Shock/turbulence interactions in dense gases

为了使英国的温室气体排放量接近2050年具有法律约束力的目标，对能源浪费和不可持续的电力生产形式进行重大打击是必不可少的。由于其吸引人的热物理特性（例如相对于分子量的大热容量、低沸点、高密度），分子复杂和致密气体（例如碳氢化合物、全氟碳化合物、硅氧烷）是热电站在低温热源（例如太阳能、生物质、地热）上有效运行的现实解决方案的核心，在这些解决方案中，它们被用作水蒸气的替代品（例如有机朗肯循环）。这类电站中的流动膨胀器部分在热力学临界点附近运行，此时声速大幅降低，膨胀器流动变成了超声速气体流动，不可避免地导致激波的形成。激波具有降低膨胀器效率的有害性质，因为它将动能耗散为热量，并通过边界层分离和增厚促进粘性损失。非常显著的是，与理想气体相反，分子复杂和致密的气体中的激波几乎是等温的，因此减轻了激波带来的部分效率损失。这种显著的性质是气体分子的活动自由度特别大的直接结果。虽然高效超音速膨胀器的前景很吸引人，但人们对近等熵冲击对湍流波动放大的影响知之甚少（湍流波动总是存在于涡轮中）。特别是，稠密气体中的激波/湍流相互作用会导致高能声波的发射，这比标准理想气体中的声波要强大得多。如果存在，这种声强迫会侵蚀预期的涡轮效率，产生振动并导致叶片过早疲劳。拟议的研究将建立对稠密气体中激波/湍流相互作用声发射的强大而基本的理解，并提供对潜在物理的新理解，这将允许开发预测工具，为未来的设计选择提供信息。

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.