Turboshaft engines employed on rotorcraft operating in dusty environments are susceptible to ingestion of dust particles lofted into a dense cloud by downwash from the helicopter rotor. When encountered in these significant quantities, the primary damage mechanism can be the accumulation of molten particles on high-pressure turbine nozzle guide vanes. This produces a layer of deposited particles which modifies the vane geometry, nozzle flow field and restricts the core flow. Over time this can manifest as compressor surge and potentially a complete loss of power from the engine. The point where this failure occurs is difficult to predict owing to a range of variables relating to the architecture of the engine, the dust ingested, and the performance requirements placed on the engine by the mission being executed.

The severity of damage due to particle deposition is a function of the physical state of the particles upon impact with the vane. This depends upon the rate at which heat is transferred to the particle from the hot gas stream and thus if it is resident in the flow for a sufficient time to allow it to partially soften or fully melt. The particle state is, therefore, a function of the NGV flow field temperature. This is something which depends strongly on the engine architecture, particularly the location of the combustor exit nozzle relative to the guide vane. The placement of this feature introduces a hot streak into the flow with a temperature gradient of up to 200K across it \[1\]. The trajectory followed by a particle through this hot streak determines the rate at which it softens and therefore, its state upon impact. Particles passing through the relatively colder part of the flow are less likely to soften and adhere whilst those passing directly through the hot streak are more likely to melt and deposit.

A low-order methodology has been developed based on numerical simulation data, which reduces the probability of a particle depositing to a simple two-equation model based on the likelihood of a particle interacting and subsequently being retained on a vane. The probabilities of interaction and retention are described as functions of non-dimensional inertial and thermal response times of each particle \[2\]. The probability of retention is underpinned by the existing particle fate model of Bons and Whitaker \[3\] where the likelihood of deposition is calculated based upon an elastic-plastic energy balance and the manner in which the particle elastic properties degrade with temperature. We demonstrate the use of this model to carry out a parametric analysis to predict the proportion of a particle size distribution which deposits on the surface of a vane for different combustor-vane architectures, bulk flow temperatures and temperature gradients across the hot streak. Compared to existing high fidelity CFD models this low-order approach has the potential to provide faster, less computationally expensive assessments of the susceptibility different engine architectures and operating states have to NGV deposition damage.

 

\[1\]          R. Prenter, A. Ameri, and J. P. Bons, ‘Computational Simulation of Deposition in a Cooled High-Pressure Turbine Stage With Hot Streaks’, *J. Turbomach.*, vol. 139, no. 9, p. 091005, Apr. 2017.

\[2\]          N. Bojdo, M. Ellis, A. Filippone, M. Jones, and A. Pawley, ‘Particle-Vane Interaction Probability in Gas Turbine Engines’, *J. Turbomach.*, vol. 141, no. 9, p. 091010, Jun. 2019.

\[3\]          S. M. Whitaker and J. P. Bons, ‘An Improved Particle Impact Model by Accounting for Rate of Strain and Stochastic Rebound’, in *ASME Turbo Expo*, Oslo, Norway, June 2018.

 


Influence of Turboshaft Engine Architecture on Ash Particle Deposition: Reduced Order Model Application

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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