Study On The Technology Of Concentration Detection Based On Differential Pressure Principle For The Agent Used In Aircraft Engine

Fire is one of important influence factors which threaten the safety of aircraft engine.To avoid the catastrophic consequences of out-of-control fire,aircraft extinguishing system is applied,and due to the cleanliness and effectiveness,gas fire extinguishing system is widely used in the aircraft.The agent concentration distribution is significant for the verification of gas fire extinguishing system.The concentration detection for the fire extinguishing agent is the core and key to the design of aircraft fire extinguishing system.At present,the concentration detection technology which has been certified by airworthiness of FAA(Federal Aviation Administration of America)is based on differential pressure principle.This sensing technique has the characteristic of fast response,wide detection range,and high precision.However,because of technical barrier,the principle of detection and its sensing performance are not well researched.In this article,we will conduct a comparatively comprehensive study by analyzing three aspects including sensing mechanism,response characteristics and influence factor,which can provide technical support for the development of sensing technique based on pressure differential principle.The main research content of this article are as following:The theoretical model of concentration detection based on differential pressure was established and perfected,and the detection mechanism was revealed.Based on the previous studies,the physical model was deduced,the physical meaning of parameters in the model was clarified,and the influences of temperature,gas properties parameter,and geometric parameter were analyzed in depth.By studying the temperature,it was found that the relative change of temperature △T/T was twice the relative change of pressure difference △Pd/Pd,and to improve the stability of detector,the way by reducing the temperature fluctuation was better than that by increasing the temperature.By studying the gas properties parameter,it was found that the gas parameter determined whether the mixture was suitable for this technique.By studying the geometric parameters,we found that increasing the geometric parameters could improve the sensitivity of the equipment,and combined with the studies of temperature and gas properties parameter,the scope of each geometric dimension in the geometric parameters is given in this paper.The agent concentration detector was developed,and the correction of physical model is verified based on the experimental results using this detector,and the response characteristics are also studied.Two type of agent are used:trifluoromethane(CBrF3)and heptafluoropropane(C3HF7).It was found that the experimental data was well consistent with the theoretical value after proper correction.However,it is difficult to obtain accurate gas properties,so it is more appropriate to use the fitting equation to calibrate the detector.Based on the established experimental platform,the detector’s stability,accuracy,sensitivity,and response/recovery time were analyzed.It was found with the increase of gas concentration,the sensitivity and accuracy decreased.According to the experimental result,the absolute error can be less than 0.3%(0-30%),and 1%(above 30%),and the response time was 0.18s,and the recovery time was 0.27s.The research of important influence factors during concentration detection were studied.The influence of geometrical construction of detector,temperature and humidity were analyzed.By studying the flow process of agent in the detector,the delay time model was established,which revealed the change rule of the response rate.It was found that for the same geometry geometric dimensioning was studied,the recovery time of the detector was greater than the response time,and as the concentration increased,both response and recovery time increased.With the increase of the diameter of current limiting structure,and the decreases of the diameter and length of the sampling tube,the response value would be greatly improved,and the response/recovery time could be improved less than 0.1s.By analyzing the effect of temperature,it was found that the effect of temperature on the response of the low concentration region was linear,while the high concentration region was non-linear;by the analysis of the effect of humidity,it was found that the humidity had little effect on the response value of the sensor.