The existence and causes of fundamental flammability limits, of combustibles have been investigated by identifying the critical states at which steady propagation of the laminar premixed flame in the doubly-infinite domain fails to be possible due to the simultaneous influence of radiative heat loss and branching-termination chain mechanisms. Flame propagation was simulated with detailed chemistry and transport properties, with radiative heat loss from H2O, CO2, and CH4, and by using the arc-length continuation technique to generate the extinction turning points. Cases studied include the flammabilities of lean methane/ air and rich hydrogen/air mixtures, and also effects of pressure, chain termination rates, radiating species, and radiation intensity on the state of flammability. Results demonstrate the existence of the extinction turning point behavior for all dependent flame parameters as the fuel concentration approaches the limits, indicating that weaker mixtures cannot sustain steady flame propagation. The limit flame speed and the flame temperature reduction, relative to the corresponding adiabatic values, are in close agreement, with the predictions from activation energy asymptotic theory. It is further shown that slightly before the turning point is reached, the normalized sensitivity of the dominant termination reaction to variations in the dominant branching reaction attains a value of unity and, starts to increase rapidly, indicating that chain mechanism is crucial in inducing the rapid weakening of the reaction intensity as the flammability limit is approached. The present study therefore unifies existing concepts of flammability limiets, based on either heat loss or chain termination, by showing that they are simultaneously operative and important at the fundamental flammbility limits.
All Science Journal Classification (ASJC) codes
- Chemical Engineering(all)
- Fuel Technology
- Energy Engineering and Power Technology
- Mechanical Engineering
- Physical and Theoretical Chemistry
- Fluid Flow and Transfer Processes