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ORIGINAL ARTICLE

Effect of dispersion behavior on the heat transfer characteristics of alumina nanofluid: an experimental investigation and development of a new correlation function Monisha Michael1 · Aparna Zagabathuni2 · Sudipta Sikdar3 · Shyamal Kumar Pabi4 · Sudipto Ghosh1 Received: 11 March 2020 / Accepted: 22 June 2020 © Islamic Azad University 2020

Abstract Present work aims to study the dispersion characteristics of ­Al2O3 nano-dispersoid in water following different periods of ultrasonication and its impact on the thermal conductivity and viscosity of the nanofluid. Nanofluids with 0.5–2 vol% of ­Al2O3 nanoparticles have been prepared by ultrasonication for varying period. A ­ l2O3 nanofluids reported a maximum thermal conductivity enhancement of 16.1% for 2 vol% of nanoparticle concentration, after an optimum ultrasonication of 2 h beyond which the thermal conductivity decreases with further ultrasonication. The optimum ultrasonication time required for uniform dispersion of nanoparticles increases with the increase in the A ­ l2O3 volume fraction. For 1.5 vol% A ­ l2O3 nanoparticle loading, the viscosity of nanofluid decreased by 33% with an increase in the sonication time from 30 to 90 min. Further increase in sonication time by 30 min resulted in 13% increase in the viscosity of A ­ l2O3 nanofluid. This decrease in the thermal conductivity enhancement and increase in the viscosity beyond the optimum ultrasonication period have been attributed to the re-agglomeration of nanoparticles which are confirmed by TEM, and DLS results carried out after different instants of ultrasonication. The occurrence of re-agglomeration is explained in terms of the convective flow associated with the ultrasonication process. Various theoretical models like Maxwell or Hamilton–Crosser models which when used to predict the thermal conductivity of nanofluid, underestimate the thermal conductivity. A new correlation is, therefore, developed on the basis of experimental results. With an R2 value of 0.9924, the correlation showed a good agreement with the present thermal conductivity data. Keywords  Nanofluids · Agglomeration · Thermal conductivity · Ultrasonication · Alumina nanoparticles · Viscosity List of symbols a Thermal diffusivity, ­m2/s C Euler’s constant c Slope of the linear section of the plot ∆T (r, t) vs ln (t) k Thermal conductivity, W/m K n No. of readings q Constant heat produced per unit time and per unit length, J/m s * Monisha Michael [email protected] 1



Indian Institute of Technology Kharagpur, Kharagpur, India

2



Department of Mechanical Engineering, Indian Institute of Technology Guwahati, Guwahati, India

3

R&D Division, Tata Steel, Jamshedpur, India

4

School of Engineering and Technology, Adamas University, Barasat, India



r Diameter of platinum wire, m s Standard deviation t Time, s u Uncertainty W Mass, kg ∆T Temperature rise over the platinum wire, K Greek letters ρ Density, kg/m3 ϕ Volume fraction, % λ Wavelength, Å Subscripts bf Base fluid nf Nanofluid

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