Low Pressure-Drop Engine Inlet Filtration System for Helicopters

The performance of helicopter gas turbines is known to deteriorate rapidly when they operate in areas where the air is laden with solid particles which usually consist of sand, minerals, dust and salt. In general, larger particles do not follow the streamlines in the engine inlet and tend to impact the blade surfaces due to their higher inertia, while smaller particles follow the gas flow.

The ingestion of larger sand particles into an aircraft engine causes extreme erosion due to the high kinetic energy of the particles.

To prevent sand from entering the compressor inlet of an aircraft engine, present technology utilizes inertial separators. As an illustration of the importance of improving the removal of fine particles in an Inlet Filtration System (IFS), note that a rise of 1% of separation efficiency can result in up to a 20% increase in the life of the engine.

To improve the efficiency of the existing approach of the IFS, Beltran designed the use of electrostatic enhancement of fine particle removal before an axial particle separator. The electrostatic separation process combines the potential advantage of providing high efficiency separation of fine particles with low pressure losses. For this process, particle charging is very important. The particle charging process is accomplished through the creation of an electric field and a corona current by applying a large potential difference between a discharge electrode and ground electrode.

Particle charging is essential to the separation process because of the electrical force, which causes the particle to migrate toward the ground electrode and is directly proportional to the charge on the particles. The most significant factors influencing particle charging are particle diameter, applied electrical field, current density and residence time. Particle separation rates for a given value of particle charge are a function of particle size, the electric field in the region of the collection electrode, gas flow rate, gas viscosity, and electrode geometry.

An effective method for charging particles is a high voltage direct-current corona. The corona in electrical separation is usually established between an active electrode, maintained at high voltage, and smooth passive electrode at ground potential. The corona is produced in the strong electric field region near the active electrode's surface. Large numbers of both positive and negative ions are formed in the active glow zone. In contrast to solid and liquid conductors, ions do not occur simultaneously in a gas but must be produced by some outside force or agent, a process generally denoted as ionization. Ionization requires electrical power (usually modest amounts since the charge is applied directly to the particles), which must be supplied from eternal sources. The dominant ion production mechanism in the corona is ionization by electron impact. At impact free electrons in the gas acquire energy from an applied electric field and collide violently with gas molecules, knocking electrons out of the molecules. The net result is the creation of additional free electrons and positively charged gas ions.

Beltran has researched and designed an electrostatic system as the Inlet Particle Separator (IPS) without adversely affecting the airflow. Therefore the electrostatic system will greatly reduce the pressure drop compared to barrier filters. Further, since electrostatic devices have wide spaces they are not prone to pluggage and the resulting increase in pressure drop which occurs from building of barrier filters.

Since Beltran combines both IPS and Elctrostatic Precipitation (ESP) techniques, it is not necessary to collect the fine particles on the opposite wall of the IPS. It is enough to change the trajectory of some particles so they would be captured by the airflow of the by-pass scavenger system.
                                                                                       Inlet filtration system test setup