| Resumo: | <p>Conjugate heat transfer (CHT), which captures the thermal interaction between the fluid and solid domains, is important for an accurate computational aerothermal analysis when there are temperature dependent fluid domain physics. In the rotating cavities in compressor internal air systems, there is a need to understand the unsteady buoyancy-driven flow and heat transfer so that the disk radial expansion (and hence change in blade tip clearance) throughout the flight cycle can be predicted. Despite the inherently conjugate nature of rotating cavity flows, coupled analyses are rare due to the challenges of using CHT with unsteady methods.</p>
<p>In this thesis, it is shown that it is necessary to use wall-resolved large eddy simulation (LES) to accurately capture the fluid domain flow structures, and to use CHT to capture the interaction of the wall heating conditions and the cavity aerodynamics. The main challenge in LES-CHT is that the timescale in the solid is significantly larger than in the fluid. Many current state-of-the-art methods try to address this by accelerating the solid domain timescale to achieve a temporally converged solution, before switching to a non-accelerated solid solution that is assumed to be correct. However, this is not the case - it is demonstrated in a simplified conduction analysis that these current time domain CHT methods still introduce a long-lasting error in the solid domain temperature.</p>
<p>As a result, a new multiscale framework for LES-CHT has been developed, building on an existing closely coupled methodology. Non-local interactions from coherent low frequency temperature fluctuations are calculated with a time-spectral solid domain solution and turbulent higher frequency fluctuations are accounted for using a wall transfer function approach, reformulated to simplify its application. This is the only framework that can consistently deal with the fluid-solid timescale disparity and capture all timescales of wall temperature fluctuation.</p>
<p>The novel framework is used in first-of-their-kind LES-CHT computations on rotating cavities, evidencing framework efficacy and providing further evidence of the previously observed issues with other LES-CHT methods. An examination of the effect of CHT on the flow structures within the rotating cavity found that thermally induced near-disk vortical instabilities are seen to only form when there is strong local heating. This effect has not been previously observed and illustrates the value of the multiscale framework for deriving new insights into heat transfer in gas turbines.</p>
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