Magnetic susceptibility based magnetic resonance estimation of micro-bubble size for the vertically upward bubbly flow

Motion-sensitized SPRITE measurements of hydrodynamic cavitation in fast pipe flow

The pressure variations experienced by a liquid flowing through a pipe constriction can, in some cases, result in the formation of a bubble cloud (i.e., hydrodynamic cavitation). Due to the nature of the bubble cloud, it is ideally measured through the use of non-optical and non-invasive techniques; therefore, it is well-suited for study by magnetic resonance imaging. This paper demonstrates the use of Conical SPRITE (a 3D, centric-scan, pure phase-encoding pulse sequence) to acquire time-averaged void fraction and velocity information about hydrodynamic cavitation for water flowing through a pipe constriction.

Magnetic Resonance Imaging measurements of a water spray upstream and downstream of a spray nozzle exit orifice

Sprays are dynamic collections of droplets dispersed in a gas, with many industrial and agricultural applications. Quantitative characterization is essential for understanding processes of spray formation and dynamics. There exists a wide range of measurement techniques to characterize sprays, from direct imaging to phase Doppler interferometry to X-rays, which provide detailed information on spray characteristics in the "far-nozzle" region (≫10 diameters of the nozzle). However, traditional methods are limited in their ability to characterize the "near-nozzle" region where the fluid may be inside the nozzle, optically dense, or incompletely atomized. Magnetic Resonance Imaging (MRI) presents potential as a non-invasive technique that is capable of measuring optically inaccessible fluid in a quantitative fashion. In this work, MRI measurements of the spray generated by ceramic flat-fan nozzles were performed. A wide range of flow speeds in the system (0.2 to >25m/s) necessitated short encoding times. A 3D Conical SPRITE and motion-sensitized 3D Conical SPRITE were employed. The signal from water inside the nozzle was well-characterized, both via proton density and velocity measurements. The signal outside the nozzle, in the near-nozzle region, was detectable, corresponding to the expected flat-fan spray pattern up to 3mm away. The results demonstrate the potential of MRI for measuring spray characteristics in areas inaccessible by other methods.

MR relaxometry of micro-bubbles in the vertical bubbly flow at a low magnetic field (0.2T)

Measurements of the vertical bubbly flow were performed at a low magnetic field of 0.2T. The void fraction data were acquired. The susceptibility-induced changes in T₂ relaxation time were analyzed using the previously introduced approaches by Sukstanskii et al. and Ziener et al., originally developed for the Magnetic Resonance analysis of randomly distributed and isolated spherical inclusions, and a simple model of a spherical particle, respectively. The CPMG signal decay due to the presence of spherical inclusions was approximated as linear vs. CPMG inter-echo times to extract the average inclusion's size information. Two equations were derived for a simplified analysis of gas-liquid systems with basic T₂ measurements, and without prior knowledge on the gas-liquid susceptibility or a need for the magnetic gradient setup. They can provide estimates for the void fraction and the average inclusion size, provided the CPMG inter-echo time requirements are met. For the control samples, there was a good agreement with the theory. For the bubbly flows, a good agreement was observed between the Magnetic Resonance and optics-based estimates for the slowest airflow rate. The deviation, however, increased for higher airflow rates. The introduced approach lends itself to the characterization of multi-phase systems such as cavitating media and well-separated bubbly flows.