Numerical Simulation Of Internal Flow Field In Differential Pressure Gas Concentration Element Of Agent

In this paper,a core concentration differential pressure converter used in the concentration detection equipment of fire extinguishing agent used in the experimental verification stage of aircraft fire extinguishing system design is studied.The advantages of this kind of equipment include small area,high measurement accuracy and fast measurement speed,but the small size also hinders the flow state of the measuring elements and the analysis of geometric parameters.At the same time,because of the complex flow field inside the components,it is difficult to analyze the theory.The optimization and selection of components often depend on experiments and experience,which consumes a lot of time and labor costs.In this paper,the flow state in the detection element is simulated numerically,and the influence of environmental factors and geometric parameters on the performance of the element is analyzed.Firstly,the Eulerian mixing flow based on Navier-Stokes fluid is selected.On the basis of constitutive equation and basic equations of hydrodynamics,RNG K-e turbulence model is introduced to make the equation closed.The semi-empirical Wilke formula is used for the viscosities of mixed gases.The volume ratio of mixed gases is used for the calculation of thermal conductivity and specific heat capacity.The Fuller formula is used for the relative diffusion coefficient to determine the most suitable exchange coefficient in the component equation.The UDF function is written for the above parameters.Assuming that the whole process of gas flow inside the element is adiabatic isentropic flow,the first boundary condition is adopted.The gas flow in the capillary is considered as laminar flow,and the pressure difference between the two sides of the flow tube is described by Poiseuille's law.The research contents include the following aspects:The flow characteristics of the flow field inside the concentration-pressure conversion element were studied.The mixing process of Halon 1301 fire extinguishing agent and nitrogen in the element was analyzed.The relationship between the physical quantities inside the element and the gas mixing process with time was studied.The physical quantity will change dramatically at the nozzle,the pressure distribution will affect the position of the measuring point,the temperature change will lead to the change of the flow coefficient of the component,the distribution of velocity field and concentration field shows the existence of stagnation zone,and stagnation zone is an important factor to prolong the response time.At the same time,the effect of the concentration on the measurement results and the performance of the components is also studied.The relationship between the geometric parameters of the element,such as the size and distribution of the inner capillary,the geometric structure of the stagnation zone and the response time and sensitivity of the element is established.Within the range of 0-100%and at intervals of 10%concentration of fire extinguishing agent,it is concluded that the smaller the volume of components and the more uniform the velocity profile of fluid are,the more advantageous it is to reduce the stagnation zone of gas,thus reducing the response time and improving the sensitivity.These conclusions provide guidance for structural optimization and design of components.