On the flame–flow interaction under distributed combustion conditions

Abstract

Colorless Distributed Combustion (CDC) has shown simultaneous benefits of high combustion efficiency, ultra-low pollutants emission, low combustion noise, uniform thermal field, and enhanced stability. Distributed combustion is fostered by reduced oxygen concentration and high temperature oxidizer to result in distributed reaction over a larger volume of the combustor and uniform thermal field. In this paper, the interaction between the velocity field (characterized through Particle Image Velocimetry) and the reaction region (identified through hydroxyl Planar Laser Induced Fluorescence) is investigated with focus on swirl assisted distributed combustion. A mixture of nitrogen and carbon dioxide was mixed with normal air upstream of the burner to simulate the hot reactive gases. The flowfield was characterized under non-reacting conditions to outline the impact of the added dilution on the flowfield. The results showed dilution to enhance both the inner recirculation and outer recirculation zones. Reacting flowfield, characterized to determine the impact of the temperature rise and density changes, showed maximum velocity region to shift downstream. Comparing the PIV data for reacting conditions with OH-PLIF revealed significant difference between normal swirl and CDC flames. In swirl flame, the flame was located around the shear layer of the entry jet (with both the inner and outer recirculation zones) where the velocity fluctuations and OH-PLIF fluctuations coincided. Flame transitioning to CDC pushed the reaction zone further downstream to locate at a position of lower velocity than what was found for swirl flames. In addition, the reaction zone occupied a much larger volume with lower signal intensity to exhibit distributed reaction. Experiments performed at same flow rates and velocities but with no reduction in oxygen concentration confirmed that the change in reaction behavior is attributed to the lower oxygen concentration rather than the increased flowrates due to dilution.

Review on MILD Combustion of Gaseous Fuel: Its Definition, Ignition, Evolution, and Emissions

“…There have been a number of specific definitions of MILD combustion and the like, which are high-temperature air combustion (HiTAC), flameless oxidation (FLOX), flameless combustion (FLC), and colorless distributed combustion (CDC). While HiTAC is related to inlet conditions, ,, the flameless , or colorless − modes are defined according to the appearance of the combustion, where no visible emission is detectable. Cavaliere and de Joannon well-justified the MILD combustion definition and made sensible comments on the HiTAC, FLOX, FLC, and CDC definitions.…”

“…Nonetheless, the definitions of these terms, made originally by different investigators, are based on various combustion properties (e.g., global and local parameters) and features (e.g., visibility and emissions). For instance, while HiTAC relies on burner-exit or furnace-inlet conditions, ,, FLC , and CDC − are judged by the combustion appearance, i.e., invisibility. So, the original boundaries of their regimes are either very loosely defined or inconsistent with each other.…”

“…Such technologies can concurrently satisfy both needs of high thermal efficiency and low exhaust emissions (e.g., NO x and CO) for application systems. , During 1990s, the MILD-like technologies were developed independently by Wünning and Wünning , (WS GmbH, Germany) and Nippon Furnace Kogyo Kaisha Ltd. (NFK-Japan) , collaborating with the International Flame Research Foundation (IFRF), , by different acronyms such as FLOX, , HiTAC, − CDC, FLC, which are to be specifically introduced in the next section. Afterward, the MILDC occurrence and characteristics have been highly extensively investigated in the international combustion community, particularly, by Blasiak’s group, , Cavaliere’s group, − Costa’s group, − Dally’s group, − Gupta’s group, − Kumar’s group, − Mi’s group, − Rota’s group, , Swaminathan’s group, − and Weber’s group, ,,− producing nearly 1000 research papers and reports in the literature (obtained by search online).…”

“…On the basis of the previous studies, ,,− this combustion is characterized by its large reacting volume in space, invisibility in appearance, good uniformity in temperature, ultralow emissions of carbon monoxide (CO) and nitric oxides (NO x ), and high silence and efficiency in burning. More specifically, the MILDC characteristics can be summarized below: Either fuel or oxidizer or both are well-diluted by recirculating the flue gas internally or externally prior to main combustion reactions taking place, so that this combustion regime occurs slowly and volumetrically while globally releasing heat homogeneously. The combustion-zone temperature distributes relatively uniformly without any significant local peaks, resulting in extremely low formations (and thus ultralow emissions) of NO x . When burning gaseous fuels and light oils, the entire furnace is bright and transparent without any distinctly visible flame fronts or spots. Moreover, this combustion regime always takes place highly steadily and silently. …”

“…To improve our understanding of this combustion, a great deal of research work on its definition, ignition, evolution, and emissions has been conducted experimentally ,,− ,− ,,,− ,,,− ,− , and numerically. ,,,− ,− ,,,− ,− ,− In particular, the ch...…”