Browsing by Author "Fereres, Sonia"
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Item Alkali Metal Loop Heat Pipe Development for Solar Dynamic Energy Conversion(48th International Conference on Environmental Systems, 2018-07-08) Fereres, Sonia; Bonnafous, Bastien; Mohaupt, Mikael; Lagier, Benjamin; Mari, Raphael; Dehombreux, Emmanuel; Guraya, Cristina; Jimenez, Cristina; Azpiroz, Xabier; De La Rosa, SoniaFuture space exploration missions and outposts on the Moon and Mars would benefit from compact, efficient, high temperature energy conversion devices such as nuclear and solar reactors. In the case of solar dynamic systems, extending operation beyond the hours of solar exposure can be achieved by incorporating Thermal Energy Storage (TES) if the concentrated radiation can be decoupled from the power conversion unit and stored. Conventional heat pipes have been previously developed for this purpose in terrestrial solar receivers and dish-Stirling systems, but the low pumping capacity provided by the capillary structure limits their operational range to horizontal configurations and short distances. Here a high temperature Loop Heat Pipe (LHP) is investigated to transport the concentrated solar radiation from a parabolic dish´s focal point to alternative locations. LHP can perform well at high power, transporting heat over large distances and in different orientations. However, the large power transport (> 10 kW) and high temperature (> 600ºC) requirements of dish-Stirling systems are above current state-of-the-art LHP technology, making this a challenge in terms of materials, fluids, and design. A trade-off study is performed taking into account system requirements, performance and cost, developing a numerical model to determine the most promising solution for a future prototype. The working fluids options at these temperatures are limited to liquid metals: mainly sodium, potassium, and cesium. Although sodium might seem like the most promising working fluid candidate, potassium is anticipated to work better within the system requirements. This paper will show through analysis that, in contrast to conventional LHP where working fluids have negligible thermal conductivity, when using a highly conductive liquid metal the parasitic heat fluxes might be extremely important. This is a novel problem, indicating that design parameter optimization has to be performed differently to ensure proper operation.Item Determining the Cause of Reduced Concurrent Flame Spread over Thin Solid Fuels in Low Pressure and Low Gravity(50th International Conference on Environmental Systems, 7/12/2021) Thomsen, Maria; Fereres, Sonia; Carmignani, Luca; Fernandez-Pello, Carlos; Ruff, Gary A.; Urban, David L.The spread of flames over the surface of solid combustible materials has been widely investigated and is known to be affected by environmental conditions. Variables such as flow condition, oxygen concentration, ambient pressure, and partial or microgravity, may change the material flammability and influence the fire dynamics. This is a critical fire safety issue for space exploration vehicles and future habitat atmospheres which will very likely have reduced pressure and enriched oxygen concentration environments, different than those currently used on the International Space Station. However, testing experimentally the materials to be used and qualified for space exploration under these conditions is a cumbersome and expensive task. The objective of this work is to provide a better understanding through numerical modeling of the dominant physico-chemical processes on the concurrent flame spread over thin fabrics under reduced ambient pressure (and in turn, buoyancy) under variable gravitational conditions. Numerical modeling is performed using the Fire Dynamics Simulator (FDS6) code with a single-step Arrhenius reaction rate for the solid phase decomposition. Different models are tested for the gas phase combustion kinetics. The model results are validated with experimental results obtained at similar reduced ambient pressure and flow conditions at 1 g. It is shown that as ambient pressure is reduced the flame spread rate over a thin fabric is also reduced, both experimentally and numerically. Numerical results are compared to an analytical approach previously developed to explain the experimental trends. Further interpretation of the model results provides information regarding the physics of the process and how they are affected by the lower pressure environments. The results of this work provide guidance for potential on-earth testing for fire safety design in spacecraft and space habitats.Item The effect of reduced pressure on the characteristics of spreading flames(50th International Conference on Environmental Systems, 7/12/2021) Carmignani, Luca; Thomsen, Maria; Fereres, Sonia; Gollner, Michael; Fernandez-Pello, Carlos; Urban, David; Ruff, GaryFlame spread over solid fuels is a canonical problem in fire science, due to its direct implications on material flammability and importance in fire development. In a microgravity environment, such as onboard a spacecraft, flames can behave very differently than on Earth. This is concerning for spaceflight life safety, especially in higher-oxygen environments. Due to the difficulties associated with microgravity testing, low-pressure environments have been proposed as an alternative to approximately replicate reduced gravity conditions because of the reduction in buoyancy. However, the roles played by gravity and pressure on flame length, standoff distance, and flame spread rate vary with the burning configuration. In concurrent flame spread, the buoyant flow enhances the spread rate by bringing the flame closer to the fuel surface and increasing the heating of the solid fuel. In opposed flame spread, the sample is preheated by the flame ahead of the flame leading edge, which is strongly affected by the surrounding flow field. In this work, we consider flames spreading over thin cotton samples in both downward (opposed) and upward (concurrent) configurations to investigate the effect of pressure (30-100 kPa) on flame characteristics, such as spread rate and standoff distance. A small forced flow is induced upward so that the flames are exposed to a mixed (forced and free) flow. By reducing pressure, flames become less bright, their standoff distance increases, and their spread rates decrease in analogy with low-gravity flames. These results could in help understanding the differences between flames at low pressure and low gravity environments for these similar, yet very different, spreading configurations. They could also provide further information about potential Earth testing of the flammability of materials in spacecraft environments.Item Modeling the Effect of Buoyancy and External Heating on the Flame Spread in Fire Resistant Fabrics(48th International Conference on Environmental Systems, 2018-07-08) Thomsen, Maria; Fereres, Sonia; Alonso Ipiña, Alain; Fernandez-Pello, Carlos; Urban, David; Ruff, GarySpacesuits are fabricated with Nomex, Kevlar and other fire resistant fabrics. The flammability behavior of these materials has been widely studied experimentally, mostly under standard sea level atmospheric conditions. However, future human space exploration vehicles and habitat environments will very likely have different environments, i.e. reduced pressure and enriched oxygen concentration. Experiments under these conditions, particularly in microgravity, can become a difficult and expensive task. Numerical investigations of the flammability of high performing fibers/fabrics may be a viable alternative to experiments. Here we present a numerical model formulated to understand the effect of environmental conditions on the flame propagation characteristics of thin fire-resistant material such as Nomex. Moreover, the effect of external radiant heating on material flammability is also studied. Thermogravimetric analysis (TGA) experiments were performed with Nomex to estimate the kinetic parameters, which were then used to model the thermal decomposition of the fabric sample using a Computational Fluid Dynamics (CFD) code, Fire Dynamics Simulator (FDS6). Two-dimensional simulations are performed using finite-rate single-step combustion kinetics in the gas phase and an Arrhenius reaction mechanism with multiple steps for the solid phase decomposition. The model results are then compared to previous experimental results at high oxygen concentrations and/or reduced pressure conditions. It is shown that with the appropriate kinetic parameters the model is able to capture the main physical aspects of the flame spread of a thin solid fuel and it provides a basis for future modeling of fire resistant fabrics for space exploration.Item Payload Concept Evaluation for Water/Oxygen production on the Moon based on Thermo- or Electro-Chemical Reduction of Lunar Regolith(50th International Conference on Environmental Systems, 7/12/2021) Fereres, Sonia; Morales, Mercedes; Denk, Thorsten; Osen, Karen; McGlen, Ryan J.; Seidel, Achim; Madakashira, Hemanth; Urbina, Diego; Binns, DavidUsing space-based resources (In-Situ Resource Utilization, ISRU) to produce life support and propulsion consumables such as oxygen or water offers the possibility of a more sustainable and cost-effective exploration of the Moon when compared to missions relying solely on material transport from Earth. Learning how to operate on the Moon with local resources, reduced gravity and a harsh environment is a stepping stone towards developing technology for a sustained human presence in space. We analyze several technical concepts to produce oxygen or water from lunar regolith. Thermochemical reduction processes of solid lunar materials using hydrogen (hydrogen reduction of ilmenite), methane (carbothermal) reduction and other processes such as molten salt electrochemistry (e.g. the FFC-Cambridge process) are evaluated. Focus is set on the technological solutions for supporting fluid management systems required to produce, separate, collect, and measure water or oxygen from solid oxide chemical/electrochemical processes, evaluating their technology readiness level, current necessary developments and their potential interaction with other life support and exploration activities. Technology de-risking plans to demonstrate fluid management system feasibility and tests in relevant environments are proposed to support establishing a human presence on the Moon sustained by local resources.