Stoichiometric Hydrogen-oxygen Research Articles

Effects of size and dilution on dynamic properties of exothermic centers

In a further examination of the concept of exothermic centers, we have calculated the effects of size and dilution of exothermic centers upon the dynamic properties of a combusting H 2O 2 mixture. The computations were carried out for a stoichiometric hydrogenoxygen mixture at an initial state of p 0 = 3.24 atm and T 0 = 1335 °K, with argon as the diluent. The adiabatic process at constant volume was shown to represent the limit of large sizes, while that of constant pressure was the limit for small sizes, the dynamic behavior of the center in an undiluted mixture, whose reacting kernel had a radius of 100 microns, being practically the same as the latter. The effects of dilution were analyzed consequently only for the adiabatic process at constant pressure. Characteristic features of exothermic power pulses as a function of exothermic energy were expressed in terms of algebraic relations with respect to the concentration of reactants over the range of 100 to 10 per cent in their volumetric fraction.

Velocity deficits and detonability limits of hydrogen-oxygen detonations

Velocity deficits and detonability limits of stoichiometric hydrogen-oxygen gas detonations have been calculated at subatmospheric pressures (50–760 torr), using a quasi-one-dimensional ZDN model. The effect of the viscous boundary layer was accounted for by allowing for cross-sectional area changes in the flow. The model was shown to predict diameter and pressure limits for a gas mixture of given composition. Some experimental results are presented; it is shown that, for this system, the experimental velocity deficits and limit pressures are larger than the computed values. The effect of the reactions of HO 2 on the velocity deficits is considered and found to be substantial. Effects of the experimentally observed transverse waves are also discussed briefly.

Hybrid Gas-Phase/Two-Phase Detonations

Abstract Experiments with shock-tube confined hybrid gas-phase/two-phase detonations are described. The reactant systems consist of mixtures of gaseous oxygen, gaseous hydrogen, and liquid diethylcyclohexane. The latter is in the form of a monodisperse spray of 480 μ diameter droplets. The detonations are shock initiated The stoichiometry of the two-phase component is held fixed and that of the gas-phase component varied. The detonation velocity and pressure structure are reported in terms of the varying reactant composition. Average and peak detonation pressures are also discussed Very pronounced changes in reaction zone pressure structure are observed as the quantity of hydrogen gas in the initial mixture is increased. In particular, at near stoichiometric hydrogen-oxygen conditions (on the lean side), and extended region of almost constant pressure is observed behind the detonation front.

Hydrogen-oxygen reaction on lanthanide oxides. II. Stoichiometric hydrogen-oxygen reaction on neodymium oxide, dysprosium oxide, and erbium oxide

Hydrogen-oxygen reaction on lanthanide oxides. II. Stoichiometric hydrogen-oxygen reaction on neodymium oxide, dysprosium oxide, and erbium oxide

The fine structure of spinning detonation

The fine structure of spinning detonation in a stoichiometric hydrogen-oxygen mixture is being investigated. It is established by means of the trace technique and high speed space-time photography that the appearance and motion of chemical reaction centers inside the spinning detonation head takes place at a frequency higher than that of the head rotation in the detonation tube. The structure both of transversal and oblique waves inside the spinning detonation head is non-uniform and shows high-frequency pulsations. Investigation of the characteristic forms of the spinning detonation fine structure shows that pulsed disturbance processes essentially contribute to the mechanism of detonation in gas mixtures.

On the shock-induced ignition of explosive gases

The paper elucidates the distinction between two regimes of ignition, one strong and the other weak, that characterize the onset of combustion in homogeneous gaseous mixtures whose reaction mechanism is controlled by a chain-branching step, such as that associated with the H+O2 collision in the hydrogen-oxygen system. On the basis of experimental observations, made primarily by the use of a stroboscopic laser-schlieren system yielding a sequence of photographic records of the entire flow field behind a reflected shock near the closed end of the tube at a repetition rate of 2 microseconds between frames, it has been revealed that strong ignition is manifested by a practically instantaneous appearance of a relatively plane pressure front associated with the generation of a flow field across the whole cross section of the tube, while mild ignition starts in the form of distinct flame kernels whose growth is comparatively slow and essentially devoid of any gasdynamic effects. The demarcation line between the two regimes of ignition is then shown to be associated with a critical value of the gradient of the induction time with respect to temperature at constant initial pressure. Specifically, for a stoichiometric hydrogen-oxygen mixture, this turns out to be −2 microseconds/°K. Although basically controlled by the same elementary processes as those that establish the second explosion limit, this so-called strong ignition limit differs from the locus of states obtained by direct extrapolation of the second limit. Moreover, it does not intersect with any of the explosion limits, encompassing a region that is fully contained within the classical explosion regime.

Transverse flame-shock interactions in an explosive gas

Interaction processes between waves generated by accelerating flames in an explosive gas mixture, contained in a rectangular cross-section duct, and a flame kernel, ignited at one of the side walls, have been observed by means of a stroboscopic laser-schlieren system. The medium was a stoichiometric hydrogen-oxygen mixture initially at a pressure of 720 mmHg (abs.) and room temperature. In a number of experimental records the existence of a stable flame-shock complex was noted-a remarkable phenomenon that could be easily misconstrued as some kind of a ‘lower mode’ detonation-and its dynamic characteristics, especially with respect to the transition to detonation, has been taken under consideration. The investigated interactions with the flame kernel included then the effects of a shock-flame complex, as well as of a plane shock. Associated with them was in all cases the generation of a triple wave intersection-a system that is recognized today as the essential element of a self-sustained detonation-and present results are therefore of particular signification to the understanding of the gas-dynamic aspects of the detonation process.

On the emission spectra from the hypersonic wave flow field in a stoichiometric hydrogen-Oxygen mixture

Following the authors' investigation on the slow oxidation of methane and using methods previously described, the effects of surfaces of various kinds on the course of the reactions have been studied at temperatures from 450° to 500° C and pressures from 100 to 400 mm Hg. The amounts of the intermediates, formaldehyde and hydrogen peroxide, have been determined. The reactions are more rapid when carried out in silica vessels or in Pyrex vessels treated with hydrofluoric acid and washed with water. In soda glass vessels or in vessels coated with potassium chloride or alkalis, the reaction proceeds much more slowly and with a long induction period; hydrogen peroxide is destroyed. In the treated vessels, the order of the reaction and the activation energy were found to be 2·3 and 39·3 kcal respectively. The effect on the reaction rate of size of vessel and of packing was determined. Almost a threefold increase in surface / volume ratio did not alter the order of the reaction rate or the formaldehyde maximum, though it slightly increased the hydrogen peroxide maximum. Packing inhibited the hydrogen peroxide formation. Experiments were also made in vessels coated with lead monoxide, boric oxide and silver and in a vessel of pure gold. Lead monoxide eliminated the peroxide and greatly diminished the reaction. Boric acid behaved rather similarly to a Pyrex surface treated with hydrogen fluoride. The gold surface prevented reaction, even at 500°C and 600 mm initial pressure. The effects of diluents such as nitrogen, argon and helium were investigated. The reaction rate and the percentage of formaldehyde increased slightly with increase in percentage of diluent. Water was found to have a slight promoting effect which was rather greater than that of carbon dioxide. The effect was relatively greater in a potassium chloride coated vessel, indicating that the diluent prevented access to the destructive surface. These and other results are discussed in relation to work on methane oxidation by Norrish, Hoare and Walsh, and others. It is concluded that other intermediates besides formaldehyde are controlling the rate and, as in the hydrogen-oxygen reaction, the radical HO2 plays an important part through the reaction HO2 + CH4→H2O2 + CH3. Alkaline and lead monoxide surfaces are particularly active in destroying HO2. As in the hydrogen-oxygen reaction, H2O2 can also lead to branching of a higher kinetic order than that introduced by OH radicals.

The onset of retonation

The onset of the retonation wave in a stoichiometric hydrogen-oxygen mixture contained in a 1 in. × 1·5 in. tube has been observed by means of schlieren streaks and instantaneous photographs. Some interesting details of transverse oscillation characteristics of spin which accompany this event have been revealed. Experimental records have been interpreted by means of a wave dynamic analysis to determine the state of the medium where the oscillations are set in. Their frequency has been found to be in agreement with the eigenvalue solution of the linear wave equation. It appears that the process has been initiated by a point explosion that is preceded by a deflagrative implosion. In this respect the phenomenon bears an interesting similarity to high frequency combustion instabilities in rocket thrust chambers, promising thus to serve as a useful tool for the study of their physicochemical aspects.

Influence of Wave Reflections on the Development of Detonation

One of the most significant features of the transition from deflagration to detonation is the dynamics of wave interactions occurring in the course of the process. Of particular interest is the effect of waves reflected from the closed end. The present study was based on a streak self-light photograph of a stoichiometric hydrogen-oxygen flame accelerating to detonation, and a simultaneous pressure transducer record taken at the closed end of the tube in the vicinity of the source of ignition. The photograph revealed considerable information on the flow field behind the flame, so that with the additional pressure measurement it was possible to deduce specific data on the changes of state in the gas, and the dynamic effects of wave reflections on the initiation of the process. The results were rationalized by means of a gas wave dynamic analysis yielding sufficiently complete wave diagrams of the phenomena to assess their importance with respect to the transition from deflagration to detonation.

Quenching diameters of some fast flames at low pressures

Quenching diameters as a function of pressure have been presented for some fast stoichiometric hydrocarbonoxygen and hydrogenoxygen flames. The order of increasing ease of quenching is acetylene, ethylene, hydrogen, propane. The overall reaction orders for all flames have been found to be very close to two.

Photography of the disturbance which propagates detonation to a charge of explosive

Detonation propagation velocity behavior and cellular structure of stoichiometric hydrogen-oxygen mixture in spiral obstacles with four roughness (ξ = 0.133, 0.231, 0.4, 0.625) are systematically examined, at the conditions outside-, near- and within the propagation limits. The experimental results indicate that, at p0 = 10 kPa (outside the limits), spirals with roughness smaller than 0.4 facilitate the initiation of detonation, but ξ = 0.625 spiral causes the detonation quench earlier than that in smooth tube without spiral, and suggests it has prohibiting effect on the detonation propagation. At p0 = 12 kPa (near the detonation limits), spirals with ξ = 0.133 and 0.231 extend the detonation limits and therefore they have the facilitating effect on the detonation propagation, but this positive effect is not obvious in the spiral with larger roughness (i.e., ξ = 0.4 and 0.625). As the initial pressure is higher than 15 kPa, under such condition detonations are well within the limits, the dual effects of promoting and prohibiting of the spiral on the detonation propagation are more obvious, which are confirmed by the cellular structures obtained from smoked foil. The ZND induction zone length (ΔI) analysis confirms ΔI is shorter at higher initial pressure, under this condition detonation is more sensitive and easier to survive from failure as it enters into the spiral section.