换热管道污垢检测回波频率优选数值模拟与实验

SIMULATION AND EXPERIMENTS ON SELECTING THE ECHO FREQUENCY IN DETECTING THE HEAT EXCHANGE TUBE FOULING

在换热管污垢检测中,多物理场耦合作用下的实验还不具备开展条件,因此,建立准确的超声波传播模型,进行回波频率优选数值模拟具有重要意义。以有限元为基础,基于COMSOL Multiphysics建立了不同换热管特征下3D有限元模型,讨论了其传播特性与换热管特征和回波激励之间变化关系,求解出多层管道特征频率与回波振型。结果表明:不同换热管特征(管材和壁厚)与压力载荷之间具有相似交互关系,采用超声时域反射法检测管道污垢厚度,至少选取5 MHz回波激励,才可抑制伪吉布斯带来的振荡问题,最优检测频率为10 MHz,且壁厚在3 mm～5 mm时,检测效果达到最佳。数值模拟与实验结果一致,对管道沉积污垢的检测误差能控制在±5%以内,为不同管材特征频率选择提供了定量检测依据。

The experiments under the coupling of the multi-physics field don’t have the conditions to be carried out in the heat exchange tubes fouling test. And it’s very important to establish the accurate ultrasonic propagation model for achieving the numerical simulation of selecting the detection frequency. Based on the finite element method,the three-dimension model with different heat exchange tubes had been established under the COMSOL Multiphysics condition. The varied relationship of the ultrasonic wave propagation,the different heat exchange tubes and echo exciting frequency were discussed respectively. The characteristic frequency and echo vibration mode of multi-layer tubes were found. The results show that the relationship between different characteristics of the heat exchange tubes( material and wall thickness) and pressure load is similar. The ultrasonic time-domain reflectometry technology to detect the thickness of the tube fouling needs to select 5 MHz echo exciting frequency at least for inhibiting the pseudo-Gibbs oscillation. The optimal detection frequency is 10 MHz,and the detection effect is the best when the thickness at 3 mm ~ 5 mm. The numerical simulation agrees with the test results,and ultrasonic detection produces an error around ±5% in examining tube fouling deposit. These results provide a basis for quantitative detecting basis to select the characteristic frequency under the different tubes.