Analisi numerica e sperimentale di fenomeni di rumore termofluidodinamico e aeroacustico

The present work collects the results of research conducted on thermo-fluid dynamics and aero-acoustic noise sources, especially on the characteristic noise of the condensing boilers combustion and centrifugal fans noise. The confined combustion cause instability phenomena which produce pressure fluctuations inside the combustion chamber, resulting in generation of noise. The instability phenomena of condensing boilers can be divided in two main types: the first one is called "Rumbling," where an interaction among flame, acoustic field and system fluid dynamics occurs; the second one is called "Hooting", which is a thermo-kinetic instability. On the first part of this paper is shown the experimental and numerical analysis of the "Hooting" in order to find: causes, characteristics, and a methodology that allows the prediction of this phenomenon. For this purpose has been made a simplified prototype of a condensing boiler to study: the main acoustic characteristics of the instability phenomenon, the fluid dynamic parameters which affect its generation and find a simple geometry to perform 3D simulation analysis. In order to evaluate the influence on the emitted spectrum of combustion chamber geometry, it has been performed an acoustic modes FEM analysis of the combustion chamber prototyped cavity. To study the reproducibility and therefore the prediction of the “Hooting” spectra sound pressure level a hybrid CFD/CAA model has been performed. The above numerical model combining a thermo-fluid dynamics simulation of combustion chemical reaction, with a frequency domain acoustic simulation considering the Lighthill’s acoustic analogy extension proposed by Curle. Beyond to this numerical method, CFD transient simulations have been performed to calculate the spectra of sound pressure levels of combustion chamber, analyzing the obtained pressure fluctuation. The last part of this work described two numerical methodologies to predict the noise level emitted from centrifugal fan outlet. The first one requires the combination of a transient CFD simulation with a CAA acoustic simulation using two different acoustic analogies. The Ffowcs Williams-Hawkins analogy estimates the emitted sound pressure level with the definition of a dipoles distribution placed on fixed and mobile hard surfaces present in a fluid domain. This numerical method enables to calculate centrifugal fans blade passing frequency noise and broadband noise as well. The Lowson theory defines an equivalent single sound source considering the pressure distribution on blades and rotation speed of the impeller. This numerical method enables to calculate only the noise contribution of blade passing frequency and its harmonics. The second numerical method considered enable to calculate the sound pressure level spectra directly from relative pressure fluctuations from CFD transient simulations. The simulation results have been compared with the experimental measurements performed on a test rig carried out according to UNI EN ISO 5136.