| As a new environmentally friendly and energy-saving clean fuel,ethanol-gasoline has been widely used around the world.Its emission characteristics as an engine fuel have been extensively studied,but the explosion safety issues caused by its flammable,explosive and volatility properties have been given less attention to.If ethanolgasoline leaks accidentally in the storage,transport and use process to form a vaporair cloud,it is very easy to cause explosion accidents with an accidental ignition,resulting in serious economic losses and casualties.From the perspective of safety application,it is crucial to control and mitigate the ethanol-gasoline vapor explosions hazard effectively.Among numerous explosion inhibition technologies,the explosion venting has been widely used since its ease of the implementation and low costs.The study of vented ethanol-gasoline vapor explosion characteristics and its influence is of some practical significances on oil/gas explosion safety.Based on the three necessary conditions(combustible,ignition and vent)for a vented explosion occurring,the influences of ethanol-gasoline reactivity,single/double ignition position and vent parameters on the flame propagation and explosion overpressure during the vented ethanol-gasoline vapor explosion process were investigated in this paper,through the self-developed small-scale vented oil/gas explosion experimental apparatus and Fluent simulation software.The flame propagation behavior and velocity were captured by High-speed photography;the CY400 high-frequency strain-gauge pressure sensors were used to record the vented explosion overpressure development;the chemical reaction products of vented ethanol-gasoline vapor explosion were analyzed by the gas chromatography-mass spectrometry(GC-MS);the flow field involved in the venting process observed to further analyze the flame propagation mechanism with the help of two-dimensional numerical simulation.The research contents and results are as follows.(1)Whole the vented ethanol-gasoline vapor explosion development process were analyzed,and four stages were discovered,including membrane burst,flame area reaching maximum,flame ejection and external flame combustion.These stages will cause different typical overpressure peak structures(Pb,Pcv,Pext,Pmax)on explosion overpressure-time curves,they were compared and their formation mechanisms were discussed.It was found that the Pmax was determined by different typical overpressure structures under various initial conditions.Numerous experimental results indicated that there is no explosion occurred outside the channel while the flame ejected but burned in the form of jet flame.The GC-MS confirmed that vented ethanol-gasoline vapor explosion reaction products were mainly alkanes of C7H8 and C8H10,including toxic and explosive toluene.(2)The effects of ethanol-gasoline volume(VE10)and chemical reactivity on its vented explosion characteristics were investigated.The results indicated that different VE10 significantly affected the flame color,the Pcv and Pmax increased and then decreased with VE10.The most reactive vapor-air reactivity within the test range was determined to be E10=2.0 mL,which reached the maximum Pmax inside and outside the channel.Additionally,the experimental results show that perfluorocaproflurosterone(C6F12O)significantly reduced the ethanol-gasoline chemical reactivity,which caused a significant decrease in flame propagation speed,and the flame front was extremely unstable and oscillated significantly,while the overpressure peak structure Pb,Pcv,Pext,Pmax and the deflagration index KG also decreased significantly.The GC-MS analysis confirmed that C6F12O interrupted the vented ethanol gasoline vapor explosion chain reaction process by decomposing into various fluorides that reacted chemically with the main reaction elements(H,O,OH).(3)On the basis of the maximum vapor-air reactivity VE10,the effect of single/dual spark positions(Ip/D)on vented ethanol-gasoline vapor explosion characteristics was investigated.Experimental results showed that the pressure rise was mostly governed by flame expansion in the vent direction under the single spark condition,whereas the opposite flame grew slowly or even stagnated,and the Pmax inside and outside the channel were linearly related to Ip/D.The dual flames expansion led to significant increases in the combustion product volume expansion rate under the dual sparks,the Pmax was almost twice the case of single spark,there is still a linear positive correlation between the Pmax and Ip/D.Interestingly,while the dual sparks were located at the same channel end,the dual flame expansion would be limited by the channel cross-section and combined into one resulting in Pmax is instead comparable to the case that single spark was located at the channel end.Under two symmetric sparks condition,dual flames developed practically simultaneously until they merged to expel,the numerical simulation results showed that the airflow between the symmetrical flames did not play a role in the flame front.The increasing trend between the Pmax and Ip/D was found to be met with a sigmoid function.In the current experimental test ranges,the maximum Pmax for the case of single spark occurred at the case of(Ip/D=8.75),the maximum Pmax under two symmetric sparks conditions occurred at the case of(Ip/D=5).(4)Based on the most reactive VE10 and the maximum Pmax for single and dual sparks positions(Ip/D),the influence of vent parameters(vent area(coefficientKv))and vent location(X/D)on the vented ethanol-gasoline vapor explosion characteristics were studied.An empirical formula for predicting Pmax based on VE10 and Kv was proposed.A correlation between the flame propagation velocity(v)vs Kv following membrane burst under single and dual sparks conditions was derived based on the energy equation.And the ratio of the unburned gas flow velocity following membrane burst under single and dual sparks conditions was also obtained,which can be used to explain the 2-fold quantitative relationship between the experimental v under single and double sparks conditions.Numerous experimental results show that Pmax and Kv under single and dual sparks conditions were both positively correlated as a power function(Pmax=aKvb)and were not affected by VE10 and X/D.The smaller Kv made internal pressure appear Helmholtz oscillation,and the increase of X/D would enhance its oscillation amplitude.The simulation results show that the flame front reversal under single/double spark conditions with larger Kv was caused by the channel wall friction on the flame front causing a vortex structure in its combustion zone,while the tulip flame appearance depended not only on Kv but also on X/D.The flame downstream of the vent that propagated with a lower velocity oscillation and the cell flame front under smaller X/D,where Kv was positively correlated with the cell size.(5)From the application engineering perspective,the dual spark compared to a single spark induced explosion resulting in a significant increase in Pmax,and its internal pressure may continue to grow after flame ejection.For this design of vent parameters,the smaller Kv and X/D,the better the internal/external pressure release effect in a channel.Notably,the X/D reduction will increase the flame retention time in the channel,but weaken the overpressure oscillation.Under symmetrical dual sparks conditions,larger Kv will inhibit the flame propagation near the vent while the vent was not centered,the external flame size and height are significantly larger than the vent centered arrangement. |
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