微小通道内流动及气溶胶沉积特性实验研究

Experimental Study on Gas Flow and Aerosol Deposition Characteristics in Micro-channel

事故后气溶胶通过安全壳微小通道穿透会发生滞留现象,在不同载气流动状态下,主导沉积机制不同,会影响气溶胶的穿透率,因此需要研究微小通道内气溶胶的载气流动特性及气溶胶沉积特性。本文建立了微小通道气溶胶穿透与滞留实验装置,开展了雷诺数为40~3600的载气流动与气溶胶沉积实验研究。在雷诺数约为700时,微小通道内会发生层流转捩。层流流动下,在大突缩比的矩形微通道中,随着雷诺数的增大,入口碰撞效应减弱,穿透率增大;小突缩比薄壁毛细管微通道入口效应弱,随着雷诺数的增大,气溶胶几乎完全穿透。在雷诺数为800~3600的湍流流动下,气溶胶沉积主要处于涡流扩散-碰撞区及惯性主导区,受到湍流涡的影响气溶胶穿透率随雷诺数的增大而减小。

Aerosol retention occurs when it penetrates through micro-leakage paths within the containment after nuclear reactor accidents.Aerosol particles are deposited by mechanisms such as gravity sedimentation,Brownian diffusion,entrance effect,and turbulence deposition,and the deposition efficiency is affected by the carrier gas flow conditions.Therefore,it is necessary to study aerosol deposition characteristics within micro-leakage paths under laminar and turbulent flow regimes.In this work,an experimental facility consisting of two circuits was established for investigating the flow characteristics and aerosol deposition behaviors.In the flow test circuit,the flow resistance characteristics of air within the Re range of 40 to 3600 are obtained by measuring the pressure differential and volume flow rate.In the aerosol test circuit,the penetration of aerosols is obtained by measuring the particle size distribution and concentration at the inlet and outlet of the micro-leakage paths.The experimental configuration encompassed the use of capillaries with diameters of 350,550,800,and 1000μm and length of 52 mm,as well as a rectangular micro-channel with dimensions of 26μm×30 mm×52 mm to simulate micro-leakage paths.The aerosol comprises polydisperse TiO 2 dust particles with a mass median diameter of less than 1μm.The results of the flow testing reveal that,in both capillaries and rectangular micro-channel,the variation curve of the C d·Re(C d is Darcy resistance coefficient)with Re significantly deviates from the traditional laminar characteristic curve within a range of±5%when the Re reaches 700,indicating that the gas flow has transitioned into turbulent flow.Under laminar flow conditions,it is observed that within capillaries with diameters of 800 and 1000μm,aerosol deposition is not evident at Re ranging from 380 to 600.The particle size distributions at the inlet and outlet are similar,with the penetration exceeding 90%.However,within rectangular micro-channels at Re between 40 and 130,as the Re increases,the aerosol penetration increases from 12%to 50%.During to the differences in size and structure feature,rectangular micro-channel has an obvious entrance effect compared with thin wall capillary.With the increase of the Re,the lift force of particles generated by the entrance gas velocity gradient leads to a reduction in entrance deposition,so the penetration is enhanced.Under turbulent flow condition,it is observed that within capillaries with diameters of 350,550,and 800μm,the aerosol penetration shows a decreasing trend as Re increases from 800 to 3600.In the test with capillary diameter of 550μm and Re of 2100,the proportion of larger particles shows significantly reduced.Aerosol deposition predominantly occurs within the vortex diffusion-impaction region and the inertia-dominated region,with the effect of turbulent eddies,the fluctuating velocity of particles towards the wall intensifies,accelerating the deposition of aerosols.