Studies of premixed flame propagation in explosion tubes (original) (raw)
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Journal of Loss Prevention in the Process Industries, 1990
Flame speeds and rates of pressure rise for gaseous explosions in a 76 mm diameter closed cylindrical vessel of large length to diameter ratio (L/D = 21.61, were quantitatively investigated. Methane, propane, ethylene and hydrogen mixtures with air were studied across their respective flammability ranges. Ignition was affected at one end of the vessel. Very fast flame speeds corresponding to high rates of pressure rise were measured in the initial 5-10% of the total explosion time. During this period 20-35% of the maximum explosion pressure was produced, and over half of the flame propagation distance was completed. Previous work has concentrated on the later stages of this type of explosion; the development of 'tulip' flames, pressure wave effects and transition to turbulence. The initial fast phase is very important and should dominate considerations in pressure relief vent design for vessels of large L/D.
The acceleration of flame propagation in a tube by an obstacle
Combustion and Flame, 1991
A quantitative determination was made of the effect of a single baffle on the characteristics of gas explosions in a 76-mm-diameter closed vessel of large length to diameter ratio (LID = 21.6). Mixtures of methane-air were predominantly used, but other gases were also investigated. Ignition was effected at one end of the vessel. Single hole plates were employed as baffles with varying blockage ratios (20%-80%). The flame speed and rate of pressure rise were greatly enhanced downstream of the baffle. The relative effect of the baffle increased with increasing blockage ratio. It was 8 times more severe with the baffle at 7D from the spark than at 14D. The flow rate of the unburned gas, set in motion ahead of the flame was determined by measuring the pressure drop across the baffle. From the unburned gas velocity the rms turbulent velocity (u') was determined using experimental correlations of grid-generated turbulence, and from this the turbulent burning velocity and turbulence factor ~ were calculated. The turbulence factor was found to be equal to the normalized rate of pressure rise. This demonstrated that current turbulent combustion theory (for high Reynolds number flows) can explain and predict the phenomena observed in such combustion regimes. Turbulent flame extinction was predicted for high blockages and experimental evidence of localized flame quenching was found. However, no total flame extinction was observed as the turbulence generated by the baffle was nonuniform and the flame could propagate round local high turbulence regions. The turbulent burning velocity was found to be as high as 110 times the laminar value. In the current literature for vent design a turbulence factor of 10 is suggested for severe cases of turbulence, The present results show the need for reviewing these guidelines if high blockages to the explosion gases exist. A method for estimating /3 more accurately is tentatively introduced and shown to give very good agreement with the experimental results.
J. Loss Prevention in the Process Industries, 2013
The separation distance (or pitch) between two successive obstacles or rows of obstacles is an important parameter in the acceleration of flame propagation and increase in explosion severity. Whilst this is generally recognised, it has received little specific attention by investigators. In this work a vented cylindrical vessel 162 mm in diameter 4.5 m long was used to study the effect of separation distance of two low blockage (30%) obstacles. The set up was demonstrated to produce overpressure through the fast flame speeds generated (i.e. in a similar mechanism to vapour cloud explosions). A worst case separation distance was found to be 1.75 m which produced close to 3 bar overpressure and a flame speed of about 500 m/s. These values were of the order of twice the overpressure and flame speed with a double obstacle separated 2.75 m (83 characteristic obstacle length scales) apart. The profile of effects with separation distance was shown to agree with the cold flow turbulence profile determined in cold flows by other researchers. However, the present results showed that the maximum effect in explosions is experienced further downstream than the position of maximum turbulence determined in the cold flow studies. It is suggested that this may be due to the convection of the turbulence profile by the propagating flame. The present results would suggest that in many previous studies of repeated obstacles the separation distance investigated might not have included the worst case set up, and therefore existing explosion protection guidelines may not be derived from worst case scenarios.
A Comparative Study of Turbulent Premixed Flames Propagating Past Repeated Obstacles
Industrial & Engineering Chemistry Research, 2012
This paper presents a comparison of the overpressures and pressure gradients obtained in turbulent premixed flames of liquified petroleum gas (LPG), compressed natural gas (CNG), and hydrogen propagating past solid obstacles. The fuel-air mixture is initially at rest and is ignited at the base of a vented chamber. For each fuel a total of eighteen configurations are studied here with various permutations of three baffles plates and an obstacle with a square cross section providing a blockage ratio of either 0.24 or 0.5. High speed laser induced fluorescence from OH (LIF-OH) imaging is also performed at a repetition rate of 5kHz providing new measurements of the evolution of the flame front along the middle section of the chamber. It is found that for all fuels the peak pressure as well as the rate change of pressure increases not only with increasing blockage ratio induced by the increasing blockage ratio, but also with decreasing separation between successive baffles. The peak pressure and its rate of change in hydrogen flames are over an order of magnitude higher than those obtained in LPG and CNG fuels. High speed images of LIF-OH show that the degree of wrinkling and contortion in the flame front also increases significantly with increasing blockages. It is evident from the LIF-OH images that the flame front relaminarises as the separation between successive obstacles increase hence explaining the pressure decrease with increasing separation.
Journal of Loss Prevention in the Process Industries, 1990
A quantitative determination was made of the influence of the blockage of a single baffle on the increase in the rate of pressure rise in a large length/diameter (L/b) ratio closed vessel explosion. A 76 mm diameter vessel with an L/a of 2 was used, and a four hole flat grid plate was used to simulate the shear layer size and turbulence levels of practical obstructions of complex shape. The maximum rate of pressure rise was investigated as a function of the baffle blockage and position, relative to the spark at the base of the vessel. The unburned gas set in motion ahead of the flame was measured by determining the differential pressure across the obstacle. This showed that mean velocities ahead of the flame were in the range 15-20 m s-' upstream of the baffle, and were caused by the high flame speed, with maximum values up to 30 m s-'. These high velocities caused large turbulence levels to be created on interaction with the grid plate, depending on the grid plate blockage. Flame acceleration occurred once the flame reached the turbulence, and this acceleration was shown to be a function of the blockage. An approximate method for relating the increase in the rate of pressure rise to turbulent burning velocity measurements was developed, using a simple method of estimating the turbulence levels generated downstream of the obstacle.
The effect of explosion venting in empty vessels
International Journal for Numerical Methods in Engineering, 1987
The two-dimensional Navier–Stokes equations, suitably amplified to include the effects of chemical reactions and turbulence, are discretized by employing a finite-volume technique. A weighted upwind/central differencing scheme, the weight depending on the grid Peclet number, is used for the convection terms. Velocities are calculated on staggered grids. The source terms are treated in a quasi-implicit manner by combining linearization of these terms with an ICE procedure providing a time-advanced pressure. The effect of turbulence is included through the eddy-viscosity concept by solving equations for turbulent kinetic energy and its rate of decay. Combustion is modelled by an equation for mass fraction of fuel containing a fuel consumption term based on mixing-limited combustion.The resulting code, called FLACS-ICE, is employed in simulating the influence of confinement on flame acceleration in premixed, stoichiometric propane-air mixtures. Calculations are performed for large (425m3), medium (35m3) and small scale (0·0036m3) vessels. Satisfactory agreement with experimental data, published by Solberg1 is obtained. Some limitations of the code are also pointed out.
Journal of Loss Prevention in The Process Industries, 2001
Results of experiments on critical conditions for flame acceleration and the deflagration-to-detonation transition in tubes with transverse venting are presented. Tests were made with hydrogen mixtures in two tubes (inner diameter of 46 and 92 mm) with obstacles. Ratios of vent area to total tube area were 0.2 and 0.4. Venting was shown to influence flame acceleration significantly. The greater the vent ratio, the more reactive the mixture necessary for development of fast flames. Critical conditions for flame acceleration in tubes with venting, expressed through a critical mixture expansion ratio σcr, were found to be σcr/σ0∼1+2α, where σ0 is the critical value for a closed tube. Critical conditions for detonation onset in a vented tube were found to be very close to those in a closed tube with similar configuration of obstacles.
International Journal of Hydrogen Energy, 2014
Spontaneous ignition processes due to high pressure hydrogen releases into air are known phenomena. The sudden expansion of pressurized hydrogen into a pipe, filled with ambient air, can lead to a spontaneous ignition with a jet fire. This paper presents results of an experimental investigation of the visible flame propagation and pressure measurements in 4 mm extension tubes of up to 1 m length attached to a bulk vessel by a rupture disc. Transparent glass tubes for visual observation and shock wave pressure sensors are used in this study. The effect of the extension tube length on the development of a stable jet fire after a spontaneous ignition is discussed.