A. Teodorczyk

Propagation mechanism of quasi-detonations

The propagation mechanism of quasi-detonations in very rough tubes is studied using high speed schlieren photography. Stoichiometric mixtures of H2, C2H4 and C3H8 in oxygen at an initial pressure range 10≤po≤160 torr are investigated in a 61.8×61.8 mm by 1.5 m long channel with two-dimensional obstacles with a height of 25.4 mm and for various obstacle spacings. The results indicate that shock reflections (transition from regular to Mach reflections) from the walls lead to re-initiation. The obstacle spacing is found to represent an effective reaction zone length (or cell length) of the quasi-detonation. At the critical condition of transition from the choking to the quasi-detonation regime, this effective reaction zone length is found to be about twice the normal cell length of the mixture in accordance with Shchelkins stability criterion for a perturbed wave. The minimum open dimension of the channel is found to be of the order of a cell size λ for transition to the quasi-detonation regime in agreement with the previous results of Peraldi17 and Gu8 for rough tubes. Photographic observations of the propagation mechanism in the choking regime reveal the absence of ignition via shock reflection. The placement of wire screens to damp the shock reflections at the channel walls suppresses the transition to quasi-detonations indicating the essential role of shock reflections. It is not clear whether the adiabatic heating or the turbulent vortex mixing associated with the shear layer wall jet by the Mach stem near the wall is the responsible mechanism for re-initiation.

Detonation attenuation by foams and wire meshes lining the walls

In the present study systematic photographic investigations were performed of detonation interactions with foams and wire meshes lining the channel walls. An initial cellular detonation wave was propagating along a damping section (acoustic absorbing walls) which removed the transverse waves associated with its cellular structure. In some cases the wave had failed and a fast deflagration wave (a shock followed by a decoupled flame) was obtained and propagated at about half the C-J detonation speed. The events were studied photographically using a high speed framing camera and smoked foils.

Flame acceleration and transition to detonation in benzene–air mixtures

Abstract We report results on flame acceleration and transition to detonation of benzene–air mixtures at room temperature. Flame acceleration experiments were carried out in a 150-mm-diameter, 3.6-m-long steel tube. The entire length of the tube is filled with circular orifice plates (blockage or obstructed area ratio of 0.43) spaced one diameter apart. The fuel concentration was varied between 1.7% and 5% by volume of benzene in the fuel–air mixture. Three regimes of propagation were observed: (1) a turbulent deflagration with typical flame speeds less than 100 m/s, (2) a “choking” regime with the flame speed corresponding to the speed of sound of the combustion products, 700 to 900 m/s, and (3) a quasi-detonation regime with a wave speed ranging from 50% to 100% of the Chapman-Jouguet value. Transition from turbulent deflagration to the choking regime occurs at an equivalence ratio of Φ = 0.65 (1.8% C 6 H 6 ) and Φ = 1.8 (4.8% C 6 H 6 ) on the lean and rich sides, respectively. Transition from the choking to the quasi-detonation regime is observed at Φ = 0.88 (2.4% C 6 H 6 ) on the lean side and Φ = 1.6 (4.3% C 6 H 6 ) on the rich side. Detonation cell widths were measured using a small charge (8 to 50 g) of solid explosive for direct initiation of the detonation in both the 150-mm-diameter tube and a larger 300-mm-diameter, 18-m-long, steel tube. Sooted foils are used for determining the cell size, which was 66 mm for a stoichiometric composition. A detailed chemical reaction scheme was used to carry out numerical solutions of the idealized Zel’dovich–von Neumann–Doring (ZND) model. The cell widths were approximately 20 times larger than the computed reaction zone lengths. The ZND model was used to examine the effects of initial temperature and dilution by steam and nitrogen, and the effects of adding hydrogen.

The structure of fast turbulent flames in very rough, obstacle-filled channels

Flames propagating in very rough confined tubes or channels are known to acheve very high speeds of propagation that can approach the speed of sound of the burnt products. The mechanisms underlying the propagation of such flames are not understood. The present paper reports the results of a detailed photographic study of such high speed flames. The experiments are carried out in rectangular channels equipped with arrays of periodically spaced obstacles to generate controlled roughness. Observations are via high speed schlieren photography. The mechanisms responsible for the high speed propagation are identified as those which cause intense turbulization of the flame. These include shock-flame interaction, Rayleigh-Taylor instabilities in an accelerating flow and auto-ignition in large recirculating eddies in the wake of obstacles.

A Theoretical Study of a Variable Compression Ratio Turbocharged Diesel Engine

A variable compression ratio concept that can give a different expansion ratio to the compression ratio has been evaluated by means of a simulation of a turbocharged diesel engine. The compression ratio is controlled by varying the ratio of the connecting rod length to the crank throw, hence the name variable crank radius/connecting rod length engine (VR/LE). The VR/LE mechanism kinematics have been defined and described, and the compression ratio and expansion ratio have been presented as a function of the eccentric phase angle (αo). A zero-dimensional engine simulation that has been the subject of comprehensive validation has been used as the basis of the VR/LE study. The effect of the compression ratio on the engine performance at fixed loads is presented. The principal benefits are a reduction in fuel consumption at part load of about 2 per cent and a reduction in ignition delay that leads to an estimated 6 dB reduction in combustion noise. The study has been conducted within the assumption of a maximum cylinder pressure of 160 bar.

Autoignition and combustion of n-hexane spray in subcritical and supercritical environments

Abstract In this study, processes of a liquid fuel spray ignition and heat release during its combustion were under investigation. The purpose of this study was to elucidate whether the ignition properties and heat release process of a liquid fuel injected into the environments of parameters exceeding its critical values differ from those obtained for subcritical regimes. Therefore, the fuel was injected into the environments of parameters below, around and above its critical values. The ignition and combustion processes were observed by monitoring the pressure in the combustion chamber and by using a high-speed camera through transparent piston. The ignition process was characterized by ignition delay, while the combustion process by heat release and rate of heat release. The ignition delay was determined by pressure rise according to tangential method. Ignition delay determined that way included both physical delay and chemical delay. Obtained results revealed stochastic nature of the spray ignition of n-hexane. No major difference in ignition delay in terms of exceeding critical parameters was noticed. The only parameter directly influencing the ignition delay was the injectant initial temperature.

Hybrid Detonations in Oats Dust Clouds in Methane-Air Mixtures

Detonation waves in oats dust clouds in methane-air mixtures were investigated experimentally. Tests were carried out in a vertical tube 4,5 m long with an 8cm internal diameter. The hybrid mixture was ignited by a shock wave generated by means of the detonation of an oxygen-hydrogen stoichiometric mixture in the driver section. Flame propagation was recorded with the use of a streak camera and pressure profiles were measured at selected tube positions. It was found that hybrid detonations are possible in oats dust clouds in methane-air ixtures for a range of methane concentrations from 8.5% to 14.5%, with oats dust concentrations not greater than 0.1 kg/m3. Moderate concentrations of oats dust were found to promote transition to detonation, but this depends on the composition of the gaseous mixture; higher concentrations act as a suppressant.

Interaction of detonation with inert gas zone

The results of experimental study on detonation interaction with the regions of low reactivity, generated by the injection of an inert gas into reactive mixture, are reported. A square cross-section 60×60 mm, 3.6 m long detonation channel was used. The experiments were done for stoichiometric H2−O2 mixture at 0.3 bar and 0.5 bar initial pressure and room temperature. The inert gas (Ar, He, N2 or CO2) was injected from 0.523 dm3 container into the main channel 1 s before ignition. The size of the inert zone was controlled by inert initial pressure. The behavior of detonation was studied using direct streak photography and pressure transducers. The study has shown that at low pressure of Ar, N2 and CO2 injection only a slight decrease of detonation velocity occurs. At higher injection pressures complete damping of detonation and flame extinguishment occur, followed by flame reigniton and DDT outside the inert zone. For low He injection pressures an increase in detonation velocity was recorded. For higher injection pressures, detonation damping occurred, followed by DDT process. The results have shown that CO2 has the strongest effect on damping 2H2+O2 detonation, with N2 and Ar in the next places, and He very far behind. The effectiveness of inert gas in detonation damping was found proportional to its molecular weight and reciprocal to its specific heat ratio. The numerical simulations of detonation propagation through inert gas zone were also performed using the one- dimensional Detonation Lagrangean code with simple energy release model. The results of simulations are in good qualitative agreement with experimental tendencies. In particular, the model has shown that the re-initiation of detonation is enhanced by smooth concentration gradients at inert/reactive interface.

Ignition of Liquid Fuel Droplet in a Hot, Stagnant Oxidizing Atmosphere-Numerical Computations

The paper describes the physical and mathematical model of the ignition of a liquid fuel droplet suddenly immersed in a hot oxidizing medium. The model was solved numerically by the finite element method. The ignition lags in terms of ambient temperature, oxygen concentration and initial droplet diameter were computed.

AN EVALUATION OF NEW PROCEDURES FOR TESTING EXPLOSION ARRESTERS

The paper first outlines the practical need for explosion arresters, including pressures arising from recent legislation. The nature of pipeline explosions is then considered, including the problem of potential transition to detonation together with the implications for arrester testing. Current testing procedures for flame and detonation arresters are then reviewed and potential problems identified. Finally, recent results from potentially more rigorous and reproducible testing approaches are discussed.