Thermoacoustic Instabilities in Large Engines: Case Study

The case study we present in this paper is related to the non-expected acoustic behavior of a set of large diesel engines. There were 8 reconverted 4-cycle Diesel engines of 10 MW of nominal power each, which generate alternating current at 50 Hz. Two years later than their installation, the engines needed another important modification to meet the NOx emission specifications. The cylinders’ chambers were modified and a turbocharger group operating at 25,500 rpm was installed in each engine. Sometime later, non-expected behaviors related to the acoustic emissions from the engines were perceived. Particularly, a “random variability” of sound pressure levels emitted by large engines operating in the same conditions with only a few hours of difference was recorded in many opportunities. When faced with this problem, there was a set of not obvious open questions. Finally, we met the answers to the most critical questions: the problem of noise was due to thermoacoustic instabilities, which is a phenomenon expected in large engines. The ideal approach to control this type of acoustic emission is the use of hybrid silencers, which function as passive silencers in high frequencies and include a solution of active noise control for low ones.