Large eddy simulation and analysis of flow and heat transfer performance in a stepped labyrinth seal considering buoyancy effects

During ramjet-mode operation of turbine-based combined-cycle engines, the internal air system is subjected to low mass flow rates and large fluid-to-solid temperature differences. Under the turbine shut-down condition associated with this operating mode, rotation-induced effects are absent, whereas thermal buoyancy can still influence the flow and heat transfer inside stepped labyrinth seals. In the present study, large eddy simulation is used to examine how buoyancy affects the flow field, heat transfer, and sealing performance of a stepped labyrinth seal. The results show that buoyancy remains non-negligible even at a globally low Richardson number, because strong local density gradients induce mixed-convection effects within the seal cavity. In buoyancy-aided regions, fluid mixing is strengthened, local vortical motions are enhanced, and thermal plumes form, leading to local temperature increases. In buoyancy-suppressed regions, mixing and turbulent transport are weakened. As a consequence, buoyancy intensifies the nonuniformity of both flow and heat transfer and modifies the sealing performance. Although the area-averaged friction coefficient and Nusselt number increase by less than 1% and 3%, respectively, buoyancy produces more evident local variations. Relative to the non-buoyant value at the same spatial location, the largest local decrease in friction coefficient is 3.8%, whereas the largest local increase is 5.16%. For the Nusselt number, the largest local decrease is 8.35% in the heat-transfer deterioration region, whereas the largest local increase is 16.42% in the heat-transfer enhancement region. The leakage coefficient also rises by 4.41%. These results indicate that, under the turbine shut-down condition, buoyancy acts mainly by amplifying local flow and thermal nonuniformities rather than by causing large changes in area-averaged quantities.