@misc{ShresthaSeidelZeuchetal., author = {Shrestha, Krishna Prasad and Seidel, Lars and Zeuch, Thomas and Mor{\´e}ac, Gladys and Dagaut, Philippe and Mauß, Fabian}, title = {On the implications of nitromethane - NOx chemistry interactions for combustion processes}, series = {Fuel}, volume = {289}, journal = {Fuel}, issn = {0016-2361}, doi = {10.1016/j.fuel.2020.119861}, pages = {8}, abstract = {In this work, we report a detailed investigation of the CH3NO2 chemistry effect on fuel-NO interactions for the fuels methane and n-heptane using a recently developed and extensively validated H2/O2/CO/NOx/NH3/CH3NO2 baseline chemistry. In general, the model predictions show good agreement with temperature profiles of major and intermediate species in jet-stirred reactor experiments and they capture the subtle effect of NO addition. For both fuels, the CH3NO2 kinetics retard the system reactivity in the low temperature range by delaying the production of key radicals like OH and HO2. This explains the retarding effect of NO for n-heptane low temperature ignition and the overprediction of reactivity enhancement by NO in earlier studies on methane combustion. For methane, the recently explored roaming mediated dissociation channel of CH3NO2 to CH3O + NO is a major reaction pathway for CH3NO2 consumption. Our analysis suggests that at higher pressure, relevant to engine conditions, the two key intermediate species HONO and CH3NO2 feature strongly increased concentrations during n-heptane combustion and they may be detectable under such conditions in combustion experiments of this fuel-NOx system. The results of this work call for detailed future investigations of the CH3NO2 chemistry effect in the context of exhaust gas recirculation, also with regard to the suppression of engine knock.}, language = {en} } @misc{FrankenSeidelShresthaetal., author = {Franken, Tim and Seidel, Lars and Shrestha, Krishna Prasad and Gonzalez Mestre, Laura Catalina and Mauß, Fabian}, title = {Multi-objective Optimization of Gasoline, Ethanol, and Methanol in Spark Ignition Engines}, abstract = {In this study, an engine and fuel co-optimization is performed to improve the efficiency and emissions of a spark ignition engine utilizing detailed reaction mechanisms and stochastic combustion modelling. The reaction mechanism for gasoline surrogates (Seidel 2017), ethanol, and methanol (Shrestha et al. 2019) is validated for experiments at different thermodynamic conditions. Liquid thermophysical properties of the RON95E10 surrogate (iso-octane, n-heptane, toluene, and ethanol mixture), ethanol, and methanol are determined using the NIST standard reference database (NIST 2018) and Yaws database (Yaws 2014). The combustion chemistry, laminar flame speed, and thermophysical data are pre-compiled in look-up tables to speed up the simulations (tabulated chemistry). The auto-ignition in the stochastic reactor model is predicted by the detailed chemistry and subsequently evaluated using the Bradley Detonation Diagram (Bradley et al. 2002, Gu et al. 2003, Neter 2019), which assigns two dimensionless parameters (resonance parameter and reactivity parameter). According to the defined developing detonation limits, the auto-ignition is either in deflagration, sub-sonic auto-ignition, or developing detonation mode. Ethanol and methanol show a knock-reducing characteristic, which is mainly due to the high heat of vaporization. The multi-objective optimization process includes mathematical algorithms for design space exploration with Uniform Latin Hypercube, pareto front convergence with Non-dominated Sorting Genetic Algorithm II (NSGA-II), and multi-criteria decision making (Deb et al. 2002). The optimization input parameter ranges are selected according to the previous sensitivity analysis, and the objectives are to minimize specific CO2 and specific CO and maximize indicated efficiency. The performance study of different optimization algorithms shows that the incorporation of metamodels is beneficial to improve the design space exploration, while keeping the optimization duration low. The comparison of different reaction mechanisms, which are applied in the optimization process, shows a strong impact on the pareto front solutions. This is due to differences in the emission formation and auto-ignition between the different reaction schemes. Overall, the engine efficiency is increased by 3.5 \% points, and specific CO2 emissions are reduced by 99 g/kWh for ethanol and 142 g/kWh for methanol combustion compared to the base case. This is achieved by advanced spark timing, lean combustion, and reduced C:H ratio of ethanol and methanol in relation to RON95E10.}, language = {en} } @misc{ShresthaGiriAdiletal., author = {Shrestha, Krishna Prasad and Giri, Binod Raj and Adil, Mohammad and Seidel, Lars and Zeuch, Thomas and Farooq, Aamir and Mauß, Fabian}, title = {Detailed Chemical Kinetic Study of Acetaldehyde Oxidation and Its Interaction with NOx}, series = {Energy \& fuels}, volume = {35}, journal = {Energy \& fuels}, number = {18}, issn = {1520-5029}, doi = {10.1021/acs.energyfuels.1c01948}, pages = {14963 -- 14983}, abstract = {This work entails a detailed modeling and experimental study for the oxidation kinetics of acetaldehyde (CH3CHO) and its interaction with NOx. The ignition behavior of CH3CHO/O2/Ar has been investigated in a shock tube over the temperature range of 1149 to 1542 K, with equivalence ratios of 0.5 and 1.0 and pressures near 1.2 bar. Absorbance-time profiles of acetaldehyde were recorded using a mid-IR laser during the autoignition measurements. A comprehensive kinetic model has been developed to quantitatively predict the oxidation of acetaldehyde and its interaction with NOx. The kinetic model has been validated using experimental data of this work and available literature data from shock tube, plug flow, and jet-stirred reactors, freely propagating, and burner-stabilized premixed flames. For better accuracy of the kinetic model, the thermochemistry of 14 important species in the acetaldehyde submechanism was calculated using ab initio methods. The heat of formation of these species was computed using atomization and isodesmic reaction schemes. For the first time, this modeling study examines the effect of NO on acetaldehyde oxidation behavior over a wide range of experimental conditions. In most cases, the proposed kinetic model captures the experimental trends remarkably well. Interestingly, the doping of NO in CH3CHO did not perturb the NTC behavior of CH3CHO in contrast to other fuels, such as n-heptane and dimethyl ether. However, for flow reactor conditions at 1 atm, doping with 504 ppm of NO was found to promote the reactivity of acetaldehyde by lowering the onset temperature for CH3CHO oxidation by ∼140 K. The hydroxyl radical is the main cause of this shift, which originates from the NO + HO2 = OH + NO2 reaction. Further evolution of hydroxyl radicals occurs via the "NO-NO2" looping mechanism and expedites the reactivity of the system. This experimental and modeling work sheds new light on acetaldehyde oxidation behavior and its interaction with NOx under combustion-relevant conditions.}, language = {en} } @misc{IssayevGiriElbazetal., author = {Issayev, Gani and Giri, Binod Raj and Elbaz, Ayman M. and Shrestha, Krishna Prasad and Mauß, Fabian and Roberts, William L. and Farooq, Aamir}, title = {Ignition delay time and laminar flame speed measurements of ammonia blended with dimethyl ether: A promising low carbon fuel blend}, series = {Renewable Energy}, volume = {181}, journal = {Renewable Energy}, issn = {1879-0682}, doi = {10.1016/j.renene.2021.09.117}, pages = {1353 -- 1370}, abstract = {Ammonia (NH3) has recently received much attention as a promising future fuel for mobility and power generation. The use of ammonia as a fueling vector can help curb global warming by cutting CO2 emissions because it is a carbon-free fuel and a hydrogen carrier with a high percentage of hydrogen atoms per unit volume. Liquid ammonia contains a higher volumetric density of hydrogen than liquid hydrogen. The low reactivity of ammonia, however, hinders its direct usage as a combustible fuel. One feasible way to boost the reactivity of ammonia is to target a dual-fuel system comprising of ammonia and a suitable combustion promoter. In this work, combustion properties of ammonia were investigated by blending it with various proportions of dimethyl ether (DME) using a rapid compression machine (RCM) and a constant volume spherical reactor (CVSR) over a wide range of experimental conditions. DME is a highly reactive fuel that may be produced in a sustainable carbon cycle with a net zero-carbon emission. Ignition delay times (IDTs) of NH3/DME blends were measured over a temperature (T) range of 649e950 K, pressures (P) of 20 and 40 bar, equivalence ratios (F) of 0.5 and 1 for a range of DME mole fractions (cDME) of 0.05e0.5 in the blends. In addition, the laminar burning velocities of NH3/DME blends were measured at P ¼ 1, 3 and 5 bar, F ¼ 0.8e1.3 and T ¼ 300 K for cDME ranging from 0.18 to 0.47. Our results suggest that DME is a good ignition promoter, resulting in a significant shortening of IDTs and an increase of flame speeds of NH3. A detailed chemical model has been developed and validated against the experimental data. Overall, our kinetic model offered reasonable predictive capabilities capturing the experimental trends over a wide range of conditions. In the worst-case scenario, our model underpredicted IDTs by a factor of ~2.5 while overpredicting laminar flame speed by ~20\%.}, language = {en} } @misc{ShresthaGiriSeideletal., author = {Shrestha, Krishna Prasad and Giri, Binod Raj and Seidel, Lars and Farooq, Aamir and Mauß, Fabian}, title = {A Kinetic Modeling Study for the Effect of NOx on Oxymethylene ethers (OMEn, n = 0 and 1) oxidation}, series = {10th European Combustion Meeting, Neapel}, journal = {10th European Combustion Meeting, Neapel}, address = {Neapel}, abstract = {We present a detailed kinetic model for the oxidation of dimethyl ether (OME0) and dimethoxymethane (OME1) in presence of NOx. We further explored the effect of NOx chemistry on the oxidation kinetics of the two OMEs. Our kinetic model is validated against the recent flow reactor data from Zhang et al. (Combust. Flame. 224 (2021) 94- 107). The results indicated that NO doping severely alters the oxidation kinetics of both fuels. The onset temperature for total fuel consumption is significantly shifted to lower temperatures for both fuels, which is in line with the experimental observation. We found that the addition of NO significantly inhibited the NTC behaviour of dimethyl ether. This inhibiting effect appears to stem from the competition between CH3OCH2O2 radical consumption by NO directly and the isomerization/dissociation reactions of CH3OCH2O2. Unlike dimethyl ether, dimethoxymethane does not exhibit a strong NTC behavior, and NO addition completely inhibited its weak NTC behavior.}, language = {en} } @misc{ShresthaGiriElbazetal., author = {Shrestha, Krishna Prasad and Giri, Binod Raj and Elbaz, Ayman M. and Issayev, Gani and Roberts, William L. and Seidel, Lars and Mauß, Fabian and Farooq, Aamir}, title = {A detailed chemical insights into the kinetics of diethyl ether enhancing ammonia combustion and the importance of NOx recycling mechanism}, series = {Fuel Communications}, volume = {10}, journal = {Fuel Communications}, issn = {2666-0520}, doi = {10.1016/j.jfueco.2022.100051}, pages = {18}, abstract = {In this work, we investigated the combustion characteristics of ammonia (NH3) by blending it with various proportions of diethyl ether (DEE). We measured laminar flame speed of various NH3/DEE blends (DEE, 10-40\% by mole) using a constant volume spherical vessel at Ti = 298 K and Pi = 3 and 5 bar and Φ = 0.8-1.3. We developed a detailed kinetic model to describe the trends of the current and previously published experimental data. For the robustness of the model, we first developed a comprehensive diethyl ether kinetic mechanism to accurately characterize neat DEE oxidation behavior. We validated the kinetic model using a large pool of experimental data comprising shock tube, rapid compression machine, jet-stirred and flow reactors, freely propagating, and burner-stabilized premixed flames. The developed kinetic model performs remarkably in capturing the combustion behavior of pure DEE and NH3. Importantly, our model captures the experimental data of laminar flame speed and ignition delay times of various NH3/DEE blends over a wide range of conditions. We found that DEE is a promising candidate to promote the combustion characteristics of NH3. A small portion of DEE (10\%) enhances the laminar flame speed of NH3 by a factor of 2 at Pi = 1 bar, Ti = 298 K, and Φ = 1.0. A further doubling of the DEE mole fraction to 20\% did not enhance the laminar flame speed of NH3 with the same propensity. At low temperatures, adding 5\% DEE in NH3 blend has significantly expedited the system reactivity by lowering the autoignition temperature. A further 5\% increment of DEE (i.e., 10\% DEE in NH3) lowers the autoignition temperature by ∼120 K to achieve the same ignition delay time. The "NOsingle bondNO2" looping mechanism predominantly drives such reactivity accelerating effect. Here, the reactions, NO + HO2 = NO2 + OH and NO2 + H = NO + OH, appear to enhance the reactive radical pool by generating OH radicals. We observed that the HNO path is favored more with increasing DEE content which eventually liberates NO. Other key reactions in "NOsingle bondNO2" looping mechanism are: CH3 + NO2 = CH3O + NO, CH3O2 + NO = CH3O + NO2, C2H5 + NO2 = C2H5O + NO, C2H5O2 + NO = C2H5O + NO2. In addition, CH3 + NH2(+M) = CH3NH2(+M) reaction is also one of the important cross-reactions which leads to the formation of HCN. Therefore, cross-reactions between the nitrogen and carbon family are crucial in accurately predicting autoignition timing. This work provides a detailed chemical insight into the NH3 and DEE interaction, which could be applied to other fuel blends of NH3. The kinetic model is also validated for several C1single bondC3 fuels including their interaction with NOx.}, language = {en} } @misc{FritscheEckartShresthaetal., author = {Fritsche, Chris and Eckart, Sven and Shrestha, Krishna Prasad and Mauß, Fabian and Krause, Hartmut}, title = {Methane flames with a substitution of 50 to 100 percent hydrogen: Experimental and numerical investigation of the temperature and pressure dependence of the laminar burning velocities}, series = {10th European Combustion Meeting, Proceedings of the European Combustion Meeting}, journal = {10th European Combustion Meeting, Proceedings of the European Combustion Meeting}, abstract = {This work reports the laminar burning velocities for CH4/H2 blends in mixtures with air utilizing the constant volume chamber at temperatures from 300 to 423 K, pressures from 2 to 10 bar and equivalence ratios from 0.7 to 2.4. A detailed chemical kinetic model based on our previous work is used to reproduce the experimental data. The chemical kinetic model can reproduce the experimental data better at lean conditions compared to rich conditions. It is observed that with an increase in H2 fraction in fuel blend, laminar burning velocities increase. Model reveals that with increasing H2 fraction in the fuel blend, formation of key radicals H, OH and O are promoted leading to higher laminar burning velocities. The most sensitive reactions are O2+H=OH+O, CO+OH=CO2+H, CH4+H=CH3+H2 and CH4+OH=CH3+H2O. It is found that as initial mixture temperature increases, the laminar burning velocity increases and shows a linear trend whereas this trend is reversed as the initial pressure increases.}, language = {en} } @misc{FritscheShresthaEckartetal., author = {Fritsche, Chris and Shrestha, Krishna Prasad and Eckart, Sven and Mauß, Fabian and Krause, Hartmut}, title = {Temperature and pressure dependency of the burning velocity in laminar premixed methanol and polyoxymethylene dimethyl ether (OME1, OME2, and OME3) flames}, series = {10th European Combustion Meeting, Proceedings of the European Combustion Meeting}, journal = {10th European Combustion Meeting, Proceedings of the European Combustion Meeting}, abstract = {This work reports the laminar burning velocities (LBV) for the liquid fuels methanol, and polyoxymethylene dimethyl ethers (OMEn, n = 1-3) in mixtures with air utilizing the heat flux burner and constant volume chamber at temperature 393 to 443 K, pressure 1 to 10 bar, and equivalence ratio 0.6 to 1.9. Laminar burning velocities for OME2 and OME3 higher than 1 bar are reported for the first time. A detailed chemical kinetic model for OME2 and OME3 was developed based on our previous work utilizing experimental data from this work. Overall, model predictions are in good agreement with the experimental data. It is previously shown that with increases in unburned gas temperature laminar burning velocity increases and shows a linear trend with respect to temperature. Further, laminar burning velocity decreases with an increase of initial pressure. The dependence of burning velocity is not linear for all the fuels investigated. For comparable temperature and pressure conditions, it was found that with an additional CH2O group the laminar burning velocities increase marginally and there is no shift of maximum laminar burning velocity with respect to equivalence ratio.}, language = {en} } @misc{ShresthaSeidelZeuchetal., author = {Shrestha, Krishna Prasad and Seidel, Lars and Zeuch, Thomas and Mauß, Fabian}, title = {Modeling of NOx Formation and Consumption during Oxidation of Small Alcohols}, series = {9th European Combustion Meeting, Proceedings of the European Combustion Meeting}, journal = {9th European Combustion Meeting, Proceedings of the European Combustion Meeting}, pages = {6}, abstract = {This work presents a newly developed kinetic mechanism extending our recent work (Shrestha et al. [1]) for the oxidation of methanol and ethanol and their fuel interaction with NO x chemistry in jet-stirred reactors, flow reactors, and burner-stabilized premixed flames. The work mainly focuses on fuel interaction with nitrogen chemistry and NO formation in laminar premixed flames. It is found that for methanol oxidation in jet-stirred reactor doping of the fuel blends with NO increase the reactivity of the system by increasing the net production of OH radicals. The increased amount of OH is formed via NO/NO 2 interconversion reaction channels NO+HO 2 ⇋NO 2 +OH, NO 2 +H⇋NO+OH, NO 2 +HO 2 ⇋HONO+O 2, followed by the thermal decomposition of HONO producing NO and OH. In burner-stabilized premixed flames studied here for methanol/air and ethanol/air, NO is mainly formed via the NCN route (CH+N 2 ⇋NCN+H) and minor contribution comes from the NNH route (NNN⇋N 2 +H).}, language = {en} } @misc{RakhiShresthaGuentheretal., author = {Rakhi, Rakhi and Shrestha, Krishna Prasad and G{\"u}nther, Vivien and Mauß, Fabian}, title = {Thermodynamic analysis to develop a detailed surface reaction mechanism}, series = {Fuel Science - From Production to Propulsion, Aachen, Germany, May 2022}, journal = {Fuel Science - From Production to Propulsion, Aachen, Germany, May 2022}, pages = {2}, abstract = {The reduction of greenhouse gasses such as CO2 and CH4 is becoming necessary due to global environmental problems. The reforming of light hydrocarbons is a particularly efficient process for producing synthesis gas, H2 and CO, from greenhouse gasses [1]. The steam reforming of methane is the most important method to produce syngas in industry by using a nickel catalyst. Nickel-based catalysts are the conventional catalysts in industrial applications due to their fast turnover rates, good availability, and low costs, however, limited by their tendency towards coke formation. In this study, a detailed surface reaction mechanism is developed for steam reform-ing of methane over nickel and results are compared with the reference data.}, language = {en} } @misc{FrankenSrivastavaLeeetal., author = {Franken, Tim and Srivastava, Vivek and Lee, Sung-Yong and Heuser, Benedikt and Shrestha, Krishna Prasad and Seidel, Lars and Mauß, Fabian}, title = {Numerical Analysis of the Combustion of Diesel, Dimethyl Ether, and Polyoxymethylene Dimethyl Ethers (OMEn, n=1-3) Using Detailed Chemistry}, series = {THIESEL 2022 : Conference on Thermo- and Fluid-Dynamics of Clean Propulsion Powerplants, 13th-16th September 2022 : conference proceedings}, journal = {THIESEL 2022 : Conference on Thermo- and Fluid-Dynamics of Clean Propulsion Powerplants, 13th-16th September 2022 : conference proceedings}, editor = {Xandra, Margot and Payri, Ra{\´u}l and Serrano, Jos{\´e} Ram{\´o}n}, publisher = {Editorial Universitat Polit{\`e}cnica de Val{\`e}ncia}, address = {Val{\`e}ncia}, isbn = {978-84-1396-055-5}, doi = {10.4995/Thiesel.2022.632801}, abstract = {New types of synthetic fuels are introduced in internal combustion engine applications to achieve carbon-neutral and ultra-low emission combustion. Dimethyl Ether (DME) and Polyoxymethylene Dimethyl Ethers (OMEn) belong to such kind of synthetic fuels. Recently, Shrestha et al. (2022) have developed a novel detailed chemistry model for OMEn (n=1-3) to predict the ignition delay time, laminar flame speed and species formation for various thermodynamic conditions. The detailed chemistry model is applied in the zero dimensional (0D) stochastic reactor model (DI-SRM) to investigate the non-premixed combustion in a 2-liter diesel engine. Further insights in the formation of unburned hydrocarbons (HC), carbon monoxide and nitrogen oxides during the combustion of OMEn fuels are obtained in this work. The combustion and emission formation of DME and OMEn (n=1-3) are investigated and compared to conventional Diesel combustion. The mixture formation is governed by an earlier vaporization of the DME and OMEn fuels, faster homogenization of the respective air-fuel mixture and higher reactivity. At the same injection pressure, the OMEn fuels obtain higher NOx but lower CO and HC emissions. High amounts of aromatics, ethene, methane formaldehyde and formic acid are found within the Diesel exhaust gas. The DME and OMEn exhaust gas contains higher fractions of formaldehyde and formic acid, and fractions of methane, methyl formate and nitromethane.}, language = {en} } @misc{FrankenShresthaSeideletal., author = {Franken, Tim and Shrestha, Krishna Prasad and Seidel, Lars and Mauß, Fabian}, title = {Effect of Gasoline-Ethanol-Water Mixtures on Auto-Ignition in a Spark Ignition Engine}, series = {International Conference on Knocking in Gasoline Engines}, journal = {International Conference on Knocking in Gasoline Engines}, editor = {Sens, Marc}, publisher = {expert}, address = {T{\"u}bingen}, isbn = {978-3-8169-3544-5}, doi = {10.24053/9783816985440}, pages = {175 -- 222}, abstract = {The climate protection plan of the European Union requires a significant reduction of CO2 emissions from the transportation sector by 2030. Today ethanol is already blended by 10vol-\% in gasoline and further increase of the ethanol content to 20vol-\% is discussed. During the ethanol production process, distillation and molecular sieving is required to remove the water concentration to achieve high-purity ethanol. However, hydrous ethanol can be beneficial to suppress knock of spark ignition engines. The hygroscopic nature of ethanol can allow to increase the water content in gasoline - water emulsions even more, without adding additional surfactants, and improve the thermal efficiency by optimized combustion phasing, while keeping the system complexity low. Hence, the effect of gasoline - ethanol - water mixtures on the auto-ignition in a single-cylinder spark ignition engine is investigated by using multi-dimensional simulation and detailed chemistry. The gasoline - ethanol mixtures are defined to keep the Research Octane Number constant, while the Motored Octane Number is decreasing. In total five surrogates are defined and investigated: E10 (10vol-\% ethanol-in-gasoline), E20, E30, E70 and E100. The water content is determined according to experimentally defined ternary diagrams that evaluated stable gasoline - ethanol - water emulsion at different gasoline - ethanol blending ratios. The auto-ignition modes of the surrogates are analyzed using the diagram, which determines if hotspots are within harmless deflagration or harmful developing detonation regime. The strongest auto-ignition is observed for the E10 surrogate, while increasing ethanol content reduces the surrogate reactivity and increases the resonance parameter. No auto-ignition of the unburnt mixture is observed for the E70 and E100 surrogates. The addition of hydrous ethanol decreased the excitation time of the surrogates, especially at low ethanol content, wherefor the reactivity parameter is significantly increased. The hotspots for E10, E20 and E30 surrogates with hydrous ethanol are found within the developing detonation regime, while hotspots of the E70 surrogate with hydrous ethanol are found in the transition regime. For the hydrous E100 surrogate no auto-ignition is predicted because of reduced temperature of the unburnt mixture due to water vaporization, which outweighs the increased reactivity due to water vapor addition.}, language = {en} }