COMBUSTION OF NORMAL AND LOW CALORIFIC FUELS IN HIGH TEMPERATURE AND OXYGEN DEFICIENT ENVIRONMENT

摘要

Combustion characteristics of two different gaseous fuels (a low calorific value fuel and methane fuel) have been examined using high temperature and low oxygen concentration combustion air. The momentum flux ratio between the fuel jet and the combustion airflow was kept constant to provide similarity in mixing between the different experimental cases to understand the role of fuel jet property on combustion. Direct flame photography, 2-D Particle image velocime-try (PIV), Light Emission Spectroscopy and chemiluminescent NO_X analyzer was used as the diagnostics. These diagnostics allowed information on global flame features, mean and rms components of axial and radial velocity, axial strain rates and vorticity, the spatial distribution of combustion intermediate species, such as, OH and CH, and overall NO_X emission levels. The results indicate a slower mixing during high temperature air combustion with low calorific value fuel as compared to methane fuel. The results showed higher turbulence levels and higher axial strain rates for low calorific fuel jets as compared to methane fuel jet during the high temperature air combustion condition. This results in less intense (or mild) combustion conditions with the result of increased flame length and volume and lower NO_X emissions. Even for the normal methane fuel high temperature and oxygen deficient combustion conditions provided lower NO_X emission. Furthermore, the high temperatures obtained for methane combustion provided lower vorticity and axial strain rates than the low calorific value fuel due to the suppression of vortical structure formation from the stronger heat release. In the case of low calorific value fuel, higher fuel jet velocity into low-density high temperature air leads to longer jet length. This jet causes a local stagnation to the upstream cross-flow to create local higher value of turbulence levels immediately upstream of the jet. The spatial distribution of the flame generated radicals (OH and CH) revealed significant ignition delay of the LCV fuel jet and a far more uniform distribution of the intermediate species. The methane fuel jet showed a prolonged reaction zone and faster ignition at high temperature and oxygen deficient conditions when compared to normal temperature air combustion of methane.