| The most frequent and serious fires in aircraft are engine nacelle pool fires,generally caused by the leakage of combustible liquids such as aviation kerosene.Halon 1301 has long been used to extinguish fires in aircraft engine nacelle,but its use in civil aviation will be restricted in 2025 due to its destructive effect on the ozone layer.Three alternatives,i.e.,HFC-125,CF3I and Novec 1230,tested by Federal Aviation Administration,have low fire extinguishing efficiency,poor anti-combustion performance.Ultra-fine dry chemical agent(UDCA)is a solid powder composed of alkali metal salts(such as sodium bicarbonate and potassium bicarbonate)and several amounts of additives with small particle size(D90,90%of the particle size ≤20 μm),good dispersion,high fire extinguishing efficiency,which is conducive to aircraft engine nacelle fire extinguishing system weight reduction.However,the doubts about high corrosiveness and poor re-ignition resistance limited its testing and application to aircraft engine naccle fires.In this paper,to solve the problem of difficult to extinguish and easy to re-ignite the engine nacelle pool fire,screening base material based on the requirements for low corrosive fire residue,modificating UDCA to be oleophobic,measuring the minimum fire extinguishing concentration of UDCA and verifying the ability to extinguish the simulated engine nacelle oil pool fire.Finally,we prepared a low corrosive fire,high efficiency and anti-reignition UDCA for engine nacelle.This work could provide theoretical and experimental supporting for the future use of UDCA as the aircraft engine fire extinguishing agent.Firstly,the sodium bicarbonate with D90 of 1 μm was determined as the base material for the preparation of UDCA.The effect of particle size on the pyrolysis kinetic parameters of sodium bicarbonate and potassium bicarbonate was studied based on thermal analysis technology.The particle size decreasesd the initial and decomposition temperatures advanced,and the thermal decomposition reaction rate was multiplied.The thermal decomposition rate of sodium bicarbonate with the D90 of 1 μm was 13 times that of potassium bicarbonate with the D90 of 1 μm,and the heat absorption capacity is 16 times that of potassium bicarbonate.Then,the differences in the corrosiveness of sodium bicarbonate and potassium bicarbonate fire extinguishing residues to four aviation metal materials(stainless steel,brass,aluminum alloy,magnesium alloy)were studied,and it was found that potassium bicarbonate fire extinguishing residues could accelerate the electrochemical corrosion of metal,especially aluminum alloys.Scendly,a one-step hydrophobic and oleophobic modification of ultrafine sodium bicarbonate was achieved by the "gel-sol-gel" method,considering the requirements of anti-reignition and storage stability of UDCA.With the help of FTIR,XPS,SEM and other means to characterize the modified sodium bicarbonate,it was found that the different surface distribution density of fluorine-containing long chains caused the spatial site resistance effect.As the proportion of fluorine-containing monomer increased,the steric hindrance effect was enhanced,and the hydrophobic and oleophobic properties were better.The fire extinguishing and anti-reignition tests showed that the oleophobic ultrafine sodium bicarbonate powder could extinguish the fire faster.The formed oleophobic powder layer played an anti-ignition effect,and the rate of fire spread on the oil pool surface was obviously inhibited.Thirdly,based on the advanced aerosol generation system,optimized cup burner structure and improved measurement method,the minimum fire extinguishing concentration of UDCA was measured more accurately.Based on Lambert’s law,the mathematical relationship between laser transmittance and UDCA concentration was established.The repeatability error was lower than 12%.During the measurement process,it was found that the loss of fire buoyancy at the moment of flame distiguishing would lead to a sudden increase of UDCA density in the measurement space and a sudden decrease of the laser transmission rate,the minimum fire extinguishing concentration of UDCA was innovatively proposed in accordance with this sudden change point.The measurement results showed that the minimum fire extinguishing concentration measurement value increased with the increase of aviation kerosene preburn time.The minimum fire extinguishing concentration of prepared FSB(functionalized sodium bicarbonate)powder was finally determined to be 31.8 g/m3,which was better than the market UDCA of 49.8 g/m3.Finally,the dynamic evolution characteristics of the pool fire after a backfacing step in engine naccle were analyzed,and the fire extinguishing ability of UDC A on the pool fire was verified and compared.The different positions of the oil pan lead to the fire spreading pattern in the recirculation area showing the "inverted L"shape to "T" shape,and then to "Z" shape transformation.The farther the oil pool was from the step,the lower the wind speed required to anchor the flame from the step,which was related to the oil and gas transport characteristics in the recirculation area.Through the fire extinguishing experiment of back-facing step diffusion pool fire experiment,the critical fire extinguishing concentration of free flow in the engine nacelle was obtained,and the characteristic mixing time of UDCA in the recirculation area was modified,and a prediction model of critical fire extinguishing concentration in the engine nacelle free flow was established.After comparison,under the same experimental conditions,the amount of UDCA required to extinguish the fire was only 10%of the mass of Halon 1301. |
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