Combustion noise analysis of a turbulent spray flame using a hybrid DNS/APE-RF approach

Abstract

Combustion generated noise of a turbulent spray flame has been investigated using a hybrid Computational Fluid Dynamics/Computational Aero-Acoustics (CFD/CAA) framework. In this two-step framework, the reacting flow-field of a flame is simulated using Direct Numerical Simulations (DNS) in the first step, while the acoustic wave propagation in a non-uniform mean flow is captured by solving the Acoustic Perturbation Equations extended to Reacting Flows (APE-RF) in the second step. The numerical approach used in this study is therefore, termed as the hybrid DNS/APE-RF approach. First, this hybrid DNS/APE-RF approach is used to simulate a benchmark experimental open turbulent non-premixed flame, and the results obtained for the non-premixed flame’s flow-field statistical quantities, as well as radiated sound intensities are extensively validated against experimental data. Next, the hybrid DNS/APE-RF approach is applied to an experimental open turbulent spray flame, for simulating its two-phase reacting flow-field along with its combustion generated acoustic field, to predict and analyze the noise radiation behavior. The DNS results of the spray flame are compared with measurements, in terms of droplet velocity statistics and gas-phase temperatures, and favorable agreement is observed. The computed acoustic power spectra of the spray flame exhibit a power law dependence of the form f⁻²˙⁴ (where f is the frequency) in the frequency range 300 Hz < f < 1000 Hz. And, the computed noise spectra of the spray flame contain a nearly constant sound pressure level plateau, for frequencies greater than 1000 Hz and up to 3000 Hz. Acoustic refraction effects induced by sound speed variations in the flame, resulting in the attenuation of high-frequency noise radiated in the flame downstream direction, are also captured in the simulation. Noise radiation characteristics of the turbulent spray flame are found to resemble those of turbulent premixed and non-premixed flames.