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Detailed Characterization of Spray Systems using Novel Laser Imaging Techniques

Periodic Reporting for period 4 - Spray-Imaging (Detailed Characterization of Spray Systems using Novel Laser Imaging Techniques)

Reporting period: 2019-09-01 to 2020-02-29

In modern society, sprays are ubiquitous; they are used for painting, cooling, misting, washing, applying chemicals, dispersing liquids, etc. In medicine, inhaled droplets must satisfy a range of size. In spray coating and painting the major challenge consists in the production of droplets which will deposit and dry into uniform layers of desired thickness. Numerous other examples related to many industrial domains could also highlight the importance of understanding the physic of spray generation.
Nevertheless, the most significant example of a spray application concerns the injection of liquid fuel into combustion engines. Internal combustion, used in cars, and gas turbines engines, used in planes, are two very important examples of devices which provide mechanical power using most often liquid fuel spray. Due to an increased desire for both efficiency improvement and reducing pollutant emission, the interest in the fuel-injection process has expanded during the last couple of decades. To burn liquid fuels efficiently, it is necessary to convert the liquid stream into a vapor stream and mix the vapor into surrounding air. Even though various alternative strategies have been suggested and tested, it is believed that combustion modes will continue to use liquid fuels especially for boat and air transportation. It is then of importance to understand liquid fuel injection using spray systems and the transition from liquid to gas in order to obtain more efficient combustion systems and reduce the emission of pollutants. This understanding is also important for the case of bio-liquid fuels such as Ethanol, Butanol, Tailor-made and Bio-diesel fuels.

Over the past two decades there has been a consequent efforts to provide both more detailed experimental data and more predictive simulation results. Despite this effort, the amount of spray information that is currently accessible remains largely limited by the lack of direct observation. In other words, the scattering nature of atomizing sprays is responsible for the blurring and hiding of any complex fluid mechanical processes. Thus, detailed information of the near-nozzle region is still missing. In addition multiple light scattering limits the possibility in measuring the droplets size within a dense cloud of droplets. As a result, the complete 3D field of droplets is rarely characterized and only local point measurements on the spray edges are provided. Finally, another quantity of importance which is almost never reported is the spray temperature from the liquid injection to the evaporation zone.

Visualizing in detail the spray dynamics in the near nozzle region, measuring both the droplets size and concentration in 3D with high spatial/temporal resolution, and determining the temperature gradients over the complete spray system, is the only possible way to fully depict a spray system. The Spray-Imaging project aims at addressing all these issues by developing and applying, three novel laser imaging techniques beyond the state of the art for the detailed characterization of various relevant spray systems. To optimize the development of those three techniques computational Monte Carlo simulation will also be initiated. The resulting unique experimental data will be well documented on an open source webpage where any modeler will have free access to them for the validation of their model.
The ultimate goal of the project is to discover and analyze unobserved fluid mechanics phenomena responsible for spray formation and to characterize in 3D the finally formed evaporating spray. The techniques needed to achieve this end do not exist today and are being developed and applied through this project. This work presents, then, a possible way to significantly increase the future knowledge of liquid-to-gas transition occurring in each atomizing spray system. It would certainly open doors for better prediction of spray behavior, ultimately leading to smarter injection devices and to the design of cleaner and more efficient liquid fuel-based combustion engines.
As described in the ERC-StG Spray-Imaging project, four specific activities have been listed. The progresses made in each of those activities are detailed below:

Activity 1)
- A technique consisting in doing structured illumination with two-modulated subimages instead of three has been developed and applied.
- A new optical set-up for two-phase structured illumination has been successfully developed
- A technique called FRAME consisting in doing structured illumination with only one modulated subimages has been developed and applied.
- Microscopic imaging using long range microscopes based has been initiated for detailed spray analysis. The technique used is based on Laser Sheet Fluorescence Microscopy but is applied for the first time in spray systems.
- A conference article on LSFM in sprays has been presented at the ILASS 2016 conference in Brighton

Activity 2)
- Using the two-phase structured illumination approach with the LIF/Mie dropsizing technique, instantaneous images of droplets size in GDI sprays running with bio-fuels could be obtained.
- The possibility of scanning the spray to obtain 3D droplet sizing has been successfully applied.
- Measuring simultaneously the size and the concentration of the droplets has been successfully applied.

Activity 3)
- For the measurement of temperature in liquids and sprays, fluorescent dyes have been chosen instead of phosphors particles.
- 2D averaged temperature in sprays has been obtained using two-color LIF structured illumination.
- The possibility of using thermographic phosphors with structured illumination under combusting situations has been done through a cooperation with researchers in TU-Darmstadt (Germany). This work has led to two journal publication.

Activity 4)
- A Monte Carlo code has been optimized and parallelized to be running on Graphics processing units.
- A webpage has been created to run simulation from an open access on-line:
- A webpage interface to visualize and analyze the results from simulation has been created. This includes new graphs/plots and histograms
All experimental results described above are beyond the current state of the art. Thanks to those novel laser imaging approaches, complex fluid mechanics phenomena which are responsible for the spray formation can now be better observed and analyzed together with the resulting 3D formed evaporating spray.

The imaging techniques needed to achieve this end did not exist prior to the start of the project. Applying those techniques presents a possible way to significantly increase the future knowledge of liquid-to-gas transition occurring in each atomizing spray system. It would certainly open doors for better prediction of spray behavior, ultimately leading to smarter injection devices and to the design of cleaner and more efficient liquid fuel-based combustion engines; which is of main concern in the recent directives from the European Union on industrial emissions. Accomplishing the project proposed here, would allow the PI to establish a world leading research laboratory for spray characterization, which could, in the future, be used for a wide variety of industrial and medical applications, as well as for the validation of numerical spray models.
Comparison between conventional laser sheet imaging and SLIPI for imaging an Ethanol GDI spray
High contrast imaging through scattering media using Structured Illumination Fourier (SIF) filtering