Model And Its Application Of Trapped Vortex Array In Enhanced Mixing Of Flammable Gas Flow

A model of trapped vortex array in enhanced mixing of flammable gas flow was proposed aiming to extend the application of lean premixed combustion in combustors with high temperature rise, hydrogen containing fuel or compact structure. A well designed profile was employed in this model to construct a controllable flow pattern which was able to enhance the mixing of flammable gas without the risks of flash back or flame holding.The flow, mixing, anti-flame holding mechanism and acoustics characteristics of this trapped vortex array model was analyzed. The flow pattern was stable and there was a weak correlation between the length of recirculation zone and Reynolds number of incoming flow. The fuel-air mixing process was greatly enhanced for the mixing non-uniformity index could drop to 0.8%　after two cascades of vanes. The study also revealed the sub-critical stability nature of reaction in this model and clarified that the anti-flame holding performance could be achieved when the Damkoler was less than 2.23 for a methane-air mixture with phi=0.55～1 and T=300～700K. This model exhibited great damping ability to acoustics waves, its transmission loss was 20dB higher than a uniform tube.A conceptual design of premixer for gas turbine combustors was proposed based on the two dimensional model flows. The performance of this premixer was investigated. It was indicated that the mixing non-uniformity of the arrayed-vanes premixer was only a half of that of the conventional swirl-based premixer despite the axial length was shorted by 30%. The experiments showed that the lean premixed flame was stable and quiet downstream the arrayed-vanes premixer. This premixer was compatible with both hydrogen containing fuel and methane. The turn down ratio was no less than 3.0. For the fuel with hydrogen concentration less than 60%, the NOx emission was less than 50mg/Nm3(@15%O2) when the burned gas temperature reached 1700K.A test of gas turbine combustor was carried out using arrayed-vanes premixer. It was showed that this trapped vortex array model was suitable for the structure and operation strategy of the conventional multi-nozzle combustor. The lean premixed flame was stable and the adjustment of the combustion modes went smoothly. It was measured that the NOx emission was less than 47mg/Nm3(@15%O2) when the exhaust temperature of the combustor was between 1400K and 1615K. It was also known that the combustion noise was successfully suppressed for the peak value of dynamic pressure within the primary zone is less than 0.42%　of the inlet pressure.