Effects of colloidal properties on sensible heat transfer in water-based titania nanofluids

Nanofluids have attracted considerable attention in recent years as effective working fluids in heat transfer applications. Nanofluids are essentially suspensions of nanoparticles in a base fluid and exhibit higher thermal conductivity than conventional heat transfer fluids.Although the relation between the thermal conductivity and volume fraction of solids in nanofluids is now relatively well known, the effects of colloidal properties such as suspension pH, particle size and aggregation remain poorly understood. In this study, the thermal conductivity of water-based TiO2 (rutile) nanofluid was studied over a range of volume fractions, temperatures, pH levels, particle sizes and sonication times. The thermal conductivity was measured using a KD2Pro apparatus. Volume fraction induced thermal conductivity enhancements were in good agreement with the published data and the effective medium theory (EMT) approximations. It was found that thermal conductivity enhancement was improved by 2% due to pH, with large spikes in enhancement being observed as the fluid pH approached the isoelectric point of TiO2. Calculated interaction energies and TEM images indicate that significant aggregation occurred during this period, and it is thought that aggregation may explain the observed enhancements due to percolation.

► Particle aggregation may cause deviations from the effective medium theory.
► Aggregation can be controlled by pH in nanofluids.
► Aggregate of 100–500 nm might perform better than discrete particles in nanofluids.