Fundamental Study of Laminar Flame Characteristics 2

Laminar lifted jet flame under autoignitive condition

At relatively high jet velocity, jet flames are detached from the jet nozzle and are stabilized by “lifted” status.
Lifted flames are observed in many practical combustion appliances (e.g., Diesel engine, gas turbine)

Isocontours of temperature and mass fraction of OH for autoignited laminar attached jet flame (left) and lifted jet flame (right)

⇒ Requires a fundamental understanding of stabilization mechanism for autoignited laminar lifted flames

to elucidate the decreasing liftoff height behavior with the increasing jet velocity
to elucidate the detailed flame structure for Tribrachial edge flame / Moderate or Intense Low oxygen Dilution (MILD) combustion
New Findings:
- high diffusive nature of hydrogen molecule mainly leads to the unusual liftoff height trend
- flame stabilization mechanism (autoignition, autoignition-assisted flame propagation) changes depending on the flame structures
- From Chemical Explosive Mode Analysis (CEMA), important variables and reaction steps for the MILD combustion and tribrachial edge flame regimes are identified.

Isocontours of temperature and mass fraction of OH for autoignited methane/hydrogen jet flames

HL_T

Liftoff and temperature variations with the increase of fuel jet velocity

Participation Index (PI) of important reactions for the MILD combustion (i.e., U = 4 m/s) and tribrachial edge flame regimes (i.e., U = 25 m/s)

to elucidate the U-shape liftoff height behavior with the variations of fuel jet velocity
to elucidate the pyrolysis effect on the liftoff and flame structure
New Findings:
- DME is pyrolyzed by smaller species such as methane, hydrogen, and the pyrolysis effect increases as fuel jet velocity decreases
- Differential diffusion effect increases for the relatively-low jet velocity, which leads to the decreasing liftoff height behavior

Liftoff and temperature variations with the increase of fuel jet velocity

Schematic for the stabilization of autoignited laminar DME jet flames with (a) decreasing and (b) increasing liftoff regimes.

The phenomenon occurring as partial oxidation reactions of fuel are sustained at low temperatures ranging from 500 to 800K
Preventing high local temperatures of the flame can block the causes of excessive soot and NOx emissions

New Findings:
- A new form of diffusion cool flame around the reaction zone where no temperature peak exists is discovered
- Various differences in ignition processes, heat release rates, and heat transfer phenomenon is analyzed.

Ignition delay time of n-heptane fuel according to the temperature and Low-temperature chemistry pathway of fuel oxidation

and self-sustaining diffusion cool flame of n-heptane fuel in the counter flow flame experimental setup

Heat release rate contour video for the Air temperature 600 K and 800 K

convection, diffusion, and reaction budgets along the centerline with temperature profiles

Variations in scalar dissipation rate of non-premixed mixture occurs in turbulence flow
Ignition delay time of non-premixed mixture is widely varied by scalar dissipation rate

⇒ Requires a fundamental understanding of the effect of turbulence on ignition delay in the view of the variation in scalar dissipation rate

To elucidate the non-linear profile of ignition delay according to vortex intensity
It can be considered that turbulence is characterized by the organized motion of multiple vortex structure

New Findings:
- Ignition which occurs on the vortex tip is retarded at the stroing vortex intensity.
- Advanced and ratardid effects of vortex on ignition delay can be represented by the ratio between the convective and chemical time scales.

그림2

Profile of ignition delay according to vortex intensity (VTI and MLI mean the vortex tip ignition and mixing layer ignition, respectively.)

그림3

Transitional iso-contour of explosive mode indicator according to vortex intensity