The Study On The Interaction Of Cold/Thermal Natural Gas Leakage Jet With Water Mist

The use of traditional fossil energy leads to increasingly serious environment problem,while natural gas,as a kind of low-carbon clean energy,has gradually become the main energy in the process of industrial production,transportation and residents and so on.However,natural gas leakage accidents may be caused by the pipeline corrosion,equipment aging,artificial destruction,et al,in the process of storage and transmission.Once a gas leakage occurs,the high-pressure gas is released rapidly to the surrounding and results in the person intoxication,even cause a fire or explosion accidents,which often can cause significant casualties and economic losses.Therefore,to develop natural gas leakage and fire explosion accident prevention work has important scientific significance and practical value.Due to its advantage of clean,smaller droplets,low water consumption,et al,water mist fire extinguishing technology has been widely used in many fields of industrial production,but the study on the fire prevention of gas leakage accident by water mist is relatively few.The study on the interaction of water mist and cold high speed gas jet is still lack,and the only part of the study on fire suppression of gas jet fire was carried out by water mist,but they only focus on the typical fire extinguishing performance from the macroscopic views.However,the flow field characteristics in the process of interaction between water mist and fire plume can reveals the performance and mechanism of fire suppression.Therefore,a series of small scale experiments and numerical simulations were conducted based on the 2D-PIV(particle image velocimetry)technology to study the interaction of water mist and the cold/thermal natural gas leakage jet based on the flow field measurement technology.In order to obtain the controlling and attenuating effect of cold gas leakage jet by water mist spray,the study on the interaction of water mist and vertical and tilted cold gas leakage jet were conducted,where Nitrogen was used instead of natural gas due to the security.In this paper,the small hole leakage scenario and the release jet speed from subsonic to sonic were considered.For vertical interaction between gas jet and mist,the gas plume lift-up distance was determined by the effective gas-spray momentum ratio ?Eff and the relative opening angle.According to the critical gas plume height,the two phase flow field could be divided into gas or spray dominant flow,where the spray dominant flow should be adopted in the practical applications.Counter-Rotating Vortex Pairs(CVP)were formed due to the entrainment effect of high velocity gas flow,which could enhance the mitigating efficiency via well gas-droplet mixing.The correlation between the lift-up distance of gas plume and the gas-spray momentum ratio can be well predicted by an exponential function.In the actual application,the spray pattern should be designed as relatively large spray cone angle,large droplet velocity and diameter in the center and small droplet diameter at the margin.For tilted interaction between gas jet and mist,the mixing effect of gas-droplet was relatively better when the intersection of gas-spray centerline was located at the middle position of the vertical flow field.For relatively larger spray cone angle and coverage,the controlling and attenuating effect of tilted gas jet was better.The velocity field structure indicated that the asymmetric gas-spray interfaces were formed at the two sides of gas plume;the shearing actions and gas-droplet mixing degree were the most violent on one side,where there is an obvious clockwise vortex structure.In addition,the vertical gas-spray interaction interface was still formed.For the interaction between water mist and thermal natural gas leakage jet,experimental study on the interaction of water mist and methane jet fire were conducted.The results of fire extinguishing experiments indicated that to gas jet fire,when the fire power is relatively small,the fire suppression effectiveness was mainly related to the spray dynamic effect.However,when the fire power is relatively large,the fire extinguishing effectiveness was related to the flame flow state and the spray dynamic effect;For spray with large droplet diameter and injection pressure,the fire extinguishing time increases first and then decrease with the increase of fire power,and existing a critical state of hardest extinguishing at certain injection pressure,and the decrease of fire extinguishing time is due to the flame blowing out with the action of spray.Compared with the circular jet nozzle,the crack nozzle jet fire was relatively difficult to be extinguished,which is due to the large flame width and better air entrainment effect in the process of fire suppression.The results of flow field measurement indicated that for spray with relative small droplet diameter and spray momentum,the flame was relatively difficult to be extinguished,and the vertical interaction boundary between water mist and fire plume was formed obviously,and the droplet velocity in spray region above flame decreased in different degree affected by the thermal plume;the interaction boundary went up with the increase of fire power,and the mist droplets cannot penetrate the fire plume directly,some droplets were entrained into the flame from the two side;the mist droplets at the spray edge would goes up and the CVP vortex structure would be formed at the two side of spray.For spray with relative large droplet diameter and spray momentum,the flame was relatively easy to be extinguished,and the interaction process between water mist and fire plume become more drastic,and the flame instability strengthened;some droplet can penetrate the flame tip and the interaction boundary in the velocity flow field was lower than the actual flame height in fire suppression,moreover,the droplet velocity in spray region were also decreased in different degree;the mist droplets at the spray edge almost went down and were not affected by thermal plume,vortex structure were also formed in the interaction boundary region.The simulation results on the interaction of water mist and vertical/tilted cold gas leakage jet based on the ANSYS Fluent 14.5 indicated that the variation trend of gas-spray interface was in agreement with experiments,but the predicted gas-spray interface position was higher than the experimental measurements,which is due the limit of mist entrainment effect by high velocity gas jet in the simulation.But for cases with relative large water flow rate and spray cone angle,the predicted results were relatively better.The simulation results on the interaction of water mist and methane jet fire indicated that FDS can predict the water spray dynamic well;The typical fire suppression process of fast and failed extinguishment cases can also be predicted well and the variation trends in the suppression process on the parameters of HRR,temperature,water vapor mass fraction were also presented.