G. Ciccarelli

Fragmentation mechanisms based on single drop steam explosion experiments using flash X-ray radiography

Flash X-ray and high-speed regular photography were used to investigate the fragmentation processes during the vapor explosion of single drops of molten metal immersed in water. For relatively low ambient flow velocities ( 45 m/s), vapor bubble growth is diminished and high-speed motion of vapor within the bubble leads to an enhanced fragmentation rate.

FLAME-QUENCHING PERFORMANCE OF CERAMIC FOAM

This paper reports on a study investigating the flame-quenching performance of ceramic foam. Experiments were performed primarily with methane oxygen mixtures in a vertical transparent tube ignited at the bottom-end which is open to the atmosphere. The flame quenching performance of the ceramic foam is compared to that of closely packed ceramic spheres. Based on the measured quenching mixture composition limits it is shown that for an equivalent flow path diameter the packed spheres performed better than the foam. The trend in the quenching limit results, as characterized by the Peclet number, could not be explained by one-dimensional thermal quenching theory. Experiments performed in a quenching medium with straight circular channels, similar in diameter as the effective flow diameter of the foam and closely packed spheres, indicates that the inherent flow tortuosity of the foam has little affect on the flame quenching phenomenon. Additional tests carried out in the straight channels with vastly different molecular weight hydrocarbon fuels, i.e., methane, ethylene and propane, show that preferential diffusion cannot explain differences in the measured limits compared to one-dimensional thermal theory predictions.

Detonation wave propagation through a single orifice plate in a circular tube

Detonation behavior associated with the propagation of a detonation wave through an orifice plate located within a circular tube is investigated. The tube and orifice diameter used in the study are 27.3 cm and 10 cm, respectively. The test gas used is hydrogen-air at 1 atmosphere and at various initial temperatures up to 650 K. Immediately after the orifice, the detonation wave decouples and either fails or reinitiates. The reinitiation process is characterized by either spontaneous initiation, initiation due to shock reflection, or deflagration-to-detonation transition (DDT). In the case of DDT, transition is preceded by the degeneration of the decoupled detonation wave to a velocity consistent with a CJ deflagration. Delineation between these various propagation regimes could not be correlated with the detonation cell size, λ, and orifice diameter, d . The data, although limited, demonstrate for the first time that the d c /λ =13 critical tube criterion obtained at room temperature may not apply at elevated temperature conditions. The evidence for this is data obtained at 500 K that shows no detonation transmission for 30% hydrogen in air that corresponds to d/λ =16.7. The tests also indicate that a simple d/λ correlation cannot be used to determine when reinitiation due to shock reflection is possible. For example, at 650 K detonation wave failure was observed for d/λ d/λ

The Influence of Initial Temperature on the Detonability Characteristics of Hydrogen-Air-Steam Mixtures*

The influence of initial mixture temperature on the detonability of hydrogen-air-steam mixtures at 0,1 MPa has been investigated experimentally. The detonability of the mixture has been characterized by its cell size. The detonation cell size has been measured in a heated detonation tube using the well-established smoked foil technique. It has been shown that for all the hydrogen-air-steam mixtures tested, increasing the initial temperature decreases the detonation cell size and thus increases the mixture‘sensitivity’to undergo detonation. It has also been shown that the mitigating effect ofsteam dilution, manifested by an increase in cell size, decreases with increasing initial temperature. A one-dimensional ZND model with full chemical kinetics was used to calculate reaction zone lengths for the mixture conditions tested. A comparison of the measured detonation cell size with the calculated reaction zone lengths indicated that a simple linear relationship between the two does not exist.

Critical tube measurements at elevated initial mixture temperatures

Experimental results obtained on the transmission of a planar detonation wave from a cylindrical tube into an unconfined volume are reported. Experiments are performed using a 9.1 mm long tube connected to an 88.9 cm inner diameter receiver vessel. Both the detonation tube and receiver vessel can be heated. Tests are performed using 10, 20, and 27.3 cm tubes. The minimum, or critical, hydrogen concentration in air that results in detonation transmission into the receiver vessel is measured at an initial pressure of 1 atm and initial temperatures of 300, 500, and 650 K. The value of the ratio of the tube diameter, d, and the critical mixture detonation cell size, u c , is found to be between 18 and 24. The data show no correlation between the value of d/ u c and the initial mixture temperature and there is no observable effect of temperature on the regularity of the soot foil line spacing. The uncertainty in the average cell size measurement of - 25% is not sufficient to explain the significant departure from the classical empirical correlation d/ u c =13. It is proposed that the d/ u c =13 is neither a unique nor adequate correlation for describing detonation diffraction in mixtures considered to have a regular or irregular detonation cellular structure.

Effect of boundary conditions on the propagation of a vapor explosion in stratified molten tin/water systems

Abstract The propagation of a vapor explosion in a stratified molten tin/water system has been investigated experimentally in two different geometrical configurations: (i) linear propagation within a narrow channel, and (ii) radial propagation in a cylindrical tank. In the narrow channel experiments, self-sustained spatial propagation of the interaction occurs with an average propagation speed of 40–50 m s −1 and with an effective mixing depth not more than 2 mm. A minimum degree of inertial confinement provided by the water aboe the tin as well as the confinement provided by the lateral walls are required to sustain a propagating interaction. If an interaction propagating within a narrow channel encounters a sudden increase in the channel width, it fails abruptly after a short distance. Interactions initiated in a relatively unconfined cylindrical geometry always fail to propagate after a certain distance. A comparison between the energetics of single molten tin drop interactions and interactions in the stratified systems suggests that similar dynamic processes occur during the first interaction cycle in each case. In both cases, the first interaction is an efficient method for mixing the tin and water and is often a precursor to a second, more violent interaction.

Numerical simulations of the flow field ahead of an accelerating flame in an obstructed channel

The 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.

Dense Particle Cloud Dispersion by a Shock Wave

A 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

Investigation of Flame Acceleration Enhancement for a Pulse Detonation Engine Initiation System

\phi _\mathrm{p} = 0.48