Dynamic Thermal Performances of Operating Limits in Heat Pipes
|關鍵字:||熱管;熱性能;動態測試;操作限制;啟動現象;Heat pipes;Thermal performance;Dynamic test;Operating limitations;Start-up phenomenon||公開日期:||2010||摘要:||近年來，由於半導體產業的蓬勃發展及相應提升的電子散熱需求，了解熱管性能已成為刻不容緩的課題。目前最具代表性的熱管測試法為檢測熱管之最大熱傳率及等效熱阻，此係傳統穩態測試法。此法已相當成熟且頗具指標性，然最大缺點為過於麻煩耗時。
為縮短檢測熱管性能之所需時間，本論文提出一全新動態測試法，期能取代傳統穩態測試法之使用。根據實驗觀測所得之暫態物理現象，本論文提出一組全新性能指標參數，分別為「decreasing slope of temperature difference S」，「dynamic descending rate M」，以及「maximum heating temperature Tmax」。藉由觀測折彎角度、填充率、及改變形狀對熱管動態及穩態性能指標的影響，動態性能指標參數之正當性已被驗證。實驗結果亦顯示相同因素對熱管性能的影響於動態測試及穩態測試間，存在非常顯著的相似性及類比性，即Tmax 與 Qmax 和M 與 Reff 之間的變化趨勢幾乎相同，數值模擬的結果亦合理解釋了動態測試之物理現象。
另一方面，本論文亦對各種熱管之操作限制進行動態測試研究。實驗結果顯示，熱管之啟動現象及黏滯極限可用「minimum transfer limit Qmin」及「minimum heating temperature Tmin」作為該現象之穩態測試及動態測試之性能參數指標，且熱管於較低的外氣溫度及熱源無循環的狀態較易達到動態黏滯極限。本論文亦針對熱管於一端瞬間置入冷源後的溫度反映進行研究。
進一步的，本論文嘗試以動態測試法研究熱管之音速極限，研究對象為一超薄之纖維毛細結構熱管(ultra-thin fibered heat pipe)。實驗結果顯示該熱管之溫度分布相當不典型，研判該熱管之音速極限與飛濺極限之操作範圍相當重疊。該熱管之溫度反應亦顯示該熱管於啟動階段，前後經歷了黏滯極限、音速極限、以及音速-飛濺混淆極限。
In recent years, a great need to know the thermal performances of the heat pipes emerges due to the flamboyant advancements in semiconductor industry and the associated booming needs of electronic cooling. A representative way to investigate the thermal performance of a heat pipe is to find its maximum heat transport capacity Qmax and the effective thermal resistance Reff. This method is referred to as conventional steady-state test, and is essentially indicative and matured. However, it’s too time/labor consuming and troublesome. With a view toward shortening the necessary time to examine the thermal performances of heat pipes, a novel dynamic test methodology is originated and developed in the present thesis, and the objective is to substitute the use of conventional steady-state test. A set of dynamic parameters of thermal performances of heat pipes is ideated from the observed transient phenomenon. They are “decreasing slope of temperature difference S”, “dynamic descending rate M”, and “maximum heating temperature Tmax”. The validities of the above parameters are verified by comparing the experimental results with those of the steady-state test through investigating the effects of bending angles, fill ratios, and shapes of heat pipes. It is found that the parameters and the influences of factors between the steady-state test and the dynamic test are remarkably analogous. The same trends could be found between Tmax and Qmax, and between M and Reff no matter which effect is studied. The numerical simulation results also reasonably explain the physical phenomenon of dynamic test. On the other hand, finding the operating limitations using the dynamic test is also of interests. Therefore, another group of heat pipes and operating conditions are investigated. Experimental results show that the start-up phenomenon and the viscous limitation could be indicated by “minimum transfer limit Qmin” and “minimum heating temperature Tmin”. It is found that the viscous limitation would be easier encountered, observed, and compared at a lower ambient temperature plus a non-circulated water heat source. Heat pipe subjected to a sudden cooling load is also investigated. Further, the sonic limitation of heat pipe is also investigated by dynamic test. An ultra-thin fibered heat pipe is adopted. Experimental results demonstrate that the operating ranges for sonic limitation and entrainment limitation overlap each other to a respectable extent for this heat pipe. The temperature responses indicate that the heat pipe experiences the transition from viscous limitation, sonic limitation, and sonic-entrainment confounded limitation during the start-up phase. Very non-typical profiles of temperature distributions are found for this heat pipe at all the conditions investigated. The corresponding explanation is addressed in the thesis. On the whole, the complete methodology and concepts for dynamic investigating the thermal performances and operating limitations of heat pipe is established and developed. The parameters and their physical meaning are testified and expounded. Conclusively speaking, the proposed dynamic test methodology is sufficient to examine the common miniature heat pipes when quality control or a pre-design of the heat pipe are needed. Empirically speaking, only 10~15 minutes are necessary to examine a heat pipe using the proposed dynamic test methodology. This is much more efficient than the steady-state test. Therefore, the proposed dynamic test could be adopted instead of the steady-state test to determine the thermal performance of heat pipes when high efficiency is of prior concern.
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