Progress in Infrared Pyrometry Measurements of Shocked Solids
Temperature measurement is one of the grand challenges still facing experimental shock physics. A shock experiment fundamentally measures E({sigma}{sub x}, {var_epsilon}{sub 11}) which is an incomplete equation of state since temperature (or entropy) remains unspecified. Ideally, one would like to experimentally determine a free energy F(T, {var_epsilon}{sub ij}) from which all other thermo-mechanical properties might be derived. In practice, temperature measurement would allow direct comparison with theory/simulation since T and {var_epsilon}{sub 11} are in most theories the underlying variables. Temperature is a sensitive measure of energy partitioning, knowledge of which would increase our understanding phase boundaries and thermally activated processes (such as chemical reactivity (including dissociation and ionization)). Temperature measurement would also allow a thermodynamically consistent coupling of hydrodynamic equations of state to the material's constitutive (deformation) behavior. The measurement of the temperature of a material that has undergone severe strains at small time-scales is extremely difficult, and we are developing a method using infrared reflectance and pyrometry. The emitted power from a warm surface is measured over a range of wavelengths using a multi-channel IR detector with a response time of {approx}0.1 {micro}s. Each channel of the detector passes the radiation from a selected wavelength interval into a detector. Pyrometers typically have anywhere from three to six channels, and not all channels may have enough signal to contribute to the measurement under any given condition. The difficulty in the measurement lies in relating the radiance (power per unit area per solid angle) in each channel to the temperature of the surface since the radiance is determined not only by the temperature, but also by the emissivity of the surface. The emissivity is not a constant for any real surface, but varies both with angle of observation and with wavelength. Thus the temperature cannot be calculated from the emitted radiance spectrum without detailed knowledge of the emissivity. Approximate temperatures may be calculated by various assumptions, such as assuming a fixed number for the emissivity (greybody), assuming the emissivity in adjacent channels is the same (the relative emissivity then drops out of the relations governing the power ratio), and by assuming a lower limit on the emissivity which yields a range of temperatures and emissivities in each channel consistent with the measurement. We hope to show that the emissivity in each wavelength channel and the temperature can be uniquely determined if in addition to measuring the emitted power we also measure the power of the reflected light from a fast (sub {micro}s) light source of known spectral content. In addition, it is not necessary to know the absolute values of the flux, either reflected or emitted. This is a great advantage because the exact geometry of the configuration may be rapidly changing or not known at all. The results of a tabletop experiment designed to demonstrate the method and initial gas gun experiments will be discussed. The goal of the tabletop experiment was to show that the temperature measurement can be accomplished in less than one s (time length of steady state behavior). We have previously shown that the emitted signals can be acquired in approximately 0.2 s. We demonstrate that we can create and measure an additional signal from an exterior light source in less than a microsecond which allows the determination of the temperature and emissivity of a heated surface.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 15013558
- Report Number(s):
- UCRL-JC-146049; TRN: US200604%%64
- Resource Relation:
- Conference: Aeroballistic Range Association, Quebec, Canada, Sep 09 - Sep 14, 2001
- Country of Publication:
- United States
- Language:
- English
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