# Numerical Investigation on Windback Seals Used in Aero Engines

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## Abstract

**:**

## 1. Introduction

## 2. Seal Geometries and Operating Conditions

_{s}was 20, the width of the scallops B was 6.3 mm and the length of the scallops L was 19.8 mm. The tip width of the helical cut was b = 0.4 mm. The labyrinth seal was purged by compressor air. In the bearing chamber, a roller bearing was rotating. In order to keep the sealing air consumption low and therefore make possible the use of an unvented bearing chamber, the pressure difference between the labyrinth seal and the sump was limited to 5 kPa at high power and to 2 kPa at idle conditions. The bearing chamber pressure was approximately atmospheric (P

_{chamber}= 1 bar) and the bearing chamber was scavenged by a scavenge pump. In order to investigate the potentials of the windback seal, different configurations and operating conditions were simulated. The qualification criteria were the seal’s ability to induce air flow in the direction towards the bearing chamber thus inhibit oil particles from overcoming the thread.

## 3. CFD Simulation

^{+}was controlled. y

^{+}is commonly used in the boundary layer theory and is defined as the ratio of the product of the friction velocity u* at the nearest wall node and of the distance y to this node to the local kinematic viscosity ν of the fluid, y

^{+}= u* y/ν with u* = (τ

_{w}/ρ)

^{1/2}and τ

_{w}the wall shear stress.

^{+}should be less than 50 and the recommendation is for values between 20 and 30. The Shear Stress Transport (SST) model was selected using with an automatic near wall treatment. SST is an ω-based model and for y

^{+}values over 11 scalable wall functions are used. y

^{+}values which are much less than 11 are important when heat transfer is involved. This is not the case in this work.

^{−6}. The Advection Scheme was selected High Resolution and for the Turbulence Numerics First Order was set. The computation was performed steady state on a Linux cluster with the use of 12 processors. The computation time was over 8 CPU hours.

## 4. State of the Art

_{d}and the ideal mass flow rate m

_{id}. m

_{id}is defined as:

_{H}proposed by Hodkinson [6] and a correction factor k

_{c}from unpublished test results:

_{c}= [n/(n−1)]

^{1/2}

_{2}= k

_{H}k

_{c}

_{d}is given in [5] in terms of graphs as a function of the Reynolds number Re and of the ratio between the clearance s and the fin tip width b.

_{10}Re

^{a}+ Log

_{10}(s/b)

^{b}and

- C
_{1}= 6.22819 × 10^{−8} - C
_{2}= −5.82396 × 10^{−5} - C
_{3}= 0.017891931 - C
_{4}= −1.986063294 - A = 52.4 and b = −61.94

_{LS}= k

_{2}C

_{d}m

_{id}

_{d}correlation. They investigated windback seals at pressure differences of 34.5, 68.9 and 103.4 kPa. These values are much higher than the targets set in this manuscript (<5 kPa). Their C

_{dw}correlation is strictly valid for the geometry given in [1]:

- a = (0.06946 + 34550s
^{1.5})^{0.5} - b = (−9.09 × 10
^{−4}+ 3.569 × 10^{−11}/s^{2}) - co = −7.475 × 10
^{−4}− 1.372 × 10^{−4}× Log_{N}(s)

_{wb}= C

_{dw}πD s [ρ

_{in}(P

_{in}− P

_{out})]

^{1/2}

## 5. Results

- (1)
- flow evaluation as a function of the operating conditions for different windback seal designs
- (2)
- oil particle ingestion downstream of the windback seal and particle tracing in order to evaluate the ability of the seal to repulse oil

#### 5.1. Comparison between Windback and Labyrinth Seal with a Scallop Free (Smooth) Rotor

_{d}is 0.536 from Equation (4), the ideal mass flow rate is 10.4 g/s from Equation (1) and the real mass flow rate is approximately 10.4 g/s as well. Thus, the mass flow deviation between the analytic and the CFD approximation is 1.6 g/s (14%). The resulting pressure difference across the labyrinth seal is 42.4 kPa (resp. PR = 1.424).

_{in}= 1.434 bar). If the calculated inlet pressure is used in Equation (7), C

_{dw}= 0.6947. This value is the numerically calculated discharge coefficient.

_{dw}= 0.4699 is calculated. This value is about 44% lower than the numerically estimated value. If C

_{dw}= 0.4699 is introduced into Equation (7) the resulting air flow rate is 8.4 g/s. This value is about 30% lower than the value derived from CFX (12 g/s). If simply the mass flow of 12 g/s is considered, Equations (6) and (7) can be solved for determining the inlet pressure and the discharge coefficient. The resulting values are P

_{in}= 1.734 bar and C

_{dw}= 0.4688 respectively. For the designer, Equations (6) and (7) yield the most pessimistic results whereas results using CFX are the most optimistic.

#### 5.2. Impact of Rotor Scallops

^{+}passes through, with m

^{+}= m

_{total}− m

_{backflow}.

#### 5.3. Impact of the Rotational Speed and Direction

_{total}). The effective mass flow m

^{+}is only 3% of the total mass flow. In the opposite direction against the direction of the helical thread (-) the seal instability begins at about −15,000 rpm whereas at −45,000 rpm the backflow rate is dominant (m

^{+}= 3%m

_{total}).

#### 5.4. Impact of the Length of the Helical Thread and of the Clearance

#### 5.5. Oil Particle Tracing

## 6. Conclusions

- (1)
- A comparison of results between this CFD survey and Morrison et al. has shown deviations in mass flow rate and discharge coefficient.
- (2)
- At very low pressure ratios backflow is a problem particularly when scallops are engraved in the rotor.
- (3)
- No evidence for backflow has been observed for a windback seal combined with rotor without scallops. This is an issue which requires further investigation.
- (4)
- Nevertheless, scallops in the rotor increase the total mass flow rate through the windback seal.
- (5)
- There are limitations concerning scallop number and size, seal clearance and rotational speeds. Increase in scallop size compared to the baseline has had a negative impact on the seal’s performance. Increase or decrease in tip clearance showed no performance deterioration. Increase in rotor speed has led to a high instability and backflow above 22,000 rpm. Nevertheless, further investigations on these issues are necessary.
- (6)
- Operational pressure ratio should be above 1.05 for unproblematic seal operation. Nevertheless, even at a pressure ratio of 1.01 no massive oil migration out of the (baseline) seal was documented. This is an important information for the designer.
- (7)
- Further numerical and also rig investigations are planned in order to investigate further design and operational aspects of windback seals.

## 7. Outlook

## Author Contributions

## Conflicts of Interest

## Nomenclature

A | Area between the fin and the stator | m^{2} |

a | Scallop depth | mm |

B | Scallop width | mm |

b | Fin tip width | mm |

CAD | Computer Aided Design | |

C_{d} | Discharge Coefficient | |

D | Fin tip diameter | mm |

d | Rotor diameter | mm |

H | Enthalpy | J/kg K |

h | Fin height | mm |

k_{H} | Hodkinson carry over factor | |

k_{c} | correction for carry over factor | |

L | Total length of CFD entity | mm |

L_{s} | Scallop length | mm |

m^{+} | Effective mass flow rate | g/s |

through seal | ||

m_{air} | Air mass flow rate | g/s |

m_{total} | Total mass flow rate | g/s |

N | Rotational speed | rpm |

N_{t} | Number of threads | |

N_{s} | Number of scallops | |

n | Number of fins | |

P | Pressure | Pa |

PR | Pressure Ratio | |

P_{s} | Static pressure | Pa |

P_{t} | Total pressure | Pa |

R | Gas constant (air:287) | J/kg K |

Re | Reynolds number | |

S | Entropy | J/kg |

s | Fin/Rotor clearance | mm |

T_{t} | Total temperature | K |

t | Pitch | mm |

U_{tan} | Tangential component of the velocity | m/s |

U_{Rotor} | Rotor circumferential | m/s |

Velocity = π N d/60 | ||

y | distance to node | mm |

y^{+} | non-dimensional wall distance | |

x | Axial coordinate | mm |

Greek letters | ||

γ | Particle diameter | mm |

τ_{w} | Wall shear stress | N/m^{2} |

## References

- Morisson, G.; Al-Ghasem, A. Experimental and Computational analysis of a gas compressor Windback seal. In Proceedings of the ASME Turbo Expo 2007: Power for Land, Sea, and Air, Montreal, QC, Canada, 14–17 May 2007. [Google Scholar]
- Lim, C.H. A Numerical and Experimental Study of Windback Seals. Ph.D. Thesis, Texas A&M University, College Station, TX, USA, 2009. [Google Scholar]
- ANSYS Inc. ANSYSVR CFX, release 11.0; ANSYS Inc.: Canonsburg, PA, USA, 2010.
- Schiller, L.; Naumann, Z. A Drag coefficient correlation. Z. Ver. Dtsch. Ing.
**1935**, 77, 318–320. [Google Scholar] - Zimmermann, H.; Wolff, K.H. Air System Correlations: Part 1—Labyrinth Seals. In Proceedings of the ASME 1998 International Gas Turbine and Aeroengine Congress and Exhibition, Stockholm, Sweden, 2–5 June 1998. [Google Scholar]
- Hodkinson, B. Estimation of the leakage through a labyrinth gland. Proc. Inst. Mech. Eng.
**1940**, 141, 283–288. [Google Scholar] [CrossRef] - Kutz, K.J.; Speer, T.M. Simulation of the Secondary Air System of Aero Engines. In Proceedings of the ASME 1992 International Gas Turbine and Aeroengine Congress and Exposition, Cologne, Germany, 1–4 June 1992. [Google Scholar]
- Farrall, M.; Simmons, K.; Hibberd, S. Experimental and numerical investigation of shear and gravity driven oil films in aero-engine bearing chambers. In Proceedings of the ASME Turbo Expo 2004: Power for Land, Sea, and Air, Vienna, Austria, 14–17 June 2004. [Google Scholar]

**Figure 1.**The windback seal components (Rotor with scallops, static part with thread and the assembly).

**Figure 4.**Variation of the Enthalpies, static pressure and velocity with the axial length of the labyrinth seal at 12 g/s.

**Figure 5.**Enthalpies, static pressure and velocity variation in the labyrinth seal as a function of entropy at 12 g/s.

**Figure 6.**Variation of the Enthalpies, static pressure and velocity in the windback seal as a function of the entropy at a mass flow of 12 g/s.

**Figure 7.**Enthalpies, static pressure and velocity variation in the windback seal as a function of the axial length.

**Figure 8.**The axial velocity distribution in a longitudinal plane cut for the labyrinth seal at 15,000 rpm with recirculation zones between the fins and flow carry over adjacent to the rotor.

**Figure 9.**The axial velocity distribution in a longitudinal plane cut for the windback seal at 15,000 rpm.

**Figure 10.**Tangential velocity components as a function of the axial length for labyrinth and windback (smooth rotor) at 12 g/s.

**Figure 11.**(

**a**) Effective mass flow m

^{+}through different seal/rotor types as a function of the pressure ratio at 15,000 rpm and s = 0.3 mm; (

**b**) Effective mass flow m

^{+}through the labyrinth seal as a function of the pressure ratio at 15,000 rpm and s = 0.3 mm.

**Figure 12.**Total and effective mass flow rates for labyrinth and windback seal with and without rotor scallops at 15,000 rpm, s = 0.3 mm as a function of the pressure ratio. Scallops initiate backflow which is considerable at small pressure ratios.

**Figure 13.**Impact of the number of scallops on the flow in a windback seal at a pressure ratio of 1.05, s = 0.3 mm and 15,000 rpm.

**Figure 14.**The impact of rotational speed and rotational direction on the flow in a windback seal at a pressure ratio of 1.05.

**Figure 15.**Mass flow rates through the windback seal at 15,000 rpm using different clearance and thread sizes.

**Figure 16.**Oil trajectories at 15,000 rpm, three different pressure ratios, s = 0.3 mm, smooth rotor.

**Figure 17.**Oil particles trajectories at 15,000 rpm, three different pressure ratios and s = 0.3 mm.

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Flouros, M.; Cottier, F.; Hirschmann, M.; Salpingidou, C.
Numerical Investigation on Windback Seals Used in Aero Engines. *Aerospace* **2018**, *5*, 12.
https://doi.org/10.3390/aerospace5010012

**AMA Style**

Flouros M, Cottier F, Hirschmann M, Salpingidou C.
Numerical Investigation on Windback Seals Used in Aero Engines. *Aerospace*. 2018; 5(1):12.
https://doi.org/10.3390/aerospace5010012

**Chicago/Turabian Style**

Flouros, Michael, Francois Cottier, Markus Hirschmann, and Christina Salpingidou.
2018. "Numerical Investigation on Windback Seals Used in Aero Engines" *Aerospace* 5, no. 1: 12.
https://doi.org/10.3390/aerospace5010012