| 1. |
- Franken, T., et al.
(författare)
-
Advanced Predictive Diesel Combustion Simulation Using Turbulence Model and Stochastic Reactor Model
- 2017
-
Ingår i: SAE Technical Papers. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191 .- 2688-3627. ; 2017-March:March
-
Tidskriftsartikel (refereegranskat)abstract
- Today numerical models are a major part of the diesel engine development. They are applied during several stages of the development process to perform extensive parameter studies and to investigate flow and combustion phenomena in detail. The models are divided by complexity and computational costs since one has to decide what the best choice for the task is. 0D models are suitable for problems with large parameter spaces and multiple operating points, e.g. engine map simulation and parameter sweeps. Therefore, it is necessary to incorporate physical models to improve the predictive capability of these models. This work focuses on turbulence and mixing modeling within a 0D direct injection stochastic reactor model. The model is based on a probability density function approach and incorporates submodels for direct fuel injection, vaporization, heat transfer, turbulent mixing and detailed chemistry. The advantage of the probability density function approach compared to mean value models is its capability to account for temperature and mixture inhomogeneities. Therefore, notional particles are introduced each with its own temperature and composition. The particle condition is changed by mixing, injection, vaporization, chemical reaction and heat transfer. Mixing is modeled using the one-dimensional Euclidean minimum spanning tree mixing model, which requires the scalar mixing frequency as input. Therefore, a turbulence model is proposed to calculate the mixing time depending on turbulent kinetic energy and its dissipation. The turbulence model accounts for density, swirl, squish and injection effects on turbulent kinetic energy within the combustion chamber. Finally, the 0D stochastic reactor model is tested for 40 different operating points distributed over the whole engine map. The results show a close match of experimental heat release rate and NOx emissions. The trends of measured CO and HC concentrations are captured qualitatively. Additionally, the 0D simulation results are compared to more detailed 3D CFD combustion simulation results for three operating points differing in engine speed and load. The comparison shows that the 0D stochastic reactor model is able to capture turbulence effects on local temperature and mixture distribution, which in turn affect NOx, CO and HC emission formation. Overall, the 0D stochastic reactor model has proven its predictive capability for the investigated diesel engine and can be assigned to tasks concerning engine map simulation and parameter sweeps.
|
|
| 2. |
- Lögdberg, Sara, et al.
(författare)
-
Hydrocarbon production via Fischer-Tropsch synthesis from H-2-poor syngas over different Fe-Co/gamma-Al2O3 bimetallic catalysts
- 2009
-
Ingår i: Applied Catalysis B: Environmental. - : Elsevier BV. - 0926-3373 .- 1873-3883. ; 89:1-2, s. 167-182
-
Tidskriftsartikel (refereegranskat)abstract
- Fischer-Tropsch synthesis (FTS) at 20 bar. and 483 K, with H-2-poor syngas (H-2/CO ratio = 1.0) in order to simulate gasified biomass, was performed over Al2O3-supported catalysts with various ratios of Fe:Co (12 wt% bimetal) prepared by co-impregnation. Co was found to be incorporated into the Fe2O3 phase after calcination, at least for the iron-rich samples, while no evidence of Fe incorporated into Co3O4 was found. Upon reduction, most probably FeCo alloys were formed in the iron-rich bimetallic samples. The degree of reduction of the catalysts showed a non-linear behavior with respect to the Fe:Co ratio, but it is obvious that Co increases the reducibility of Fe. Alloying Co with small/moderate amounts of Fe improved the FT activity compared to the 100% Co catalyst at low conversion levels. Alloying Fe with small/moderate amounts of Co lowered the FT activity, but increased the relative water-gas-shift (WGS) activity compared to the 100% Fe catalyst. However, the overall WGS activity was very low for all catalysts, even with external water addition to the feed, resulting in low FT productivities (per gram catalyst) due to the low partial pressure of H-2. A higher Fe:Co ratio in the bimetallic catalyst generally resulted in higher relative WGS activity, but did not lower the H-2/CO usage ratio to the desired value of 1.0. For the Fe-containing catalysts, the space-time yield of hydrocarbons (HCs) decreased with increasing partial pressure of water or reduced space velocity, indicating an inhibition of water on the FT activity, most often resulting in low FT productivity under the conditions with highest relative WGS activity (usage ratios closest to the inlet H-2/CO ratio). Moreover, the co-impregnation technique resulted in a surface enrichment of Fe, at least for the Co-rich samples, covering the Co sites. For the bimetallic catalysts, both FT and WGS activities rapidly declined at high partial pressure of water due to deactivation by oxidation and sintering. However, the results indicate that WGS and FT proceeded over sites of different nature in the bimetallic catalysts. The bimetallic catalysts showed essentially no synergy effects with respect to HC selectivities and olefin/paraffin ratios, which partly can be explained by the use of a sub-stoichiometric H-2/CO ratio as feed. The higher the Fe content, the lower were the C5+ selectivity and C-3 olefin/paraffin ratio. Water addition increased the C5+ selectivity and C-3 Olefin/paraffin ratio and reduced the CH4 selectivity.
|
|
| 3. |
- Alrifaiy, Ahmed, et al.
(författare)
-
A lab-on-a-chip for hypoxic patch clamp measurements combined with optical tweezers and spectroscopy : first investigations of single biological cells
- 2015
-
Ingår i: Biomedical engineering online. - : Springer Science and Business Media LLC. - 1475-925X. ; 14
-
Tidskriftsartikel (refereegranskat)abstract
- The response and the reaction of the brain system to hypoxia is a vital research subject that requires special instrumentation. With this research subject in focus, a new multifunctional lab-on-a-chip (LOC) system with control over the oxygen content for studies on biological cells was developed. The chip was designed to incorporate the patch clamp technique, optical tweezers and absorption spectroscopy. The performance of the LOC was tested by a series of experiments. The oxygen content within the channels of the LOC was monitored by an oxygen sensor and verified by simultaneously studying the oxygenation state of chicken red blood cells (RBCs) with absorption spectra. The chicken RBCs were manipulated optically and steered in three dimensions towards a patch-clamp micropipette in a closed microfluidic channel. The oxygen level within the channels could be changed from a normoxic value of 18% O 2 to an anoxic value of 0.0-0.5% O 2. A time series of 3 experiments were performed, showing that the spectral transfer from the oxygenated to the deoxygenated state occurred after about 227 ± 1 s and a fully developed deoxygenated spectrum was observed after 298 ± 1 s, a mean value of 3 experiments. The tightness of the chamber to oxygen diffusion was verified by stopping the flow into the channel system while continuously recording absorption spectra showing an unchanged deoxygenated state during 5400 ± 2 s. A transfer of the oxygenated absorption spectra was achieved after 426 ± 1 s when exposing the cell to normoxic buffer. This showed the long time viability of the investigated cells. Successful patching and sealing were established on a trapped RBC and the whole-cell access (Ra) and membrane (Rm) resistances were measured to be 5.033 ± 0.412 M Ω and 889.7 ± 1.74 M Ω respectively.
|
|
| 4. |
- Blomberg, Sara, et al.
(författare)
-
Generation and oxidation of aerosol deposited PdAg nanoparticles
- 2013
-
Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 616, s. 186-191
-
Tidskriftsartikel (refereegranskat)abstract
- PdAg nanoparticles with a diameter of 10 nm have been generated by an aerosol particle method, and supported on a silica substrate. By using a combination of X-ray Energy Dispersive Spectroscopy and X-ray Photoelectron Spectroscopy it is shown that the size distribution of the particles is narrow and that the two metals form an alloy with a mixture of 75% Pd and 25% Ag. Under oxidizing conditions, Pd is found to segregate to the surface and a thin PdO like oxide is formed similar to the surface oxide previously reported on extended PdAg and pure Pd surfaces. (C) 2013 Elsevier B.V. All rights reserved.
|
|
| 5. |
|
|
| 6. |
|
|
| 7. |
- Fernandes, V. R., et al.
(författare)
-
Reversed Hysteresis during CO Oxidation over Pd75Ag25(100)
- 2016
-
Ingår i: ACS Catalysis. - : American Chemical Society (ACS). - 2155-5435. ; 6:7, s. 4154-4161
-
Tidskriftsartikel (refereegranskat)abstract
- CO oxidation over Pd(100) and Pd75Ag25(100) has been investigated by a combination of near-ambient pressure X-ray photoelectron spectroscopy, quadrupole mass spectrometry, density functional theory calculations, and microkinetic modeling. For both surfaces, hysteresis is observed in the CO2 formation during the heating and cooling cycles. Whereas normal hysteresis with light-off temperature higher than extinction temperature is present for Pd(100), reversed hysteresis is observed for Pd75Ag25(100). The reversed hysteresis can be explained by dynamic changes in the surface composition. At the beginning of the heating ramp, the surface is rich in palladium, which gives a CO coverage that poisons the surface until the desorption rate becomes sufficiently high. The thermodynamic preference for an Ag-rich surface in the absence of adsorbates promotes diffusion of Ag from the bulk to the surface as CO desorbs. During the cooling ramp, an appreciable surface coverage is reached at temperatures too low for efficient diffusion of Ag back into the bulk. The high concentration of Ag in the surface leads to a high extinction temperature and, consequently, the reversed hysteresis.
|
|
| 8. |
- Franken, T., et al.
(författare)
-
Multi-Objective Optimization of Fuel Consumption and NOx Emissions with Reliability Analysis Using a Stochastic Reactor Model
- 2019
-
Ingår i: SAE Technical Papers. - 400 Commonwealth Drive, Warrendale, PA, United States : SAE International. - 0148-7191 .- 2688-3627. ; 2019-April:April
-
Tidskriftsartikel (refereegranskat)abstract
- The introduction of a physics-based zero-dimensional stochastic reactor model combined with tabulated chemistry enables the simulation-supported development of future compression-ignited engines. The stochastic reactor model mimics mixture and temperature inhomogeneities induced by turbulence, direct injection and heat transfer. Thus, it is possible to improve the prediction of NOx emissions compared to common mean-value models. To reduce the number of designs to be evaluated during the simulation-based multi-objective optimization, genetic algorithms are proven to be an effective tool. Based on an initial set of designs, the algorithm aims to evolve the designs to find the best parameters for the given constraints and objectives. The extension by response surface models improves the prediction of the best possible Pareto Front, while the time of optimization is kept low. This work presents a novel methodology to couple the stochastic reactor model and the Non-dominated Sorting Genetic Algorithm. First, the stochastic reactor model is calibrated for 10 low, medium and high load operating points at various engine speeds. Second, each operating point is optimized to find the lowest fuel consumption and specific NOx emissions. The optimization input parameters are the temperature at intake valve closure, the compression ratio, the start of injection, the injection pressure and exhaust gas recirculation rate. Additionally, it is ensured that the maximum peak cylinder pressure and turbine inlet temperature are not exceeded. This enables a safe operation of the engine and exhaust aftertreatment system under the optimized conditions. Subsequently, a reliability analysis is performed to estimate the effect of off-nominal conditions on the objectives and constraints. The novel multi-objective optimization methodology has proven to deliver reasonable results. The zero-dimensional stochastic reactor model with tabulated chemistry is a fast running physics-based model that allow to run large optimization problems in a short amount of time. The combination with the reliability analysis also strengthens the confidence in the simulation-based optimized engine operation parameters.
|
|
| 9. |
- Franken, T., et al.
(författare)
-
Multi-objective optimization of water injection in spark-ignition engines using the stochastic reactor model with tabulated chemistry
- 2019
-
Ingår i: International Journal of Engine Research. - : SAGE Publications. - 1468-0874 .- 2041-3149. ; 20:10, s. 1089-1100
-
Tidskriftsartikel (refereegranskat)abstract
- Water injection is investigated for turbocharged spark-ignition engines to reduce knock probability and enable higher engine efficiency. The novel approach of this work is the development of a simulation-based optimization process combining the advantages of detailed chemistry, the stochastic reactor model and genetic optimization to assess water injection. The fast running quasi-dimensional stochastic reactor model with tabulated chemistry accounts for water effects on laminar flame speed and combustion chemistry. The stochastic reactor model is coupled with the Non-dominated Sorting Genetic Algorithm to find an optimum set of operating conditions for high engine efficiency. Subsequently, the feasibility of the simulation-based optimization process is tested for a three-dimensional computational fluid dynamic numerical test case. The newly proposed optimization method predicts a trade-off between fuel efficiency and low knock probability, which highlights the present target conflict for spark-ignition engine development. Overall, the optimization shows that water injection is beneficial to decrease fuel consumption and knock probability at the same time. The application of the fast running quasi-dimensional stochastic reactor model allows to run large optimization problems with low computational costs. The incorporation with the Non-dominated Sorting Genetic Algorithm shows a well-performing multi-objective optimization and an optimized set of engine operating parameters with water injection and high compression ratio is found.
|
|
| 10. |
- Lundgren, Edvin, et al.
(författare)
-
Surface composition of clean and oxidized Pd75Ag25(100) from photoelectron spectroscopy and density functional theory calculations
- 2012
-
Ingår i: Surface Science. - : Elsevier BV. - 0039-6028. ; 606:23-24, s. 1777-1782
-
Tidskriftsartikel (refereegranskat)abstract
- High resolution photoelectron spectroscopy and density functional theory calculations have been used to study the composition of clean and oxidized Pd75Ag25( 100). The results for the clean surface confirm earlier reports of surface segregation by Wouda et al. (1998), where the top most layers are rich in Ag. The Pd 3d core level component from the surface region is observed at higher binding energies than the contribution from the bulk which is found to be a signature of Pd embedded in Ag. Low energy electron diffraction and scanning tunneling microscopy measurements reveal that oxidation of the Pd75Ag25(100) surface results in a (root 5 x root 5)R27 degrees-O structure similar to the one reported for Pd(100). The calculations suggest that the stable structure is a PdO(101) monolayer supported on a (100) surface rich in Ag at the interface to the stoichiometric alloy. The calculated core level shifts for the oxidized surface are in good agreement with the experimental observations.
|
|
