Study On Low False-alarm Smoke Detection Technology Based On Asymmetry Ratio For Aircraft Cargo Compartment

The false alarms of aircraft cargo compartment fire smoke detection systems not only threaten flight safety,but also cause huge economic losses.The high false alarm rate of smoke detectors needs to be solved urgently.The non-fire aerosols are the main reason for the high false alarm rate of smoke detectors.In the environment of aircraft cargo compartment,dust aerosol and water mist are two of the most common nuisance aerosols.Previous studies have shown that the discrimination and identification of nuisance aerosols requires comprehensive consideration of factors such as the wavelength of the light source,polarization,and the receiving angle of scattered light.The ratio of forward to backward scattered light intensity(ie asymmetry ratio)can be used to distinguish different types of aerosol.In comparison with the angular distribution measurement of scattered light intensity,ordinary smoke detectors are easier to measure the asymmetry ratio.On the other hand,the smoke detector has the problem of unbalanced response to black and white smoke,which affects the detection performance.This thesis focuses on two typical nuisance aerosols(dust aerosol and water droplet aerosol)in the aircraft cargo compartment.By using theoretical analysis,numerical calculation and experimental measurement methods to carry out the aircraft cargo compartment low false-alarm smoke detection technology research based on the asymmetric ratio.The main research works are as follows:(1)Theoretical analysis of the changing law of the asymmetry ratio,using numerical calculation methods to study the scattering characteristics of different types of particles,and verifying the changing law of the asymmetry ratio.Based on the Lorenz-Mie scattering theory,the theoretical calculation formula of the asymmetry ratio is derived,and the asymmetry ratio of fire smoke particles and nuisance particles is analyzed with the size parameter(0.1-50).It is found that the asymmetry ratio increases with the size parameter at the beginning.After reaching the first maximum value,it begins to show the law of periodic oscillation,and the oscillation amplitude and frequency are related to the refractive index of particle.According to the optical property characteristics of fire smoke particles(smoldering/pyrolysis smoke particles and black carbon particles)and typical nuisance particles(dust particles and water droplets)in the aircraft cargo compartment,the light scattering model is established and numerical calculation methods are used to study the scattering characteristics under different wavelengths(405-940 nm)and polarization direction(vertical and horizontal).The results show that:under non-polarized light scattering conditions,the asymmetry ratio of smoldering/pyrolysis smoke particles has maximum value(82.1)near the size parameter 1.75 which is significantly larger than that of black carbon particles(4.7).The asymmetry ratio of black carbon particles is relatively small due to strong absorption.Therefore,the two types of fire smoke particles can be distinguished according to the asymmetry ratio,and the asymmetry ratio of the two nuisance particles decreases overall with the increase of the size parameter;under the condition of polarized light scattering,wavelengths and particle sizes will affect the relative size of the vertical polarization and horizontal polarization direction corresponding to the asymmetry ratio.Among them,the asymmetry ratio of large water droplets in the vertical polarization is significantly smaller than the horizontal polarization.The numerical calculation results are consistent with the theoretical analysis results,and it is proposed that the asymmetry ratio change trend under different wavelengths and polarization directions can be used to distinguish fire smoke particles from nuisance particles.(2)Development of smoke detector experimental prototype.According to the theoretical analysis and numerical calculation results of asymmetry ratio,in order to distinguish fire smoke particles and interfering particles,450 and 135° are selected as the scattering light receiving angle and 405 nm and 850 nm as the light source wavelength.Dual-emission dual-receiving detection optical path and corresponding optical chamber were designed.In order to raise the signal-to-noise ratio,the structures of condenser and light absorber were designed separately,and the optical chamber was optimized by optical simulation method to reduce the stray light caused by ambient light.By assembling and debugging the optical chamber and the working circuit,the development of smoke detector experimental prototype is finally completed.(3)Study on the discrimination and identification of nuisance aerosols based on the asymmetry ratio under non-polarized light scattering condition.According to SAE AS 8036A,an aerosol scattering experimental device was developed to carry out light scattering experiments.Non-polarized light scattering is beneficial to simplify the hardware structure of smoke detectors and reduce the cost.Therefore,this thesis first studies the nuisance aerosol identification method based on the asymmetry ratio under non-polarized light scattering conditions.Representative fire smokes(standard test fire smoke and common engineering plastics ABS,PET,etc.)and aircraft cargo compartment typical nuisance aerosols(ISO standard test dust A1-A4,water mist,etc.)were selected.The scattering signals of aerosols and corresponding extinction rate signal were achieved by experiments.And the asymmetry ratio and its relative standard deviation were obtained by calculating.The linear fitting method is used to obtain the forward and backward scattering sensitivity for each aerosol.The results of scattering sensitivity show that the size of dust particles and water droplets in water mist is significantly larger than fire smoke particles(<1 um),but the size of water droplets in water vapor(1-2 um)is smaller than water mist(about 10 um).The results of the asymmetry ratio show that the asymmetry ratio changing trend with wavelength can be used to distinguish dust aerosols;the asymmetry ratio of water vapor is significantly larger than other aerosols,thus it can be distinguished according to the asymmetry ratio.The size of water droplets in the water mist is large(about 10 um),and the asymmetry ratio obtained is close to smoldering/pyrolysis smoke,which cannot be effectively distinguished.On the other hand,black smoke and white smoke can be distinguished according to the size of the asymmetry ratio,so that the algorithm alarm threshold can be adjusted according to the difference in scattering sensitivity to achieve balanced response.The above results are consistent with theoretical analysis and numerical calculation results.(4)Study on the discrimination and identification of nuisance aerosols based on the asymmetry ratio under polarized light scattering condition.Using the asymmetry ratio under the non-polarized light scattering condition can not effectively distinguish the water mist.Therefore,the nuisance aerosol identification method based on the asymmetry ratio under the polarized light scattering condition is studied.The smoke detector experimental prototype is further improved by using polarized light sources,and light scattering experiments under vertical polarization and horizontal polarization directions were carried out.The results show that the asymmetry ratio of water mist and water vapor in vertical polarization direction is smaller than horizontal polarization direction.The results are consistent with the numerical calculation results.By comparing with the results of other aerosols,optical path design method with a short-wavelength(405 nm)light source emitting vertical polarization beam and a long-wavelength(850 nm)horizontal polarization beam emitting is proposed to discriminate the typical nuisance aerosols in the aircraft cargo compartment.Aerosol type identification algorithm based on the asymmetry ratio under polarized light scattering is proposed.First,the nuisance aerosols are distinguished according to the relative size relationship of the asymmetry ratio,and then the black and white smoke are further distinguished according to the asymmetry ratio.The research results of this thesis can provide theoretical guidances and data supports for low false-alarm smoke detection technology in aircraft cargo compartment,and can also provide references for aerosol identification methods and optical property characteristics research.