Studies of turbulent flame propagation and chemistry interaction at elevated temperatures and high Reynolds numbers

A new high temperature, high Reynolds number, Reactor Assisted Turbulent Slot (RATS) burner has been developed providing a well-defined flow geometry and fuel/oxidizer reactivity to explore the new flame regimes experienced in propulsion engines that result from turbulence-chemistry interaction at elevated temperature and large ignition Damköhler numbers. The RATS burner enables turbulent flame studies for both gaseous and liquid fuels up to 700 K. The present study intends to answer one question: how does low temperature reactivity of large hydrocarbon fuels affect turbulent combustion regimes? The turbulent flow characteristics of the new burner at room temperature have been quantified using hot wire anemometry revealing moderate levels of turbulence with u'/U spanning 8 - 15 % depending on the turbulent generation geometry. Experimental methods were validated by measuring and comparing turbulent flame speeds of methane/air flames at bulk Reynolds numbers exceeding 20,000. Turbulent flame speeds, structures, and regimes of methane-air and n-heptane-air mixtures at elevated temperatures are investigated. The results demonstrate two different turbulent combustion regimes, a conventional thin reaction zone turbulent flame regime, and a new low temperature ignition (LTI) turbulent flame regime. The results from the methane/air and low reactant temperature n-heptane/air flames indicate a conventional turbulent flamelet regime and agree with previously reported flame speed data in the literature. However, the results of n-heptane/air flames at elevated reactant temperatures demonstrate a LTI flame regime and a distinct flame speed dependence on turbulent intensity. The present results indicate that the low temperature reactivity of large hydrocarbons can lead a new turbulent flame regime as the ignition Damköhler number is increased.