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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 3 - Spray-Imaging (Detailed Characterization of Spray Systems using Novel Laser Imaging Techniques)

Teaser

Summary

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 and cryogenic sprays are used to remove heat during laser surgery. In spray coating and painting the major challenge consists in the production of droplets which will deposit, spread and dry into uniform layers of desired thickness. Numerous other examples (e.g. agricultural sprays, spray drying and spray cooling) 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. Mixture preparation has a controlling impact on emissions. Overly fuel rich mixing zones can produce large amounts of soot (black smoke); mixing zones that fall outside the flammability limits produce hydrocarbon and CO emissions, while mixing zones near the stoichiometric ratio are very hot, producing high NOx emissions. 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 effort from researchers of the spray community 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 respons

Work performed

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 Keynote presentation detailing the approach has been given at the International Symposium on Applications of Laser Techniques to Fluid Mechanics in July 2016.
- A technique called FRAME consisting in doing structured illumination with only one modulated subimages has been developed and applied.
- The advantage of FRAME is the possibility of doing multiple exposures from one recording allowing series of 4 images recorded simultaneously.
- A presentation of the technique has been given at the 36th International Symposium on Combustion in August 2016.
- One article on FRAME is now being published in 2016 in the Proceedings of the Combustion Institute
- 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
- A journal article on this topic is now being written.

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 results have been presented at the ILASS 2016 conference in Brighton.
- A journal article has been recently been accepted for publication in Optics Letters.
- The possibility of scanning the spray to obtain 3D droplet sizing has been successfully applied.
- The results have been presented in a Conference article at the International Symposium on Applications of Laser Techniques to Fluid Mechanics in July 2016.
- Current work consists in measuring simultaneously the size and the concentration of the droplets.

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.
- Results have been reported in an article published in Optics Express in January 2016.
- The possibility of doing instantaneous temperature measurement using two or one modulated structured illumination sub-image is currently on-going.
- The investigation of new fluorescent dyes which are temperature sensitive has been initiated through a cooperation with researchers in LEMTA, Nancy (France)
- The possibility of using thermographic phosphors with structured illumination under combusting situations has been initiated through a cooperation with researchers in TU-Darmstadt (Germany).

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
- The possibility in running several simulations at the same time has been initiated
- A collaborative work with the University of California, Irvine, has been done to calculate the temporal response of an ultra-short pulse crossing various scattering systems.
- Creating a multi-user login system on the on-line webpage and initiating the launch of the webpage to be accessible by anyone online (planned for the end of year 2016)

Final results

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.