Numerical Model on the Effects of Gravity on Diffusion Flames By: Crisen McKenzie

Introduction Motivation Project Goal Objectives Approach Understanding how flames are effected by gravity Spacecraft safety Better understanding of the combustion process Project Goal Perform calculations to see flame shape and behavior in a microgravity environment. Objectives Understanding how gravity affects flame behavior Better understanding of Flame shape Species ratio though the combustion process Approach Literature review Physical model Non premixed flame model Gravitational model / Microgravity model Results

Literature Search Turns Glassman Paul D Ronney Good theoretical knowledge of diffusion flames No numerical analysis calculation Glassman Theoretical knowledge of diffusion flames No numerical analysis Paul D Ronney Uses Roper model to calculate flame shape Experimentation analysis done on flames in microgravity (SOFBALL) NASA Glenn Research Center Understanding of what research is being done for flame under microgravity condition and why it is so important S.S.Krishnan Analyzed Roper model for flame shape. Performed experimental analysis of flames in microgravity

Non premixed diffusion flame Combustion occurring with fuel and air mixing together through diffusion.

Normal Gravity 9.81 m/s2 Buoyancy - the upward force on an object or fluid when immersed in another fluid. In diffusion flame buoyancy causes upward motion of heated less dense gases. Convection occurs due to the mass transfer of heated gases up towards regions of unheated gas

Microgravity State of receiving minute gravitational effects Pure diffusion and initial mass flow rate govern the momentum equation Ignore buoyancy forces Very slow convection

Convection Inside the Flame Convection occurs as the Paraffin gas combusts with air. The products heat and rise in the air caused by buoyancy forces since the heated air is less dense than the cooler air around it. As the heated products rise, they start to cool due to heat transfer with the cooler air around it. As the products leave the flame cooler air takes it place. This cycle keeps the flame burning. If not then the flame will not get its supply of oxygen and die

Governing Equations Conservation of mass Conservation of momentum Axial Radial Species Conservation Conservation of energy Buoyancy term- with greater acceleration of gravity of less dense gas the buoyancy increases

Numerical Model Candle Flame Model Products, Air, and, Unburned gas Porous Wick OX OX F Candle Flame Model

Numerical Model Fluent 6.3 Reacting Flows non-premixed flame model Steady State Axisymmetric flame Fuel – methane NOTE: methane was used instead of paraffin wax because it was not available in fluent MFR = 0.3 grams/sec Fuel inlet mixture fraction of 1 Oxidizer – Air MFR = 0.1 grams/sec 23% oxygen 77% nitrogen Oxidizer mixture fraction of 0

Candle Flame Temperature Normal Gravity Microgravity Effects of Buoyancy on temperature profile. Heated gases are lifted up do to buoyancy and cooled down through convection.

CO2 Distribution Normal Gravity Microgravity From the gravity side one can see that the heated products are convecting outward away from the flame, while in microgravity the products are more contained by the entrance of the fuel. This will result in the choking of the flame from lack of oxygen. If oxygen is not supplied due to convection the flame will die.

Exaggerated Candle Flame Model Heated gas 1g 100g This exaggerated model shows what the candle flame profile would look like after being exposed to 100 times earth’s gravity. This model allows for a clear view of how buoyancy affects candle flame profiles. One cane also see the light region on top of the flame to show heated gas escaping to flame profile.

Conclusion Flames are deeply affected by buoyancy forces. Without it a flame could die. Buoyancy helps gives the flame its needed supply of oxygen. This type of information is needed for when we have combustion in space or for fire prevention here on earth. When dealing with combustion in space it is better to use premixed flames rather than diffusive, since oxygen is readily supplied to the pre-mixed flames.

Improvements Next step is to model the soot formation in the surrounding air. Extend my study using varying mixture fractions coming from the wick Do an unsteady case showing the production of the products and how they dissipate over time. Also with the unsteady case show the time it takes for the flame in microgravity to go out do to choking from lack of oxygen Find a way to model paraffin wax from solid state, to liquid, to gas in order to fully see the candle flame characteristics. Because we used a simple gas as methane this model would not be an accurate model on describing candle flames.