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3073 |
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A substrate surface thermocouple for thermal spraying |
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Michael Marr / University of Toronto
Department of Mechanical and Industrial Engineering, Canada Olivera Kesler / University of Toronto, Canada |
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A thermocouple was developed for measuring surface temperatures of steel substrates during plasma spraying. Under high heat flux conditions, the accuracy of conventional surface thermocouples diminishes because their thermal properties are different from those of the substrate. This difference causes the hot junction temperature to deviate from the substrate surface temperature. To compensate for the property mismatch, our thermocouple includes a highly conductive copper layer to equalize the hot junction and substrate surface temperatures. Even under large heat fluxes, the copper layer ensures thermocouple accuracy. To make the thermocouple, nicrosil and nisil (type N) wires are embedded with ceramic cement in two individual holes through the substrate. The cement electrically isolates the wires from the substrate. The wire tips are trimmed flush with the surface, and then the copper layer is deposited across the surface by plasma spraying. In addition to providing for temperature equalization, the copper also creates an electrical connection between the wire tips to form the hot junction. Due to its intrusive nature, the thermocouple is for diagnostic purposes and not meant to be incorporated into functional coatings. Finite element modeling was performed to determine the optimal thickness of the copper layer. The effective thermal conductivity of the copper was estimated from image analysis of sample coatings. The optimal thickness was determined to be 38 micrometres. With the estimated thermal properties, the model found that the thermocouple can measure the magnitude of temperature swings due to a passing plasma jet within +/- 3 percent. However, due to various uncertainties, thermocouple accuracy was estimated as +19/-10 percent for measuring temperature swings. For measuring average temperatures, the thermocouple is very accurate because large heat fluxes occur only as the plasma torch is directly in front of the substrate. Experimental surface temperatures measured with the optimized thermocouple are presented. |