高溫對流場中多成分生質油滴暫態燃燒之數值分析

Transient Combustion of Multicomponent Biofuel Droplets in a Hot Convective Environment

劉昭忠(Chao-Chung Liu) ； 王宣富(Hsuan-Fu Wang) ； 蔡欣倫(Hsin-Luen Tsai)

《燃燒季刊》 81期 (2013/05) Pp. 27-43

https://doi.org/10.30041/CQ.201305_(81).0003

摘要

本研究以數值方法計算多成分生質油滴的燃燒過程。在數值方法上以hyperbolic tangent function產生不均勻的格點分佈，並以有限體積法對氣液兩相的統御方程式做離散化，採用SIMPLER演算法疊代求出氣液兩相之流場，而對流項、黏滯項和擴散項等相關物理量皆以Power-law Scheme處理，在燃燒過程中，油滴變小所需的移動格點，與外界流場因阻力作用而氣流減速均有被考慮。在與文獻資料驗證吻合後，計算n-octane和ethanol不同比例組成油滴的燃燒過程。當油滴內部經過加熱後，會在油滴後部的氣相流場中點火燃燒形成尾部火焰，隨著時間增加，因油滴和周圍流場阻力之故，使兩者間的相對速度逐漸變小，導致油滴之火焰形態，從尾部火焰經過渡火焰至包圍火焰。在加熱過程中，油滴內部流場會有渦流產生，溫度和不同成分的濃度，因受流場作用，也會依渦流形狀分佈。比較三種不同組成的多成分油滴蒸發率，發現因n-octane和ethanol不同混合比造成油滴之化學反應速率和混溶後燃燒熱的大小變化，對生質油滴火焰變化型態和蒸發率快慢有其重要的影響。

關鍵字

多成分 ； 生質油 ； 油滴

並列摘要

Unsteady combustion and internal heating of a multicomponent biofuel droplet in a high-temperature flow are studied numerically and validated consistently with the literature. In the developed numerical method, the governing equations of the gas and the liquid phases are fully solved and linked at the interface. In addition, a moving grid system and the particle deceleration are also considered to account for the droplet regression and impact of drag force. The obtained results show that the convective droplet is ignited with a wake flame after a heating-up period. With increasing time the relative velocity between the droplet and surrounding flow becomes smaller due to the drag. As a result, the flame changes from a wake type, through a transition type, eventually to an envelope one. With regard to the droplet interior, internal heat transfer and composition of different mass fraction are always dominated by internal circulation and unsteady heating prevails over the entire droplet lifetime. In comparison with the vaporization fluxes of three different blending biofuel droplets, the numerical results reveal that the single-step global finite rate chemical reaction and mixed combustion heat of n-octane and ethanol biofuel blends play the important role for dominating the transient behavior of biofuel droplet combustion.

並列關鍵字

Multicomponent ； biofuel ； droplet