Modelling sub-grid scale features in congested engine ventilation zones

In the increasingly congested accessory zones of gas turbine engine casings, it is important that the several temperature-sensitive components, like the electronic engine control unit (EEC), are bathed in an appropriate ventilation environment. Additionally it must be ensured that heat sources, like the geometrically complex gear box components, furthest from the inflows do not sit in stagnant zones. In this paper CFD methods have been used to study in detail the ventilation and heat transfer environment of one particular zone - that of the fan casing in the engine nacelle of a high by-pass turbofan. A particular challenge was the appropriate modelling of the extensive pipe systems that existed in this environment, ensuring that their impact on the flow field and heat transfer was suitably taken into account. Whilst in past practice large components and ducts have been modelled in CFD studies, the small scale pipe systems and electrical harnesses do not lend themselves easily to explicit modelling strategies. In this work a methodology is presented whereby the effects of all small scale pipe systems within the zone are represented using a sub-grid modelling approach. The momentum drag and heat release associated with all small scale pipes have been modelled and their impact on the ventilation and heat transfer characteristics of the accessory zone environment assessed. Comparisons made with the explicit methodology, not employing sub-grid models, have revealed that the small scale pipe systems have a significant impact on the flow and heat distribution, particularly around the EEC. Finally, limited comparisons with similar test rig flow visualisation data have been made, confirming the overall flow pattern within the zone. The work also suggests approaches in which the sub-grid methodology may be extended and verified for engine design purposes.