R.C. Hendricks

Glenn Research Center, Cleveland, Ohio

M.J. Braun and D. Deng

University of Akron, Akron, Ohio

J.A. Hendricks

Diligent Design, Toledo, Ohio

Compliant Turbomachine Sealing

NASA/TM—2011-214040

May 2011

54–ISROMAC–11
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R.C. Hendricks

Glenn Research Center, Cleveland, Ohio

M.J. Braun and D. Deng

University of Akron, Akron, Ohio

J.A. Hendricks

Diligent Design, Toledo, Ohio

Compliant Turbomachine Sealing

NASA/TM—2011-214040

May 2011

54–ISROMAC–11

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

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Acknowledgments
NASA/TM—2011-214040 1 

Compliant Turbomachine Sealing 

Robert C. Hendricks 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Minel J. Braun

University of Akron 

Akron, Ohio 44325 

D. Deng

University of Akron 

Akron, Ohio 44325 

John A. Hendricks

Diligent Design 

Toledo, Ohio 43614 

Abstract 

Sealing interface materials and coatings are sacrificial, 

giving up their integrity for the benefit of the component. Seals 

that are compliant while still controlling leakage, dynamics, 

and coolant flows are sought to enhance turbomachine 

performance. Herein we investigate the leaf-seal configuration. 

While the leaf seal is classified as contacting, a ready 

modification using the leaf-housing arrangement in conjunction 

with an interface film rider (a bore seal, for example) provides 

for a film-riding noncontact seal. The leaf housing and leaf 

elements can be made from a variety of materials from plastic 

to ceramic. Four simplistic models are used to identify the 

physics essential to controlling leakage. Corroborated by CFD, 

these results provide design parameters for applications to 

within reasonable engineering certainty. Some potential 

improvements are proposed. 

Introduction 

Brush, finger, and foil seals are representative of compliant 

seals. Their modeling and applications and other compliant seal 

configurations are described elsewhere (e.g., see [1] to [4]). 

Herein we consider some design parameters for the leaf seal 

described by Nakane et al. [5], Flower [6], and Steinetz and 

Sirocky [7] with some potential for modifications such as 

adding an elastic retainer at the outer perimeter, a film-riding 

ring interface, and side-wall sealing (Figs. 1 and 2). 

Several types of retainers and film-riding rings can be 

envisioned, including the bump foil, garter springs, and sealing 

elastomers, with shaft or wave-riding interfaces to enable the 

linear and rotary motions depending on the stiffness 

requirements, leaf geometry, and rotor interface. 

Static and dynamic interface clearance and wear characteristics can be controlled by adjusting elastomer tension. With 

active control such adjustments can be implemented upon 

demand through use of piezoelectric actuators for example and 

a tension band (ring) acting on the elastomer. By controlling the 

retainer ring tension, the effective leaf seal stiffness can be 

higher than the brush seal and float within the housing 

confinement. This implies, for example, that Si3N4 wafers or 

composites or coated plates can be used. It also implies that 

functionally graded materials can be used as plates. Segmented 

seals could be made similar to those for the brush seals and like 

brush seals can be waxed to facilitate installation, and the wear 

interface can be coated. 

The importance of the positioning and functioning of 

O-ring or piston-ring-type side-wall seals both on the low- and 

high-pressure side walls cannot be overemphasized. These 

rings must be close to the interface to mitigate leaf leakage and 

facilitate leaf dynamics, otherwise excessive leakage can be 

expected (Fig. 2). 

Herein, we (1) present and discuss experimental leakage data 

for four simplistic models that capture the essential leaf and 

side-wall physics, (2) corroborate these findings by 

computational fluid dynamics (CFD) modeling, and (3) suggest 

some potential improvements. 

Modeling 

Usual practice is to make and test models that are scalable 

with high-order accuracy. For exploratory purposes however, 

the concept herein is that it is only necessary for the model to 

capture the fundamental physical parameters of a component 

and validate experimental data via CFD. Once validated, one 

can then predict with reasonable engineering certainty actual 

physical components for engines. 
NASA/TM—2011-214040 2 

With that in mind we undertook to fabricate several models 

that captured the essence of leaf sealing, acquired flow data 

over a small range in pressure differentials, and determined 

how well these data are represented by CFD modeling. These 

models are all static configurations, as rotation accounts for a 

small decrease in the leakage and dynamics will, if hysteresis 

is involved, enhance leakage. We first want to establish some 

concepts for the leakages. 

Four such models were fabricated based on the principles of 

leaf geometry relative to a bore seal configuration. The first 

model is that of a 74-leaf, tight packed configuration (Fig. 3). 

These leaves in theory overlap, yet in practice the surfaces 

deform and are packed about the simulated shaft. In each case 

an elastomer is used to simulate the outer wave spring that sets 

the leaves into contact with the shaft and restores sealing during 

seal dynamics. Here the elastomer also held the leaves into 

place while facilitating assembly and disassembly. In a real 

configuration, waxing of the leaves would serve this function 

and would burn out with operation; the wave spring then 

becomes effective. 

Decreasing the number of leaves to 63 (Fig. 4) allows a 

configuration without overlapping geometries and similar to 

 
NASA/TM—2011-214040 3 

Fig. 3, except that at the outside perimeter, the leaf spacing 

is alternately increased to account for the change in 

circumferential spacing. 

Details of the leaf configurations are given in Table 1. 

TABLE 1. LEAF GEOMETRIES IN SEALS OF FIGS. 3 TO 6 

[See schematic in Fig. 7.] 

Number of leaves, N 533 74 63 63 

Spacing uniform uniform alternating alternating

spring 

Figure 5 3 4 6 

Thickness, t, in. 0.008 0.052 0.052 0.052 

Leaf angle, θ, deg. a 40 to 60 33 25/32 22/25 

Flow area, in.2 0.2274 0.19 0.228 0.228 

Outside diameter, OD 4.38 4.38 4.38 4.38 

Inside diameter, ID 1.9 1.9 1.9 1.9 

Leaf length, Lo, in.b 1.5 1.5 1.5 1.5 

Leaf width, W, in. 1.125 1.125 1.125 1.125 

Fence O-ring diam., in. 2.25 2.25 2.25 2.25 

a

 Angle measured from tangent plane to leaf. 

b No attempt was made to contour the leaf tip at the leaf-shaft interface. 

The third and fourth configurations are based on how leaf 

geometry effects seal leakage. When the leaves are made thin 

and flexible, which is characteristic of a low-stiffness seal, the 

number of leaves becomes large (Fig. 5). For leaves of 

0.008-in. thickness, compared to the 0.052-in. thickness of 

Figs. 3 and 4, the number of leaves packed into the seal is 

N = 533. 

The fourth configuration considered a closing of the intraleaf 

triangular spacing generated by the differences between the 

outer diameter (OD) and inner diameter (ID) providing a leaf 

spring whereby the inner leaf can be more compatible with the 

shaft interface and the other leaf spacers of different materials. 

In Fig. 6(a) the leaves are aligned such as to provide point 

contact when the principle leaf is flexed while in Fig. 6(b) a 

line of contact provides additional support to the principle leaf. 

Leakages for the model of Fig. 6 are similar to the model of 

Fig. 4, but CFD was not done on this configuration. 

 
NASA/TM—2011-214040 4 

These arrangements permit a wider use of materials at the 

rotor interface and would be beneficial for the case where the 

leaves are used to spring load a wave-rider rotor, for example. In 

the wave-rider design, an annular ring (or ring segment) with 

an inscribed wave [8] would provide a film shielding the shaft 

from contact while the leaves provide sealing and compliance 

(Fig. 1(b)). 

Analysis 

Flow Through a Slit 

Consider a leaf. In general it will be tapered or made in some 

manner to compensate for the inner and outer circumference. In 

a brush seal riding on a shaft at its inner circumference, the 

bristle pack itself is tapered (thicker at shaft and thinner at outer 

diameter). As the average taper is at this point unknown, we 

shall consider the geometry at the midplane of the leaf to be 

representative. This is conservative in that the housing will 

cover the leaf pack at this point, yet it may compensate for 

neglecting leakage at the shaft interface. For modeling 

convenience, we use the leaf midplane and simulate the leakage 

path as that of a slit (Fig. 7). 

In the 74-leaf configuration (Fig. 3), the tight-packed leaves 

are deformed near the rotor interface. In the 63-leaf 

configuration (Fig. 4), which is essentially the 74-leaf with 11 

leaves removed, tight packing is provided without deformation. 

The 533-leaf configuration (Fig. 5) is tight packed with some 

deformation principally between the seal dam and the interface. 

Flexibility and sizing of the leaf materials near the interface 

becomes important as an error of 0.00005 in. in average 

thickness can result in a 0.043-in. overall error in packing. For 

some real seal configurations with some 2000 leaves, such an 

error would be an intolerable 0.1 in. 

The 63-spring-leaf configuration is slightly underpacked (Fig. 6). 

The leakage flow is simplistically given for an aperture flow 

model as 















 γ =Δρ= e

D eeD RT

2 PMACPACm (1) 

and the exit Mach number is given as 

















 −γ















− 













 =

γ −γ

1

2 1

1

e

t e P

P M (2) 

where 

 CD is the flow coefficient 

 A is flow area 

ρ is density 

 P is pressure 

 Pe is exit pressure 

γ is the ratio of specific heats 

 R is the gas constant 

 Te is the exit temperature 

 Pt is total pressure 

The parameter CDA accounts for many unknowns including 

actual flow area, leaf geometry, friction, and tolerances. 

Determining the actual values for these parameters is difficult 

with only an estimate made for the flow area A. CD is then 

determined empirically. 

CFD Model 

The commercial numerical code CFD-ACE+ (CFD Corp., 

Huntsville, AL) was used to model the 533-, 74-, and 63-leaf 

configurations. Using the CAD modeling program, geometries 

for the leaf seal can be constructed. These models serve as the 

basis for the flow calculations and determination of both flow 

areas and leaf angle assuming that the leaf contact points with 

simulated rotor are fixed and leaf interference is allowable. The 

wire frame model and pressure profiles are illustrated in Fig. 8. 

The boundary conditions of the triangular volumes are solid 

wall from the OD to the sealing dam, as are the leaf interfaces 

and rotor. At the inlet and exit leakage gap the pressure is 

constant with atmospheric at the exit. The pressure drop is 

significant at the inlet and exit with little to moderate changes 

over the width of the leaf, somewhat reminiscent of a labyrinth 

cavity. Thus, for these seal configurations, the radial position 

and sealing capability of the inlet and outlet O-ring, pistonring-type, or rubbing contact side seal become very important. 

Variations in flow area and leaf angle with the number of 

leaves for fixed-leaf, simulated-shaft, and housing geometries 

is illustrated in Fig. 9. 

Grid independence was checked. One needs to also take care 

in specifying that the inlet pressures and temperatures are 

stagnation values; otherwise the code will not converge 
NASA/TM—2011-214040 5 

properly. Also checked were the effects of turbulence and leaf 

blunt-edge inlet (by adding a plenum region), which lowered 

flow rates between 8 and 10 percent in each case for the N = 63 

leaf configuration. These real effects are to be considered in 

leaf-seal design and indicate a flow rate lower than predicted. 

Results 

Flow within the leaf seal, like the labyrinth seal, is relatively 

unperturbed and dominated by the exit piston-ring or O-ring 

side-wall sealing. The difference between the outer and inner 

circumference engenders large unimpeded flow apertures as 

illustrated by the streak lines and velocities shown in Fig. 10 

and the near constant pressure in the cavity region shown in 

Fig. 8. Figs. 10(a) and (b) illustrate the main flow path with low 

circulation in the leaf cavity on the centerline between leaves at 

1 psi pressure drop for the configurations N = 63 leaves at 28° 

and N = 533 leaves at 50°, respectively. In both cases, the flow 

rapidly expands through the inlet and the exit, but for N = 533 

the flow is near the sealing dam at the inlet and more uniform 

at the exit. Readily apparent are the differences in the 

circulation patterns within the cavity region: in Fig. 10(a) it is 

more inertia dominant and in Fig. 10(b), more viscous 

dominated. Figs. 10(c) and 10 (d) contrast the differences in 

streak lines within the leaf cavity and illustrate a more uniform 

behavior for the N = 533 configuration. This also serves to 

illustrate both the function and importance of locating the sidewall seals (piston-type, O-ring, or sliding contact seal) close to 

the rotor interface. 

As can be seen from Fig. 11, the leakage is marginally 

decreased as the packing of the leaves is increased from 63 to 74, 

yet a considerable decrease is seen for N = 533. The temperature 

is ambient and the pressures are inlet-stagnation values. 

For the N = 74 model (Fig. 3) the simplistic leakage model 

gives <CD> = 0.7 over the range of data which is in reasonable 

agreement with other known aperture flow experiments. The 

CFD solution for this same model provides a reasonably good 

prediction to within 8 percent of the data. The effects of 

turbulence were checked and the effects of entry losses were 

found by adding an upstream plenum region. Each were found 

to decrease the leakage by 8 to 10 percent, which places 

experiment and analysis in reasonably good agreement. The 

effects of irregular leaves, interface contact and laminate 

irregularities of the experimental model can all augment 

leakage, yet are difficult to assess. 

CFD simulations illustrate that the leaf angle and leaf 

interference can play a role in leakage as illustrated in Figs. 9 

and 11 for the N = 63 leaf configuration where the angle is 

changed from 33° to 28° to 23°. The leaf angle is measured 

from the tangent plane to the simulated rotor. Because of the 

inserted spacers, the leaf angle alternated from 25° to 32° every 

other leaf, and as such 28° would be a reasonable average (see 

Table 1). It is seen that the data follow the CFD solution to a 

point and then depart to a lower leakage more closely 

associated with the 25° angle and may be affected by 

turbulence. Correcting the N = 63 leaf at 28° for inlet losses and 

turbulence underpredicts leakage, in good agreement with 

experimental results (Fig. 11). There are many flaws in the 

crude experimental models, yet the principle physical 
NASA/TM—2011-214040 6 

 
NASA/TM—2011-214040 7 

parameters have been retained and are captured by the CFD 

model. 

For N = 533, the leaves are distorted and curved between 40° 

to 60°, where 50° seems most reasonable. For this case, the 

predictions are less promising, and model losses are significant. 

Leakages for leaf angles of 45°, 50°, and 55° are shown in 

Fig. 11. 

Potential Improvements in Design 

Suggested design improvements include diamond like 

coatings (DLC) [8] that could be applied to the leaf interfaces 

to resist abrasion and provide low friction if the operating 

temperatures are less than 500 °C (930 °F). For example, in a 

wave bearing application, Dimofte et al. [8] tested coatings 

applied to both the rotor and stator: (1) silicon diamondlike 

carbon (SiDLC), (2) diamondlike carbon (DLC), (3) tungsten 

carbide/carbon (WC/C), and (4) titanium carbide (TiC). SiDLC 

performed well over 1000 start-stop cycles followed by an 

additional 50 oil-off cycles where friction torque lockup 

occurred in about 10 min. WC/C also did well except at oil-off, 

because of higher friction. TiC failed at oil-off, while DLC ran 

20 min, but seized on the first cycle. For new coated bearings 

operating at low supply pressures both SiDLC and WC/C 

performed well, yet at oil-off WC/C degraded rapidly (3 min) 

versus 21⁄2 hr for SiDLC. Tribologically, SiDLC performed very 

well for the wave bearing tested and currently would be the 

coating of choice for the plates and the rotor surface; however, 

statistically significant data are still required [9].

Improvements in the design may be found in the use of 

spring-loaded side-plate sealing, for example, similar to a 

piston ring. This will reduce the flow through the open 

triangular passages between leaves due to the change in radii. 

In the leaf seal experiments, filter paper side-wall seals were 

used, and of course this is a static configuration. 

The use of the OD wave spring will be useful in many cases 

to provide fresh interface, as this is a contact seal and will wear 

like the brushes in an electric motor, a little at a time, until they 

wear out. It will be necessary to provide leaf configurations that 

will not disengage from the holder. 

It may be that radial triangular-shaped leaves will also work, 

as they would be bidirectional for rotation; that is, the shaft can 

be rotated either way without damage. The OD wave spring 

would keep them in line, yet low-friction interfaces would still 

be needed, otherwise they would lock up; a SiDLC coating 

would be of great value on the leaves, yet make the seal more 

costly. So there will become a tradeoff between closing down 

the triangular area due to differences in radii and the restoring 

moments or leaf hysteresis. The leaves could be made 

triangular with a small bulge (like the head of a pin or needle) 

that will space them a slight distance apart and keep them from 

friction locking. These plates can then be quite short and would 

make a very compact seal with high length-to-diameter ratio 

(L/D) versus the conventional ones with low L/D. 

In our models, the leaf interfaces are not contoured to the 

shaft. In a real seal, this would become a necessity except in the 

case of a radial leaf configuration. Radial leaves do not need to 

be a "solid pack" but could be optimized between hysteresis 

effects, the weight penalty, and the effectiveness of sealing. 

This same criteria can be applied to other configurations as well 

as the use of side plates. 

The end plates or side plates need to have ID that are a few 

percent—about 5 percent—greater than the largest clearance 

requirement to ensure noncontact. In most cases a single 

labyrinth tooth upstream and downstream would suffice to 

provide sure sealing in case of leaf failures; however, in 

extreme cases up to three at each end can be used; for that 

matter any number can be used and one essentially has a leafaugmented labyrinth seal rather than a labyrinth augmented leaf 

seal. Usually the labyrinth is more effective on the rotor with 

abradable materials for the stator interface. Here the labyrinth is 

less effective yet essential to the safety of the turbomachine. 

Leaf-seal leakages could be improved If a compliant material 

could be found to fill the triangular gaps between the leaves 

while still enabling interface dynamics. 

As cited prior, the materials of the leaf-spring seal can differ. 

The material interfacing with the shaft needs to have good wear 

characteristics and withstand a lot of heat. The shorter ones 

may be of different materials. It becomes a manufacturing-costweight benefits issue. 

Patents by Shinohara et al. (Japan) [10], Flower (Cross Mfg, 

England) [6], and Steinetz and Sirocky (United States) [7] need 

be considered prior to manufacturing. Herein we have added to 

these concepts and then compared the modeling methods along 

with the results and suggested methods of improvements. 

Conclusions 

Experimental data from simple models using convenient 

working fluids, (e.g., air and water) that capture the essential 

physics, supported and verified by CFD, were used to provide 

scalable parameters for design of leaf-seal configurations for 

applications, to within reasonable engineering certainty. Simple 

experimental tests on four leaf-seal configurations have 

identified principle physical parameters, and CFD was 

employed to verify that these parameters can be estimated and 

applied to sealing designs. 

Leaf-seal internal flow and pressure drop characteristics are 

somewhat similar to a labyrinth seal cavity yet thin leaf flow 

characteristics tend to be dominated by viscous effects with a 

skewed flow expansion at the inlet. 

Leaf-seal leakage can be similar to brush seal leakages, 

and indeed the patents are derivatives of those issued for 

brush seals. 

Several potential improvements have been delineated 

including dynamic or spring-loaded side plates, a compliant 

interleaf filler, sandwiching the leaves between sets of labyrinth 

teeth, and a floating-leaf-pad interface seal providing a filmriding noncontact seal. 

References 

[1] Hendricks, R.C., et al.: Brush Seals in Vehicle Tribology. 

17th-Leeds-Lyon Symposium on Tribology, paper IX, Leeds 

England, 1990, pp. 231−242. 
NASA/TM—2011-214040 8 

[2] Braun, Minel J.; and Choy, Fred K.: Hybrid Floating 

Brush Seal. Patent 5,997,004, Dec. 7, 1999. 

[3] Salehi, M., et al.: The Application of Foil Seals to a Gas 

Turbine Engine. AIAA−1999−2821, 1999. 

[4] Hendricks, R.C.; Steinetz, B.M.; and Braun, M.J.: 

Turbomachine Sealing and Secondary Flows. 

NASA/TM⎯2004-211991, 2004. 

[5] Nakane, H., et al.: The Development of High-Performance 

Leaf Seals. J. Eng. Gas Turbines Power, vol. 126, no. 2, 2004, 

pp. 342−350. 

[6] Flower, Ralph F.J.: Brush Seal With Asymmetrical 

Elements. Patent 5,135,237, Aug. 4, 1992. 

[7] Steinetz, Bruce M.; and Sirocky, Paul J.: High Temperature 

Flexible Seal. Patent No. 4,917,302, Apr. 17, 1990. 

[8] Dimofte, F., et al.: Test of PVD Coatings in the Wave 

Bearing Environment Part I: One Thousand Start and Stop 

Tests and Oil-Off Tests at the End. Part II: Test of Coatings 

Under Low Oil Supply Pressure Down to Zero. Submitted to 

STLE. 

[9] Vlcek, B.L.; Hendricks, R.C.; and Zaretsky, E.V.: 

Determination of Rolling-Element Fatigue Life from Computer 

Generated Bearing Tests. Tribology Transactions (STLE), 

vol. 46, no. 4, 2003, pp. 479–493. 

[10]Shinohara, Tanehiro, et al.: Shaft Seal and Turbine Using 

the Same. U.S. Patent 6,343,792 B1, Feb. 2002. 
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13. SUPPLEMENTARY NOTES

14. ABSTRACT

Sealing interface materials and coatings are sacrificial, giving up their integrity for the benefit of the component. Seals that are compliant 

while still controlling leakage, dynamics, and coolant flows are sought to enhance turbomachine performance. Herein we investigate the 

leaf-seal configuration. While the leaf seal is classified as contacting, a ready modification using the leaf-housing arrangement in 

conjunction with an interface film rider (a bore seal, for example) provides for a film-riding noncontact seal. The leaf housing and leaf 

elements can be made from a variety of materials from plastic to ceramic. Four simplistic models are used to identify the physics essential to 

controlling leakage. Corroborated by CFD, these results provide design parameters for applications to within reasonable engineering 

certainty. Some potential improvements are proposed. 

15. SUBJECT TERMS

Seals; Coatings; Thin films; Turbomachines; Life 

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