Emissivity characteristics of aluminum alloy surfaces and assessment of multispectral radiation thermometry (MRT) emissivity models
Experiments were performed to examine the emissivity characteristics of aluminum alloy samples over the spectral range of 2.05 to 4.72 μm and temperatures of 600, 700 and 800 K. AL 1100, 7150, 7075 and 2024 samples with polished, 6-μm, and 14-μm surface finishes were tested. Additionally, extruded and saw-cut samples were tested to examine the effects of extreme roughness on emissivity. Overall, aluminum alloys are shown to buck the general trend of increasing emissivity with increasing temperature for metallic surfaces in the infrared range. The emissivity of aluminum samples generally decreased between 600 and 700 K and increased between 700 and 800 K. The latter increase is attributed to surface discoloration at 800 K. For the polished, 6-μm and 14-μm samples, the emissivity decreased appreciably between 2.05 and 3.5 μm, and increased slightly between 3.5 and 4.72 μm. Spectral variations were far less pronounced for the extruded and saw-cut surfaces. The experimental results were used to assess the accuracy of eighteen multispectral radiation thermometry (MRT) emissivity models for temperature measurement. It is shown that drastic changes in the shape of emissivity distribution preclude the use of a single function to accurately represent every band of the measured spectrum. Better predictions were achieved using the simplest form of MRT emissivity models and minimum number of wavelengths were required by the model. Overall, two relatively simple models provided the best overall predictions for different alloys, temperatures and surface roughnesses. This study also explored the relationship between the emissivity of aluminum alloy surfaces and surface roughness. Different theories governing this relationship were reviewed and categorized. A previous model by Agababov was found to be an effective means for both characterizing surface roughness and incorporating roughness features in emissivity models. Four emissivity models were examined based on their proven accuracy at determining both emissivity and surface temperature using the MRT technique. Those models were tested both in their basic form and modified with the Agababov roughness function for accuracy in inferring surface temperatures of AL 7075 samples. The results show that modified models yield better accuracy in temperature prediction.
Mudawar, Purdue University.
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