Opposed-Flow Flame Spread in Different Environments Subrata (Sooby) Bhattacharjee San Diego State University

Acknowledgement Profs. Kazunori Wakai and Shuhei Takahashi, Gifu University, Japan Dr. Sandra Olson, NASA Glenn Research Center. Team Members (graduate): Chris Paolini, Tuan Nguyen, Won Chul Jung, Cristian Cortes, Richard Ayala, Chuck Parme Team Members (undergraduate): Derrick, Cody, Isaac, Tahir and Mark. ( Support from NASA and Japan Government is gratefully acknowledged )

Overview What is opposed-flow flame spread? Flame spread in different environment. Recent experiments at MGLAB, Japan Mechanism of flame spread. Length scales and time scales. Spread rate in normal gravity. Spread rate in microgravity The quiescent limit Future plan.

Upward or any other flow-assisted flame spread becomes large and turbulent very quickly. Opposed-flow flame spread is also known as laminar flame spread.

AFP: = 0.08 mm = 1.8 mm/s Downward Spread Experiment, SDSU Combustion Laboratory PMMA: = 10 mm = 0.06 mm/s

Gravity Level: 1.e-6g Environment: O 2 /N 2 mixture at 1.0 atm. Flow Velocity: 50 mm/s Fuel: Thick PMMA (Black) Spread Rate: 0.45 mm/s mm Sounding Rocket Experiment Spread Over PMMA: Infrared Image at 2.7 

Fuel: Thin AFP, =0.08 mm = 4.4 mm/s Thick PMMA Image sequence showing extinction Vigorous steady propagation. Experiments Aboard Shuttle: O2: 50% (Vol.), P=1 atm.

Front view camera Side view camera Fuel holder O 2 port N 2 port Vacuum pump port Manometer port Apparatus for normal-gravity experiments CCD camera Air Honeycomb Fan PMMA ： 30mm x 10mm x 15,50,125  m Fuel holder Igniter (Ni-Cr wire) Apparatus for micro-gravity experiments conducted with the 4.5sec trop-tower (100meter-drop) of MGLAB in Japan. Igniter (Ni-Cr wire) Fuel holder Vacuum O2O2 CCD camera Air Igniter (Ni-Cr wire) VfVf VfVf Fuel holder VgVg V g ~300mm/sec PMMA ： 30mm x 10mm x 15,50,125  m

Front view Back view Video camera Sample holder (sample size: 6cm x 1cm) Fan & Motor Motor controller Solenoid coil to remove the igniter at the onset of MG

AssembleMove to drop shaftClose the capsule Attach the transceiverReady to drop

Ignite the sample 1.6 sec before MG. Remove the igniter 0.3sec before MG. The onset of MG Declaration G in the friction damper. MG for 4.5 sec Typical sequence of the drop experiment

PMMA: = 0.025mm = 10 mm/s (Downward spread) = 4.1 mm/s (MGLAB drop tower) O2: 30%, 1 atm.

PMMA: = 0.025mm = 22.8 mm/s (Downward spread) = 18.9 mm/s (MGLAB drop tower) O2: 50%, 1 atm.

Mechanism of Flame Spread Fuel vapor O 2 /N 2 mixture Flame seeks out the stoichiometric locations The flame spreads forward by preheating the virgin fuel ahead. Virgin Fuel

Mechanism of Flame Spread O 2 /N 2 mixture The rate of spread depends on how fast the flame can heat up the solid fuel from ambient temperature to vaporization temperature. Virgin Fuel Vaporization Temperature,

Forward Heat Transfer Pathways: Domination of Gas-to-solid Conduction (GSC) Preheat Layer Pyrolysis Layer Gas-to- Solid Conduction Solid-Forward Conduction The Leading Edge

Gas-phase conduction being the driving force, Zooming on the Leading Edge

Length Scales - Continued

Heated Layer Thickness – Gas Phase

Heated Layer Thickness – Solid Phase

Vaporization Temperature, Ambient Temperature, The Characteristic Heating Rate Sensible heating (sh) rate required to heat up the unburned fuel from to Heating rate due to gas-to-solid (gsc) conduction: Flame Temperature,

Conduction-limited or thermal spread rate: Flame Temperature, Spread Rate Expressions Vaporization Temperature, For semi-infinite solid,

Conduction-limited spread rate: Flame Temperature, Vaporization Temperature, For thermally thin solid, Spread Rate Expressions

Hang-distance, the distance between the flame front and the pyrolysis front, is ignored in de Ris solution. Flame front. Pyrolysis front Hang-Distance Correction for Thin Fuels [Bhattacharjee, Combustion and Flame, 94] The Extended Simplified Theory (EST) retains the same form as the de Ris expression and recommends for evaluating properties.

Thick Fuel Spread Rate (EST): Replace the forced or buoyancy induced boundary layer with an equivalent slug flow. Extended Simplified Theory – Thick Fuels [Bhattacharjee et al., 26 th Symp] The Extended Simplified Theory (EST) retains the same form as the de Ris expression and recommends for evaluating properties. Introduce a correction for the lifted flame through

There are Hardly Any Studies on Transition in Literature Most thin fuel studies were done with cellulose Most thick fuel studies were done with PMMA

At low opposing velocity, critical thickness can be a hundred time larger, removing the difficulty of creating thin samples. Thin-fuel formula Thick-fuel formula The intersection produces: It Maybe Easier to Study Transition in the Absence of Buoyancy

Thoery, Numerical Simulation and Existing Data Spread Rate [cm/s] where,

for thermally-thin fuel and for thermally-thick fuel Downward spread rate vs. fuel half-thickness in normal-gravity where

Non-dimensional downward spread rate vs. non-dimensional fuel half-thickness

Solid Forward Conduction (sfc) Gas to Solid Conduction (gsc) Gas to Environment Radiation (ger) Gas to Solid Radiation (gsr) Solid to Environment Radiation (ser) Parallel Heat Transfer Mechanisms

Gas to Solid Conduction (gsc) The characteristic heat is the heat required to raise the solid-phase control volume from to. Gas-to-surface conduction time: Time Scales

Solid Forward Conduction (sfc) Gas to Solid Conduction (gsc) Relative dominance of GSC over SFC

Solid Residence Time: Gas to Solid Conduction (gsc) Solid to Environment Radiation (ser) The radiation number is inversely proportional to the velocity scale. In the absence of buoyancy, radiation can become important. Radiative Term Becomes Important in Microgravity

Mild Opposing Flow: Computational Results for Thin AFP As the opposing flow velocity decreases, the radiative effects reduces the spread rate

Mild Opposing Flow: MGLAB Data for Thin PMMA

Gas to Solid Conduction (gsc) Solid to Environment Radiation (ser) Include the radiative losses in the energy balance equation: Algebraic manipulation leads to: Spread Rate in the Microgravity Regime

Gas to Solid Conduction (gsc) Solid to Environment Radiation (ser) The minimum thickness of the heated layer can be estimated as: All fuels, regardless of physical thickness, must be thermally thin in the quiescent limit. The Quiescent Microgravity Limit: Fuel Thickness

Gas to Solid Conduction (gsc) Solid to Environment Radiation (ser) The spread rate can be obtained from the energy balance that includes radiation. where, The Quiescent Microgravity Limit: Spread Rate reduces to:

In a quiescent environment steady spread rate cannot occur for The Quiescent Limit: Extinction Criterion

Extinction criterion proposed is supported by the limited amount of data we have acquired thus far. The Quiescent Limit: MGLAB Experiments

Oxygen/Nitrog en Mixture A B Flow Modifier reduces the entrance length. Average velocity Centerline velocity Control Thermocouple: The conveyor belt holding the fuel is spooled from roller A to B so as to maintain a constant thermocouple temperature. Igniter for opposed- flow spread. The fuel is spooled from A to B Igniter for concurrent- flow spread. The fuel is spooled from B to A Spot Radiometer Imaging window backlit with IR radiation Smoke Wire IR Source with beam expander IR Camera with a rotating filter wheel containing 4.3  m and 2.8  m filters of varying trasmittance. Top View Side View C D B A C Thin PMMA sheet (thickness 200  m or less) attached on a conveyor belt. E

Future Work The MGLAB data suffers from limited low-g duration (4.5 s) to distinguish steady spread from a spreading extinction. Only space experiment can establish the microgravity and quiescent formulas proposed. While this work predicts extinction for fuel with thickness greater than a certain critical thickness, the pathway to extinction is not clear. Detailed infrared emission and absorption photography will be used to establish the role played by radiation. Numerical modeling and a comprehensive set of data with flow velocity, oxygen level, ambient pressure and fuel thickness as parameters from an ambitious flight experiment will be used to quantify the transition between thin and thick fuels, thermal, microgravity and quiescent regimes, and wind opposed and wind aided spread. A novel experimental set up is being built at SDSU, where the fuel is moved relative to the flame so as to keep the flame stationary with respect to the laboratory. The absorption pyrometry is being developed at Gifu.