International Journal of Engineering, Management & Sciences (IJEMS) ISSN-2348 –3733, Volume -2, Issue-3, March 2015
Thermal Conductivity Enhancement in Oxide Nanofluids –a Mathematical Model Khagendra Kumar Upman Abstract— The study investigated the impact of the nanoparticle size which has been suggested to be an important factor the results were found to be in concord with the experimental observations. The values of the thermal conductivity for different nanofluid combinations were calculated using the expression developed in this study and they agreed with published experimental data. From the study, it was observed that Brownian motion is significant only when the volume fraction is less than 1 % in case of TiO2&ZnO and 4 % in case of Al2O3. The combination of the base fluid and nanoparticles to from nanoclusters is expected provide better heat transfer solution than the conventional fluids , Hence it is concluded that adding nanosized materials to base fluids enhances thermal properties and makes them more suitable to heat exchanger applications as well as for many industrial applications also .
Index Terms— Thermal brownian motion, nanofluids.
conductivity,
because of these present limitations for better understanding the heat transfer mechanism and effect of different parameters on thermal conductivity of nanofluids more studies have to be carried out. In this Paper, new models have been developed to measure the thermal conductivity of Al2O3- water and ZnO and TiO2-water nano fluids. Models have been developed by considering the fact thatthermal conductivity of nanofluid is depends on so many parameters like effect of temperature , volume fraction , size of nano particles , particle density , viscosity , thermal conductivity of particle and as well as base fluid . Rem (Modified Reynolds number) which is a dimensionless quantity based on Boltzmann constant. II. PRESENT MODEL FOR THERMAL CONDUCTIVITY Thermal conductivity of a nanofluid, knf, is given by:-
nanoparticle,
I. INTRODUCTION So far no general mechanisms to have been formulated to understand the strange behavior of the nano fluids including the highly improved effective thermal conductivity, this technology isstill limited for commercial use because there is yet no proven standardized design process for accurately predicting important heat transfer properties. Developing a reliable fundamental model for the thermal conductivity of nanofluids has always been a challenging task for researchers. Early attempts to explain this behavior have made use of the classical model of Maxwell. This model is generally applicable to dilute suspensions with micro particles but when applied to nanofluids the models predicted lower thermal conductivity enhancement as compared to the experimental observations. Several authors extended the Maxwell’s theory such as Bruggeman (1935); Jeffrey (1973); Yu and Choi (2003); Koo and Kleinstreuer (2004); Xie et al. (2005) are some theoretical models and Chon et al. (2005); Li and Peterson (2006); Mintsa et al. (2009) and Teng et al. (2010) are some empirical models. These models are not so accurate and stable against a wide range of experimental data. So
Manuscript received March 24, 2015. Khagendra Kumar Upman, M.Tech Scholar, Department of Mechanical Engineering, AIET Jaipur
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(1) For development of thermal conductivity model first we have to analyze the Brownian motion of nanoparticle. The particles suspended in the liquid are very small, Brownian movement of the particles is quite possible.The root-mean-square velocity (vN) of a Brownian particle can be defined as (2) It can be written as:
(3) (mpis particle mass =
)
Now we consider the effect of the convection of theliquid near the particles due to their Brownian movement. The Reynolds number based on vN given by Eq. (4) can bewritten as: (4) These variables in equation (1) can be expressed in non-dimensional terms as: Knf=f[
, Rem, φ]
(5)
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