地下河不同流量状态下溶质运移的参数及模拟

Parameters and modeling of solute transport under different flow conditions in subterranean river

在官村地下河流域一段长为750m的管道进行了十一次弥散试验,以考察岩溶管道流的溶质运移行为.采用NaCl作为示踪剂,利用电导仪记录浓度的变化和流速仪测量管道流的流量.试验表明溶质的平均运移速度为1.3~5.8cm·s-1,通过弥散试验求的管道平均截面面积在不同流量时差别不大,表明岩溶管道处于有压管流状态,并且雷诺数大于20000,据此判断管道流为紊流.弥散系数与溶质平均运移速度之间为近似线性关系.利用经典对流弥散方程以及由试验结果取得的参数模拟溶质的穿透曲线,发现模拟结果的溶质运移总比实际滞后,这种现象与模拟时管道形状的平均化处理无关,是用管道截面平均流速代替管道截面实际流速产生的.改用代表暂存区机理的溶质运移模型模拟试验结果,将管道水流截面分为主流区和不流动区,模拟结果与实际吻合,这解释了溶质平均运移时间提前和穿透曲线出现拖尾的原因.

Eleven tracer tests were conducted under different discharge conditions in a karst conduit in Guancun subterranean river.50 kg of NaCl was injected into a karst window in impulse manner every time.Specific electrical conductivity was auto-recorded,and discharge was measured with current meter.QTRACER2 was used to estimate parameters of conduit,flow and dispersion through breakthrough curves.The section value of conduit varies little among the tests,indicating the conduit is filled with water even in the lowest discharge.Reynolds numbers in all tests are bigger than 20000,which are exceeded the threshold value of turbulence flow.Dispersion coefficient is linear with the mean tracer velocity.The classic one dimension advection dispersion equation was used to simulate the break-through curves.The results show the modeling curves have bigger tracer transit time than the actual values,causing by the actual velocity of conduit section was replaced by the mean velocity,rather than resulting from variable conduit shape.To reduce this error,a transit storage model of OTIS was used to simulate and calculate parameters.The section of conduit was divided into main channel and storage zone.The OTIS model is successful for simulating these tests.