59061 - Flow Fields, Emission and Stabilization in Premixed Centrally-Staged Swirl Flames With Different Air Split Ratios 

The flow distribution is crucial to the performance of combustors, especially the air split between the pilot and main stages in centrally-staged swirl combustors. The flow fields, emission levels and lean blowout (LBO) limits were investigated experimentally under different air split ratios (ASR, the ratio of the pilot stage air mass flow rate to the total air mass flow rate) at a fixed equivalence ratio of 0.8 in premixed centrally-staged swirl flames.

The flame macrostructures were captured by a CH* chemiluminescence high speed camera and the corresponding results were processed by Abel deconvolution. In addition, the reacting flow fields were obtained by planer Particle Image Velocimetry (PIV) technique for a better understanding on the aerodynamic structure of the centrally-staged swirl flames. There are three types of recirculation zones: primary recirculation zone (PRZ), lip recirculation zone (LRZ) and corner recirculation zone (CRZ). The size of the reacting primary recirculation zone (PRZ) becomes larger as more air is distributed to the pilot stage. This is caused by the fact that the majority of the pilot fluid participates in formation of the PRZ and the penetrability of the pilot jet becomes stronger. The main flame is stabilized upon the LRZ and the pilot flame anchors at the lip structure.

The emission levels of NOx and CO were measured by a gas analyser. It turns out that the global emission of the NOx increases while the CO decreases, which is because of a longer residence time of the radicals within a larger PRZ. On the other hand, an increased residence time allows the reactions more complete and accelerates the oxidation process of CO to CO₂ which decreases the CO emission levels.

The LBO limits of centrally-staged swirl flames are also studied at an equivalence ratio range of main and pilot stage from 0.8 to 0.1. The results show that the boundary of the LBO limits is extended and more flames can be stabilized at a lower pilot equivalence ratio as the air split ratio increases, which demonstrates the stabilization effect of the pilot flame because it can provide radicals and heat for reactions. The bulk velocity of the main stage fluid reduces also leading to an easier way to stabilize the flame. Most importantly, the larger size of the PRZ contributes to provide a more stable area for the reactions and eventually can broaden the stability boundary.

In brief, a larger air split ratio can provide a bigger PRZ and thus can provide a more stable reaction area, which can broaden the LBO limits eventually. However, NOx emissions need to considered to select the best compromise between flame stability margins and reduced emission levels. The above findings can be a reference to choose an appropriate air split ratio in the early stage design of areo-engine combustors.