A Comprehensive Study of Multiple Interactive Parallel Premixed Methane Slot Jet Flames on Lean Combustion

• Main Authors: Ho-Chuan Lin, 林河川
• Other Authors: Yei-Chin Chao
• Format: Others
• Language: en_US
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/71496489633220071370

博士 === 國立成功大學 === 航空太空工程學系碩博士班 === 97 === This study aims to investigate interactive parallel lean and sub-limit lean premixed methane-air flames issued from two or three rectangular slot burners with variable jet spacing, equivalence ratio and burner exit speed. The twin-jet flames consist of two identical slot flames which are called symmetrical interactive flames. The triple-jet flames, with a same jet spacing, consist of two identical slot flames in outboard side on lean combustion and one center flame on sub-limit lean combustion in the center. The outboard and center jet flames form non-symmetrical interactive flames. The flowfield and combustion chemical reactions are predicted by detailed numerical simulation with Skeletal and GRI–Mech 3.0 reaction mechanisms. Numerical results such as velocity streamlines, temperature, flame height and flame shape are validated with those obtained by experimental particle image velocimetry (PIV) and flame measurements. When moved closer beyond a threshold jet spacing, twin-jet flames become interactive and both flames tilt outward in appearance. This symmetrical flame interaction provides a wider operation range. Numerical predictions found that there are three different interactive flow fields: entrainment, recirculation and reverse flows according to jet-to-jet spacing. At the reverse flow stage, a stagnating flowfield termed lateral impingement is generated along the symmetrical axis between the flames, which is similar but not identical to that found in the counterflow flames. Regardless of the extent of flame interaction or the degrees of flame tilting angle, the interacting postflame flowfield creates a restriction to slow down itself, reduces the convective heat to the down stream and increases the conductive heat transfer to the upstream unburned mixture. This is the main mechanism to enhance the flame stabilization, especially in lean conditions. The stabilization mechanism of the interactive twin-jet flames is also enhanced by other crucial factors such as low dissipation of heat, inter-change of chemical species between two flames, and import chemical species from main flame to burner rim. In contrast to the twin-jet flames, the triple flames, with jet spacing L/d = 2 and equivalence ratio of φ = 0.88 outboard and φ = 0.7 ~ 0.3 for inboard flame, can be classified as a non-symmetrical lateral impingement. The interactive postflame consists of one weak flow in the center and one strong flow in the outside. The interactive thermal field also combines one hot flow and one cold flow. It is verified that the sub-limit lean center flame is mainly sustained by the hot products of outboard lean flames. The strong outboard postflame twists the flow direction of center postflame in a sharp angel to the flame sheet. Usually the flow direction is nearly normal to the flame sheet. This twisted flame is called cascade flame. The cascade flame accommodates the slow flame speed of the sub-limit mixture, extents the residence time and allows the chemical species to perform the cross streamline combustion. The cascade flame is a kind of negative lateral impinged flame. The center flame base is supported by the outboard flame with the same method as the twin-jet flames. This sub-limit lean flame is mainly burnt through the path of HO2 / CH3O with branching reaction step of O2 + H-> O + OH. The experimental study of sub-limit lean combustion is difficult to conduct due to the strong contrast ratio between two non-symmetrical flames. Thus, the current numerical method is one of the feasible methods to study the sub-limit lean combustion.