PERFORMANCE ANALYSIS OF STRAIGHT TOOTH LABYRINTH SEALS

[rojilthomas]

PERFORMANCE ANALYSIS OF STRAIGHT TOOTH LABYRINTH SEALS

Submitted by

RAJESH S L
S1 M.TECH MECHANICAL ENGG. ( MACHINE DESIGN )
ROLL NO: ME10 MD11

Department of Mechanical Engineering
SCT College of Engineering, Thiruvananthapuram 18
APRIL 2011

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ABSTRACT


Labyrinth seals are used to provide a tortuous passage to help prevent leakage by allowing the fluid to pass through a long and difficult gap. Labyrinth seals on rotating shafts provide non contact sealing action by controlling the passage of the fluid through large no. of chambers by centrifugal motion as well as by the formation of controlled fluid vortices. The fluid that gets escaped from the main chamber becomes entrapped in the cavities of labyrinth where it is forced into a vortex like motion. In the present work, this obstruction to the leakage flow is due to straight tooth labyrinth seals on the stator. The main objective of the work is to evaluate the performance of straight tooth labyrinth seals in CFD, thereby calculating the leakage flow rate and compare it with the theoretical calculated leakage flow rate using mathematical results.

1. INTRODUCTION
All bearings function in association with some form of sealing device. The primary function of a seal is to limit the loss of lubricant or the process fluid from systems and to prevent contamination of a system by the operating environment. Seals are among the mechanical components for which wear is a prevailing failure mode. However, in the case of contact seals, wear during initial operation can be essential in achieving the optimum mating of surfaces and, therefore, control of leakage. With continued operation, after break-in, wear is usually in the mild regime and the wear rates are quite uniform; thus wear life may be predicted from typical operating data. In Fig.1.1, a face seal configuration is shown.

Fig.1.1. A face seal configuration

Solid contact takes place between two annular flat surfaces where one element of the primary sealing interface rotates with a shaft and the other is stationary. This contact gives rise to a series of phenomena, such as wear, friction and frictional heating. Similar problems occur with shaft riding or circumferential seals, both with carbon and other materials for rings and for elastomeric lip seals. Lubrication of the sealing interface varies from nil to full hydrodynamic and wear can vary accordingly. Wear of abradable shroud materials is utilized to achieve minimum operating clearance for labyrinth seals and other gas-path components like turbine or compressor blade tips to achieve minimum leakage. The functions of seals are also of great importance to the operation of all other lubricated mechanical components. Wear in seals can occur by a variety of mechanisms. A cause of wear in many types of mechanical systems is contamination by abrasive particles that enter the system through the seals.

Design features in seals that exclude external contamination from mechanical systems may be of vital importance. Seals are also important to energy conservation design in all types of machines. The most effective leakage control for contact seals is achieved with a minimum leakage gap and when both sliding faces, moving and stationary are flat and parallel. This condition is perhaps never achieved. That is probably fortunate, since a modest degree of waviness or nodal distortion can give rise to fluid film lubrication that would not be anticipated with the idealized geometry. With distortion, wear of either internal or external edges can cause the nose piece to form a leakage gap that can be convergent or divergent. Changes in the leakage gap geometry have significant effects on the mechanics of leakage, on the pressure balance, and on the susceptibility of lubrication failure and destructive wear. One of the wear mechanisms which occur in seals is adhesive wear. With adhesive wear, the size of the wear particles increases with face loading. An anomaly of sealing is that as the closing forces on the sealing faces are increased to reduce the leakage gap, the real effect can be larger wear particles that establish and increase the gap height and thereby increase leakage. Also, greater closing force can introduce surface protuberances or nodes from local frictional heating, termed thermoelastic instability that may determine the leakage gap height. The leakage flow through a sealing gap obeys the usual fluid mechanics concepts for flow. In addition, there are likely boundary layer interactions with the surfaces in an immediate proximity.

2. OBJECTIVES
The objective of the work is to evaluate the performance of straight tooth labyrinth seals in CFD, thereby calculating the leakage flow rate and compare it with the theoretical calculated leakage flow rate using mathematical relations.Computational Fluid Dynamics is used to justify the use of bulk cavity variables, and to analyze the details of the flow in a single cavity under steady state, axisymmetric conditions.

Fig.2. Section view of labyrinth seal

Labyrinth seals are used extensively in turbo machinery to prevent high pressure gas from flowing into a region of low pressure. There is a significant problem associated with such seals that the present work attempts to answer. The problem is the accurate prediction of the leakage flow rate through the seal. This leakage, which depends on a great variety of parameters such as geometry of the teeth, number of cavities, pressure differences, temperature, and type of gas, etc. is inevitably present even in the case of abradable seals. Its correct prediction and control is crucial for the efficient and economic operation of turbo machinery.

3. LITERATURE REVIEW
Sealing parts have been specially designed and placed in turbomachinery to suppress the leakage flow from the high pressure to the low pressure regions, which otherwise deteriorates the engine efficiency. Compared with the other two sealing configurations, e.g., the tip clearance between the rotor and the casing, the sealing clearance between the hooked stator and the shaft, the labyrinth seal between the casing and the shaft as shown in Fig.3 has received widespread attention. By axially arranging a series of knife teeth on the casing (shaft) surface with a small clearance to the shaft (casing) surface, the labyrinth seal is constructed with an axially serpentine flow path, which restricts the leakage but smoothes sliding motion of the shaft surface. In practice, the available labyrinth seals are usually resulted from the compromise between complexity of teeth arrangement and technologies of manufacturing and fabrication. No doubt that in-depth understanding of influence of the teeth arrangement on the leakage flow is of great significance. Numerous studies were made to investigate the fluid flow through the labyrinth seals which are placed between the casing and the shaft. Among various seal geometries, the simple straight-through seal as shown in Fig. 3(a) was usually adopted in previous researches for convenience.
The first reliable prediction was due to Stoff [1] who treated the case of turbulent flow through a labyrinth seal of the grooved shaft type. He was able to show how much the flow pattern in the labyrinth is affected by the ratio of the axial leakage velocity to the shaft peripheral speed. By using a finite volume method, he employed the Navier-Stokes equations to predict the water leakage through a straight-through seal. Subsequently, improvement in reducing the false diffusion [1] was made by Rhode [2]. Later, optimization of the labyrinth seals by using numerical parametric studies was conducted by Rhode et al. [2].He also presented rub-groove width and depth effects on flow predictions for straight-through labyrinth seals. For the laminar case El-Gamal et al. [3] were able to examine the pressure drop and how pattern for a number of geometrical configurations of seals. Their results showed how important the effect of shaft rotation is on the performance of the seals. References 1 and 3 however, were not able to divorce the effect of shaft rotation from the mutual effect of geometrical configuration and clearance size on the performance of the seal when the shaft was at rest. As well, Yucel and Kazakia [4] presented approaches for predicting leakage of the fluid flow through a straight-through seals. For the case of the influence of the induced aerodynamic forcing on rotor dynamics, a school of rotor dynamic coefficients for the straight-through seal were determined by using a one-dimensional control volume method, in which a whole seal cavity was regarded as a single control volume. A literature survey reveals that most previous numerical endeavors were attempted to delineate the fluid flow through the simple straight-through seal. The modern industrialization of the practical turbomachinery systems necessitates complexity of the high-performance labyrinth seals with special arrangement of knife teeth. Ding investigated the effects of the labyrinth clearance size, tooth width, cavity depth, and tooth shapes on leakage flow rate in labyrinth seals. Further, Wang [5] investigated the compressible fluid leakage flow through two labyrinth seals, namely the interlocking seal and the stepped seal, using Computational Fluid Dynamics (CFD) analysis.