Supercritical fuel jets - resolving controversy

新的高效发动机燃烧模式有望显著减少交通工具对气候变化的影响。所有这些模式都依赖于向燃烧室直接喷射燃料，因此燃料/空气混合物的制备在发动机效率(即减少的二氧化碳产生)和污染物排放(包括导致气候变化的另一个因素--烟尘)方面是一个控制过程。随着这些新的燃烧模式推向更高的压力和温度，燃料喷流似乎可能经历了热力学变化，在喷流边缘变得超临界。如果这是真的，它将彻底改变我们目前对燃料/空气混合物制备的理解，这将对发动机的性能和设计产生重大影响。目前，对于这种变化是否会发生，以及它是否会发生，以及如何最好地理解它，几个理论小组存在分歧。这个项目将解决这些分歧，它将导致对喷油器和发动机适应这一潜在新现实所需的理解。除了汽车行业，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非常重要，我们的目标也是与这些领域的研究人员合作。该项目包括四个主要部分。将根据达姆施塔特技术大学正在运行的成功设计，设计和建造一个专门的、光学可接近的单元。层流液体射流(有通向流体/气体界面的通道)将向下流过该腔室，该腔室可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，以表征流场的化学成分随时间和位置的变化。接下来，将发展激光诱导热声学(LITA)。随着条件的变化，LITA将被用来测量声速作为位置和时间的函数。声速在临界点达到最小，随着压力和温度超过临界点，声速急剧增加，因此它可以作为热力学状态的重要标志。最后，Förster共振能量转移(FRET)将被评估为一种观察喷流接近超临界状态时密度(平均自由程)变化的方法。这样的变化也被认为是这个州的一个独特标志。这个项目是专门为我们的理论合作伙伴(伦敦城市大学、桑迪亚国家实验室、斯坦福大学和威斯康星大学)设计的，他们参与了这项提案的规划讨论。因此，我们将利用这些实验来提供学术界或工业界从未有过的定量信息。这些信息将从两个方面提供明确的、定量的结果：1)行业可以利用这些结果来重新思考他们的混合物制备策略，2)理论家可以使用这些结果来告知和验证模型。最终，这些模型将交付给行业。如前所述，还有许多其他主题领域对类似的问题感兴趣。一旦该系统运行良好，我们将接触在相关领域工作的英国研究人员，并为合作研究提供这一设施。美国空军科学研究办公室已承诺为这一努力提供部分支持，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.