Almost seventy percent of deaths in accidental fires are caused by inhalation of toxins such as CO and soot that form during underventilated burning. The COSMIC project examines the formation mechanisms of CO and soot during underventilated combustion, by using laminar, inverse diffusion flames (IDFs) formed between an air jet and surrounding fuel. An IDF mimics underventilated combustion because carbon-containing species that form on the fuel side of the flame can escape without passing through an oxidizing flame tip. Laminar flames are studied so that relatively simple models and experiments can be combined. Microgravity will be used because buoyancy-induced instabilities impede systematic variation of IDF operation conditions in normal gravity.

Previous work done on IDFs in microgravity by Linda G. Blevins give great insight into this phenomenon and serve as a foundation for our research. Computations of the effects of gravity on methane flame shapes were used to select an appropriate burner design. Calculations were performed using direct numerical simulation of the time-dependent Navier Stokes and conserved variable equations for an axisymmetric laminar flame. The simulation employs assumptions of low Mach number, infinite-rate chemical kinetics, unity Lewis number, variable thermophysical properties, a semi-infinite surrounding fuel-stream, and negligible radiation heat transfer.

A small burner that produced flames less than 1-mm long was unaffected by gravity, but was found to be undesirable because a wide variety of IDF operating conditions could not be achieved. In this project we want a broad variety of operating conditions that yield different flames in normal as well as micro-gravity. The preferable larger burner consists of a 1-cm diameter central air jet surrounded by a 3-cm diameter co-annular fuel tube. A nitrogen curtain flows through a 6.4-cm tube surrounding the burner. The N2 prevents secondary flames from forming between fuel and room air. Flow straightening beads and honeycomb are present in the air and fuel passages, while the N2 flow is smoothed with a fine-mesh screen. The air and fuel tubes are sharpened to knife-like edges to facilitate flame attachment. In a recent paper written by Dr. Blevins one methane (CH4) and one ethylene (C2H2) flame were studied. For the ethylene flame 10cm/s average velocity airflow and 50 cm/s fuel flow were used. Flame heights were estimated from digital photographs. For the ethylene flame, soot particles were collected on a 3-mm, 400-mesh copper microscope grid coated with amorphous carbon film. The soot particles were analyzed using a Philips CM300FEG Scanning Transmission Microscope (STEM) operating at 300 kV with a 1-nm probe.

The figure above shows calculated temperature contours for the 1-g and 0-g CH4 flames. Path particles are shown in white. The 0-g flame length is about 2.2-cm, while the 1-g flame length is 2.1-cm. The computed 0-g flame is longer and more rounded than the 1-g flame. This is due to the absence of buoyancy and natural convection due to gravity. The particle paths in the figure above show that soot particles formed low in the flame move away from the flame for IDFs and can escape with minimal oxidation for both gravity conditions. Particles in the 1-g flame spend less time at high temperatures than those in the 0-g flame. Soot sampling for the STEM was taken from the flame tip. Particles appeared liquid like. The liquid structure is similar to soot collected in underventilated flame exhaust streams. U.C. Berkeley students Mark Mikofski and PapaMagatte Diagne will study the implications of this predicted trend for soot structure.

The specific project tasks are (1) to attempt to stabilize IDFs in a series of 2.2-second drop tower experiments and capture the events with a video camera for assessment of their stability, (2) to gain an understanding of CO and soot formation in IDFs by analyzing the post-combustion products of CH4 and C2H4 under normal and low gravity conditions, (3) to use a fiber-optic-coupled CO sensor to monitor the IDF tips during microgravity combustion, and (4) to model the flow field and soot particle temperature-time histories in both normal and low gravity conditions using an existing NIST computer program. Post flame amounts of CO and soot will be collected and measured. The soot will be analyzed for organic and graphitic content, for carbon and hydrogen content, and for primary sphere size, agglomerate size, and structure. The fiber-optic CO sensor will be based on near-infrared tunable diode laser absorption. The computer code features a particle transport model, which includes the effects of inertial, thermophoretic, and gravitational forces. The computed flow conditions will be combined with the measured CO and soot formation to gain insight into the conditions leading to the formation of these species during underventilated combustion.

Inverse diffusion flame in normal gravity

Inverse diffusion flame in micro gravity

Combustion chamber

Drop rig