One hundred milliseconds, maybe 120, is all the time that Florian Weckenmann has for his experiment. An engineer at the University of Stuttgart’s Institute of Aerospace Thermodynamics, he is investigating how individual droplets of Acetone behave near the critical point at which the substance’s liquid and gas phases have the same density and are therefore indistinguishable. What may seem at first blush to be a rather vaporous pursuit has in fact very practical applications. More precise knowledge of droplet behaviour could, for example, boost the efficiency of diesel engines and aerospace propulsion units, Weckenmann said. “Droplet Research has been going on for some time,” said Weckenmann’s colleague Andreas Birkefeld. The Research is aimed at enriching scientists’ understanding of droplets and improving measuring techniques and calculation models. “But the more extreme the conditions become, the more imprecise are the findings.” As an example of the extremes being studied, the small but sturdy test chamber used for the experiments is designed to withstand pressures exceeding 60 bar - atmospheric pressure at sea level is equal to about 1 bar. “The Auto industry has been puzzling over this topic for years” and developers have made “quantum leaps,” remarked a spokesman for the Association of German Engineers (VDI). As technology advances, the relevant droplets get smaller, but computer models have become more accurate, too, he pointed out. An understanding of the thermodynamic processes involved is key. “You can only simulate them on a computer when you’ve got an idea of what happens when a droplet vapourises,” the VDI spokesman said. Matthias Brock, a spokesman for the Germany-based luxury automaker Daimler, while talking of effects on engine efficiency, said that the atomisation process and vapourisation influenced emission behaviour. “Unwanted side effects, such as oil dilution during catalytic converter regeneration, are likewise strongly influenced by vapourisation,” he said. “These effects can increase with larger proportions of biodiesel, for instance.” The Droplet Research in Stuttgart is part of a joint project with the Darmstadt University of Technology and German Aerospace Centre (DLR). Initially focused on individual droplets, it will now turn more to droplet groups and then sprays. Each research site has a different area of study. Under investigation by scientists in Darmstadt, for example, is how droplets behave on the surface of an insulator - such as dew on high-tension wires - and whether they attack it under the influence of strong electric fields. The DLR is looking at the injection of sprays into rocket thrust chambers.
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