MEASUREMENT OF THE MIXING QUALITY IN PREMIX COMBUSTORS

The problem of mixing fuel and air is the essential point of low emiss ion combustion in gas turbines. In premixed combustors and fuel staged combustors the quality of the fuel-air mixture is the determinant par ameter for the amount of emissions of nitric oxides (NOx). The nearly perfect preparation of the fuel-air mixture is also a condition for tr ouble-free operation in catalytical combustion. Prevaporization of liq uid fuels hampers the process of mixing. So the investigative work at the Department of Steam and Gas Turbines at the University of Bochum c oncentrated on experiments with liquid fuels. The results show that th ere is a great potential of reducing NOx emission even with liquid fue ls and reveal the key role of prevaporization and mixing. The experime nts were carried out at a premix combustion test rig at moderate press ure. By using the technique of planar-laser induced fluorescence (LIF) , highly time and spatially resolved measurements of fuel concentratio ns were yielded from the experiments. The optical measurements showed the structure of the mixture held of fuel and air in the zone downstre am of the flameholder. The pollutant emissions were simultaneously mon itored with conventional gas analysers. As the main result, the strong dependence of the pollutant emissions on the mixture could be clearly revealed. On one hand the homogeneity over the cross-section of the c ombustor was the main condition for low emission combustion. On the ot her hand the time-resolved two-dimensional LIF images of the turbulent mixture field showed that the instationary distribution also had a co nsiderable influence on the rate of emissions. Even the mixture of sta tic mixers contained spatial and temporal inhomogeneities, which could be observed by using the LIE-technique but not with conventional meth ods. (C) 1998 Elsevier Science Ltd. All rights reserved.