H. Niazmand, M. Charjouei Moghadam, A. Behravan ENTECH '14 The International Energy Technologies Conference , Turkey, Istanbul, Dec 2014 Investigation on flow and heat transfer of CNT/transformer oil nanofluids E. Ebrahimnia-Bajestan Department of Energy, Graduate University of Advanced Technology Kerman, Iran Ehsan.Ebrahimnia@gmail.com H. Niazmand, M. Charjouei Moghadam, A. Behravan

Overview Experimental Section Numerical Section The flow and heat transfer of the CNT/transformer oil have been numerically simulated for the straight and 90° pipe employing the single-phase approach CNT/Transformer Oil nanofluid is prepared in two volume fractions of 0.0004 and 0.0041. The effect of Temperature and volume fractions is investigated on nanofluid Thermal conductivity and viscosity The measured thermophysical properties have been used for developing the nanofluid thermophysical properties correlations that are employed in the numerical simulation

Introduction Transformer main components of the electricity transmission networks Heat production in wire and coil Temperature rise: Damages the paper insulation and liquid insulation medium Is a limiting factor for transformer operation (breakdown) Reduces the Lifetime of the transformer Temperature control of the transformer is a essential need. Transformer oil electrical insulating medium (Prevent electrical problems) Heat transfer fluid

Introduction Cooling Limitation: Low thermal conductivity of Transformer oil Solution: Nanofluids Nanofluids Uniform and stable dispersion of nanoparticles into liquids

Literature review Most of the studies in the literature are done on the water and ethylene glycol based nanofluids. The oil based nanofluids are mostly been considered for their electrical and magnetic characteristics. (Jeon and Lee, 2014, Lee et al., 2012, Miao et al., 2013, Mansour et al., 2012, Timko et al., 2012, Dong et al., 2013) Thermophysical properties of the oil based nanofluid Investigator(s) Base fluid Particle type Investigation Botha et al., 2011 Transformer oil Silver-Silica Thermal conductivity increment and reduction in viscosity Li et al., 2011 Copper Both thermal conductivity and viscosity have increased Singh and Kundan, 2013 Al2O3

Literature review convective heat transfer of oil based nanofluids. Investigator(s) Base fluid Particle type Geometery Investigation Liu, 2011 Transformer oil Cu Straight pipe Convective heat transfer of nanofluid at different flow temperatures and nanoparticle size was examined Srinivasan and Saraswathi, 2012 BN 76% enhancement of the nanofluid convective heat transformer of with respect to its base fluid CNT nanoparticles nanofluids used in different conditions Investigator(s) Base fluid Particle type Investigation Nazari et al., 2014 Water and Ethylene glycol Alumina and CNT Comparing different types of nanofluids in CPU cooling Ashtiani et al., 2012 Transformer oil MW-CNT Heat transfer characteristics of MW-CNT/transformer oil nanofluid inside flattened tubes under uniform wall temperature condition Rahman et al., 2014 Water CNT Effect of solid volume fraction and tilt angle in a quarter circular solar thermal collectors filled with CNT–water nanofluid Amiri et al., 2014 Pool Boiling heat transfer of CNT/water nanofluid Chen and Xie, 2009 Oil Optimized thermal conductivity enhancement

Challenges an motivations Enhancement of low thermal conductivity of transformer oil Natural or force convection of oil in transformers heat transfer fluids with low viscosity Thermophysical properties of oil based nanofluid Convective heat transfer of oil based nanofluid Outlines of this study: Measurement of viscosity and thermal conductivity of CNT/transformer oil Numerical study of convective heat transfer of the nanofluids employing measurement thermophysical data

Nanofluid Preparation using two-step method Functionalized CNT Surfactant free Ultrasonication (100W and 28kHz) Different particle concentration of 0.001wt.% and 0.01wt% (0.0004vol.% and 0.0041vol.%)

Thermophysical Properties Measurement DV-III ultra viscometer for measuring nanofluid viscosity KD2 Device for measuring nanofluid thermal conductivity

Viscosity measurement results and the correlated relation Viscosity strongly decreases with temperature No significant effect of particle concentration Smaller pressure drops in higher volume fractions

Thermal conductivity measurement results and the correlated relation The thermal conductivity increases with the increment of volume fractions because: Higher thermal conductivity of CNTs in comparison with transformer oil Brownian motion of nanoparticles into the base fluid The bulk motion due to the Brownian motion of nanoparticles that results in a strong fluid mixing and heat transfer improvement Shape effect of nanoparticles

Numerical Modeling Laminar forced convective heat transfer Constant heat flux (3000 W/m2) Single phase model Proposed correlations for thermal conductivity and viscosity is applied Both pipes have the same length (2.05m) and inner diameter (7.8mm)

The Governing equations Continuity equation Momentum equation Energy equation

Nanofluid effective thermophysical properties relations used in the governing equations Density Heat Capacity Thermal Conductivity Viscosity

Thermophysical properties of the transformer oil and CNT Thermophysical properties of the transformer oil have been obtained from the curve fitting to the measured data. As for the density and specific heat, the curve fits have been done to the data of (Susa. 2005). Thermophysical properties of nanoparicle Particle Type ρ (g/cm3) K(W/mk) Inner diameter (nm) Outer diameter (nm) Length (µm) CNT 2.1 2000 5-10 10-20 30

Numerical Model Validation 90 curved pipe: Re = 300 , De = 122 , D = 8mm , Rϲ = 24mm , α = 1/6 VAN DE VOSSE, F., VAN STEENHOVEN, A., SEGAL, A. & JANSSEN, J. 1989. A finite element analysis of the steady laminar entrance flow in a 90 curved tube. International journal for numerical methods in fluids, 9, 275-287.

Results and discussion velocity contours

Results and discussion Temperature contours

Results and discussion Averaged heat transfer coefficient versus Reynolds number for the straight pipe Enhancement of heat transfer characteristics with Reynolds number and volume fraction

Results and discussion Averaged heat transfer coefficient versus Reynolds number for the 90° curved pipe Development of secondary flows in the curved pipe Better mixing of the flow Increased heat transfer

Results and discussion The nanofluid heat transfer increment ratio with respect to the base fluid

Conclusion Thermal conductivity of the CNT/Transformer oil nanofluid increases with volume fraction and temperature. Viscosity of the CNT/Transformer oil nanofluid decreases with temperature and is not influenced by volume fraction which makes the usage of this kind of nanofluids more efficient in power transformers as a result of smaller pressure drops. Convective heat transfer of the nanofluid is increased by using the nanofluid with respect to its base fluid. The heat transfer enhancement is greater in the 90° curved pipe due to the development of secondary flows which results in the better mixing of the flow.

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