Towards a Quieter Low Pressure Turbine: Design Characteristics and Prediction Needs

2019

Acoustics

In future Ultra-High By-Pass Ratio turboengines, the turbomachinery noise (fan and turbine stages mainly) is expected to increase significantly. A review of analytical models and numerical methods to yield both tonal and broadband contributions of such noise sources is presented. The former rely on hybrid methods coupling gust response over very thin flat plates of finite chord length, either isolated or in cascade, and acoustic analogies in free-field and in a duct. The latter yields tonal noise with unsteady Reynolds-Averaged Navier–Stokes (u-RANS) simulations, and broadband noise with Large Eddy Simulations (LES). The analytical models are shown to provide good and fast first sound estimates at pre-design stages, and to easily separate the different noise sources. The u-RANS simulations are now able to give accurate estimates of tonal noise of the most complex asymmetric, heterogeneous fan-Outlet Guiding Vane (OGV) configurations. Wall-modeled LES on rescaled stage configurations have now been achieved on all components: a low-pressure compressor stage, a transonic high-pressure turbine stage and a fan-OGV configuration with good overall sound power level predictions for the latter. In this case, hybrid Lattice–Boltzmann/very large-eddy simulations also appear to be an excellent alternative to yield both contributions accurately at once.

Section: Introductionmentioning

confidence: 99%

Turbomachinery Noise Predictions: Present and Future

2019

Acoustics

“…Modern turboengine architectures involve lean, partially premixed and more unstable combustors, 1 along with fewer turbine stages to prevent the propagation of combustion noise outside of the engine. 2 Similarly, turboshaft engines have even fewer turbine stages and lower ejection velocity and consequently no jet noise to mask the combustion noise. 3,4 As a result of such technological changes, the latter noise source is becoming a major nuisance that needs to be understood and controlled to meet future regulations.…”

Section: Introductionmentioning

confidence: 99%

Large-Eddy-simulation prediction of indirect combustion noise in the entropy wave generator experiment

International Journal of Spray and Combustion Dynamics

Becerril

Gicquel

2017

Compact and non-compact analytical solutions of the subsonic operating point of the entropy wave generator experiment are compared with detailed numerical results obtained by large Eddy simulations. Two energy deposition methods are presented to account for the experimental ignition sequence and geometry: a single-block deposition as previously used and a delayed deposition that reproduces the experimental protocle closely. The unknown inlet acoustic reflection coefficient is assumed to be fully reflective to be more physically consistent with the actual experimental setup. The time delay between the activation of the heating modules must be considered to retrieve the temperature signal measured at the vibrometer and pressure signals at the microphones. Moreover, pressure signals extracted from the large Eddy simulations in the outlet duct using the delayed ignition model clearly reproduce the experimental signals better than the analytical models. An additional simulation with actual temperature fluctuations directly injected at the inlet of the computational domain clearly shows that the pressure fluctuations produced by the acceleration of the hot slug yields indirect noise almost entirely. Finally, the entropy spot is shown to be distorted when convecting through the turbulent flow in the entropy wave generator nozzle. Its amplitude decreases and its shape is dispersed, but hardly any dissipation occurs. The distortion appears to be negligible through the nozzle and become important only when convected over a long distance in the downstream duct. As the dominant frequencies of the entropy wave generator entropy forcing are very low, the effects of dispersion by the mean flow are however weak.

“…1 This has led to what Nesbitt calls the ''turbine noise storm'' in modern low-pressure turbines. 2 Consequently, the noise contribution of both turbine stages and combustion chamber is expected to increase drastically and to become relevant noise sources at all operating conditions. Combustion noise is the low-frequency noise generated in the combustion chamber of gas turbines and comes from two main mechanisms.…”

Section: Introductionmentioning

confidence: 99%

Noise mechanisms in a transonic high-pressure turbine stage

Wang

Sanjosé

International Journal of Aeroacoustics

et al. 2016

In modern ultra-high by-pass ratio turboengines, the noise contribution of both turbine stages and combustion chamber is expected to increase drastically. In the present work, both noise sources are evaluated in the realistic, fully three-dimensional transonic high-pressure turbine stage MT1 using large-eddy simulations (LES). An analysis of the basic flow field and the different turbine noise generation mechanisms is performed for two configurations: one with a steady inflow and one with an unsteady inflow, where a plane entropy wave train at a given frequency is injected at the inlet and propagates across the stage generating indirect noise. The noise is evaluated through Fourier analysis, dynamic mode decomposition of the flow field, and estimates of propagation coefficients. The steady case show three different dominant noise mechanisms: the usual stator wake brushing of the rotor blades, the several pulsating shocks in the stage and the vortex shedding. The forced results have additional tonal noise caused by the indirect noise mechanism generated by the acceleration and distortion of entropy spots. They are compared with previous two-dimensional (2D) simulations of a similar stator/rotor configuration, as well as with the compact theory. Results show that the upstream propagating entropy noise is reduced due to the choked turbine nozzle guide vane. Downstream acoustic waves are found to be of similar strength as the 2D case and larger than the blade passing frequency of the wake-interaction mechanism, highlighting the potential impact of indirect combustion noise on the overall noise signature of the engine.