Although engineering correlations exist for the effects of small-scale roughness on heat transfer and shear stress, the complexity of flow physics for elements whose height exceeds the sonic line of hypersonic boundary layers is largely unknown. Additionally, the superposition of multiple scales of roughness in largely unexplored. This study investigates the heat transfer augmentation of two scales of patterned roughness, both individually and combined, where the large-scale roughness exceeds 20% of the boundary-layer height. These mimic technical roughnesses from the original experiments from Nikuradse low-speed experiments. The experiments were undertaken in a Ludwieg tunnel at a nominal Mach number of 5 and unit Reynolds number of [Formula: see text]. Determination of the Stanton numbers were performed by combining measurements of the freestream total temperatures with the measurement of the heat transfer from calorimeter and thin-film heat transfer gauges. Results indicate trends toward bulk heat transfer augmentation of order 20, 40, and 50% with increased [Formula: see text] for the small-, large-, and multiscale roughness patterns tested, respectively. For the large-scale roughness pattern investigated, significant heat transfer suppression was measured in the troughs between the elements; enhancement was measured to be substantial on the peaks, with notable difference distinguished between the spans and the intersections of the elements.

This paper presents experimental ice accretion measurements alongside numerical simulations, using the National Research Council Canada’s morphogenetic approach, on a pitot probe geometry at varying icing conditions. In previous publications, the morphogenetic approach for the numerical simulation of ice accretion has shown promise for pitot probe applications, potentially reducing the number of wind tunnel entries, and therefore cost, of the development cycle. An experimental campaign has been completed, providing ice shapes on a representative pitot probe model. Comparison of the experimental and numerical ice shapes indicate that the morphogenetic model is able to generate the complex ice shapes seen experimentally for real-world icing conditions on a fully 3D geometry, closely matching both ice features and total ice thicknesses.

This paper presents the application of the morphogenetic approach for numerical modelling of ice accretion to the test cases of the 1st AIAA Ice Prediction Workshop. The morphogenetic approach differs in a number of key ways from traditional icing codes, allowing it to generate distributed and non-uniform features ice shapes as seen experimentally. Computational fluid dynamics (CFD) and drop trajectory modelling were added prior to the morphogenetic ice accretion code. Making use of the natural surface roughness generated by the morphogenetic approach, a novel technique was implemented to capture the increase in surface roughness with accreted ice, allowing the CFD solution to capture this transient behavior. Time dependency was also simulated for one case, capturing well the growth of the glaze horn features, while other rime cases were successfully simulated using a single time step.

Transient thermochromic liquid crystal (TLC) experiments can provide high-fidelity spatially resolved heat transfer data for complex geometries, particularly where infrared techniques cannot be applied. One challenge when applying transient methods to internal geometries is the local definition of the driving gas temperature. The transient nature of the streamwise driving gas temperature profile has led to comparisons with steady-state computational fluid dynamics being questioned. This paper explores simulating the temporal behavior of transient TLC experiments directly. A novel technique is developed to account for differences in the gas and solid time scales, where surface temperature is calculated at each spatial location analytically from the surface heat flux, using an impulse response method assuming one dimensional, semi-infinite conduction. Postprocessing of the simulated surface temperature history is performed using the same method as experimentally, allowing for direct comparison. This analytical thermal boundary condition (ATBC) is applied to simulate a transient TLC experiment of a stationary superscaled rib turbulated internal cooling passage in a gas turbine engine. Traditional steady-state simulations were also performed with constant temporal and spatial temperature boundary conditions. Results show that calculations using the new ATBC and traditional steady-state method give very similar Nusselt number distributions and mean values in relation to the experimental data, suggesting the larger discrepancy between simulations and the experiments is not the definition of the driving gas temperature. Analysis of transient variation of Nusselt number indicated brief highly localized maximum variations up to 40%, although this was not found to significantly affect the mean values, and passage-averaged values converged to within 0.5% of the final value within 0.2 s.

Stochastic particle tracking models coupled to RANS fluid simulations are frequently used to simulate particulate transport and hence predict component damage in gas turbines. In simple flows 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. To date, the CRW model has used turbulent flow statistics determined from DNS in channels and experiments in pipes. Robust extension of the CRW model to accelerating flows modelled using RANS is important to enable its use in design studies of rotating engine-realistic geometries of complex curvature. This paper builds on previous work by the authors to use turbulent statistics in the CRW model directly from Reynolds Stress Models (RSM) in RANS simulations. Further improvements are made to this technique to account for strong gradients in Reynolds Stresses in all directions; improve the robustness of the model to the chosen time-step; and to eliminate the need for DNS/experimentally derived statistical flow properties. The effect of these changes were studied using a commercial CFD solver for a simple pipe flow, for which integral deposition prediction accuracy equal to that using the original CRW was achieved. These changes enable the CRW to be applied to more complex flow cases. To demonstrate why this development is important, in a more complex flow case with acceleration, deposition in a turbulent 90° bend was investigated. Critical differences in the predicted deposition are apparent when the results are compared to the alternative tracking models suitable for RANS solutions. The modified CRW model was the only model which captured the more complex deposition distribution, as predicted by published LES studies. Particle tracking models need to be accurate in the spatial distribution of deposition they predict in order to enable more sophisticated engineering design studies.

This paper presents an experimental investigation into the spatial deposition of particles in internal flows relevant to the secondary air system (S-bends) at engine representative temperatures and velocities. The confidence in results from computational deposition models is limited by previous workers use of integral deposition measurements to validate their predictions. Measuring spatially resolved deposition measurements means this data can provide more rigorous test data for the validation of numerical deposition models. In the presented results, Arizona Road Dust is injected into the test piece at engine representative temperatures by mixing cold particle laden flow from a generator with hot particle-free flow. The test pieces are preheated and sit within an oven for the duration of the test, creating near steady state conditions. The tests are conducted at realistic metal temperatures (1230 K) and gas temperatures (830 K). For each test, the integrated deposition fraction and spatial map of deposition height is reported. The effects of bend curvature, Reynolds number, and metal temperature are studied. It was found that tests conducted at engine cooling passage representative temperatures exhibit some different fundamental trends in deposition seen when compared to cold experiments. Further, clear trends in the spatial distribution of deposits were apparent. The experiments provide a promising basis for a local deposition data validation set for numerical work with improved resolution compared to integral deposition fraction results.

A significant number of historical engine powerloss events have recently been attributed to ingestion of high altitude ice crystals, prompting regulators to expand engine certification envelopes to incorporate 'ice crystal icing' conditions. There has been a resulting effort by OEMs and academia to develop analytical and semi-empirical models for the phenomenon, partly through use of rig testing. The current study presents results and analysis of experiments conducted in the National Research Council's Research Altitude Test Facility (RATFac). The experiments used a simplified compressor stator vane test article, designed to produce data to build semi-empirical models and validate an existing ice crystal icing code. Accretion growth rates, extracted from backlit shadowgraphy, are presented as a function of test condition, and the algorithm of a new image processing technique using Canny filtering is discussed. Wet bulb temperature, Mach number, particle size and test article angle of attack were systematically varied. In line with previous experiments, the accretion growth rate was observed to be strongly dependent upon bulk particle melt ratio, with a peak growth rate at approximately 10% melt ratio. If leading edge accretions shed during the test, the growth rate of the second accretion would be greater than the first, regardless of test condition, due to the cooling of the substrate surface during the first accretion. The rate of erosion was found to correlate with bulk particle kinetic energy. The highest growth rates were observed for positive angles of attack, at both the leading edge and pressure surface. In contrast, at negative angles of attack growth rates were minimized, attributed to unfavourable accretion conditions on the suction surface. Finally, a qualitative assessment of the accretion quality and build/shed behavior as a function of test condition is presented.

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.

Creating robust empirical and computational models of the process of deposition of salts, dust, sand and volcanic ash has gained increased importance over the last two decades as civil aircraft flights in regions with particulate laden atmospheres have increased. This is associated with increased costs of maintenance to engine suppliers in a market where there is pressure from carriers to continue to fly. Thus, knowledge of the build-up of particulates within the engine over long or multiple deposition events is required in addition to predicting its onset. In this paper deposition in idealised geometries typical of internal cooling passages is examined. The fluid phase is modelled using the commercial flow solver FLUENT and a simple RANS approach. The discrete phase was then solved using Lagrangian particle tracking and a continuous random walk model using one-way coupling. Following identification of deposition fluxes, the local surface of the solid domain was modified using a bespoke cell transformation process. Particular care was taken to distribute deposited mass appropriately to surface cells to avoid large discontinuities at the boundaries. The model was implemented using user-defined functions. The functionality of the technique, is demonstrated through application two impingement cooling geometries for which experimental validation data are available. Here the solution was highly sensitive to the changing target surface geometry as deposition advanced temporally. Fair agreement was found with the experimental data of Burwash et al.[1] though the level of accretion found was an order of magnitude too high, highlighting the need to combine this approach with accurate stick-bounce and shedding models. Significant changes in deposition locations were observed as the deposition site grew in size. Comparison to a second validation case, by Clum et al [2], was used to test further the effect of deposition on the local flow field. Again, good qualitative agreement was obtained. The procedure is shown to create believable deposits of volcanic ash for all cases tested, without many of the typical problems encountered with mesh morphing - overlapping volumes and indeterminate boundary layer resolution. For the commercial computational fluid dynamic (CFD) code used, the process of identifying cells which are to be modified and their neighbours is, disappointingly, an order of magnitude slower than using a mesh morphing strategy. The procedure does, however maintain high, known, resolution throughout the thermal boundary layer, will allow the redistribution of particles to take into account features such as the fusing of neighbouring accretions and the breakaway of deposits from the surface as they grow.

The ingestion of micron-sized particulates into gas turbine engines is known to cause a number of damage mechanisms, from in-flight engine to component life-reduction. Numerical simulations are presented of particle deposition experiments at temperatures representative of gas turbine secondary air systems. The continuous random walk (CRW) model is applied to simulate the fluid turbulent fluctuations observed by the particle in a steady simulation. The CRW model assessed for the first time under an external body force, namely gravitational conditions perpendicular to the bulk flow. The introduction of the external body force is seen to require a redevelopment of the numerical integration time step. This captures the crossing trajectories effect. At ambient isothermal conditions, simulation-experimental differences were seen to be 0.9-49.7%; at high temperature isothermal conditions the differences were seen to be 12.6-50.1%. Simulations of non-isothermal flows were able to capture thermophoretic effects well through the use of the Beresnev and Chernyak thermophoretic coefficient, rather than the widely-used Talbot coefficient.

Transient liquid crystal experiments can provide high fidelity heat transfer data for complex geometries, particularly for internal geometries where IR cameras cannot be applied. However, one main drawback of applying the technique to internal geometries is the accurate definition of the driving gas temperature and the need to define it locally in the streamwise direction. Additionally, due to transient changes in the driving gas streamwise profile, the comparison to steady state CFD has been questioned. This paper explores simulating the transient behaviour of the experiment directly. A novel technique to account for differences in the applicable time scales is developed, where the solid surface temperature is calculated analytically using the impulse response method for a semi-infinite conduction and coupled to CFD solver directly. This is compared to the application of transient conjugate heat transfer. Both numerical methods are applied to simulate a transient liquid crystal experiment of a stationary super-scaled rib turbulated internal cooling passage. The surface temperature from the numerical results is post-processed using the method applied in the experiment to ensure direct comparison. Results show that calculations using the new analytical method and steady state gave very similar Nusselt number distributions and mean value in relation to the experimental data. Analysis of transient variation of Nusselt number indicated localised maximum variations up to 40%, though this was not found to significantly effect the minimised global values.

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 has 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, using a variety of flow coupling models, 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. Numerical models of small particulate turbulent motion and stick/bounce models are developed and integrated within commercial software to perform 3D fluid simulations to capture the deposition behaviour. The technique is implemented using a novel, coupled deposition-dynamic mesh morphing approach to the simulation of particulate-laden flows using RANS modelling of the bulk fluid, and Lagrangian-based particulate tracking. On an iterative basis the calculated particle deposition distributions are used to modify the surface topology by altering the locations of surface nodes. The mesh, continuous phase solution, and particle tracking are then recalculated, from which the mesh is again modified. The sensitivity to the modelling time steps employed is explored. This mesh morphing technique is further refined through the application of the particle stick-bounce model of Bons et al. [1] and the Continuous Random Walk model. An impingement geometry case is used to assess the validity of the technique, and a passage with film cooling holes is interrogated. The paper illustrates that for engine realistic levels of internal deposition this can lead to a significant disparity in the local aerodynamic flow field. Modelling of several internal flow fields have been investigated to illustrate the use of the technique. Differences are seen for all of the sticking and solid phase motion models employed. Notably, there are real discrepancies in using commercial and bespoke models. At small solid particle sizes considerable disparity is observed between the discrete and continuous random walk modelling approaches, while the position and level of accretion is altered through the use of a non-isotropic stick and bounce model.

This paper reports an experimental and numerical study of the development and coupling of aerodynamic flows and heat transfer within a model ribbed internal cooling passage to provide insight into the development of secondary flows. Static instrumentation was installed at the end of a long smooth passage and used to measure local flow features in a series of experiments where ribs were incrementally added upstream. This improves test turnaround time and allows higher-resolution heat transfer coefficient distributions to be captured, using a hybrid transient liquid crystal technique. A composite heat transfer coefficient distribution for a 12-rib-pitch passage is reported: notably the behaviour is dominated by the development of the secondary flow in the passage throughout. Both the aerodynamic and heat transfer test data were compared to numerical simulations developed using a commercial computational fluid dynamics solver. By conducting a number of simulations it was possible to interrogate the validity of the underlying assumptions of the experimental strategy; their validity is discussed. The results capture the developing size and strength of the vortical structures in secondary flow. The local flow field was shown to be strongly coupled to the enhancement of heat transfer coefficient. Comparison of the experimental and numerical data generally shows excellent agreement in the level of heat transfer coefficient predicted, though the numerical simulations fail to capture some local enhancement on both the ribbed and smooth surfaces. Where this was the case, the coupled flow and heat transfer measurements were able to identify missing velocity field characteristics.

Internal cooling passages of turbine blades have long been at risk to blockage through the deposition of sand and dust during fleet service life. The ingestion of high volumes of volcanic ash (VA) therefore poses a real risk to engine operability. An additional difficulty is that the cooling system is frequently impossible to inspect in order to assess the level of deposition. This paper reports results from experiments carried out at typical high pressure (HP) turbine blade metal temperatures (1163 K to 1293 K) and coolant inlet temperatures (800 K to 900 K) in engine scale models of a turbine cooling passage with film-cooling offtakes. Volcanic ash samples from the 2010 Eyjafjallajökull eruption were used for the majority of the experiments conducted. A further ash sample from the Chaiten eruption allowed the effect of changing ash chemical composition to be investigated. The experimental rig allows the metered delivery of volcanic ash through the coolant system at the start of a test. The key metric indicating blockage is the flow parameter (FP), which can be determined over a range of pressure ratios (1.01–1.06) before and after each experiment, with visual inspection used to determine the deposition location. Results from the experiments have determined the threshold metal temperature at which blockage occurs for the ash samples available, and characterize the reduction of flow parameter with changing particle size distribution, blade metal temperature, ash sample composition, film-cooling hole configuration and pressure ratio across the holes. There is qualitative evidence that hole geometry can be manipulated to decrease the likelihood of blockage. A discrete phase computational fluid dynamics (CFD) model implemented in Fluent has allowed the trajectory of the ash particles within the coolant passages to be modeled, and these results are used to help explain the behavior observed.

Threats to engine integrity and life from deposition of environmental particulates that can reach the turbine cooling systems (i.e. < 10 micron) have become increasing important within the aero-engine industry, with an increase of flight paths crossing sandy, tropical storm-infested, or polluted airspaces. This has led to studies in the turbomachinery community investigating environmental particulate deposition, largely applying the Discrete Random Walk (DRW) model in CFD simulations of air paths. However, this model was conceived to model droplet dispersion in bulk flow regimes, and therefore has fundamental limitations for deposition studies. One significant limitation is an insensitivity to particle size in the turbulent deposition size regime, where deposition is strongly linked to particle size. This is highlighted within this study through comparisons to published experimental data.Progress made within the wider particulate deposition community has recently led to the development and application of the Continuous Random Walk (CRW) model. This new model provides significantly improved predictions of particle deposition seen experimentally in comparison to the DRW for low temperature pipe flow experiments. However, the CRW model is not without its difficulties. This paper highlights the sensitivities within the CRW model and actions taken to alleviate them where possible. For validation of the model at gas turbine conditions, it should be assessed at engine-representative conditions. These include high-temperature and swirling flows, with thermophoretic and wall-roughness effects. Thermophoresis is a particle force experienced in the negative direction of the temperature gradient, and can strongly effect deposition efficiency from certain flows. Previous validation of the model has centred on low temperatures and pipe flow conditions. Presented here is the validation process which is currently being undertaken to assess the model at gas turbine-relevant conditions. Discussion centres on the underlying principles of the model, how to apply this model appropriately to gas turbine flows and initial assessment for flows seen in secondary air systems. Verification of model assumptions is undertaken, including demonstrating that the effect of boundary layer modelling of anisotropic turbulence is shown to be Reynolds-independent. The integration time step for numerical solution of the non-dimensional Langevin equation is redefined, showing improvement against existing definitions for the available low temperature pipe flow data. The grid dependence of particle deposition in numerical simulations is presented and shown to be more significant for particle conditions in the diffusional deposition regime. Finally, the model is applied to an engine-representative geometry to demonstrate the improvement in sensitivity to particle size that the CRW offers over the DRW for wall-bounded flows.