時差式超音波流量計的聲場分析與驗證

Analysis and Validation of Sound Fields for Transit-time Ultrasonic Flowmeters

薛政竑(Cheng-Hung Hsueh)

指導教授 ： 尹慶中

國立交通大學/工學院/機械工程系所/碩士(2019年)

摘要

超音波流量計是製程控制的重要量測儀器，管流內部的超音波幾何反射無法解釋V型架設夾管式超音波流量計遭遇的特定現象。例如：接收訊號為兩組音頻脈波群，並參雜許多的微小振盪。本論文結合理論分析及實驗驗證，探討超音波在管流及周圍管壁的傳遞。理論分析包含基于廣義射線追蹤的訊號重建及有限元素分析，模擬管流及周圍管壁的聲場。聲波傳遞的一次路徑包含在管線內的正向傳遞及一次反射，二次路徑包含管壁內的額外兩次反射路徑。兩路徑超音波干涉造成大口徑聚氯乙烯管線的接收訊號分成兩組波群。相較之下，金屬或較小口徑管線的訊號干涉不明顯。應用二維有限元素分析模擬聲場，結果發現初始激發的平面聲波發散成柱面波，若傳遞距離增加，接收訊號會出現較長時間的振盪。模擬與靜止水管實驗結果具有很好的吻合度。流體聲速不明，嚴重影響管夾式流量計的安裝及流量的估算。若發射與接收探頭的距離變化，接收訊號高於閥值的週期數會跟著改變，最小週期數對應探頭的最佳安裝距離。不論管線及管流材質如何，這項方法都驗證有效。

關鍵字

有限元素分析 ； 廣義射線追蹤 ； 訊號重建 ； 聲場 ； 超音波流量計

並列摘要

Ultrasonic flow meter is an important measuring instrument for process control. Geometric reflections of ultrasound in flows cannot explain some specific phenomena occurred in practical V-type installation for large-diameter pipes. For example, the received signal splits into two major tone-burst groups with a large number of small oscillations. This thesis presents a coordinated theoretical and experimental investigation of ultrasound propagation in the flows and surrounding pipe walls. The simulations include signal reconstruction based on generalized ray tracing and finite-element analysis of sound field in flows and pipes. The primary path of ultrasound traveling in the pipe consists of once forward and reflection over the pipe. The secondary path is composed of excess twice reflections across the walls of pipe. Ultrasound from both paths interfere such that signal splits into two groups for large-diameter polyvinylchloride (PVC) pipes. By contrast, the interference is not obvious in metal or pipes of small diameter. Simulated sound field using two-dimensional finite-element analysis indicates the transmitted plane waves are diverging into cylindrical waves if travel distance increases. Therefore, longer durations of small ringing are induced in received signal. Both results achieved by simulations and still water experiments are in very good agreement. The acoustic speed in the flow has a significant influence on the installation of clamp-on flow meter and determination of flow quantity. The cycle number of received signal beyond the threshold changes if the distance between transmitted and received transducers varies. The minimum indicates the best installation distance. This method has been validated to be true no matter what pipe materials and flows.

並列關鍵字

finite-element analysis ； generalized ray tracing ； signal reconstruction ； sound field ； ultrasonic flow meter

參考文獻

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