Al2O3/Water Nanofluid as Coolant in Double-Tube Heat Exchanger

Mr.V.S. PATNAIK Presented By, (Guide) K.S.SUMAN KRISHNA KANTH

Advanced Flow and Heat-Transfer Challenges

• Cooling becomes one of the top technical challenges faced by high-tech industries such as microelectronics, transportation, manufacturing, and metrology.

• Conventional method to increase heat flux rates: extended surfaces such as fins and micro-channels increasing flow rates increases pumping power.

• However, current design solutions already push available technology to its limits.

• New Technologies and new advanced fluids with potential to improve flow & thermal characteristics are of critical importance.

• Nanofluids are promising to meet and enhance the challenges.

Concept of Nanofluids

• Conventional heat transfer fluids have inherently poor thermal conductivity compared to solids.

• Conventional fluids that contain mm- or m-sized particles do not work with the emerging “miniaturized” technologies because they can clog the tiny channels of these devices.

• Modern nanotechnology provides opportunities to produce nanoparticles.

• Nanofluids are a new class of advanced heat-transfer fluids engineered by dispersing nanoparticles smaller than 100 nm (nanometer) in diameter in conventional heat transfer fluids.

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Thermal conductivity of typical materialsTh

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Material

0.15 0.25 0.61

1-Engine Oil2-Ethylene Glycol3-Water4-Alumina5-Silicon6-Aluminum7-Copper8-Silver9-Carbon

Solids have thermal conductivitiesthat are orders of magnitude larger than those of conventional heat transfer fluids.

NANOFLUIDS• A recent advancement in nanotechnology has been the

introduction of nanofluids, that is, colloidal suspensions of nanometer-sized solid particles instead of common working fluids.

• Nanofluids were first innovated by Choi and Eastman in 1995 at the Argonne National Laboratory, USA.

• Nanofluids have novel properties that make them potentially useful in many applications in heat transfer, including microelectronics, fuel cells, pharmaceutical processes, and hybrid-powered engines, engine cooling/vehicle thermal management, domestic refrigerator, chiller, heat exchanger, nuclear reactor coolant, in grinding, machining, in space technology, defense and ships, and in boiler flue gas temperature reduction

Why Use Nanoparticles?• The basic concept of dispersing solid particles in fluids to enhance thermal

conductivity can be traced back to Maxwell in the 19th Century.

• Studies of thermal conductivity of suspensions have been confined to mm- or mm-sized particles.

• The major challenge is the rapid settling of these particles in fluids.

• Nanoparticles stay suspended much longer than micro-particles and, if below a threshold level and/or enhanced with surfactants/stabilizers, remain in suspension almost indefinitely.

• Furthermore, the surface area per unit volume of nanoparticles is much larger than that of microparticles .

• These properties can be utilized to develop stable suspensions with enhanced flow, heat-transfer, and other characteristics.

Materials for Nanoparticles and Base Fluids

1. Nanoparticle materials include:– Oxide ceramics – Al2O3, CuO– Metal carbides – SiC– Nitrides – AlN, SiN– Metals – Al, Cu

2. Base fluids include:– Water– Ethylene- or tri-ethylene-glycols and other coolants– Oil and other lubricants

Methods for Producing Nanoparticles/Nanofluids

Two nanofluid production methods has been developed in ANL to allow selection of the most appropriate nanoparticle material for a particular application.

• In two-step process for oxide nanoparticles (“Kool-Aid” method), nanoparticles are produced by evaporation and inert-gas condensation processing, and then dispersed (mixed, including mechanical agitation and sonification) in base fluid.

• A patented one-step process simultaneously makes and disperses nanoparticles directly into base fluid; best for metallic nanofluids.

• Other methods: Chem. Vapor Evaporation; Chem. Synthesis.

• Realizing the modest thermal conductivity enhancement in conventional nanofluids, a team of researchers at Indira Gandhi Centre for Atomic Research Centre, Kalpakkam claimed developing a new class of magnetically polarizable nanofluids where the thermal conductivity enhancement up to 300% of base fluids is demonstrated.

The objective of this paper is to provide improvements through nanofluids in place of pure working fluid in heat exchangers with a view of decreasing the mass flowrate for providing the same heat exchange capacity.

• 7nm- Al2O3 nanoparticle with concentration up to 2vol.% has been selected as a coolant in a typical horizontal double-tube heat exchanger because of its good thermal properties and easy availability.

• Water has been chosen as heat transfer base fluid.• Al2O3 nanoparticles are generally considered as safe material

for human being and animals that are actually used in the cosmetic products and water treatment.

• In addition, Al2O3 nanoparticles are stabilized in the various ranges of PH.

Thermophysical properties of nanoparticle and base fluid

• Density of Al2O3/water nanofluid can be calculated using mass balance

ρnf =(1 – φ)ρbf + φρp

• specific heat of nanofluids cp,nf =[(1 – φ)ρbfcp,bf + φρpcp,p ]/ρnf

Thermophysical properties of nanoparticle and base fluid

• The viscosity of nanofluidμnf = μbf(1 + aφ)

Where a is the slope of the relative viscosity of the particle volume fraction Value of a calculated from experimental results of Chun et al.

is a=15.4150.• The thermal conductivity of nanofluid is the kang model

which is expressed as knf = kbf

Mathematical Modeling HOT SOLVENT SIDE CALCULATION:• The rate of heat transferred to the hot solvent in a double-

tube heat exchanger can be written as follows : Q.

given = m˙ hcp,h(T1 − T2) ≡m˙nfcp,nf(t2 – t1)

• The heat exchange capacity of exchanger (Q.given ) is equal to

15.376kW, the inlet outlet temperatures of hot solvent stream are equal to 400C and 300C respectively, the flowrate of hot solvent stream is 0.8kg.s-1 and its specific heat capacity is equal to 1922 Jkg-1K-1.

• The heat transfer coefficient of hot solvent flowing inside the tube under a turbulent regime(Re>10000) can be calculated as

• where Di is the internal diameter of the internal tube(μnf/μwnf)0.14 is the viscosity correction factor. In the previousequation the Reynolds and Prandtl numbers and heat transfer coefficient of hot solvent are calculated as follows :

Nanofluids Side Calculation• The heat transfer coefficient of the nanofluid as coolant flowing in the

annular can be calculated considering the turbulent Nusselt number presented by Li and Xuan as follows:

• where Ped is the nanofluid Peclet number and is defined in the following form:

• where dp is the diameter of the nanoparticles and αnf is the nanofluids thermal diffusivity which is defined as follows:

• The Reynolds and Prandtl numbers in are calculated considering the nanofluid properties as follows:

• where Deq is the equivalent diameter which is expressed in the following form:

where Ds is the internal diameter of the external tube.

• Tw is calculated by equating the heat transfer rates at both sides of the tube wall as follows:

• The friction factor of Al2O3/water nanofluid can be calculated using the formula presented as follows :

Where

• The pressure drop (Δpnf) and pumping power (PP) for Al2O3/water nanofluid used as a coolant in a double-tube heat exchanger are calculated as follows

• where L is the length of the tube, D|eq is the equivalent diameter of an

annulus given by D|eq= Ds − Do, and as is the annular flow area.

Total Heat Transfer Area and Coefficient Calculation

• The total heat transfer coefficient can be calculated as follows:

• where Rf is the fouling resistance, hh,o is heat transfer coefficient of hot solvent that referred to the external area, and hnf is the heat transfer coefficient of the nanofluid coolant. In this work, the fouling resistance is assumed to be 5×10−4m2KW−1.

• The total heat transfer area of a double-tube heat exchanger, A, is computed from the following equation:

• where U is the total heat transfer coefficient and F is the temperature correction factor, which in the case of the countercurrent flow can be taken equal to 1.

• The thermal conductivity of Al2O3 /water nanofluid with different concentrations (0-2% volume fraction) has been calculated using Kang model.

• As can be seen, thermal conductivity increases with increasing the nanoparticles volume concentration.

• Results show that heat transfer coeffcient and Nusselt number can be enhanced by adding nanoparticles to the base fluid.

• For volume concentration of 2%, the heat transfer coefficient increase about 64.65%, while the increase of thermal conducitivity is below 40%

• Increasing the particles concentration raises the fluid viscosity and decreases the Reynolds number and consequently decreases the heat transfer coefficient

• It can be concluded that the change in the coolant heat transfer coefficient is more than the change in the fluid viscosity with increasing nanoparticles loading in the base fluid.

• This figure reveals that as the concentration increases, the effect of increasing nanoparticles concentration on changing the thermal conductivity is lower than changing the heat transfer coefficient.

• The total heat transfer coefficient of the Al2O3/water nanofluid for volume concentrations in the range of 0.1% to 2% increases by 0.55%–3.5%.

• The total heat transfer coefficient is high when the probability of collision between nanoparticles and the wall of the heat exchanger has increased under higher concentration conditions. It confirms that nanofluids have considerablepotential to use in cooling systems.

• Figure 7 shows the reduction percent of wall temperature and heat transfer area in a double-tube heat exchanger that utilizes Al2O3/water nanofluid as a coolant under turbulent flow conditions.

• The wall temperature and total heat transfer area decrease with the increasing of volume concentration of nanoparticles. For example, the reduction percent of wall temperature at 0.5%, 1%, 1.5%, and 2% volume concentrations is about 5.35%, 9.32%, 11.74%, and 13.72%, respectively. Moreover, the reduction of the total heat transfer area at 2% volume

concentration is about 3.35%.

• This figure reveals that at the same heat exchange capacity, the flowrate of nanofluid coolant decreases with the increasing concentration of nanoparticles. For a volume concentration range of 0.1% to 2%, the mass flowrate decreases by 4.73% to 24.5%.

• The results show that nanofluid friction factor and pressure drop increase with increasing nanoparticles loading in the base show that using the Al2O3/water nanofluid at higher particle volume fraction creates a small penalty in pressure drop.

CONCLUSIONS

• The results confirm that nanofluid offers higher heat performance than water and therefore can reduce the total heat transfer area and also coolant flowrate for providing the same heat exchange capacity.

• Inorder to determine the feasibility of Al2O3/water nanofluid as a coolant in a double-tube heat exchanger, the effects of nanoparticles on the friciton factor, pressure drop, and pumping power have been evaluated.

• The results show that using Al2O3/water nanofluid at higher particle volume fraction creates a small penalty in pressure drop.

REFERENCES[1] S. U. S. Choi and J. A. Eastman, “Enhancing thermal conductivity of fluids with nanoparticles,” in ASME International Mechanical Engineering Congress and Exhibition, San Francisco, Calif, USA, 1995.[2] M. Shafahi, V. Bianco, K. Vafai, and O. Manca, “An investigation of the thermal performance of cylindrical heat pipes using nanofluids,” International Journal of Heat and Mass Transfer, vol. 53, no. 1–3, pp. 376–383, 2010.[3] P. Naphon , P. Assadamongkol, T. Borirak , “Experimental investigation of titanium nanofluids on the heat pipe thermal efficiency”, International Communications in Heat and Mass Transfer ,vol 35, pp.1316–1319, 2008.[4] Navid Bozorgan, Mostafa Mafi, and Nariman Bozorgan, “Performance Evaluation of Al2O3 /Water Nanofluid as Coolant in a Double-Tube Heat Exchanger Flowing under a Turbulent Flow Regime,” Advances in Mechanical Engineering, vol. 2012, Article ID 891382, 8 pages, 2012.

Q &A