Velocity bias in intrusive gas-liquid flow measurements

B Hohermuth,M Kramer,D Valero,S Felder

doi:10.1038/s41467-021-24231-4

Abstract

Gas–liquid flows occur in many natural environments such as breaking waves, river rapids and human-made systems, including nuclear reactors and water treatment or conveyance infrastructure. Such two-phase flows are commonly investigated using phase-detection intrusive probes, yielding velocities that are considered to be directly representative of bubble velocities. Using different state-of-the-art instruments and analysis algorithms, we show that bubble–probe interactions lead to an underestimation of the real bubble velocity due to surface tension. To overcome this velocity bias, a correction method is formulated based on a force balance on the bubble. The proposed methodology allows to assess the bubble–probe interaction bias for various types of gas-liquid flows and to recover the undisturbed real bubble velocity. We show that the velocity bias is strong in laboratory scale investigations and therefore may affect the extrapolation of results to full scale. The correction method increases the accuracy of bubble velocity estimations, thereby enabling a deeper understanding of fundamental gas-liquid flow processes.

Highlights

Gas–liquid flows occur in many natural environments such as breaking waves, river rapids and human-made systems, including nuclear reactors and water treatment or conveyance infrastructure
The two needle tips of a double-tip probe are separated by a distance Δx in probe-wise direction and changes in physical properties, such as electric resistance or optical refraction, are synchronously sampled
Measured velocities may be subject to different velocity biases, which include (i) statistical velocity bias due to the fact that more particles impact the probe tips at high velocities[17,18], (ii) velocity bias due to the misalignment of the probe tips with flow streamlines[19] and (iii) velocity bias due to particle-probe interaction[20]

Summary

Introduction

Gas–liquid flows occur in many natural environments such as breaking waves, river rapids and human-made systems, including nuclear reactors and water treatment or conveyance infrastructure. In highly aerated open-channel flows, the void fraction ranges from almost zero to unity and the most probable travel time of gas–liquid interfaces is typically obtained through a crosscorrelation analysis, allowing the estimation of mean velocities, averaged over the sampling period. Bias (iii) is due to interactions between dispersedphase particles (bubbles/droplets) and the probe tips Different mechanisms such as blinding, drifting and crawling have been recognized to affect the measured properties at low bubble Reynolds numbers (Reb ≲ 102)[20,23,24,25,26], while impact and crawling forces are anticipated to dominate in high Reynolds number flows (Reb ≳ 103). The blinding effect primarily leads to an underestimation of void fraction[24] and the effects of drifting are similar to the misalignment bias (ii)

Methods

Results

Conclusion