Study On Transient Radiative Heat Flux Meter With Double Thermal Guarded Plates

Heat flux simulation and measurement are very important in spacecraft ground thermal test. Compared to common heat flux meters, such as Gardon meter, SB meter, thin film meter and et al, heat flux meters for spacecraft thermal test has a feature of being used in condition of vacuum and low temperature. The heat flux meters for in spacecraft ground thermal test include adiabatic heat flux meter and isothermal screen heat flux meter. Adiabatic heat flux meter, also known as black foil, belongs to steady state type heat flux meter. It is not suitable for transient heat flux measurement due to its long response time. Besides, the measurement result is affected by the temperature of heat flux meter mounting plate. And the thermal conductivity of insulation material has to be measured with high accuracy for heat loss correction. Isothermal screen heat flux meter can be used for transient heat flux measurement if thermal receiver heat capacity term associated with temperature is applied. However, isothermal screen heat flux meter is not suitable for low heat flux measurement and its calibration is difficult. Transient adiabatic heat flux meter also has the defects that the measurement result is also affected by the mounting plate temperature and its insulation material conductivity needs to be measured. The above heat flux meters for thermal test give heat flux according to the temperature change of the thermal receiver, which is different from the Gardon meter which gives heat flux according to linear relation between the input heat flux signal and the output voltage signal. As a result, common problem of lumped heat capacity applicability associated with thermal receiver temperature uniformity exists in the above heat flux meters. Aiming at the shortcomings of the existing heat flux meters for spacecraft thermal test, this thesis carried out the theoretical and experimental research of a novel transient radiative heat flux meter.A novel heat flux meter thermal receiver with double thermal guarded plates was proposed. It was covered with non-spectrally selective coating. The spectral absorptivity of the coating was measured by spectrophotometer and Fourier transform infrared spectrometer, the results showed the absorptivity is higher than 0.95 in the range of solar spectrum and IR spectrum. The central thermal receiver and the first thermal guarded plate have the same material, thickness and coating, and are both suspended in the vacuum. Thus, the changes of their temperatures are synchronous, which makes the edge heat loss of central thermal receiver negligible. Thermal receiver temperature uniformity is verified by both FEM simulation and IR thermal image experiment, which makes lumped heat capacity method suitable to the novel heat flux meter. Radiation heat transfer is applied as heat loss correction method for double thermal guarded heat flux meter, which has a better response characteristic than adiabatic transient heat flux meter and does not require the measurement of insulation material thermal conductivity. In order to guarantee the entire heat flux meter mechanical strength and fix the thermal receiver and multilayer insulation blankets, PTFE pipe is used as heat flux meter substrate. As a result of the low thermal conductivity and heat conduction area, thermal receiver temperature is not affected by the heat flux meter mounting plate temperature. The measurement principle was proposed according to the structural features of this novel heat flux meter. The performances of the novel heat flux meter were theoretically investigated for the conditions of step square-wave heat, non-uniform heat and on-orbit spacecraft external heat by using FEM simulation.To calibrate the heat flux meter, a vacuum heat sink container with built-in blackbody furnace and Xenon lamp was developed. The effective emissivity and heat flux non-uniformity of blackbody furnace aperture were proved to be 0.999 and 3.5% by Monte-Carlo ray tracing method. The uncertainty of the radiative heat flux of the blackbody furnace was estimated less than 0.4%. The stability and reliability of thermocouple in the heat sink container was verified as a temperature measurement reference point by both numerical simulation and experiment. The two sensor coefficients for heat flux measurement were calibrated by using steady state method and transient method, respectively. The uncertainty analysis was carried out based on both theory and experimental data. The experimental results of black body furnace and xenon lamp show that the measurement deviation is within 5% and the response time is less than 10s. The non-spectrally selectivity was also proved.To further improve the performance of the transient radiative heat flux meter with guarded plates, a numerical simulation was carried out for different materials and constructions. A mold was designed and produced for thermal receiver manufacture. The modified heat flux meter was calibrated and experimentally verified. The results show that its absolute average measurement deviation from the actual value under the low heat flux range of 60Wm-2 is 0.8Wm-2, which is lower than the original transient radiative heat flux meter with double thermal guarded plates.