Hn Phylaktou

The acceleration of flame propagation in a tube by an obstacle

Abstract A quantitative determination was made of the effect of a single baffle of on the characteristics of gas explosions in a 76-mm-diameter closed vessel of large length to diameter ratio ( L D = 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 β 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 β more accurately is tentatively introduced and shown to give very good agreement with the experimental results.

Venting of gas explosion through relief ducts: Interaction between internal and external explosions

Relief ducts fitted to venting openings is a widespread configuration in the industrial practice. The presence of a duct has been reported to severely increase the violence of the vented explosion posing a problem for the proper design of the venting device. Several studies have reported the leading importance--in the whole complex explosion phenomenology--of a secondary explosion in the duct. Modern approaches in the study of simply vented explosions (without ducts) have focused on the study of the interaction between internal and external explosion as a key issue in the mechanisms of pressure generation. The issue is even more relevant when a duct is fitted to the vent due the confined nature of the external explosion. In this work the interaction between internal and external events is experimentally investigated for gas explosions vented through a relief duct. The work has aimed at studying mechanisms underlying the pressure rise of this venting configuration. The study has put the emphasis on the mutual nature of the interaction. A larger scale than laboratory has been investigated allowing drawing results with a greater degree of generality with respect to data so far presented in literature.

Gas explosions in long closed vessels

Abstract The majority of experimental data on which current vent design practice is based, has been obtained in compact vessels of length lo diameter ratio (L/D) less than 3. This leads to great uncertainty when designing vents for larger L/D vessels. The need for more information on explosions in long enclosures has been noted in recent critical reviews and official guides. In this work methane/air explosions were investigated in closed vessels of large L/D with the aim of providing experimental data relevant to the explosion protection of such vessels. A 76mm diameter tube of an L/D of 21.6 and a 162 mm diameter tube of variable L/D (6.2-18.4) were employed. High rates of pressure rise, associated with fast flame speeds and significant overpressures, characterized the very early stages of these explosions, indicating the need for fast and effective pressure relief within the initial 10% of the total explosion time, Lower flame speeds and lower rates of pressure rise succeeded the fast initial phase. Sma...

Fast flame speeds and rates of pressure rise in the initial period of gas explosions in large L/D cylindrical enclosures

Abstract 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 = 21.6), 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 .

Gas explosions in linked vessels

Abstract In process plants, vessels handling flammable mixtures are often interlinked through pipeline systems. Explosions communicating between vessels connected in this way have occurred, often with devastating results in the secondary vessel. There is very little data on the explosion development in these geometries and no guidelines for their protection against this hazard. Some data are presented here for methane/air explosions in a 0.5 m diameter, 0.5 m long vessel connected to an identical vessel through a 76 mm diameter, 1.7 m long tube. Detailed pressure measurements were taken in both chambers and the flame propagation in the system was recorded. The initial laminar combustion in the first vessel induced a gas flow through the linking pipe into the second vessel. For 10% methane/air mixtures the pipe gas velocities ranged from 70 to 130 m/s. This high velocity flow generated high turbulence levels in the pipe and in the second compartment. When the flame encountered this turbulence it was accelerated to 370 m/s and resulted in a violent explosion in the second vessel. This in turn generated an even faster flow back into the ignition vessel resulting in an equally violent deflagration. With ignition at the centre of one of the vessels a 4 to 6-fold increase in the violence (compared to what it would have been in a single vessel) was measured in both vessels. With ignition at the end of the vessel the enhancement factor in the second vessel was about 17. This was twice the maximum reported in the literature. The venting implications were discussed and it was concluded that prevention of the explosion transmission to the second vessel should be a priority, in order to avoid an explosion which could be impossible to vent.

Side-vented gas explosions in a long vessel: the effect of vent position

Abstract Currently, the experimental data on the influence of vent position on the overpressure development during gas explosions in large length to diameter ratio (L/D) vessels are very sparse. Design guides such as NFPA 68 recommend the use of side-venting, but this is based on few experimental data and it was the objective of the present work to provide further information on vented explosions in tube configurations with side vents. Methane-air explosions (10 gas by volume) were undertaken in large L/D vessels and the influence of the side-venting position relative to the spark was investigated and compared with end-venting. The pressure development during side-venting was investigated and five stages of the explosion development were identified. The five pressure peaks were identified as follows: P1 due to the initial elongated flame acceleration, P2 due to the turbulence flame acceleration in the downstream gases ahead of the flame set in motion by the initial explosion gas expansion, P3 due to the flame acceleration around the 90° bend of the side vent, P4 due to the external gas cloud explosion downstream of the vent, and P5 due to the oscillatory combustion of trapped unburnt mixture in the vessel between the side vent and the far end of the tube. All these pressure peaks were not present for each vent position and the effect depended on the distance of the side vent from the ignition source. Very high flame speeds and overpressures were measured as the distance between the side vent position and the spark was increased. In the end-venting explosions the maximum overpressure was higher than the side-vented tests, except for the case where the side vent was placed at the middle of the vessel, which had a similar overpressure to end-venting. Induced gas velocities were also measured and associated turbulent parameters were calculated and were found to increase with an increase in the distance of the vent position from the spark.

Explosion enhancement through a 90° curved bend

Abstract Junctions and bends are commonly incorporated in pipeline systems conveying potentially explosive mixtures. Very little data exists on how these affect the development and transmission of an accidental explosion. There is therefore great uncertainty when designing explosion protection measures for such systems. A 3 m long, 162 mm diameter tube, closed at both ends and incorporating a 90° curved bend was used to investigate the influence of the bend on the development of gaseous explosions. Methane/air mixtures of 10% and 5.7% by volume were used. The mixture was ignited at one end of the tube at 5 mm or 53 mm from the flange. It was found that for all explosions, the flame moved faster around the inner wall of the bend than the outer and hence it was elongated. This gave rise to an overall acceleration of the flame and a significant increase in the rate of pressure rise. The enhancement factor due to the bend ranged from 4 to 6 for the 10% mixture and it was equivalent to the effect of an orifice plate with a 20% blockage in the path of the flame. These findings highlight the need for more work on bends of different shapes and in different layouts.

The influence of flow blockage on the rate of pressure rise in large L/D cylindrical closed vessel explosions

Abstract 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/D) ratio closed vessel explosion. A 76 mm diameter vessel with an L/D 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−1 upstream of the baffle, and were caused by the high flame speed, with maximum values up to 30 m s−1. 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.

Torrefaction Effects on the Reactivity and Explosibility of Woody Biomass

Biomass is increasingly being used as a fuel for power generation. Torrefaction has helped improve some of the drawbacks associated with the use of raw biomass by making it easier to grind, increasing energy density and reducing transport costs. However, data relevant to the safe handling against the fire and explosion risks associated with the use of biomass is scarce, and in the case of torrefied biomass non-existent (in the open literature). In this work, the effect of torrefaction severity over the explosibility and reactivity of 5 samples of torrefied wood biomass was investigated. All samples were milled using the same procedure to nominally below 63μm.The Leeds modified Hartmann apparatus was used to measure the minimum explosible concentrations (MEC) as well as the rates of pressure rise and flame speeds inside the vessel over a range of concentrations for each of the samples. The results showed a small reduction in reactivity with increasing torrefaction severity and this was attributed to the reduction of volatile content whilst it is thought an increase in the fraction of fines (due to the increase in brittleness) for the more torrefied samples, moderated the effect of volatile content reduction on the reactivity by providing finer particles, easier to burn. The MEC was found to be around 0.2 equivalence ratio (similar to raw biomass), which is less than half that reported for coal.

Duct-vented Propaneiair Explosions With Central And Rear Ignition

It is commonplace in industrial installations to have duct vented vessels, the design of which is often based upon the premise that central ignition will provide the worst case scenario. This research investigates duct-vented explosions using a vented test chamber of 200 l capacity fitted with a 1m long vent pipe, discharging into a large (50 m 3 ) dump volume with rear and central ignition. Propane-air mixtures over a range of concentrations (Ф=0.8-1.6) have been used. Results show that while there is no significant difference in maximum pressure in the test vessel for rear and central ignition, rear ignition consistently produces the worst case in terms of rates of pressure rise and flame-speeds in the duct. In addition, the detailed records of pressure traces and flame position showed a direct relationship between the induced gas velocity in the duct prior to the flame arrival and the subsequent rate of pressure rise in the vessel. The implications of the findings for practical systems are briefly discussed.