Studies On The Characteristics And Application Of The Speckles Formed By Illuminating Nanofluids With Laser Beam

Nanofluids or liquids with suspended nanoparticles are likely to be the future heat transfer media as their thermal conductivities are significantly higher than those of the parent liquids even if the nanoparticle concentrations are negligible. The movement of nanoparticles in fluids is one of the main factors that result in the remarkable thermal conductivity of nanofluids; however, there is no suitable method to detect the movements of nanoparticles in nanofluids till now. In this paper, the formation-mechanism and characteristics of the speckles, which are formed by illuminating nanofluids with laser beam, are investigated. Laser Speckle Velocimetry (LSV) is presented to measure the velocities of nanoparticles in nanofluids. Moreover, experiments are carried out measure the velocities of nanoparticles in directional nanofluids with LSV.According to the optical characteristics of nanoparticle, LSV is proposed to measure the velocities of nanoparticles in nanofluids, and traditional LSV system is modified in order to be competent for detecting the movements of nanoparticles. Light sheet is changed to plane light source, and CCD digital camera is placed perpendicular to the incident light.By the means of experiments and numerical simulations, it is verified that speckles can be formed when nanofluids are illuminated by laser beam. First, clear speckles are formed by illuminating static nanofluids containing proper volume fraction of nanoparticles. Then, classic-scattering-model and photon-nanoparticle-collision-model are respectively established based on the wave-particle duality of light. The formation of speckles with nanofluids is simulated respectively on these two models, and the formation mechanism of speckles is figured out.According to the experimental setup, a physical model is established. Based on statistical properties of speckles, the relationship between the movements of naoparticles and the movements of speckles on the screen is studied by formula deduction. A conclusion is drawn that the velocities of nanoparticles is identical to the velocities of speckles in Fresnel region. Thereafter, the experimental system is set up, and speckle patterns sequences are recorded with high speed CCD camera.Two consecutive speckle patterns are processed with computer programs to obtain the motion vector diagram of speckles, thereby the motion of nanoparticles. Speckle patterns are first preprocessed with homomorphic filter and image threshold. Then they are correlated with global searching algorithm and several block-matching algorithms (BMA) to obtain the motion vector diagram of speckles. Furthermore, sub-pixel algorithm is introduced and the resolution is improved to 0.1 pixel. Finally, the velocities of speckles are obtained, thereby the movements of nanoparticles.The research consequences may offer theoretical and experimental references to further study of the movement of nanoparticles in fluids with Laser Speckle Velocimetry or some new optical method.