Mixture formation and combustion of alcohols and other biogenic fuels in mixture-controlled combustion processes

Methods

The fuel injection process in modern diesel engines is a complex two phase problem. The cause effect chain (Injection-Evaporation-Mixing-Combustion) is strongly coupled and needs a variety of models to capture the physical and chemical modes of action. For the numerical discretization method, the finite volume method (FVM) is utilized. The physical domain is resolved by small discrete volumes that are called cells. The so-called Navier-Stokes- Equations are solved in order to obtain the flow field information in each of these cells. Additionally, equations for energy, chemical reactions and evaporation are solved. In contrast to the so-called Eulerian-Eulerian (e.g., Volume of Fluids (VOF)) simulation, which resolves both liquid and gaseous phase in Eulerian coordinates, the less computational expensive Eulerian-Lagrangian approach is used in this project. In this approach, the gaseous phase is solved in Eulerian coordinates and the liquid phase is described by lagrangian particles that interact with the gas phase. Each particle, a so-called parcel, represents a number of droplets that share the same properties (temperature, size, velocity). The coupling between parcels and gas phase involves momentum and evaporation source terms. In spray combustion, the mixture formation is an important quantity. The quality of the mixture field determines the accuracy of the combustion and pollutant formation simulation. Therefore, high fidelity Large Eddy Simulations (LES) are frequently used in literature. A large amount of the turbulent structures/eddies are resolved (high grid resolution), only a smaller amount of turbulence has to be modeled using appropriate turbulence models, leading to the need of HPC-resources. The last part in the simulation is the auto ignition process in spray flames. Chemical reactions are often computed using detailed chemistry solver, but in this project the so-called flamelet model is used, which allows to speed up the simulation significantly. The idea behind the flamelet model is that a turbulent flame can be represented by a manifold of one- dimensional flame structures, so-called flamelets. The great advantage of the flamelet model lies in the ability to precompute the flamelet solutions for all relevant thermo-chemical states and store them in look up tables. During the simulation the chemical source terms can be looked up, instead of solving a large amount of additional differential equations, using appropriate access variables.

Results

This project delivered important information on the diesel relevant spray combustion of biogenetic and synthetic fuels. The influence of significantly different thermophysical properties of renewable diesel fuels have been elucidated in detail. The influence of the increased latent heat of evaporation on ignition has been discovered and a new combustion model has been developed. Furthermore, the cause effect chain of different injector nozzle sizes has been elucidated and explained. Lastly, a new combustion model for “dual-fuel” combustion systems has been developed.

Discussion

The findings of this project are crucial for the further development of combustion engines using renewable fuels. As a novelty in this work, the influence of thermophysical properties of biogenic and synthetic fuels on mixture formation and combustion have been elucidated. For the first time the cause effect chain of mixture formation and combustion has been elucidated in great detail.
The new models can be utilized to reduce the computational cost for predevelopment of combustion devices that use renewable biofuels. The “dual-fuel” combustion model can be utilized to accelerate the development of retrofit solutions in large marine engines to reduce emissions.

Additional Project Information

DFG classification: 404 Fluid Mechanics, Technical Thermodynamics and Thermal Energy Engineering
Software: OpenFOAM, The C++, ULF, ParaView
Cluster: Lichtenberg

Publications

Haspel, P; Gierth, S; Popp, S; Scholtissek, A;  Rieß, S; Wensing, M; Hasse, C. Large eddy simulation of OME3 and OME4 spray combustion under heavy-duty conditions (2023). https://doi.org/10.1016/j.fuel.2023.129097.

Haspel, P. PhD Thesis: Comparison of turbulent reactive spray characteristics of different renewable fuels using Large Eddy Simulation, Simulation of reactive Thermo-Fluids (STFS), Department of Mechanical Engineering, TU Darmstadt (2023)

Haspel, P. Combined numerical and experimental study of 1-octanol spray flames at ECN Spray A conditions using measured Vapor Liquid Equilibrium data, 18th International Conference on Numerical Combustion, San Diego (2022)

Haspel, P. Evaluation of the Unsteady Flamelet Progress Variable Approach in Large Eddy Simulation of the ECN Spray A, ECN Webmeeting, ECN Webmeeting (2022)