An Experimental Investigation of Diluted and Highly Preheated Flames

Abstract

An effective method of reducing soot and harmful emissions from practical combustion devices is Exhaust Gas Recirculation which cycles the hot combustion products to heat and dilute the reactants of the chemical process. Due to the effectiveness of this practice, there is an ongoing effort to achieve a better understanding of combustion under preheated and diluted conditions with the aim of more accurately modeling and predicting the behavior of practical combustion devices. This study provides an experimental investigation of dimethyl ether/air jet flames subjected to both diluted and preheated conditions. This study analyzes both premixed and diffusion flames to comment on flame behavior. This investigation includes the design methodology and construction of a laboratory burner that can produce stable premixed and diffusion flames while controlling the reactant temperatures and oxygen concentration in the reactant stream. This burner, referred to as the Vitiator Assisted Preheat Oxygen Reducing (VAPOR) burner, has produced novel preheat temperatures of premixed, Bunsen-type flames. This study reports the chemiluminescence and formaldehyde PLIF measurements of the premixed and diffusion flames produced by the VAPOR burner. Both measurements are used to present observations on the effects of preheating and dilution on flame properties. The chemiluminescence measurements provide information on the geometric structure of the flames and are used to characterize the turbulent flame speed describing the global consumption rate of the premixed flames. The normalized flame height for premixed flames was measured to decrease by 19% as the preheat temperature was increased from 600K - 1000K and increase by 14% as the oxygen concentration was reduced from 21%-15% for a preheat temperature of 600K. The normalized flame height for diffusion flames was measured to decrease by 7% as the preheat temperature was increased from 600 K - 1100K. The normalized global turbulent premixed flame speed decreased by 37% as the preheat temperature was increased from 600K - 1000K with an oxygen concentration of 21%, and increased 13% as the oxygen concentration was reduced from 21%-15% for a preheat temperature of 600K. The PLIF measurements provide insight into the preheat layer structure of the flames at different conditions and describe the local turbulence behavior through the calculation of flame surface density, the surface area ratio of turbulent and laminar flames, and the local turbulent consumption rate. Both premixed and diffusion cases exhibited preheat layers that were distributed upstream to the combustor inlet at preheat temperatures of 900 K and 1100 K, respectively. The normalized local turbulent premixed flame speed was measured to increase by 17% as preheat temperature was increased from 600K - 900K. These measurements are accompanied by CHEMKIN studies that provided the characteristics of laminar flame speed and flame zone chemical structure. These results are viewed in the broader context of other experimental investigations studying the effects of preheat and dilution.