Dash Amitosh, Hogendoorn Willian, Poelma Christian
Multiphase Systems (Process and Energy) Mechanical, Maritime and Materials Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands.
Exp Fluids. 2021;62(4):85. doi: 10.1007/s00348-020-03132-0. Epub 2021 Mar 31.
We discuss empirical techniques to extract quantitative particle volume fraction profiles in particle-laden flows using an ultrasound transducer. A key step involves probing several uniform suspensions with varying bulk volume fractions from which two key volume fraction dependent calibration parameters are identified: the peak backscatter amplitude (acoustic energy backscattered by the initial layer of the suspension) and the amplitude attenuation rate (rate at which the acoustic energy decays with depth owing to scattering losses). These properties can then be used to reconstruct spatially varying particle volume fraction profiles. Such an empirical approach allows circumventing detailed theoretical models which characterize the interaction between ultrasound and suspensions, which are not universally applicable. We assess the reconstruction techniques via synthetic volume fraction profiles and a known particle-laden suspension immobilized in a gel. While qualitative trends can be easily picked up, the following factors compromise the quantitative accuracy: (1) initial reconstruction errors made in the near-wall regions can propagate and grow along the reconstruction direction, (2) multiple scattering can create artefacts which may affect the reconstruction, and (3) the accuracy of the reconstruction is very sensitive to the goodness of the calibration. Despite these issues, application of the technique to particle-laden pipe flows shows the presence of a core with reduced particle volume fractions in laminar flows, whose prominence reduces as the flow becomes turbulent. This observation is associated with inertia-induced radial migration of particles away from the pipe axis and is observed in flows with bulk volume fractions as high as 0.08. Even transitional flows with low levels of intermittency are not devoid of this depleted core. In conclusion, ultrasonic particle volume fraction profiling can play a key complementary role to ultrasound-based velocimetry in studying the internal features of particle-laden flows.
我们讨论了使用超声换能器在含颗粒流中提取定量颗粒体积分数分布的实验技术。关键步骤包括探测几种具有不同总体积分数的均匀悬浮液，从中确定两个与体积分数相关的关键校准参数：峰值背向散射振幅（悬浮液初始层反向散射的声能）和振幅衰减率（由于散射损耗声能随深度衰减的速率）。然后可以利用这些特性来重建空间变化的颗粒体积分数分布。这种实验方法可以避免使用表征超声与悬浮液之间相互作用的详细理论模型，因为这些模型并非普遍适用。我们通过合成体积分数分布和固定在凝胶中的已知含颗粒悬浮液来评估重建技术。虽然定性趋势很容易识别，但以下因素会影响定量精度：（1）近壁区域的初始重建误差会沿重建方向传播并增大，（2）多次散射会产生可能影响重建的伪像，（3）重建精度对校准的优劣非常敏感。尽管存在这些问题，但将该技术应用于含颗粒管道流时，结果表明在层流中存在颗粒体积分数降低的核心区域，随着流动变为湍流，该核心区域的突出程度会降低。这一观察结果与颗粒因惯性引起的远离管道轴线的径向迁移有关，并且在总体积分数高达0.08的流动中也能观察到。即使是间歇性较低的过渡流也存在这种贫化核心。总之，在研究含颗粒流的内部特征时，超声颗粒体积分数剖面测量可以对基于超声的测速起到关键的补充作用。
