Development and Validation of a Numerical Model of the CO2 Dry-ice Blasting Process for Aircraft Engine Cleaning Applications

On-wing cleaning of engine compressors for commercial aircraft is a required maintenance task which results in greater operating efficiency and lower emission rates. It is typically carried out by injection of water and detergents into the intake of an engine while the engine is being cranked by the starter. Two drawbacks of this process are the risk of icing in cold weather and the collection and treatment of the water effluent. The dry-ice blasting process, a cleaning system which uses pressurized air and CO2 dry-ice particles as cleaning agent, has been proposed as an alternative method which does not suffer the above drawbacks but is potentially capable of efficient cleaning. In this context, such a cleaning system is currently being developed by Lufthansa Technik in association with Hochschule Darmstadt and DIT. This work focuses on the development and validation of a numerical model of this process, which can be used to improve the understanding of the complex multiphase flow phenomena involved and to assess the cleaning physics. Appropriate multiphase flow set-ups and new particle breakup and erosion models are developed. These new models will facilitate the numerical prediction of particle behaviour and defouling erosion rates during the defouling process. An appropriate simulation set-up for the particle laden injection system flow simulations using the Euler-Lagrange method is investigated. Three possible injection systems with various air flow velocities and particle loading densities are considered. These systems are investigated by means of high-speed camera (HSC) experiments and the predicted results are compared to the experimental in order to find the best numerical set-up. An improvement to the particle drag force formulation is proposed for highly pressurized air-flows. A new particle breakup model for dry-ice in Euler-Lagrange simulations is developed. This model is theoretically derived from an energy balance and un-derpinned with data from HSC experiments. It includes velocity, impact angle and target temperature as factors determining breakup behaviour of dry-ice particles impinging solid walls. A new defouling erosion model utilizing an energy balance approach and based on a range of experiments with several types of actual and artificial fouling material is developed and tested. The particle breakup and the erosion model are implemented into the commercial CFD code Ansys CFX. Verification and validation studies of both new models are presented. The validation of the new models uses data acquired in a specially-designed wind-tunnel experiment. All main findings and models are used in a final application case study where the new dry-ice based cleaning procedure is applied to a GE-CF6-50 test engine. Comparison of numerical results to data from air-flow, particle tracking and defouling experiments is also presented for this case

An Energy-based Approach to Assess and Predict Erosive Airfoil Defouling

A dynamic indentation experiment is presented for assessment of the adhesive behavior of a range of coatings in erosive defouling of commercial aircraft engines using CO2 dry-ice. A series of experiments is presented in which particles made from a reference material (polyoxymethylene – POM) and from CO2 dry-ice are made to impact compressor airfoils under a range of impact angle and velocity conditions. The airfoils investigated are coated with an indicator material (PTFE), which is typically used to visualise the defouling effect in large scale compressor defouling experiments. In addition, fouled compressor airfoils taken from service and coated with a fouling typically found in low-pressure compressor stages are investigated. The energy required for the reference particles (POM) to create a defouling effect for the different coatings is determined by an experimental evaluation of their coefficient of restitution. This energy requirement is assumed to be fouling specific. Empirical defouling functions are presented. They correlate the defouling effect for both particle materials under various impact conditions. The empirical correlations are developed into a simulation procedure to predict particle impact erosion and energy dissipation of coated surfaces in numerical indentation simulations

A VALIDATION STUDY FOR A NEW EROSION MODEL TO PREDICT EROSIVE AIRFOIL DEFOULING

A new defouling erosion model for Lagrangian particle tracking is used to predict defouling of amor-phous, heterogeneous coatings such as those typically found in aircraft compressors. The main problem description, the mathematical formulation and the underpinning experiment of the model are presented in a previous communication by the authors. In this work, the Ansys CFX implementation of the model is described and an experiment is presented for the validation of the model. Air flows laden with a number of dry-ice particles are observed in an optically accessible stream channel containing a flat plate target. The defouling process of these particles is recorded with HSCs and the main parameters, such as indentation size in fouling layers, are processed and compared to corresponding numerical results. The model parameters considered are particle impact velocity and angle as well as particle and fouling material. Typical coatings which are relevant to commercial aircraft defouling processes are investi-gated. The target plate angle and the air velocity are varied and dry-ice particles of random size and shape are injected into the flow. The experiment is set up in a wind-tunnel test-rig and all recordings are made using two HSCs, a digital camera and Prandtl probe measurement. Experimental and numerical defouling results show good overall agreement for steep target angles but significant deviations for low target angles. Potential improvement to the defouling erosion model is discussed based on these results. The model as presented is used in large-scale compressor defouling simulations in the development process of on-wing aircraft maintenance system