管夾式超音波流量測量之換能器引致聲場研究

Abstract

流量計是工業節能感控系統量測與驗證中最重要的儀表之一，非侵入式的超音波流量計更為首要的應用工具。管夾式超音波流量計大多為Z型與V型架設，頻率範圍在1-2 MHz，依據聲波在管流內的傳遞時間計算流速及流量。斜束超音波入射於液固耦合介面，不僅會發生幾何反射，還會產生滲漏導波，後者折射進入耦合液體與前者干涉，形成非幾何反射聲場。超音波流量計探頭設計時，窄束剪力波探頭需避開非幾何反射聲場，以增加精確度，寬束藍姆波探頭則利用滲漏導波達到增加聲場覆蓋率之需求。本文數值解析有限寬度之超音波聲束傾斜入射於固液耦合的管壁產生之反射及穿透聲場，找出適當的入射角參數。以市售之參考探頭及自製壓電探頭為分析對象，從數值結果解釋探頭設計原理與重要設計因子，並加入實驗佐證。

Flow meter is one of the important instruments in industrial sensing and control systems for energy saving measurement and verification. Non-invasive clamped-on ultrasonic flow meter is the primary choice among many application instruments. Both Z-type and V-type configurations are usually employed in ultrasonic flow measurement, in which the operating frequency range is from 1 to 2 MHz. The flow velocity can be determined through the time of flights of obliquely incident ultrasound propagating in pipe flow. Nonspecular reflection of a bounded acoustic beam is caused by the interference of geometric reflection and associated leaky guided waves propagating along the interface between liquid and the tube. Ultrasonic transducers developed for time-of-flight flow meter can be categorized into two groups, narrow beam shear wave and wide beam Lamb wave. Higher accuracy can be achieved by the transducer based on narrow beam shear waves if nonspecular acoustic reflection is avoided. On the other hand, the transducer of broad beam Lamb waves can increase coverage of ultrasonic field transmitting across low acoustic impedance flow by stimulating leaky guided wave along the pipe. This thesis numerically analyzes both reflected and transmitted beam profiles induced by a Gaussian acoustic beam obliquely incident on the fluid coupled pipe. The working principle and several important design factors of ultrasonic transducers for flow meters have been studies and verified through experimental exploration on the house-made piezoelectric transducers and both types of products available in industry. The appropriate angles of incidence have been determined for above mentioned ultrasonic transducers.