Natural convection prediction closely relevant to flexible operations (e.g. fast and frequent startups and showdowns) of gas turbines and steam turbines presents considerable challenges. The strong inter-dependence between fluid and solid parts points to the need for conjugate heat-transfer (CHT) methods. However, the long time scales of the practical operation processes of interest, and the fundamental fluid-solid time scale disparity raise general issues regarding the computational costs of the CHT methods. In particular, if a high-fidelity flow model (e.g. LES) needing to resolve smaller time scales of turbulence is adopted, we also face an additional question regarding the consistency and accuracy of the fluid-solid interface treatment.
In this paper, we address the issues by the means of a loosely coupled CHT procedure based on the multi-scale methodology recently proposed for transient conjugate heat transfer predictions. The multi-scale framework provides an efficient way for accurately solving problems with a huge scale disparity. A particular emphasis of the present work is on efficient and accurate transient CHT solutions in conjunction with the turbulence eddy resolved modelling (LES) for natural convection. A multi-scale flow decomposition associated with the corresponding time step split is adopted. The resultant triple timing formation of the flow equations can be solved efficiently for the fluid-solid coupled system with very disparate time scales. The methodology will be presented with case studies supported by a new interface analysis to underpin the problem statement and motivation of the present work, and to demonstrate the validity and effectiveness of the methodology and implemented procedure.
Multi-scale time integration for transient conjugate heat transfer