Supercritical fuel jets - resolving controversy

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New high efficiency engine combustion modes hold the promise to significantly reduce the contribution of transport vehicles to climate change. All of these modes rely upon direct injection of fuel into the combustion chamber, and so fuel/air mixture preparation is a controlling process in terms of engine efficiency (i.e. reduced CO2 production) and pollutant emissions (including soot, another contributor to climate change). As these new combustion modes push to higher pressure and temperature, it would appear that the fuel jets probably undergo a thermodynamic change, becoming supercritical at the edges of the jet. If that were true, it would completely change our current understanding of fuel/air mixture preparation, and that would have a significant effect on engine performance and design. At this time several theory groups are in disagreement on whether or not this change happens, and if it does how best to understand it. This project will resolve those disagreements, and it will lead to the understanding required to adapt both fuel injectors and engines to this potential new reality.Aside from the motor industry, supercritical mixing is important to the pharmaceutical, food processing, catalyst production, and other nanomaterials industries, and our goal is to team with researchers in these areas as well.This project has four main parts. A specialized, optically-accessible cell will be designed and built based on a successful design under operation at the Technical University of Darmstadt. A laminar liquid jet (with clear access to the fluid/gas interface) will flow down through this chamber, which can be set to various pressures and temperatures below and above the liquid critical point. A line-Raman scattering instrument will be developed in order to characterize the chemical composition of the flowfield as a function of time and position. Next, laser induced thermal acoustics (LITA) will be developed. LITA will be used to measure the sound speed as a function of position and time as conditions are varied. The sound speed reaches a minimum at the critical point, and increases very steeply as pressure and temperature go above the critical point, so it can be a significant marker for thermodynamic states. Finally, Förster resonance energy transfer (FRET) will be evaluated as a way to observe changes in the density (mean free path) as the jet approaches a supercritical state. Such a change is considered to be a distinctive marker for this state as well. This program has been designed specifically for our theory partners (City University London, Sandia National Labs, Stanford University, and University of Wisconsin), who have taken part in planning discussions for this proposal. We will thus use the experiments to provide quantitative information never before available to academia or to industry. The information will provide unambiguous, quantitative results with two aspects: 1) Industry can use them to re-think their mixture preparation strategies, while 2) theoreticians can use the results to inform and validate models. Ultimately those models will be delivered to industry.As mentioned, there are many other subject areas interested in similar problems. Once this system is operating well we will approach UK researchers working in related areas and offer this facility for collaborative research.The US Air Force Office of Scientific Research has committed to support partially this effort, with $360,000.