Alexander Penn

Last Name

Penn

First Name

Alexander

ORCID

0000-0001-5596-6310

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Publications 1 - 10 of 37

Magnetic Resonance Imaging and Modeling of Liquid-Solid Fluidization
Boyce, Christopher M.; Penn, Alexander; Prüssmann, Klaas P.; et al. (2018)
8th World Congress on Particle Technology Proceedings
Regimes of jetting and bubbling in a fluidized bed studied using real-time magnetic resonance imaging
Penn, Alexander; Boyce, Christopher M.; Prüssmann, Klaas P.; et al. (2020)
Chemical Engineering Journal
Real-time magnetic resonance imaging was used to study the different flow regimes which occur in a fluidized bed containing a gas injection system. The gas flow rates through the main distributor and a central orifice were varied independently. We identified six different regimes of bubbling and jetting behavior: (1) freely bubbling, (2) permanent jet, (3) spouting, (4) pulsating jet, (5) pulsating jet with bubble collapse and (6) pulsating jet and freely bubbling. While regimes (1–4) have been described previously in the literature, regimes (5) and (6) are described here for the first time. To construct a regime map, the Froude number (Fr) and the ratio of the superficial gas velocity to the minimum fluidization velocity (U/Umf) were used to describe the system. We observed that bubbles formed predominantly, when U/Umf > 1. Further, we propose an empirical model that predicts the length of jets in the permanent jet regime as a function of Fr and background gas flow as (see equation in the version of the publisher). The proposed model is in good agreement with tomographic measurements in smaller 3D systems reported in the literature, indicating that the non-dimensionalized description of jet length using a Fr number is valid throughout a large range of system diameters. Moreover, the bubble breakoff frequency of the pulsating jet regime was assessed by Fourier analysis, demonstrating that the frequency increases with increasing Fr before plateauing.
Detection of Fluidization with Real-Time Magnetic Resonance Imaging
Penn, Alexander; Boyce, Christopher M.; Prüssmann, Klaas P.; et al. (2018)
8th World Congress on Particle Technology Proceedings
Rayleigh-Taylor Instability in Vibrated Gas-Fluidized Particles
Boyce, Christopher M.; McLaren, Christopher; Kovar, Thomas; et al. (2019)
2019 AIChE Annual Meeting Proceedings
A Rayleigh-Taylor-like instability1,2 forms in a binary set of particles of different density subject to upward gas flow and vertical vibration3. â€œFingersâ€ and â€œbubblesâ€ of light particles rise through heavy particles despite a lack of effective surface tension between the two types of particles. CFD-DEM simulations show that gas flow channels through fingers of lighter, larger particles due to a higher permeability of these particles to gas flow, locally increasing drag and causing these fingers to grow3. We demonstrate that the same physical mechanism causes an isolated â€œbubbleâ€ of light grains to rise and an isolated â€œdropletâ€ of heavy grains to break up forming a binary branching pattern3. We explore how the combination of vibration and gas flow causes a bubble-free fluidization state in Geldart Group B and D particles and how this state can lead to other structured flow patterns.
Real-time magnetic resonance imaging of fluidized beds with internals
Penn, Alexander; Boyce, Christopher M.; Conzelmann, Nicholas; et al. (2019)
Chemical Engineering Science
Temperature distribution in a gas-solid fixed bed probed by rapid magnetic resonance imaging
Serial, M. Raquel; Benders, Stefan; Rotzetter, Perrine; et al. (2023)
Chemical Engineering Science
Controlling the temperature distribution inside catalytic fixed bed reactors is crucial for yield optimization and process stability. Yet, in situ temperature measurements with spatial and temporal resolution are still challenging. In this work, we perform temperature measurements in a cylindrical fixed bed reactor by combining the capabilities of real-time magnetic resonance imaging (MRI) with the temperature-dependent proton resonance frequency (PRF) shift of water. Three-dimensional (3D) temperature maps are acquired while heating the bed from room temperature to 60°C using hot air. The obtained results show a clear temperature gradient along the axial and radial dimensions and agree with optical temperature probe measurements with an average error of +/- 1.5°C. We believe that the MR thermometry methodology presented here opens new perspectives for the fundamental study of mass and heat transfer in gas–solid fixed beds and in the future might be extended to the study of reactive gas–solid systems.
Link between packing morphology and the distribution of contact forces and stresses in packings of highly nonconvex particles
Conzelmann, Nicholas; Penn, Alexander; Partl, Manfred N.; et al. (2020)
Physical Review E
An external load on a particle packing is distributed internally through a heterogeneous network of particle contacts. This contact force distribution determines the stability of the particle packing and the resulting structure. Here, we investigate the homogeneity of the contact force distribution in packings of highly nonconvex particles both in two-dimensional (2D) and three-dimensional (3D) packings. A recently developed discrete element method is used to model packings of nonconvex particles of varying sphericity. Our results establish that in 3D packings the distribution of the contact forces in the normal direction becomes increasingly heterogeneous with decreasing particle sphericity. However, in 2D packings the contact force distribution is independent of particle sphericity, indicating that results obtained in 2D packings cannot be extrapolated readily to 3D packings. Radial distribution functions show that the crystallinity in 3D packings decreases with decreasing particle sphericity. We link the decreasing homogeneity of the contact force distributions to the decreasing crystallinity of 3D packings. These findings are complementary to the previously observed link between the heterogeneity of the contact force distribution and a decreasing packing crystallinity due to an increasing polydispersity of spherical particles.
Asynchronous bubble pinch-off pattern arising in fluidized beds due to jet interaction: A magnetic resonance imaging and computational modeling study
Penn, Alexander; Padash, Azin; Lehnert, M.; et al. (2020)
Physical Review Fluids
Rapid magnetic resonance imaging is used to study the interaction between two gas jets injected into a 3D incipiently fluidized bed. At large separation distances and in cases with larger particles, bubbles pinch off from the two jets simultaneously with one another. At small separation distances with smaller particles, a jet grows at one orifice while a jet pinches off to form a bubble at the other orifice, resulting in bubbles pinching off the two jets nearly completely out-of-phase from one another. Discrete particle simulations coupled with computational fluid dynamics reproduce these two patterns. Simulations demonstrate that the asynchronous pattern emerges due to drag forces on the particles causing particle inertia dominate dissipation, causing motion of particles between the jets to become unstable. Specifically, when one jet is growing, it forces particles to move toward the neighboring jet, causing a bubble to break off from the neighboring jet.
Magnetic Resonance Imaging of Wet Fluidization
Boyce, Christopher M.; Penn, Alexander; Prüssmann, Klaas P.; et al. (2017)
Real-Time Magnetic Resonance Imaging of Bubble Behavior and Particle Velocity in Fluidized Beds
Penn, Alexander; Boyce, Christopher M.; Kovar, Thomas; et al. (2018)
Industrial & Engineering Chemistry Research

Publications 1 - 10 of 37

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