279135 Magnetic Resonance Imaging (MRI) of Jets in Gas-Solid Systems
Owing to their excellent heat and mass transfer characteristics, fluidized beds have various applications in industry ranging from the cracking of hydrocarbons over the gasification of carbonaceous fuels, to the coating of tablets. However, since gas-fluidized beds are visually opaque it is intrinsically difficult to perform measurements in these systems. Only a few measurement techniques exist that can probe non-intrusively the dynamics of gas-fluidized beds, such as positron emission particle tracking (Stein et al. 1997), electron capacitance tomography (Du, Waristo and Fan, 2006), X-ray attenuation (Rowe and Yocono, 1976) and magnetic resonance imaging (MRI) (Müller et al. 2008).
In this study we apply magnetic resonance imaging to image the formation of jets in gas-solid systems. MRI is an imaging technique that is able to measure both the solids' distribution and the velocity of the solids in granular systems.
In this study beds, constructed of PMMA, of square cross section (L = 46.8 mm and L = 72 mm) were used. The orifice was also of square geometry with Lo=3.6 mm. The following MRI detectable seeds were used as particles: (i) Iceland poppy seeds dp = 0.74 ± 0.08 mm,(ii) opium poppy seeds dp = 0.94 ± 0.04 mm, and (iii) mustard seeds dp = 1.34 ± 0.18 mm).
MRI measurements were performed in a Philips 3T Achieva system equipped with dual Quasar gradients. The maximal gradient strength and slew rate were 80mT/m and 200 mT/m/ms, respectively. A spin echo sequence of repetition time 1500 ms, echo time 12.4 ms and flip angle 60º was used. The voxel size was 1 mm x 1 mm x 1 mm.
Müller et al. (2009) found that the start-up procedure crucially influenced jet heights. In this study two different start up procedures were evaluated: (I) Filling of the bed with particles at a low orifice velocity followed by a stepwise increase of the orifice velocity and (II) filling of the bed at a high orifice velocity followed by a stepwise reduction of the orifice velocity. The MRI measurements revealed that the jet height differed by a factor of two between the two start -up procedures investigated. Subsequently all experiments were performed using procedure (II) since this procedure was determined as more relevant for practical applications and showed an excellent reproducibility.
Further measurements were performed to determine the jet length as a function of (i) fill level, (ii) particle diameter, (iii) bed dimensions and (iiii) orifice velocity. It was observed that the reduction of the orifice velocity led to a rapid transition from a spouting bed to a stable jet in a packed bed, of jet height approximately half the height of the bed. Subsequently, the jet height decreased roughly linearly with decreasing orifice velocity. Interestingly, the fill level did not influence the jet height. However, with increasing fill level the transition to a spouting bed shifted to higher orifice velocities. As expected the jet height increased with increasing orifice velocity and decreased with increasing particle diameter. Additional, in particular for the larger particles, a substantial difference in the jet height (at a given orifice velocity) was observed for beds with different cross sectional areas. Based on the MRI measurements we postulate that once a critical ratio of bed dimension to particle diameter of approximately 70 is obtained, the jet length becomes independent of the bed dimensions, i.e. above that critical ratio the walls of the bed do not affect the jet height.
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