The GTL has two spin pit facilities used to investigate the effect of blade tip-rubs, blade damping, and a variety of other questions. Each facility is highly configurable to accomodate a range of hardware and research interests. The Compressor Spin Pit Facility (CSPF) came first and is sized to accomodate components from the high-pressure compressor of a gas turbine engine, while the Large Spin Pit Facility (LSPF) is designed to investigate larger discs including jet engine bypass fans up to 60-inches in diameter and power turbine components.
Compressor Spin Pit Facility
The Compressor Spin Pit Facility (CSPF) is designed to investigate a range of aeromechanic phenomena for gas turbine engine hardware rotating at full engine speed. Installed in early 1999, the CSPF is intended to generate the data needed to validate emerging modeling and design tools for the development of aircraft gas turbine engines. Currently, the facility is applied to the study of blade-casing rub-in-systems of the type used on contemporary gas turbines to improve the tip clearance behavior for the engine's entire life span.
The CSPF consists of an in-ground containment tank that supports a highly rigid spindle rated at 20,000 rpm. Engine rotors undergoing investigation are mounted on the spindle with an adapter and can carry a variety of sensors for the desired measurements. Transducer power and signals are transmitted to and from controlling equipment through a high speed slip-ring. Additional mechanisms required in the experiments are attached inside the containment tank.
The CSPF is capable of accommodating medium to large compressor and turbine rotors (maximum diameter 34-in). In the ongoing compressor disk investigation, the facility is equipped with a metric fast-acting mechanism that has been developed to perform blade rubs at full engine speed. The facility is designed to allow insertion of a segment of engine casing into the path of the single or multiple bladed disk with predetermined blade incursion into the casing. The casing sector is mounted on a metric unit based on three piezoelectric load cells and attached to a support ring that swivels in the horizontal plane. The single throw by a fast acting gas piston attached to a cam follower is coupled to the support ring and brings the casing in and out of the path of the rotating blade for a few rubs over a time interval of about 20 milliseconds. The incursion of the rubs is prescribed by setting the initial position of the mechanism in increments of 0.2 thousandths of an inch.
Among the many phenomena that can be investigated in the CSPF are asymmetric and full rotor clearance closure for actual engine stages. In single blade asymmetric clearance closure investigations, a rapid contact of the bladed rotor with its housing is simulated as would occur in an abrupt flight maneuver. In full rotor clearance closure, an extended contact of the bladed rotor with its housing is simulated as could be experienced in some flight phases due to temporary thermal imbalances in different parts of the engine.
Blade-to-case rub can degrade the performance of jet engines through the introduction of high amplitude shaft vibration and severe blade/seal wear. It can even lead to catastrophic failure of the whole engine in the worst occurrence. Aerodynamic requirements dictate that engines operate with the minimum blade tip clearances that are mechanically practical. However, the smaller the blade tip clearances, the higher the possibility for blade-to-case rubbing during operation. Complex blade-casing rub-in-systems are used to improve the tip clearance and maintain greater gap uniformity over the life of the engine. Typically a rub-in-system in the compressor section of the engine may consist of a specific circumferential area of the metal alloy case shaped to accept coatings of materials selected for in-service wear and, when required, fire shielding interactions. In the experiments at the GTL, single blade rubs have obtained the same wear pattern in the facility as observed in field occurrences. Detailed measurements of blade stresses and casing forces have been obtained and modeling is in progress to improve predictions of the structural modal response.
In addition to tip rubs, the CSPF can be used for material characterization for engine blade-out dynamic modeling. Blade-out events are caused by material failure due to fatigue, a bird strike, or some other foreign object damage. Successful blade-out certification testing is routinely required before an engine is accepted into service.
The newest experimental facility at the Gas Turbine Laboratory is the Large Spin Pit Facility (LSPF). Building on the intense interest in data from the Compressor Spin Pit (CSPF) and lessons learned about the experimental process, this new facility is large enough to accommodate scaled bladed rotors up to 60 inches in diameter. Like the CSPF, its primary purpose is to study the physics of blade tip-rubs and other aeromechanic events.
Compared to the CSPF, distinctive features of the LSPF are
- a progressive incursion mechanism that allows control of the incursion rate and depth instead of just depth and
- optical access windows for high-speed imaging of the rub event.
In addition to continuing aeromechanics research, this facility can also be used for internal heat transfer experiments investigating the flow through large-scale blade cooling passages.