LAMINAR DIFFUSION FLAMES IN EARTH GRAVITY(1g) AND MICROGRAVITY (µg) Ashish H. Mistry December 10th 2009

MOTIVATION AND GOAL In 1997 ,Fire on board the Mir space station. To understand behavior of flames in 1g & µg (microgravity). Fire safety of future manned space missions.

Diffusion Flame in 1g Fuel along flame axis diffuses radially outward while oxygen diffuses inwards. Fuel rich zone Φ >1. Flames surface Φ=1. Oxidizer rich zone Φ<1. Variable Density –Buoyancy. Buoyancy convection accelerates the flow, causes narrowing of the flame . Momentum + Buoyancy controlled . Soot- fine black particles chiefly composed of carbon produced by incomplete combustion.

Diffusion Flame in µg µg = a condition in space of virtual absence of gravity. Diffusion control the overall combustion process. Absence of buoyancy Momentum controlled. Density & Temp. remains constant. Flame have spherical shape.

Flame Length and Velocity of Gases Flame height ~ Fuel Volumetric flow rate whether the flame is momentum or buoyancy controlled Time to the flame tip is depend on what the controlling force is For 1g : t → QF , V= V0 +Vb For µg: t → tD , V= V0 tD (µg) > tD (1g)

Velocity profile for 1g & µg Roper analysis As we decrease gravity , velocity of gases also decreases and become constant for microgravity

Modified Roper Model Assumptions Axial diffusion is neglected Axial convection is included Radial diffusion is included Radial convection is included Axial velocity, vz is assumed to vary in the axial direction but is a constant at each z location The flame sheet is located where the mixture is stoichiometric

Modified Roper Model Conservation of a conserved scalar (Cylindrical Cord.) r, z, vr and vz radial and axial coordinates and velocities C= Coupling function D, mass diffusivity Conserved scalar approach, concentration utilizing mixture fraction Mixture Fraction: The mass fraction of material having its origin in the fuel stream Eq(3) Boundary condions for Eq(1) (1) (1) (2) In the far field (3)

Modified Roper Model Utilizing the transformations we convert from r and z to a new η and θ coordinate system rD is the characteristic scale of diffusion After simplification and transformation eq. 1 becomes eq. 4 (4) (5) In the far field

Modified Roper Model (6) Fuel Cst Ethane (C2H6) 0.057 Propane (C3H8) Eq. (4) is now subject to the boundary conditions eq (5) following the previous boundary conditions eq(3) we get equation (6) I0 = Bessel function of the first kind and zero order Take C=Cst to get flame shape (6) Fuel Cst Ethane (C2H6) 0.057 Propane (C3H8) 0.040 n- Butane (C4H10) 0.031

Modified Roper Model This shows that as H/C ratio decreases flame length increases

Diffusion flames in 1g With Increasing oxygen, soot formation decreases Flame length decrease

Diffusion flame in µg With Increasing oxygen, soot formation increases If enough oxygen is seating in upstream of flame than flame propagates in the direction of pure oxygen and break ups into little flame lets. So any movement in space station cause still air to flow and it provides fresh air (oxygen) to flame may result in big fire in space station.

Referances ‘Analytical predictions of shapes of laminar diffusion flames in microgravity and earth gravity’ by S.S. Krishnan, J.M. Abshire, P.B. Sunderland c, Z.-G. Yuan and J.P. Gore Combustion Theory and Modeling Vol. 12, No. 4, August 2008, 605–620 ‘Numerical study on the effect of gravity on flame shape and radiation in laminar diffusion flames’ by Marc R.J. Charest , Clinton P.T. Groth, ¨Omer L. G¨ulder at Proceedings of Combustion Institute - Canadian Section Spring Technical Meeting University of Toronto, Ontario May 12-14,2008 ‘Oxygen and Fuel Jet Diffusion Flame Studies in Microgravity Motivated By Spacecraft Oxygen Storage Fire Safety’ by S.S. Krishnan, J.M. Abshire, P.B. Sunderland c, Z.-G. Yuan and J.P. Gore ‘Microgravity Combustion: Fire in Free Fall’ , Author Howard D. Ross ‘An Introduction to Combustion: Concepts and Applications’, Author Stephen.R.Turns. ‘Combustion’, Author Irvin Glassman. NASA Connect : connect.larc.nasa.gov/ www.me.berkeley.edu/mcl/

Thank you