Experimental Measurements of Heat Transfer in Aero-Engine Compressor Rotors

Abstract

The climate crisis necessitates reducing aircraft emissions. Goals set by ACARE Flightpath 2050 require net zero CO2 operation and 90% reduction in NOx emissions by 2050 relative to year 2000 levels. This is achievable with modern engine architectures, such as ultra high bypass ratio engines and geared turbofans. However, increased bypass ratios require a reduction in engine core size, reducing clearances, which increases the sensitivity to flow disruptions. To maximise the operating efficiency of an aero-engine, temperature and stress distributions have to be modelled accurately. In the compressor, blade tip clearance depends on the radial expansion of the compressor discs, which is directly coupled to conjugate heat transfer in co-rotating discs governed by unsteady and unstable buoyancy-induced flow in the inter-disc cavities.

This thesis discusses a series of experimental studies using the Bath Compressor Cavity Rig, which simulates a generic axial compressor at fluid-dynamically scaled conditions. The rig features a titanium disc pack instrumented with thermocouples, heat flux gauges and unsteady pressure transducers collecting data in the frame of reference of the rotating discs. Modular cob attachments allow for modifying the cavity geometry. Two electric heater assemblies create varying temperature gradients on the discs. The rig was specifically designed to generate heat transfer of practical interest to the engine designer and validate computational codes. Experimental data was supported by theoretical modelling and enabled the development of a reduced-order model able to accurately calculate the radial distribution of disc and fluid-core temperatures.

The flow structure in a rotating cavity typically consists of one or multiple pairs of counter-rotating vortices separated by hot and cold radial plumes. The unsteady, self-organising flow is driven by buoyancy and Coriolis forces at Grashof numbers ∼10^13. Additionally, the cavity core can interact with the bore flow from the Secondary Air System. The degree of this interaction depends on the cavity geometry at low radius; a non-linear variation in exchange flow with the cavity opening width was observed experimentally. When the mass and enthalpy exchange between the cavity and the throughflow was limited by installing aluminium cob attachments, conduction significantly increased disc temperatures at low radius. At high radius, compressibility effects led to an increase in core temperature. Increased core temperature suppressed the shroud heat transfer, which in some cases led to flow stratification.

A strong relationship between the shroud heat transfer and the flow structure formation was observed experimentally for the first time under transient conditions. The strength, rotational frequency, stability and number of the unsteady vortex structures in the cavity changed with rotational Reynolds and Grashof numbers. A critical Rossby number separated two flow regimes. At subcritical Ro, the vortices were only present when the flow in the rotating cavity was dominated by buoyancy; structures disappeared when the shroud heat flux reduced to zero. At supercritical Ro, the flow was dominated by a toroidal vortex, even under quasi-isothermal conditions. The shroud heat transfer, represented by the Nusselt number, as well as the radial mass flow rate in the rotating core were correlated against Gr. Remarkably, consistent correlations were revealed for both steady state and transient conditions over a wide range of Gr.

Varying heating configurations were tested to simulate effects associated with different compressor stages. Axial temperature gradient was introduced by offsetting the shroud heater assembly to replicate the inter-stage temperature rise in the main gas path. The disc temperature distribution was influenced by the throughflow impingement at low radius and significant conduction on the shroud. The flow structure behaviour was consistent with findings from symmetrically heated open cavities at lower β∆T. A novel method for calculating the buoyancy parameter in cavities with an axial temperature gradient allowed for accordant comparison of fluid dynamics and temperature distributions with symmetrically heated cavities. The proposed analysis method supports the application of reduced order models taking into account only the radial temperature gradient to differentially heated cases with reasonable confidence.

Heating the throughflow created an inverted radial temperature gradient, which occurs in the Low and Intermediate Pressure compressors. Unexpectedly, the new data revealed the flow structure in cavities with positive and inverted temperature differences were fundamentally similar, albeit with reversed radial temperature profiles. The flow structure for the inverted temperature gradient continued to be governed by buoyancy due to disc heat transfer at low radius, despite near-zero shroud heat flux. Differences in slip and vortex symmetry were observed between cases with opposite temperature gradients as a result of momentum exchange with the axial throughflow and the circumferential pressure gradient.

Results presented herein provide information about the fundamental fluid dynamic phenomena affecting compressor cavities under previously untested conditions. The new insights are of importance for the determination of thermal stresses in discs, engine life, compressor blade clearance and efficiency.

Date of Award	10 Dec 2025
Original language	English
Awarding Institution	University of Bath
Sponsors	EPSRC & Rolls-Royce PLC
Supervisor	James Scobie (Supervisor), Gary Lock (Supervisor) & Hui Tang (Supervisor)