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Rotorcraft Engine Air Particle Separation

Bojdo, Nicholas Michael

[Thesis]. Manchester, UK: The University of Manchester; 2012.

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Abstract

The present work draws together all current literature on particle separating devices and presents a review of the current research on rotor downwash-induced dust clouds. There are three types of particle separating device: vortex tube separators; inlet barrier filters; and inlet particle separators. Of the three, the latter has the longest development history; the former two are relatively new retrofit technologies. Consequently, the latter is well-represented in the literature, especially by computational fluid dynamics simulations, whereas the other two technologies, with specific application to rotorcraft, are found to be lacking in theoretical or numerical analyses. Due to their growing attendance on many rotorcraft currently in operation, they are selected for deeper investigation in the present work.The inlet barrier filter comprises a pleated filter element through which engine bound air flows, permitting the capture of particles. The filter is pleated to increase its surface area, which reduces the pressure loss and increases the mass retention capability. As particles are captured, the filter's particle removal rate increases at the expense of pressure loss. The act of pleating introduces a secondary source of pressure loss, which gives rise to an optimum pleat shape for minimum pressure drop. Another optimum shape exists for maximum mass retention. The two optimum points however are not aligned. In the design of inlet barrier filters both factors are important. The present work proposes a new method for designing and analysing barrier filters. It is found that increasing the filter area by 20% increases cycle life by 46%. The inherent inertial separation ability of side-facing intakes decreases as particles become finer; for the same fine sand, forward-facing intakes ingest 30% less particulate than side-facing intakes. Knowledge of ingestion rates affords the prediction of filter endurance. A filter for one helicopter is predicted to last 8.5 minutes in a cloud of 0.5 grams of dust per cubic metre, before the pressure loss reaches 3000 Pascals. This equates to 22 dust landings.An analytical model is adapted to determine the performance of vortex tube separators for rotorcraft engine protection. Vortex tubes spin particles to the periphery by a helical vane, whose pitch is found to be the main agent of efficacy. In order to remove particles a scavenge flow must be enacted, which draws a percentage of the inlet flow. This is also common to the inlet particle separator. Results generated from vortex tube theory, and data taken from literature on inlet particle separators permit a comparison of the three devices. The vortex tube separators are found to achieve the lowest pressure drop, while the barrier filters exhibit the highest particle removal rate. The inlet particle separator creates the lowest drag. The barrier filter and vortex tube separators are much superior to the inlet particle separator in improving the engine lifetime, based on erosion by uncaptured particles. The erosion rate predicted when vortex tube separators are used is two times that of a barrier filter, however the latter experiences a temporal (but recoverable post-cleaning) loss of approximately 1% power.

Layman's Abstract

The similarity between a Bedouin tribesman and Blackhawk helicopter may not be apparent at first. But consider the extreme environment that they endure, and you may see the connection. The desert hinterland provides numerous problems for their `vital organs', which must continue to function amidst swirling dust and sand clouds. To protect his lungs, the Bedouin tribesman wraps his head with a cloth Keffiyeh; to protect its engines, the Blackhawk helicopter employs an engine inlet sand filter.However, just as a cloth Keffiyeh is difficult to breathe through, so too is a helicopter sand filter. As air is drawn through a filter cloth, its flow is resisted by friction from the individual pores of the fabric. To maintain the required intake of air, more work is required to overcome this resistance. In the case of the Bedouin tribesman, he must inhale more strongly; in the case of the Blackhawk helicopter, more engine power is required. In the latter, this leaves less power available to lift the payload, thus reducing the helicopter's capability. To complicate matters, as a filter traps more dust particles on its surface, the resistance increases. The work presented investigates this resistance, which can be influenced by the filter structure, local environment, and the helicopter's operation. By using computers to simulate the air flowing into the engine intake and through the filter, we can calculate the energy lost to friction. This allows us to predict the loss of engine power that a helicopter experiences when using these devices.