Weak Extinction in Low NOx Gas Turbine Combustion

Well mixed low NOx gas turbines are limited, in the operational range of the low NOx mode, by the weak extinction and CO limits of the flame stabiliser used. The operational range of the combustor in the <10ppm low NOx mode is set by the range of equivalence ratios over which ultra low NOx without acoustic resonance can be achieved. This paper reviews the available data on weak extinction in well mixed low NOx combustion systems and presents some new data. Atmospheric pressure weak extinction data is shown to be similar to weak extinction at pressure for similar stabiliser designs and reference velocities. For low NOx gas turbine combustion it is demonstrated that all the best weak extinctions are identical to the lean flammability limit for laminar flames. Weak extinction is where the flow velocity exceeds the turbulent burning velocity and data on weak extinction is used as a measure of the mean turbulent burning velocity and shown to correlate with turbulent burning velocity data and theories. Methods of predicting the peak turbulence generated downstream of a flame stabiliser are outlined, based on grid plate measurements of turbulence and pressure loss. It is shown that a wide range of premixed flame stabilisers including swirling and non-swirling flame stabilisers have a weak extinction that can be predicted using this method.