Renewable bio-derived alternative fuels are seen as a viable option to reduce net greenhouse gas emissions, and also provide energy security by relying on locally-sourced feedstock. Unlike conventional aircraft fuels, which maintain tight bounds on fuel specs, such as physical and chemical properties, alternative fuels might have widely varying composition and properties. This variability introduces uncertainty in their utility, both for fuel certification purposes and their ultimate use as transportation fuel. Hence, the practical use of alternative fuels is predicated on the availability of reliable tools that can estimate performance given some basic information about the physical and chemical composition. The prediction of fuel emissions is important. In this program, this issue of predictability of fuel performance was addressed. The overarching objective was to develop physics-based models that are fuel-composition sensitive, such that aircraft combustors can be directly simulated to estimate emissions performance.
A fully validated physics-based model for simulating emissions from aircraft combustors was sought. For this purpose, an atomistic to full-scale modeling program was commissioned. Chemistry models that describe both fuel oxidation and particulate emissions from gas turbines were developed. Due to the sensitivity of emissions formation from turbulent mixing, the unsteady and three-dimensional large eddy simulation (LES) approach was used for modeling the full-scale combustors. A critical bottleneck in assessing models was the lack of high-fidelity validation data for turbulence-combustion interaction in alternative fuels based flames. For this purpose, novel laser-diagnostics approaches were used to simultaneously measure soot-velocity-temperature fields that vastly enhanced validation capabilities.
The main outcomes of this program were: a) a surrogate-fuel based description of alternative fuels, which allows the development of fuel oxidation mechanisms for any given fuel composition, b) a molecular-dynamics driven oxidation mechanism for soot particulates, which allows fuel-sensitive prediction of particulate emissions, c) a comprehensive LES model for soot-turbulence interaction, developed and implemented in an open source solver and applicable to full-scale gas turbine combustor simulations, and d) an extensive database of high-fidelity laser-diagnostics based planar imaging of alternative-fuels burning turbulent flames, with simultaneous measurement of key validation quantities.
This project significantly advanced the fundamental understanding of emissions formation in aircraft combustors. The main products from this program are a) a comprehensive and openly distributed simulation software for emissions from aircraft combustors, b) an extensive database of high quality validation data, c) fundamental models for soot formation, fuel chemistry, and turbulence-chemistry interactions. Each of these have broader applied beyond aircraft engines, in power generation, chemical processing, and atmospheric transport of particulates.