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THE INFLUENCE OF MULTI‐WALLED CARBON NANOTUBES ON SINGLE‐PHASE HEAT TRANSFER AND PRESSURE DROP CHARACTERISTICS IN THETRANSITIONAL FLOW REGIME OF SMOOTH

TUBES

Kersten Grote

Supervisors: Prof. J.P Meyer (UP)Dr. T.J McKrell (MIT)

Layout of Presentation Purpose of the study Previous work Experimental setup Test section Validation of the experimental system Preparation of the nanofluids Properties of the nanofluids Heat transfer results of the nanofluids Friction factor results of the nanofluids Pressure drop results of the nanofluids Performance evaluation of the nanofluids Conclusion

Purpose of Study

Exponential growth in communication, electronics and computing technologies

Conventional method is to increase the cooling rate by increasing the surface area

Research is being done on microscale heat transfer Find new novel ways for innovative cooling technology

Purpose of Study

Low heat transfer performance of conventional fluids Thermal conductivity of a fluid Introduction of solid particles into the base fluid Suspension of millimeter‐ or micrometer‐sized particles are

well known Use nanometer‐sized particles to overcome aforementioned

problems

Previous WorkAuthor Flow Range Nanofluid Results

Pak and Cho (1998) Turbulentγ‐Al2O3 – waterTiO2 ‐ water

45% enhancement75% enhancement

Li and Xuan (2002) Laminar and turbulent Cu‐water 60% enhancement

Wen and Ding (2004) Laminar γ‐Al2O3 ‐ water45% enhancementfor developing flow

Yang et al. (2005) Laminar Graphite‐water22% enhancement at 50˚C15% enhancement at 70˚C

Ding et al. (2006) Laminar MWCNT‐water350% enhancementat the entrance region

Garg et al. (2009) Laminar MWCNT ‐ water 32% enhancement

Kim et al. (2009)Laminar andturbulent

γ‐Al2O3 – waterA/C‐water

14% enhancement, laminar7% enhancement, laminar

Anoop et al. (2009) Laminar γ‐Al2O3 – water25% enhancement for 45 nm11% enhancement for 150 nm

Duangthongsuk andWongwises (2010)

Turbulent TiO2 ‐ water26% enhancement for 1 vol%‐14% enhancement for 2 vol%

Ferrouillat et al. (2011) Turbulent SiO2 ‐ water 50% enhancement for 19 vol%

Experimental Setup

Test section

•ID 5.16 mm•OD 6.44 mm•Length 1 m•Input power 212 W (13 000 W/m2)

•13 Wall thermocouples•One inlet and exit thermocouple

Validation of Experimental System

Reliability and accuracy of the experimentalsystem was tested with distilled water beforeany MWCNT‐water nanofluid experimentswere conducted

Validation was done for adiabatic , diabaticfriction factors and heat transfer


Adiabatic friction factors were compared toBlasius and Poiseuille friction factor:

Validation of Experimental SystemFriction Factor ‐ Adiabatic:


Diabatic friction factors were compared to Blasius, Allen and Eckert (1964) and Poiseuille friction factor:

Validation of Experimental SystemFriction Factor ‐ Diabatic:

Validation of Experimental SystemHeat transfer:

The results were compared to the correlations developed by Ghajar and Tam (1994) for laminar and turbulent flow

For the transitional flow regime the correlation developed by Ghajar and Tam (1994) was modified to account for a developing length inlet condition

Validation of Experimental SystemHeat Transfer:

Preparing the Nanofluid

Carbon nanotubes were used in the current study as the nanoparticles

Different types of carbon nanotubes: Single‐walled carbon nanotubes (SWCNT) Double‐walled carbon nanotubes (DWCNT) Multi‐walled carbon nanotubes (MWCNT)

They have the highest thermal conductivity compared to water (MWCNT – k = 3 000 W/m˚C; Water – k = 0.61 W/m˚C)

Using MWCNT in this case, due to being the cheapest out of the family of carbon nanotubes


Physical dimensions of the nanoparticles: Outside diameter 10‐20 nm Inside diameter 3‐5 nm Length 10‐30 µm

Test three different volume concentrations: 0.33 vol% 0.75 vol% 1.00 vol%


Preparation of the nanofluid was based on the work by Garget al. (2009)

Study concentrated on the sonication time of the nanofluid Tested four different sonicated samples of MWCNT‐water

nanofluids The four samples consisted out of 0.25%wt GA and 1wt%

MWCNT – which is a 1:4 ratio The sample that was sonicated for 40min showed the best

increase in convective heat transfer

Properties of the MWCNT‐waternanofluids


From prediction models, the thermal conductivity can be written as follows:

Ck is a constant and is depended on the experimental data


Einstein developed a correlation for the viscosity of dilute suspensions ( < 5 vol%) for spherical particles:

Einstein equation can be modified to include ellipsoidal particles:

Heat Transfer Results of the MWCNT‐water nanofluids

Comparison of Results to existing correlations

Laminar


Turbulent


Transition

Friction Factor Results of the MWCNT‐water nanofluids

Pressure Drop Results of the MWCNT‐water nanofluids

Performance evaluation of the MWCNT‐water nanofluids

Prasher et al. (2006) challenged the idea whether there is any benefit using nanofluids as heat transfer fluids.

They considered the conservative case where hnf = hbf and developed the design equation for nanofluids:

If ΔPnf /ΔPbf > 1 then the nanofluid is worse as a heat transfer fluid than the base fluid, however if it is ΔPnf /ΔPbf < 1, then it is a better heat transfer fluid

Performance evaluation of the MWCNT‐water nanofluids

In turbulent flow ‐ ΔPnf /ΔPbf ≈ 1 In laminar flow ‐ ΔPnf /ΔPbf > 1. Nanofluids are better suited for the turbulent flow regime

due to the shear thinning behaviour In the laminar flow regime they are worse heat transfer fluids

than the base fluid For the conservative case of Nunf = Nubf and desiring that ΔPnf

not exceed ΔPbf

Conclusion

Three different MWCNT‐water nanofluid volume concentrations were tested for the late laminar, transition and early turbulent flow regime

Heat transfer and pressure drop measurements were taken for a Reynolds number range of 1 000 ‐ 8 000 and compared to that of distilled water

From the preparation of the nanofluids it was seen that the 1.0 vol% MWCNT‐water nanofluid was unstable compared to the other two concentrations

The performance evaluation showed that the current nanofluids are better suited for turbulent flow but overall they are a worse heat transfer fluid compared to the base fluid

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