Extending Fluorescence Thermometry to Measuring Wall Surface Temperatures Using Evanescent-Wave Illumination

Cooling microelectronics with heat flux values of hundreds of kW/cm² over hot spots with typical dimensions well below 1 mm will require new single- and two-phase thermal management technologies with micron-scale addressability. However, experimental studies of thermal transport through micro- and mini-channels report a wide range of Nusselt numbers even in laminar single-phase flows, presumably due in part to variations in channel geometry and surface roughness. These variations make constructing accurate numerical models for what would be otherwise straightforward computational simulations challenging. There is, therefore, a need for experimental techniques that can measure both bulk fluid and wall surface temperatures at micron-scale spatial resolution without disturbing the flow in both heat transfer and microfluidics applications. We report here the evaluation of a nonintrusive technique, fluorescence thermometry (FT), to determine wall surface and bulk fluid temperatures with a spatial resolution of O(10 μm) for water flowing through a heated channel. Fluorescence thermometry is typically used to estimate water temperature fields 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. Water temperature profiles were measured in steady Poiseuille flow at Reynolds numbers of 3.3 and 8.3 through a 1 mm² heated minichannel. Water temperatures in the bulk flow (i.e., away from the walls) were measured using DFT with an average uncertainty of 0.2 °C at a spatial resolution of 30 μm. 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 water temperatures at an average distance of ∼70 nm from the wall are identical within experimental uncertainty to the wall surface temperature.