Keywords

1 Introduction

The civil aircraft Pneumatic system usually use the intermediate-pressure or high-pressure ports to bleed air, the maximum bleed temperature could be higher than 500 ℃ [1]. To cooling down the temperature, the fan air is also bled as the cold source [2]. In order to maximize the heat exchange capability in limited space, usually a cross-flow precooler is used. The fan air is heated and then discharged by the exhaust louver; the system diagram is shown in Fig. 1.

Fig. 1
A model diagram of heat exchanger system. The labeled parts are the precoder, hot path inlet, cold path inlet, hot path outlet, compensating pipe, seat, and cold path exhaust louver.

Diagram of heat exchanger system of pneumatic system

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A compensation pipe should be used to connect the precooler and louver in order to compensate the manufacture and installation tolerance. Based on the engineering experience, The pipe design requirement should consider following issues:

  • High Temperature: with the Engine performance upgrading, the working temperature of pneumatic also raised, now the exhaust temperature usually higher than 200 ℃ in the civil aircrafts.

  • High Vibration level: the engine vibration will transfer to pylon, and the plenum should withstand the vibration.

  • Large Air Flow: to cooling down the high bleed air, the air flow can reach to 1–2 kg/s.

  • High Layout Requirement: fuel, hydraulic, fire protection, Pneumatic pipes are arranged in the limited pylon box, the space is very strapped.

  • Elastic compensation: considering the manufacturing and installation tolerances of upstream equipment and downstream structure, the plenum should be design with a compensation.

To meet all the requirement listed above, usually the material of silicone rubber reinforced with glass fiber is used. This material combines the advantage of silicon rubber and glass fiber [3, 4], however the rubber aging problem would also appear.

2 Design Requirement

Tables 1 and 2 shown a type of compensation pipe design requirement and related material information.

Table 1 Design requirement of compensation pipe
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Table 2 Material information
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3 Analysis

3.1 Thermal Aging Analysis

Thermal Aging mechanism. Silicon rubber will occur high temperature aging failure in the long-term high temperature environment. According to the domestic and foreign scholars, the high temperature resistance of silicon rubber is mainly affected by two factors: one mainly occurs in an oxygen-free high temperature environment, the main chain will break and rearrange the silicon-oxygen bond, so that the silicon rubber will soften and aging. Another mainly occurs in an aerobic high temperature environment, and the organic side group will have thermal oxygen aging, so that the silicon rubber will harden and aging.

The compensation pipe is normally exposed to the air, and combined with the phenomenon of failure component losing elasticity and hardening, it can be determined that the product has hardened and aged, the reaction equation is shown in Fig. 2 [5, 6].

Fig. 2
Two chemical reactions describe how the high-temperature resistance of silicon rubber is mainly affected by an oxygen-free high-temperature environment and an aerobic high-temperature environment.

Equation of hot oxygen reaction

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Thermal Analysis. System performance analysis shows the average temperature could meet the material requirement, but the cross-flow precooler will lead to the temperature stratification, so the CFD should also performed to analysis the temperature field. The temperature stratification was supposed as linear distribution and the boundary conditions and turbulence model are shown in Table 3.

Table 3 Parameter of thermal analysis
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As Fig. 3 shows, the maximum temperature could reach to 279 ℃，which is beyond the limit temperature 232 ℃.

Fig. 3
Three heat maps present the distribution of temperature on the surface of the compensation duct. A. Temperature ranges between 159.94 and 254.13 degrees Celsius. B. Temperature ranges between 182.83 and 279.33 degrees Celsius. C. Temperature ranges between 148.58 and 252.11 degrees Celsius. The temperature is highest near the openings and decreases outward.

CFD calculation of compensation duct

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3.2 High-Speed Air with Sand Dust

Abrasion mechanism. According to the research, both sand dust and high-speed air flow can cause abrasion on the surface of silicon rubber, but the abrasion mechanism of the two effects is not the same. The abrasion under the sand dust is caused by the impact of particles and friction and wear, and its abrasion level is affected by the hardness and concentration of particles. The abrasion under the impact of high-speed air flow is caused by the surface damage and stripping wear caused by the gas molecules strike [7, 8].

Flow field analysis. In order to identify the exact speed in the compensation pipe, flow field analysis is performed as Table 4. Three critical cases show the average speed in the pipe is about 200 m/s, the maximum could reach to 300 m/s as Fig. 4. The comparison in Fig. 5 shows the simulated high-speed area is in good agreement with the actual white area of the product.

Table 4 Boundary conditions of flow field calculation
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Fig. 4
Three heat maps present the variation in velocity in the compensation duct. A. Velocity ranges between 0 and 300 degrees Celsius. B. Velocity ranges between 0 and 265 meters per second. C. Velocity ranges between 0 and 300 meters per second.

Flow field calculation of compensation duct

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Fig. 5
Left. 3 heat maps of the compensation pipe present the flow field analysis for 200, 375, and 600 micrometers. Right. A photograph of a compensation pipe.

CFD result compared with the component

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Abrasion test. In order to verify the impact of the high-speed air with sand dust, an abrasion test is performed, the test requirement is accordance with ISO12103 [9]. Test instruments and results show in Fig. 6 and Table 5.

Fig. 6
4 photographs of the abrasion test display the sample sand projection device, skin temperature sensor, and heat air device.

Diagram of sand dust test

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Table 5 Test results
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The test result shown the blowing surface became white and the internal glass fiber exposed, this phenomenon is the same as the airline products.

The test evidences that the high velocity and dust flow indeed wear the silicon rubber. When the velocity reach to 200 m/s, never less the room temperature or high temperature, the air flow with sand dust has a serious impact on the abrasion of silicon rubber, and at the velocity of 150 m/s, the abrasion is significantly weakened.

4 Conclusion

Aiming at the problem of pneumatic compensation pipe rupture problem, this paper focuses on the impact of thermal aging and sand dust. Based on the CFD calculation and abrasion test, the root cause of rupture is identified. Because of the technology limitation, the area which to suffer high temperature up to 250 ℃ and velocity up to 150 m/s is not suitable to use silicon rubber.