鈦液滴在空氣中的燃燒數值模擬

Abstract

奈米二氧化鈦為廣泛使用之奈米粉體，製造奈米二氧化鈦的方法有許多，其中氣相合成技術為有潛力的製造二氧化鈦技術之一。 在本研究中，發展了一個鈦液滴在空氣氣流中燃燒之數值方法，本方法包含熱力學放熱模型並以計算流體力學軟體STAR-CD模擬流場、液氣界面之熱傳與質傳現象，以分層近似方法描述液氣界面之熱傳與質傳之邊界條件，並求取非穩態之流場、溫度場、濃度分佈。最後在氣相中求解GDE方程式(包含核凝、膠結與表面成長)以預測形成之二氧化鈦微粒之粒徑分佈。 結果顯示，氧氣被鈦液滴完全吸收，而氮氣溶解度受存在於液滴中氧的影響且有氮氣的釋放，內部環流對氮、氧原子在液滴內部的傳輸有重要的影響。液滴產生的TixOy蒸氣，由求取此系統中最小吉布斯自由能得知，蒸氣中存在最多的物種為TiO2。此研究中計算了TiO2蒸氣在空間中的分佈，發現二氧化鈦蒸氣過飽和區開始於約距離液滴表面大於17μm之外的位置。微粒成長於燃燒初期主要以燒結、聚合現象為主，而後以膠結現象為主，二氧化鈦一次微粒之數目中間粒徑(CMD)介於8.80-22.84nm，幾何標準偏差(GSD)介於1.49-1.52間，模擬結果與實驗結果相符。

Nano-TiO2 particles are one of the most widely used nanopowder. There are many methods for producing nano-TiO2 particles, in which the gas-phase synthesis is one of the most promising method. In this thesis, a numerical method has been developed to simulate the combustion process of a single metal titanium droplet in the ambient air. This model consists of a thermodynamic heat release submodel, the STAR-CD CFD package was used to simulate the flow field, heat and mass transfer in both the gas and liquid phases. Furthermore, a multiple thin layer boundary description was used to consider the heat and mass transfer across the liquid-air phase boundary. The transient flow field, temperature field, species concentration distribution were solved. The formation of the TiO2, including the nucleation, coagulation and surface growth, was simulated by solving the GDE equation. Simulated results indicate that oxygen absorption by the Ti droplet is perfect but the solubility of nitrogen is affected by existing oxygen in the droplet. Besides, nitrogen will be released from the Ti droplet. Internal circulation plays an important role on the uniformity of O and N atoms in the droplet. By calculating the minimum Gibbs free energy in the TixOy system, TiO2 is found to have the highest concentration in TixOy vapor. The concentration field of TiO2 shows that TiO2 supersaturation positions start from 17 μm behind the Ti droplet surface. In the initial stage of combustion, the major mechanisms of particle growth are sintering and coalescence, which coagulation is the major mechanism at a later period. The count median diameter of the primary particles of TiO2 is found between 8.80-22.84 nm, and geometric standard deviation is between 1.49-1.52. The simulated results are in good agreement with the experimental data.