CBET Award Achievements
Notable Accomplishments from CBET Awards

Flames on Edge!

Sandip Ghosal, Northwestern University

Background:  Studying flame phenomena can have significant multidisciplinary impacts in understanding, predicting, and controlling flames.  In particular, sharp edges can affect how flames react to their environment.  What happens to simple, laminar flames along sharp edges?  As a building-block approach to future studies on turbulent diffusion flame phenomena, the Ghosal Group at Northwestern University specifically studied strain-induced local extinction and re-ignition by observing flames on simpler surfaces.

The Group studied strain-induced local extinction and re-ignition through laboratory research and by using numerical simulations.  Certain flame characteristics are a result of the propagation of the “edge” or boundary between burning and extinction zones.  The Ghosal Group sought to obtain a better theoretical understanding of edge-flame phenomena.

Results:  The Ghosal Group adopted a simple model involving circular flame holes and disks on a laminar counter-flow diffusion flame and solved the problem numerically.  The Group found a relationship between the strain rate and the radius of the hole.  The Group discovered that if the diffusion flame had a strain rate intermediate between the ignition and extinction limits, then a critical hole radius exists (analogous results hold for flame disks) and above which the hole opened up and below which it closed.  As the strain rate increases, the critical radius decreases but is finite at all values of the strain rate between the ignition and extinction limits.

In the final stage of the collapse, the speed of propagation of points situated on diametrically opposite sides of the collapsing hole increases rapidly due to overlap of their pre-heat zones.  The mechanism is identical to the analogous acceleration of a pair of planar pre-mixed colliding flames for which a similarity solution was derived for unity Lewis numbers.  The departure of the Lewis number from unity leads to some interesting unsteady effects.

Sandip Ghosal Image 1   Sandip Ghosal Image 2

Top view of a flame hole
Top view of a flame disk
Sandip Ghosal Image 3   Sandip Ghosal Image 4

Cross-sectional view of a flame hole
Cross-sectional view of a flame disk
Credit, all images:  Ghosal, S. and Lu, Z.; Northwestern University

Scientific Uniqueness:  The Ghosal Group was able to study laminar flow of flames along an edge.  This work will provide a fundamental basis in flame research and serves as a building-block for future studies.  The Group was able to model strain-induced local extinction and re-ignition numerically as the edges of flames shrink and grow.

Impact on Industry and/or Society:  Manifestations of turbulent diffusion flames are everywhere.  A few examples are propulsion systems, such as aircraft gas turbine engines and rocket engines; energy generation systems, such as gas-turbine generators; automobile engines; and large-scale accidental fires, such as forest fires.  Consequently, a large research effort is dedicated to the computer modeling of flames in order to enhance our ability to understand, predict and control them.  Local ignition, extinction and propagation of fronts within a turbulent flame are the most difficult things to model.  Fundamental studies such as the work of the Ghosal Group will lead to improved models for such large-scale simulations.

Potential Economic Impact: Improved understanding of the combustion process leads to better and more efficient design of devices that rely on combustion.  More efficient designs will lower both the operational cost as well as the environmental cost in terms of emission of greenhouse gases and other harmful products of combustion.

This work is notable because this Ghosal Group is helping to lay the foundation in flame research through a building-block approach by studying how sharp edges of flames shrink or grow.

This project addresses the NSF Strategic Goals of:  (1) Discovery and (2) Research InfrastructureThe Ghosal Group's contributions to fundamental flame research will lead to better predictive, large-scale simulations.  This building-block approach will have impact on future flame models.

Program Officer:   Phillip R. Westmoreland
NSF Award Numbers:   0121051
Award Title:   Dynamics of Ignition and Extinction Fronts on Diffusion Flame Sheets
PI Name:   Sandip Ghosal
Institution Names:   Northwestern University
Program Element:   1407

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This Nugget was Updated on 27 June 2008.