Thin Zone Research Articles

Evaluation of Quadrature-based Moment Methods in turbulent premixed combustion

Transported probability density function (PDF) methods are widely used to model turbulent flames characterized by strong turbulence-chemistry interactions. Numerical methods directly resolving the PDF are commonly used, such as the Lagrangian particle or the stochastic fields (SF) approach. However, especially for premixed combustion configurations, characterized by high reaction rates and thin reaction zones, a fine PDF resolution is required, both in physical and in composition space, leading to high numerical costs. An alternative approach to solve a PDF is the method of moments, which has shown to be numerically efficient in a wide range of applications. In this work, two Quadrature-based Moment closures are evaluated in the context of turbulent premixed combustion. The Quadrature-based Moment Methods (QMOM) and the recently developed Extended QMOM (EQMOM) are used in combination with a tabulated chemistry approach to approximate the composition PDF. Both closures are first applied to an established benchmark case for PDF methods, a plug-flow reactor with imperfect mixing, and compared to reference results obtained from Lagrangian particle and SF approaches. Second, a set of turbulent premixed methane-air flames are simulated, varying the Karlovitz number and the turbulent length scale. The turbulent flame speeds obtained are compared with SF reference solutions. Further, spatial resolution requirements for simulating these premixed flames using QMOM are investigated and compared with the requirements of SF. The results demonstrate that both QMOM and EQMOM approaches are well suited to reproduce the turbulent flame properties. Additionally, it is shown that moment methods require lower spatial resolution compared to SF method.

Experimental investigation on ammonia combustion behavior in a spark-ignition engine by means of laminar and turbulent expanding flames

Ammonia combustion appears as a meaningful way to retrieve stored amounts of excess variable renewable energy, and the spark-ignition (SI) engine has been proposed as a practical conversion system. The present work aims at elucidating the combustion characteristics of ammonia blends in engine-relevant turbulent conditions. To that end, laminar and turbulent flame experiments were conducted in a constant-volume vessel at engine-relevant conditions of 445 K and 0.54 MPa to assess the combustion behavior of ammonia/hydrogen/air, ammonia/methane/air and methane/hydrogen/air mixtures observed in an all-metal single-cylinder SI engine. Results show that the respective accelerating or decelerating effects of hydrogen or methane enrichment observed in the SI engine could not be sufficiently explained by the measured laminar burning velocities of the mixtures. Since the latter are very low, the studied combustion regimes are at the boundary between the thin and broken reaction zones regimes, and thus strongly influenced by flame-turbulence interactions. The quantification of the flame response to turbulence shows much higher effects for ammonia blends, than for methane-based fuels. The aforementioned opposite effects of ammonia enrichment with hydrogen or methane are observed on the turbulent burning velocity during the turbulent flame experiments and correlated to the thermochemical properties of the reactants and the flame sensitivity to stretch. The latter may explain an unexpected bending effect on the turbulent-to-laminar velocity ratio when increasing the hydrogen fraction in the ammonia/hydrogen blend. Nevertheless, a very good correlation of the turbulent velocity was found with the Karlovitz and Damköhler numbers, that suggests that ammonia combustion in SI engines may be described following the usual turbulent combustion models. This encourages further investigations on ammonia combustion for the optimization of practical systems, by means of dedicated experiments and numerical simulations.

Modelling of a turbulent premixed flame series using a new MMC-LES model with a shadow position reference variable

A new multiple mapping conditioning model with a shadow position reference variable for localising mixing is implemented in a large eddy simulation solver and validated against three experimental turbulent premixed flames. The shadow position used here has diffusion coefficient which is linked to the turbulent propagation speed. Localisation of mixing produces a flamelet-like inner structure in shadow position space and the structure in physical space is determined by the subgrid dispersion of shadow and physical positions. The lengths of the predicted flames match the trends observed for the experimental flames, and the predicted inner structures are consistent with what is expected for these flames in the thin reaction zones regime. The statistical predictions are in very good quantitative agreement with the experimental data for velocity and major species. Errors for reaction progress variable, turbulent kinetic energy and minor species (carbon monoxide) are generally larger yet mostly within the ranges reported in the literature.

Control of fracture networks on a coastal karstic aquifer: a case study from northeastern Yucatán Peninsula (Mexico)

The Yucatan Peninsula karst aquifer in southeastern Mexico is important because it is the only source of freshwater supply in the region. Along the eastern coast, the aquifer behaves as a shallow unconfined aquifer, and one of its main characteristics is the development of a complex network of karstic conduits. Electrical resistivity tomography (ERT) is a geophysical technique that provides reliable information on aquifer properties that can be applied to karst. With this in mind, ERT surveys (with dipole-dipole and Wenner arrays) together with vertical profiles of groundwater electrical conductivity were obtained in the northeast of Yucatan Peninsula in the Akumal zone. The objective was to evaluate the characteristic structure of the unsaturated and saturated areas, and to determine the importance of factors at a local level that affect the thickness of the freshwater lens, as well as groundwater flow directions near the coast. The results of the ERT sections show a thin vadose zone with resistivity values greater than 1,052 Ωm and a saturated zone with resistivity values that vary from 1 to 1,052 Ωm, and show the presence of dissolution conduits and fracture zones that are affecting the freshwater lens of the aquifer. In addition, water-table measurements show the role of fracture networks in the groundwater flow. The overall results indicate conditions for a possible microbasin in the Akumal zone, highlighting the importance of local fractures, which could work as groundwater divides along the eastern coast of the Yucatan Peninsula.

Deep Residual Networks for Flamelet/progress Variable Tabulation with Application to a Piloted Flame with Inhomogeneous Inlet

ABSTRACT In this work, a deep neural network is presented which is trained on flamelet/progress variable (FPV) tables and validated in a combustion large eddy simulation (LES) of the Sydney/Sandia flame with inhomogeneous inlets. Using data scaling and transformation techniques, as well as novel network architectures as the residual skip connection we are able to store all combustion relevant quantities in one single network, thus reducing effectively the memory footprint compared to the FPV tables, while keeping data retrieval times similar to table interpolation. The accuracy of the deep neural networks (DNN) is compared to its FPV counterpart and achieves excellent agreement. The DNN is also able to accurately predict the stiff progress variable source term in the thin reaction zone. Finally, the DNN thermochemistry representation is validated in combustion simulations of a 2D laminar premixed flame and a 3D LES of the Sydney/Sandia piloted jet flame with inhomogeneous inlet. The DNN results are in very good agreement with the conventional tabulated FPV simulations and show a promising way to efficiently reduce the storage size of high dimensional pretabulated thermochemical state space in reactive flow simulations. Abbreviations: ANN: Artificial neural network; API: Application programming interface; CFD: Computational fluid dynamics; CFL: Courant-Friedrichs-Lewy; CPU: Central processing unit; DL: Deep learning; DNN: Deep neural network; FGM: Flamelet generated manifold; FLOPS: Floating point operation per second; FPV: Flamelet/progress variable; GPU: Graphics processing unit; HPC: High-performance computing; ILDM: Intrinsic low-dimensional manifold; LES: Large eddy simulation; MAE: Mean absolute error; MPI: Message passing interface; MSE: Mean squared error; PDF: Probability density function; SGS: Subgrid scale; SOM: Self-organizing map; TCI: Turbulence–chemistry interaction.

Dr. Gardiner. Dystocia from occlusion of cervix uteri. (Lancet, 26 May, 1894, p. 1300). Obstruction to childbirth due to pharyngeal overgrowth

Dr. G. was invited to the pregnant woman for the second time, on the occasion of pains, which were taken by others for birth, but turned out to be intestinal. On internal examination, the uterine neck turned out to be overgrown; instead of a throat hole, one felt a tubercle, the size of a mustard grain, through which it was impossible to pass the thinnest zones.

A priori DNS study of applicability of flamelet concept to predicting mean concentrations of species in turbulent premixed flames at various Karlovitz numbers

Complex-chemistry direct numerical simulation (DNS) data obtained earlier from lean hydrogen-air flames associated with corrugated flame (case A), thin reaction zone (case B), and broken reaction zone (case C) regimes of turbulent burning are analysed to directly assess capabilities of the flamelet approach to predict mean concentrations of species in a premixed turbulent flame. The approach consists in averaging dependencies of mole fractions, reaction rates, temperature, and density on a single combustion progress variable c, which are all obtained from the unperturbed laminar flame. For this purpose, four alternative definitions of c are probed and two probability density functions (PDFs) are adopted, i.e. either an actual PDF extracted directly from the DNS data or a presumed β-function PDF obtained using the DNS data on the first two moments of the c(x, t)-field. Results show that the mean density and mean mole fractions of H2, O2, and H2O are well predicted using both PDFs for each c, although the predictive capabilities are little worse in case C. In cases A and B, the use of the actual PDF and the fuel-based c also offers an opportunity to well predict mean mole fractions of O and H, whereas the mean mole fraction of OH is slightly underestimated. In the highly turbulent case C, the same approach performs worse, but still appears to be acceptable for evaluating the mean radical concentrations. The use of the β-function PDFs or another combustion progress variable yields substantially worse results for these radicals. When compared to the mean mole fractions, the mean rate of product creation, i.e. the source term in the transport equation for the mean combustion progress variable, is worse predicted even for a quantity (species concentration or temperature) adopted to define c and using the actual PDF. Consequently, turbulent burning velocity is not predicted either.

Decadal Viscoelastic Postseismic Deformation of the 1964 Mw9.2 Alaska Earthquake

AbstractViscoelastic postseismic deformation after the 1964 Mw9.2 Alaska earthquake extends thousands of kilometers from the rupture region and lasts for decades, providing unique opportunities to better understand the three‐dimensional rheological properties of the Alaska subduction zone. We have optimized a three‐dimensional viscoelastic finite element model to study processes that control the postseismic deformation of the 1964 event. The model includes an elastic continental plate and an elastic oceanic plate, a two‐layered viscoelastic oceanic upper mantle, and a uniform viscoelastic mantle wedge. Stress‐driven afterslip is simulated by a thin weak shear zone. The viscoelastic relaxation of the upper mantle and shear zone is represented by the bi‐viscous Burgers rheology. The model has determined the viscosities of the mantle wedge and shear zone to be 3 × 1019 Pa s and 8 × 1016 Pa s, respectively. The afterslip takes place mostly within the first 5 years after the earthquake and is up to 4 m, equivalent to a modeled earthquake of Mw8.5. Model results reveal a spatial and temporal correlation between the afterslip distribution and later slow slip events. The model predicts that the surface deformation about 200 years after the earthquake will be controlled mostly by the relocking of the fault. Further tests on the impact of lateral variation in the mantle wedge viscosity indicate that the viscosity in the continental upper mantle thousands of kilometers from the rupture area may be about an order of magnitude higher than that of the mantle wedge in the subduction zone.

On the mechanisms that sustain the inception of attached cavitation

Abstract

Attached and lifted flame stabilization in a linear array of swirl injectors

Effective flame anchoring maximizes the low-emissions operating window of gas-turbine systems featuring closely spaced, multiple lean flames on single injector units. The complex flame-flow interactions inherent to such designs are not well understood, in particular, at part-load gas turbine conditions. In the present work, large eddy simulation (LES) was used to investigate flames in an optically accessible, confined, linear array of five industrial swirl injector elements. Well-anchored, partially attached and fully lifted flames were observed and three cases corresponding to these were selected for modeling. The dynamically thickened flame (DTF) model was used with a newly developed reduced chemical kinetics scheme that contains low temperature reaction pathways. Chemical explosive mode analysis (CEMA) shows that the flames exhibit thin ignition zones near their base, while a more distributed region of strongly positive modes occur downstream when a transition to partial blow-off takes place. Extinctions are frequent in this region and create broken reaction zones which seem to exhibit both autoignition as well as flamelet characteristics. Further decrease in reactivity leads to a fully lifted flame. It is clear that part-load conditions are best investigated by models that are combustion regime independent.

Numerical Simulation of Methane and Propane Reforming Over a Porous Rh/Al2O3 Catalyst in Stagnation-Flows: Impact of Internal and External Mass Transfer Limitations on Species Profiles

Hydrogen production by catalytic partial oxidation and steam reforming of methane and propane towards synthesis gas are numerically investigated in stagnation-flow over a disc coated with a porous Rh/Al2O3 layer. A one-dimensional flow field is coupled with three models for internal diffusion and with a 62-step surface reaction mechanism. Numerical simulations are conducted with the recently developed computer code DETCHEMSTAG. Dusty-Gas model, a reaction-diffusion model and a simple effectiveness factor model, are alternatively used in simulations to study the internal mass transfer inside the 100 µm thick washcoat layer. Numerically predicted species profiles in the external boundary layer agree well with the recently published experimental data. All three models for internal diffusion exhibit strong species concentration gradients in the catalyst layer. In partial oxidation conditions, a thin total oxidation zone occurs close to the gas-washcoat interface, followed by a zone of steam and dry reforming of methane. Increasing the reactor pressure and decreasing the inlet flow velocity increases/decreases the external/internal mass transfer limitations. The comparison of reaction-diffusion and Dusty-Gas model results reveal the insignificance of convective flow on species transport inside the washcoat. Simulations, which additionally solve a heat transport equation, do not show any temperature gradients inside the washcoat.

Theoretical and numerical investigations of swirl enhanced short natural draft dry cooling towers

A new draft equation involving swirling motions for natural draft dry cooling towers (NDDCTs) is derived and solved analytically. A 2-D axisymmetric model for a short NDDCT is built and computational fluid dynamics (CFD) simulations are carried out to verify the theoretical predictions. Results show that the optimized location for swirl input is at the tower outlet to avoid the swirl decay. It is noted that the swirl influence on the air flow draft velocity gradually becomes significant especially when the dimensionless input swirl ratio exceeds 2. The theoretical predictions generally agree with the numerical results, but deviate when: (1) vortices adjacent to the wall occur in the presence of excessively strong swirl, in the case with swirl created by a thin source zone; (2) cold air inflow penetrates due to the significant heat exchanger resistance coefficient; (3) vortex breakdown appears, in the case with swirl filling the whole tower. All the aforementioned unfavourable phenomena are local effects and thus cannot be predicted by the draft equation unless proper local resistance terms are added.

COMPARATIVE EVALUATION OF RADICULAR DENTINE THICKNESS IN DANGER ZONE OF MANDIBULAR FIRST MOLAR AFTER INSTRUMENTATION WITH TWO DIFFERENT ROTARY SYSTEMS USING CBCT - AN INVITRO STUDY

Introduction: The mandibular first molar poses a series of anatomic challenges such as multiple canals, isthmuses, lateral canals, and apical ramifications. Additionally, the distal surface of mesial root presents a thin zone, which is 4-6 mm cervical to canal orifice with a thin area of dentin termed as “danger zone,” as it increases the risk of perforation of furcal dentin during mechanical instrumentation.
 Aim: To measure the amount of the dentin removed after biomechanical preparation with XP-Endoshaper and Neoendo flex files in the danger zone of the mandibular first molar using CBCT.
 Methodology: Twenty-four mandibular first molars, the mesial roots of which presented a curvature of 20-30 degrees, were selected using the Schneider method. Samples were instrumented with Neoendo rotary file system (n=12) and XP-Endoshaper rotary single file system (n=12). The samples were scanned using CBCT before and after instrumentation. The analyzed parameter includes the remaining dentin thickness at the danger zone of the mandibular 1st molar. The obtained data were subjected to statistical analysis.
 Results: There was no significant difference in remaining dentine thickness between the two rotary systems (p > 0.05).
 Conclusion: XP-Endoshaper revealed better results than Neoendo rotary files. However, both rotary file systems showed no significant difference in the remaining dentin thickness in the danger zone of the mandibular first molar.
 Keywords: Danger zone, Remaining dentine thickness, XP EndoShaper, NeoEndo, CBCT.

The role of differential diffusion during early flame kernel development under engine conditions - part I: Analysis of the heat-release-rate response

Although experimental evidence for the correlation between early flame kernel development and cycle-to-cycle variations (CCV) in spark ignition (SI) engines was provided long ago, there is still a lack of fundamental understanding of early flame/turbulence interactions, and accurate models for full engine simulations do not exist. Since the flame kernel is initiated with small size, i.e. with large positive curvature, differential diffusion is expected to severely alter early flame growth in non-unity-Lewis-number (Le ≠ 1) mixtures as typically used in engines. In this work, a DNS database of developing iso-octane/air flame kernels and planar flames has been established with flame conditions representative for stoichiometric engine part-load operation. Differential diffusion effects on the global heat release rate are analyzed by relating the present findings to equivalent flames computed in the Le=1 limit. It is shown that in the early kernel development phase, the normal propagation velocity is significantly reduced with detrimental consequences on the global burning rate of the flame kernel. Besides this impact on the overall mass burning rate, the initial production of flame surface area by the normal propagation term in the flame area balance equation is noticeably reduced. By using the optimal estimator concept, it is shown that strong fluctuations in local heat release rate inherent to Le ≠ 1 flames in the thin reaction zones regime are mainly contained in the parameters local equivalence ratio, enthalpy, and H-radical mass fraction. Differential diffusion couples the evolution of these parameters to the unsteady flame geometry and structure, which is analyzed in Part II of the present study (Falkenstein et al., Combust. Flame, 2020).

Applicability of extrapolation relations for curvature and stretch rate dependences of displacement speed for statistically planar turbulent premixed flames

A three-dimensional Direct Numerical Simulation (DNS) database of statistically planar turbulent premixed flames covering the wrinkled/corrugated flamelets and the thin reaction zones regimes of premixed turbulent combustion has been utilised to assess the performances of the extrapolation relations, which approximate the stretch rate and curvature dependences of density-weighted displacement speed . It has been found that remains negatively correlated with curvature and exhibits a significantly non-linear relation with stretch rate for all cases considered here. As a result of these behaviours, an extrapolation relation, which assumes a linear stretch rate dependence of density-weighted displacement speed has been found to be inadequate. However, an alternative extrapolation relation which assumes a linear curvature dependence of has been found to be more successful in capturing local behaviour of the density-weighted displacement speed because it allows for a non-linear stretch rate dependence of . The extrapolation relations, which express as non-linear functions of either curvature or stretch rate, have been found to perform better than the linear extrapolation relations. The Markstein length for all the extrapolation relations except for the one, which assumes a linear stretch rate dependence of , has been found to be in good agreement with an analytical expression for . An extrapolation relation which involves solving a non-linear equation in terms of stretch rate has been found to be sensitive to the initial guess and an alternative high-order polynomial-based extrapolation relation gives rise to overshoots and undershoots and thus the goodness of fit of these relations suffers for high turbulence intensities. An alternative extrapolation relation has been proposed in this analysis, which explicitly accounts for the non-linear curvature dependence of the combined reaction and normal diffusion components of and satisfactorily captures the statistical behaviours of for all cases considered here.

From surface morphologies to inner structures: insights into hypermobility of the Nixu rock avalanche, southern Tibet, China

Large rock avalanches often result in many deaths and catastrophic damage to infrastructure far from source. No consensus has yet been reached on mechanisms causing their extraordinarily long runout since the first study at the Elm Slide, Switzerland. Much current understanding of rock avalanche kinematics and dynamics has been gained from observations of their surface morphologies and inner structures. Based on field surveys, high-resolution satellite image interpretation, and laboratory sieving, we analyzed the ancient, earthquake-induced Nixu rock avalanche in southern Tibet, China, which has unique exposures of both morphological features and inner structures of the deposits. The study revealed that (1) the Nixu rock avalanche probably could be disintegrated into three emplacement stages, transforming from an initial deep-seated rockslide into a large secondary debris avalanche, plus a relatively small third debris avalanche. The spreading of the secondary debris avalanche contributed to the subsequent long-runout propagation. (2) The morphological features observed in successive proximal to distal exposures (such as scarps, flowbands, transverse ridges, grid grooves, hummocks, and splash zones) reflect the change in stress-strain state of debris in motion from tension, to compression, to shear and tension. (3) The complicated inner structures exposed along river banks (such as jigsaw structures, fragmented clasts, inner shear zones, conjunction faults, convoluted lamination, decollements, sand injections, and intrusions) indicate the existence of a thin basal shear zone and a pressurized entrained substrate between the substrate and rock avalanche debris. (4) This suggests turbulent flow with bulldozing in the front, but a laminar flow under a simple shear model for the main body. (5) The analysis suggests that the rock avalanche’s extraordinary mobility involves a combination of momentum transfer, substrate entrainment, liquefied basal shearing, internal shearing, and rock fragmentation.

Impacts of non-uniform filament feed spacers characteristics on the hydraulic and anti-fouling performances in the spacer-filled membrane channels: Experiment and numerical simulation

Feed spacer is universally used in spiral-wound nanofiltration (NF) and reverse osmosis (RO) membrane modules. It can separate membrane sheets, create flow channels, promote turbulence and enhance mass transfer. However, it also induces increased pressure drop across the flow channel, and generates dead zones for biofilm growth at specific locations. Optimization of feed spacer geometries is highly desirable for energy saving and biofouling control. In this study, four kinds of commercial feed spacers featured with non-uniform filaments were compared in terms of hydraulic and anti-fouling performances. Computational fluid dynamics (CFD) simulations were launched to give insights into the impacts of feed spacer characteristics on the flow field. Results show that the hydraulic performance was substantially affected by the number of filament layers (single or dual layer), the non-uniformity of filament diameter and the width of thinning zones. The design of single layer feed spacer of non-uniform filaments was not recommended due to high flow resistance and poor anti-fouling performance. The feed spacer structure of alternating filament diameter contributed to reducing dead zones and alleviating membrane fouling. The thinning zones located adjacent to the filament junctions achieved better anti-fouling performance, as it disturbed the dead zones and partially washed away the deposited foulants. This study demonstrates for the first time that the characteristics of non-uniform filament feed spacer had a crucial impact on the hydraulic and anti-fouling performances, and suggests that more emphasis should be laid on number of filament layers, variation of filament diameter and width and positioning of thinning zones for the optimization of feed spacer geometries in the future.

Quantitative imaging of sodium concentrations in soil-root systems using magnetic resonance imaging (MRI)

AimsDemonstrating the potential of MRI as a 3D, non-invasive and continuous measurement technique to map Na+ concentration distributions in soil and around roots.MethodsDissolved NaCl in soil and soil-plant systems was mapped by 3D 23Na-MRI. The lower limit of detectability in saturated and unsaturated porous media was evaluated, followed by evaporation experiments to test the quantification. Finally, Na+ enrichment around tomato roots, irrigated with saline solution under low/high transpiration rates (LT, HT), was imaged in parallel to the root system,.ResultsA spin echo pulse sequence allowed the quantitative mapping of the volume concentration of NaCl in sandy porous medium. Evaporation experiments showed slight enrichment in the top surface layer, plus uniform temporal enrichment in the deeper layers. In the tomato experiments, enrichment was more distinct under HT than under LT. Concentration-distance correlation curves revealed thin enrichment zones ranging a few mm around the roots.ConclusionsMRI can map Na+ non-invasively in 3D at relevant concentrations for root activity. Visualizing water content, roots and Na+ on the same scale is possible, despite limitations of different scanning times and resolution. This opens a route for further quantitative investigations of salt enrichment processes in soil and soil-plant systems.

Flame Surface Density Transport Statistics for High Pressure Turbulent Premixed Bunsen Flames in the Context of Large Eddy Simulation

ABSTRACT The implications of elevated pressure on the statistical behavior of the flame surface density (FSD) transport statistics together with the behavior of selected established sub-models of the unclosed terms of the FSD transport equation have been analyzed in the context of large eddy simulation. For this purpose, five turbulent premixed Bunsen flames have been considered from an existing database. Four of the Bunsen flames are characterized by different pressure levels and are located on the boundary of the wrinkled and the corrugated flamelet (CF) regimes to allow for the possibility and clear identification of combustion instabilities that are often observed at high pressures. The fifth flame is in the thin reaction zones regime and serves as a reference case for the purpose of comparison. For a given filter width to flame thickness ratio, the terms of the FSD transport equation and their models behave in a qualitatively similar manner for different pressure levels. However, as the flame thickness decreases with increasing pressure, it is unlikely that a high pressure flame will be simulated for the same spatial resolution (i.e. LES filter width) normalized by flame thickness as that of an atmospheric flame. It is more likely that the spatial resolution remains constant and in this case, the modeling becomes much more challenging for higher pressures: the magnitudes of the sub-grid contributions increase for larger filter width to flame thickness ratio and thus the accuracy of sub-grid modeling is expected to play a more important role in determining the fidelity of the simulations. The flames considered in this work feature a relatively low ratio of turbulent velocity fluctuations to laminar flame speed, and under these conditions, positive values of the sub-grid curvature term and negative values of the strain rate term are observed toward the leading edge of the flame brush. This behavior cannot be captured by the well-established existing models for the sub-grid curvature term as these model expressions yield deterministically negative values. Similarly, existing models for the tangential strain term yield deterministically positive values. Detailed explanations have been provided for the observed behaviors of the unclosed terms of the FSD transport equations and their respective model predictions.

A new approach to simulate the fluid dynamics in a wiped film evaporator using Modelica

Wiped film evaporators (WFE) or agitated thin film evaporators are efficient equipment for separating highly thermosensitive and viscous mixtures. There are different approaches to tailor residence time distribution (RTD) for specific applications and to maximise heat and mass transfer by equipment design or process parameter adjustment, like liquid load or wiper speed. However, the fundamental principles of fluid dynamics are only known to a limited extent. Although flow patterns and mass and heat transfer are highly interdependent, the effects of different wipers on liquid flow and, finally, on residence time behaviour are poorly understood. In this work, a new model describing fluid dynamics and its impact on RTD is presented. The model is implemented in the equation-based modelling language Modelica. Several system variables – e.g. film thickness, hold-up or velocity profiles within the liquid film – can be estimated. The effect of varying process parameters, like liquid load and fluid temperature, can also be evaluated. The model has been calibrated with results from RTD experiments in a WFE using a roller wiper. It is capable of describing the residence time behaviour of the roller wiper system with good accuracy. Mean residence times can be reproduced within approx. ±20%. Further results, such as film thickness or hold-up, provide additional insight into the fluid dynamics of characteristic regions – like bow wave, wiping and thin film zone – and the effects of several process parameters.