Conjugate Heat Transfer Study of a Biaxial Rig: Application to the Lifing of HP Turbine Disc Firtrees

Abstract

A full conjugate heat transfer Computational Fluid Dynamics (CFD) study of a Biaxial rig enclosure used for the lifing of the high pressure turbine disc firtrees of a large turbofan engine has been undertaken. Initial rig tests had demonstrated challenges associated with test repeatability and unexpected thermal gradients in some components. The detailed CFD analysis was used to gain an appropriate understanding of the overall heat balance for the assembly, requiring all three modes of heat transfer as well as the fluid dynamics about the disc-blade components. Moreover the detailed modelling methodology presented for the twin short wave infrared emitters allowed the balance of radiation to convection heat transfer from the heaters, essential to the whole study, to be appropriately captured. It was observed that radiation setup the bulk assembly temperature levels (> 900K). Convection heat transfer very much influenced component temperature gradients (10K for disc front-to-back surfaces and >80K for blade front-to-back surfaces). The gap sizes between the assembly and cooling blocks associated with rig setup had a strong influence on the convection currents and flow structures about the assembly and were an important factor in test repeatability. Strong cooler inflows were entrained under the blades, which in the tests had to be compensated by asymmetric top-to-bottom heater power setting of 1.5–2 folds. Once appropriate thermal operating conditions of the assembly were achieved in the tests, the model was tuned to achieve a satisfactory match against the more reliable test thermocouple temperatures, to about -/+10K for the most important locations. Thereafter the model component thermal field was used for follow-on stress analysis, required in critical component lifing and eventual engine certification. This circumvented the need for separate thermal modelling and provided efficient utilization of man-time resource.