Turbulent Combustion Parameters in Gas Explosions with Two Obstacles with Variable Separation Distance

Most of the congested gas explosions studies have focused on quantifying global flame acceleration and maximum overpressure through obstacle groupings rather than detailed analysis of the flame propagation through the individual elements of the congested region. Fundamental data of the turbulent flow and combustion parameters would aid better understanding of gas explosion phenomena and mechanisms in the presence of obstacles in addition to the traditional flame speeds and overpressures that are usually reported. In this work we report near stoichiometric methane/air explosion tests in an elongated vented cylindrical vessel 162 mm internal diameter with an overall length-to-diameter, L/D of 27.7. Single and double obstacles (both hole and flat-bar types) of 20-40% blockage ratios, BR with variable obstacle scale were used. The spacing between the obstacles was systematically varied from 0.5 m to 2.75 m. Turbulence parameters were estimated from pressure differential measurements and geometrical obstacle dimensions. This enabled the calculation of the explosions induced gas velocities, rms turbulent velocity, turbulent Reynolds number and Karlovitz number. This allowed the current data to be plotted on a premixed turbulent combustion regimes diagram. The bulk of the data fell in the thickened-wrinkled flames regime. The influence of the calculated Karlovitz number on the measured overpressures was analysed and was related to obstacle separation distance and obstacle scale characteristics

Flame Acceleration in Tube Explosions with up to Three Flat-bar Obstacles with Variable Obstacle Separation Distance

The effect of obstacle separation distance on the severity of gas explosions has received little methodical study. It was the aim of this work to investigate the influence of obstacle spacing of up to three flat-bar obstacles. The tests were performed using methane-air (10% by vol.), in an elongated vented cylindrical vessel 162 mm internal diameter with an overall length-todiameter, L/D, of 27.7. The obstacles had either 2 or 4 flat-bars and presenting 20% blockage ratio to the flow path. The different number of flat-bars for the same blockage achieved a change of the obstacle scale which was also part of this investigation. The first two obstacles were kept at the established optimum spacing and only the spacing between the second and third obstacles was varied. The profiles of maximum flame speed and overpressure with separation distance were 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 at 80 to 100 obstacle scales about 3 times further downstream than the position of maximum turbulence determined in the cold flow studies. Similar trends were observed for the flames speeds. In both cases the optimum spacing between the second and third obstacles corresponded to the same optimum spacing found for the first two obstacles demonstrating that the optimum separation distance does not change with number of obstacles. In planning the layout of new installations, the worst case separation distance needs to be avoided but incorporated when assessing the risk to existing set-ups. The results clearly demonstrate that high congestion in a given layout does not necessarily imply higher explosion severity as traditionally assumed. Less congested but optimally separated obstructions can lead to higher overpressures

Prediction of distance to maximum intensity of turbulence generated by grid plate obstacles in explosion-induced flows

The interaction of unburnt gas flow induced in an explosion with an obstacle results in the production of turbulence downstream of the obstacle and the acceleration of the flame when it reaches this turbulence. Currently, there are inadequate experimental measurements of these turbulent flows in gas explosions due to transient nature of explosion flows and the connected harsh conditions. Hence, majority of measurements of turbulent properties downstream of obstacles are done using steady-state flows rather than transient flows. Consequently, an empirical based correlation to predict distance to maximum intensity of turbulence downstream of an obstacle in an explosion-induced flow using the available steady state experiments was developed in this study. The correlation would serve as a prerequisite for determining an optimum spacing between obstacles thereby determining worst case gas explosions overpressure and flame speeds. Using a limited experimental work on systematic study of obstacle spacing, the correlation was validated against 13 different test conditions. A ratio of the optimum spacing from the experiment, xexp to the predicted optimum spacing, xpred for all the tests was between 2-4. This shows that a factor of three higher than the xpred would be required to produce optimum obstacle spacing that will lead to maximum explosion severity. In planning the layout of new installations, it is appropriate to identify the relevant worst case obstacle separation in order to avoid it. In assessing the risk to existing installations and taking appropriate mitigation measures it is important to evaluate such risk on the basis of a clear understanding of the effects of separation distance and congestion. It is therefore suggested that the various new correlations obtained from this work be subjected to further rigorous validation from relevant experimental data prior to been applied as design tools

The effect of vent size and congestion in large-scale vented natural gas/air explosions

A typical building consists of a number of rooms; often with windows of different size and failure pressure and obstructions in the form of furniture and décor, separated by partition walls with interconnecting doorways. Consequently, the maximum pressure developed in a gas explosion would be dependent upon the individual characteristics of the building. In this research, a large-scale experimental programme has been undertaken at the DNV GL Spadeadam Test Site to determine the effects of vent size and congestion on vented gas explosions. Thirty-eight stoichiometric natural gas/air explosions were carried out in a 182 m3 explosion chamber of L/D = 2 and KA = 1, 2, 4 and 9. Congestion was varied by placing a number of 180 mm diameter polyethylene pipes within the explosion chamber, providing a volume congestion between 0 and 5% and cross-sectional area blockages ranging between 0 and 40%. The series of tests produced peak explosion overpressures of between 70 mbar and 3.7 bar with corresponding maximum flame speeds in the range 35 - 395 m/s at a distance of 7 m from the ignition point. The experiments demonstrated that it is possible to generate overpressures greater than 200 mbar with volume blockages of as little as 0.57%, if there is not sufficient outflow through the inadvertent venting process. The size and failure pressure of potential vent openings, and the degree of congestion within a building, are key factors in whether or not a building will sustain structural damage following a gas explosion. Given that the average volume blockage in a room in a UK inhabited building is in the order of 17%, it is clear that without the use of large windows of low failure pressure, buildings will continue to be susceptible to significant structural damage during an accidental gas explosion

Effects of obstacle separation distance on gas explosions

The separation distance (pitch) between obstacles is an area that has not received adequate attention by gas explosion researchers despite general recognition of the important role it plays in determining the explosion severity. Either too large or too small a separation distance between the obstacles would lead to lower explosion severity. Therefore obstacles would need to have “optimal” separation distance to produce the worst case explosions overpressures and flame speeds. Most studies to date with multi-obstacles had the obstacles too closely packed resulting in data that most likely do not represent the worst case scenarios. The major objective of this project was to investigate the influence of spacing between obstacles in gas explosions by systematically varying the distance in order to determine the worst case separation that will produce the maximum explosion severity. A long vented cylindrical vessel 162 mm internal diameter with an overall length to diameter ratio (L/D) of 27 was used in the experimental study. The vessel was closed at the ignition end and its open end connected to a large cylindrical dump-vessel with a volume of 50 m3. The spacing between the obstacles in the test vessel was systematically varied from 0.25 m to 2.75 m. The influence of obstacle spacing was studied with obstacles of different blockage ratios, shapes, number and scale. Tests were carried out with methane, propane, ethylene and hydrogen mixtures with air. A correlation was developed and applied in this research to predict the position to maximum intensity of turbulence downstream of an obstacle, xmax dimensionalised with obstacle scale, b as a function of obstacle blockage ratio, BR, using steady state experiments from the limited available data in the literature as, ( ) for t/d < 0.6 (thin/sharp obstacles) A clearly defined separation distance which gave the most severe explosions in terms of both maximum flame speed and overpressure was found in this research. The profile of effects with separation distance agreed 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, after the flame has moved passed the obstacle. The predicted model on position to maximum intensity of turbulence from cold flow data agreed with the worst case obstacle separation distance in the current research if multiplied by a factor of three. Turbulence parameters were estimated from pressure differential measurements and geometrical obstacle dimensions. This enabled the calculation of the explosions induced gas velocities, r.m.s turbulent velocity, turbulent Reynolds number and Karlovitz number. By expressing these parameters in terms of turbulent combustion regimes, the bulk of the tests in this study was shown to be within the thickened-wrinkled flames regime. Turbulent burning velocity, ST models with dependence on obstacle scale, higher than the ones in the existing gas explosion scaling techniques were obtained as, for single hole-obstacles for single flat-bar obstacles From the newly obtained ST correlation for single flat-bar obstacles, an overpressure correlation, P for scaling relationship was derived and validated against both small and large scale experimental data and the results were encouraging. [( √ ) ][ ] In planning the layout of new installations, it is appropriate to identify the relevant worst case obstacle separation in order to avoid it. In assessing the risk to existing installations and taking appropriate mitigation measures it is important to evaluate such risk on the basis of a clear understanding of the effects of separation distance and congestion. The present research 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 correspond to worst case scenarios