Research On Low-temperature Performance And Two-phase Flow Injection Mechanism Of Liquid Nitrogen Fire-fighting Equipment

With the continuous development of Chinese economy,technology and science and technology,the construction of public safety facilities is also constantly improving.Fire extinguishing vehicles,as an important equipment for extinguishing fires,are also being constantly updated.Fire trucks can be divided into three generations,the liquid nitrogen fire-fighting equipment studied in this article belongs to the third-generation fire-fighting vehicle.It is a fire-fighting vehicle that sprays high-flux particulate fire extinguishing agent and it ejects a high rate of "Cloud-like"fire extinguishing agents.The "cloud-shaped" fire extinguishing agents come in contact with large areas of flames,and also change the tactics of firefighters entering the building to attack and extinguish the fire.This paper mainly studied the temperature distribution characteristics of cryogenic valve and the injection mechanism of gas-liquid two-phase flow in liquid nitrogen firefighting equipment.Taking the low-temperature cut-off valve as an example,the model establishment and meshing were performed,and a steady-state thermal analysis was performed to obtain the temperature distribution and heat flow distribution of the low-temperature valve.For the morphological changes of the two-phase flow inside the nozzle,Laval nozzles and ordinary nozzles with different size parameters were selected to analyze the flow field.Based on the geometric basin model,the VOF simulation model was used to simulate the flow field,and the pressure,density,and speed changes of the two-phase flow was obtained.Comparing the jet velocities of different nozzle sizes,the variation of two-phase flow velocity with nozzle size was obtained,proposed the optimum nozzle size.In addition,nozzle recoil calculations were also performed.It was found that the recoil force of the nozzle is proportional to the flow rate and inversely proportional to the outlet area.In order to further study the distribution characteristics of the spray downstream of the nozzle,the formation mechanism of liquid mist was analyzed,including the formation of liquid film and the breakage of droplets.At the same time,Three mathematical models of droplet breakup were deduced.By calculating the two-phase Weber number,determined the droplet breaks in this paper.In the simulation,the liquid film thickness of the two-phase flow needs to be determined first.By calculating the two-phase flow parameters at the nozzle outlet cross section with different gas-liquid ratios and different liquid flow rates,the variation of gas content and liquid film thickness at outlet cross-sections were studied.The liquid film thickness decreases with the increase of the gas-liquid mass ratio,and has nothing to do with the liquid flow rate.And at the same gas-liquid ratio,the greater the liquid flow rate,the shorter the liquid film fluctuation period.In addition,the results of the study also show that the parameters of the outlet section change periodically with time.Finally,the DPM model was used to simulate the continuous and discrete phases of the spray field.Modeling and numerical simulation of the fluid field of the spray of water mist were set up,and the droplet distribution parameters of the spray field are obtained..The spray characteristics at different liquid flow rates and different gas-liquid ratios were studied,and their influence on the droplet distribution was obtained.The best gas-liquid ratio was obtained.The research in this paper has important theoretical significance for the design and optimization of liquid nitrogen firefighting equipment.