Flame Extinguishment Experiment - 2

The primary goal of the FLEX-2 flight experiments is to extend the results of FLEX-1 to fuels and environmental conditions that mimic real combustor conditions, as well as, provide benchmark experimental data on bi-component droplet combustion over a wide range of initial conditions in order to validate numerical simulations and to develop new theoretical models. Detailed experimental objectives can be further itemized as: 1. Investigate the role of liquid-phase mixing on the burning characteristics of bi-component fuel droplets. 2. Examine the steady and unsteady liquid and gas-phase phenomena in pure fuel combustion. 3. Study flame extinction mechanisms and their boundaries. 4. Analyze the soot formation process, including soot volume fraction soot-shell dynamics. 5. Determine the influence of droplet interactions on the flammability limits and extinction behavior of droplet arrays. 6. Validate theoretical models of droplet combustion through comprehensive experimental data collection in microgravity environments.

The chamber was filled with the appropriate atmosphere, which was varied in pressure between 0.5 atm to 3.0 atm and O2 concentration between 0.1 to 0.4 mole fraction with the diluent gas making up the balance. A settling time of approximately 2 minutes elapsed prior to initiating the test in order to ensure that the temperature and pressure of the chamber gases have stabilized. This settling time was followed by the dispensing of a predetermined amount of fuel (based on the target droplet size) onto the support fiber for those tests in which the droplet is not freely deployed. When sufficient fuel has been dispensed the dispensing needles were retracted and a dwell period of at least 10 seconds was allowed for the droplet internal fluid motion induced by deployment to subside. This was then followed by initiating power to the ignitor for a selectable amount of time ranging from 1 second to 5 seconds after which the ignitor was retracted from the field of view. If the flow field is to be generated by translating the droplet then droplet motion would commence at the same time that the ignitor is retracted. A near real-time download of the color camera video was employed in order to verify successful droplet deployment, ignition, and overall progress of the experiment. Pressure and temperature data of the chamber environment was also recorded in near real time. The overall investigation approach can be summarized as: 1. Test parameters are determined, including pressure, temperature, fuel mixture, and humidity. 2. Fill the combustion chamber with the selected gas mixtures. 3. Deploy the droplet(s) carefully controlling the droplet diameter. 4. Allow the droplet to stabilize in the microgravity environment. 5. Ignite the droplet in a uniform and controlled fashion. 6. Monitor combustions and track flame evolution using high speed photography. 7. Monitor and adjust experimental conditions as needed. 8. Analyze data. The independently varied experimental parameters during FLEX-2 experiments were 1) Fuel type 2) Ambient gas composition 3) Ambient Pressure 4) Droplet initial diameter 5) Droplet translation velocity (and acceleration) 6) Droplet spacing and initial size in a binary linear array.

Fairly sophisticated numerical simulations of convective droplet combustion have been developed in the last decade where the key contribution is the inclusion of full Navier-Stokes equations for the gas-phase flow field. However, due to the inherent complexity of the problem, varying degrees of approximations have been made in treating the chemistry, radiation, and the coupling between the gas and liquid phases. The validity of the assumptions or the hypothesis used in these simulation have not been confirmed by using benchmark data. By conducting these experiments in a microgravity environment, the effects of buoyancy are minimized, and the droplet can be characterized only as a function of thermocapillary or solutalcapillary stresses allowing for a clear observation of the fundamental combustion processes, The present FLEX-2 experiments using bi-component mixtures. In addition, the microgravity environment allows study of such phenomena as the transitions between radiative and diffusive extension of the flame. The primary goal of the flight experiments is to provide benchmark experimental data on bi-component droplet combustion over a wide range of initial conditions that could be used to validate numerical simulations and to develop new theoretical models.

The combustion of liquid fuels is the overwhelming energy source in the transportation sector. Insights gained from these studies can lead to more efficient and cleaner combustion technologies which are crucial for reducing pollution and greenhouse gas emissions. By improving fuel efficiency and developing better fuels, these experiments can contribute to: 1) More sustainable energy production. 2) Future combustor designs with minimal carbon footprint, maximum fuel efficiency and less pollutant emissions. 3) Allow quantitative evaluation of the performance of future fuels in combustors through the development of surrogates (mixtures of pure fuels that simulate the behavior of real fuels). 4) Validation of jet engine models by engine manufacturers with support from prior droplet results.