線陣列超音波換能器於質子交換膜燃料電池石墨雙極板的聲場

Abstract

質子交換膜燃料電池反應生成水的管理是相關產品研發的一項重要課題，本研究應用數值方法分析陣列超音波穿透石墨流道雙極板，模擬超音波可視化監測反應生成水。石墨流道雙極板特性經接觸式超音波方法量測，確認應為石墨複材，材料特性接近等向性材料。並以相同之實驗架構獲得石墨流道板不同區域與狀態之超音波反射訊號，當流道內有水存在時，超音波回波訊號具明顯之差異。使用聚焦式沒水超音波探頭對石墨複材流道板進行線掃描，結果顯示聚焦探頭能清楚辨別不同流道內水的分佈。 為設計合適的陣列探頭，探討單晶壓電換能器在半無限域固體基材中激發的聲場指向性，作為設計陣列超音波換能器的依據。以有限元素法分別對於介質為等向性固體之石墨複材及異向性純石墨進行聲場指向性的模擬分析。純石墨材料的軸向與橫向剛性差異太大，單晶壓電探頭激發的聲場主瓣較窄，聲波相位干涉產生的側瓣不明顯，聲束的擴散角小，石墨複材則相反。根據分析結果設計陣列超音波換能器，再利用暫態分析模擬設計之效果，確認可辨識不同區域水的分佈。

Water management is an important issue to improve performance of proton-exchange-membrane (PEM) fuel cells. In order to understand the feasibility of monitoring water production in PEM fuel cells using ultrasonic visualization, this thesis numerically studies linear array induced ultrasound propagation in graphite bipolar plates with machined flow channels. The material properties of graphite bipolar flow channels were experimentally characterized. Experimental results indicate that it is nearly an isotropic material. Comparing the ultrasonic signals acquired in different states and areas of graphite bipolar plates, the reflected echoes have significant changes when water appears in flow channels. According to the B-scan signals of graphite bipolar plates using immersion focusing ultrasonic transducer, the distribution of water can be clearly determined. Based on the investigation of the acoustic field induced by single element piezoelectric transducer attached on surface of semi-infinite solids, a linear array ultrasonic transducer was further designed. Both graphite composite and pure graphite are considered as acoustic media under investigation. Single element generated ultrasound in pure graphite has narrow main lobe due to its hexagonal anisotropy. The side lobe produced by acoustic phase interference is relatively small. It results that acoustic directivity of sound beam has smaller divergence angle in pure graphite than graphite composite. Transient analysis for acoustic field induced by linear array ultrasonic transducer in graphite bipolar plate indicates that the water distribution in the flow channels can be successfully discriminated.