The efficiency of spray systems designed for various applications is determined by the size/shape distribution and velocity of the droplets formed. For example, in the combustion process, smaller droplets imply higher vaporization rates and subsequently more efficient combustion with fewer emissions. Despite the importance of droplet size and velocity distributions in industrial applications, there are fundamental questions which still remain unanswered. Given certain conditions, including fluid properties and geometry, it is unclear what size and shape of the droplets are expected to be generated. What is the effect of ultra‐high pressure and turbulence on these structures? How do these structures evolve? What is the effect of in‐nozzle phenomena on the formation of sprays? This proposal aims at answering these questions trough the development of advanced numerical tools that will be capable of representing in a unified manner in‐nozzle and subsequent spray formation mechanisms and will be valid for both sub and supercritical conditions.
The break‐up phenomenon is topologically extremely complex due to the close coupling of diverse physical phenomena (fluid mechanics, thermodynamics, and surface effects) over a wide range of lengths and time scales: from the nozzle geometry (mm), to the smallest droplets (μm). Even with the most powerful computers it is impossible to solve the governing equations directly for all the scales involved. The current project is based on a novel idea that instead of treating droplets and ligaments as individual quantities, we can view the whole injection process from inside the nozzle up to droplet break up cascade as a continuous evolution process of a liquid surface that can be generated or destroyed locally by various factors such as turbulence, shear forces, local bubble collapse etc. This proposal introduces the concept of a sub‐grid PDF of the droplet surface density jointly with liquid and vapour volume fraction in a Large Eddy Simulation context (LES). The major potential of the approach is that it bypasses the spherical vision of the liquid structures that compose the SGS spray and a priori does not include any region dependent assumptions. Moreover, it represents a three‐fluid system (including a vapour equation), to extend the model for the in‐nozzle phenomena.
The project objectives are:
- Derive the generalised surface density model
- Incorporate in the model, SGS models based on the findings of experiments and DNS currently performed at the AEC and supported from two ongoing EPSRC grants
- Extensively validate the model against data close to realistic operating conditions of engines provided by the partner industry, Ricardo
- Explore the effect of fuel variability
- Use the developed framework as a virtual testing tool to shed light at the transitional mechanisms of sub to super‐critical conditions
- Derive an appropriate non‐dimensional quantity that characterises the transition in order to help generalise the project outcomes to various applications
The PhD studentship will provide outstanding training in numerical modelling and fluid dynamics, with potential applications for numerous spray systems. The successful candidate will be based within the AEC and will undertake leading‐edge modelling research in using state‐of‐the‐art facilities. The PhD student will present findings at project review meetings with the industry partner (Ricardo), as well as at national and international conferences and contribute to high impact journal publications.