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Technique for Heat Flux Determination

Technique for Heat Flux Determination. John Heim Vanderbilt University. Motivation. Accurate characterization of heating loads is crucial. Heat flux can provide meaningful information concerning the environment that a full-scale structure will experience.

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Technique for Heat Flux Determination

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  1. Technique for Heat Flux Determination John Heim Vanderbilt University

  2. Motivation • Accurate characterization of heating loads is crucial. • Heat flux can provide meaningful information concerning the environment that a full-scale structure will experience. • Temperature does not typically scale well.

  3. Heat Flux Gauge Uses thermopiles to measure temperature differences which can be converted to heat fluxes. Lagging response times, flow disturbance, and Calibration Calorimeter Lagging response times, flow disturbance. Complicated because heat load is not uniform across a surface due to lateral heat conduction. Standard Techniques

  4. Thermochromatic Liquid Crystals Layers of crystals in thin films aligning based on temperature gradients. Calibration, difficult to apply, and difficult to measure accurately. Inverse Method Appropriate data reduction is a balancing act between introducing smoothing bias and amplification of noise Solutions may contain unacceptable errors. Standard Techniques

  5. New Heat Flux Determination Approach • Measure a different quantity • HEATING RATE- the time rate of change of temperature for a given location and time. • Data reduction is inherently more stable. • Inverse method techniques

  6. New Problem • No method exists to measure heating rate directly. • Describe a technique to measure heating rate. • Demonstrate a stable method for estimating heat flux from a measured heating rate.

  7. Thermographic Phosphors • Rare-earth-doped ceramics that fluoresce when exposed to light. • Intensity, frequency line shift, and decay rate are all temperature dependent. • The strong dependence of the decay rate on temperature will be leveraged to acquire a heating rate.

  8. Heating Rate • Measure the intensity of phosphor emission. • Temperature is not constant; the normalized intensity is: • The heating rate can now be found:

  9. Phosphor Activity Excitation Intensity Phosphors Decay Time

  10. Theory • The infinite series solution for the normalized temperature is: • The solution for the heating rate :

  11. Data Reduction Procedures Q q Yn QYn QY Hd QHd H Hn QHn

  12. Triangular Heat Flux From Surface Temperature

  13. Triangular Heat Flux From Heating Rate

  14. Square Heat Flux

  15. Exact Heat Flux vs. Estimated Heat Flux

  16. Conclusion • Improved heat flux estimates by reducing instabilities inherent in temperature to heat flux data reduction methods. • By measuring heat rate the integral equation for heat flux becomes a Volterra equation of the second kind. • More stable solutions which can accommodate more noise then a temperature approach.

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