COMBUSTION OF LOW CALORIFIC FUELS IN HIGH TEMPERATURE OXYGEN DEFICIENT ENVIRONMENT

摘要

Combustion characteristics of two different gaseous fuels (a low calorific value fuel and methane fuel)have been examined. The experiments are performed in an experimental high temperature air combustionfacility using gas fuel jet injected to a transverse flow of high temperature combustion air having lowerthan normal oxygen concentration. The composition and temperature of the combustion air flow can becontrolled and varied in order to simulate phenomena that occur in high temperature air combustionconditions. The momentum flux ratio between the fuel jet and the combustion air flow was kept constantto provide similarity between the different experimental cases and to understand the role of fuel jetproperty on mixing and combustion. Direct flame photography was used to obtain information on theglobal flame features. A 2-D Particle image velocimetry (PIV) was used to provide information on theinstantaneous flow field for the different cases investigated. This diagnostics allowed information onmean and rms axial and radial velocity, axial, radial and total strain rates and vorticity. The NOXemissions levels were also recorded.The results provide detailed information of the fluid dynamics that occurs during high temperature aircombustion conditions. The results indicate a slower mixing during high temperature air combustion witha low calorific value fuel as compared to methane fuel. The results showed higher turbulence levels andhigher axial strain rates for low calorific fuel jets as compared to methane fuel jet during the hightemperature air combustion condition. This leads to less intense (or mild) combustion conditions toprovide increased flame length and volume and lower NOX emissions for the low calorific value fuel.Even for the normal methane fuel high temperature air combustion conditions mean reduced emissions ofNOX. In contrast the high temperatures obtained for methane combustion leads to lower vorticity andaxial strain rates than the low calorific value fuel as the stronger heat release suppresses the formation ofvortical structures. In the case of low calorific value fuel, higher fuel jet velocity into low density hightemperature air leads to prolonged jet length. This jet causes a local stagnation to the upstream cross flowto create local higher value of turbulence levels immediately upstream of the jet. In this upstream regionof the jet, the turbulence levels, axial strain rates, and vorticity are much higher for the low calorific valuefuel than methane fuel.