VeLoNOx (Very Low NOx) burners have been developed by Ansaldo Energia to satisfy the most restrictive emission limits of many countries. The stability field range is a key element for the design of such combustion systems; ever-deeper knowledge is therefore essential. The impact on stability of flame transition is of paramount relevance. Usually, flame transition occurs during load ramp: two different flame shapes take place in a well-defined load range. When loading up to base load condition, stability requires proper choice of flame shape. Observation of flame transition in a full-scale pressure test-rig implies a) unambiguous observation of an alteration in the flame shape b) measurement of both the burner area discharge coefficient “Alfa” (on the air side) and of a temperature drop on the walls of the combustion chamber.

The aim of the present work was to verify that RANS simulations with particular setting of the physical models can provide us with realistic description of the global flame behaviour in the full-scale, pressurised test rig even during the crucial flame transition at partial base load. This aim was achieved. CFD RANS has led to two different solutions with different shape, just like in the experiment. The main global parameters of the combustion system undergo similar changes and with similar magnitude in CFD results and in the experiment. In particular, the burner discharge coefficient “Alfa” increases with flame transition from “open” to “closed” flame, while the combustion chamber wall temperature decreases with flame transition from “open” to “closed” flame. It means that this CFD RANS simulation provides us with a reliable flow and temperature fields which can be used as meaningful input data for the more detailed analysis of both thermal load at combustion chamber wall and CO and NOx emission level, thermo-acoustic behaviour of the combustion system by means of more specific tools. Such information could be useful when designing more efficient, less polluting combustion systems.

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