Taking an engine's temperature

Taking an engine's temperature

by Stephen W. Allison, David L. Beshears, Michael R. Cates, Bruce W. Noel, and W. Dale Turley

When properly applied as a thin coating, thermally sensitive phosphors can monitor the temperature of ceramic surfaces inside an engine

Ceramic and ceramic-coated components will be of increasing importance in the advanced engines now under development. For noncontact high-temperature measurements of ceramic components and in engine environments with high- thermal-radiation backgrounds, phosphor thermometry is particularly useful. Unlike pyrometry, it is unaffected by emissivity and relatively immune to blackbody emission and electromagnetic backgrounds. The technique is fundamentally an absolute measurement of temperature and can be extended over essentially the entire range of the solid state of many materials.

An optical arrangement for thermographic phosphor thermometry delivers the excitation light and returns the fluorescence. Timing electronics are necessary if a laser must be synchronized with a rotating surface.

Our collaboration has generally addressed remote, high- temperature, and high-rotational-speed surfaces such as those in turbines and combustion engines. Heat flux can also be measured with a multilayer of phosphor. The remote applications have included distances ranging from a few centimeters to as much as 5 meters between the phosphor- coated surface and collection optics.

A phosphor is a solid with an impurity, known as an activator, added. For the fluorescence to be produced, energy must somehow be deposited in the phosphor. If the energy source is pulsed, the fluorescence will persist for a characteristic period, which often exhibits a striking change with temperature.

Any system for thermographic phosphor thermometry will consist of the following basic components: a light source, an optical arrangement to deliver the excitation light and return the fluorescence, a detector that converts the fluorescence to its electrical analog, and a data acquisition and analysis system. Timing electronics are necessary if a laser must be synchronized with a rotating surface.

The method for attaching the phosphor to a surface is crucial to turbine blade and vane applications because the combination of high temperature, vibration, and abrasion can remove a phosphor coating.

Because many phosphors are refractory materials, their surface application to thermal-barrier coatings is particularly attractive for both durability and aerodynamics. It should be possible to incorporate phosphor into many ceramic components. Continuing developments are leading toward the practical use of high-temperature heat- flux gauges and thin-film crystalline layers.

The above was adapted from an article by Stephen W. Allison, a senior scientist, David L. Beshears, a senior engineer, and Michael R. Cates, a senior scientist, all in the Photonics and Measurement Systems Group at Lockheed Martin Energy Research in Oak Ridge, Tenn.; Bruce W. Noel, a consultant with Noel Associates in Espanola, N.M.; and W. Dale Turley, a group leader at the Special Technologies Laboratory in Santa Barbara, Calif. The full text may be found in the January 1997 issue of Mechanical Engineering magazine. © 1997 ASME International.© To obtain a copy of this issue, click here.