Clase IV

Clase IVTipos de LlamaLlama laminar premezcladaLlama turbulenta premezcladaLlama laminar difusivaLlama turbulenta difusivaPropagacin de la Llama en un medio Heterogneo (Llama turbulenta premezclada)

Propagacin de la Llama en un medio Heterogneo (Llama turbulenta premezclada) g is the thermal diffusivity of fully vaporized fuelair mixturef is the mass fraction of fuel initially present as vapour. C2 and C3 are drop-size distribution parameters, equal to 0.41 and 0.56B mass transfer numberD32 Sauter mean diameter

Propagacin de la Llama en un medio Heterogneo (Llama turbulenta premezclada) The flame speed can be also expressed in respect to the evaporation constant ()

For a monodisperse spray of fuel drops and air

Propagacin de la Llama en un medio Heterogneo (Llama turbulenta premezclada) The first term characterizes the evaporation rate, and thus depends on fuel volatility, mean drop size, and vapour fraction. The second characterizes the chemical reaction rate.

Propagacin de la Llama en un medio Heterogneo (Llama turbulenta premezclada) If conditions are such that chemical reaction rates are limiting to flame speed, the flame speed reverts to the normal burning velocity for the fully evaporated mixture

If the reaction time is small in comparison to the time required for evaporation, the equations predict that flame speed is inversely proportional to mean drop size

The previous equations are still valid for turbulent mixtures of fuel drops and air, provided that SL is replaced by ST, and eff , which takes account of the role of turbulence in enhancing evaporation rates, is used instead of

Influence of fuel/air ratio and mean drop size on flame speed

Influence of mainstream velocity and mean drop size on flame speed.Heat up period

Variation of fuel temperature during drop lifetime

Evaporation rate curves for kerosene and JP 4 at 2000 K and 1 atmThe heat-up period is much longer for many fuels at high ambient pressures and temperaturesfuels containing several different petroleum compounds may not experience steady-state evaporation during their lifetimeEvaporation of a single drop

that corresponds to the slope of the lines drawn before

The average rate of evaporation during the drop lifetime is obtained fromThe drop lifetime is also readily obtained by assuming is constant

Godsave:Evaporation of a single drop Spalding:

Values of may be used to determine the transfer number B.

Other effectConvective Effects

Where ReD is the drop Reynolds number; typically around 5.

Effective Evaporation ConstantThis variable considers the effect of the heat-up period and the beneficial effect of forced convection (Chin and Lefebvre)

Variation of effective evaporation constant with normal boiling point at normal atmosphericPressure

For gas turbine combustors, where the value of u is usually high enough to affect evaporation rates, U in this figure should be replaced with u.

Spray evaporationFor a volume of air, V, containing n fuel drops of Sauter mean diameter Do, the average rate of fuel evaporation can be expressed as:

F/A ratio

Ignition theory

Two aspects of ignition

Gaseous Mixtures

Gaseous MixturesFor the spark kernel to survive and propagate unaided throughout a gaseous mixture

Case 1Case 2

Gaseous MixturesThe minimum ignition energy Emin is

Case 1Case 2

Relationship between Emin and dq for both quiescent and flowing mixtures. = 1.0.Gaseous Mixture

Influence of turbulence intensity on minimum ignition energy for propaneair mixturesGaseous Mixtures

Influence of pressure and oxygen concentration on quenching distance for propane - Velocity = 15.3 m/s.Heterogeneous MixturesHeavy fuel oil, U = 0, Bst =1.5

is the thermal diffusivityD32 is Sauter mean diameterHeterogeneous Mixtures

U = 15 m/s, = 0.65,SMD = 60 m

Property effects

Quenching Diameter and Ignition Energy

Spontaneous ignition delay Time interval between creation of a combustible mixture and flame onset

Spontaneous ignition delay

Effect of pressureSpontaneous ignition delay

Effect of fuel concentration

Spontaneous ignition delay

FlashbackFlashback occurring in the free stream (compressor surge or combustion instability)

Flashback occurring through the low-velocity flow in the boundary layer along the walls of the premixing section, and

Flashback driven by combustion-induced vortex breakdown (CIVB) in swirl-stabilized combustors.Flashback

Critical boundary velocity gradient

d=DiametereB= BlowoffF=Flashback Boundary layer

Turbulent boundary layer