arXiv subjects

Holger Grosshans

Publications and source records attributed to Holger Grosshans.

At least 19 recordsLinked to original sources

arXiv2026

Divergent Impact Charging of Polymer Particles

When a particle contacts a surface of another material, it is commonly believed that the particle acquires an impact charge that scales inversely with its pre-impact charge and whose polarity is set by the materials. We show that this belief holds for conductive particles but fails for polymers. For polymers, the impact charge increases linearly with the particle's pre-impact charge. Its polarity is not determined by the materials but by the pre-impact particle charge relative to a divergence point at which the net charge transfer reverses. We attribute this divergence to the attraction of surrounding ions to the particle surface. These attracted ions carry polarity opposite to that of the particle, and their amount scales with the particle charge. They transfer to the opposing surface during contact, thereby defining the impact charge. We propose a phenomenological model for the divergent impact charge arising from this mechanism. Finally, we reexamine previous measurements and show that they support this mechanism.

cond-mat.soft↗

arXiv2026

Effect of Reynolds number on triboelectric particle charging in turbulent channel flow

Triboelectric charging in particle-laden flows is a complex interplay of fluid and particle dynamics, collision mechanics, and electrostatics. In this study, we introduce triboFoam, an open-source solver built on the OpenFOAM framework, designed to simulate triboelectric charging in particle-laden turbulent flows. We validate triboFoam using Direct Numerical Simulations (DNS) of a fully developed turbulent channel flow at a friction Reynolds number of $Re_τ= 180$. The results demonstrate good agreement with DNS data for particle concentration profiles and charge distributions. Then, we investigate the influence of Reynolds number on particle distribution and charging behaviour using Large-Eddy Simulations (LES) at varying friction Reynolds numbers up to $Re_τ= 550$. Our findings reveal that higher Reynolds numbers lead to increased near-wall particle concentrations and enhanced charging rates, attributed to intensified turbulent fluctuations and elevated impact velocities. Finally, an empirical correlation is proposed to predict the average particle charging rate as a function of Reynolds number and particle diameter. With this work, we provide a tool for simulating triboelectric charging in complex geometries and turbulent flows, advancing the understanding of electrostatic phenomena in particle-laden systems. The empirical correlation offers practical insights for predicting charging behaviour in industrial applications and thus can contribute to improved safety and efficiency in processes involving particulate matter.

physics.flu-dyn↗

arXiv2026

The effect of discontinuous injection on particle back-flow in pneumatic conveying systems

Pneumatic conveying is used in many process industries to transport dry, granular, and powdered solids. The triboelectrification of particles during conveying causes particle agglomeration, spark discharges, and disruptions in particle flow, making particles move upstream against the fluid flow. The effect of frequency of particle injection on particle backflow is studied using CFD-DEM simulations. Conveying flow in a square-shaped duct with fluid frictional Reynold's number equal to 180, particle Stokes number equal to 8, and individual particle charge equal to 504 fC, is simulated with different particle injection frequencies. The proportion of particles moving upstream is found to increase as the delay period between injections increases, and the effect of the length of the injection period is minimal. Further, particles moving upstream are situated in low-drag zones at the corners of the duct where the electrostatic force dominates. In conclusion, the delay period between discontinuous injections plays a major role in particle backflow. The findings of the article are important for industrial processes with discontinuous injection of particles with a risk of particle accumulation within the conveying boundary.

physics.flu-dyn↗

arXiv2026

CFD study of particle back-flow in pneumatic conveying systems due to triboelectrification

In industrial plants, pneumatic conveying systems are often used to convey particles from one location to another. Surprisingly, in bench-scale experiments, clusters of particles sometimes flow backward or upstream in the conveying channel. In this paper, the effect of electrostatic charge and forces on particle back-flow was investigated. Different conveying conditions with varying particle charges were simulated using computational fluid dynamics (CFD), and the resulting flow patterns were compared with CFD simulations of uncharged particles. In a channel flow with periodic boundary conditions in the streamwise and spanwise directions, it was found that electrostatic forces drive particles into low-velocity regions but do not reverse their flow. When transporting particles through a finite-length duct, electrostatic forces cause particles to settle close to the duct's inlet. Finally, when particles were injected into the duct in a pulse, back-flow was observed once particles obtained a charge of 5.04 femto-coulombs or more. The electrostatic forces decelerated the particles at the tail of the pulse and ultimately reversed their direction, whereas the particles at the head of the pulse were accelerated. Thus, it was concluded that electrostatic forces can cause particle back-flow in pneumatic conveying systems if particles are fed discontinuously.

physics.flu-dyn↗

arXiv2026

ML enhanced measurement of the electrostatic charge distribution of powder conveyed through a duct

The electrostatic charge acquired by powders during transport through ducts can cause devastating dust explosions. Our recently developed laser-optical measurement technique can resolve the powder charge along a one-dimensional (1D) path. However, the charge across the duct's complete two-dimensional (2D) cross-section, which is the critical parameter for process safety, is generally unavailable due to limited optical access. To estimate the complete powder charge distribution in a conveying duct, we propose a machine learning (ML) approach using a shallow neural network (SNN). The ML algorithm is trained with cross-sectional data extracted from four different three-dimensional direct numerical simulations of a turbulent duct flow with varying particle size. Through this training with simulation data, the ML algorithm can estimate the powder charge distribution in the duct's cross-section based on only 1D measurements. The results reveal an average $L^1$-error of the reconstructed 2D cross-section of 1.63 %.

physics.flu-dyn↗

arXiv2025

Unifying same- and different-material particle charging through stochastic scaling

Triboelectric charging of insulating particles through contact is critical in diverse physical and engineering processes, from dust storms and volcanic eruptions to industrial powder handling. However, many experiments over the years have consistently revealed counterintuitive charging patterns, including variable impact charge under identical conditions, charge sign reversal with repeated impacts, and bipolar charging of differently sized particles. Existing computational models cannot predict these patterns; they either rely on oversimplified heuristics or require inaccessible detailed surface properties. We present a stochastic scaling model (SSM) for particle charging that unifies same-material (particle-particle) and different-material (particle-wall) charging in a single theoretical framework. The model grounds in a physics-based stochastic closure by the mean, variance, skewness, and minimum impact charge measured in a highly-controlled reference experiment. To test the SSM, we implemented it in an open-source Lagrangian-Eulerian CFD solver. When simulating 300 000 insulating particles transported by turbulent wall-bounded flows, the SSM takes less than 0.01% of the CPU time. By scaling the statistical parameters of the reference impact to each collision, the new model reproduces the complex charging patterns observed in experiments without requiring surface-level first-principles inputs. The SSM offers a physically grounded route to large-scale simulations of electrostatic effects across many fields of particle-laden flows.

physics.comp-ph↗

arXiv2025

Secondary flows drive triboelectric powder charging in pneumatic conveying

Highly resolved simulations reveal the fundamental influence of a carrier fluid's flow dynamics on triboelectric powder charging. We found that particles transported through a square-shaped duct charge faster than in a channel flow caused by secondary flows that led to more severe particle-wall collisions. Specifically, particles with a Stokes number of 4.69 achieve 85 % of their equilibrium charge approximately 1.5 times faster in duct flow than in channel flow. Also, charge distribution is more uniform in a duct cross-section compared with a channel cross-section. In channel flow, particles are trapped near the walls and collide frequently due to limited movement in the wall-normal direction, causing localized charge buildup. In contrast, duct flow promotes better mixing through secondary flows, reducing repeating collisions and providing uniform charge distribution across the cross-section. Upon charging, electrostatic forces significantly reshape particle behaviour and distribution. Once the powder achieves half of its equilibrium charge, particles increasingly accumulate at the wall, leading to a reduced concentration in the central region. These changes in particle distribution have a noticeable impact on the surrounding fluid phase and alter the overall flow dynamics. These findings open the possibility for a new measure to control powder charging by imposing a specific pattern.

physics.flu-dyn↗

arXiv2025

A Novel Apparatus For Particle-Particle Single Contact Electrification Experiments

The experiment of a single contact between two sub-centimeter high-speed particles is often difficult to execute, especially if the collision must be physically and electrically isolated, as is the case for triboelectrification studies. Apparatuses designed for this type of experiment fall short of providing high-speed isolated collisions with a high probability of contact. In this article, we propose a novel apparatus that combines pneumatic conveying and acoustic levitation to provide an electrically and physically isolated, high impact speed collision between two sub-centimeter particles with a collision success rate of 93 %. We can control the pre-contact charge, material, and size of both particles, and the impact speed and angle. Test results show that charge transfer between two insulator particles is not solely driven by contact potential difference; it is a stochastic process that requires large datasets to resolve and understand. Our new apparatus can efficiently generate these datasets and provide new insights on the stochastic nature of charge transfer, and the effect of each of the collision parameters mentioned earlier on particle-particle charge transfer.

physics.ins-det↗

arXiv2025

Influence of the particle distribution on dust explosions in the 20 L sphere

It is essential to standardize the safety characteristics of dust explosions to mitigate their impact on the process industries. The 20 L sphere primarily investigates the safety characteristics, namely explosion pressure (P_ex) and the rate of pressure rise ((dP/dt)_ex), of dust explosions at the laboratory level. Ensuring uniform dust distribution inside the sphere is essential for accurate data acquisition and standardization. However, whirls created by the incoming flow through the nozzle yield particles to concentrate near the wall before ignition. This study simulated the explosion inside a 20 L sphere to investigate the impact of near-wall particle concentration on the safety characteristics. The OpenFOAM model based on the Euler-Lagrangian approach was benchmarked against experimental data of lycopodium dust explosions. A novel radial homogeneity parameter Phi (0 <= Phi <= 1) quantifies the near-wall particle concentration. The parameter Phi is calculated using a power law based on the radial component of particle coordinates, where Phi = 1 indicates a uniform distribution, and Phi = 0 represents all particles concentrating on the wall. Different particle distributions (Phi = 0.1, 0.2, ..., 1) are initiated before ignition. As Phi decreases from 1, P_ex and (dP/dt)_ex first decrease, but beyond a certain point, both parameters increase. At Phi = 0.1, both P_ex and (dP/dt)_ex reach their highest values, which are 1.75% and 10.1% higher than the uniform distribution, respectively. The lowest values arise at Phi = 0.7, with reductions of 0.25% and 5.6% compared to the uniform distribution. Thus, high near-wall concentrations enhance explosion intensity, while moderate concentrations result in lower intensity than the uniform distribution.

physics.flu-dyn↗

arXiv2025

Triboelectric Charging Model for Particles with Rough Surfaces

The triboelectric charging of particles depends on the contact area of the particle and the contacting surface. Even though the surface topology determines the real contact area, particle charging models do not account for surface roughness. In this paper, we combine contact mechanics and triboelectrification models to predict the charging of rough particles. First, a laser confocal microscope measured the statistical descriptors of polyethylene (PE) particles surface topology. Then, we descriptors particle surfaces by distributing spheroidal asperities on the smooth particle core until the surface roughness reaches the measured value. The Hertz contact mechanics model predicts the deformation of the asperity-covered particle and the resulting real contact area in dependence on impact velocity. Finally, we introduced the real contact area into the condenser model for triboelectric particle charging. The accuracy of the new model predictions was demonstrated by comparing it to a more complex surface reconstructions that account for the fractal surface topology. Furthermore, the model's predicted particle saturation charges agree well with our shaker experiments and with experimental data in the literature on the charging of plane surfaces. The developed triboelectric charging model for particles with rough surfaces is simple and requires only standard descriptors of the surface topology; thus, it suits large-scale simulations of electrifying powder flows.

cond-mat.soft↗

arXiv2024

Unmasking hidden ignition sources: A new approach to finding extreme charge peaks in powder processing

Powders acquire a high electrostatic charge during transport and processing. Consequently, in the aftermath of dust explosions, electrostatic discharge is often suspected to be the ignition source. However, definite proof is usually lacking since the rise of electrostatic charge cannot be seen or smelled, and the explosion destroys valuable evidence. Moreover, conventional methods to measure the bulk charge of powder flows, such as the Faraday pail, provide only the aggregate charge for the entire particle ensemble. Our simulations show that, depending on the flow conditions, contacts between particles lead to bipolar charging. Bipolar charged powder remains overall neutral; thus, a Faraday pail detects no danger, even though individual particles are highly charged. To address this gap, we have developed a measurement technology to resolve the powder charge spatially. The first measurements have revealed a critical discovery: a localized charge peak near the inner wall of the conveying duct is 85 times higher than the average charge that would be measured using the Faraday pail. This finding underscores the possibility of extremely high local charges that can serve as ignition sources, even though they remain undetected by conventional measurement systems. Our new technology offers a solution by spatially resolving the charge distribution within powder flows, unmasking hidden ignition sources, and preventing catastrophic incidents in the industry.

physics.app-ph↗

arXiv2024

Spatially resolved measurement of the electrostatic charge of turbulent powder flows

This article reports on measurements of the electrostatic charge of particles in a turbulent duct flow. In contrast to previous charge measurements, which do not apply to turbulent flows or give only the sum of all particles' charges, the new method resolves the charge of a turbulent powder flow spatially. The experiment consists of a Particle Tracking Velocimetry (PTV) system and electrode plates that generate an electric field. By comparing particle velocities and accelerations with and without the electric field, the time-averaged local particle charge profile is derived. Spatially resolving the charge profiles unveiled bipolar particle flow. The average of the charge profiles agreed well with a conventional Faraday pail measurement, demonstrating the accuracy of our measurements. However, the peak value of the charge profiles was 76 times higher than the average of the particles' charge.

physics.flu-dyn↗

arXiv2024

Suppression and Control of Bipolar Powder Charging by Turbulence

Current models predict particles of the same material but different sizes to charge bipolar upon contacts; the resulting charge peaks endanger process safety. However, we found wall-bounded turbulence to suppress the powder's electrostatic charging. Aerodynamic forces skew the collision frequency and narrow the charge distribution's bandwidth. Bipolar charging reduces, especially in moderately polydisperse systems of a low Stokes number. Not the smallest but mid-sized particles charge most negatively. Moreover, turbulence separates charge, producing pockets of high electric potential in low-vorticity regions.

physics.flu-dyn↗

arXiv2024

Influence of the Reynolds number from $Re_τ= 150$ to $210$ on size-dependent bipolar charging

We recently found wall-bounded turbulence to suppress and control bipolar triboelectric charging of particles of identical material. This control is due to fluid modifying the motion of light particles. Thus, the particles' charge distribution depends on their Stokes number distribution. More specifically, fluid forces narrow the bandwidth of the charge distribution, and bipolar charging reduces dramatically. Consequently, not the smallest but mid-sized particles collect the most negative charge. However, the influence of the Reynolds number or particle concentration on bipolar charging of polydisperse particles is unknown. This paper presents the charging simulations of same-material particles the in different wall-bounded flows. In a comprehensive study, we vary the Reynolds number from $Re_τ=$ $150$ to $210$ and the particle number density from $4 \times 10^9 \ \mathrm{m}^{-3}$ to $1 \times 10^{10} \ \mathrm{m}^{-3}$ to further explore the influence of the carrier flow on bipolar charging. We model charge transfer based on the balance of transferable charge species. Such species can represent adsorbed ions transferred during collisions or free electrons captured into a lower energy state on the other surface. The turbulent flow is modeled via Direct Numerical Simulations (DNS) and is coupled to the particulate phase modeled via the Discrete Element Method (DEM). Overall, our multiphysics approach couples the fluid dynamics, electric field, triboelectric charging, and particle momentum into one complex simulation.

physics.flu-dyn↗

arXiv2024

Polarity switch of PMMA powder transported through a PMMA duct

During pneumatic conveying, powder electrifies rapidly due to the high flow velocities. In our experiments, the particles even charge if the conveying duct is made of the same material, which might be caused by triboelectrification between two asymmetric contact surfaces. Surprisingly, we found the airflow rate to determine the polarity of the overall powder charge. This study investigates the charging of microscale PMMA particles in turbulent flows passing through a square PMMA duct. The particles are spherical and monodisperse. A Faraday at the duct outlet measured the total charge of the particles. At low flow velocities, the particles charged negatively after passing through the duct. However, the powder's overall charge switched to a positive polarity when increasing the flow velocity.

cond-mat.soft↗

arXiv2023

Airborne Virus Transmission: Increased Spreading Due to Formation of Hollow Particles

The globally supported social distancing rules to prevent airborne transmission of COVID-19 assume small saliva droplets evaporate fast and large ones, which contain most viral copies, fall fast to the ground. However, during evaporation, solutes distribute non-uniformly within the droplets. We developed a numerical model to predict saliva droplet drying in different environments. We represent saliva droplets as a solution of NaCl mixed with water. In a hot and dry ambiance, the solutes form a shell on the droplets' surface, producing light, hollow particles. These hollow particles have a larger cross-sectional area compared to their solid counterparts and can float longer and travel farther in the air. We introduced the "hollowness factor," which serves as a measure of the ratio of the volume of a hollow particle and the volume of a solid residue formed during droplet drying. Through our investigations, we determined that under specific conditions, namely an ambient humidity level of 10% and a temperature of 40°C, the highest hollowness factor observed was 1.610. This finding indicates that in the case of hollow particle formation, the droplet nucleus expands by a factor of 1.610 compared to its original size.

physics.flu-dyn↗

arXiv2023

Effect of Drag Force Modeling on the Flow of Electrostatically Charged Particles

In CFD simulations of two-phase flows, accurate drag force modeling is essential for predicting particle dynamics. However, a generally valid formulation is lacking, as all available drag force correlations have been established for specific flow situations. In particular, these correlations have not been evaluated for particle-laden flows subjected to electrostatic forces. The paper reports the effect of drag force modeling on the flow of electrically charged particles. To this end, we implemented different drag force correlations to the open-source CFD tool pafiX. Then, we performed highly-resolved Direct Numerical Simulations (DNS) using the Eulerian-Lagrangian approach of a particle-laden channel flow with the friction Reynolds number of 180. The simulations generally revealed a strong influence of the precise drag correlation on particles in the near-wall region and a minor effect on the particles far from the walls. Due to their turbophoretic drift, particles accumulate close to the channel walls. For uncharged particles, the simulations show large deviations of the particle concentration profile in the near-wall region depending on the drag force correlation. Therefore, the disturbance of the flow surrounding a particle by a nearby wall or other particles is important for its drag. Driven by electrostatic forces, charged particles accumulate even closer to the wall. Contrary to the uncharged cases, when the particles carry a high charge (in our case one femto-coulomb), we found minor effects of drag force modeling on particle concentration profiles. In conclusion, for the investigated conditions, we propose to account for the effect of nearby particles and walls on the drag of low- or uncharged particles.

physics.flu-dyn↗

arXiv2022

Recent progress in CFD modeling of powder flow charging during pneumatic conveying

Thus far, Computational Fluid Dynamics (CFD) simulations fail to predict the electrostatic charging of particle-gas flows reliably. The lack of a predictive tool leads to powder operations prone to deposits and discharges, making chemical plants unsustainable and prime candidates for explosions. This paper reviews the rapid progress of numerical models in recent years, their limitations, and outlines future research. In particular, the discussion includes CFD models for the physics and chemistry of particle electrification. The condenser model is most popular today in CFD simulations of powder flow electrification but fails to predict most of its features. New experiments led to advanced models, such as the non-uniform charge model, which resolves the local charge distribution on non-conductive particle surfaces. Further, models relying on the surface state theory predicted bipolar charging of polydisperse particles made of the same material. While these models were usually implemented in CFD tools using an Eulerian-Lagrangian strategy, recently Eulerian methods successfully described powder charging. The Eulerian framework is computationally efficient when handling complete powders; thus, Eulerian methods can pave the way from academic studies to application, simulating full-scale powder processing units. Overall, even though CFD models for powder flow charging improved, major hurdles toward a predictive tool remain.

physics.flu-dyn↗