The Study Of Laser Speckle Image Analysis To Measure The Average Velocity Of Nanofluids

At present,nanofluids are a new type of medium with good heat transfer performance in the energy field,and the research on its thermal conductivity is one of the hotspots in the field of energy.Researchers believe that exploring the dynamic characteristics of nanoparticles in fluids is of far-reaching significance for exploring the thermal conductivity mechanism of Nanofluids and improving the thermal conductivity of nanofluids.From the perspective of experimental research and image analysis theory,the average velocity of nanofluids is calculated by analyzing laser speckle images,and then the method of studying the dynamic characteristics of nanoparticles is discussed.The main research contents are as follows:The correlation between laser speckle image and nanoparticles in fluids is analyzed from the optical characteristics of speckles,and the information extracted from the image is used to characterize the dynamic characteristics of nanoparticles in fluids.A method of measuring the flow velocity of nanofluids by using laser speckle image analysis is presented.Based on the traditional simulated optical grating spatial filtering method,an improved simulated optical grating spatial filtering model is established by introducing the method of dividing the interpretation window in cross-correlation algorithm and the principle of gray level conservation in optical flow algorithm.Based on the principle of gray level conservation,the sequence speckle image is transformed into gray value matrix.The sensitivity region is selected by the method of interpretation window.The selected sensitivity region is sampled continuously,and the spectrum characteristic curve is obtained by Fourier transform.The low frequency characteristic terms is separated by wavelet packet decomposition,using inverse transform to reconstruct the spectrum characteristic curve of the separated low-frequency feature terms,the peak frequency of the signal can be obtained.Finally,the average velocity of nanofluids is calculated according to the improved filtering velocity measurement principle.We built the laser speckle circulating flow measurement system for circular pipe channel,and two kinds of nanofluids,water-based copper oxide and water-basedalumina,were prepared and their stability characteristics were evaluated.The factors affecting the image sharpness of dynamic speckle experiment were analyzed by static experiment of laser speckle,including the types of nanofluids,concentration range,laser power and temperature.According to the determined experimental conditions,the corresponding dynamic speckle sequence images of nanofluids are obtained at different flow rates and temperatures,and the sequence image analysis and the calculation of experimental speed are completed.The relative error of the calculation is obtained by comparing with the theoretical calculation speed.From the main distribution range of the errors(1.13%-8.21%)between the two sets of data and the average relative errors of 4.89% and 4.52%,the feasibility of this method is proved.In addition,in order to verify the influence of temperature on the calculation error of average velocity of nanofluids,the relative errors between theoretical calculation speed and experimental calculation speed were compared at temperatures of 20?,40? and 55?.It was verified that in a certain temperature range,the increase of temperature would aggravate Brownian movement in fluids,however,this method has no significant effect on reducing the relative error of the average velocity of nanofluids.