Frequency Response Characteristics of an Active Heat Flux Gage

The transient response and frequency response of a constant-temperature platinum film gage are computationally modeled for application to heat flux measurement. The probe consists of a thin platinum film (sensor) deposited on a Pyrex substrate, and coated with aluminum oxide. The probe is exposed to a convective environment, and the power required to maintain the sensor at a constant temperature is a direct indication of the local, instantaneous heat transfer rate. In application, the probe is mounted in a heated, high thermal conductivity material, creating an isothermal heat transfer surface. A two-dimensional numerical model was developed to represent the sensor, the Pyrex substrate and the coating. Ideally, the probe would be operated with the platinum at identically the same temperature as the isothermal surface. In the present study, the effects of non-ideal operating conditions, resulting in differences between the sensor and surface temperature, are examined. Frequency response characteristics are presented in a nondimensional form. The results of this modeling effort clearly indicate the importance of precise control over the sensor temperature in employing the present method for heat flux measurement. With the sensor temperature equal to the isothermal surface temperature, the probe calibration is insensitive to the heat transfer rate over a wide range of heat transfer coefficients. However, a 0.5°C difference between the sensor and surface temperatures yields a change in the calibration of approximately 20 percent over a range of heat transfer coefficient of 500 W/m²K. At an input frequency of 10 Hz and an average heat transfer coefficient of 175 W/m²K, amplitude errors increase from 3 percent to 35 percent as the temperature difference changes from zero to 1°C. These results are useful guide to calibration, operation, and data reduction in active heat flux measurement.