8 research outputs found
Direct detonation initiation in hydrogen/air mixture: effects of compositional gradient and hotspot condition
Two-dimensional simulations are conducted to investigate the direct
initiation of cylindrical detonation in hydrogen/air mixtures with detailed
chemistry. The effects of hotspot condition and mixture composition gradient on
detonation initiation are studied. Different hotspot pressure and composition
are first considered in the uniform mixture. It is found that detonation
initiation fails for low hotspot pressures and supercritical regime dominates
with high hotspot pressures. Detonation is directly initiated from the reactive
hotspot, whilst it is ignited somewhere beyond the nonreactive hotspots. Two
cell diverging patterns (i.e., abrupt and gradual) are identified and the
detailed mechanisms are analyzed. Moreover, cell coalescence occurs if many
irregular cells are generated initially, which promotes the local cell growing.
We also consider nonuniform detonable mixtures. The results show that the
initiated detonation experiences self-sustaining propagation, highly unstable
propagation, and extinction in mixtures with a linearly decreasing equivalence
ratio along the radial direction respectively, i.e., 1 to 0.9, 1 to 0.5 and 1
to 0. Moreover, the hydrodynamic structure analysis shows that, for the
self-sustaining detonations, the hydrodynamic thickness increases at the
overdriven stage, decreases as the cells are generated, and eventually become
almost constant at the cell diverging stage, within which the sonic plane shows
a sawtooth pattern. However, in the detonation extinction cases, the
hydrodynamic thickness continuously increases, and no sawtooth sonic plane can
be observed
Numerical Simulation of Hot Jet Detonation with Different Ignition Positions
Ignition position is an important factor affecting flame propagation and deflagration-to-detonation transition (DDT). In this study, 2D reactive Navier–Stokes numerical studies have been performed to investigate the effects of ignition position on hot jet detonation initiation. Through the stages of hot jet formation, vortex-flame interaction and detonation wave formation, the mechanism of the hot jet detonation initiation is analyzed in detail. The results indicate that the vortexes formed by hot jet entrain flame to increase the flame area rapidly, thus accelerating energy release and the formation of the detonation wave. With changing the ignition position from top to wall inside the hot jet tube, the faster velocity of hot jet will promote the vortex to entrain jet flame earlier, and the DDT time and distance will decrease. In addition, the effect of different wall ignition positions (from 0 mm to 150 mm away from top of hot jet tube) on DDT is also studied. When the ignition source is 30 mm away from the top of hot jet tube, the distance to initiate detonation wave is the shortest due to the highest jet intensity, the DDT time and distance are about 41.45% and 30.77% less than the top ignition