One of the 3.5-kW laser rigs at Glenn. The inset shows the stainless steel specimen holder that sandwiches a 1-in.-diameter test coupon that is held by loops of platinum wire.
The laser rigs at the NASA Glenn Research Center provide a relatively simple, affordable, and versatile approach to high-heat-flux testing. The laboratory consists of three lasers--two different 3.5-kW lasers and a 1.5-kW laser that has a 7.5-kW enhanced pulse capability--as well as a variety of test stations. These industrial carbon dioxide lasers deliver energy at 10.6 μm, an ideal wavelength for testing zirconia-based or other oxide ceramics because these ceramics tend to be opaque to that wavelength. The laser rigs can achieve engine-level transient and steady-state temperature and thermal gradients. A wide variety of test configurations have been employed, with the most important configuration to date involving the high-heat-flux testing of 1-in.-diameter buttons of the type typically used for furnace testing. A variety of laser beam heating patterns have been employed, including uniform, trapezoidal, and gaussian--with the latter pattern being similar to the heating pattern from a torch test.
The term "trapezoidal" refers to the cross section of a heating pattern that is uniform over a central region (for example, 0.75 in.) and that tapers to low power toward the edge of the specimen. If considered in three dimensions, this geometry can also be referred to as a "truncated cone." The uniform and trapezoidal heating patterns employ a faceted integrating lens combined with specimen or lens rotation to even out irregularities in the beam pattern that are caused by diffraction. The maximum heat flux that can be delivered to the specimen generally increases as the heating pattern is changed from uniform to trapezoidal to gaussian. As a point of reference, the trapezoidal beam configuration can routinely pass a heat flux of about 0.7×106 Btu/hr-ft2 (220 W/cm2) through the specimen, and this could be increased to approximately 1.0×106 Btu/hr-ft2 (315 W/cm2) with modification to the beam delivery and backside cooling systems. Also, thinner substrates and substrate roughness facilitate the removal of the higher heat fluxes from the backsides of the specimens.
Example of a controlled heating rate approximating the heating profiles in a gas turbine engine.
In addition to durability testing, the rigs are usually used to measure thermal conductivity. The uniform beam configuration provides the most straightforward measurement of conductivity. However, other beam geometries can also be used for conductivity measurement of calibration to correct for the loss of one-dimensional heat-transfer conditions. One benefit from recording conductivity is that the plot of conductivity versus time is an excellent way to monitor specimen health. An increasing plot of conductivity versus time generally indicates sintering, whereas a decreasing plot generally indicates delamination cracking.
Example of thermal conductivity--measured in the laser rig--of two specimens, one that held up very well to the high-temperature exposure and another that failed during testing.
The rigs have proven to be especially useful for qualifying combustor-section thermal barrier coatings for engine tests. They can readily achieve and accurately monitor the required combustor-level heat fluxes and the thermal conditions of coatings and substrates in an engine. Also, one of the rigs can be configured to allow ramped heating to represent the heating rates that the coating would experience in the engine, and an intermediate-level heat flux that represents idle conditions can be programmed. As a result, a heating profile can mimic the actual exposure of coatings in a gas turbine engine. This year, the rigs helped engine designers to qualify coating vendors, to demonstrate coating system durability, and to determine appropriate coating thicknesses. They also provided thermal conductivity versus time-at-temperature design data.
Find out more about the research of Glennís Durability and Protective Coatings Branch: http://www.grc.nasa.gov/WWW/EDB/
Dr. Robert A. Miller, 216-433-3298, Robert.A.Miller@nasa.gov; and Dr. Dongming Zhu, 216-433-5422, Dongming.Zhu@nasa.gov
Authors: Dr. Robert A. Miller and Dr. Dongming Zhu
Headquarters program office: Aeronautics Research
Programs/Projects: VSP, UEET
Last updated: October 16, 2006
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