Shock/turbulence interactions in dense gases

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.

The Earth's surface receives enough solar radiation in 10 minutes to fulfill the world's annual electricity consumption. Yet, the actual contribution of solar energy to the mix is below 1%. One particular technology, known as thermal solar power (TSP), uses the sun's radiation to heat up the working fluid of a steam turbine, a method that could produce 20% of Europe's electricity consumption by 2050, including in the UK. Using molecularly-complex and dense gases as working fluids in the turbine would enable TSP power stations to run on low-temperature heat source, a technology which could dominate the kW to MW power-output market. However, emission of strong acoustic waves from the shock/turbulence interactions, inevitably leading to varying inlet-flow angles and vibrations of the turbine, can cause premature blade fatigue and halve the turbine efficiency and the solar radiation to electricity conversion currently assumed. Developing a tool to avoid triggering such vibrations is therefore invaluable to TSP manufacturers, but also to energy policy makers so as to ensure that their assumptions on expected energy performances are as accurate as possible. Therefore, in addition to the aforementioned academic beneficiaries, software developers in the area of computational fluid dynamics (CFD) are directly targeted by the proposed research, and this is reflected by the partnership with a leading CFD company. Developing a low-cost and reliable technique to capture and predict possible acoustic emission from the shock, easily fitting in standard CFD packages, would be extremely valuable to turbine designers. Ultimately, power station operators, their customers, and environmental policy makers, would all benefit from the availability of more efficient and robust turbine designs.