Numerical and Experimental Study of Turbulent Mixing in Colorless Distributed Combustion Systems

Abstract

Development of efficient and low-emission colorless distributed combustion (CDC) systems for gas turbine applications require careful examination of the effects of various parameters on the fuel-air mixing and combustion in these systems. These effects are systematically studied by experimental and numerical simulations of turbulent flow, mixing and combustion in a laboratory-scale CDC system. The high air temperature CDC has shown to significantly reduce the NOx and hydrocarbon emissions while improving the reaction pattern factor and stability with low pressure drop and noise without using any flame stabilizer. Numerical simulations conducted in this study are based on the large eddy simulation (LES) and high order numerical methods. The main objective is to investigate the flow field and fuel-air mixing within the combustor by LES for the same conditions as experiments. The numerical results establish the reliability of the computational model for the CDC simulations. They also indicate the importance of fuel injector configuration and show the effects it has on the velocity, scalar and temperature fields within the combustor. HE Colorless Distributed Combustion (CDC) has been demonstrated to provide ultra-low NOx and CO emissions, improved pattern factor and reduced combustion noise in stationary gas turbine combustors. 1-3 The flame in the CDC is homogeneous and is formed in the entire combustion volume instead of a limited reaction zone. 1-4 The key factor to achieve CDC is the controlled flow distribution, reduce ignition delay, and rapid injection and mixing of fuel and air jets to promote the distributed reaction in the entire combustion volume without any flame stabilizer. 1-3 Large gas recirculation and high turbulent mixing rates are desirable and helpful for achieving the distributed reaction and for avoiding hot spot zones in the flame. 9 Air and fuel jets are normally injected separately to prevent the formation of thin reaction zones between the heated air stream and the fuel jet. Both air and fuel jets entrain hot and diluted burned gases to a desirable degree and with controlled shear layer mixing, to establish “complete” mixing of fuel and oxygen and suitable fuel-air mixture temperature in the entire combustor priori to autoignition 1 . The CDC may be established by various fuel and air injection configurations. The performance of non-premixed CDC for various geometries and fuel-air injection configurations has been studied experimentally at the University of Maryland (UMD). The focus of this study was to develop CDC for stationary gas turbine combustors, operating in the thermal intensity range of 5–85 MW/m 3 -atm. The thermal load was scaled with volume as well as the operating pressure. 7,8 The present work uses the LES model for detailed and systematic study of the turbulent flow pattern and fuel-air mixing in the UMD CDC system. The numerical predictions are first compared with the available experimental data for the case without fuel jet injection. The CDC system is then simulated by LES for various fuel-air jet configurations, and inflow air conditions.