On the acoustic response of a generic gas turbine fuel injector passage

A current trend in the design of modern aero engines is the transition towards leaner combustion as a solution to satisfy increasingly stringent emission regulations. Lean combustion systems are often more susceptible to thermoacoustic instability and the fuel injector can play a critical role. This paper presents an analytical study on the unsteady air flow through a generic injector passage in response to incident acoustic waves. The injector passage is represented by a simplified geometry which comprises the main geometrical passage features. The unsteady flow through the passage is obtained by combining the elemental solutions for different parts of the passage. This enables the transfer impedance of the injector passage to be determined and the effects of different design parameters on the sensitivity of the air flow to acoustic perturbations to be examined. The convective wave associated with the unsteady swirl vane wakes is also visited and compared with the results from the numerical simulations obtained in previous works. In addition to helping derive design practices for injector passages from the perspective of thermoacoustic instability, the current analysis can also be applied as a preliminary design tool to assess the acoustic characteristics for an injector passage of the axial swirler type

Volumetric PIV measurement for capturing the port flow characteristics within annular gas turbine combustors

© 2020, The Author(s). Abstract: The three-dimensional flows within a full featured, unmodified annular gas turbine combustor have been investigated using a scanned stereoscopic PIV measurement technique. Volumetric measurements have been achieved by rigidly translating a stereoscopic PIV system to scan measurements around the combustor, permitting reconstruction of volumetric single-point statistics. Delivering the measurements in this way allows the measurement of larger volumes than are accessible using techniques relying upon high depth of field imaging. The shallow depth of field achieved in the stereoscopic configuration furthermore permits measurements in close proximity to highly detailed geometry. The measurements performed have then been used to assess the performance of the combustor port flows, which are central to the emissions performance and temperature/velocity profile at turbine inlet. Substantially differing performance was observed in the primary ports with circumferential position, which was found to influence the behaviour of the second secondary port jets. The measurements indicated that the interaction between the primary and secondary jets occurred due to variations in the external boundary conditions imposed by the annular passages in which the combustor is located. Graphic abstract: [Figure not available: see fulltext.]

Annular diffusers with large downstream blockage effects for gas turbine combustion applications

In engineering applications, diffuser performance is significantly affected by its boundary conditions. In a gas turbine combustion system, the space envelope is limited, the inlet conditions are generated by upstream turbomachinery, and the downstream geometry is complex and in close proximity. Published work discusses the impact of compressor-generated inlet conditions, but little work has been undertaken on designing diffusers to accommodate a complex downstream geometry. This paper considers the design of an annular diffuser in the presence of a large downstream blockage. This is most applicable in the combustion system of a low-emission landbased aero-derivative gas turbine, where immediately downstream of the diffuser approximately 85% of the flow moves outboard and 15% moves inboard to supply the various flame-tube and turbine-cooling features. Several diffuser concepts are numerically developed and demonstrate 1) the interaction between the diffuser and downstream geometry and 2) how this varies with changes in diffuser geometry. A preferred concept is experimentally evaluated on a low-speed facility that simulates the combustion system and provides compressorgenerated inlet conditions. A conventionally designed aero-derivative diffuser system is also evaluated and, with reference to this datum, the system total pressure losses are reduced by between 20 and 35%

Acoustic quasi-steady response of thin walled perforated liners with bias and grazing flows

This paper considers the acoustic performance of a passive damper in which acoustic energy is absorbed by orifices located within a thin plate (i.e. a perforated liner). The perforated liner, which incorporates orifices of length to diameter ratios of ~0.2, is supplied with flow from a passage. This enables the liner to be subject to a flow that grazes the upstream side of each liner orifice. Flow can also pass through each orifice to create a bias flow. Hence the liner can be subjected to a range of grazing and bias flow combinations. Two types of liners were investigated which incorporated either simple plain or ‘skewed’ orifices. For the mean flow field, data is presented which shows that the mean discharge coefficient of each liner is determined by the grazing to bias flow velocity ratio. In addition, measurements of the unsteady flow field through each liner were also undertaken and mainly presented in terms of the measured admittance. For a given liner geometry, the admittance values were found to be comparable for a given Strouhal number (with the exception of the lowest bias to grazing flow velocity ratio tested) which has also been noted by other authors. The paper shows that this is consistent with the unsteady orifice flow being associated with variations in both the velocity and the area of the vena contracta downstream of each orifice. These same basic characteristics were observed for both of the liner geometries tested. This provides a relatively simple means of predicting the acoustic liner characteristics over the specified operating range

The application of porous media to simulate the upstream effects of gas turbine injector swirl vanes

Numerical simulations are an invaluable means of evaluating design solutions. This is especially true in the initial design phase of a project where several simulations may be required as part of an optimisation study. The design of aircraft gas turbine combustor external aerodynamics frequently calls upon the services of numerical methods to visualise the existing flow field, and develop architectures which improve the performance of the system. Many of these performance improvements are driven by the desire to reduce fuel burn and cut emissions lowering the environmental impact of aviation. The gas turbine combustion chamber is, however, reasonably complex geometrically and requires a high fidelity model to resolve small geometric details. The fuel injector is the most geometrically complex component, requiring around 20% of the mesh cells of the entire domain. This makes it expensive to model in terms of both requisite computational resource and run time. Most modern aircraft gas turbines utilise swirling flow fields to stabilise the flame front in the combustion liner. The swirl cone is generally generated using fixed angle vane rows within the injector. It is these small features that are responsible for the requisite high mesh cell count. This paper presents a numerical method for replacing the injector swirl vane passages with mathematically porous volumes which replicate the required pressure drop. Modelling using porous media is preferential to modelling the fully featured injector as it allows a significant reduction in the size of the computational domain and number of cells. Additionally the simplification makes the geometry easier to change, scale and re-mesh during development. This in turn allows significant time savings which serve ultimately to expedite the design process. This method has been rigorously tested through a range of approach conditions and flow conditions to ensure that it is robust enough for use in the design process. The loss in accuracy owing to the simplification has been demonstrated to be less than 4.4%, for all tested flow fields. This error is dependent on the flow conditions and is generally much less for passages fed with representative levels of upstream distortion

Quantifying ageing effects in thermochromic liquid crystal thermography as applied to transient convective heat transfer experiments

Thermochromic liquid crystal (TLC) thermography is used in transient heat transfer experiments to determine distributions of convective heat transfer coefficient (HTC) inside models of internally cooled gas turbine engine components. As these components become more geometrically complex, the application of TLC thermography becomes increasingly challenging and additional sources of experimental uncertainty grow to be significant. The present work quantifies the uncertainties introduced by TLC ageing using a state-of-the-art imaging system and a new postprocessing methodology that are optimised for the intensity-based method of analysing TLC data. A coating comprising multiple TLCs with different active temperature ranges is considered and subject to 33 repeated thermal cycles. These repeated cycles are shown to increase the random and systematic uncertainties in the TLC measurements, resulting in consequent increases in the uncertainties associated with calculated HTCs. Increases in systematic uncertainty are caused by reflectance in the measured wavelength band moving to different temperatures, while increases in random uncertainty are related to changes in individual crystals or crystal clusters with ageing. Approaches to calibrating out increases in systematic uncertainty are proposed and recommended, but increases in random uncertainty will always persist unless the TLC coating is removed and reapplied

Intercooled aero-gas-turbine duct aerodynamics: core air delivery ducts

The development of radical new aero engine technologies will be key to delivering the step-changes in aircraft environmental performance required to meet future emissions legislation. Intercooling has the potential for higher overall pressure ratios, enabling reduced fuel consumption, and/or lower compressor delivery air temperatures and therefore reduced NOx. This paper considers the aerodynamics associated with the complex ducting system that would be required to transfer flow from the core engine path to the heat exchanger system. The cycle benefits associated with intercooling could be offset by the pressure losses within this ducting system and/or any detrimental effect the system has on the surrounding components. A suitable branched S-shaped duct system has been numerically developed which diffuses and delivers the flow from the engine core to discrete intercooler modules. A novel swirling duct concept was used to locally open larger spacing between certain duct branches in order to provide engine core access whilst hiding the resultant pressure field from the upstream turbomachinery. The candidate duct system was experimentally evaluated on a bespoke low speed, fully annular isothermal test facility. Aerodynamic measurements demonstrated the ability of the design to meet the stringent aerodynamic and geometric constraints

Measurements and computational fluid dynamics predictions of the acoustic impedance of orifices

The response of orifices to incident acoustic waves, which is important for many engineering applications, is investigated with an approach combining both experimental measurements and numerical simulations. This paper presents experimental data on acoustic impedance of orifices, which is subsequently used for validation of a numerical technique developed for the purpose of predicting the acoustic response of a range of geometries with moderate computational cost. Measurements are conducted for orifices with length to diameter ratios, L/D, of 0.5, 5 and 10. The experimental data is obtained for a range of frequencies using a configuration in which a mean (or bias) flow passes from a duct through the test orifices before issuing into a plenum. Acoustic waves are provided by a sound generator on the upstream side of the orifices. Computational fluid dynamics (CFD) calculations of the same configuration have also been performed. These have been undertaken using an unsteady Reynolds averaged Navier–Stokes (URANS) approach with a pressure based compressible formulation with appropriate characteristic based boundary conditions to simulate the correct acoustic behaviour at the boundaries. The CFD predictions are in very good agreement with the experimental data, predicting the correct trend with both frequency and orifice L/D in a way not seen with analytical models. The CFD was also able to successfully predict a negative resistance, and hence a reflection coefficient greater than unity for the L/D=0.5L/D=0.5 case

Acoustic performance of a resonating perforated liner with incident axial and circumferential acoustic modes

Perforated liners are a common form of passive damping device used in engineering applications to damp acoustic pressure fluctuations. The liner has many orifices arranged over the surface with a rear cavity, where the liner can be designed to resonate akin to an array of Helmholtz resonators in parallel. However, whilst a Helmholtz resonator is insensitive to the incident mode, the large surface area and rear cavity of a perforated liner can generate internal mode shapes that affect the acoustic performance. This paper presents a quasi-one-dimensional analytical model capable of capturing the variation in acoustic performance as the internal cavity segmentation is altered with incident higher-order acoustic modes in a narrow annular duct. Thus, the model can allow the generation of circumferential mode shapes. The model shows, when the liner is highly segmented circumferentially, the liner behaviour is akin to that with an incident axial wave. The segmentation causes the internal cavity pressure to fluctuate uniformly at a similar frequency to a Helmholtz resonator with the same effective cavity dimensions. When the cavity length is significant relative to the wavelength, circumferential mode shapes are generated within the cavity and the frequency of resonance increases based on the circumferential frequency component. The model is then compared to an example experimental data set obtained from a facility designed to allow circumferential modes to cut-on simultaneously with an axial mode. A description of the facility and the multi-microphone decomposition method applied to decompose simultaneous modes of unknown orders and relative magnitudes is presented. The model has good agreement with the experimental results for a small cavity segmentation, although there is deviation observed at high frequencies when the cavity length becomes significant relative to the circumferential wavelength

Measurements of fuel thickness for prefilming atomisers at elevated pressure

© 2020 Elsevier Ltd This work describes an experimental study of the fuel flows on the prefilmer of an aerospace gas turbine airblast atomiser at elevated pressure. The work identifies the physics leading to contradictory findings within the literature. This concerns an important atomisation boundary condition, whether the thickness of the fuel film on the prefilming surface influences the downstream drop size distribution. Analysis of the experimental data shows that fuel film thickness becomes uncorrelated with the downstream drop size if surface tension forces dominate inertia at the prefilmer tip. Fuel film thickness however provides the initial length scale for primary atomization if fuel inertia exceeds surface tension forces. It is the high inertial conditions that are associated with gas turbine operation, but the low inertial conditions that are readily achievable at laboratory scale through momentum flux scaling. Additionally, a detailed statistical description of the fuel flow has been provided for the atomiser tested. This reveals the importance of upstream hydrodynamic and aerodynamic boundary conditions on the probability of a ligament forming. Surprisingly, operating pressure is shown to have limited effect on the probability of ligament formation, a significant advantage for future modelling of the primary atomization processes