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This paper describes the development phases of an annular type combustor for heavy-duty gas turbine applications. High cycle efficiency and low emissions are required over a wide range of load conditions, with the consequence of reducing margin to thermo-acoustic instability onset and lean blow-out. In addition, in lean premixed combustors, the increased fuel air mixing times required to keep emissions low, may lead to undesired ignition or flashback into the fuel burner ducts. All these aspects are matter of this work and focus is on fuel burner design modifications which allowed dry emissions reduction while maintaining a sufficiently wide safe operation window. A synergic effort has been put in place, involving experimental campaigns and CFD simulations, with the purpose of assessing design changes initially and doing screening. In the meanwhile, numerical practices have taken benefits form the experience growth. Results of past work on similar components has been leveraged too. Test campaign involved different scale facilities, from single burner through full annular combustor up to full scale prototype engine. The progressive reduction of viable option for combustor components design changes, due to high impact of such modifications during the gas turbine late development phases, forced designers to concentrate efforts onto fuel burner optimization, looking for efficient ways to implement modifications and assess their effectiveness of combustion system performances. Emissions trends, blow-out and flashback margin for several burner designs are reported. Numerical analysis results are also shown, which revealed to be well aligned with the experimental outcomes, allowing burner optimized solution to be identified. Finally, characterization with respect to fuel gas composition is shown as well as sensitivity to different operating conditions.

This work was aimed at measuring the resulting electric forces acting on a gas bubble growing quasistatically, with gas supplied to it through a circular orifice, and to compare them with their theoretical counterparts. An experimental apparatus has been set up, consisting essentially of an orifice drilled in a flat stainless steel plate submerged in the test fluid (FC-72). A dedicated gas injection system allowed creating slowly growing or even static bubbles of any desired volume, up to the detachment volume. An electric field could be imposed by means of a washer-shaped electrode laid parallel to the surface and centered on the orifice. The apparatus could be operated with the orifice both in upward and in downward direction to investigate the favorable or adverse role of the buoyancy force. Data were acquired via a highresolution video camera, equipped with a microscopic lens, and were digitized and processed via dedicated software, implemented in Matlab. The resulting forces acting on the bubbles were derived from bubble shape and size. The data in the absence of electric field were compared with their theoretical counterpart to validate the method and the image processing technique and showed an excellent agreement. In a second phase, data with electric field were acquired. The resulting electric force was evaluated from the force balance, as the opposite of the sum of all the other forces acting on the bubble. The values of measured electric forces showed excellent agreement with the theoretical evaluations.