Browsing by Author "Ciccarelli, Gaby"
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- ItemOpen AccessCombustion in a horizontal channel partially filled with porous media(Shock Waves, 2008) Johansen, Craig; Ciccarelli, GabyExperiments were carried out to investigate the combustion propagation phenomenon in a horizontal channel partially filled with ceramic-oxide spherical beads. A 1.22 m long, 43 mm nominally thick layer of spherical beads is located at the ignition end of a 2.44 m long, 76 mm square channel. Tests were performed with 6.4 and 12.7 mm diameter beads. A flame is ignited at the bead end wall by an automotive spark ignition system. Flame propagation and pressure measurements are obtained via ionization probes and piezoelectric pressure transducers mounted on the top and bottom surfaces of the channel. High-speed schlieren video was used to visualize the structure of the explosion front. Experiments were performed with a 31% nitrogen diluted stoichiometric methane–oxygen mixture at room temperature and at an initial pressure in the range of 15–50 kPa. For initial pressures of 15 and 20 kPa the flame accelerates to a velocity close to the speed of sound in the combustion products. For initial pressure of 30 kPa and higher DDT occurs in the gap above the bead layer. An explosion front propagating at a velocity just under the CJ detonation velocity is detected in the bead layer even though the bead layer pore size is much smaller than the detonation cell size. It is demonstrated that flame propagation within the bead layer is the driving force behind the very rapid flame acceleration observed, however the DDT event occurring in the gap above the bead layer is not affected by the bead layer porosity. Schlieren video indicates that the structure of the explosion front varies across the channel height and with propagation distance down the channel.
- ItemOpen AccessDense particle cloud dispersion by a shock wave(Shock Waves, 2013) Kellenberger, Mark; Johansen, Craig; Ciccarelli, Gaby; Zhang, FannA dense particle flow is generated by the interaction of a shock wave with an initially stationary packed granular bed. High-speed particle dispersion research is motivated by the energy release enhancement of explosives containing solid particles. The initial packed granular bed is produced by compressing loose powder into a wafer with a particle volume fraction of Φ = 0.48. The wafer is positioned inside the shock tube, uniformly filling the entire cross-section. This results in a clean experiment where no flow obstructing support structures are present. Through high-speed shadowgraph imaging and pressure measurements along the length of the channel, detailed information about the particle shock interaction was obtained. Due to the limited strength of the incident shock wave, no transmitted shock wave is produced. The initial “solid-like” response of the particle wafer acceleration forms a series of compression waves that eventually coalesce to form a shock wave. Breakup is initiated along the periphery of the wafer as the result of shear that forms due to the fixed boundary condition. Particle break-up is initiated by local failure sites that result in the formation of particle jets that extend ahead of the accelerating, largely intact, wafer core. In a circular tube the failure sites are uniformly distributed along the wafer circumference. In a square channel, the failure sites, and the subsequent particle jets, initially form at the corners due to the enhanced shear. The wafer breakup subsequently spreads to the edges forming a highly non-uniform particle cloud.
- ItemOpen AccessFlame acceleration enhancement by distributed ignition points(Journal of Propulsion and Power, 2005) Ciccarelli, Gaby; Johansen, Craig; Hickey, MarkThis paper reports on the investigation of a novel method for promoting flame acceleration leading to detonation initiation in a tube. A common method used to initiate a detonation wave is via flame acceleration in an obstacle laden tube. Previous studies with fuel-air mixtures have shown that the measured detonation run-up distance, and corresponding run-up time, is too long for a PDE application. The objective of the present investigation is to enhance the flame acceleration process that leads to DDT by using multi-point ignition. Experiments were performed in a 3.05 m long, 14 cm inner-diameter tube equipped with a primary igniter mounted centrally on the tube endplate. Equally spaced orifice plates were placed in the first 2 m of the tube. A bank of four circumferentially equally spaced automotive spark plugs are located after each of the first three orifice plates. The firing time of each igniter bank is variable. The results indicate that flame acceleration is augmented early in the tube and maintained to the end. The reduction in the distance required for the flame to accelerate to a velocity on the order of the speed of sound in the combustion products is modest, on the order of 10%. However, the reduction in the time required to reach this velocity is much more pronounced which has an impact on the PDE cycle frequency. Flame acceleration was further enhanced by replacing the first few orifice plates with perforated plates with the same total flow area, e.g., the flame run-up distance was shortened by 30%. However, detonation initiation was not observed over the 3 m length of the tube in stoichiometric propane-air mixtures.
- ItemOpen AccessModeling the Initial Flame Acceleration in an Obstructed Channel using Large Eddy Simulation(2013) Johansen, Craig; Ciccarelli, Gaby
- ItemOpen AccessNumerical simulations of the flow field ahead of an accelerating flame in an obstructed channel(2010) Johansen, Craig; Ciccarelli, GabyThe development of the unburned gas flow field ahead of a flame front in an obstructed channel was investigated using large eddy simulation (LES). The standard Smagorinsky–Lilly and dynamic Smagorinsky–Lilly subgrid models were used in these simulations. The geometry is essentially two-dimensional. The fence-type obstacles were placed on the top and bottom surfaces of a square cross-section channel, equally spaced along the channel length at the channel height. The laminar rollup of a vortex downstream of each obstacle, transition to turbulence, and growth of a recirculation zone between consecutive obstacles were observed in the simulations. By restricting the simulations to the early stages of the flame acceleration and by varying the domain width and domain length, the three-dimensionality of the vortex rollup process was investigated. It was found that initially the rollup process was two-dimensional and unaffected by the domain length and width. As the recirculation zone grew to fill the streamwise gap between obstacles, the length and width of the computational domain started to affect the simulation results. Three-dimensional flow structures formed within the shear layer, which was generated near the obstacle tips, and the core flow was affected by large-scale turbulence. The simulation predictions were compared to experimental schlieren images of the convection of helium tracer. The development of recirculation zones resulted in the formation of contraction and expansion regions near the obstacles, which significantly affected the centerline gas velocity. Oscillations in the centerline unburned gas velocity were found to be the dominate cause for the experimentally observed early flame-tip velocity oscillations. At later simulation times, regular oscillations in the unburned streamwise gas velocity were not observed, which is contrary to the experimental evidence. This suggests that fluctuations in the burning rate might be the source of the late flame-tip velocity oscillations. The effect of the obstacle blockage ratio (BR) on the development of the unburned gas flow field was also investigated by varying the obstacle height. Simulation predictions show favorable agreement with the experimental results and indicate that turbulence production increases with increasing obstacle BR.
- ItemOpen AccessVisualization of the unburned gas flow field ahead of an accelerating flame in an obstructed square channel(Combustion and Flame, 2009) Johansen, Craig; Ciccarelli, GabyThe effect of blockage ratio on the early phase of the flame acceleration process was investigated in an obstructed square cross-section channel. Flame acceleration was promoted by an array of top and bottom-surface mounted obstacles that were distributed along the entire channel length at an equal spacing corresponding to one channel height. It was determined that flame acceleration is more pronounced for higher blockage obstacles during the initial stage of flame acceleration up to a flame velocity below the speed of sound of the reactants. The progression of the flame shape and flame area was determined by constructing a series of three dimensional flame surface models using synchronized orthogonal schlieren images. A novel schlieren based photographic technique was used to visualize the unburned gas flow field ahead of the flame front. A small amount of helium gas is injected into the channel before ignition, and the evolution of the helium diluted unburned gas pocket is tracked simultaneously with the flame front. Using this technique the formation of a vortex downstream of each obstacle was observed. The size of the vortex increases with time until it reaches the channel wall and completely spans the distance between adjacent obstacles. A shear layer develops separating the core flow from the recirculation zone between the obstacles. The evolution of oscillations in centerline flame velocity is discussed in the context of the development of these flow structures in the unburned gas.