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Research Papers: Experimental Techniques

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

[+] Author and Article Information
Myeongsub Kim1

 G. W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0405minami@gatech.edu

Minami Yoda2

 G. W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0405minami@gatech.edu

1

Current address: Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, ON Canada M5S 3G8.

2

Corresponding author.

J. Heat Transfer 134(1), 011601 (Oct 27, 2011) (8 pages) doi:10.1115/1.4004871 History: Received June 17, 2010; Accepted August 11, 2011; Published October 27, 2011; Online October 27, 2011

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.

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Figures

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Figure 1

(a) Sketch showing an isometric view of the PDMS-glass channel and the ITO heater and two orthogonal planar slices of the channel (b) parallel and (c) normal to the flow (not to scale). Two different channels, with the edge of the heater at z=-0.1 mm and z=-1.0 mm, respectively, were used in these studies. The dotted rectangle in (b) and the dotted line in (c) represent the imaged region (IR). All dimensions are given in millimeter.

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Figure 2

Plot of average normalized fluorescence intensity of the emissions from Fl for the EFT calibrations I¯/I¯20 (▴) as a function of solution temperature T. Equation 1 is given by the solid line, and the error bars represent the standard deviations.

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Figure 3

Graph showing the average fluorescence intensity normalized by its value at 20 °C I¯/I¯20 for Fl (▴) and SrB (•) in the DFT calibrations, as well as ratio of the emission intensities from these two species I' (▪), all as a function of T. Equation 2 is given by the solid line. The error bars for the I¯/I¯20 data denote their standard deviations.

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Figure 4

Pseudocolor temperature map obtained with EFT using Fl for Poiseuille flow at Re = 3.3 at over a 100 μm (x) × 120 μm (z) region at a spatial resolution of 1 μm. The center of the region shown is at (x,z)= (0, 0.16 mm).

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Figure 5

Averaged 0.95 mm (x) × 0.467 mm (z) grayscale images of the (a) Fl and (b) SrB emissions and (c) the resultant pseudocolor temperature distribution in the water next to the channel side wall at Re = 8.3. The center of the region shown in all three images is at (x, z) = (0, 0.23 mm), with y≈20 μm.

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Figure 6

Fluid temperatures T as a function of normalized channel coordinate z/L spanning a 100 μm segment of the channel measured by EFT (▴) compared with the predictions from FLUENT® at both y = 0 (•) (i.e., the wall) and y=50 μm (▪) for (a) Re = 3.3 and (b) Re = 8.3. The uncertainty of 0.16 °C in the EFT results is smaller than the size of the symbols, and is therefore not shown in these graphs.

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Figure 7

Graph of water temperature T over a physical z-distance of 0.5 mm as a function of normalized channel coordinate z/L measured by DFT using volume illumination (▴) and predicted by numerical simulations at the wall, or y = 0 (•) and at y = 50 μm (▪) at (a) Re = 3.3 and (b) Re = 8.3. The error bars represent the uncertainty of 0.21 °C in the DFT results.

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