Visualization of SiO2-water nanofluid flow characteristics in backward facing step using PIV

The conventional tests of heat transferring for nanofluids have confirmed that, except the increase of thermal conductivity, there also exists an additional strengthening mechanism responsible for the heat transfer enhancement. The strengthening mechanism is the changes in the flow characteristics due to the interaction between the nanoparticles and the base fluid, which is also the main factor that affects the heat transferring of nanofluids. To reveal the strengthening mechanism, it is necessary to study the flow characteristics further. The past visualization experiment just gives the flow patterns of the nanofluids, and cannot quantitatively reveal the enhancement degree of the fluids after adding the nanoparticles. In this paper, by using PIV (particle image velocimetry), the flow characteristics of SiO2-water nanofluids in two-dimensional backward step flow are quantitatively studied. The results show that, under the same Reynolds number, the number of vortexes, vorticity and turbulent kinetic energy increase by the addition of nanoparticles. With the increasing of nanoparticle volume fraction, the turbulence becomes more intense, and the flow characteristics are constantly changing. For the vol. 3% SiO2-water nanofluids, the number of vortexes increases by 3, the average vorticity increases by 22.1%, and the average turbulent kinetic energy increases by 368% compared with pure water, when the Reynolds number is 5000. Therefore, it can be concluded that the mass and momentum transfer in nanofluids is enhanced, resulting in the heat transfer enhancement.