燃燒效應對於背向階梯流場駐焰特性之影響

本文採用大渦數值模擬的方法，計算具壁面噴流之背向階梯流場，並探討燃燒效應對於流場駐焰特性的影響，而類似的研究鮮見於公開文獻。本文之計算方法係採用有限體積法，配合交替使用之MacCormack與修正型Godunov數值方法，搭配Smagorinsky次尺度紊流模式來求解可壓縮Navier-Stokes方程式，文中使用之燃燒模式爲有限快、單步驟化學反應模式。在本研究中，藉由數值視流技巧（煙線、徑線及瞬時渦度場）將流場結構視覺化，以瞭解入口空氣與壁面噴流之混合機制，並追蹤流體質點來決定入口空氣與壁面燃料於迴流區中的駐留時間。經研究發現燃燒效應對於左右流場特性之渦旋結構及其動態均有相當大的影響力，不但會減弱剪層引入之能力以及渦旋結構之渦度，因而延遲空氣與燃料之混合效率，更會分別延長壁面噴流流體質點及縮短入口空氣流體質點於迴流區內之駐留時間，進而減少迴流區與其外部流場間質量、動量及能量之交換。

關鍵字

背向階梯；駐留時間；渦旋結構；數值視流

並列摘要

In view of lacking flame-holding characteristic studies of a backstep flow using numerical simulations, the present study therefore performs a large-eddy simulation on such an issue that is important for combustor design. The time-dependent 2-D compressible conservation equations were solved directly using the Smagorinsky's subgrid-scale turbulence model. The numerical code used the finite volume technique, which involved alternating in time the second-order, explicit MacCormack's and modified Godunov's scheme. The main task is to reveal the dynamic flow structure and mixing mechanism through numerical flow visualizations such as streaklines, pathlines and instantaneous vorticity contours. The intra-separation bubble residence times of both air and fuel particles released from the axial air inlet and bottom porous wall, respectively, are subsequently quantified based on probability density functions. The vortices dynamics and vortical structures, which are predominant to the flow patterns, are influenced by the combustion effects. The shear entrainment and the vorticity of the vortices are both weakened by the heat release to cause the reduction of fuel-air mixing efficiency, the increase of the fuel particles' residence time, and the decrease of the air particles' residence time suggesting the lost of turbulent heat, mass, and momentum transfer between outer flow and separation bubble.

並列關鍵字

Backstep flow ； Residence time ； Vortical structure ； Numerical flow visualization

國際替代計量

燃燒效應對於背向階梯流場駐焰特性之影響

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