Combined wavelet-based optical flow velocimetry and temperature field measurements in alumina-water nanofluids during transient turbulent natural convection

This paper presents the development and demonstration of simultaneous planar measurements of instantaneous temperature and velocity fields using Planar Laser-Induced Fluorescence (PLIF) in alumina-water nanofluids under transient turbulent natural convection. Specifically, the 2-colour/2-dye LIF technique is employed to measure instantaneous temperature fields in the central region of a cubic convection cell. Two fluorescent dyes (Rhodamine B and Rhodamine 110) are excited at 532 nm by an Nd:YAG laser sheet. The intensity ratio of the emitted fluorescent intensities at two different spectral bands quantified the instantaneous temperature distribution with temperature sensitivity of ∼3 %/°C. Furthermore, wavelet-based Optical Flow Velocimetry (wOFV) is applied to the instantaneous temperature fields to estimate local velocity. This will enable new understanding on the correlation between heat transfer performance, thermal plume dynamics, and flow characteristics of pure water and alumina-water nanofluids under transient conditions of turbulent natural convection (Rayleigh number ∼10 9 ). However, these insights are not included in the present paper, which focuses solely on the development and application of the measurement technique. To the best of our knowledge, this is the first successful application of 2-colour LIF temperature measurements in nanofluids, offering insights into the behaviour of the instantaneous and time-averaged temperature and velocity fields. It is noted that due to the opaqueness of denser nanofluids, we limited the maximum nanoparticle concentration of the nanofluid to 0.0025 vol%. The present methodology can be applied to other nanofluid systems of interest. • Alumina-water nanofluids are studied under conditions of transient turbulent natural convection. • 2-colour/2-dye planar laser-induced fluorescence is employed to measure instantaneous liquid temperature. • Non-invasive instantaneous planar liquid temperature measurements demonstrated in nanofluids for the first time. • Uncertainty of temperature measurement ( ± 1.51 °C) reduced through a developed frame-rejection algorithm. • Wavelet-based optical flow velocimetry is applied to the nanofluid thermography images.