title: Comparison of turbulent reactive spray characteristics of different renewable fuels using Large Eddy Simulation
Creators: name: Haspel, Philip Michael
Publisher: name: UNSPECIFIED
Year: 2023

description: The human-caused climate change imposes many challenges for future generations. It is commonly agreed to stop the global warming process, and new technologies have to be found to reduce the footprint of greenhouse gases. A high share of greenhouse gas emissions comes from the transport sector. In particular, carbon dioxide CO2 from the combustion of fossil fuels in engines contributes significantly to global warming. The reduction of emissions in the transport sector can either be achieved by decarbonization, eliminating energy carriers containing carbon, or defossilation, including combustion of carbon-neutral fuels. The defossilation pathway is currently favored, and many fuels from renewable sources are in the focus of research. Two very promising carbon-neutral Diesel fuels are 1-Octanol, which is produced from biogenic feedstock, and the group of Polyoxymethylene ethers (OME) that are synthesized from green hydrogen and ambient carbon dioxide. Both fuels are considered backstop technologies, which makes them very interesting in sustainable energy production. Both fuels exhibit changed thermophysical and chemical kinetic properties compared to conventional fossil fuels. The latent heat of evaporation of 1-Octanol is significantly increased compared to Diesel. Also, OME shows higher latent heat of evaporation and a significantly increased vapor pressure. The reactivity of 1-Octanol is reduced, while OME is considered a high-reactive fuel due to its high level of oxygenation. Both fuels show no soot formation and can be utilized in blends with regular fuels to meet new emission regulations. Furthermore, emissions of CO, CO2, NOx and soot can be significantly reduced with 1-Octanol and OME. In the present thesis, the spray flame ignition is examined for the renewable fuels 1-Octanol and OME and compared with the Diesel surrogate n-Dodecane. The investigation utilizes a high-fidelity Large Eddy Simulation framework coupled with a tabulated flamelet-generated manifold combustion model. In particular, the influence of the changed thermophysical properties on mixture formation is elucidated. Further, the effect of the changed thermophysical properties on ignition is investigated. Also, the influence of the changed chemical kinetic properties on ignition is examined. The impact of the latent heat of evaporation on ignition will be elucidated. The flame structures of 1-Octanol and OMEmix are compared to the Diesel reference fuel n-Dodecane. The analysis is performed in an automotive, heavy-duty and marine injector with increasing nozzle sizes, and the influence of the nozzle size on ignition is discussed. Excellent agreement of the Large Eddy Simulations under inert conditions with experimental data regarding liquid penetration and vapor penetration length is achieved. The mixture formation analysis of the automotive Engine Combustion Network (ECN) Spray A3 injector shows that 1-Octanol and n-Dodecane exhibit a similar mixture formation process, while OMEmix shows higher values of the scalar dissipation rate and a narrower spray shape. The mixture formation process in the heavy-duty ECN Spray D and marine injector from Woodward L'Orange show delayed mixture formation. The large particle diameter leads to reduced drag and consecutive less momentum exchange and heat transfer to the liquid phase. The temperature distribution of the gas phase clearly shows that the heat loss due to evaporation of 1-Octanol is pronounced. The comparison of the adiabatic mixing line assumed in the combustion model and the temperature distribution in the spray revealed that the higher heat capacity of n-Dodecane inherently leads to a more concave shape of the adiabatic mixing line. This fuel property makes n-Dodecane less sensitive to spray cooling effects on ignition. The flame structure is first investigated utilizing laminar non-premixed 1D flamelet simulations. OMEmix shows the highest reactivity and the lowest ignition delay times for different scalar dissipation rates at stoichiometry. Furthermore, the highest ignition limit is observed for OMEmix. Compared to OMEmix, the ignition delay time of n-Dodecane is increased, and its ignition limit is significantly lower. 1-Octanol shows the highest ignition delay times at lower scalar dissipation rates at stoichiometry. In the proximity of the ignition limit of n-Dodecane, the ignition delay time of 1-Octanol is shorter, which is explained by an increased reactivity of 1-Octanol during the high-temperature ignition. A novel flamelet model is derived that incorporates the heat losses due to evaporation based on the results from the analysis of the gas temperature distribution of the spray. In contrast to methods from the literature, the presented model is physically consistent and does not change the spray flame structure. The novel flamelet model is utilized in the reactive spray simulations for all fuels and injectors investigated. The typical onset of ignition over the spray head in ECN Spray A3 has been confirmed for all fuels. In contrast, the start of ignition at the spray flanks has been observed in the ECN Spray D and the marine injector from Woodward L'Orange. In this thesis, a cause-effect mechanism has been identified that explains the different ignition locations. The onset of ignition in mixture fraction space is similar for each fuel in all injectors. This finding suggests that the mixture formation process dominates the ignition location. The comparison of different-sized nozzles shows that the lowest ignition delay time is found for Spray A3, and the ignition delay time of the larger nozzles is increased. The trend from the flamelet simulation of the lowest ignition delay time for OMEmix is also observed in the spray flame. OMEmix exhibits a significantly different mixture formation and ignition behavior than 1-Octanol and n-Dodecane, due to its high stoichiometric mixture fraction. The mixture formation process of n-Dodecane and 1-Octanol is similar, while the ignition delay time of n-Dodecane is shorter than that of 1-Octanol. The influence of heat loss due to evaporation is very prominent for 1-Octanol. The flamelet model without the heat-loss correction underestimates the ignition delay time by 25%. The heat-loss corrected model perfectly aligns with the experimental ignition delay time. Overall, this thesis contributes to the understanding of the ignition of spray flames with renewable Diesel fuels. Significant differences in the mixture formation process have been identified and explained with the changed thermophysical properties. A novel flamelet model incorporating heat loss due to latent heat of evaporation is developed and successfully utilized in the simulation of reactive sprays in LES. Perfect agreement by means of ignition delay time and flame structure has been achieved. This thesis significantly contributes to a deeper understanding of renewable fuels in the context of defossilation. The results of this thesis can be utilized to develop new technologies that reduce greenhouse gases in the transport sector and slow down global warming.