David R. H. Gillespie

The secondary air system and cooling passages of gas turbine components are prone to blockage from sand and dust. Prediction of deposition requires accurate models of particle transport and thermo-mechanical interaction with walls. Bounce stick models predict whether a particle will bounce or stick upon impact and calculate rebound trajectories if applicable. This paper proposes a bounce stick model that uses analytical solutions of adhesion, plastic deformation and viscoelasticity to time-resolve collision physics. The Discrete-Element Methods (DEM) model shows good agreement when compared to experimental studies and FEA simulations, requiring minimal parametric fitting. Further comparison is made to the best resolved and industry standard models available. In addition to coefficients of restitution, other variables crucial to modelling rebound, for example angular velocity, are predicted. The time-stepping explicit approach allows coupling between internal processes during contact and shows that viscoelasticity plays a significant role in adhesion. Modelling time-dependent internal variables like wall-normal force allow the modelling of non-spherical particles, the physics of which has been shown by previous work to differ significantly from that of spheres. These effects have not been captured well, if at all, by other models. The spherical version of this model has previously been shown to produce good predictions of deposition distribution in CFD. The developments presented here allow the inclusion of more complex collision physics which will improve numerical predictions of deposition. The accompanying second part to this paper demonstrates this with the addition of surface roughness modelling.

Cooling passages and the secondary air system of gas turbine components are prone to blockage from sand, dust and ash. The ability to model deposition of non-spherical particles accurately using CFD would allow the prediction of in-service performance degradation and the design of deposition-tolerant hardware. This paper implements a Continuous Random Walk (CRW) transport model with a Discrete Elements Methods (DEM) based bounce stick model to predict deposition in CFD. These are the first such simulations in the open literature to incorporate particle shape and surface roughness. The resulting spatial trends of deposition are compared with experimental data of particle deposition in an S-bend. The case considered is representative of the flow and metal conditions, particle size and loading distribution, and passage geometry seen in HPT blade cooling passages. Numerical predictions of deposition distribution show good qualitative agreement with experiments. The approach is therefore suitable to assess and compare the susceptibility of components to particulate damage in industry. To the authors' knowledge, this is the first example in the open literature of experimental comparison of deposition trends in CFD. The approach provided reasonable first-order estimations of quantitative deposition heights, which may be used to estimate component-level degradation in-engine. More accurate quantitative predictions of deposition will require a suitable deposit evolution and erosion model and improved material data, which are not yet available in the literature. The performance of the DEM bounce stick model was favourable compared to energy-based alternatives.

Particle deposition is a major damage mechanism for gas turbine components, especially in the secondary air system. Predicting the transport and deposition of ingested atmospheric contaminants is of great interest in component-level simulations. Bounce stick models predict deposition upon collision with a wall during Lagrangian particle tracking; they consider a range of physical phenomena, including van der Waals forces and plastic deformation. The effect of the rotating frame of reference on particle collision physics has thus far been neglected in the literature despite the significant centrifugal (CF) loads experienced by many components of interest for deposition studies, for example, turbine blades. The collision physics of the rotating frame are discussed here using low-order models and Monte Carlo style simulations. The present work aims to provide a conceptual framework for the inclusion of CF forces in collision physics. No significant effect was found on the rebound velocities of particles experiencing “worst case” CF forces. However, differences were observed in the sticking probability between concave and convex rotating surfaces, where the CF forces act in opposing directions to either push the particle into the surface or detach it. A force-based analogy of the popular critical velocity model was developed to study the phenomenon. It was used in a Monte Carlo style simulation of a rotor disk, finding that the variation of critical velocity due to CF forces was likely to bias deposition toward concave surfaces. The collision physics of the rotating frame were found to be unintuitive, with complex implications for modeling deposition in gas turbine components. However, they were consequential in determining the distribution of deposition through the system, hence should be included in particle deposition simulations in computational fluid dynamics (CFD).

Stochastic particle tracking models utilising RANS simulations are frequently used to simulate particulate matter damage in gas turbines. The Continuous Random Walk (CRW) model has been shown to model particulate motion in the diffusion-impaction regime significantly more accurately than Discrete Random Walk implementations in simple flows, however, the model uses turbulent flow

Prediction of contact temperature between two materials in high-speed rubbing contact is essential to model wear during unlubricated contact. Conventionally, assumptions of either a steady or an annular heat source are used for slow and high speed rotation, respectively. In this paper, a rotating heating source is solved using an in-house finite element method code. This captures the full geometry and rotating speed of the rubbing bodies. Transient heat transfer is modeled quasi-statically, eliminating the need for a transient 3D simulation. This model is shown to be suitable for contact temperature prediction over a wide range of rotating speeds, anisotropic thermal conductivity, and nonuniform thermal boundary conditions. The model calculates heat partition accurately for a thin rotating disk and short pin combination, which cannot be predicted using the existing analytical solutions. The method is validated against ansys mechanical and experimental infrared thermography. Results demonstrate that the annular source assumption significantly underpredicts contact temperature, especially at the rubbing interface. Explicit modeling of a thin disk results in higher heat partition coefficients compared with the commonplace semi-infinite length assumption on both static and rotating components. The thermal anisotropy of tuft-on-disk configurations is evaluated and compared to a uniform pin-on-disk configuration. Despite the effective thermal conductivity (ETC) in the bristle tuft being approximately 1 order of magnitude lower than along the bristle length (treating the bristle pack as a porous medium), its impact on heat partition and contact temperature is shown to be limited.

The ingestion and deposition of solid particulates within gas turbine engines has become a very significant concern for both designers and operators in recent times. Frequently aircraft are operated in environments where sand, ash, dust, and salt are present, which can drive damage mechanisms from long term component degradation to in-flight flame-out. Experiments are presented to assess deposition characteristics of sodium chloride (NaCl) at gas turbine secondary air system temperature conditions in horizontal pipe flow. Monodisperse NaCl particles were generated in the size range 2.0–6.5 µm, with gas temperatures 390–480 °C, and metal temperatures 355–730 °C. Two engine-representative surface roughnesses were assessed. An experimental technique for the measurement of deposited NaCl based on solution conductivity was developed and validated. Experiments were carried out under isothermal and nonisothermal/thermophoretic conditions. An initial experimental campaign was conducted under ambient and isothermal conditions; high temperature isothermal results showed good similarity. Under thermophoretic conditions, deposition rates varied by up to several orders of magnitude compared to isothermal rates.

It has been recognized in recent years that high altitude atmospheric ice crystals pose a threat to aircraft engines. Instances of damage, surge, and shutdown have been recorded at altitudes significantly greater than those associated with supercooled water icing. It is believed that solid ice particles can accrete inside the core compressor, although the exact mechanism by which this occurs remains poorly understood. Development of analytical and empirical models of the ice crystal icing phenomenon is necessary for both future engine design and this-generation engine certification. A comprehensive model will require the integration of a number of aerodynamic, thermodynamic, and mechanical components. This paper studies one such component, specifically the thermodynamic and mechanical processes experienced by ice particles impinging on a warm surface. Results are presented from an experimental campaign using a heated and instrumented flat plate. The plate was installed in the Altitude Icing Wind Tunnel (AIWT) at the National Research Council of Canada (NRC). This facility is capable of replicating ice crystal conditions at altitudes up to 9 km and Mach numbers up to 0.55. The heated plate is designed to measure the heat flux from a surface at temperatures representative of the early core compressor, under varying convective and icing heat loads. Heat transfer enhancement was observed to rise approximately linearly with both total water content (TWC) and particle diameter over the ranges tested. A Stokes number greater than unity proved to be a useful parameter in determining whether heat transfer enhancement would occur. A particle energy parameter was used to estimate the likelihood of fragmentation. Results showed that when particles were both ballistic and likely to fragment, heat transfer enhancement was independent of both Mach and Reynolds numbers over the ranges tested.

Thermal closure of the engine casing is widely used to minimize undesirable blade tip leakage flows thus improving jet engine performance. This may be achieved using an impingement cooling scheme on the external casing wall, provided by manifolds attached to the outside of the engine. The assembly tolerance of these components leads to variation in the standoff distance between the manifold and the casing, and its effects on casing contraction must be understood to allow build tolerance to be specified. For cooling arrangements with promising performance, the variation in closure with standoff distance of z/d = 1–6 were investigated through a mixture of extensive numerical modeling and experimental validation. A cooling manifold, typical of that adopted by several engine companies, incorporating three different arrays of short cooling holes (chosen from previous study by Choi et al. (2016, “The Relative Performance of External Casing Impingement Cooling Arrangements for Thermal Control of Blade Tip Clearance,” ASME J. Turbomach., 138(3), p. 031005.)) and thermal control dummy flanges were considered. Typical contractions of 0.5–2.2 mm are achieved from the 0.02–0.35 kg/s of the current casing cooling flows. The variation in heat transfer coefficient observed with standoff distance is much lower for the sparse array investigated compared to previous designs employing arrays typical of blade cooling configurations. The reason for this is explained through interrogation of the local flow field and resultant heat transfer coefficient. This implies that acceptable control of the circumferential uniformity of case cooling can be achieved with relatively large assembly tolerance of the manifold relative to the casing.

The presence and accretion of airborne particulates, including ash, sand, dust, and other compounds, in gas turbine engines can adversely affect performance and life of components. Engine experience and experimental work have shown that the thickness of accreted layers of these particulates can become large relative to the engine components on which they form. Numerical simulation to date has largely ignored the effects of resultant changes in the passage geometry due to the build-up of deposited particles. This paper will focus on updating the boundaries of the flow volume geometry by integrating the deposited volume of particulates on the solid surface. The technique is implemented using a novel, coupled deposition-dynamic mesh morphing (DMM) approach to the simulation of particulate-laden flows using Reynolds-averaged Navier–Stokes modeling of the bulk fluid, and Lagrangian-based particulate tracking. On an iterative basis, the particle deposition distributions are used to modify the surface topology by altering the locations of surface nodes, which modifies the mesh. The continuous phase solution and particle tracking are then recalculated. The sensitivity to the modeling time steps employed is explored. An impingement geometry case is used to assess the validity of the technique, and a passage with film cooling holes is interrogated. Differences are seen for all sticking and solid phase motion models employed. At small solid particle sizes, considerable disparity is observed between the particle motion modeling approaches, while the position and level of accretion is altered through the use of a nonisotropic stick and bounce model.