Numerical Investigation to Support the Design of a Flat Plate Honeycomb Seal Test Rig

Among the various type of seals used in gas turbine secondary air system to guarantee sufficient confinement of the main gas path, honeycomb seals well perform in terms of enhanced stability and reduced leakage flow. Reliable estimates of the sealing performance of honeycomb packs employed in industrial gas and steam turbines, are however missing in literature, thus, in order to evaluate the complete characteristic curve of the seals in the wide range of working conditions, an experimental campaign is planned. This work reports the findings of the numerical investigation exploited to properly design such test rig. Computations are performed with the steady-state RANS solver implemented in Ansys CFX ^® using k-ω SST turbulence model with automatic wall treatment and exploiting symmetry condition when possible. Due to the generally large amount of honeycomb cells typically present in real seals, it would be convenient to treat the sealing effect of the honeycomb pack as an increased distributed friction factor on the plain top surface that is why the simplest configuration, the honeycomb facing a flat plate, is employed in this paper. The geometry of the hexagonal cell and the investigated clearances were chosen to well represent actual honeycomb packs employed in industrial compressors. First the pressure distribution within the seal was analysed verifying that downstream the first 5 rows of cells where entrance effects are predominant, the relative pressure drop is almost constant thus the use of an equivalent friction factor is appropriate to characterize the seal. Furthermore the calculated pressure field was used to assess potential effects of pressure probe positioning. Subsequent analysis focused on the characterization of the friction factor as function of the Reynolds number with the aim of establishing the proper geometrical scaling to achieve flow conditions similar to real turbine most critical ones. The eventual direct influence of both geometrical scaling and operating conditions was investigated as well. Additional CFD computations were used to assess the entrance length effects and the spanwise extension of the honeycomb pack. Finally the different behaviour of the honeycomb sealing depending on the hexagonal cell arrangement was evaluated both in terms of flow structure and friction factor showing an increase of 15% circa with the facing edge arrangement.