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    Please use this identifier to cite or link to this item: http://tkuir.lib.tku.edu.tw:8080/dspace/handle/987654321/35962

    Title: Causes of cavitation phenomena in mechanical heart valves
    Other Titles: 機械心瓣穴蝕成因探討
    Authors: 羅啟文;Lo, Chi-wen
    Contributors: 淡江大學水資源及環境工程學系博士班
    盧博堅;Lu, Po-chien
    Keywords: 機械心瓣;穴蝕;水錘效應;擠壓流效應;渦流效應;文氏效應;mechanical heart valves;cavitation;water hammer effect;squeeze flow effect;vortex effect;Venturi effect
    Date: 2008
    Issue Date: 2010-01-11 07:24:12 (UTC+8)
    Abstract: 從過去的研究指出,造成機械心瓣發生穴蝕的機制包括水錘效應(water hammer effect)、擠壓流效應(squeeze flow effect)、渦流效應(vortex effect)與文氏效應(Venturi effect)。由於加速狀態下的穴蝕現象較為明顯,同時過去尚未有學者針對加速狀態下的機械心瓣穴蝕成因做探討,因此本研究首先利用不同設計的機械心瓣在加速狀態下研究穴蝕的成因。然而加速狀態下無法說明真實體內的情況,因此本研究隨後在生理條件下針對包括直接量測造成穴蝕發生時的擠壓流與關閉後渦流效應與穴蝕的關係進行研究。機械心瓣在關閉的瞬間包含所有可能造成穴蝕的效應都可能會發生,因此難以了解這些效應何者為最主要的效應以及這些效應是否單獨能造成穴蝕的發生,因此本研究設計一個能模擬單葉片機械心瓣關閉的裝置,同時能將上述可能造成穴蝕的成因各自分離與合併進行研究。本研究中主要利用strobe lighting technique的方法觀察穴蝕汽泡影像、利用高頻之壓力計針對穴蝕汽泡的高頻壓力進行、利用高感度之CCD雷射位移計量測心瓣葉片的關閉速度、利用雷射都卜勒流速儀與質點影像流速儀來量測流場的速度與渦流現象。
    綜合上述各個研究的結果指出單獨水錘效應、擠壓流效應均可造成穴蝕的發生,但是單獨水錘效應需要較大之關閉速度才能造成。而當兩個效應伴隨產生時,此時所需要的關閉速度較低,此說明了擠壓流的效應應為機械心瓣穴蝕發生的主因,而水錘效應則有加成的效果。本研究中所直接量測心瓣關閉瞬間之擠壓流流速遠小於利用數值模擬計算出來之流速,造成此結果可能是因為流體加速度項所造成,然而此加速度項不易進行量測。本研究中量測心瓣關閉後渦流中心的壓降僅約25 mmHg,此不足以單獨造成穴蝕的發生,因此渦流效應應不是主因,但在穴蝕的發生上具有加成的效果。
    Prior research shows that the factors which induce cavitation include water hammer, squeeze flow, vortex and Venturi effects. Because the MHV cavitation in accelerated condition is more intense than in physiologic condition, and investigators never studied the mechanisms of cavitation formation in accelerated condition in past years. Therefore, in this study, we utilized the different design of MHVs testing under accelerated condition and studied the mechanisms related to cavitation at first. However, accelerated testing may not accurately reflect in vivo performance. Hence, we tried to measure the squeeze flow velocity directly and calculate the pressure drop during MHV closure related to MHV cavitation in physiologic condition later. During the MHV closure, it may combine several mechanisms to induce MHV cavitation formation. Because it is very difficult to study one of the mechanisms mentioned above alone, we designed a simplified model to simulate the closing behavior of a monoleaflet valve. This model allowed us to modify various parameters and measure flow velocities and pressure changes by separating or combining the various flow effects mentioned above. In this study, the strobe lighting technique (SLT) was used to capture the image of cavitation bubbles, the high-frequency pressure transducer was mounted very close to the leaflet surface to obtain the transient pressure signal, and the laser Doppler velocimetry (LDV) and particle image velocimetry (PIV) were used to measure the flow velocity and vortex phenomenon.
    Results showed that both squeeze flow and water hammer individually are sufficient to generate cavitation bubbles, with the latter requiring higher velocities. When both squeeze flow and water hammer effects are compounded, cavitation occurs at lower closing velocity. This indicated that the dominate mechanism which induce MHV cavitation inception is the squeeze flow effect, and the water hammer effect may play a minor role in MHV cavitation. The maximum pressure drop in the vortex center is roughly 25 mmHg. Since cavitation formation requires the local pressure to drop below vapor pressure, our results clearly showed that vortex formation cannot provide significant contribution to MHV cavitation.
    Appears in Collections:[Graduate Institute & Department of Water Resources and Environmental Engineering] Thesis

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