Fluorescence Thermometry for Measuring Wall Surface and Bulk Fluid Temperatures

Cooling the next generation of microelectronics with heat fluxes of more than 1 kW/cm² over hot spots less than 10³ μm² in area will require new single- and two-phase thermal management technologies with micron-scale addressability. Thermal transport models using heat transfer correlations may be the most efficient approach for the initial design and optimization of such micron-scale heat exchangers which will likely involve arrays of microchannels. It is unclear, however, whether classic macroscale convective heat transfer correlations are applicable to these devices given their complex geometries and the possibility of significant thermal coupling between channels. There is therefore a need for new techniques that can measure both bulk fluid and wall surface temperatures at micron-scale spatial resolution without disturbing the flow of coolant. We report here the use of a nonintrusive technique, fluorescence thermometry (FT), to determine bulk fluid temperatures and, for the first time, wall surface temperatures, with a spatial resolution of O(10 μm) for water flowing through a heated channel. Fluorescence thermometry is typically used to estimate temperature distributions in water flows based on variations in the emission intensity of a fluorophore dissolved in the water. The accuracy of FT can be improved by taking the ratio of the emission signals from two different fluorophores (dual-tracer FT, or DFT) to eliminate variations in the signal due to (spatial and temporal) variations in the excitation intensity. In this work, two temperature-sensitive fluorophores, fluorescein and sulforhodamine B, with emission intensities that increase and decrease, respectively, with increasing temperature, are used to further improve the accuracy of the temperature measurements. Temperature profiles were measured in the steady Poiseuille flow of water at Reynolds numbers of 3.3 and 8.3 through a 1 mm square channel heated with a thin-fim heater. Temperatures in the bulk flow were measured using DFT with an average uncertainty of 0.2 °C at a spatial resolution of 30 μm. Fluid temperatures within the first 0.3 μm next to the wall were measured using evanescent-wave illumination of a single temperature-sensitive fluorophore with an average uncertainty of less than 0.2 °C at a spatial resolution of 10 μm. The results are compared with numerical predictions, which suggest that the fluid temperatures within 0.3 μm are effectively the wall surface temperature.