John H.S. Lee

Disruption of Cnp1 uncouples oligodendroglial functions in axonal support and myelination

Myelination of axons by oligodendrocytes enables rapid impulse propagation in the central nervous system. But long-term interactions between axons and their myelin sheaths are poorly understood. Here we show that Cnp1, which encodes 2′,3′-cyclic nucleotide phosphodiesterase in oligodendrocytes, is essential for axonal survival but not for myelin assembly. In the absence of glial cyclic nucleotide phosphodiesterase, mice developed axonal swellings and neurodegeneration throughout the brain, leading to hydrocephalus and premature death. But, in contrast to previously studied myelin mutants, the ultrastructure, periodicity and physical stability of myelin were not altered in these mice. Genetically, the chief function of glia in supporting axonal integrity can thus be completely uncoupled from its function in maintaining compact myelin. Oligodendrocyte dysfunction, such as that in multiple sclerosis lesions, may suffice to cause secondary axonal loss.

The critical tube diameter for detonation failure in hydrocarbon-air mixtures

Abstract Critical tube diameters dc for the successful transformation of a planar to a spherical detonation have been measured in nine gaseous fuels (CH4, C2H2, C2H4, C2H6, C3H6, C3H8, C4H10, MAPP and H2) in stoichiometric fuel-oxygen mixtures diluted with nitrogen at atmospheric initial pressure. In agreement with the previous work of Matsui and Lee. acetylene is found to have the smallest critical tube diameter of 12 cm at stoichiometric composition with air. However, in contrast with the earlier estimate of Matsui and Lee, hydrogen is now found to be the second most sensitive fuel with a critical tube diameter of 20 cm. Both ethylene and MAPP, with critical diameters of about 38 and 53 cm, respectively, have nearly the same sensitivity. The heavier hydrocarbons (i.e., C4H10, C2H6, C3H6, and C3H8) are found to belong to the same sensitivity class with critical diameters of 64, 67, 70, and 70 cm, respectively. Direct measurements of detonation cell sizes λ in both atmospheric fuel-oxygen-nitrogen mixtures and subatmospheric fuel-oxygen mixtures demonstrate that the observation of Mitrofanov and Soloukhin in low-pressure ( p 0 ⋍ 80 Torr) C2H2O2 mixtures that dc3- 13λ is valid for other fuels as well and can be used as an empirical law to estimate critical tube diameter from cell size data. Based on the preseent results for the critical tube diameter, initiation energies have been estimated using the work-done formula of Lee and Matsui to evaluate the detonation hazard numbers DH as proposed previously by Matsui and Lee. For stoichiometric fuel-air mixtures at 1 atm initially, acetylene ranks first (DH = 8.4 × 105), followed by hydrogen (DH = 3.4 × 106) and ethylene oxide (DH = 1.1 × 107) and MAPP (DH = 7.2 × 107) have nearly the same sensitivity. The other alkanes (DH = 1.5 × 108 for ethane, DH = 1.7 × 108 for n-butane, DH = 1.73 × 108 for propane) and the alkenepropylene (DH = 1.6 × 108) belong to the same sensitivity class.

Photochemical initiation of gaseous detonations

Abstract Direct initiation of detonations in gaseous mixtures of C 2 H 2 -O 2 , H 2 -O 2 and H 2 -Cl 2 in the pressure range of 10–150 torr using flash photolysis was studied. Similar to blast initiation using a concentrated powerful energy source, it was found that for photochemical initiation, there exists a certain threshold of flash intensity and energy for each mixture at any given initial pressure and composition below which a deflagration is formed. At the critical threshold, however, a fully developed detonation is rapidly formed in the immediate vicinity of the window of incident UV radiation. However, at super critical flash energies, the amplitude of the detonation formed decreases and combustion of the entire irradiated volume approaches a constant volume explosion. It was found that photo-chemical initiation requires both a certain minimum peak value of the free radical concentration generated by the photo-dissociation as well as an appropriate gradient of this free radical distribution. The minimum peak radical concentration permits rapid reaction rates for the generation of strong pressure waves, while the gradient is necessary for the amplification of the shock waves to a detonation. If the gradient is absent and the free radicals are uniformly distributed in the mixture, then the entire volume simply explodes as in a constant volume process. The present study reveals that the mechanism of photochemical initiation is one of proper temporal synchronization of the chemical energy release to the shock wave as it propagates through the mixture. In analogy to the LASER, the term SWACER is introduced to represent this mechanism of Shock Wave Amplication by Coherent Energy Release. There are strong indications that this SWACER mechanism is universal and plays the main role in the formation of detonations whenever a powerful concentrated external source is not used to generate a strong shock wave in the explosive.

Flame acceleration due to turbulence produced by obstacles

Abstract This paper reports on an investigation of the influence of obstacles on the propagation of freely expanding cylindrical flames. The flame speed is found to depend critically on the obstacle configuration and flame speeds up to 130 m/sec in stoichiometric methane-air mixtures are readily achieved by placing appropriate turbulence producing obstacles in the flame path. This is ∼ 24 times the flame speed observed with no obstacles. The dramatic influence of obstacles is interpreted in terms of the positive feedback coupling between the flame itself and the turbulence and flow field distortions produced by the obstacles. It is also shown that the flame is unable to maintain its large turbulent flame speed without repeated obstacles to provide flow field distortions and turbulence continuously.

Pressure development due to turbulent flame propagation in large-scale methaneair explosions

Abstract The results of large-scale methaneair explosion tests performed at Raufoss, Norway (July–September 1980) are described and discussed. These tests were performed in a vented 50-m 3 vessel (a tube 2.5 m in diameter and 10 m long, open at one end) with regularly spaced obstacles in the form of orifice plates providing blockage ratios from 0.16 to 0.84 to the flow in the tube. It is observed that even relatively small repeated obstacles of height 0.1 m (blockage ratio 0.16) have a dramatic influence on the violence of the explosion, generating explosion overpressures larger than 1 bar in the tube. With no obstacles, the maximum overpressure observed in the tube was about 0.1 bar, to be compared with overpressures up to 8.8 bar recorded with repeated obstacles. The results obtained by a systematic variation of obstacle configurations are described and discussed in terms of a generalized venting model which takes into account the folding of the flame due to the presence of obstacles. The results are also compared with safe vent area criteria for central ignition of initially quiescent explosive mixture in near-spherical containers. The present criteria are found to be totally inadequate for large-scale explosions in obstacle environments.

Criteria for transition to detonation in tubes

Transition from high speed flame to detonation in tubes was studied in an extensive series of experiments with the aim being to establish quantitative limiting criteria for the onset of transition. The experiments were carried out in three long tubes each with a different diameter. The tubes were 18 meters each and had internal diameters of 5, 15 and 30 cm, respectively. A matrix of fuel-air mixtures at atmospheric initial pressure and room temperature was studied over a broad range of equivalence ratios. The fuels were hydrogen, acetylene, ethylene, propane and methane. High speed flame propagation and transition to detonation were achieved in a controlled manner within each tube using the well-known flame acceleration technique of obstructing obstacles pioneered long ago in the experiments of Wheeler. In the present experiments the entire tube length was filled with orifice ring obstacles, equispaced one tube diameter apart, to ensure that the maximum terminal flame speed is achieved in all cases within the available length of each tube. The results show that transition to detonation in tubes invariably occurs from a minimum level of flame speed corresponding roughly to the speed of sound of the combustion products. Since the flame speed in a tube is directly coupled to the flow field that it generates ahead of itself, this minimum flame velocity requirement implies that an adequate intensity of turbulent shear mixing is required to form the required explosive pocket of gas inherent in the genesis of detonation. There is also a necessary condition for transition to detonation in that the minimum transverse tube dimension, corresponding to the orifice opening diameter d in this study, must be sufficiently large to accomodate at least one transverse cell width characteristic of the mixture in the tube. That is, the quantitative criterion for transition is that λ/ d ≤1. Once established, the detonation wave in the tube within the obstacle field is observed to propagate at a steady velocity with a substantial velocity deficit which can be as high as 40% below the theoretical C-J value. In the limit when d /λ→13, the detonation propagation asymptotically approaches the C-J level as expected.

On the measure of the relative detonation hazards of gaseous fuel-oxygen and air mixtures∘

The critical energy for direct initiation of spherical detonation for eight gaseous fuels (C 2 H 2 , C 2 H 4 , C 2 H 4 O, C 3 H 6 , C 2 H 6 , C 3 H 8 , CH 4 and H 2 ) have been measured using a planar detonation from a linear tube for initiation. On the basis of the minimum value of the critical energy (corresponding to about the stoichiometric composition) a dimensionless parameter D H is defined by the ratio of the minimum energy of the fuel to that of acetylene-oxygen mixture. The magnitude of D H is then used for comparing the relative detonation sensitivity of the various fuels. Based on the values of D H for fuel-oxygen mixtures, it is found that ethylene oxide with a value of D H ⋍10 is about 10 times less sensitive than acetylene (D H =1). The olefins (i.e., ethylene and propylene) having values of D H ⋍10 2 are about 100 times less sensitive than acetylene. The alkanes (i.e., propane, ethane, etc.) have values of D H ⋍10 3 with the exception of methane which is particularly insensitive with a value of D H ⋍10 5 . Hydrogen is found to be similar to the normal alkanes with a value of D H ⋍10 3 . Fuel-air mixtures in general have values of D H about 10 6 times larger than the corresponding values for the same fuel with pure oxygen. The relative sensitivities of the various fuels remain the same for fuel-air mixtures as in the case of fuel-oxygen mixtures.

High speed turbulent deflagrations and transition to detonation in H2air mixtures

Abstract An experimental investigation on flame acceleration and transition to detonation in H 2 air mixtures has been carried out in a tube which had a 5 cm cross-sectional diameter and was 11 m long. Obstacles in the form of a spiral coil (6 mm diameter tubing, pitch 5 cm, blockage ratio BR = 0.44) and repeated orifice plates spaced 5 cm apart with blockage ratios of BR = 0.44 and 0.6 were used. The obstacle section was 3 m long. The compositional range of H 2 in air extended from 10 to 45%, the initial pressure of the experiment was 1 atm, and the mixture was at room temperature. The results indicate that steady-state flame (or detonation) speeds are attained over a flame travel of 10–40 tube diameters. For H 2 ≲ 13% maximum flame speeds are subsonic, typically below 200 m/s. A sharp transition occurs at about 13% H 2 when the flame speed reaches supersonic values. A second transition to the so-called quasi-detonation regime occurs near the stoichiometric composition when the flame speed reaches a critical value of the order of 800 m/s. The maximum value of the averaged pressure is found to be between the normal C-J detonation pressure and the constant volume explosion value. Of particular interest is the observation that at a critical composition of about 17% H 2 transition to normal C-J detonation occurs when the flame exits into the smooth obstacle-free portion of the tube. For compositions below 17% H 2 , the high speed turbulent deflagration is observed to decay in this portion of the tube. The detonation cell size for 17% H 2 is about 150 mm and corresponds closely to the value of πD that has been proposed to designate the onset of single-head spinning detonation, in this case for the 5 cm diameter tube used. This supports the limit criterion, namely, that for confined detonations in tubes, the onset of single-head spin gives the limiting composition for stable propagation of a detonation wave.

The failure mechanism of gaseous detonations: Experiments in porous wall tubes

Abstract To clarify the important role of the transverse wave structure in real detonations, we conducted experiments in porous wall tubes, as to attenuate the detonation’s transverse waves. Flow visualization and measurements of the attenuation and failure processes for three typical unstable mixtures (oxy-acetylene, oxy-methane, and oxy-propane mixtures), characterized by their highly irregular frontal structure, revealed the ongoing competition between transverse wave elimination at the porous wall and re-amplification of triple points within the reaction zone. At critical conditions, when these two effects balance, a unique failure limit is found, namely d∗ ≈ 4λ. Below this limit, the detonations fail. These experiments thus illustrate that transverse wave interactions are essential in the ignition and propagation mechanism for such unstable detonations. In comparison, experiments with argon-diluted detonations displaying a regular cellular structure with weaker transverse waves indicate that their transverse waves do not play a significant role in their propagation mechanism. The failure of these stable detonations is due to the global curvature mechanism caused by the mass divergence into the porous wall, leading to the slow distribution of frontal curvature. The results obtained in diluted and undiluted mixtures are also compared with a model taking into account the mass divergence at the permeable tube walls. Very good agreement is found for the argon-diluted mixtures, while the agreement for the unstable mixtures is very poor. This further indicates that the weak transverse wave structure in argon-diluted detonations, unlike the undiluted ones, does not significantly contribute to the ignition mechanism. Hence these argon-diluted detonations can be well approximated by a ZND reaction zone structure.

The hydrodynamic structure of unstable cellular detonations

The study analyses the cellular reaction zone structure of unstable methane–oxygen detonations, which are characterized by large hydrodynamic fluctuations and unreacted pockets with a fine structure. Complementary series of experiments and numerical simulations are presented, which illustrate the important role of hydrodynamic instabilities and diffusive phenomena in dictating the global reaction rate in detonations. The quantitative comparison between experiment and numerics also permits identification of the current limitations of numerical simulations in capturing these effects. Simulations are also performed for parameters corresponding to weakly unstable cellular detonations, which are used for comparison and validation. The numerical and experimental results are used to guide the formulation of a stochastic steady one-dimensional representation for such detonation waves. The numerically obtained flow fields were Favre-averaged in time and space. The resulting one-dimensional profiles for the mean values and fluctuations reveal the two important length scales, the first being associated with the chemical exothermicity and the second (the ‘hydrodynamic thickness’) with the slower dissipation of the hydrodynamic fluctuations, which govern the location of the average sonic surface. This second length scale is found to be much longer than that predicted by one-dimensional reaction zone calculations.