Detailed fluctuation theorem for mesoscopic modeling

The detailed fluctuation theorem is derived. The basic assumptions are phase space incompressibility (Liouville’s theorem) and time reversibility on the microscopic level. The theorem relates the conditional probability to end up in a mesoscopic state [Formula presented] at time [Formula presented], starting from [Formula presented] at time [Formula presented], to the time-reversed process. The ratio of these two probability densities is related to the entropy difference of the two mesoscopic states. The fluctuation theorem remains valid even far from equilibrium as long as the local equilibrium condition is obeyed. It is shown that the theorem imposes constraints on the form mesoscopic equations can take. For stochastic differential equations a generalized kinetic form is derived. The fluctuation theorem can be used to derive thermodynamically consistent simulation techniques. At the end of this paper the relation with the GENERIC formalism is discussed.</p

Elimination of time step effects in DPD

The equilibrium statistics of quantities computed by means of DPD (dissipative particle dynamics) are usually very sensitive to the time step used in the simulation. In this letter we show how to eliminate this sensitivity by considering the irreversible dynamics in DPD as the limiting case of a thermostat that leaves Maxwell-Boltzmann distribution invariant even for finite time steps. The remaining dependency on time step is solely due to the discretization of the conservative part of the dynamics and is independent of the thermostat

Modeling the drying process of porous catalysts:impact of viscosity and surface tension

The distribution of catalytically active species in heterogeneous porous catalysts strongly influences their performance and durability within industrial reactors. A drying model for investigating this redistribution is developed and subsequently the impact of non-linear couplings of liquid viscosity and surface tension on hydrodynamics and redistribution of species during the drying process investigated. The gained insights are then used to exert control of the redistribution within the presented model by solely varying external process parameters

Fluidized bed gas-solid heat transfer using a CFD-DEM coarse-graining technique

Computational Fluid Dynamics - Discrete Element Method (CFD-DEM) is extensively used for modeling heat transfer in gas-solid fluidized beds. However, CFD-DEM is computationally expensive, leading to a restriction regarding the number of simulated particles. Coarse-grained CFD-DEM is a technique to circumvent this constraint, allowing one to simulate larger fluidized beds. In this work, a scaling law used for coarse-graining hydrodynamics is generalized to gas-solid heat-transfer. This approach for coarse-graining heat transfer is tested using three different superficial gas velocities where the coarse-grained particle temperatures and Nusselt numbers are obtained. The particle temperature shows good correspondence with the original system for all cases and the Nusselt number is accurately predicted by the coarse-graining scaling law

Multi-scale simulations for predicting materials properties of a cross-linked polymer

In this paper we aim at predicting material properties of a cross-linked polymer by using multi-scale simulations and to compare the elastic properties and glass transition temperature with experimentally observed values. To that purpose we use an epoxy polymer for which the starting point is a mesoscopic simulation of its cross-linked structure realized by Dissipative Particle Dynamics (DPD) simulations, as recently improved to conserve local densities properly. This results in a coarse-grained structure of this thermoset polymer, relaxed at a large length- and long time-scale. Such a mesoscopic simulation is important as otherwise insufficient relaxation of the structures occurs for a later and proper comparison with experimental properties. Allowing further simulations at the atomistic scale using molecular dynamics (or any other method) to obtain material properties, a reverse-mapping procedure is required to insert atomistic detail into the coarse-grained structures. Hence, an efficient and reliable reverse-mapping procedure was implemented to be able to connect these two types of simulation. For the epoxy polymer chosen, Poisson’s ratio, the elastic modulus, the glass transition temperature and the thermal expansion coefficients of the glassy and rubbery state resulting from the equilibrated reverse-mapped structure, match the experimental values well. Overall, the paper reports a fast and straightforward procedure to bridge a mesoscopic structure to experimentally observed material properties, which can be applied to any system of interest

Experimental study on vibrating fluidized bed solids drying

Enhancing the gas–solid contacting inside a fluidized bed leads to improved solids drying characteristics. This can be achieved by mechanical vibration, resulting in so-called vibro-fluidized beds. In this study, experiments in a pseudo-2D vibro-fluidized bed setup are performed in order to better understand this improved drying behavior. A coupled particle image velocimetry - infrared thermography technique is applied to characterize the local solids velocity and temperature fields. The added vibration results in a significantly increased solids drying rate compared to a traditional gas-fluidized bed due to enhanced meso-scale particle agitation. Furthermore, it is shown that the gas-bubble appearance and the particle temperature standard deviation are highly dependent on the applied vibration amplitude and frequency.Enhancing the gas–solid contacting inside a fluidized bed leads to improved solids drying characteristics. This can be achieved by mechanical vibration, resulting in so-called vibro-fluidized beds. In this study, experiments in a pseudo-2D vibro-fluidized bed setup are performed in order to better understand this improved drying behavior. A coupled particle image velocimetry - infrared thermography technique is applied to characterize the local solids velocity and temperature fields. The added vibration results in a significantly increased solids drying rate compared to a traditional gas-fluidized bed due to enhanced meso-scale particle agitation. Furthermore, it is shown that the gas-bubble appearance and the particle temperature standard deviation are highly dependent on the applied vibration amplitude and frequency

Riser hydrodynamics and cluster characterization by Particle Image Velocimetry (PIV) and Digital Image Analysis (DIA) coupling

We present a recently developed DIA technique that can provide full-field hydrodynamic measurements of a pseudo-2D lab scale riser reactor. The main strength of this DIA technique is the full-field measurement of solids volume fraction under riser flow conditions. It provides high quality data to perform cluster detection and characterization. In depth knowledge of the behavior of clusters is important because these heterogeneities have a large influence on mass transfer phenomena. Riser hydrodynamics have been widely investigated in the last decades. Several experimental techniques have been employed to obtain hydrodynamic data of heterogeneity due to cluster formation in a riser flow. However, cluster-related phenomena are difficult to quantify by most of these techniques that only provide local information or/and are limited by the high number of sensors needed to describe the full flow field of the system. Full-field DIA techniques were previously applied on lab scale units under bubbling fluidization regime, where a calibration with a known solids weight was needed (1). However, under riser flow conditions, this is inherently changing and it lacks a robust methodology to quantify solids concentration for these systems. The conventional full-field DIA techniques also had severe difficulties to filter image imperfections that arise, e.g., due to inhomogeneous lighting. Please click Additional Files below to see the full abstract