Elaine S. Oran

De∞agration-to-Detonation Transition in H2-Air Mixtures: Efiect of Blockage Ratio

We analyze the efiect of blockage ratio on ∞ame acceleration and de∞agration-to-detonation transition (DDT) in hydrogen-air mixtures using two-dimensional numerical simulations. The numerical model is based on reactive Navier-Stokes equations coupled to a one-step Arrhenius kinetics of energy release. The simulations show that the distance to DDT does not signiflcantly change for blockage ratios BR = 0:31i0:56, but increases sharply outside of this interval. This is a result of two main competing efiects: larger obstacles promote the ∞ow and ∞ame acceleration, but they also weaken difiracting shocks. We analyze the evolution of the total energy-release rate in the system between the ∞ame ignition and the DDT and show that it grows by a factor of 1000{2000, mostly due to the ∞ame surface increase. The distance to DDT seems to be insensitive to the ignition mode, and this can be explained by the ∞ow choking that limits the in∞uence of the upstream ∞ow on the downstream.

Deflagration-to-Detonation Transition in Premixed H2-Air in Channels with Obstacles

We study ∞ame acceleration and de∞agration-to-detonation transition (DDT) in obstructed channels using 2D reactive Navier-Stokes numerical simulations. The energy release rate for the stoichiometric hydrogen-air mixture is modeled by one-step Arrhenius kinetics. Computations performed for channels with symmetrical and staggered obstacle conflgurations show two main efiects of obstacle spacing S. First, more obstacles per unit length create more perturbations that increase the ∞ame surface area more quickly, and therefore the ∞ame speed grows faster. Second, DDT occurs more easily when the obstacle spacing is large enough for Mach stems to form. These two efiects are responsible for three difierent regimes of ∞ame acceleration and DDT observed in simulations: (1) detonation is ignited when a Mach stem formed by the difiracting shock re∞ecting from the bottom wall collides with an obstacle, (2) Mach stems do not form, and the detonation is not ignited, and (3) Mach stems do not form, but the leading shock becomes strong enough to ignite a detonation by a direct collision with the top part of an obstacle. Regime (3) is observed for small S and involves multiple isolated detonations that appear between obstacles and play a key role in flnal stages of ∞ame and shock acceleration. For staggered obstacle conflgurations, we observe resonance phenomena that signiflcantly reduce the DDT time when S=2 is comparable to the channel width in Regime (1). Simulations also show that basic processes responsible for DDT phenomena are the same for obstructed channels and 2D arrays of obstacles, even though flnal detonation ignition modes may be difierent. Efiects of imposed symmetry and stochasticity on DDT phenomena are also considered.

Flame Acceleration in Narrow Tubes: Eect of Wall Temperature on Propulsion Characteristics

We use reactive Navier-Stokes numerical simulations to study the propagation of laminar ames in two-dimensional channels closed at one end. We consider three types of boundary conditions for channel walls: adiabatic, isothermal, and with a limited heat loss coecien t that was varied to model dieren t levels of material insulation. We also vary the channel width and compare resulting outo w velocities. The maximum ame acceleration and outo w velocity occur for adiabatic walls. As the heat loss coecien t increases, energy losses to the walls from the hot, burned material behind the ame front reduce ame acceleration and and outo w velocity. For isothermal walls, there is no signican t ame acceleration and the ame may be quenched. Previous computations showed the maximum ame acceleration in channels with adiabatic walls occurred when the channel was about v e times larger than the reaction zone of a laminar ame. As the energy-loss rate to the walls increases, the maximum outo w velocity decreases, and the position of the maximum shifts towards larger channels. The results indicate that the choice of the particular insulating material is critical for achieving the desired level of ame acceleration in practical propulsion devices.

Simulation of Deflagration-to-Detonation Transition in Premixed CH 4 -Air in Large-Scale Channels with Obstacles

The deflagration-to-detonation (DDT) transition of a stoichiometric methane-air gas mixture in a channel with obstacles is simulated using a reduced single-step reaction mechanism. The parameters of this chemical model are calibrated to produce properties of laminar flames and planar detonation waves that correspond to existing experimental and theoretical data. The model is further calibrated using experimental data of DDT in obstructed tubes. Two distinct regimes of flame propagation are identified: the “quasidetonation” regime characterized by repeated initiations and failures of detonations and the “choking” regime for which DDT does not occur. The development of a flame into a particular propagation regime depends on the channel geometry. The physical mechanisms controlling the DDT are found to be the same as those identified for hydrogen-air mixtures, namely the formation of strong shock waves and Mach stems that locally raise the temperature in a region of unburned fuel above the ignition point, although the initiation of detonations depends strongly on the model parameters.