70 research outputs found
Thermal modelling of Selective Laser Melting: A semi-analytical approach
No full textSelective Laser Melting (SLM) is a 3D-printing method to produce metal parts. In SLM a part is manufactured layer-by-layer by selectively melting a metal powder with a laser. Although SLM is a promising process for the production of high-quality parts, the implementation in high-tech industry is held back by a few barriers. One of the main barriers in the adoption of the process as a new production method are the high residual stresses and large deformations that arise in a part during manufacturing. The residual stresses may limit the load resistance and contribute to the formation of thermal cracks. Furthermore, it is difficult to ensure a constant quality during the build-process, resulting in e.g. varying porosity throughout the part. To obtain a better understanding of the process and predict processing and structural part characteristics prior to production, various modelling techniques have been proposed in literature. The majority of these models attempt to capture the movement of the laser by means of the finite element method (FEM). However, the fine discretization required to capture the steep thermal gradients in the vicinity of the moving laser spot, renders these types of models computationally too expensive to perform a full build-analysis of a component. Moreover, the transient nature of the heat transfer problem also increases the computational requirements. The aim of this research is to develop an efficient and simple, yet reasonably accurate thermal model of the SLM process, that is able to describe the influence of the process parameters on the level of residual stresses and the amount of porosity. To reach this objective, a semi-analytical modelling approach is proposed. The moving laser is represented with a set of point sources for which the closed form analytical solution is known. This solution efficiently captures the transient nature of the heat transfer problem and the steep thermal gradients in the vicinity of the moving laser. Since analytical solutions for a point source are limited to simple problems with specific types of boundary and initial conditions, a complementary correction field is superposed to the analytical solution to prescribe boundary conditions of interest. Predictions of the analytical part of the semi-analytical model are compared to experimental data, and it is found that, for the majority of the data, the predictions are within 10\% of the experimental values. Furthermore, results of the semi-analytical model are compared to an infinite series solution for a simple problem and show good correspondence, indicating that the implementation of the model is correct. A sensitivity analysis of the analytical part of the model to changes in the material properties, process parameters and scanning strategy is performed. It reveals that these factors significantly influence the melt pool dimensions and the maximum spatial thermal gradients that occur over time. It is suspected that the maximum spatial thermal gradient over time is a measure for the residual stresses in the end-product and that the stability of the melt pool influences the amount of porosity. Consequently, it is expected that by controlling the process parameters and scanning strategy, the porosity and residual stresses can be controlled. Single and multi-layer predictions are presented using the semi-analytical model. The simulations demonstrate that the scanning of overlying layers has a significant impact on the thermal history in the layers below, resulting in additional cycles of rapid heating and cooling. Furthermore, it is found that the time between the scanning of subsequent layers is sufficient for the material to cool down to the build-chamber temperature. This means that subsequent layers can be modelled separately. In an initial speed comparison with respect to a FEM-model the semi-analytical model is shown to have the potential to be much faster (up to three orders of magnitude) than a standard FEM-model. Consequently, it is a promising method to investigate e.g. the porosity and the residual stresses throughout several layers, and may potentially be used for the process optimization of small problems. However, in the current implementation the coarse numerical discretization limits the resolution at which the temperature profile can be evaluated. This limits the accuracy with which the melt pool dimensions and the maximum thermal gradient can be predicted. Concluding, the semi-analytical model provides a reasonably accurate thermal description of the SLM process. It is a promising method to investigate the amount of porosity and the level of residual stresses as introduced during the SLM process in a computationally efficient manner. Some recommendations for future research are given.Engineering MechanicsPrecision and Microsystems EngineeringMechanical, Maritime and Materials Engineerin
Developing and evaluating a model for Surfactant-Foam Flooding
No full textMany reservoirs are considered unsuitable candidates for Enhanced Oil Recovery. Surfactant-Foam flooding, also referred to as Low-Tension-Gas flooding, could be a solution to enhance the oil recovery in tight, saline or high temperature reservoirs. SF-flooding combines reduction of the oil-water interfacial tension by surfactant, with mobility control provided by foam. While coreflood experiments have been performed, no Surfactant-Foam model has been developed yet. Therefore, the objective of this research is to develop and evaluate a conceptual model for Surfactant-Foam flooding. Several available foam- and surfactant flooding models are analyzed. By comparing the various approaches, two models are selected based on the identified modelling objectives, model complexity and available experimental data. In this study a Surfactant-Foam model is developed that combines an implicit-texture foam model with a two-phase effective surfactant model. The model assumes foam is in local-equilibrium and correlates salinity and a minimum surfactant concentration with a lowered water-oil interfacial tension. The model does not explicitly model the micro-emulsion phase. Furthermore, the model includes surfactant adsorption and salt and surfactant dispersion. An attempt to model foam diversion with surfactant dispersion has been made, but results are limited due to numerical instability. For every coreflood the effective dispersion coefficient is determined by fitting the experimental effluent salinity data with a solution to the 1D advection-diffusion equation. The performance of the combined model is evaluated with a reservoir simulator. According to the simulation study the surfactant model affects the gas relative permeability through a correlation with the connate water saturation. The defined chemical connate water saturation always affects the gas mobility, either by reducing the gas relative permeability, or by minimizing the impact of the limiting capillary pressure. The results show that an experimentally applied salinity gradient, correlated with a decrease and increase in water-oil interfacial tension, cannot be modelled with capillary number dependent relative permeability curves. In this research it is assumed that the water-oil interfacial cannot increase, after it achieved an ultra-low value. According to the current desaturation approach, oil prefers to flow in the presence of water in stead of gas at ultra-low interfacial tension. More research is required to investigate if this can be related with physics, or if it is purely a modelling artifact. Furthermore, the simulation study shows that the foam matching parameters of the implicit-texture model require optimization for cores with a mutual difference in connate water saturation, due to variation of the impact of the limiting capillary pressure. Simulations are performed to history match experimental data and are aimed to match the measured pressure drop, oil production and effluent salinity profiles. As Surfactant-Foam flooding is taking its first steps in the laboratory, the accuracy and amount of available data is limited. To improve the validity of the model parameters, coreflood experiments in which the foam and surfactant processes are decoupled were interpreted. The surfactant concentration and water saturation are identified as the main drivers affecting the foam texture. Waterflooding pressure data was used to determine the non-chemical relative permeability parameters. The analyzed Surfactant-Foam corefloods are conducted in tight Indiana Limestone cores. Carbonates often have a complex pore structure due to their dual porosity. The experimental data suggests the presence of heterogeneities such as high permeable zones. Therefore, a one-dimensional model is expanded to a two-dimensional model, represented by two layers. Both the one and two-dimensional Surfactant-Foam model achieve a reasonable history match with the cumulative oil recovery and pressure gradient. The two-dimensional model, with a small difference in flow capacity between the layers, successfully matches the size of the oil cut. With the two-dimensional model an improved match with the effluent salinity profile can be achieved at the expense of the oil cut and recovery match. The contribution of measurement errors and heterogeneities to the effluent salinity profile requires more research. As the experimental data are subject to a high level of uncertainty, more corefloods should be performed to identify the most suitable geological representation of the Indiana Limestone.Petroleum EngineeringGeoscience & EngineeringCivil Engineering and Geoscience
A semi-analytical thermal modelling approach for selective laser melting
No full textSelective laser melting (SLM) wherein a metal part is built in a layer-by-layer manner in a powder bed is a promising and versatile way for manufacturing components with complex geometry. However, components built by SLM suffer from substantial deformation of the part and residual stresses. Residual stresses arise due to temperature gradients inherent to the process and the accompanying deformation. It is well known that the SLM process parameters and the laser scanning strategy have a substantial effect on the temperature transients of the part and henceforth on the degree of deformations and residual stresses. In order to provide a tool to investigate this relation, a semi-analytical thermal model of the SLM process is presented which determines the temperature evolution in a 3D part by way of representing the moving laser spot with a finite number of point heat sources. The solution of the thermal problem is constructed from the superposition of analytical solutions for point sources which are known in semi-infinite space and complimentary numerical/analytical fields to impose the boundary conditions. The unique property of the formulation is that numerical discretisation of the problem domain is decoupled from the steep gradients in the temperature field associated with localised laser heat input. This enables accurate and numerically tractable simulation of the process. The predictions of this semi-analytical model are validated by experiments and the exact solution known for a simple thermal problem. Simulations for building a complete layer using two different scanning patterns and subsequently building of multiple layers with constant and rotating scanning patterns in successive layers are performed. The computational efficiency of the semi-analytical tool is assessed which demonstrates its potential to gain physical insight in the full SLM process with acceptable computational costs.Accepted Author ManuscriptComputational Design and Mechanic
Packed modulation loops to reduce band broadening in two-dimensional liquid chromatography
No full textModulation interfaces employing sample loops are applied in many hyphenated separations such as two-dimensional liquid chromatography (2D-LC). When the first-dimension effluent in 2D-LC is eluted from the modulation loop, dispersion effects occur due to differences in the laminar flow velocity of the filling and emptying flow. These effects were recently studied by Moussa et al. whom recommended the use of coiled loops to promote radial diffusion and reduce this effect. In the 1980s, Coq et al. investigated the use of packed loops, which also promote radial diffusion, in large volume injection 1D-LC. Unfortunately, this concept was never investigated in the context of 2D-LC modulation. Our work evaluates use of packed loops in 2D-LC modulation and compares them to unpacked coiled and uncoiled modulation loops. The effect of the solvents, loop volume, differences in filling and emptying rates, and loop elution direction on the elution profile was investigated. Statistical moments were used as a pragmatic tool to quantify elution profile characteristics. Decreased dispersion was observed in all cases for the packed loops compared to unpacked loops and unpacked coiled loops. In particular for larger loop volumes the dispersion was reduced significantly. Furthermore, countercurrent elution resulted in narrower elution profiles in all cases compared to concurrent elution. We found that packed modulation loops are of high interested when analytes are not refocussed in the second-dimension separation (e.g. for size-exclusion chromatography). Moreover, our work suggests that the use of packed loops may aid in prevention of loop overfilling.</p
Natural manganese deposits as catalyst for decomposing hydrogen peroxide
No full textDrinking water companies (are intending to) implement advanced oxidation processes (AOP) in their treatment schemes to increase the barrier against organic micropollutants (OMPs). It is necessary to decompose the excessive hydrogen peroxide after applying AOP to avoid negative effects in the following, often biological, treatment steps. A drinking water company in the western part of the Netherlands investigated decomposition of about 5.75 mg L?1 hydrogen peroxide in pre-treated Meuse river water with different catalysts on pilot scale. In down flow operation, the necessary reactor empty bed contact time (EBCT) with the commonly used granulated activated carbon (GAC) and waste ground water filter gravel (MCFgw) were found equal with 149 s, corresponding with a conversion rate constant r of 0.021 s?1. The EBCT of the fine coating of ground water filter gravel (MC) was significantly shorter with a little more than 10 s (r = 0.30 s?1). In up flow operation, with a flow rate of 20 m h?1, the EBCT of coating MC increased till about 100 s (r = 0.031 s?1), from which can be concluded, that the performance of this waste material is better compared with GAC, in both up and down flow operation. The necessary EBCT at average filtration rate of full scale dual layer filter material (MCFsw) amounted to 209 s (r= 0.015 s?1). Regarding the average residence time in the full scale filters of 700 s, applying AOP in front of the filters could be an interesting alternative which makes a separate decomposition installation superfluous, on the condition that the primary functions of the filters are not affected.Water ManagementCivil Engineering and Geoscience
The etiology of esophageal cancer in high- and low- risk areas of Jiangsu province, China
No full text [Background]Esophageal cancer (EC) remains one of the most common and fatal malignancies worldwide. The geographic variation in EC occurrence is striking, and China is an area with one of the highest incidences of EC. A number of epidemiological studies have been conducted toward EC in the past decades, results suggested that tobacco smoking, alcohol drinking, unhealthy dietary factors and chronic injuries of the esophageal mucosa are important in the development of this disease. Genetic polymorphisms in enzymes involved in metabolism of carcinogens may also influence individual susceptibility. However, the effects of major lifestyle and hereditary risk factors on the development of EC remain poorly understood in China. Moreover, little attention has been paid to the etiological heterogeneity between similar areas with great risk gradient. [Methods]From 2003 to 2007, a large population-based case-control study of EC has been conducted in a selected high-risk area and a selected low-risk area of Jiangsu Province, one of the highest cancer incidence areas in China. In total, 1,520 cases and 3,879 controls were recruited. In this thesis, we evaluated the role of major lifestyle factors such as tobacco smoking, alcohol drinking and dietary factors, as well as inherited determinants including family history of cancer and genetic polymorphisms of alcohol-metabolizing related genes on the risk of EC. In addition, we investigated how much of the risk gradient between two areas could be explained by variation in the distributions of major risk factors. [Results] Tobacco smoking and alcohol drinking moderately increased the risk of EC, while the positive associations were only found among men but not among women. Dietary factors were observed to play important roles in the development of EC. Specific dietary habits i.e., fast eating speed, and hot eating and/or drinking substantially elevated EC risk and could explain more than 20% of EC cases each. High intake of salty foods and fried foods, low consumption of raw garlic were also observed to increase the risk of EC. In addition to environmental and lifestyle factors, we confirmed that a positive family history can significantly increase EC risk, and found the inheritance may modify the effect of some unhealthy lifestyles. Moreover, we further explored the relationship between EC and single nucleotide polymorphismsof ADH1B, ADH1C and ALDH2 genes. Results showed that the slow metabolizing ADH1B G allele, ADH1C G allele and ALDH2 A allele significantly increased EC risk among moderate-to-heavy alcohol drinkers, and a significant interaction was observed between ALDH2 gene and alcohol consumption. Lastly, we found that more than 60% of EC cases could be attributable to major lifestyle risk factors in the study population; furthermore, dissimilar distribution of several lifestyle factors, together with variations of hereditary factors may be largely responsible for the incidence difference between two study areas. [Conclusion]The findings in this thesis confirm that unhealthy lifestyles including smoking, alcohol drinking and some dietary factors are the predominant risk factors of EC in China, and a large proportion of incidence difference between regions at varying risk could be attributed to the different prevalence of lifestyle factors. As most of the identified risk factors are modifiable, these could be translated into risk reduction prevention programs in China, and a substantial proportion of new EC cases are expected to be prevented by eliminating or avoiding these risk factors in the population. </p
Detection challenges in quantitative polymer analysis by liquid chromatography
No full textAccurate quantification of polymer distributions is one of the main challenges in polymer analysis by liquid chromatography. The response of contemporary detectors is typically influenced by compositional features such as molecular weight, chain composition, end groups, and branching. This renders the accurate quantification of complex polymers of which there are no standards available, extremely challenging. Moreover, any (programmed) change in mobile-phase composition may further limit the applicability of detection techniques. Current methods often rely on refractive index detection, which is not accurate when dealing with complex samples as the refractive-index increment is often unknown. We review current and emerging detection methods in liquid chromatography with the aim of identifying detectors, which can be applied to the quantitative analysis of complex polymers
Online hyphenation of size-exclusion chromatography and pyrolysis-gas chromatography for polymer characterization
No full textAn understanding of the composition and molecular heterogeneities of complex industrial polymers forms the basis of gaining control of the physical properties of materials. In the current work we report on the development of an online method to hyphenate liquid polymer chromatography with pyrolysis-GC (Py-GC). The designed workflow included a 10-port valve for fractionation of the first-dimension effluent. Collected fractions were transferred to the Py-GC by means of a second LC pump, a 6-port valve was used to control injection in the Py-GC, allowing the second pump to operate continuously. The optimized large volume injection (LVI) method was capable of analyzing 117 µL of the LC effluent in a 6 min GC separation with a total cycle time of 8.45 min. This resulted in a total run time of 2.1 h while obtaining 15 Py-GC runs over the molar mass separation.The method was demonstrated on various real-life samples including a complex industrial copolymer with a bimodal molar mass distribution. The developed method was used to monitor the relative concentration of 5 different monomers over the molar mass distribution. Furthermore, the molar mass-dependent distribution of a low abundant comonomer (styrene, <1% of total composition) was demonstrated, highlighting the low detection limits and increased resolving power of this approach over e.g. online NMR or IR spectroscopy. The developed method provides a flexible and widely applicable approach to LC-Py-GC hyphenation without having to resort to costly and specialized instrumentation
Development of a chemometric approach to improve the accuracy of copolymer sequence information obtained from pyrolysis gas-chromatography mass-spectrometry data
No full textNumber-average copolymer sequence length information can be obtained by pyrolysis-gas chromatography (Py-GC) by comparing the ratios of formed oligomers (i.e. dimers and trimers). The formation constants of the oligomers and their detection efficiency are not constant for all fragments, however. This can lead to unrepresentative peak ratios in the chromatogram. In these cases, calibration with an external method (e.g. NMR) is required. In this work, we introduce an algorithm that improves the copolymer sequence accuracy yielded from chromatograms with unrepresentative peak areas. The algorithm even functions in cases where oligomer data is missing as the rate of formation of certain oligomers is too low to detect them. One Py-GC measurement and one NMR measurement are required to train the developed algorithm for the determination of average monomer reactivity ratios and relative pyrolysis constants. Afterwards, Py-GC measurements of copolymers containing the same monomers, albeit with different compositions, can be corrected using the previously estimated constants. The algorithm was tested on various styrene-acrylate copolymers, yielding more accurate sequence information, even when limited oligomer information was available.</p
Dynamic(18)F-fluorocholine PET/CT for parathyroid imaging in patients with primary hyperparathyroidism
No full textObjective In the past few years,F-18-fluorocholine PET/CT has been established as a promising imaging technique for preoperative localization of parathyroid adenomas, but the optimal time point to start PET/CT acquisition after tracer injection is yet unknown. The aim of the present study was to assess the optimal time frame to acquire the PET/CT images and to evaluate the ability of dynamic imaging to differentiate parathyroid adenomas from active lymph nodes, a common cause for false-positive scan results. Patients and methods Patients with primary hyperparathyroidism who had undergone a dynamic(18)F-fluorocholine PET/CT positive for parathyroid disease and who subsequently underwent successful parathyroidectomy were retrospectively included in this study. On the 20 minutes dynamic images, standardized uptake value measurements were acquired per 1 minute frame for the parathyroid adenoma, the thyroid gland, blood pool activity, and, if present, lymph node activity. Results A total of 101 patients were included in this study. Time-activity curves showed a decrease of activity in parathyroid and thyroid glands, with faster wash-out from the thyroid gland and on average a stable, lower activity in lymph nodes. Blood pool activity was particularly present in the first 2 minutes. Differentiation of a parathyroid adenoma from active lymph nodes was best before 5 minutes, but no definitive cutoff value could be determined. Differentiation of a parathyroid adenoma from the thyroid gland was best after 10 minutes. Conclusion Dynamic imaging starting at the early time point of 2 minutes after injection of(18)F-fluorocholine is useful for characterization of hyperfunctioning parathyroid glands.</p
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