Matlab programmes for inclined aperture

These are the Matlab programmes to calculate the acoustic effective length of finite-length inclined circular aperture(s).</p

The aerodynamic response of fuel injector passages to incident acoustic waves

Modern low emission combustion systems are more prone to combustion instabilities due to operation at lean conditions. The response of the airflow passing through the injector to incident acoustic waves is therefore of interest. Airflow fluctuations can initiate, for example, perturbations in stoichiometry and velocity that are subsequently delivered into the heat release region. In the case of liquid fuelled gas turbines the atomisation process will also be affected. Such effects can lead to further unsteady heat release and the generation of acoustic waves, thereby leading to combustion instability. This paper describes experimental measurements and the development of a numerical methodology by which the unsteady airflow response of complex, modern, low emission fuel injectors can be characterised. Single and two passage injector configurations have been investigated which broadly capture many of the features associated with modern fuel injectors. Although targeted at low emission (lean burn) liquid fuelled injector geometries, the methodology developed is thought applicable to a wide range of injector configurations. Initially experimental measurements were used to characterise the overall acoustic impedance of each injector design over a range of frequencies. Such information is also required for the low order thermo-acoustic network models, as typically used in the design process, to predict the stability of the combustion system. In addition to the experimental measurements a methodology was developed using unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations in which acoustic boundary conditions were implemented to reproduce the experimental scenarios. Interrogation of the pressure field enabled similar data analysis techniques to be applied to the numerical data for determining the injector acoustic characteristics. Fidelity of the numerical simulations is confirmed by the excellent agreement between the experimental data and numerical simulations. Furthermore, the unsteady flow field within the passages is difficult to access experimentally, but can be examined in more detail from the simulation results. In this way an improved understanding of the passage flows and their individual responses to the incident acoustic pressure waves can be obtained. The numerical approach is aimed at providing a computationally efficient and economic tool for predicting the acoustic characteristics of the complex geometries typical of modern fuel injector designs. Using this tool injector designs with different acoustic response characteristics can be developed relatively quickly

Low-frequency still-air acoustic inertia of inclined circular aperture in an infinite flat plate of finite thickness

The acoustic inertia of the canonical configuration of inclined circular aperture(s) in a finite-thickness plate at the low frequency limit is investigated under the inviscid still-air condition. A hybrid approach combining modal expansion and boundary element method is applied to calculate the effective length of the aperture as a quantitative characterisation of its acoustic inertia. These calculations, covering different inclination angles up to 75°, are performed for a single isolated aperture with a selected range of plate thickness and the periodic aperture arrays of aligned and staggered arrangements with a selected range of aperture spacing respectively. The results are validated by the simulations carried out with the commercial software COMSOL. The parametric studies of geometric dimensions included in this work provide representative results for typical acoustic related engineering applications of finite-length inclined apertures. A link for access to the MATLAB programmes implementing the calculations is provided for interested readers. As an example of application, the hybrid method described in this work is used for the acoustic modelling of a representative multi-perforated liner studied in the literature for the problem of thermo-acoustic instability. The results compared well against those obtained from the Computational Fluid Dynamics simulations reported in the literature.</p

Low-frequency acoustic radiation from a flanged circular pipe at an inclined angle

The generic problem of low-frequency acoustic radiation through quiescent air from a circular pipe that is inclined with respect to its exit flange is studied in this work. The exit flange is taken to extend as an infinite plane away from the pipe opening. The analysis implements a hybrid method that combines modal expansions with the boundary element method. The reflection coefficient and pipe end correction for Helmholtz numbers (based on the pipe radius) less than 2.5 are calculated for various inclination angles up to 75°. Calculations are validated using simulations from the finite-element solver of the commercial software package COMSOL. The reflection coefficient and end correction predictions agree closely with the validation simulations yet differ notably from the results available in the literature. The solution obtained from the hybrid method is subsequently used to analyse the acoustic field at the pipe exit and in the downstream space. The key aspects of the governing physics pertaining to practical engineering applications at low frequencies are captured in a low-order approximation, which significantly reduces the degrees of freedom of the problem and provides generally good estimates of the reflection coefficient and end correction, as well as the downstream acoustic field

The response to incident acoustic waves of the flow field produced by a multi-passage lean-burn aero-engine fuel injector

© 2017 ASME. Previous work has shown that compressible unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations, with suitable acoustic boundary conditions, are capable of correctly predicting the acoustic impedance of simplified fuel injectors. In this work the method developed is applied to simulating the acoustically forced flow in and downstream of a realistic multi-passage fuel injector. The simulations are validated by compar-ing the impedance of the injector with data obtained experimen-tally by a multi-microphone technique. Such results can then be used in conjunction with a suitable low-order thermo-Acoustic network model to predict the stability of combustors. However the validated simulations can also be used to reveal further de-tails about the effect of acoustic forcing on the flow field. The velocity flow field produced by the injector with and without acoustic forcing is analysed using snapshot POD to de-termine the large scale energy containing structures within the flow. In the non-Acoustically forced simulations it was found that the first four POD modes correspond to two rotating spi-ral modes, designated as the m=1 and m=2 modes with a peak frequency content of 450 Hz for the first mode and 1000 Hz for the second mode corresponding to experimental Hot-Wire mea-surements made in a separate study. It is hypothesised that these spiral modes will affect the atomisation, evaporation and mixing of the fuel in subsequent planned two-phase simulations. POD analysis of the flow subjected to 300 Hz, 300 Pa acoustic excita-tion shows that the first four POD modes correspond to similarly shaped spiral modes. The acoustic excitation is responsible for the appearance of 4 POD modes within the injector body that correspond to two push-pull velocity modes with axes of symme-try perpendicular to each other. The acoustic forcing also pro-duces two additional POD modes that most likely represent the non-linear interaction between the push-pull and spiral modes. Further analysis of the fluctuations in pressure, mass flow rate, angular velocity and swirl number, within the passages and at the injector exit plane, show that the fluctuations in pressure and mass flow rate average across the passages while variations in angular velocity and swirl number sum across the passages. The relationship between mass flow rate, angular velocity and swirl number is discussed with reference to general observations of the sensitivity of flames to fluctuations in these quantities