Investigation of Non-Spherical Droplets in High-Pressure Fuel Sprays

Understanding the mechanisms that lead to the breakup and evaporation of liquids is a key step towards the design of efficient and clean combustion systems. The complexity of the processes involved in the atomisation of Diesel fuels is such that many facets involved are still not understood.

The morphological composition of a typical Diesel spray includes structures such as ligaments, amorphous and spherical droplets, but the quantity of fuel occupied by perfectly spherical droplets can represent a small proportion of the total injected volume. These relatively large non-spherical structures have never been thoroughly investigated and documented in high-pressure sprays, even though the increase in heat transfer surface area of deformed droplets is an influential factor for predicting the correct trend of evaporating Diesel sprays. The characterisation of fuel spray droplets is generally conducted using laser diagnostics that can measure droplet diameters with a high level of accuracy, but they are fundamentally unable to measure the size or shape of non-spherical droplets and ligaments. Hence the data obtained through these diagnostic techniques provide a partial and biased characterisation of the spray. The experimental bias towards spherical droplets is compounded by the complexity of modelling the heating and evaporation of deformed droplets. Consequently, theoretical models for liquid fuel atomisation and vaporisation are based on a number of simplifying hypotheses including the assumption of dispersed spherical droplets.

Our proposal seeks to initiate a step change in the description of petroleum and bio fuel spray formation by developing diagnostics and numerical models specifically focused on non-spherical droplets and ligaments. Our approach will build upon recent advances with microscopic imaging to build novel diagnostics and algorithms that can measure the shape, size, velocity and gaseous surrounding of individual droplets and ligaments. This morphological classification, along with the velocity measurements, will be used to develop new phenomenological and numerical models for spray breakup, heating and evaporation. The models will then be implemented into computational fluid dynamics (CFD) codes to simulate spray mixing under modern engine conditions, and generate information where optical diagnostics cannot be applied. These goals will be achieved by combining the expertise of the academic and industrial partners with that of international experts from the University of Bergamo, CORIA, and Moscow State University.

The project's concerted approach, aimed at removing the experimental and numerical biases towards spherical droplets, will establish a unique world leading research capability with potential impact for numerous practical spray applications. The project would underpin research in areas that rely upon the atomisation or evaporation of liquids, including the efficient delivery of liquid fuel, pharmaceutical drugs, cryogens, lubricants and selective catalytic reductants.

The project will generate new diagnostic methods, models and knowledge which can be transferred to other applications that rely on liquid atomisation and evaporation. Removing the experimental and numerical biases towards spherical droplets would pave the way to a more comprehensive understanding of atomisation and evaporation processes in real sprays. Such new knowledge would lead to more accurate physical and numerical models of spray formation, which can be applied to other spray systems applications.

The oil industry will benefit through an improved understanding of the effect of liquid physical properties on spray formation. This would underpin the development of new fuels and fuel additive formulations that atomise and mix more efficiently with the in-cylinder gas.

Engine manufacturers and combustion system developers will also benefit from new findings and models for the atomisation and mixing of petroleum and renewable fuels. Advanced phenomenological and validated numerical models will improve the predictive capabilities of commercial Computational Fluid Dynamics (CFD) tools.

Findings from the project would have transferable applications to spray systems used in the marine industry. New marine Diesel engines must meet increasingly stringent emissions legislations, which could lead to changes in both fuel and lubricant formulations. The project's diagnostic methods for deformed droplets could be applied to gain new insight into the characteristics of oil sprays used to lubricate large bore Diesel engines.

The general public will be the ultimate beneficiary through the improved quality of life and cleaner environment as a result of higher efficiency and greener vehicles.