應用微奈米結構增強沸騰熱傳

Abstract

沸騰是一個常見的相變化過程。由於其有效的熱傳特性，沸騰熱傳已被廣泛應用於能源系統 中。例如全世界大約百分之四十的熱機(heat engines)利用朗肯循環(Rankine cycle)。沸騰熱傳的 效用被臨界熱通量(Critical Heat Flux)與熱傳導係數(Heat Transfer Coefficient)所決定。臨界 熱通量決定熱機的最大產生電力而熱傳導係數決定沸騰熱傳的有效性。儘管已經有很多的研究致力 於提升沸騰熱傳的性能，目前實驗所得到的最高的臨界熱通量與熱傳導系數還是比根據動力學所預 測的理論計算最大值低許多。此外由於沸騰是一個相當複雜的物理現象，研究學者們對於產生臨界 熱通量的物理機制還不是完全了解。本研究計畫將學習在覆蓋微\奈米線陣列的表面上的池沸騰現 象。由於此微奈米線陣列可以提供很大的毛細力，理論上在此表面上應可得到較高的臨界熱通量。 此外本計畫將藉由改變微\奈線的大小及形狀，有系統的檢驗臨界熱通量產生的物理機制。本計畫另 提出一多尺度的流體系統用以提升沸騰熱傳的效能。此多尺度的流體系統包含一微尺度流體管道陣 列與另一微\奈米線副陣列。此微尺度流體管道陣列可以減少此系統的流體阻力，同時此微\奈米線 副陣列可以提供大的毛細力。此設計理論上可以提升此系統的最大毛細限(Capillary Limit)因而可 以提高沸騰熱傳的臨界熱通量。此外此多尺度流體系統將產生許多的微尺度空穴於此微管道的交界 處，此大量的微尺度空穴可以作為沸騰所需的表面空穴，因此在此表面應可提高其熱傳導係數。由 於以上原因，此多尺度流體系統應可提高沸騰熱傳的臨界熱通量與熱傳導係數。

Boiling is a common phase-change process. Due to the effectiveness of heat transfer, boiling is widely exploited in power generation. For example, about 40 percent of the total power generated by heat engines in the world is through the Rankine cycle. The efficacy of the boiling heat transfer is represented by the critical heat flux (CHF) and the heat transfer coefficient (HTC). The CHF sets an upper limit of the maximum-generated power of the Rankine-cycle heat engines; the HTC determines the effectiveness of the boiling heat transfer. Despite intensive studies on the enhancement of boiling heat transfer, the values of CHF and HTC are still far below the theoretical predictions of kinetic theory. In addition, due to the complex process of boiling, the mechanism causing CHF eludes our full understanding. In this project, I plan to study pool boiling on surfaces coated with micro/nano wire arrays. The large capillary force provided by the micro/nano arrays could hypothetically enhance the CHF. The mechanism causing the CHF will be systematically analyzed by tailoring the morphology of the micro/nano wires. Furthermore, a multi-scale flow system is proposed to further enhance CHF and HTC, simultaneously. This multi-scale flow system is composed of an array of micro-channels and a sub-array of micro/nano wires. The array of the large micro-channels can reduce the overall flow resistance, whereas the sub-array of the micro/nano wires can provide a large capillary force. Thus, a larger capillary limit can be obtained. In addition, a large number of micro-scale cavities formed at the intersections of the micro-channels can be the activated nucleation sites at a low wall temperature. As a result, the CHF and HTC can be greatly enhanced by the multi-scale flow system.