Laser Speckle Zero Measurement Of The Experimental Study Of The State Of Motion Of Nanoparticles In Nanofluids

Nanofluids, as the new next-generation coolants, have attracted great attention from researchers worldwide. However, the fundamental understanding of the mechanisms of enhanced heat transfer in nanofluids, which is one of the research focuses in heat transfer field, is still a mystery calling for new and comprehensive theories to explain. Based on the existing studies of mechanism of nanofluids so far, the movement of nanoparticles is one of the important factors controlling the thermal conductivity of nanofluids. In this paper, combined with the idea of Laser Speckle Velocimetry (LSV), experiments are carried out to measure the velocities of nano-particles in directional nanofluids with Zero-Crossing Method.Based on the statistical characteristics of laser speckle and the theoretical dynamic speckle analysis, methods using dynamic speckle are proposed to measure the velocities of nanoparticles in nanofluids. Since the Zero-Crossing method has two advantages that the optical system is very simple and that the measurements of the object velocity are carried out in real time, the principle of Zero-Crossing method is described, and the relationship between the numbers of Zero-Crossing per second of speckle intensity fluctuation and the velocities of the object is obtained.In order to remove the influences of noise in the signals and the baseline drift on the Zero-Crossing numbers, the signals are de-noised by wavelet transform and processed by subtracting the linear fitting values. The obtained signals are verified by calculating the velocity of moving nanofluids. Then, the relationship between velocities of nanoparticles and average velocities of the directional nanofluids flow is obtained by statistical experimental results, which obtains the conclusion that nanoparticles show the super following feature in nanofluids, which suggests it's feasible and valuable method for measuring directional moving fluids. Moreover, in order to measure the direction of the nanofluids flow, the Zero-Crossing method used by TEM10 mode-like laser is proposed and it is verified reliable by experimental results.The results of this research may provide theoretical and experimental references for researches on measuring the velocity and direction of moving fluid and understanding of the mechanisms of enhanced heat transfer in nanofluids.