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Vedha Nayagam

Publications and source records attributed to Vedha Nayagam.

12 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2024

Simplified Structure Models for Premixed n-Alkane Cool Flames

The chemical kinetics of cool flames in mixtures of normal alkanes, oxygen, and nitrogen recently have been reduced to obtain simplified asymptotic flame-structure models that can be used to compare theoretical predictions with experimental measurements. A key step that affects flame structures strongly, namely the abstraction of a hydrogen atom from the alkyl radical by an oxygen molecule, had not been taken into account in those models. This deficiency has now been remedied for diffusion flames. The present contribution reports the corresponding remedy for premixed-flame analyses. In the new model, non-zero chemical reaction rates are restricted to narrow regions in the vicinity of the maximum temperature, whereas previously they extended throughout the preheat zone. Comparisons of the new predictions with experimental burning-velocity measurements seem favorable.

premixed cool flames↗
NASA NTRS2021

Topical: Solid Fuel Combustion in Partial and Micro-Gravity

The risk of fire remains an ever-present danger in spaceflight. Most fire safety hazards originate in or eventually involve solid fuels, whether they be cellulosic (e.g., cotton fabric), hydrocarbons (e.g., plastics) or high-energy density electrode materials (e.g., batteries). A key approach to ensuring safety has been to focus on reducing the potential flammability of these materials – achieved by limiting their ignitability, potential for flame spread, and ultimate heat-release potential if ignited [1-3]. This approach has been relatively effective despite several close calls [4]. The limits of our understanding, however, are continually being challenged as future spaceflight missions incorporate partial gravity, enhanced oxygen, new types and classes of materials (e.g., composites), and higher energy-density batteries. This presents both an exciting scientific opportunity to enhance our understanding of solid fuel combustion processes while also posing a dire threat to future long-duration missions to the Moon and Mars.

Michael Gollner↗
NASA NTRS2021

Topical: Challenges and Research Needs for Micro- and Partial-Gravity Fires

Spacecraft fire safety has always been an essential component for any successful space mission. The importance increases exponentially for longer duration missions, such as the upcoming Artemis missions to the Moon and the “next giant leap” to Mars. If fire occurs in the spacecraft, terrestrial help for recovery may not be possible. Crew members have limited options to suppress and escape the fires and the associated vitiated atmosphere. The partial gravity conditions after landing on the Moon or Mars, as well as the high oxygen concentration proposed for the Lunar habitat, bring in additional challenges for spacecraft fire safety. To ensure safety and mission success, there is an urgent need to advance the knowledge of fire behavior in micro and partial gravity. This will also improve the understanding of how buoyancy flow plays a role in fire behavior, leading to a more complete theory of fire dynamics for Earth applications.

Ya-Ting Liao↗
NASA NTRS2020

High Pressure Transcritical Combustion (HPTC Combustion)

Combustion is the predominant source of the energy in the US and worldwide, and an overwhelming majority in the transportation sector, particularly liquid hydrocarbons (i.e., gasoline, diesel and jet fuel). Engine manufacturers move to higher operating pressures, above and beyond the critical pressures of the fuel, in order to optimize performance, maximize efficiency and reduce pollutant formation. The design of the next generation of engines, however, are limited by a lack of fundamental understanding of the thermo-physical properties, fluid behavior and chemical kinetics at these high pressures. Terrestrial laboratory studies are hampered by the ubiquitous buoyant flow that renders fundamental experiments difficult and/or impossible to interpret. Studying high pressure transcritical phenomena in microgravity allows researchers to investigate the fundamental aspects of phase change, chemical kinetics and property evaluation in a simplified geometry without the complicating influence of a buoyancy-induced flow.

Combustion↗
NASA NTRSDate not supplied

Effects of Ambient Inert Gas Properties on Cool-Flame Combustion of Normal-Alkane Droplets in Microgravity

Cool-flame-supported quasi-steady droplet combustion was first observed during droplet combustion experiments conducted onboard the International Space Station (ISS) in the Combustion Integrated Rack (CIR) with n-alkane fuels. These cool flames, not visible to the naked eye, result from the existence of the negative-temperature-coefficient (NTC) region during the combustion n-alkane fuels when the chemical-kinetic pathways experience a transition from the low-temperature chemistry to the normal, high-temperature-controlled chemistry. In this study, we present results for the cool-flame-supported droplet burning rates and extinction diameters for n-alkane fuels (n-heptane, n-octane, n-decane, and n-dodecane) in inert-diluent-substituted ambient environments. In these ISS experiments, some of the nitrogen in the ambient gas mixture is replaced by an inert gas, namely, carbon dioxide, helium, or xenon. Different diluents were observed to exert remarkably different influences on the cool-flame combustion of fuel droplets. The results show that the cool-flame extinction diameter is drastically increased with helium dilution compared to N2, CO2 or Xe, and Xe dilution leads to longer burning with a smaller extinction diameter. Unlike hot flames, the cool-flame burning rates are controlled by the cool-flame temperature established in the NTC region by the chemical kinetics, rather than by equilibrium thermodynamics, and the thermo-physical properties of the ambient gases, which control the heat feed back to the liquid droplet. The cool-flame extinction is triggered by the instability that occurs at the low-temperature end of the NTC region, below which cool diffusion flames become statically unstable. A quasi-steady theoretical model, presented earlier, which involves a six-step reduced-chemistry mechanism, valid around the lower end of the NTC region, is shown to explain the observed experimental trends qualitatively.

microgravity↗
NASA NTRSDate not supplied

Asymptotic Analysis of Premixed N-alkane Cool Flame Propagation

A simplified chemical-kinetic cool-flame mechanism for n-alkanes developed recently is applied to premixed laminar cool-flame deflagrations. The present contribution derives the structure of the associated freely propagating cool premixed flame controlled by the low-temperature chemistry and develops formulas for calculating the corresponding laminar burning velocity. Application of activation-energy asymptotics reveals finite-rate chemistry without heat release occurring throughout the preheat zone and leakage of both fuel and oxygen through the thin heat-release zone, there being zones of consumption of an intermediate species on each side of the heat-release zone, thicker than that zone but still thin compared with the preheat-zone thickness. Predicted laminar burning velocities are compared with recently reported measurements for n-dodecane, performed in a newly deigned high-pressure ignition apparatus, resulting in reasonable agreement.

Cool flames↗
NASA NTRSDate not supplied

Asymptotic Analysis of Premixed N-alkane Cool Flame Propagation

A simplified chemical-kinetic cool-flame mechanism for n-alkanes developed recently is applied to premixed laminar cool-flame deflagrations. The present contribution derives the structure of the associated freely propagating cool premixed flame controlled by the low-temperature chemistry and develops formulas for calculating the corresponding laminar burning velocity. Application of activation-energy asymptotics reveals finite-rate chemistry without heat release occurring throughout the preheat zone and leakage of both fuel and oxygen through the thin heat-release zone, there being zones of consumption of an intermediate species on each side of the heat-release zone, thicker than that zone but still thin compared with the preheat-zone thickness. Predicted laminar burning velocities are compared with recently reported measurements for n-dodecane, performed in a newly deigned high-pressure ignition apparatus, resulting in reasonable agreement.

cool flames↗
NASA NTRSDate not supplied

The Effects of Pressure and Optical Thickness on Radiative Losses in Spherical Diffusion Flames in Microgravity

In microgravity combustion, heat loss due to radiation plays a large role. Combustion products are able to accumulate in the flame region and radiate heat, while the heat release from the flame remains relatively constant. This causes the flame temperature to decrease until combustion can no longer be sustained, a phenomenon known as radiative extinction.

Kendyl A. Waddell↗