January 30, 2015

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Compressible Reacting Flows

Energy transport in chemically reacting flows can occur through both thermal conduction and gasdynamic heating. In low Mach number reacting flows the pressure across a flame is nearly constant and thermal conduction and molecular diffusion are the dominant heat and mass transport mechanisms. Our research considers cases in where heat is transported by both gasdynamic heating as well as thermal conduction. Acoustic timescale detonation initiation is one such case that demonstrates the importance of gasdynamic heating.

A reactive mixture is initially at rest in thermal equilibrium with slip walls on the top, bottom and left boundaries. Heat is deposited in the lower left hand corner of the domain on a timescale similar to the local acoustic time of the deposition region. Compression waves propagate away and heat the reactive mixture. Surface instabilities form between the reacted and unreacted gas through Richtmyer-Meshkov and Kelvin-Helmholtz instabilities.

Shock wave interaction with dense particle cloud

Little if any information exists that details what occurs on the acoustic timescale as a shock wave impacts a dense non-compacted particle cloud, particularly inside and just behind the particle cloud. We use numerical simulations to investigate and characterize this interaction. Results are compared with a simple one-dimensional model to determine how accurate the model is during this early interaction.

A shock wave $latex M=1.67$ impacts a dense particle curtain creating reflected and transmitted shock fronts. A contact discontinuity is formed as the transmitted shock emerges from the trailing edge of the particle curtain. The flow between the contact and the trailing edge of the curtain is unsteady and contains numerous vortical structures. Additionally, shock reflections continue to reverberate inside the curtain and emit compression waves both upstream and downstream of the particle curtain.

Cell formation - lean hydrogen combustion

Premixed flames develop thermo-diffusive instabilities when the diffusivity of the fuel is different from the rest of the mixture. This results in preferential diffusion. Even when a uniform premixed composition is used, the local equivalence ratio across a flame front will not be constant. Thus different parts of the flame burn at different speeds, which can act as stabilizing or destabilizing mechanisms.

A sinusoidally perturbed premixed laminar flame, burning a mixture of hydrogen and air (equivalence ratio $\latex \phi=0.4$), forms cells on the surface of the flame front. The simulation is performed using the flamelet progress variable combined with the mixture fraction to track the differential diffusion of hydrogen gas.