Guoqing LI, Shen ZHANG, Zhong KANG, Yanfeng ZHANG,Xingen LU, Junqiang ZHU

a Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China

b School of Aeronautics and Astronautics, University of Chinese Academy of Sciences, Beijing 100190, China

c School of Energy, Power and Mechanical Engineering, North China Electric Power University, Beijing 102206, China

KEYWORDS Aero-engine;Carbon seal;Durability test;Leakage;Rotating;Wear

Abstract Experimental investigation has been performed to study the leakage and wear characteristics of carbon seal working at high circumferential speed.To expand the scope of application,two newly designed carbon seals were compared: #1 Carbon Seal (CS1) with the inner diameter of 136 mm and including 4 segments, #2 Carbon Seal (CS2) with the inner diameter of 212 mm and including 6 segments. Air leakage tests were firstly conducted in the Medium-speed Seal Test Rig. The pressure ratio changed from 1.04 to 2.02 with the rotating speed varying from 0 to 18300 r/min. Of paramount concern was the durability test, including 300 h running time accumulated by three different working conditions,which was separately implemented on each carbon seal.The morphology variation of the friction surface, wear and leakage were recorded. Results indicated that the leakage monotonously increases with the pressure ratio and decreases with the rotating speed.Comparing with CS1,more typical features exist on the friction surface of CS2,which are generated by more severe wear.Continually,leakage characteristics deteriorate.Furthermore,fitted formula has been educed for the life prediction of carbon seal,which could provide some supports for aero-engine design.

1. Introduction

For modern aero-engine, public awareness of environmental hazards and severe emissions limits strongly drive the design requirements for sealing applications. High-performance and long-life seals become more and more important and urgent.Valentine and Povinelli1pointed out that the seals of aeroengine in the 1980′s with co-rotational rotors should operate at gas temperature within 922 K to 1032 K range and circumferential speed within 152 m/s to 183 m/s range.Nowadays,the above parameters have been increased by a great deal. Chupp et al.2systematically summarized the characteristics of different seals and pointed out that newly developed seal systems exhibit limited capability to satisfy advanced requirements without imposing intense system innovations. To meet the requirements of tomorrow’s aero-engine, seal characteristics are urgently needed to develop. Carbon seal is an effective technology in the bearing chamber to prevent the oil leakage.Usually, carbon seal is composed of several carbon segment,circumferential spring, axial spring, baffle and snap ring.Rotating characteristics including leakage, friction and wear directly decide the lifetime and application when carbon seal works at severe environment such as high temperature, high pressure and high rotating speed.

Carbon seal,born in 1967 in NASA,3is widely used for gas and liquid applications. The leakage characteristics of carbon seal situates between labyrinth seal and mechanical seal.4Comparing with the labyrinth seal, pointed out by P&W,5air leakage of carbon seal could be reduced by at least 60%.Results given by GE6proved further that air leakage of carbon seal rates from 75% to 95% lower than can be achieved with‘‘best” state-of-the-art labyrinth seal. Early, NASA Lewis Research Center carried out many researches on carbon seal.7–9Hady and Ludwig7developed a new self-acting carbon seal. Unexpectedly, air leakage at the circumferential speed to 75 m/s and pressure differential to 700 kPa reaches 4.25×10-4m3/s due to thermal distortions and clearance open, which is approximately twice that of the conventional carbon seal.Continually, the applications of carbon seal in small gas turbine engines were considered by Ludwig.8Air leakage is as large as 27 g/s at the circumferential speed to 122 m/s and pressure differential to 140 kPa for a worn-out carbon seal. Pressure differential plays more important role in the leakage than that of rotating speed. On this basis, Goldring9added step-type geometry on the inner surface of the carbon seal.Oil leakage was recorded and was within acceptable limits.After that,carbon seal working in special state was studied further.10–12Shaughnessy and Dobek10investigated the high misalignment carbon seal for the fan drive gear system. More compliant carbon seal was conceived to work under the misalignment increased in steps up to 2.667 mm radial &0.5° angular. Yan et al.11studied the effect of deflect angles on the leakage of carbon seal. Increasing the deflect angles enlarges the sealing clearance and then, facilitates leakage.Vinogradov et al.12focused on the leakage of carbon seal in the transition from take-off to cruise mode. Ring deflection occurs at different working state, which generates different leakage. Besides, the static characteristics of carbon seal were theoretically analysed by Arghir and Mariot.13The lift force,contact characteristics between the rotor and the segments were talked about in detail. Parametric calculations proved the importance of pad waviness, of the pocket depth and of the spring force on the characteristics of carbon seal.

Due to the high cost of test, only a few of studies on wear characteristics of carbon seal, especially the durability characteristics,could be found in the open literatures.In 1972,Hady and Ludwig.7carried out 120 h durability test for a newly developed self-acting carbon seal. The segments are lifted off the shaft under the function of the lift forces so that the wear is only one-tenth of conventional carbon seal.A 150 h durability test of the self-acting carbon seal was conducted at circumferential speeds to 145 m/s, air pressure to 1240 kPa, and air temperature to 408 K.8The wear of seal was not measurable.An additional 100 h test also revealed non-measurable wear.However, the carbon seal wears out at pressure differential above 414 kPa and rotating speeds above 107 m/s. In 1977,300 h durability test was firstly provided by NASA at circumferential speed ranging from 25.4 m/s to 50.8 m/s.9Wear of carbon segment was checked after the initial 63 hours run and at the end of the test and was found to be negligible.Only a slight burnish at the inner diameter of the carbon seal was captured. The shaft surface was in excellent condition. After that, some typical features of wear were found. As AlliedSignal14pointed out, blistering, being a chronic problem facing the sealing applications for many decades, is a typical coke related failure of carbon seal yielded to wear.Dinzburg15analysed the effects of carbonization on the leakage and wear of dynamic shaft seals. According to the visual inspection after 225 h bench test, carbonization phenomena is observed at the seal-lip area. Certainly, many studies were presented from the perspective of carbon material properties.16–18Strom et al.16conducted friction and wear tests at the circumferential speed to 50.8 m/s with some impregnated mechanical carbons to determine their performance at high temperatures. Oxidation plays an important role in friction and wear behaviour in high temperatures.On this basis,load to 500 g was imposed by Allen17and the circumferential speed reduced to 1.33 m/s at a specimen temperature of 923 K. results indicated that hardness is not a major factor in determining the friction and wear under these conditions. As an oxidation inhibitor, the inclusion of boron carbide has a strong influence on wear rate.To reduce wear, pressure balancing and incorporation of hydrodynamic lift could be introduced to positive contact seal.18. Li et al.19numerically simulated the opening force of the circumferential carbon seal. When the depth of Rayleigh step is at 0.01 mm,the maximum open force is achieved.With the wear of the carbon seal, the depth of Rayleigh step is reduced and the open force becomes smaller and smaller,which contributes to the leakage.

The above analysis indicated that the theory of carbon seal is relatively mature and complete. The force distribution around the carbon seal is clear.In recent years,more attention is paid to the design of dynamic pressure groove to improve the lift force and further to decrease the leakage of carbon seal.Besides, the misalignment, feasibility and material of the carbon seal have also been investigated.Limited by the test capability and measurement accuracy, few investigations could be found in open literatures for carbon seal working in high circumferential speed, especially the wear characteristics for life prediction are not fully revealed. On the basis of the study in finger seal,20labyrinth seal21and floating ring seal,22a more complicated environment, nearly close to the actual aeroengine, is supplied to investigate the leakage and wear characteristics of carbon seal. Two newly designed carbon seals,named #1 Carbon Seal (CS1) and #2 Carbon Seal (CS2), are compared. 300 h durability test for both CS1 and CS2 is carried out, respectively. At last, both air leakage and wear are achieved.

2. Experimental details

In order to investigate the leakage and wear characteristics of two different carbon seals,experiments have been done to test the leakage and durability at different rotating speed and pressure ratio. Finally, the life prediction model could be established for the contacted carbon seal which may provide help for the aero-engine design. In this section, the test rig and method would be descripted in detail.

2.1. Test rig

The Medium-speed Sealing Test Rig, located in Institute of Engineering Thermophysics, Chinese Academy of Sciences,was employed to study the leakage, friction and wear characteristics of carbon seal.Recently,an aggressive effort has been launched in the test rig to reveal the characteristics of many kind of seals such as Labyrinth Seal,Brush Seal,Floating Ring Seal and Carbon Seal.

Fig. 1 shows the layout of the Medium-speed Sealing Test Rig. Two flow circuits including air and oil were realized in the test rig.The sealing air was supplied by an air blower with a maximum flow rate of 18300 L/min and maximum pressure of 1200 kPa. A heater with a power of 100 kW was used to provide hot sealing air. An integrated oil system was built to supply oil at different flow rate(Qo),pressure(Po)and temperature (To). The oil temperature was controlled by the heater placed in oil tank.

A 25 kW high-speed motor with the maximum rotating speed of 20000 r/min was employed to supply power.In detail,the test section is shown in Fig. 2. The rotor was rigidly connected to the high speed motor by the conical shaft output(Taper 1:19.992). Then, a dial gauge was employed to test the run-out value of the cylindrical surface of the rotor.In this test,the value is as low as 0.02 mm,which guarantees the concentric run-out of the shaft system. Two cavities, including high pressure cavity in the left and low pressure cavity in the right, were separated by the carbon seal which was placed on the mounting seat.

Fig. 2 Test section.

Fig. 1 Layout of middle-speed sealing test rig.

To model the bearing chamber of aero-engines,oil provided by the oil pump was injected towards the rotating shaft to generate oil/air mixture in the low pressure cavity and then, converged to the bottom and was drained by the return pump.The mass flow rate of the oil is adjusted to keep the volume fraction consistent with the real aero-engines so that an actual bearing cavity environment is modeled. Meanwhile, the oil injection is also used to cool the rotor to remove the heat generated by rotating friction. High-pressure air was injected into the high pressure cavity to prevent the oil from leaking. With the pressure ratio increasing, more air would pass through the sealing clearance and enter the low pressure cavity, most of which would be drained by the vent pipes placed on the wall of the low pressure cavity as shown in Fig. 2.

The temperature of sealing air (Ta) and oil (To) was separately recorded in the air inlet and oil inlet. The temperature of high pressure cavity (Th) and low pressure cavity (Tl) was achieved by averaging the values of three K-type thermocouples, which were placed in the high pressure cavity and low pressure cavity at regular intervals, respectively.

In the same time, the pressure parameters were also monitored in the test. Near the test positions of Ta, To, Thand Tl,the pressure of sealing air (Pa), oil (Po), high pressure cavity(Ph)and low pressure cavity(Pl)were all collected.In addition,the volume flow rates of sealing air(Qa)and oil(Qo)were measured by flowmeters. Hall sensor, fixed at the end of the highspeed motor, was used to pick up the rotating speed (Ω). To ensure the test safety, an acceleration sensor was employed to monitor the vibration (V) of the test section. When the vibration exceeds the limitation,the high-speed motor will stop automatically.

As is shown in Fig.3,two different seal constructions were compared: #1 Carbon Seal (CS1) with the inner diameter of 136 mm and including 4 segments,#2 Carbon Seal(CS2)with the inner diameter of 212 mm and including 6 segments.

There are 4 pads on each of the segment for both CS1 and CS2.Differently,several dynamic grooves,which are expected to improve the sealing performance,appear on the pads of CS2 while no dynamic grooves exist on the pads of CS1. Besides,the widths of the lip and pad are also different for CS1 and CS2. Geometric details of the two carbon seals are listed in Table 1.

Fig. 3 Configurations of two different carbon seals.

2.2. Experimental method

To simulate the actual working environment of aero-engines,different parameters including pressure, temperature and rotating speed were adjusted in the test rig (Table 2). For CS1, the rotating speed varied from 0 to 18300 r/min and the temperature of the sealing air changed from 293 K to 383 K. The pressure ratio between the high pressure cavity and the low pressure cavity differed from 1.04 to 1.71. For CS2, the rotating speed changed from 0 to 13200 r/min with the temperature range of 293 K to 413 K and pressure ratio range of 1.04 to 2.02. Meanwhile, the parameters of the oil injection were also altered.

Leakage characteristics were investigated by collecting the leakage of sealing air from high pressure cavity to low pressure cavity without any oil injection.The leakage( ˙m)was recorded by mass flowmeter with the accuracy of 0.8%RD + 0.2%FS,which connected to the vent pipe to leakage(shown in Fig.2).At the same time,the accuracy of the K-type thermocouples is±0.4%T and it is ±0.2%FS for the pressure transmitters.

The pressure ratio is defined as follows:

Here, Phis the pressure of high pressure cavity, MPa; Plis the pressure of low pressure cavity, MPa.

To analyze leakage characteristics of the two carbon seals in both stationary state and rotating state, the leakage coefficient, considering the effect of rotating diameter and working conditions as is given in Ref. 20, is employed:

Table 2 Working conditions.

The maximum measurement uncertainty of the leakage coefficient is ±1.2%.

Durability tests were carried out according to the working conditions listed in Table 3 and Table 4. For CS1, different parameters were realized in the sequence of CS101 - CS102 -CS103,which was a complete cycle with the total running time of 50 h.For CS2,the sequence of the durability test was CS201-CS202-CS203 which also equaled to 50 h.An accumulative running time of 300 h was realized by implementing 6 cycles for both CS1 and CS2, respectively.

In the durability test, 20X microscope was employed to observe the morphology variation of the friction surface on the carbon segments. High-precision Length Measurement with the accuracy of ± (0.1 + L/2000) μm was used to test the thickness (B) of the carbon segment after each cycle. As is shown in Figs. 4, 6 points were evenly chosen for each segment and also, repeatability of the measurement was carried out.

For CS1, the thickness of each segment is defined as B1ni.

Here, n stands for the number of carbon segments,n = 1,2,...,4. i means the number of cycles, i = 0,1,...,6.i = 0 is the initial state before the durability test, which is the basis to calculate the wear. j represents the number of points chosen for each segment to conduct the thickness measurements, j = 1,2,...,6.

Eq. (5) shows the thickness matrix of the carbon segments.

The accumulative wear is calculated by the thickness change as follows:

For CS2,the same process is executed to calculate the wear.The only difference is that there are 6 carbon segments, not 4 segments.

The final wear rate can be calculated as follows:

Here, w stands for the wear rate in terms of wear volume,m3/(N.m);Vδis integral wear volume associated with the inner diameter and averaged wear of the carbon seal, m3; F is the normal force, N; L is the sliding distance, m.

Finally, the wear rate of CS1 and CS2 are both achieved.The measurement uncertainty of the wear rate can be calculated by

The maximum measurement uncertainty of the wear rate is±0.32%.

3. Experimental results and discussion

Leakage characteristics are firstly investigated by elaborate measurements.Then,morphology variation of the friction surface between the carbon segments and rotor are recorded in the durability test. Moreover, the leakage and wear characteristics of the two different carbon seals are focused specially.Both the leakage and wear are achieved experimentally.

3.1. Leakage characteristics

Leakage, sealing air escaping from the high pressure cavity to low pressure cavity by way of the carbon seal,is investigated at different pressure ratio and rotating speed. Before the leakage tests, short-time run-in has been done for the carbon seal and rotor to avoid the effects of manufacturing and assembling defects. In addition, leakage collected in the durability test is also analyzed to show the dependence upon accumulative running time (see 3.2.3).

Table 3 Durability test cycles for CS1.

Table 4 Durability test cycles for CS2.

Fig.4 Distribution of measurement points on a carbon segment.

3.1.1. Distribution of vibration

For rotating test, vibration is a key parameter to monitor whether the operation is in safety or not. Here, the vibration is recorded by an acceleration sensor in both the leakage test and durability test. As the rotor is connected rigidly to the high-speed motor,the sensor installed in the motor can directly reflect the radial excursion of the rotor. Fig. 5 gives out the vibration distribution for both CS1 and CS2. The abscissa Ω is the rotating speed and the ordinate V is the vibration acceleration.

As is plotted in Fig. 5, the rotating speed varies from 0 to 18,300 r/min for CS1. It is shown that there are two critical speeds at 8000 r/min and 14500 r/min where the vibration values are relatively larger. After that, the vibration is continuously stable. For CS2, only one critical speed is captured as is plotted in Fig. 5. According to the working parameters in Table 2 and 3, the rotor is able to avoid the critical speed in the test. Therefore, the vibration nearly has no effect on the leakage.

From Fig. 2(b) we can see that the rotor-bearing system of the test is a typical cantilever structure.The rotordynamic performance is mainly decided by the rotor structure of the highspeed motor.23The geometric difference between CS1 and CS2 has little effect on the high-speed motor so that the vibration magnitude is almost the same.

Fig. 5 Distribution of vibration vs rotating speed.

Fig. 6 Repetition of leakage measurement.

It needs to be emphasized that, as the durability test lasts 300 h for both CS1 and CS2, large vibration is not allowed to ensure the safety of the test.In addition,the radial excursion of the rotor could affect the leakage distribution and it will be analyzed in another paper where different vibration environments are modeled by external excitation.

3.1.2. Repetition of leakage measurement

Fig.6 shows the leakage repetition for both CS1 and CS2.The abscissa PR is the pressure ratio and the ordinate ˙m is the leakage magnitude. For CS1, the rotating speed is set at 18300 r/min with the pressure ratio varying from 1.04 to 1.71. From Fig. 6 we can see that the maximum relative deviation of the leakage magnitude, occurring at PR = 1.32, is about 2.92%.Fig. 6 also gives out the leakage repetition of CS2. The rotating speed is fixed at 13,200 r/min with the pressure ratio varying from 1.04 to 2.02. As is plotted in Fig. 6, the maximum relative deviation appears at PR = 1.68 with a value of 1.54%.Hence,good repeatability and accuracy for the leakage analysis are provided for both CS1 and CS2.

3.1.3. Distribution of leakage at different pressure ratio PR

Fig. 7 shows the distribution of the leakage at different pressure ratio for CS1 and CS2, respectively. For CS1, the tests are carried out at the pressure ratio of 1.04, 1.23, 1.32, 1.52 and 1.71 as is shown in Fig.7(a). Basically, the leakage coefficient monotonously increases with the increase of pressure ratio, which is consistent to the results given by literature.7–9Fig. 7(b) indicates the leakage distribution of CS2 at the pressure ratio of 1.04,1.23,1.32,1.52,1.68 and 2.02.Increasing the pressure ratio can directly strengthen the penetration ability of the high pressure air so that the leakage coefficient is increased.

3.1.4. Distribution of leakage at different rotating speed Ω

Fig. 7 also demonstrates the dependence of leakage upon rotating speed in both stationary state and rotating state, separately.For CS1,the tests are carried out at the rotating speed of 0 r/min, 13300 r/min, 16800 r/min and 18300 r/min as is shown in Fig.7(a).Obviously,leakage coefficient in stationary state is relatively larger than those in rotating state. With the increase of rotating speed, very slow decrease of leakage coefficient is captured further. For CS2, the tests are conducted at the rotating speed of 0 r/min, 9200 r/min, 12200 r/min and 13200 r/min as is shown in Fig. 7(b). Like CS1, rotation contributes to the decrease of leakage coefficient.Increasing rotating speed mainly enlarges the centrifugal force, which decreases the sealing clearance. Therefore, the leakage from the clearance is reduced.

Fig. 7 Distribution of leakage vs pressure ratio.

3.2. Durability characteristics

The durability tests last 300 h for each carbon seal to explore the wear growth. Of particular interest is the morphology observation of the friction surface and wear difference between CS1 and CS2. Air leakage is monitored during the durability tests.Similarly,short-time run-in is performed before the durability tests, too.

3.2.1. Morphology variation of the friction surface

The morphology observations of the friction surface on the carbon segments, recorded by 20× microscope, are done at 0 h, 150 h and 300 h in the durability tests for both CS1 and CS2, respectively.

(1) CS1

Fig. 8 shows the morphology variation of the friction surface on the four carbon segments of CS1. In Fig. 8(a), 8(d),8(g) and 8(j), the initial images of the friction surface are presented to be the basis of the comparison. No trace generated by friction can be found on the images.Everything looks neat.In Fig.8(b),8(e),8(h)and 8(k),the images after 150 h durability test are exhibited. The texture is still very clear and seems like no friction happened. In Fig. 8(c), 8(f), 8(i) and 8(l), the images after 300 h durability test are shown. Still, few typical friction features can be captured.

Apparently, the influence of 300 h durability test on the morphology variation of the friction surface is almost negligible, which agrees well with the results in Refs8–9and can be used for subsequent analysis of the wear and leakage characteristics.

(2) CS2

Fig. 9 gives out the friction surface variation of the 6 carbon segments of CS2. At 0 h, the textures of the friction surface are very clear and clean. The circumferential grooves on the surface are regular and uniform from CS2-01 to CS2-06.No friction trace can be found on the surfaces of the 6 carbon segments. However, several friction features can be observed after 150 h durability test. Firstly, the textures of the friction surface become blurred and irregular.Secondly,the finish surfaces of the carbon segments are damaged due to the grinding in the friction progress,which causes the friction surface to be rough. Thirdly, the colors are darkened when the finish surfaces are damaged. Specifically, some typical features are indicated.

For CS2-01,local black spots appear on the friction surface as plotted in Fig. 9(b). For CS2-02 to CS2-04 as shown in Fig.9(e),Fig.9(h)and Fig.9(k),some pits are captured,which can mainly be attributed to the inhomogeneity of the carbon materials. For CS2-05, the boundary of the circumferential grooves becomes indistinguishable. Compared with the morphology at 150 h, some new features come into view when the 300 h durability test is completed.For CS2-01,a slight burnish mentioned in Ref9, appears as plotted in Fig. 9(c). For CS2-02 in Fig. 9(f) and CS2-06 in Fig. 9(r), some position of the circumferential groove is filled by debris. For CS2-03 in Fig. 9(i), there are a lot of diagonal stripes. For CS2-04 in Fig. 9(l), there are some non-uniform spots below. For CS2-05, a black spot appears in the large circumferential groove.Even though, unlike the presentation of blistering mentioned by Ullah,14no coke related failure appears on the surface of CS2.

Fig. 8 Variation of friction surface during the durability test (carbon segments of CS1).

Therefore, the morphology variations of CS2, compared with those of CS1, are obvious and complicated. Overall, the friction generates different surface features on the carbon segments, which could be attributed to some differences between CS1 and CS2.Firstly,the existence of the dynamic grooves on the pads of CS2 results in incomplete friction surface, which accelerates the wear of CS2. Secondly, it is possibly the operating condition difference between CS1 and CS2.It can be calculated that the maximum circumferential speed of CS1 is 130.3 m/s while it is 146.5 m/s for CS2, larger than that of CS1. In addition, the difference of manufacturing processes between CS1 and CS2 also should be considered.

3.2.2. Wear

Fig.10 shows the wear rate with accumulative running time for both CS1 and CS2. Wear rate of each segment is separately represented by the histogram. Meanwhile, averaged wear rate of the whole carbon seal is specially indicated by the black bar.Besides,another black line for the averaged wear rate is shown for the detailed analysis. Herein, the finger seal in Ref. 20 is used for comparison to explore a general method to predict the lifetime of carbon-based contact seal. Just like the carbon-based finger seal in Ref. 20, relationship between wear and accumulative running time for carbon seal also indicates that the most severe wear happens at the beginning of the durability test.

Fig. 9 Variation of friction surface during the durability test (carbon segments of CS2).

In accordance with the carbon-based finger seal in Ref. 20,the distribution of the wear growth can also be divided into four periods: Sharp Wear, Progressive Wear, Stable Wear and Permanent Wear.Sharp Wear lasts for the initial 50 hours.In this period, the initial wear of the carbon seal is rapid and most of the wear of the full life-cycle occurs, which coincides with the results given by Ref. 7. Take CS1 for example, the averaged wear rate is 1.78 × 10-16m3/(N?m) at t = 50 h.Meanwhile, the wear inhomogeneity of the 4 segments is very prominent. The deviation from the averaged wear rate covers the range from-6.9%to+5.2%.Progressive Wear continues to the end of the 3rd cycle, that is 150 h. In this period, the wear rate decreases greatly. At t = 150 h, an averaged wear rate of 7.81×10-17m3/(N?m)is achieved for CS1.At the same time, the wear inhomogeneity becomes very small. After that,Stable Wear starts acting until the finish of 6th cycle.For CS1 at t=300 h,the averaged wear rate is 4.44×10-17m3/(N?m),which is only 25%of that at t=50 h.At last,it is Permanent Wear until the end of the life. Predictably,the carbon seal has the potential to work for a longer life, which is in accordance with the results in Refs. 8,9.

Fig. 10 Distribution of wear rate vs accumulative running time.

For CS2, the overall distribution of the wear is almost the same as that of CS1. Several differences mainly include:Firstly, the wear rate of CS2 is greatly larger than that of CS1 as is shown in Fig. 11. The overall averaged wear rate in 300 h is 5.50 × 10-16m3/(N?m) while it is only 4.44 × 10-17m3/(N?m),about one-twelfth of CS2.Secondly,the wear inhomogeneity of the 6 segments in the initial 50 h is more prominent than that of CS1. A range from -6.9% to +6.7% has been calculated from Fig. 10(b). Combined with the morphology observation given in 3.2.1, we can analyze the phenomenon in detail. As is observed in the durability test, no significant change occurs on the friction surface of CS1 while many complicated and clear features appear on the friction surface of CS2. That is, the wear of CS2 is more severe than that of CS1.

Fig. 11 Comparison of wear between CS1, CS2 and finger seal 20.

The relationship between the wear rate and accumulative running time of the two carbon seals could be fitted according to the averaged wear rate line.

For CS1

Here, w is the wear rate, m3/(N.m); t is the accumulative running time, h. The relative deviation between the averaged wear rate (test data) and the fitted formula is ±5%.

Calculated from Eq. (11) for CS1,it is found that the wear rate is only 3.23×10-18m3/(N?m)at t=10000 h,which is the designed life of CS1. According to the above wear rate, the wear growth could also be calculated and further proved that the carbon seal could work for a long life-time. Like CS1, the wear rate of CS2 at t = 10000 h could be calculated and analyzed. Predictably, the carbon seal has the potential to work for a long life, which is in accordance with the results in Refs.8,9.

In order to further explore the general wear characteristics of the carbon-based contact seal, the difference between carbon seal and carbon-based finger seal20is also clarified. To ensure the comparability, the wear of finger seal in hot state is employed here (Fig. 11).

Fig. 12 Distribution of leakage vs accumulative running time:effect of rotating speed.

Fig. 13 Distribution of leakage vs accumulative running time: effect of pressure ratio.

At the initial 50 h, the wear rate of the two carbon seals is dramatically smaller than that of the finger seal.With the accumulative running time, all of the wear rate begins to decrease.At t=300 h,the wear rate of CS2 begins to approach that of the finger seal, but it is still very small for CS1. This can be explained from the structure of carbon seal and finger seal.Firstly, both of these seals are contact-type seals. Secondly,the stiffness of carbon seal is decided by the circumferential spring (see Fig. 3) while it is up to the number and geometry of fingers and the clearance between the fingers for finger seal.Different stiffness leads to different friction. Comparing with the finger seal, the adaptability and adjustability are better for the carbon seal. Thirdly, although both the carbon seal and finger seal are based on carbon,there are still some differences in material properties and manufacturing process.Totally,the wear resistance is relatively better for carbon seal.

3.2.3. Leakage

As is expected, leakage is also recorded in the durability test.Measurement of leakage,sealing air from high pressure cavity to low pressure cavity, is performed after each cycle. Fig. 12 shows the effect of rotating speed at a fixed pressure ratio of PR = 1.32.

Intuitively, leakage coefficient increases slowly with the running time for both CS1 and CS2, no matter in stationary state or rotating state. Consistent with the conclusion given by NASA Lewis Research Center,8rotation still promotes the reduction of leakage coefficient in the durability test since centrifugal force tends to close the clearance. From the comparison between CS1 and CS2, we can see that the leakage coefficient of CS1 is far less than that of CS2. Firstly, the circumference of CS2 is larger than that of CS1, which enlarges the leakage area. Secondly, more wear (see section 3.2.2) and terrible friction surface (see section 3.2.1) appear on CS2 with the accumulative running time so that the leakage characteristics are greatly deteriorated.

Fig. 13 indicates the leakage distribution at different pressure ratio in the durability test. Both stationary state and rotating state are investigated. As Fig. 13(a) and (b) show,the pressure ratio varies from 1.04 to 1.71 for CS1. With the accumulation of running time,leakage coefficient becomes larger, too. At the same time, larger pressure ratio generates higher leakage coefficient.Fig.13(c)and(d)give out the leakage distribution for CS2.The pressure ratio changes from 1.04 to 2.02.Take PR=1.32 for example,from t=0 to t=300 h,a total increase for CS2 is 0.0167 kg?K0.5/(MPa?m?s)while it is only 0.0059 kg?K0.5/(MPa?m?s) for CS1 in stationary state.

For CS2,the rotating speed of 13,200 rpm corresponding to a circumferential speed of 146.5 m/s, is chosen as Fig. 13(d)shows. For CS1, it is at the rotating speed of 18,300 r/min(see Fig. 13(b)) which equals to a circumferential speed of 130.3 m/s.That is,there is not much difference of circumferential speed between CS2 and CS1,which means that the leakage in rotating state is comparable. The leakage coefficient distributions at PR = 1.32 are compared again. Results show that a total increase of 0.0082 kg?K0.5/(MPa?m?s)happens for CS2,while it is 0.0062 kg?K0.5/(MPa?m?s) for CS1. Therefore, CS1 exhibits better than CS2 in the durability test. The above results prove that the dynamic grooves(shown in Fig.4(d)and Fig.9),which are expected to decrease the leakage,fail to fulfil their function and should be improved in future.

In addition,results of the durability test show that the effect of pressure ratio on air leakage is more significant than the rotating speed for carbon seal, which agrees well with the results given by Ludwig.8

4. Conclusions

Leakage and durability tests were carried out firstly in the Medium-speed Sealing Test Rig to analyze the characteristics of two different carbon seals (CS1 and CS2). Different pressure ratio and rotating speed were compared in detail to indicate the typical leakage characteristics. Different cycles were implemented on both CS1 and CS2 to achieve the durability characteristics. Several conclusions could be drawn from the above analysis:

In the range addressed all air leakages increase progressively with the pressure ratio increasing and reduce slowly with the rotating speed increasing. Comparing with CS2, CS1 behaves better in the leakage characteristics.

In the durability test, morphology observations of the friction surface are conducted for CS1 and CS2,respectively. For CS2,many typical features are captured in the progress of friction while there are few changes for CS1,which can be applied to clarify the different wear and leakage characteristics.

Wear is achieved for the two carbon seals. Just like the carbon-based finger seal studied in,20the largest wear rate occurs in the early period of the durability test for both CS1 and CS2. Through the wear characteristics comparison with finger seal, the fitted formulas could be achieved to give a relatively accurate life prediction method for the carbon-based contact seal.

Interestingly, the wear of CS2 is more severe than that of CS1, which just contributes to the complicated morphology variation and poor leakage characteristics of CS2 in the durability test.

Combined with Ref. 20, life prediction method is summarized for carbon-based contact seal. Due to the complexity and high-cost of durability test, the method developed here provides direct support for designers and researchers in aeroengines.More analysis about friction and heat transfer of carbon seal is on the way.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 51976214) and National Science and Technology Major Project (2017-IV-0010-0047).