Characterization of Fluorescence Tracers for Thermometry and Film Thickness Measurements in Liquid Coolants Relevant for Thermal Management of Electric and Electronic Components

Abstract

This study investigated a novel two-color LIF (laser-induced fluorescence) technique for thermometry in coolants relevant for electric components. In principle, this diagnostic enables thermometry in liquid flows but also a simultaneous determination of film thickness and film temperature, which is relevant, e.g., for jet impingement cooled electric components. Temperature measurements are based on a temperature-sensitive intensity ratio of special tracers realized by suitable band pass filters within the respective emission spectra. For this purpose, the heat transfer fluids Fragoltherm F12, Marlotherm LH, and a water–glycol mixture WG20 (80 vol.% water, 20 vol.% glycol) and its individual components were doped with suitable tracers. The tracer Eosin-Y was utilized for polar coolants (water, WG20, and glycol) and Nile red was utilized for non-polar solvents (Fragoltherm F12 and Marlotherm LH). The spectral LIF intensities were recorded for a wide range of temperatures (253–393 K), which are relevant for cooling of electric motors, batteries, and power electronics. Furthermore, absorption spectra were analyzed as well. The temperature-dependent fluorescence measurements revealed different behavior for the polar and non-polar solvents. A temperature increase in the polar solvents (water, WG20, glycol) led to a spectral shift of the emission peaks of Eosin-Y towards longer wavelengths (red-shifted), while the peaks of Nile red in the non-polar solvents (Fragoltherm F12 and Marlotherm LH) showed an opposite behavior and were blue-shifted. The highest average temperature sensitivity was achieved for Marlotherm LH (4.22%/K), followed by glycol (1.99%/K), WG20 (1.80%/K), water (1.62%/K), and Fragoltherm F12 (1.12%/K). These sensitivities are similar to or even much higher than the literature data of other LIF tracers, which were, however, not determined in those coolants. Consequently, the two novel proposed dyes for the studied heat transfer liquids enable a reliable temperature determination.

Spectral Investigations of Fluorescence Tracers in Automotive and Aviation Fuels under Cryogenic Conditions

“…Similar emission behavior of the dyes studied has already been reported in one of our previous investigations [50], where the peaks for Eosin-Y (dissolved in water; water + 20 vol.% glycol and water + 40 vol.% glycol) were shifted toward longer wavelengths with increasing temperature as well, while the peaks of Nile red dissolved in oils such as Fragoltherm F12 and Marlotherm LH were shifted toward shorter wavelengths.…”

“…Earlier absorbance and fluorescence measurements of the group were conducted in a micro-cell, which was driven with a recirculating thermostat [28,43,50]. Herein, the lower temperature limit was governed by the mass flow and the viscosity of the recirculating thermostat used.…”

“…Koegl et al characterized suitable tracers for two-color thermometry for cooling applications for electric and electronic components. They determined the temperature sensitivities of Eosin-Y in water (1.62%/K), a water-glycol mixture (80 vol.% water, 20 vol.% glycol; 1.80%/K), and Nile red in Marlotherm LH (4.22%/K) and Fragoltherm F12 (1.99%/K) [50]. Vetrano et al calculated the temperature sensitivity of rhodamine B in ethanol (297 K-328 K) to be 0.7%/K [33].…”

A novel laser-induced fluorescence concept for thermometry measurements in machining lubricoolants

“…They measured the temperature sensitivity of water (1.62%/K), water-glycol mixture (80 vol.% water, 20 vol.% glycol; 1.80%/K) with Eosin-Y. Otherwise, nile red in Marlotherm LH and in Fragoltherm F12 showed a temperature sensitivity of 4.22%/K and 1.99 %/K, respectively [62]. Prenting et al characterized dyes for two-color LIF thermometry (296 K-348 K) for spray applications.…”

Droplet thermometry based on an optimized two dye two-color laser-induced fluorescence concept

“…2c-LIF was successfully applied to measure averaged temperature field images in the SpraySyn flame. Other auspicious dyes in polar and non-polar liquids for thermometry are, for example, Eosin-Y with a sensitivity of 1.6%/K in water (2.0%/K in glycol) and Nile red (for non-polar heat transfer oils such as Marlotherm LH with a sensitivity of 4.2%/K), both measured in a tempered cuvette [19]. However, both dyes showed MDRs in the LIF signal in droplet sizing experiments based on the LIF/Mie ratio [20][21][22].…”