Insight into the contribution of rotating Brownian motion of nonspherical particle to the thermal conductivity enhancement of nanofluid

Up to now, most thermal conductivity models of nanofluid considering Brownian motion assumed that particles are spherical. It is obviously not the cases in the practical application. In our study, we successfully derived the equation of angular velocity of rotating Brownian motion (ω) as a function of particle size, mass and resistance coefficient and resistance moment coefficient etc. based on Langevin equation and energy equipartition theorem. By using a rotating Reynolds number Rer, the effect of rotating Brownian motion of cubic particles was evaluated when the model is used to predict thermal conductivity of nanofluid. What' more, the thermal conductivity was chosen to experimentally verify the possible contribution of the rotating motion of nonspherical particles for the first time. Cubic nanoparticles of 30, 50 and 60 nm in sizes have been prepared and the thermal conductivities for their colloid suspensions were experimentally investigated. It was found that the prediction of the thermal conductivities for the cubic nanoparticle suspension “considering ω” is in very good agreement with the experimental values, while that for “without considering ω” case is much smaller than the experimental values. A decreasing and an increasing trend against cubic lengths were found for the two cases, respectively. When the cubic length is close to 1000 nm, the difference between the two cases is almost disappeared. The thermal conductivities become nearly constant with further increase of particle sizes. Our finding is rationalized by considering the competition between two key factors influencing the thermal conductivity, i.e., the interfacial thermal resistance and the Re induced by Brownian motion. The conclusion of our present work is expected to be especially valuable if the particle are of irregular shape and have the sizes below micrometers which are supposed to be the cases for many practical applications.