Engineering Analyses for Gas Pocket Damper Seals and Combined Labyrinth-Brush Seals
Pocket damper seals are used as balance drum, interstage and impeller eye seals to restrict leakage flow rate from a high-pressure region to a low-pressure region in compressors and turbines. 
Combined labyrinth-brush seals are common in aircraft engines and steam turbines to increase plant efficiency and reduce leakage in a similar size labyrinth seal.
SPONSOR: Turbomachinery Research Consortium (TRC)
OBJECTIVE: To develop computational models for prediction of leakage, drag power loss and force coefficients of fully portioned PDS and combined labyrinth-brush seals. In the first year, leakage models will be developed and verified, and in the second year the computational models will advance to predict rotordynamic force coefficients.
Pocket damper seals (PDS), adding baffles in (alternating) circumferential cavities of labyrinth seals have demonstrated enormous benefits in seal stability by providing physically large damping coefficients .
A Fully-partitioned pocket damper seal (FPDS) has axial baffles covering the whole seal extent and the pockets are separated by thick small clearance regions (not sharp teeth) which can amount to 20% of the whole sealing area.
These seals, as shown in Fig. 1, when properly designed, can act as damper bearings by offering also large stiffness coefficients. In experimental comparisons  with a labyrinth seal and a honeycomb seal, the FPDS showed the largest destabilizing cross-coupled stiffnesses, strong functions of inlet pre-swirl and rotor speed.
Fig. 1. Schematic view of a conventional pocket damper seal (PDS) and a fully-partitioned pocket damper seal (FPDS).
Ertas, B.H., Vance, J.M., 2007, “Rotordynamic Force Coefficients for a New Damper Seal Design,” ASME J. Trib. 129, pp. 365-374
Fig. 2 shows a combined labyrinth-brush seal. Combined labyrinth-brush seal may increase plant efficiency and reduce the leakage in a similar size labyrinth seal. In addition, some steam turbines presently incorporate a hybrid seal composed of a BS installed mid-way of a multiple-teeth labyrinth seal. The hybrid BS-LS  affords significant sealing improvements over conventional labyrinth seals. “Additionally, when retrofit into labyrinth seals with radial movement, the individual labyrinth seal segments are free to move radially independently of one another during transients.” 
Fig. 2. Schematic view of a combined labyrinth-brush seal.
Patent Number: WO 2004/023008, Inventors: R.P. Menendez and H. King, Publication date: 18 March 2004
 Li, J., 1999, “A
Bulk-flow Model of Multiple-Blade, Multiple-Pocket Gas Damper Seals,” Ph. D
 Childs, D. W.,
Vance, J. M., 1997, “Annular Gas Seals and Rotordynamics of Compressors and
Turbines”, Proc.26th Turbomachinery Symposium,
 Ertas, B.H., Delgado, A., and Vannini, G., 2011, “Rotordynamic Force Coefficients for Three Types of Annular Gas Seals with Inlet Preswirl and High Pressure Differential Ratio,” ASME GT2011-45556.
 Skinner, D.R., et al., 2001, “Combined Brush Seal and Labyrinth Seal Segment for Rotary Machines,” US Patent 6,257,586 B1 (see also http://www.turbocare.com/retractable_brush_seals.html