Quiescent Mixture Research Articles

Linear wave propagation of fast and slow modes in mixtures of liquid and gas bubbles

A new set of space-averaged equations that governs the dynamics of mixtures of liquid and gas bubbles is derived, where a surface-averaged liquid pressure at the gas–liquid interface is used as a variable as well as volume-averaged pressures, and the liquid compressibility is taken into account. To verify the validity of the model equations, the propagation of linear plane wave in a quiescent mixture is studied theoretically and numerically. It is shown that the compressibility of liquid induces two propagation modes, a fast mode and a slow mode, for all real wave numbers, and if one assumes that the liquid is incompressible there only exists the slow mode. Since the incompressibility approximation has been used for liquid phase in previous studies, the fast mode has not been investigated in detail. In the present study, several important characters of the slow and fast modes are clarified. In particular, it is demonstrated that the amplitude of the fast mode is not always small and it can become large when a typical wave number exceeds a critical value.

Vented gaseous deflagrations: modelling of translating inertial vent covers

Abstract A model of explosion pressure build up in enclosures with translating inertial vent covers is presented. The previous approach, valid for inertia-free vents, is advanced by appending to it a new model of translating inertial vent cover displacement. The model and CINDY code are validated against experiments by Hochst and Leuckel (J. Loss Prev. Process Ind. 11 (1998) 89) in a 50-m 3 vessel with vertically translating covers with surface densities of 42 and 89 kg/m 2 at conditions of initially quiescent and turbulent mixtures. It is demonstrated for the first time that modelling of the vent cover jet effect is crucial for prediction of interdependent pressure-time and cover displacement-time transients, whereas air drag force and cushioning effects are negligible. The model was used further to investigate the influence of vent cover surface density on venting generated turbulence, via comparisons with experimental data of Cooper et al. (Combust. Flame 65 (1986) 1) in a 1-m 3 enclosure with vertically translating covers of various surface densities up to 200 kg/m 2 . The increase of the turbulence factor, i.e. total premixed flame front wrinkling factor, with cover inertia is obtained and explained.

Flame development along a straight vortex

Flame development along a straight vortex was studied experimentally to elucidate the effects of the maximum circumferential velocity and the density ratio of the flame. A pair of straight vortices was produced in nitrogen-diluted stoichiometric hydrogen-oxygen mixtures with density ratios ranging from 5.3 to 7.2. The velocity field measured by particle image velocimetry (PIV) showed that the vortex tube had a mean maximum circumferential velocity ranging from 18.0 to 35.8 m/s and had a mean core diameter ranging from 5 to 6 mm. One of the vortices was ignited at the core by a focused laser at 193 nm without disturbing the flow field. The flame propagated along the axis of the straight vortex at a speed much higher than in the radial direction or in the quiescent mixture. The axial propagation velocity increased over time and became nearly constant when the half-axial length of the flame was larger than the core diameter of the vortex tube. The axial propagation velocity at steady state was roughly proportional to the maximum circumferential velocity and to the density ratio minus unity. The axial propagation velocity in the initial stage increased with the square root of the half-axial length of the flame as well as with the maximum circumferential velocity and the density ratio minus unity.

Visualization of Dust Explosion Under Microgravity Conditions

The objective of this work was to apply visualization methods to the experimental study of cornstarch dust-air mixture combustion in a closed vessel volume under microgravity conditions. A dispersion system with a small scale of turbulence was used in the experiments. A gas igniter initiated combustion of the dust-air mixture in the central or lop pan of the vessel. Flame propagation through the quiescent mixture was recorded by a high-speed video camera. Experiments showed a very irregular flame front and irregular distribution of the regions with local reactions of re-burning behind the flame front, at a later stage of combustion. Heat transfer from the hot combustion products to the walls is shown to have an important role in the combustion development. The maximum pressure and maximum rate of pressure rise were higher for flame propagation from the vessel center than for flame developed from the top part of the vessel. The reason for smaller increase of the rate of pressure rise, for the flame developed from the top of the vessel, in comparison with that developed from the vessel center, was much faster increase of the contact surface of the combustion gases with the vessel walls. It was found that in dust flames only small part of heat was released at the flame front, the remaining part being released far behind it.

Gasdynamic stimulation of combustion of lean fuel mixtures. 1. experimental study

We studied combustion modes experimentally for a lean fuel mixture of propane-air, including combustion of a quiescent mixture, combustion with annular gas swirling, and combustion stimulated by the jets of a burning gas that are injected into the combustion chamber (pulsed jet combustion — PJC). It is shown that, in providing a maximum combustion rate, the PJC mode has an obvious advantage in initiation of combustion of extremely lean fuel mixtures over other gasdynamic combustion modes.

Performance Of Selected Agents In Suppressing Explosions Of Near- Stoichiometric Methane-air Mixtures

This study was carried out to obtain data on the performance characteristics of two powders (sodium bicarbonate [SB] and monoammonium phosphate [MAP]) and water as explosion suppression agents, and to understand the effects of geometric variables on suppression system performance. Tests were carried out in a 2.5-m3 test vessel with 10% (by volume) methanebair mixtures, both under initially quiescent and turbulent conditions. Tests with the two powder agents (SB and MAP) at a concentration of 1200 g/m3 resulted in similarly successful suppression of quiescent mixtures, with small increases in final overpressures as the detectionlsystem trigger pressure was increased (from 0.07 to 0.55 barg [I to 8 psig]). Successf~~l suppressions were also obtained with turbulent mixtures, although with greater overpressures than for quiescent mixtures. Water at ambient temperature (approximately 20°C) was not successful at suppressing the explosion, even when carbon dioxide was added to the charge. In tests where the suppression failed, the pressure inside the vessel reached the value corresponding to fully unvented conditions. Heated water (at or above 230°C [446'F]) performed reasonably well as a suppression agent, both in quiescent and turbulent tests, for trigger pressures up to 0.55 barg (8 psig). The introduction of the superheated water into the vessel, however, contributed about 0.83 barg ( I 2 psig) to the total pressure increase.

An experiment of the combustion characteristics with laser-induced spark ignition

The combustion characteristics and minimum ignition energies using laser-induced spark ignition were demonstrated for quiescent methane-air mixtures in an optically-accessible, constant volume combustion chamber. Initial pressure and equivalence ratio as well as spark energy were varied in order to explore the flame behavior with laser-induced spark ignition. Shadowgraphs for the early stages of combustion process showed that the flame kernel becomes separated into two, one of which grows back towards the laser source. Eventually after a short period, the two flame kernels developed into two flame fronts propagating individually, which is unique in laser-induced spark ignition. For a given mixture, lower initial mixture pressure and higher spark energy resulted in shorter flame initiation period and faster flame propagation. The results of minimum ignition energies for laser ignition shows higher values than electric discharge results, however, the difference decreases toward lean and rich flammability limits.

Gaseous-Propellant Microthrusters. II. Venting Prior to Completion of Burn

We present results from an inviscid one-dimensional model of the quasi-steady adiabatic dame propagation within a closed cylindrical container, as a consequence of a localized ignition in an initially homogeneous and quiescent mixture of reactive ideal gases. We continue the description after the instantaneous removal of an effectively massless diaphragm permits discharge from a sonic orifice, located at either the ignition end or the opposite end of the container. In a companion study (part I, published separately), the efflux begins only after the burn of the contents is complete; here, in contrast, the diaphragm is instantaneously removed when a pressure is achieved which is less than the adiabatic pressure for complete combustion of the contents. Key results include the temporal histories of the fraction of the initial mass that is burned within the container, the fraction of the initial mass that has exited the container, and the flux of momentum associated with the efflux of mass (for the case in which the container itself does not move). For cases of interest, involving a single simple hydrocarbon vapor in a fuel-lean mixture with oxygen and a diluent, at an initial pressure in excess of one atmosphere, the pressure increases mono-tonically (and appreciably) until diaphragm rupture; thereafter, the pressure typically decreases with efflux, and the flame speed across the mixture typically increases with the decreasing pressure. Thus, flameout (so the combustion within the container of the initial contents is incomplete) is neither coincident with diaphragm rupture nor owing to a chemical-kinetic consequence of the decrease of the (spatially virtually uniform) pressure. Rather, flameout is owing to the efflux of all hot burned gas from the container, so there is no preheating of residual cold unburned mixture to sustain propagation of the deflagration within the container.

Effective viscosities of a phase-separating binary mixture imposed to shear

Abstract We report measurements of effective viscosities in a near-critical phase-separating binary mixture in the presence of a flow field. Our experimental results suggest that the viscosity associated with a flow-induced quasi-stationary, two-phase structure obeys Onuki's formula, in which a delicate balance between shear and growing domains should be achieved, resulting in significantly extended domain structures in the flow direction. It is observed that the domains break up into spherical ones smaller in size than those in a quiescent mixture when the shear is turned off. This implies that the undulation associated with a tube-like concentration domain could be suppressed by shear.

On the effect of venting large vessels with mass inert panels

A numerical analysis indicated that in some situations venting by rather heavy venting devices such as panels or explosion doors will lead to substantial increase of the peak pressure in the considered enclosure compared to venting with burst disks or membranes of negligible mass. Vent sizing guidelines like the VDI 3673 consider a vent capability of explosion doors which has to be tested prior to their use. To study the effect of mass inertia of explosion doors or panels moved by translation, a set of experiments was carried out in a 50 m3 silo with a height to diameter ratio H/D=4. The experiments showed, however, partially unexpected results. The peak pressure pred increased when changing membranes with inert devices (turbulent mixtures). When the mass of the devices was varied, no variation of pred was detected, though it could be demonstrated that the time necessary for full opening of the vent increased with higher mass. When venting with inert devices (quiescent gas mixtures at the instant of ignition), there are even situations which led to lower explosion pressures compared with membrane venting. Panels moved by translation are faster than explosion doors, but the flow through the vent seems to be less disturbed during venting in the case of explosion doors. The effect of pressure generation and flame front propagation during a vented explosion is dominated by conditions other than the mass of the devices, especially with the turbulence level of the mixture at the instant of ignition.

Combustion Characteristics of Lean Limit Flame Propagation in a Long Tube Under Microgravity.

Studies on combustion of extremely lean hydrocarbon-air mixtures are very important from the viewpoint of pollution control which are prompting the development of new types of combustors and internal combustion engines. However, accurate data on combustion characteristics of such mixtures are scarce owing to difficulties inherent in conventional measuring techniques. It is well known that the flame behavior is strongly affected by buoyancy under normal gravity. This becomes more critical in the vicinity of flammability limits where flame propagation velocities are very low. Consequently, data such as flammability limits obtained under normal gravity are unreliable. In this experimental investigation carried out with a quiescent mixture contained in a long tube, flame stability and buoyancy effects in the vicinity of flammability limit have been detemined. Furthermore the study has been extended to include the effect of surface roughness on flame propagation. The experimental results show that, (1) Under microgravity, an axially symmetrical flame propagation is possible in a long tube, even at mixture conditions near the flammability limits and the combustion characteristics can be accurately determined using this method, (2) Propagation velocity with the rough surface is higher than with the smooth surface.

The Effects of Electric Fields on Ignition Delay of Combustible Mixtures in Hot Surface Ignition.

The present study was carried out to reveal the possibility of controlling the ignition delay of combustible mixtures by applying electric fields. A thin nickel wire was used as a hot surface to ignite the mixtures. It was suddenly heated up and then its temperature was kept constant. Quiescent propane-air mixtures were used as combustible mixtures. DC electric fields were applied between the nickel wire and the outer electrode plates parallel with the nickel wire. As the applied voltage to the electrode plates increased, both the mean values and the fluctuations of the ignition delay decreased regardless of the electric polarities. Without electric fields, both the mean ignition delay and the fluctuation varied with the equivalence ratio of the mixture. With increasing the applied voltage up to =2kV, the variations became to disappear. When the surface temperature was low, the stoichiometric mixture could be ignited only by applying the electric fields.

The Effects of DC Electric Fields on Ignition Delay of a Hydrogen-Air Mixture in Hot Surface Ignition.

The present study was carried out to reveal the possibility of controlling the ignition delay of hydrogen-air mixtures by applying electric fields. A quiescent stoichiometric hydrogen-air mixture was ignited by a suddenly heated thin wire of nickel or tungsten. DC electric fields were applied between the wire and outer electrode plates parallel with the wire. The mean ignition delay was calculated stochastically from the measured ignition delays which scattered considerably. Both with the nickel wire and with the tungsten wire, positive voltages applied to the outer electrode plates resulted in almost the same change of the mean ignition delay ; The mean ignition delay showed a slight increase at the lower applied voltages, while it became very short at the highest applied voltages. When the nickel wire was used and negative voltages were applied, the mean ignition delay decreased even at the lower voltages. However, it showed little change with the tungsten wire and the negative applied voltages.

Validation of scaling techniques for experimental vapor cloud explosion investigations

AbstractWithin the project “MERGE”, an acronym for Modelling and Experimental Research into Gas Explosions, eight institutes from five different European countries co‐operated to increase the understanding of vapor cloud explosions and to improve the sophisticated computerized prediction techniques that were, until this project, under separate development at several institutes.The objectives of the project were: to provide a consistent set of data on the influence of obstacles on flame propagation in initially quiescent mixtures at various geometrical scales; to improve, compare and validate numerical models for vapor cloud explosion simulation; to validate scaling techniques for explosion experiments; to investigate and to model jet explosions. The second item is discussed in this report.

Numerical modeling of spark ignition and flame initiation in a quiescent methane-air mixture

The initiation of a spark kernel and the subsequent propagation of a self-sustained flame in an internal combustion engine have been investigated numerically. A theoretical model which employs a two-dimensional cylindrical coordinate system and assumes axial symmetry has been developed. It considers the various physical and chemical phenomena associated with the ignition process and employs a detailed chemical reaction scheme for a methane-air mixture which contains 29 chemical species and 97 reaction steps. The thermodynamics and transport properties of the plasma at high temperatures are evaluated by a statistical thermodynamics approach, while assuming local thermodynamic equilibrium. Using the PHOENICS and the CHEMKIN codes, the appropriate conservation equations are solved in the domain of solution. It was concluded that the kernel growth can be described as a two-step process. In the early short stage (1–5 μs) the mass and energy transfer processes are very much dominated by the pressure wave and the violently expanding plasma kernel, while the contribution of the chemical reactions is negligible. This stage is followed by a much longer period in which diffusion and thermal conduction control mass and energy transfer as the flame becomes gradually self-sustained. Owing to the heat release by chemical reactions, the expansion of the combustible mixture is accelerated at the beginning of the diffusive stage.

Numerical investigation of the ignition of dilute fuel sprays including detailed chemistry

One-dimensional unsteady numerical calculations have been performed to study the ignition of dilute fuel sprays. Recent investigations in this field employ global kinetic rate expressions to model the chemical reactions occuring in the gas phase. In the present paper a detailed kinetic mechanism is employed to describe the ignition process of dilute methanol sprays in air, the mechanism for methanol comprises 23 species and 168 elementary reactions. The present investigation accounts for unsteady droplet heating and vaporization using a distillation-limit model; temperature dependence of liquid phase properties is included. The study investigates uniform sprays; both monodisperse and bidisperse sprays are considered. The quiescent mixture of fuel and air is ignited by assuming hot air and cold fuel. The present study concerns ignition of a fuel spray in a closed volume. Effects of varying parameters such as initial gas temperature, initial fuel-vapor concentration, pressure, droplet size in both monodisperse and bidisperse sprays as well as equivalence ratio on both ignition-delay time and on spray lifetime are investigated. It is shown that there is a minimum ignition-delay time in dependence of both equivalence ratio and of initial droplet size for various conditions. Ignition-delay time of a bidisperse spray is dominated by the characteristics of the smaller droplets. There is a two-stage heating for small droplets caused by gas-phase characteristics. Fuel-vapor accumulation leads to delayed ignition and to higher gas temperatures for small droplets. The results of the calculations give profiles of radical concentrations such as OH as well as concentration of pollutants.

Analysis of ignition mechanism of combustible mixtures by composite sparks

Formation process of flame kernels produced by composite sparks in a quiescent propane-air mixture is numerically simulated by using a set of partial differential equations on two-dimensional cylindrical coordinates. Simulation is done with emphasis on physical effects such as gas movements generated by spark discharge and heat transfer from the flame kernel to spark electrode surfaces. Although chemical reaction is considered only by an overall reaction, the present simulation is found to be useful for understanding of physical effects in the following points: the flow pattern near spark gaps is an important factor that governs the flame kernel structure, the flow pattern is affected by the spark electrode diameter, gap width, and spark duration, and the calculated variation in the minimum ignition energy agrees qualitatively with the experimental variation, and the existence of the optimum spark duration is well confirmed. As for composite sparks, it is found that the superiority in ignition ability of composite sparks over capacitance sparks depends on spark electrode diameter and gap width.

On the flame propagation in a rotating flow field

Using a glass tube into which a combustible mixture is tangentially injected from one end, the phenomenon of flame propagation in a rotating flow field has been experimentally investigated. The regions for the flame propagation are mapped as functions of the fuel concentration and the flow velocity, and the flame speeds are also determined. The results show that, when the tangential injection velocity is small, the propagation range in the fuel concentration becomes narrower with an increase of the flow velocity, whereas when the injection velocity is large, and thereby the rotation is strong, the range becomes wider as the flow velocity is increased; an axisymmetric flame, convex towards the oncoming mixture, is formed at the open end, and it projects into the glass tube eventually to the closed end. An interesting point is that the concentration limit is extended by the flow rotation to that for a quiescent mixture in lean methane or rich propane mixtures, but not in rich methane or lean propane mixtures. In the former mixtures, the head of the flame is intensified in burning, whereas in the latter mixtures, the head is weakened. In addition, it is found that the flame speed almost linearly increases with an increase of the tangential velocity. These results are discussed on the basis of the vortex bursting mechanism proposed by Chomiak and Lewis number considerations.

Numerical Simulation of Electric Spark Ignition in Atmospheric Pressure Methane-Air Mixtures

Abstract A model has been developed to describe the growth of an ignition kernel produced by an electric spark in quiescent methane-air mixtures at atmospheric pressure. The model involves the solution of the conservation equations of mass, momentum, and energy in a chemically reacting fluid in one-dimensional spherical coordinates. Detailed chemical kinetics for methane oxidation are included in the equations, along with an accurate method of calculating the transport coefficients. The electric spark energy is added in the form of internal energy; ionization in the discharge is neglected. The temporal and spatial dependence of this internal energy addition can be taken from the current, voltage, and duration of the discharge which produces the ignition kernel being modeled. The agreement with experimental results for arc and glow discharges is quite good.

Concentration and Pressure Effects on the Decomposition Rate of Aqueous Hydroxylammonium Nitrate Solutionsa

Abstract Abstract— Decomposition experiments have been performed on liquid propellants consisting of mixtures of hydroxylammonium nitrate (HAN) and water. Electric discharge initiation was used to produce a propagating, exothermic decomposition wave front, which was then observed by cineshadowgraphy. Experiments were conducted on quiescent HAN-water mixtures of 3.12 to 13.00M HAN in a 1.8 x 1.0mm rectangular strand burner over pressures of 6 to 34 MPa. A lean propagation limit was found to exist at a concentration of between 3.12 and 5.10 M HAN. Measured overall decomposition rates decreased with increased pressure in an unstable manner with instabilities ranging in size from tens of micrometers to the burner width. It was found that the overall decomposition rates were a function of both the HAN concentration and the density ratio, ϱproduct/ϱreactant (which is proportional to pressure), across the decomposition wave. This density ratio ettect is interpreted as affecting the hydrodynamic stability of the ...