@article{21947, abstract = {{Wall slip is a long-known phenomenon in the field of rheology. Nevertheless, the origin and the evolution are not completely clear yet. Regarding suspensions, the effect becomes even more complicated, because different mechanisms like pure slip or slip due to particle migration have to be taken into account. Furthermore, suspensions themselves show many flow anomalies and the isolation of slip is complicated. In order to develop working physical models, further insight is necessary. In this work, we measured experimentally the wall slip velocities of different highly filled suspensions in a rectangular slit die directly with respect to the particle concentration and the particle size. The slip velocities were obtained using a particle image velocimetry (PIV) system. The suspensions consisting of a castor oil–cinnamon oil blend and PMMA particles were matched in terms of refractive indexes to appear transparent. Hereby, possible optical path lengths larger than 15 mm were achieved. The slip velocities were found to be in a quadratic relation to the wall shear stress. Furthermore, the overall flow rate as well as the particle concentration has a direct influence on the slip. Concerning the shear stress, there seem to be two regions of slip with different physical characteristics. Furthermore, we estimated the slip layer thickness directly from the velocity profiles and propose a new interpretation. The PIV technique is used to investigate the viscosity and implicit the concentration profile in the slit die. It is shown that the particle migration process is quite fast.}}, author = {{Jesinghausen, Steffen and Weiffen, Rene and Schmid, Hans-Joachim}}, issn = {{0723-4864}}, journal = {{Experiments in Fluids}}, keywords = {{Rheology, Wall Slip, Slip, apparent slip, suspension}}, title = {{{Direct measurement of wall slip and slip layer thickness of non-Brownian hard-sphere suspensions in rectangular channel flows}}}, doi = {{10.1007/s00348-016-2241-6}}, year = {{2016}}, } @article{25140, author = {{Pieper, Sven and Schmid, Hans-Joachim}}, issn = {{0377-0257}}, journal = {{Journal of Non-Newtonian Fluid Mechanics}}, pages = {{1--7}}, title = {{{Layer-formation of non-colloidal suspensions in a parallel plate rheometer under steady shear}}}, doi = {{10.1016/j.jnnfm.2016.04.004}}, year = {{2016}}, } @article{26108, abstract = {{The highly efficient filtration of ultrafine dust emitted by biomass combustion processes with a baghouse filter has been successfully tested in the last years. To protect the filter material from the very small and sticky fine dust particles and to guarantee very high total collection efficiencies (> 99 %) in a long-term stable process, the use of a precoat is necessary. Tests done in a laboratory and a real-application plant show that the reuse of precoat materials can lead to significant savings. Considering the influences of different combustion processes, the associated precoat efficiencies could be calculated. With these characteristic ratios, it is possible to evaluate different process settings. Hence, the amount and the cost of the needed precoat could be reduced significantly.}}, author = {{Schiller, Sascha and Hellmich, Christoph and Schmid, Hans-Joachim}}, issn = {{0930-7516}}, journal = {{Chemical Engineering & Technology}}, pages = {{491--498}}, title = {{{Evaluation of the Efficiency of Filtration Processes Using Precoat Materials}}}, doi = {{10.1002/ceat.201500385}}, year = {{2015}}, } @inbook{26119, abstract = {{The emphasis of the study presented is on a new process of particle extraction to transfer magnetite nanoparticles from an aqueous into an immiscible organic phase directly through the liquid-liquid interface. For the production of high-quality organosols, stabilized colloidal and functionalized particles are required in a liquid organic phase. The mechanism of phase transfer is initiated by adsorption and chemical binding of surfactants (fatty acids) at the particle surface. The resulting physico-chemical dispersion of the hydrophobically modified particles leads to the formation of the stabilized organic colloid, or organosol. The aim here is to demonstrate the entire chain of the transfer process in a continuous miniplant, which comprises particle synthesis, conditioning, and transfer, and which uses a drop column for extraction and as a transfer device. Based on the investigation of the governing principles and the material parameters, the results obtained for the transfer kinetics in the individual contact devices (centrifuge, single-drop column, and drop column for different operations) are used for the dimensioning of the entire process chain.}}, author = {{Erler, Jacqueline V. and Machunsky, Stefanie and Franke, Steffen and Grimm, Philipp and Schmid, Hans-Joachim and Peuker, Urs A.}}, booktitle = {{Colloid Process Engineering}}, title = {{{Process Development of a Liquid-Liquid Phase Transfer of Colloidal Particles for Production of High-Quality Organosols}}}, doi = {{10.1007/978-3-319-15129-8_16}}, year = {{2015}}, } @article{26121, abstract = {{The conditioning of the aerosol particle population into a bipolar charge equilibrium is an essential prerequisite to calculate the particle number size distribution using mobility particle size spectrometers. This is commonly realized by diffusion charging of bipolar air ions generated by e.g. a 85Kr source. Because of strict legal regulations on radioactive sources in several countries, soft-X-ray (SXR) appears as a suitable alternative. However, multiple measurements showed a systematical and significant difference between the particle charge distribution delivered by a radioactive source and an SXR charger, respectively. In this investigation, a calibrated particle charge distribution, suitable for the SXR chargers, was calculated based on the Fuchs model. An approximation analogous to the commonly used Wiedensohler approximation formula (Wiedensohler, 1988) was computed. The use of the new SXR approximation of the bipolar charge equilibrium for the inversion of an electrical mobility distribution to a particle number size distribution improves the comparability of these results, compared to measurements involving a 85Kr charger or to bipolar chargers using radioactive material in general. A systematic error in case of using the SXR charger could be eliminated and hence the root mean square deviation could be reduced from 13% using the common parameters for both charger types to 7% using the new SXR approximation for the SXR bipolar charger.}}, author = {{Tigges, L. and Wiedensohler, A. and Weinhold, K. and Gandhi, J. and Schmid, Hans-Joachim}}, issn = {{0021-8502}}, journal = {{Journal of Aerosol Science}}, pages = {{77--86}}, title = {{{Bipolar charge distribution of a soft X-ray diffusion charger}}}, doi = {{10.1016/j.jaerosci.2015.07.002}}, year = {{2015}}, } @article{26122, abstract = {{An essential part of mobility particle size spectroscopy is the prediction of the aerosol charge distribution in a highly concentrated bipolar ion environment. This charge distribution can be readily calculated, but is depending on several environmental conditions. These influences are investigated theoretically. The first part of this work deals with a sensitivity analysis using the Fuchs model (Fuchs, 1963) to determine the variation of the resulting charge distributions depending on the input parameters. It is demonstrated, that the main influencing variable is the difference between the positive and negative ion mobility. A sensitivity analysis reveals that a reasonable variation of the ion mobilities may lead to variations of the particle density distribution up to±20%. The second part investigates the evolution of the charge distribution along the tubing downstream of the bipolar charger exit starting with equal ion concentrations for positive and negative ions. Due to wall losses depending on ion mobility a non-equilibrium charge distribution is developing along the plumbing. The evolution of the particle charge distribution is determined using a coupled population balance model. Even though the non-equilibrium character is clearly shown, it turns out that this effect is negligible at conditions typical for particle size measurements.}}, author = {{Tigges, L. and Jain, A. and Schmid, Hans-Joachim}}, issn = {{0021-8502}}, journal = {{Journal of Aerosol Science}}, pages = {{119--134}}, title = {{{On the bipolar charge distribution used for mobility particle sizing: Theoretical considerations}}}, doi = {{10.1016/j.jaerosci.2015.05.010}}, year = {{2015}}, } @article{26123, abstract = {{The precipitation of sticky and ultrafine particles has become increasingly important. Biomass burners are one important example for ultrafine dust emission sources with ever growing importance. Therefore, a baghouse filter has been developed, which combines excellent separation efficiency (> 99%, clean air dust loading of < 1 mg/m3) with convenience in operation. However, in order to prevent clogging of the filter cloth by sticky and ultrafine particles, it is necessary to use a precoat layer (e.g. hydrated limestone powder). If this technology is applied to larger scale processes, e.g. biomass burning for industrial drying processes, the reuse of the precoat material can generate significant savings. Therefore, extended tests on recycling of used precoat material have been performed. Particularly, the influence of precoat injection parameters and various mixing strategies of used and virgin powder for refreshing the precoat material have been investigated. Different mixtures have been characterised by their ability to disintegrate, flowability and filtration behaviour. It is clearly demonstrated that upon redispersing the used precoat fine dust mainly adheres to the coarse precoat with only a limited number of dust agglomerates being produced in addition. For each kind of precoat a minimum amount is determined in order to ensure a long-term stable process. This way a saving potential of between 40–67% has been found. An economic and ecologic process has been developed to precipitate ultrafine dust in a baghouse filter system using precoat materials.}}, author = {{Schiller, Sascha and Schmid, Hans-Joachim}}, issn = {{0032-5910}}, journal = {{Powder Technology}}, pages = {{96--105}}, title = {{{Highly efficient filtration of ultrafine dust in baghouse filters using precoat materials}}}, doi = {{10.1016/j.powtec.2015.03.048}}, year = {{2015}}, } @article{26124, abstract = {{Capillary forces between particles often dominate other adhesion forces. However, the calculation of the shape of capillary bridges and the resulting force is complex, so often assumptions and approximations are used. These assumptions are not useful for nanoscale particles. Therefore, a simulation method was established to calculate numerically the shape of the meniscus and derive the corresponding capillary bridge force. The main focus are nanoscaled particles with liquid bridges formed by condensing air humidity. The dependence of the capillary force on various parameters such as particle sizes, contact angle and humidity was investigated and it is demonstrated that often-used assumptions cannot be used for nanoscaled particles without remarkable mistake.}}, author = {{Dörmann, Michael and Schmid, Hans-Joachim}}, issn = {{1877-7058}}, journal = {{Procedia Engineering}}, pages = {{14--23}}, title = {{{Simulation of Capillary Bridges between Particles}}}, doi = {{10.1016/j.proeng.2015.01.102}}, year = {{2015}}, } @inproceedings{22178, author = {{Delfs, Patrick and Li, Z. and Schmid, Hans-Joachim}}, booktitle = {{26th Annual International Solid Freeform Fabrication Symposium}}, pages = {{514--526}}, title = {{{Mass finishing of laser sintered parts}}}, doi = {{http://utw10945.utweb.utexas.edu/sites/default/files/2015/2015-41-Delfs.pdf}}, volume = {{26}}, year = {{2015}}, } @inproceedings{22179, abstract = {{The temperature distribution and history within laser sintered part cakes is an important aspect regarding the process quality and reproducibility of the polymer laser sintering process. Especially the temperature history during the build and cooling phase is decisive for powder ageing effects and the development of part quality characteristics. In this work, a measurement system for three-dimensional in-process temperature measurements is set up and the influence of different parameters on the inner part cake temperature distribution and history is analyzed. Important factors are not only geometrical build job parameters like the part packing density and build height, but also process parameters like the layer thickness and bulk powder density. Individual in-process temperature profiles at different positions within a part cake are finally correlated with powder ageing effects. The results of this work help to understand the temperature history dependency of powder and part properties and can therefore be used to develop optimized process controls.}}, author = {{Josupeit, Stefan and Schmid, Hans-Joachim}}, booktitle = {{26th Annual International Solid Freeform Fabrication Symposium}}, pages = {{190--199}}, title = {{{Temperature History within Laser Sintered Part Cakes and its Influence on Process Quality}}}, doi = {{http://utw10945.utweb.utexas.edu/sites/default/files/2015/2015-15-Josupeit.pdf}}, volume = {{26}}, year = {{2015}}, } @inproceedings{22187, abstract = {{Due to long process times at high temperatures, unmolten polyamide 12 material ages during the manufacturing process. Hence, it needs to be refreshed with new material for further build cycles. In application, refresh rates of about 50 % are commonly used. In this work, the recycling optimized material PA 2221 from EOS is analyzed along a series of 13 build and refresh cycles using a reduced refresh rate of 32 %. Before and after every build, the powder is analyzed regarding melt properties determined by MVR and DSC measurements. Thereby, in-process ageing effects are investigated and the steady-state conditions are determined accordingly. In addition, powder properties are directly linked to resulting mechanical and geometrical part properties. Key findings are a robust DSC measurement method for polyamide 12 powder, constant “circulated” material properties after three build/refresh cycles and robust tensile properties along the whole tested powder life cycle. As a result, process conditions of PA 2221 using reduced refresh rates can be derived from this work.}}, author = {{Josupeit, Stefan and Lohn, Johannes and Hermann, E. and Gessler, Monika and Tenbrink, S. and Schmid, Hans-Joachim}}, booktitle = {{26th Annual International Solid Freeform Fabrication Symposium}}, pages = {{540--549}}, title = {{{Material Properties of Laser Sintered Polyamide 12 as Function of Build Cycles Using Low Refresh Rates}}}, doi = {{http://utw10945.utweb.utexas.edu/sites/default/files/2015/2015-43-Josupeit.pdf}}, volume = {{26}}, year = {{2015}}, } @inproceedings{22189, abstract = {{The layered structure of Additive Manufacturing processes results in a stairstepping effect of the surface topographies. In general the impact of this effect strongly depends on the build angle of a surface, whereas the overall surface roughness is caused by the resolution of the specific AM process. The aim of this work is the prediction of the surface quality in dependence of the building orientation of a part. These results can finally be used to optimize the orientation to get a desired surface quality. As not every area of a part can be optimized, a predetermination of areas can be used to improve the surface quality of important areas. The model uses the digital STL format of a part as this is necessary for all AM machines to build it. Each triangle is assigned with a roughness value and by testing different orientations the best one can be found. This approach needs a database for the surface qualities. This must be done separately for each Additive Manufacturing process and is shown exemplary with a surface topography simulation for the laser sintering process.}}, author = {{Delfs, Patrick and Toews, Marcel and Schmid, Hans-Joachim}}, booktitle = {{26th Annual International Solid Freeform Fabrication Symposium}}, pages = {{1334--1344}}, title = {{{Surface roughness optimized alignment of parts for additive manufacturing processes}}}, doi = {{http://utw10945.utweb.utexas.edu/sites/default/files/2015/2015-107-Delfs.pdf}}, volume = {{26}}, year = {{2015}}, } @inproceedings{22199, abstract = {{Powder ageing effects due to thermal load usually lead to a limited recyclability of unmolten polyamide 12 powders in the laser sintering process. In this work, the recycling optimized material PA 2221 is analyzed regarding its ageing behavior along a series of more than ten build/refresh cycles using a refresh ratio of about 30%. Thereby, the melt viscosity is measured before and after every cycle and the “steady-state” condition is determined accordingly. In addition, the material properties of parts manufactured with PA 2221 are discussed dependent on different process parameters and compared to parts made of standard PA 2200 material with a refresh rate of about 50%. These parameters include the mechanical properties as well as for example the impact strength and geometrical properties as function of layer thickness and testing temperature. As a result, the optimal process conditions and achievable material properties for PA 2221 can be derived from this work.}}, author = {{Josupeit, Stefan and Tutzschky, S. and Gessler, M. and Schmid, Hans-Joachim}}, booktitle = {{Proceedings of the Rapid Tech 2015}}, isbn = {{978-3-662-48473-9}}, pages = {{63--78}}, publisher = {{Springer Vieweg}}, title = {{{Powder ageing and material properties of laser sintered polyamide 12 using low refresh rates}}}, doi = {{10.1007/978-3-662-48473-9_5}}, year = {{2015}}, } @inproceedings{23074, author = {{Koke, Isabel and Dibblee, Katharina and Hankeln, Frederik and Seidel, Stefan and Trächtler, Ansgar and Schmid, Hans-Joachim}}, booktitle = {{ 44. Jahrestagung der Deutschen Gesellschaft für Hochschuldidaktik - Hochschuldidaktik im Dialog, Paderborn}}, title = {{{Tutorenprogramm und Vertiefungsberatung stellen Weichen in entscheidenden Phasen des Student Life-Cycles in Maschinenbaustudiengängen}}}, year = {{2015}}, } @article{26125, abstract = {{We investigated an explanation for the length dependence of apparent wall slip in capillary and slit die experiments that had been implied in earlier studies. Firstly, we used a very long slit die fitted with six equidistant pressure transducers and an adjustable outlet. It was found that, all other parameters being equal, the impact of pressure on the flow behavior of suspensions was not different from the impact on the unfilled matrix fluid. Particle-related properties, such as apparent wall slip, were therefore not influenced by pressure as assumed by other authors. Secondly, we determined wall slip velocities by both the Mooney method and numerical simulation, based on an implementation of Phillips constitutive equation for particle migration. This led to a similar length dependence of wall slip as had been reported in an earlier work. We concluded that this was due to the transient nature of particle migration and its interrelationship with apparent wall slip, rather than an influence of pressure.}}, author = {{Pieper, Sven and Kirchhoff, Nadine and Schmid, Hans-Joachim}}, issn = {{0035-4511}}, journal = {{Rheologica Acta}}, pages = {{69--75}}, title = {{{Absence of pressure sensitivity of apparent wall slip in pressure-driven flow of non-colloidal suspensions}}}, doi = {{10.1007/s00397-014-0815-x}}, year = {{2014}}, } @article{26126, abstract = {{Ultrafine dust separation from different sources like industry, traffic, or private households has become increasingly important in the last decade. A compact baghouse filter system has been developed which is suitable for pellet heaters. For filtration, a precoat material is required to prevent clogging of the filter media by the ultrafine dust particles. In order to ensure the best performance of this filter system, different combinations of filter media and precoat materials, e.g., grade efficiencies and the cake area load for the various filter media, were investigated in a special test rig. With this highly efficient technology, extremely high separation efficiencies of > 99 % and ultrafine dust concentrations of > 1 mg cm−3 could be reached in a long-term stable process.}}, author = {{Schiller, Sascha and Schmid, Hans-Joachim}}, issn = {{0930-7516}}, journal = {{Chemical Engineering & Technology}}, pages = {{1009--1020}}, title = {{{Ultrafine Dust Filtration Using Precoat Materials Considering the Influence of Filter Media}}}, doi = {{10.1002/ceat.201300856}}, year = {{2014}}, } @article{26127, abstract = {{Capillary forces are very important as they exceed in general other adhesion forces. But at the same time the exact calculation of these forces is very complex, so often assumptions and approximations are used. Previous research was done with regard to micrometer sized particles, but the behavior of nanoscale particles is different. Hence, the results for micrometer sized particles cannot be directly transferred when considering nanoscale particles. Therefore, a simulation method was developed to calculate numerically the shape of a rotationally symmetrical capillary bridge between two spherical particles or a particle and a plate. The capillary bridge in the gap between the particles is formed due to capillary condensation and is in thermodynamic equilibrium with the gas phase. Hence the Kelvin equation and the Young–Laplace equation can be used to calculate the profile of the capillary bridge, depending on the relative humidity of the surrounding air. The bridge profile consists of several elements that are determined consecutively and interpolated linearly. After the shape is determined, the volume and force, divided into capillary pressure force and surface tension force, can be calculated. The validation of this numerical model will be shown by comparison with several different analytical calculations for micrometer-sized particles. Furthermore, it is demonstrated that two often used approximations, (1) the toroidal approximation and (2) the use of an effective radius, cannot be used for nanoscale particles without remarkable mistake. It will be discussed how the capillary force and its components depend on different parameters, like particle size, relative humidity, contact angle, and distance, respectively. The rupture of a capillary bridge due to particle separation will also be presented.}}, author = {{Dörmann, Michael and Schmid, Hans-Joachim}}, issn = {{0743-7463}}, journal = {{Langmuir}}, pages = {{1055--1062}}, title = {{{Simulation of Capillary Bridges between Nanoscale Particles}}}, doi = {{10.1021/la404409k}}, year = {{2014}}, } @inproceedings{22176, abstract = {{One barrier of laser sintering (LS) to become the main process for Direct Manufacturing (DM) is the surface quality of LS parts. Hence, the property which has to be improved is the rough surfaces of LS parts due to the layered structure. Another additional effect is the incomplete melting of powder particles on the surface due to the high process temperature. In this paper we demonstrate our approach of a theoretical model for the topography of LS part surfaces. We investigated the surface roughness as a function of surface orientation. Considering that the model involves further variables as layer thickness, particle density and particle size distribution to describe the topography precisely. Experimental results were used to optimize and check the results of the model.}}, author = {{Delfs, Patrick and Herale, A.A. and Li, Z. and Schmid, Hans-Joachim}}, booktitle = {{25th Annual International Solid Freeform Fabrication Symposium}}, pages = {{1250--1258}}, title = {{{Simulation of the Surface Topography on Laser Sintered Polymer Parts}}}, doi = {{http://utw10945.utweb.utexas.edu/sites/default/files/2014-098-Delfs.pdf}}, volume = {{25}}, year = {{2014}}, } @inproceedings{22177, abstract = {{An uneven temperature distribution and varying cooling rates at different positions within the part cake are two of the most important challenges regarding the part quality and reproducibility of the polymer laser sintering process. In the presented work, a temperature measurement system is implemented within an EOSINT P395 laser sintering system. It allows the determination of a three dimensional temperature distribution and history during the full build and cooling process. The influence of important job parameters, for example the packing density, job height and layer thickness, can be figured out. In combination with a finite element simulation of the cooling process, the temperature measurement will be the basis for optimized process controls.}}, author = {{Josupeit, Stefan and Schmid, Hans-Joachim}}, booktitle = {{25th Annual International Solid Freeform Fabrication Symposium}}, pages = {{49--58}}, title = {{{Three-dimensional in-process temperature measurement of laser sintered part cakes}}}, doi = {{http://utw10945.utweb.utexas.edu/sites/default/files/2014-006-Josupeit.pdf}}, volume = {{25}}, year = {{2014}}, } @inproceedings{22184, abstract = {{Polymer laser sintering is one of the most important additive manufacturing technologies for the tool-less production of three-dimensional prototypes and end-use parts. In this process, parts are manufactured layerwise out of a polymer powder by laser exposure. After the building process, these parts are located within a loose bulk powder cake. Due to long process times and high process temperatures, this powder ages thermally, which reduces the recyclability of the material. As a result, mixtures of used and virgin powder ("refreshed" powder) with a mixture ratio of approximately 50% are commonly used in the industry. The goal of this work is to determine the exact influence of different powder ages on resulting part quality characteristics, especially the mechanical behavior and the surface quality. Therefore, refreshed powder with different qualities adjusted by the melt volume rate (MVR) was processed along a defined process quality chain. To analyze the part qualities, mechanical tensile and profilometer tests were performed. The focus is on an application-oriented test set-up to ensure the usability of the results in the industry. The material used is polyamide 12 (PA 2200) processed on an EOSINT P395 laser sintering system from EOS GmbH, Krailling, Germany.}}, author = {{Josupeit, Stefan and Rüsenberg, Stefan and Rupp, N. and Gessler, Monika and Schmid, Hans-Joachim}}, booktitle = {{72nd Annual Technical Conference of the Society of Plastics Engineers (ANTEC 2014)}}, isbn = {{978-1-634-39708-7}}, number = {{3}}, pages = {{2383--2385}}, title = {{{Thermal ageing of polyamide 12 used for polymer laser sintering - influence on part quality characteristics}}}, doi = {{https://www.researchgate.net/publication/283135483_Thermal_ageing_of_polyamide_12_used_for_polymer_laser_sintering_-_Influence_on_part_quality_characteristics}}, volume = {{72}}, year = {{2014}}, }