Thermal and emission characteristics of an inverse diffusion flame with circumferentially arranged fuel ports

Abstract

An inverse diffusion flame (IDF) exhibits the characteristics of both partially premixed flame and diffusion flame. However, it is less liable to liftoff or flashback compared with premixed flame, and it burns cleaner compared with diffusion flame. The study is mainly experimental in nature. The objective is to determine the structure, thermal and emission characteristics of an IDF with circumferentially arranged fuel ports (CAP). Test rigs were set up to generate the CAP IDF, to measure the flame length, the flame temperature and the combustion species concentrations of the free jet; and to measure the heat flux of the impinging CAP IDF. According to the flame appearance under the influence of the air jet Reynolds number, Reair, and the overall equivalence ratio, o, the CAP IDF was classified into six types. Among them, the Type IV and Type V which occur at Reair > 2000 are of particular interest. They consist of a short entrainment zone and an elongated flame tower, separated by a narrow neck. Fuel is entrained and impinges on the central air jet, thus enhances air/fuel mixing which improves the flame's thermal performance. Type V flame has a dual flame structure due to its high level of fuel supply. The flame lengths were measured and an empirical relationship was derived to correlate the normalized flame length with the ratio of the air and fuel jet velocities. The flame temperature and centerline O2, CO2, CO and NOx concentrations were measured to provide information on the thermal and emission characteristics of the CAP IDF. There is a significant reduction of centerline NOx concentration in the fuel-rich combustion zone of the Type V flame at o>=2.2, which is an indication of NOx reburning. The same phenomenon was also observed in the analysis of the NOx emission index (EINOx). An empirical relationship correlating the EINOx formation rate per unit volume with o was derived. The shape, thermal and emission characteristics of the CAP IDF were compared with an equivalent CoA IDF, with co-axial air and fuel jets. The results show that there is always a better air/fuel mixing in the CAP IDF. The CAP burner tends to shorten the flame length and extends the partially premixed combustion characteristics to broader ranges of Reair and o. Moreover, the temperature of the CAP IDF is generally higher, indicating more intense combustion. The heat transfer characteristics of the impinging CAP IDF were investigated. The results show that the maximum heat flux is always associated with the visual partially premixed reaction zone impinging on the target surface. While in the fuel-rich combustion region and the post-flame region, a relatively flat heat flux plateau is formed which is an advantage for heating applications. A numerical simulation on the non-reacting flow, as well as on the reacting flow, of the CAP IDF was carried out to supplement the experimental results. The non-reacting flow simulation enhances our understanding on the air/fuel mixing effectiveness. The reacting flow simulation shows that the simulated results agree well with the experimental results at both o=1.0 and o=1.6, except that the simulated temperature is under estimated at the central region. In summary, this study has clarified the visual, physical and thermodynamic structure of an inverse diffusion flame with circumferentially-arranged fuel ports, which is a novel design. The NOx-reburning mechanism has been identified in the IDF having a dual flame structure, which is useful for low-NOx combustion applications. Two useful empirical correlations, one for the flame length, and the other for the EINOx formation rate per unit volume of the flame, have been developed.