1 / 16

ME 525: Combustion Lecture 17: Flame Speed Parameters, Quenching, Flammability, Ignition, Stabilization

ME 525: Combustion Lecture 17: Flame Speed Parameters, Quenching, Flammability, Ignition, Stabilization. Flame speeds of different fuels, with oxygen enhancement, and with different inert gases Laminar flame quenching with heat loss Minimum ignition energy for spark ignition.

meg
Download Presentation

ME 525: Combustion Lecture 17: Flame Speed Parameters, Quenching, Flammability, Ignition, Stabilization

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. ME 525: CombustionLecture 17: Flame Speed Parameters, Quenching, Flammability, Ignition, Stabilization • Flame speeds of different fuels, with oxygen enhancement, and • with different inert gases • Laminar flame quenching with heat loss • Minimum ignition energy for spark ignition

  2. Flame Speeds based on Simplified Chemistry • The laminar flame speed increases with increasing thermal diffusivity and increasing volumetric chemical reaction rate.

  3. Structure of Premixed Hydrogen Flames • Laminar flame speeds for different fuels Glassman, Combustion, 2nd Edition SL (cm/s)

  4. Laminar Flame Speeds Glassman, Combustion, 2nd Edition • Can increase volumetric fuel reaction rate by increasing O2 content of air.

  5. Laminar Flame Speeds Glassman, Combustion, 2nd Edition • Can increase flame speed by increasing thermal diffusivity a or final flame temperature Tb:

  6. Laminar Flame Speeds Glassman, Combustion, 2nd Edition

  7. Quenching of Laminar Flames • Critical diameter of a circular tube where flame extinguishes rather than propagates is the quenching distance. Wall of tube extinguishes flame by heat transfer and radical recombination.

  8. Quenching of Laminar Flames • Critical diameter d of a circular tube where flame extinguishes rather than propagates is the quenching distance. Wall of tube extinguishes flame by heat transfer and radical recombination. Critical diameter is determined by observing whether or not a flame stabilized above the burner will flashback when reactant flow is shut off (velocity goes to zero).

  9. Quenching of Laminar Flames Plane Parallel Plates Burner Slot

  10. Quenching of Laminar Flames • Consider heat balance on laminar flame confined between two parallel walls, flame quenches when heat lost to the walls equals heat generated by combustion:

  11. Quenching of Laminar Flames • Lower bound for gradient at the wall: • Substituting:

  12. Quenching of Laminar Flames • Assumption: Tw= Tu • But: • Substituting:

  13. Minimum Ignition Energy for Spark Ignition • Assume that a spherical volume of premixed gases is heated to Tb by a spark. There is a critical radius Rcrit below which heat losses to the surrounding gas will be too high for the flame to propagate.

  14. Minimum Ignition Energy for Spark Ignition • Equate energy liberated by reaction (same as energy supplied by spark) to heat lost to surrounding gases to determine Rcrit:

  15. Minimum Ignition Energy for Spark Ignition • From solution of heat conduction equation for an infinite hollow sphere (see Incropera and Dewitt): • Using:

  16. Minimum Ignition Energy for Spark Ignition • We obtain: Link to Example Problem Link to Solution to Example Problem

More Related