Scientists at von Karman Institute in Belgium contracted LayerWise to produce a scaled turbine inlet guide vane model for a turbine research project. LayerWise built the metal-vane specimen as a single part, complete with internal cooling cavity and fine instrumentation channels.
|Fig. 1. Shock waves formed at the trailing vane edge reduce turbine efficiency and add additional mechanical challenges.|
Research based on detailed simulation and testing concludes that turbine cooling could be improved by ejecting a pulsating stream instead of a continuous stream through the trailing edge. At the same time, the pulsed cooling significantly reduces the intensity of the shock waves. This opens up opportunities for jet-engine and power-plant turbine manufacturers to achieve higher turbine expansions, resulting in more compact engines and reduced development costs.
|Fig. 2. Research based on detailed simulation and testing concludes that turbine cooling could be improved by ejecting a pulsating stream through the trailing edge instead of a continuous stream.|
Pulsated vs. Continuous Cooling
Turbine blades in jet engines and power plants are internally cooled because of their exposure to high-temperature gas flow, which is directly discharged from the combustion chamber. Shock waves formed at the trailing vane edge generate strong stator/rotor interactions that reduce turbine efficiency and add additional mechanical challenges (Fig. 1).
The current research at the von Karman Institute focuses on pulsated cooling versus continuous cooling. Scientists selected and characterized the different building blocks needed to acquire detailed insight into this new concept of pulsated turbine cooling (Fig. 2).
|Fig. 3. LayerWise produced the entire metal vane, including internal cooling cavities and instrumentation channels, as one unit in a single production step.|
Building Blocks for Fluid-Dynamics Research
A mechanical pulsating valve delivering an adequate margin of frequencies and amplitudes generates the pulsating cooling air. The airflow travels through a model of a high-pressure inlet guide vane produced by LayerWise (Fig. 3), circulating all along its length before being ejected through a slot at the trailing edge. It is a simplified and scaled turbine inlet guide vane model that is derived from a real geometry.
Professor Paniagua and his team studied numerically the entire setup using fluid-dynamics simulation software. The complete experimental setup was modeled, including piping, pulsating valve and blade cavity. The fluid-dynamics model was used to extend the experimental investigation beyond the limits of the current setup, mainly in the upper frequency provided by the valve. Subsequently, experiments were carried out to verify the numerical results.
|Fig. 4. The SLM-produced vane contains dedicated fittings and channels to ensure that instrumentation sensors and wiring do not influence wind-tunnel experiments.|
Turbine Inlet Guide Vane
Building the physical model of the turbine inlet guide vane was a real challenge. LayerWise, a company focusing on selective laser melting (SLM), produced the vane according to von Karman Institute specifications. Quite impressive is that the vane was manufactured as one unit in a single production step, including all internal cooling cavities and instrumentation channels.
Tom De Bruyne, LayerWise sales manager, explains that the required geometry was out of reach for conventional metalworking processes. “SLM reverses the entire production process by building up material in layers instead of removing it in different metalworking steps. By pinpointing metal-powder particles with a powerful high-precision laser, the particles quickly and fully melt so that new material properly attaches without glue or binder liquid. As the laser can access any desired location at any time, we were able to produce the complex geometry of the vane, including all internal cooling cavities and instrumentation channels.”
The heavily instrumented vane is designed to allow high density in the measurements. The pressure sensors include both pressure tabs and kulite’sTM unsteady pressure sensors. De Bruyne says that the SLM-produced vane contains dedicated fittings and channels to ensure that instrumentation sensors and wiring do not influence wind-tunnel experiments (Fig. 4). SLM is the only metalworking technology that is capable of incorporating these subtle features, which guarantee the validity of the measurement data. In this regard, the researchers at von Karman Institute valued the proactive interaction with LayerWise, which helped a great deal in timely achieving the geometric vane requirements.
|Fig. 5. During wind-tunnel testing, the sensors in the vane as well as in the upper and lower wind-tunnel flow channels collected all the experimental data.|
Engine-Representative Test Conditions
In the wind tunnel, the aerodynamics performance of the vane is experimentally verified under representative engine conditions. As a single experiment only lasts about half a second, the von Karman Institute opted for a Schlieren imaging setup to picture the shock waves formed at the vane trailing-edge base region. In the design of the vane, the trailing-edge diameter had been enlarged to 5 millimeters to obtain the required spatial resolution for Schlieren experimental imaging and temporal resolution for vortex shedding.
During the wind-tunnel tests, the sensors in the vane as well as in the upper and lower wind-tunnel flow channels collected all the experimental data (Fig. 5). The coolant vane airflow is generated using a rotating valve operating with a perforated rotating disc, delivering a pulsated high-pressure airflow up to 200 Hertz.
|Fig. 6. Computed fluid-dynamics simulation predicts a 70% reduction in shock intensity between continuous (left) and pulsated cooling (right). Experimental verification of the aerodynamics performance of the vane under representative engine conditions confirms the heavy reduction tendency, as shown on these Schlieren shock-wave images.|
Improved Vane Cooling and Shock-Wave Control
Wind-tunnel test results, including both time-averaged and time-resolved results, helped the aerodynamicists to understand and prove the complex physics involved. According to Paniagua, the relationship between pulsated cooling and shock-wave behavior is also quite revolutionary. Computed fluid-dynamics simulation predicts a 70% reduction in shock intensity with experimental data confirming the heavy reduction tendency.
The conclusion of this successful research project is that shock waves can be adequately controlled by optimizing the cooling pulsation timing and amplitude. This offers great potential for jet-engine and power-plant turbine manufacturers to develop more compact engines exhibiting higher reliability and thrust/weight ratio. IH
For more information: Contact Tom De Bruyne, sales manager, LayerWise N.V., Kapeldreef 60, 3001 Leuven, Belgium; tel: +32 (0)16 298 420; fax: +32 (0)16 298 319; e-mail: email@example.com; web: www.layerwise.com. The author, Rob Snoeijs, is a technology writer for LayerWise N.V.; e-mail: firstname.lastname@example.org