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 aircraft cargo compartment,aerosols formed by fine dust particles and water droplets 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 detection light source,polarization,and the receiving angle of scattered light.The ratio of forward to backward scattered light intensity(i.e.,asymmetry ratio,AR)can be utilized to distinguish different types of aerosols.In comparison with the angular distribution measurement of scattered light intensity,common smoke detectors are easier to measure the asymmetry ratio.Thus,This method is more suitable for aircraft cargo hold fire smoke detection system.On the other hand,the smoke detector also has the problem of unbalanced response to black and white smokes,which affects the detection performance.Therefore,this thesis focuses on two typical nuisance aerosols(dust aerosol and water droplet aerosol)presented in the aircraft cargo compartment.By using the 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 asymmetry 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 formulas of the asymmetry ratio were derived,and the asymmetry ratios with the size parameter(0.1-50)of fire smoke particles and nuisance particles were analyzed.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 models were established and numerical calculation methods were adopted to study the scattering characteristics under different wavelengths(405-940 nm)and polarization direction(vertical and horizontal).The results show that:under the 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 the strong absorption.Therefore,the two types of fire smoke particles can be distinguished according to the asymmetry ratio value,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 that in the horizontal polarization.The numerical calculation results are consistent with the theoretical analysis results,and it was proposed that the change trend of asymmetry ratio under different wavelengths and polarization directions can be utilized 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 from nuisance particles,45° and 135° are selected as the scattering light receiving angles and 405 nm and 850 nm as the light source wavelengths.Dual-emission dual-receiving detection optical paths and corresponding optical chamber were designed.In order to increase the signal-to-noise ratio,the structures of condenser and light absorber were designed,respectively,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 was 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 conduct the light scattering experiments.Non-polarized light scattering is beneficial to simplify the hardware structure of smoke detector and reduce the cost.Thus,this thesis first studied the nuisance aerosol identification method based on the asymmetry ratio under non-polarized light scattering conditions.Several representative fire smokes(standard test fire smoke and smokes generated by 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 and corresponding extinction rate signal of aerosols were obtained by experiments.And the asymmetry ratios and its relative standard deviations were calculated.The linear fitting method was 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 utilized 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 very large(about 10 um),and the asymmetry ratio obtained is close to smoldering/pyrolysis smoke,which cannot be effectively distinguished.But 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 the 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 was studied.The smoke detector experimental prototype was further modified by applying 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 that in horizontal polarization direction.The results are consistent with the numerical calculation results.By comparing with the results of other aerosols,the design method of detection optical path with a short-wavelength(405 nm)light source emitting vertical polarization beam and a long-wavelength(850 nm)horizontal polarization beam emitting was 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 was further 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 guidance and data support for low false-alarm smoke detection technology in aircraft cargo compartment,and also can provide a reference for aerosol identification method and optical property characteristics research.