Terminal energy distribution of blast waves from bursting spheres

The calculation results for the total energy delivered to the surroundings by the burst of an idealized massless sphere containing an ideal gas are presented. The logic development of various formulas for sphere energy is also presented. For all types of sphere bursts the fraction of the total initial energy available in the sphere that is delivered to the surroundings is shown to lie between that delivered for the constant pressure addition of energy to a source region and that delivered by isentropic expansion of the sphere. The relative value of E sub/Q increases at fixed sphere pressure/surrounding pressure as sphere temperature increases because the velocity of sound increases

Behavior of the lean methane-air flame at zero-gravity

A special rig was designed and constructed to be compatible with the NASA Lewis Research Center Airborne Research Laboratory to allow the study of the effect of gravity on the behavior of lean limit in a standard 50.4 mm (2 in.) internal diameter tube when the mixtures are ignited at the open end and propagate towards the closed end of the tube. The lean limit at zero gravity was found to be 5.10% methane and the flame was found to extenguish in a manner previously observed for downward propagating flames at one g. It was observed that g-jitter could be maintained at less than + or 0.04 g on most zero g trajectories. All of propagating lean limit flames were found to be sporadically cellularly unstable at zero g. There was no observable correlation between the occurrence of g-jitter and the lean limit, average propagation speed of the flame through the tube or the occurrence of cellular instability

The characterization and evaluation of accidental explosions

Accidental explosions are discussed from a number of viewpoints. First, all accidental explosions, intentional explosions and natural explosions are characterized by type. Second, the nature of the blast wave produced by an ideal (point source or HE) explosion is discussed to form a basis for describing how other explosion processes yield deviations from ideal blast wave behavior. The current status blast damage mechanism evaluation is also discussed. Third, the current status of our understanding of each different category of accidental explosions is discussed in some detail

The Use of a Laser Doppler Velocimeter in a Standard Flammability Tube

The use of the Laser Doppler Velocimeter, (LDV), to measure the flow associated with the passage of a flame through a standard flammability limit tube (SFLT) was studied. Four major results are presented: (1) it is shown that by using standard ray tracing calculations, the displacement of the LDV volume and the fringe rotation within the experimental error of measurement can be predicted; (2) the flow velocity vector field associated with passage of an upward propagating flame in an SFLT is determined; (3) it is determined that the use of a light interruption technique to track particles is not feasible; and (4) it is shown that a 25 mW laser is adequate for LDV measurements in the Shuttle or Spacelab

Effect of thermal expansion on the linear stability of planar premixed flames for a simple chain-branching model: The high activation energy asymptotic limit

The linear stability of freely propagating, adiabatic, planar premixed ames is investigated in the context of a simple chain-branching chemistry model consisting of a chain-branching reaction step and a completion reaction step. The role of chain-branching is governed by a crossover temperature. Hydrodynamic effects, induced by thermal expansion, are taken into account and the results compared and contrasted with those from a previous purely thermal-di�usive constant density linear stability study. It is shown that when thermal expansion is properly accounted for, a region of stable ames predicted by the constant density model disappears, and instead the ame is unstable to a long-wavelength cellular instability. For a pulsating mode, however, thermal expansion is shown to have only a weak e�ect on the critical fuel Lewis number required for instability. These e�ects of thermal expansion on the two-step chain-branching ame are shown to be qualitatively similar to those on the standard one-step reaction model. Indeed, as found by constant density studies, in the limit that the chain-branching crossover temperature tends to the adiabatic ame temperature, the two-step model can be described to leading order by the one-step model with a suitably de�ned e�ective activation energy

An experimental study of reactive turbulent mixing

An experimental study of two coaxial gas streams, which react very rapidly, was performed to investigate the mixing characteristics of turbulent flow fields. The center stream consisted of a CO-N2 mixture and the outer annular stream consisted of air vitiated by H2 combustion. The streams were at equal velocity (50 m/sec) and temperature (1280 K). Turbulence measurements were obtained using hot film anemometry. A sampling probe was used to obtain time averaged gas compositions. Six different turbulence generators were placed in the annular passage to alter the flow field mixing characteristics. The turbulence generators affected the bulk mixing of the streams and the extent of CO conversion to different degrees. The effects can be related to the average eddy size (integral scale) and the bulk mixing. Higher extents of conversion of CO to CO2 were found be increasing the bulk mixing and decreasing the average eddy size

The mechanisms of flame holding in the wake of a bluff body

The flame holding mechanism for lean methane- and lean propane-air flames is examined under conditions where the recirculation zone is absent. The main objective of this work is to study the holding process in detail in an attempt to determine the mechanism of flame holding and also the conditions where this mechanism is viable and when it fails and blow-off occurs. Inverted flames held in the wake of a flat strip were studied. Experiments with different sizes of flame holders were performed. The velocity flow field was determined using a laser Doppler velocimetry technique. Equation of continuity was used to calculate the flame temperature from the change in area of flow streamlines before and after the flame. Observations of the inverted flame itself were obtained using schlieren and direct photography. Results show that there are different mechanisms operative at the time of blow-off for lean propane and methane flames. Blow-off or extinction occurs for lean propane-air flame in spite of the reaction going to completion and the disparity between the heat loss and the gain in mass diffusion in the reaction zone i.e., Le 1.0 causes the flame to blow-off. For methane-air flame the controlling factor or blow-off is incomplete reaction due to higher blowing rate leading to reduced residence time in the reaction zone

Effect of a zero g environment on flammability limits as determined using a standard flammability tube apparatus

Flammability limits in a zero gravity environment were defined. Key aspects of a possible spacelab experiment were investigated analytically, experimentally on the bench, and in drop tower facilities. A conceptual design for a spacelab experiment was developed

The behavior of fuel-lean premixed flames in a standard flammability limit tube under controlled gravity conditions

Fuel-lean flames in methane-air mixtures from 4.90 to 6.20 volume percent fuel and propane-air mixtures from 1.90 to 3.00 volume percent fuel were studied in the vicinity of the limit for a variety of gravity conditions. The limits were determined and the behavior of the flames studied for one g upward, one g downward, and zero g propagation. Photographic records of all flammability tube firings were obtained. The structure and behavior of these flames were detailed including the variations of the curvature of the flame front, the skirt length, and the occurrence of cellular instabilities with varying gravity conditions. The effect of ignition was also discussed. A survey of flame speeds as a function of mixture strength was made over a range of lean mixture compositions for each of the fuels studied. The results were presented graphically with those obtained by other researchers. The flame speed for constant fractional gravity loadings were plotted as a function of gravity loadings from 0.0 up to 2.0 g's against flame speeds extracted from the transient gravity flame histories for corresponding gravity loadings. The effects of varying gravity conditions on the extinguishment process for upward and downward propagating flames were investigated

Magnetic Field Dependent Tunneling in Glasses

We report on experiments giving evidence for quantum effects of electromagnetic flux in barium alumosilicate glass. In contrast to expectation, below 100 mK the dielectric response becomes sensitive to magnetic fields. The experimental findings include both, the complete lifting of the dielectric saturation by weak magnetic fields and oscillations of the dielectric response in the low temperature resonant regime. As origin of these effects we suggest that the magnetic induction field violates the time reversal invariance leading to a flux periodicity in the energy levels of tunneling systems. At low temperatures, this effect is strongly enhanced by the interaction between tunneling systems and thus becomes measurable.Comment: 4 pages, 4 figure