermodynamics, Vol 22, pp 89-106

Bieliński, H.; Mikielewicz J (2004) The effect of geometrical parameters on the mass flux in

a two phase thermosyphon loop heated from one side., Archives of Thermodynamics,

Vol 29, No 1, pp 59-68

Bieliński, H.; Mikielewicz J (2004) Natural circulation in two-phase thermosyphon loop

heated from below., Archives of Thermodynamics, Vol 25, No 3, pp 15-26

Bieliński, H.; Mikielewicz, J (2005) A two-phase thermosyphon loop with side heating,

Inżynieria Chemiczna i Procesowa., Vol 26, pp 339-351 (in Polish)

Bieliński, H.; Mikielewicz, J (2010) Energetic analysis of natural circulation in the

closed loop thermosyphon with minichannels, Archiwum Energetyki, Tom XL, No

3, pp.3-10,

Bieliński, H.; Mikielewicz, J (2010) Computer cooling using a two phase minichannel

thermosyphon loop heated from horizontal and vertical sides and cooled from

vertical side., Archives of Thermodynamics, Vol 31(2010), No 4, pp 51-59

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Bieliński, H.; Mikielewicz, J (2010) A Two Phase Thermosyphon Loop With Minichannels

Heated From Vertical Side And Cooled From Horizontal Side, Inżynieria Chemiczna

i Procesowa., Vol 31, pp 535-551

Bieliński, H.; Mikielewicz, J (2011) Natural Circulation in Single and Two Phase

Thermosyphon Loop with Conventional Tubes and Minichannels, published by InTech (ISBN 978-953-307-550-1) in book Heat Transfer Mathematical Modeling, Numerical Methods and Information Technology, Edited by A Belmiloudi, pp 475-496,

Blanchard, D.B., Ligrani, P.M., Gale, B.K (2004) Performance and Development of a Miniature

Rotary Shaft Pump (RSP)., 2004 ASME International Mechanical Engineering Congress and RD&D Expo, November 13-20, 2004, Anaheim, California USA

Chen, K (1988) Design of Plane-Type Bi-directional Thermal Diode., ASME J of Solar Energy

Engineering, Vol 110

Churchill, S.W (1977) Friction-Factor Equation Spans all Fluid Flow Regimes., Chem Eng.,

pp 91-92

El-Hajal, J.; Thome, J.R & Cavalini A (2003) Condensation in horizontal tubes, part 1;

two-phase flow pattern map., Int J Heat Mass Transfer, Vol 46, No 18, pp 3349-3363 Greif, R (1988) Natural Circulation Loops., Journal of Heat Transfer, Vol 110, pp 1243-1257

Madejski, J.; Mikielewicz, J (1971) Liquid Fin - a New Device for Heat Transfer Equipment,

Int J Heat Mass Transfer, Vol 14, pp 357-363

Mikielewicz, D.; Mikielewicz, J & Tesmar J (2007) Improved semi-empirical method for

determination of heat transfer coefficient in flow boiling in conventional and small

diameter tubes., Inter J Heat Mass Transfe , Vol 50, pp 3949-3956

Mikielewicz J (1995) Modelling of the heat-flow processes., Polska Akademia Nauk Instytut

Maszyn Przepływowych, Seria Cieplne Maszyny Przepływowe, Vol 17, Ossolineum

Misale, M.; Garibaldi, P.; Passos, J.C.; Ghisi de Bitencourt, G (2007) Experiments in a

Single-Phase Natural Circulation Mini-Loop., Experimental Thermal and Fluid Science, Vol

31, pp 1111-1120

Ramos, E.; Sen, M & Trevino, C (1985) A steady-state analysis for variable area one- and

two-phase thermosyphon loops, Int J Heat Mass Transfer, Vol 28, No 9, pp 1711-1719

Saitoh, S.; Daiguji, H & Hihara, E (2007) Correlation for Boiling Heat Transfer of R-134a in

Horizontal Tubes Including Effect of Tube Diameter., Int J Heat Mass Tr., Vol 50,

pp 5215-5225

Tang, L.; Ohadi, M.M & Johnson, A.T (2000) Flow condensation in smooth and microfin

tubes with HCFC-22, HFC-134a, and HFC-410 refrigerants, Part II: Design

equations Journal of Enhanced Heat Transfer, Vol 7, pp 311-325

Tran, T.N.; Chyu, M.C.; Wambsganss, M.W.; & France D.M (2000) Two –phase pressure

drop of refrigerants during flow boiling in small channels: an experimental

investigations and correlation development., Int J Multiphase Flow, Vol 26, No 11,

pp 1739-1754

Vijayan, P.K.; Gartia, M.R.; Pilkhwal, D.S.; Rao, G.S.S.P & Saha D (2005) Steady State

Behaviour Of Single-Phase And Two-Phase Natural Circulation Loops 2nd RCM

on the IAEA CRP ,Corvallis, Oregon State University, USA

Zhang, M.; Webb, R.L (2001) Correlation of two-phase friction for refrigerants in

small-diameter tubes Experimental Thermal and Fluid Science, Vol 25, pp 131-139

Zvirin, Y (1981) A Review of Natural Circulation Loops in PWR and Other Systems.,

Nuclear Engineering Design, Vol 67, pp 203-225

Part 3 Nanofluids

1 Introduction

1.1 A need for energy saving

The global warming and nuclear or ecological disasters are some current events that show

us that it is urgent to better consider renewable energy sources Unfortunately, as shown byfigures of the International Energy Agency (IEA), clean energies like solar, geothermal or windpower represent today only a negligible fraction of the energy balance of the planet During

2008, the share of renewable energies accounted for 86 Mtoe, only 0.7% of the 12,267 Mtoe ofglobal consumption Unfortunately, the vital transition from fossil fuels to renewable energies

is very costly in time and energy, as evidenced by such high costs of design and fabication

of photovoltaic panels Thus it is accepted today that a more systematic use of renewableenergy is not sufficient to meet the energy challenge for the future, we must develop otherways such as for example improving the energy efficiency, an area where heat transfers play

an important role

In many industrial and technical applications, ranging from the cooling of the engines andhigh power transformers to heat exchangers used in solar hot water panels or in refrigeration

systems, the low thermal conductivity k of most heat transfer fluids like water, oils or

ethylene-glycol is a significant obstacle for an efficient transfer of thermal energy (Table 1)

liquids:

EthylenGlycol(EG)

Glycerol(Gl) Water (Wa)

ThermalCompound(TC)

Table 1 Thermal conductivities k of some common materials at RT.

The improvement of heat transfer efficiency is an important step to achieve energy savings

and, in so doing, address future global energy needs According to Fourier’s law jQ = − k ∇ T,

an increase of the thermal conductivity k will result in an increase of the conductive heat flux.

Thus one way to address the challenge of energy saving is to combine the transport properties

of some common liquids with the high thermal conductivity of some common metals (Table 1)such as copper or novel forms of carbon such as nanotubes (CNT) These composite materialsinvolve the stable suspension of highly conducting materials in nanoparticulate form to the

17 Nanofluids for Heat Transfer

Rodolphe Heyd

CRMD UMR6619 CNRS/Orléans University

France

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2 Will-be-set-by-IN-TECH

fluid of interest and are named nanofluids, a term introduced by Choi in 1995 (Choi, 1995).

A nanoparticle (NP) is commonly defined as an assembly of bounded atoms with at leastone of its characteristic dimensions smaller than 100 nm Due to their very high surface

to volume ratio, nanoparticles exhibit some remarkable and sometimes new physical andchemical properties, in some way intermediate between those of isolated atoms and those

of bulk material

1.2 Some applications and interests of nanocomposites

Since the first report on the synthesis of nanotubes by Iijima in 1991 (Iijima, 1991), there hasbeen a sharp increase of scientific interest about the properties of the nanomaterials and theirpossible uses in many technical and scientific areas, ranging from heat exchange, coolingand lubrication to the vectorization of therapeutic molecules against cancer and biochemicalsensing or imaging The metal or metal oxides nanoparticles are certainly the most widelyused in these application areas

It has been experimentally proved that the suspension in a liquid of some kinds ofnanoparticles, even in very small proportions (<1% by volume), is capable of increasing thethermal conductivity of the latter by nearly 200% in the case of carbon nanotubes (Casquillas,2008; Choi et al., 2001), and approximately 40% in the case of copper oxide nanoparticles(Eastman et al., 2001) Since 2001, many studies have been conducted on this new class

of fluids to provide a better understanding of the mechanisms involved, and thus enablethe development of more efficient heat transfer fluids The high thermal conductivity ofthe nanofluids designates them as potential candidates for replacement of the heat carrierfluids used in heat exchangers in order to improve their performances It should be notedthat certain limitations may reduce the positive impact of nanofluids Thus the study of theperformance of cooling in the dynamic regime showed that the addition of nanoparticles in

a liquid increases its viscosity and thereby induces harmful losses (Yang et al., 2005) On theother hand, the loss of stability in time of some nanofluids may result in the agglomeration

of the nanoparticles and lead to a modification in their thermal conduction properties and torisks of deposits as well as to the various disadvantages of heterogeneous fluid-flow, likeabrasion and obstruction Nevertheless, in the current state of the researches, these twoeffects are less important with the use of the nanofluids than with the use of the conventionalsuspensions of microparticles (Daungthongsuk & Wongwises, 2007) We must not forget totake into account the high ecological cost of the synthesis of the NPs, which often involves alarge number of chemical contaminants Green route to the synthesis of the NPs using naturalsubstances should be further developed (Darroudi et al., 2010)

2 Preparation of thermal nanofluids

2.1 Metal nanoparticles synthesis

2.1.1 Presentation

Various physical and chemical techniques are available for producing metal nanoparticles.These different methods make it possible to obtain free nanoparticles, coated by a polymer orencapsulated into a host matrix like mesoporous silica for example In this last case, they areprotected from the outside atmosphere and so from the oxidation As a result of their veryhigh surface to volume ratio, NPs are extremely reactive and oxidize much faster than in thebulk state The encapsulation also avoids an eventual agglomeration of the nanoparticles

Nanofluids for Heat Transfer 3

as aggregates (clusters) whose physico-chemical properties are similar to that of the bulkmaterial and are therefore much less interesting The choice of a synthesis method is dictated

by the ultimate use of nanoparticles as: nanofluids, sensors, magnetic tapes, therapeuticmolecules vectors,etc Key factors for this choice are generally: the size, shape, yield andfinal state like powder, colloidal suspension or polymer film

2.1.2 Physical route

The simplest physical method consists to subdivide a bulk material up to the nanometricscale However, this method has significant limitations because it does not allow precisecontrol of size distributions To better control the size and morphology, we can use othermore sophisticated physical methods such as:

• the sputtering of a target material, for example with the aid of a plasma (cathodesputtering), or with an intense laser beam (laser ablation) K Sakuma and K Ishii havesynthesized magnetic nanoparticles of Co-Pt and Fe with sizes ranging from 4 to 6 nm(Sakuma & Ishii, 2009)

• the heating at very high temperatures (thermal evaporation) of a material in order that theatoms constituting the material evaporate Then adequate cooling of the vapors allowsagglomeration of the vapor atoms into nanoparticles (Singh et al., 2002)

The physical methods often require expensive equipments for a yield of nanoparticles oftenvery limited The synthesized nanoparticles are mostly deposited or bonded on a substrate

2.1.3 Chemical route

Many syntheses by the chemical route are available today and have the advantage of beinggenerally simple to implement, quantitative and often inexpensive Metallic NPs are oftenobtained via the reduction of metallic ions contained in compounds like silver nitrate, copperchloride, chloroauric acid, bismuth chloride, etc

We only mention here a few chemical methods chosen among the most widely used for thesynthesis of metal and metal oxides NPs:

Reduction with polymers: schematically, the synthesis of metal nanoparticles (M) from a

solution of M+ions results from the gradual reduction of these ions by a weak reducingpolymer (suitable to control the final particle size) such as PVA (polyvinyl alcohol) or PEO(polyethylene oxide) The metal clusters thus obtained are eventually extracted from thehost polymer matrix by simple heating The size of the synthesized metal nanoparticlesmainly depends on the molecular weight of the polymer and of the type of metal ions For

example with PVA (M w =10000) we obtained (Hadaoui et al., 2009) silver nanoparticleswith a diameter ranging from 10 to 30 nm and copper nanoparticles with a diameter ofabout 80 nm

Gamma radiolysis: the principle of radiolytic synthesis of nanoparticles consists in reducing

the metal ions contained in a solution through intermediate species (usually electrons)produced by radiolysis The synthesis can be described in three parts (i) radiolysis of thesolvent, (ii) reduction reaction of metal ions by species produced by radiolysis followed by(iii) coalescence of the produced atoms (Benoit et al., 2009; Ramnani et al., 2007; Temgire

et al., 2011)

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Thermal decomposition: the synthesis by the thermal decomposition of an organometallic

precursor allows to elaborate various systems of nanoparticles (Chen et al., 2007; Liu

et al., 2007; Roca et al., 2006; Sun et al., 2004) or carbon nanotubes (Govindaraj & Rao,2002) This method is widely used because of its ease and of the reproducibility ofthe synthesis, as well as the uniformity in shape and size of the synthesized particles.Metal particles such as Au, Ag, Cu, Co, Fe, FePt, and oxides such as copper oxides,magnetite and other ferrites have been synthesized by this method It mainly consists

of the decomposition of an organometallic precursor dissolved in an organic solvent (liketrioctylamine, oleylamine, etc.) with high boiling points and containing some surfactants(so called capping ligands) like oleic acid, lauric acid, etc By binding to the surface of theNPs, these surfactants give rise to a steric barrier against aggregation, limiting the growingphase of the nanoparticles Basing on the choice of the ligand properties (molecular length,decomposition temperature) and on the ligand/precursor ratio, it is possible to control thesize and size distribution of the synthesized NPs (Yin et al., 2004)

Using the thermal decomposition of the acetylacetonate copper precursor dissolved inoleylamine in the presence of oleic acid, we have synthesized copper oxide nanoparticles

of mean diameter 7 nm with a quasi-spherical shape and low size dispersion (Fig 1)

Fig 1 TEM picture of copper oxides nanoparticles synthesized by the thermal

decomposition of acetylacetonate copper precursor dissolved in oleylamine (Hadaoui, 2010)

2.1.4 Characterization of the nanoparticles

Depending on the final state of the nanoparticles, there are several techniques to visualizeand characterize them: the X-ray diffraction, electron microscopy (TEM, cryo-TEM, etc.), theatomic force microscopy, photoelectron spectroscopy like XPS More macroscopic methodslike IR spectroscopy and UV-visible spectroscopy are interesting too in the case where there is

a plasmonic resonance depending on the size of the NPs like for example in the case of silverand gold

The Dynamic Light Scattering (DLS) is a well established technique to measure hydrodynamicsizes, polydispersities and aggregation effects of nanoparticles dispersed in a colloidalsuspension This method is based on the measurement of the laser light scattering fluctuationsdue to the Brownian motion of the suspended NPs In the case of opaque nanofluids, only thebackscattering mode of DLS is able to provide informations on NPs characteristics

Nanofluids for Heat Transfer 5

2.2 Stability of colloidal suspensions

2.2.1 Presentation

The nanofluids belong to the class of Solid/Liquid colloidal systems where a solid phase isvery finely dispersed in a continous liquid phase Most of nanofluids are prepared by directinjection of nanoparticles in the host liquid, depending on the nature of this liquid (water,ethylene glycol (EG), oils, glycerol, etc.) it may be necessary to add chemicals to the solution

to avoid coagulation and ensure its stability by balancing internal forces exerted on particlesand slowing down agglomeration rates This addition can dramatically change the physicalproperties of the base liquid and give disappointing results

2.2.2 Isolated spherical particle immersed in a fluid

We consider a spherical particle of radius a p, densityρ p, immersed in a fluid of densityρ fanddynamical viscosityη, placed at rest in the gravitational field g assumed to be uniform (Fig.

2(a)) Under the effect of its weight P=ρ p V pg and of the buoyancy FA = − ρ f V pg due to the

fluid, the particle moves with velocity v that obeys to the equation of motion m pdvdt =ΔF+Fv,whereΔF = P+FA =V p(ρ p − ρ f)g and Fvis the viscous drag exerted by the fluid on theparticle In the limit of laminar flow at very low Reynolds numbers Re=ρ f v2a p/η1, we

can write the Stokes law for a sphere as Fv = −6πap ηv We deduce from these hypotheses

the following equation satisfied by the velocity of the sphere:

Based on previous results, we can preserve the stability of water-based nanofluids by limiting

a p , that is by limiting the agglomeration of nanoparticles In the case of viscous host media (like

glycerol or gels), stability is generally guaranteed, even for large agglomerates

2.2.3 Coagulation of nanoparticles

2.2.3.1 Presentation

The coagulation between two particles may occur if:

1 the particles are brought close enough from each other in order to coagulate When acolloid is not stable, the coagulation rate depends of the frequency at which the particles

collide This dynamic process is mainly a function of the thermal motion of the particles, of

the fluid velocity (coagulation due to shear), of its viscosity and of the inter-particles forces(colloidal forces)

2 during the collision the energy of the system is lowered by this process This decrease

in energy originates from the forces, called colloidal forces, acting between the particles

in suspension The colloidal forces are mainly composed of electrostatic repulsive forces

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