Chapter 5 Flames ME 460 Fuels and Combustion

Chapter 5: Flames ME 460 Fuels and Combustion Laminar Premixed Flames Laminar Flame Theory Turbulent Premixed Flames Explosion Limits Diffusion Flames

Laminar Premixed Flames Laminar Flame Theory Turbulent Premixed Flames Explosion Limits Diffusion Flames

(the familiar) Bunsen Burner Flame slot burner Vtube Vl = Vtube sin α

Laminar Flame Speed rich side Effect of Stoichiometry 1. flame speeds of hydrocarbon fuels with air are within the range of 35 – 50 cm/s (1/2 meter per second) 2. flame speed is generally highest at the stoichiometric point (where temperatures are the highest – ignoring dissociation) 3. flame speed of hydrogen – air mixture is very high and slightly skewed to the rich side of stoichiometry 4. flame speed is affected by humidity 5. flame speed in oxygen mixtures is very high – ten times that of air

Combustion Tube ignition spark + h y d r o c a r b o n - + a i r m i x t u r e -

Flame Temperature and Flammability Limits 1. flame temperatures of hydrocarbon fuel – air mixtures fall within the range of 2000 – 2500 K 2. flame temperatures are maximum on the slightly rich side – why? 3. flame temperatures are a function of bond strength of the fuel 4. flame temperatures are a function of diluent, re: CO-O 2 5. flame temperatures are a function of heat capacity of the fuel and oxidant mixture rich side

Flammability Limits e. g. methane and air not necessarily symmetrical stoichiometric rich lean f/fs by mass or volume

Flammability Limits

Flammability Limits Effect of Adding Inert Gas such as CO 2 rich limit water added combustion no combustion Methane – Air from the USBM lean limit Another look

Flammability Limits U. S. Bureau of Mines Data Methane/Air and Various Diluents no combustion 1. methane flammability in air is 5 – 15% concentration by volume 2. diluent concentration decreases flammability limit combustion no combustion 3. diluent heat capacity decreases flammability limit 4. CO 2 has the least effect while N 2 and He have the greatest effect 5. humidity has an effect (as expected)

Laminar Flame Speed and Pressure equation 5. 2 Temperature 1. flame speed increases with temperature 2. flame speed decreases with increasing pressure 3. flame speed increases with decreased heat transfer ambient condition 4. flame speed decreases with increasing density (heat capacity)

Flame Stability flame stability is obviously important in steady state combustion such as burners and gas turbines

Laminar Premixed Flames Laminar Flame Theory Turbulent Premixed Flames Explosion Limits Diffusion Flames

Laminar Flame Modeling flow flame

Fundamental Equations continuity momentum energy

Calculated Flame Profiles

Simplified Model energy equation in the preheat zone solving with k and cp constant

assume chemical reaction starts at Tig, then the boundary conditions are differentiating and setting x = 0 (slope of the curve at the ignition point yields: (5. 16)

linearizing the temperature in the reaction zone (5. 17) (5. 16) equating 5. 17 with 5. 16 yields the result of Mallard and Le Chatelier (1883) laminar flame speed (5. 18)

The reaction thickness depends on the reaction rate. A reaction time can be determined based upon the average global reaction rate The thickness of the laminar flame is: (5. 20) Thus flame speed depends upon 1. thermal diffusivity 2. reaction rate 3. initial fuel concentration 4. flame temperature 5. ignition temperature 6. initial temperature Substituting 5. 20 into 5. 18 yields:

Simplified Model Results

Laminar Premixed Flames Laminar Flame Theory Turbulent Premixed Flames Explosion Limits Diffusion Flames

Turbulent Flames • turbulent combustion is caused by: ▫ high levels of heat release, rapid temperature increases, and rapid expansion of combustion products ▫ external factors • turbulence can increase flame speed and heat release 5 -50 times that of the laminar flame speed • turbulence is characterized by: ▫ wrinkled flamelets – an extension of the laminar flame ▫ thickened wrinkled flames – thickened reaction zone ▫ thickened flames – distributed reaction zone containing small lumps of reactants that are entrained within combustion products

Turbulent Parameters Kolmogorov Length Ll

Damköhler eddy turnover time = integral scale/mean fluctuating velocity Damköhler Number – ratio of eddy turnover to reaction zone chemical reaction time = flame thickness/laminar flame speed

Turbulent Combustion Diagram Karlovitz Number, re: reference below from Poinsot, T. , et. al. , ‘Diagrams of Premixed Turbulent Combustion Based on Direct Simulation’, Twenty-Third Symposium (International) on Combustion/The Combustion Institute, 1990)/pp. 613 -619

Fig 5. 12 Diagram of a turbulent premixed flame front, reactants at 300 K and products at 2000 K (a) wrinkled flame (b) thickened wrinkled flame (c) thickened flame

Turbulent Flame Sketch

Turbulent Flame Speeds

Laminar Premixed Flames Laminar Flame Theory Turbulent Premixed Flames Explosion Limits Diffusion Flames

Explosion Limits When a combustible gas mixture is heated above the autoignition temperature an explosion (a detonation) rather than a propagating flame )a deflagration) may occur. ▫ accompanied delay (a dwell period) ▫ detonation or deflagration is temperature and pressure sensitive ▫ no flame zone ▫ possible two stage reaction process (before and then during) the detonation reaction ▫ kinetics, although important may not be fully understood.