A compensation method for the disturbance in the temperature field caused by subsurface thermocouples
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R boiling heat transfer plays an important role in metallurgical and materials manufacturing operations to control the temperature of the product during processing as well as its final microstructure. The modeling, design, and control of these quenching operations require an accurate quantification of the heat transfer occurring at the boundary. Quantification of the heat-transfer boundary condition (heat flux or heat-transfer coefficient) is rendered especially challenging by the rapid nonlinear changes in the heat flux with variations of the component surface temperature. Researchers are increasingly turning to inverse heat conduction (IHC) methods to quantify the surface heat flux during a quench experiment. The IHC analysis estimates the heat-transfer boundary condition based on the temperature-time history at a known interior location in the material during quenching. Traditionally, thermocouples have been used to measure the sample thermal history. Typically, thermocouples consist of two wires of different metals welded together at their extremity, the so-called thermocouple junction. Away from the junction, the thermocouple wires are surrounded by an insulating material covered by a metallic sheath. The electrical potential difference generated between the two wires is related to the temperature at the thermocouple junction. The measurement of the voltage at the ends of the thermocouple therefore corresponds to a measurement of the temperature at the welded tip. The measured thermal history at a known location in the sample can be used in conjunction with inverse heat-transfer analysis to estimate the surface heat flux or heat-transfer coefficient. The usual method used to instrument a sample is to install the thermocouples at the base of holes drilled close to the surface (subsurface thermocouples). The holes used to instrument subsurface thermocouples generally have a much lower thermal conductivity than the surrounding material and create a disturbance in the E. CARON, Graduate Student, and M.A. WELLS, Associate Professor, are with the Department of Materials Engineering, The University of British Columbia, Vancouver, BC, Canada V6T 1Z4. Contact e-mail: etienne@ cmpe.ubc.ca D. LI, Principal Research Engineer, is with Belvac Production Machinery Inc., Lynchburg, VA 24502. Manuscript submitted May 28, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
local temperature field when instrumented parallel to the heat flux.[1] As shown in Figure 1, for the case of a quenched surface, the transfer of heat from the back of the sample is rendered more difficult by the presence of the thermocouple hole. This leads to a significantly greater cooling at the thermocouple tip and hence to a temperature discrepancy compared to the undisturbed case, i.e., the temperature measured at the same location in a sample without a thermocouple. The disturbance created by subsurface thermocouples was observed in the so-called hot spot formed at the surface of aerospatial components exposed to heating during re-entry[2] as well as in instrumented s
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