In this study, the flow behavior of deionized water through the staggered circular micro pin fin arrays with three longitudinal spacings (S-L = 2D, 3D and 4D) is investigated using flow visualization technology of micro particle image velocimetry (micro-PIV) in the range of Re = 100-800. The streamline distribution and velocity field in the three micro pin fin arrays are obtained. Experimental results indicate that the longitudinal spacing has considerable effect on both the extension of the wake region and velocity field around pin fins. The small longitudinal spacing hinders the extension of the wake region behind the pin fin and delays the vortex shedding. The micro pin fin array with S-L = 2D provides maximum velocity span and transverse velocity, indicating intense local fluid mixing. The flow and heat transfer characteristics in the microchannel with a single circular micro pin fin are also studied. By comparison, the feature of the wake region in the micro pin fin array with a large longitudinal spacing is similar to that in the flow past a single micro pin fin. Moreover, vortex shedding occurs in the micro pin fin array at higher Reynolds number. The correlation between velocity field and temperature field around the pin fin is investigated. The belt zone with enhanced heat transfer around the pin fin is consistent with the distribution of fluid with high velocities. Vortex shedding can obviously enhance the heat transfer downstream of the micro pin fin.

A novel measurement system for mixing property of binary mixtures in three-dimensional fluidized beds is developed based on capacitance probe method. The mixing processes at multi-positions of the bed are acquired simultaneously. A new dispersion coefficient is proposed to characterize the local dispersion of particles and a new mixing index is proposed to evaluate the local mixing quality in three-dimensional fluidized beds. The effect of convection and diffusion mechanism on particle mixing is discussed separately. Results show that the governing mechanism of particle mixing at the center and top of the beds is convection; meanwhile the governing mechanism for particle mixing at the bottom and near the wall is diffusion. The radial dispersion coefficient at the half-radius of the bed is mainly between 0.0038 and 0.026 m2/s, which is about 1.5 times that near the wall. The vertical dispersion coefficient is about 2.5 times that the radial dispersion coefficient.

A novel measurement system for radial particle mixing in annular fluidized beds is designed on the basis of the capacitance probe method. Mixing parameters at different radial positions are acquired. The effects of the convection/diffusion mechanism on radial mixing are analyzed individually. It is found that the governing mechanism of mixing at the axial line is convection; meanwhile, diffusion is the governing mechanism of mixing near the wall. The effect of convection on radial mixing at the upper part is more important than that at the lower part. The radial dispersion coefficient ranges from 0.006 to 0.072 m2/s. At the upper part, the radial dispersion coefficient at half-radius is between 0.016 and 0.072 m2/s and that near the wall is 0.006 and 0.028 m2/s, which is four times and twice that at the same radial position and at the lower part, respectively. The radial dispersion coefficient is about 1.5 times that in two-dimensional fluidized beds.

Measuring the particle mixing parameters at multi positions in three-dimensional fluidized beds continuously remains a challenging task. A novel measurement method for the mixing and segregation of particles inside three-dimensional fluidized beds is developed based on capacitance probe. The measurement error is generally below 7%. The particle mixing parameters and dispersion coefficients at multi-positions of the three-dimensional fluidized bed are acquired. The effect of convection and diffusion mechanism on particle mixing is discussed. Results show that the governing mechanism of particle mixing at center and top of the bed is convection; meanwhile the governing mechanism for particle mixing at bottom and near the wall is diffusion. The radial dispersion coefficient at half-radius of the bed is mainly between 0.0038 and 0.026 m2/s, which is about 1.5 times that near the wall. The axial dispersion coefficient is mainly between 0.004-0.056 m2/s, which is about 2.3 times that in radial direction.

基于显微粒子成像测速(Micro-PIV)技术,对微通道内单柱绕流特性展开实验研究,分析了10＜ Re＜ 350范围内不同高度流层的速度场、涡量场及旋涡特性.结果 表明:微尺度绕流现象相比宏观尺度存在滞后,首次出现旋涡的第一临界Re约为10.随着Re的增大,尾流区长度不断增加,旋涡尺度逐渐增大,旋涡中心位置向下游延伸.涡量强度随Re的增加而提高,涡量向下游扩散能力增强,高涡量区变窄.不同高度流层的速度场与涡量场存在差别,体现出三维效应.