Shock/turbulence interactions in dense gases

为了减少英国温室气体排放接近具有法律约束力的2050年目标，对能源浪费和不可持续的电力生产形式进行重大打击至关重要。分子复杂、密度大的气体(如碳氢化合物、全氟碳化合物、硅氧烷等)因其具有吸引人的热物理性质(如相对分子质量的大热容、低沸点、高密度)而成为热电厂利用低温热源(如太阳能、生物质、地热)高效运行的现实解决方案的核心，在低温热源中，它们被用作水蒸汽的替代品(如有机朗肯循环)。这类电站中的膨胀机部分工作在热力学临界点附近，在那里声速大大降低，使膨胀机流动变成高超音速气流，不可避免地导致激波的形成。冲击波具有不利的性质，即通过将动能耗散成热来降低膨胀机的效率，并通过边界层分离和增厚促进粘性损失。非常值得注意的是，与理想气体相反，分子复杂的稠密气体中的冲击波可以几乎等温，从而缓解了冲击波造成的部分效率损失。这种非凡的性质是气体分子异常多的活跃自由度的直接结果。虽然高效超音速膨胀机的前景很吸引人，但人们对近等熵激波对湍流波动(涡轮机中总是存在的)的放大的影响知之甚少。特别是，稠密气体中的激波/湍流相互作用可以导致高能声波的发射，这比标准理想气体中的声波要强得多。如果存在这种声学强迫，可能会侵蚀预期的涡轮机效率，产生振动，并导致叶片过早疲劳。拟议中的研究将建立对稠密气体中激波/湍流相互作用发出的声音的强有力和基本的理解，并提供对基本物理的新理解，这将允许开发预测工具，为未来的设计选择提供参考。

To reduce the UK's greenhouse-gas emissions anywhere near the legally-binding 2050 targets, a major attack on both energy wastes and unsustainable forms of electricity production is essential. Owing to their appealing thermo-physical properties (e.g. large heat capacity relatively to the molecular weight, low boiling point, elevated density), molecularly-complex and dense gases (e.g. hydrocarbons, perfluorocarbons, siloxanes) are at the heart of realistic solutions for thermal power stations to operate efficiently on low-temperature heat sources (e.g. solar, biomass, geothermal), where they are used as substitute for water steam (e.g. organic Rankine cycle). Flow expanders in such power stations partially operate in the vicinity of the thermodynamic critical point, where the speed of sound is substantially reduced, turning the expander flow into a highly supersonic gas flow, inevitably leading to the formation of shock waves.Shock waves have the detrimental property of degrading the expander efficiency by dissipating kinetic energy into heat, and by promoting viscous losses through boundary-layer separation and thickening. Quite remarkably, and contrary to ideal gases, shock waves in molecularly-complex and dense gases can be made almost isothermal, therefore relieving part of the efficiency losses imparted by the shock wave. This remarkable property is a direct consequence of the exceptionally large number of active degrees of freedom of the gas molecule. While the prospect of efficient supersonic expanders is appealing, little is known on the implication near-isentropic shocks have on the amplification of turbulence fluctuations (which are always present in turbines). In particular, shock/turbulence interactions in dense gases can lead to the emission of energetic acoustic waves, which are significantly more powerful than in standard ideal gases. If present, such acoustic forcing can erode the expected turbine efficiency, generate vibrations and cause premature blade fatigue. The proposed research will establish a robust and fundamental understanding of sound emission from shock/turbulence interactions in dense gases, and provide a new understanding of the underlying physics, which will allow the development of predictive tools that can inform future design choices.