Supercritical fuel jets - resolving controversy

新型高效发动机燃烧模式有望大幅减少运输车辆对气候变化的影响。所有这些模式都依赖于将燃料直接喷射到燃烧室中，因此燃料/空气混合物制备是发动机效率（即减少的CO2产生）和污染物排放（包括烟尘，气候变化的另一个贡献者）方面的控制过程。随着这些新的燃烧模式推向更高的压力和温度，似乎燃料射流可能经历热力学变化，在射流边缘处变得超临界。如果这是真的，它将完全改变我们目前对燃料/空气混合物制备的理解，这将对发动机性能和设计产生重大影响。目前，一些理论团体对这种变化是否会发生以及如何最好地理解它存在分歧。该项目将解决这些分歧，并将导致对燃料喷射器和发动机适应这种潜在新现实所需的理解。除了汽车工业，超临界混合对制药，食品加工，催化剂生产，和其他纳米材料行业，我们的目标是与这些领域的研究人员合作。这个项目有四个主要部分。将根据达姆施塔特技术大学正在运作的一项成功设计，设计和建造一个专门的光学无障碍牢房。层流液体射流（清楚地进入流体/气体界面）将向下流过该室，该室可以设置为低于和高于液体临界点的各种压力和温度。将开发一种线拉曼散射仪器，以表征流场的化学成分随时间和位置的变化。接下来，将开发激光诱导热声学（LITA）。当条件变化时，LITA将用于测量作为位置和时间函数的声速。声速在临界点处达到最小值，并且随着压力和温度超过临界点而急剧增加，因此它可以是热力学状态的重要标志。最后，福斯特共振能量转移（FRET）将作为一种观察密度变化（平均自由程）的方式进行评估，因为射流接近超临界状态。这种变化也被认为是这种状态的一个独特标志。该计划是专门为我们的理论合作伙伴（城市大学伦敦，桑迪亚国家实验室，斯坦福大学和威斯康星州大学）设计的，他们参加了该提案的规划讨论。因此，我们将利用这些实验提供学术界或工业界以前从未获得过的定量信息。这些信息将从两个方面提供明确的定量结果：1）行业可以使用它们来重新思考他们的混合物制备策略，而2）理论家可以使用这些结果来通知和验证模型。最终这些模型将被交付给工业界。如前所述，还有许多其他学科领域对类似的问题感兴趣。一旦这个系统运行良好，我们将接触在相关领域工作的英国研究人员，并提供这个设施进行合作研究。美国空军科学研究办公室已承诺部分支持这项工作，提供36万美元。

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.