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A new measurement system called “pulsed 2D-2cLIF-EET” has been developed to study temperature fields inside micro-droplets. Pulsed fluorescence excitation allows motion blur suppression and thus simultaneous measurement of droplet size and temperature. Occurrence of morphology-dependent resonances and subsequent stimulated dye emission are accounte...

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... Rhodamine B works well in ethanol and water [43][44][45][46][47]. Fluorescein is often used in combination with water and ethanol [48][49][50]. Pyrromethene is mainly used in alkanes (dodecane), ketones (3-pentanone), and alcohols [45,[51][52][53][54][55][56]. Coumarin is mainly used for two-color LIF thermometry in ethanol [42]. ...
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This study investigates 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 for non-polar solvents (Fragoltherm F12 and Marlotherm LH). The spectral LIF intensities were recorded for a wide range of temperatures (253 K – 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 reveal different behavior for the polar and non-polar solvents. A temperature increase of the polar solvents (water, WG20, glycol) leads to a spectral shift of the emission peaks of Eosin-Y towards larger wavelengths (red-shifted), while the peaks of nile red in the non-polar solvents (Fragoltherm F12 and Marlotherm LH) show an opposite behavior and are 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 or even much higher than 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.
... This results in a distinct signal drop for higher dye concentrations in this spectral emission band; see inserted diagram in Fig. 5. In the spectral regions were reabsorption takes place, the existence of MDR is expected to be less probable [36,62,63]. ...
... However, in the previous work only images were taken and no spectral information about the MDR peak was reported. The utilization of a second dye with an absorption band located at the wavelengths where MDRs usually occur, leads to a suppression of the lasing signal [62,63]. In the case of Nile red dissolved in iso-octane/ethanol blends, Solvent Blue 38 (CAS: 1328-51-4, Sigma Aldrich) may be a valuable choice, in order to suppress MDRs. ...
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The present study deals with droplet sizing based on laser-induced fluorescence (LIF) and Mie scattering for varied polarization of the utilized laser (parallel or perpendicular). The polarization-dependent LIF/Mie ratio is studied for micrometric droplets (25–60 µm) produced with a droplet generator. The investigations were carried out with the dye Nile red dissolved in ethanol and ethanol/iso-octane mixtures. A spectral absorption and fluorescence characterization at various dye and ethanol concentrations is carried out in a cuvette in order to identify reabsorption effects. The LIF|| droplet images (index ||: parallel polarization) show a more homogeneous intensity distribution in the droplets and slightly stronger morphology-dependent resonances (MDRs) in comparison to LIF (index: perpendicular polarization). The spectral LIF emissions reveal a dependence of the MDR on the ethanol admixture. The larger the ethanol content, the lower the MDR peak, which is also shifted further to the red part of the spectrum. The Mie droplet signal images are mainly characterized by two distinct glare points, one at the entrance of the laser light (reflection) and one at the exit (first-order refraction). The Mie images show a more pronounced entrance glare point, in comparison to Mie||, where the exit glare point is more pronounced. These observations are in accordance with the theory. The calibration curve of the micro droplet signals revealed a volumetric trend of the LIF signals and a slightly higher LIF signal and sensitivity in comparison to LIF||. The signal Mie follows roughly a quadratic trend on average, while Mie|| follows a linear trend. Consequently, the calculated LIF/Mie ratio shows a linear trend, whereas the LIF||/Mie|| ratio shows a quadratic trend, which confirms theoretical calculations. A numerical simulation of the Mie signal at various detection angles shows a good agreement with the experimental data at large apertures.
... The researchers developed a new measurement system called 'pulsed 2D-2cLIF-EET' to study temperature fields inside micro-droplets. The MDR and stimulating dye emission are accounted for by using energy transfer [29]. ...
... Therefore, the change in the refractive index and radius of microsphere led to the change (shift) in the resonance (MDR) of the microsphere. Many applications of MDR have been shown in the same manner of interpretation [2,12,23,[27][28][29][30]32]. Lately, the MDR techniques are used to determine the size and composition of core-shell particles. ...
... The MDR modes can be calculated by using the theory of MDR, the coefficients of exponential-like increasing functions have to vanish [33]. The MDRs concept has also been applied to optical-biosensors used in indicating the locations for a high-density photon in micro-droplet [29,33]. The concept also has been used to investigate the size and composition of glassy aerosol microspheres. ...
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In many applications constant or piecewise constant refractive index profiles are used to study the scattering of plane electromagnetic waves by a spherical object. When the structured media has variable refractive indices, this is more of a challenge. In this paper, we investigate the morphology dependent resonances for the scattering of electromagnetic waves from two concentric spheres when the outer shell has a variable refractive index. The resonance analysis is applied to the general solutions of the radial Debye potential for both transverse magnetic and transverse electric modes. Finally, the analytic conditions to determine the resonance locations for this system are derived in the closed form of both modes. Our numerical results are provided with discussion.
... 3D (3 dimension) fluorescence visualization of dynamical flow process can be further achieved with the help of reconfiguration coded incoherent light-sheet array technique (Ren et al. 2020). To achieve higher temperature measurement accuracy, two-color or multicolor LIF method, using ratio of multi-fluorescence or phosphorescence bands' intensities with different temperature sensitivities, is proposed to eliminate interference from destabilization of laser intensity and dye molecular concentration, which also have a strong impact on fluorescence or phosphorescence intensity (Jeong et al. 2009;Labergue et al. 2013;Dunnand et al. 2011;Palmer et al. 2018). With the help of structured laser illumination planar imaging method, unwanted background can be further eliminated effectively (Mishra et al. 2015). ...
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In this work, a two-color laser-induced fluorescence (LIF) thermometry approach for 2D (2 dimension) temperature field visualization in gas–liquid system is presented, which is still challenging in complex flow field changing rapidly with time and space. Our experiment shows that temperature deviation from two-color LIF image misalignment in few pixels scale can be the main source for temperature measurement error, especially at gas–liquid boundaries with high relative gradient value. In spatial modulated PL images with 0.2/pixel relative gradient value, temperature deviation estimated to be ~ 85.0 ℃ at temperature 25.0 ℃ under 1 pixel position, which is far from measurement error observed in uniformly PL intensity distributed images (6.0 ℃ maximum measurement error in 20.0–100.0 ℃). According to this problem, a nonlinear image registration method based on optical flow algorithm is proposed to enhance image registration accuracy. Relative ratio deviation is significantly reduced from maximum value 60 to < 10% in position deviation ranging from 1 to 15 pixels and corresponding temperature deviation value can be effectively reduced from maximum value ~ 75 ℃ to < 10.0 ℃ in the temperature ranging from 19.0 to 94.0 ℃. Finally, accurate 2D temperature distribution data and corresponding heat flux in liquid around hot steam bubble can be visualized by this convenient and low-cost thermometry method. Graphical abstract
... Because of their high quantum efficiencies and excellent solubility in hydrocarbons, alcohols, and water, organic dyes are often a preferred choice. Two-color LIF (2cLIF) thermometry is one of the approaches for temperature imaging, where the different temperature dependence of the fluorescence signal in two selected wavelength regions of the fluorescence spectrum is exploited to determine the temperature from the intensity ratio [25][26][27]. Several investigations on tracers for LIF have been done recently [28][29][30]. ...
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... Fluorescent dye emission also experiences MDRs and causes stimulated and amplified emission at the frequencies of the resonance mode. This unspontaneous fluorescence emission makes measurements of fluorescence intensity inaccurate [333]. However, research so far has analyzed only symmetrical morphologies, such as spheres and cylinders [334]. ...
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Evaporation of liquid is important in a diverse range of engineering applications, such as ink-jet printing, pesticide spraying, micro- and nanofabrication, thin-film coatings, biochemical assays, deposition of DNA/RNA microarrays, the manufacture of novel optical and electronic materials, and cooling microelectronics and power electronic devices. In particular, evaporation at the microscale has attracted increasing interest as an effective cooling strategy for overcoming the thermal challenges in high heat flux microelectronics devices. A large number of studies have demonstrated the prospect of evaporative heat transfer methods to tackle die-level hotspots reaching 1 kW/cm2 on each high-power tier in a 3D microelectronics device. Furthermore, evaporation at the microscale (thin-film evaporation) can achieve higher heat removal than evaporation at macroscale. However, evaporation is a complicated process involving several physical transport phenomena, and their dominance can vary with variations in device dimensions and other system parameters. This article reviews the literature on the factors affecting microscale evaporation, which include the properties and temperature of the solid substrate, vapor transport in the gas domain, microconvection, and engineered surface features. Techniques to enhance evaporative heat transfer are highlighted. Extending the contact line region effectively enhances evaporative heat transfer, and this technique is employed in surface coatings or micro- and nanostructures, wicking structures, and micro- and nanoporous membranes. The evaporation rate can also be enhanced by manipulating the meniscus shape to provide an energy barrier at sharp edges of micro- and nanostructures. This review also summarizes the theoretical models for estimating evaporation rates, then discusses the physical transport processes associated with evaporation and their corresponding thermal resistances. Because non-invasive, high resolution temperature measurement and visualization are critical for implementing evaporative cooling in high heat flux applications, state-of-art techniques are also discussed. Laser-induced fluorescence techniques are judged the most advanced for temperature measurement, and particle image velocimetry (PIV) is the most advanced means of flow field visualization. This review identifies the most promising evaporative cooling techniques for next generation of ultra-high heat flux microelectronics applications. It also compares the performance of these cooling technologies in a regime plot, providing useful information for designing effective cooling solutions. We end by summarizing the current challenges and discussing the outlook for evaporative cooling technologies, then consider future research needs.
... When considering the experimental results, we analyzed the possibility of morphology dependent resonances (MDRs) or lasing effect. This phenomenon usually manifests itself either with small droplets (with a radius of under 50 μm) or with relatively high specific power of the laser radiation source [43][44][45] . The specific radiation power in the experiments approximated 3.5 W/cm 2 , and the size of the minimum recorded separate droplets (as mentioned before) was Rd = 25-50 μm. ...
... The specific radiation power in the experiments approximated 3.5 W/cm 2 , and the size of the minimum recorded separate droplets (as mentioned before) was Rd = 25-50 μm. Such parameters, according to Ref. [44] , indicate the minimization of the laser resonance inside micro-droplets and, thus, of the temperature measurement error. At the same time, an emulsion droplet consists of water micro- droplets evenly distributed in tetradecane, which is generally similar to a micro-spray cloud analyzed in Ref. [43] . ...
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In this research, we measure the temperatures of the non-combustible component (water) in a fuel droplet by 2-Color LIF before its micro-explosive fragmentation. We use two types of droplets based on water and tetradecane: a two-component immiscible droplet with water in the core and tetradecane as the envelope; a pre-mix emulsion. In both cases, the relative volume fraction of water in a droplet is 9%, and that of the combustible component (tetradecane) is 91%. To provide the micro-explosive dispersion of droplets, we use a scheme with the conductive heating in the range of 250 °C to 550 °C. Using the high-speed 2-Color LIF technique, we observe the coalescence of water micro-droplets in emulsion droplets and intense disruption of the water core in a two-component droplet when heated. The temperature in two zones are measured. The temperature difference may range from 1 °C to 10 °C. In the puffing regime, the droplet temperature is 1–3 °C higher than in the micro-explosion regime at the same temperatures of the heated metal substrate surface. Within a certain interval of the heating time, the temperatures of water in a two-component droplet and in an emulsion droplet become comparable.
... Afterwards, these glare points disappear probably due to extinction effects caused by the increasing dye concentration in combination with the low laser fluence. Morphology-dependent resonance (MDR) effects [5,30], which are common for pulsed laser operation, were not observed during the The LIF signal inside the droplet is larger in the right part, which is because of absorption and extinction effects within the droplet. Within the first 60 s, small glare points at the entrance and exit of the droplet are caused by Mie-scattering enhanced reabsorption effects. ...
... Afterwards, these glare points disappear probably due to extinction effects caused by the increasing dye concentration in combination with the low laser fluence. Morphology-dependent resonance (MDR) effects [5,30], which are common for pulsed laser operation, were not observed during the measurements, which is related to the low laser fluences applied. A lens effect due to the droplet interface leads to a focusing of the incidental laser beam [4]. ...
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Laser-induced fluorescence (LIF) spectroscopy using dyes is frequently applied for characterization of liquids and two-phase flows. The technique is utilized e.g., for mixing studies, thermometry or droplet sizing. One major application of the LIF technique combined with Mie-scattering is the planar measurement of droplet sizes in spray systems. However, its uncertainty is determined, among others, by varying dye concentration and temperature changes occurring during mixing and droplet evaporation. Systematic experimental investigations are necessary to determine the influence of dye enrichment effects on the LIF-signal of single droplets. For these investigations, the fluorescence dye Eosin-Y is dissolved in water and ethanol, which are typical solvents and working fluids in bio-medical applications and power engineering. A photo-physical characterization of the mixtures under various conditions was conducted using a spectrometric LIF setup and a micro cell. For ethanol, a small temperature dependency of the Eosin-Y LIF signal is observed up to 373 K. Photo-dissociation of Eosin-Y is negligible for solution in ethanol while it is distinct in water. The LIF signals of the single droplets are studied with an acoustic levitator. Effects of droplet evaporation, droplet deformation and varying dye concentration on the LIF-signal are studied. The single droplet measurements revealed a complex change of the fluorescence signal with reduced droplet size. This is due to droplet deformations leading to variations in the internal illumination field as well as dye enrichment during evaporation.
... Rights reserved. (2020) and mainly pyrromethene and its derivatives (e.g., 597-8C9, 597-C8) are applied, but the fluorescence strongly depends on temperature so that this dye is mainly used for thermometry (Depredurand et al. 2008(Depredurand et al. , 2010Palmer et al. 2018). The tracer Nile red (C 20 H 18 N 2 O 2 ) is one promising tracer, which is commonly used in microfluidic and in biology applications (Zhang et al. 2018;Lin et al. 2014;Greenspan and Fowler 1985). ...
... Obviously, the MDR effect is less pronounced for the selected laser fluence, dye and droplet size range as well as dye concentration in comparison to droplet studies with pyrromethene 597-8C9 [see e.g. (Palmer et al. 2018)]. Slightly more distinct MDR-effects were observed for E20 in comparison to the base fuel Toliso (see Fig. 4, e.g. for 50 µm diameter). ...
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A novel planar droplet sizing (PDS) technique based on laser-induced fluorescence (LIF) and Mie-scattering is utilized for the characterization of the spray structure under gasoline direct-injection spark-ignition (DISI) conditions. Fuel effects on the spray structure and cyclic variations are studied for a gasoline surrogate fuel (Toliso, consisting of 65 vol.% isooctane and 35 vol.% toluene) and the gasoline-ethanol blend E20 (20 vol.% ethanol admixture). Sauter mean diameter (SMD) results are compared with those from phase-Doppler anemometry (PDA) measurements showing good agreement especially at early points in time (up to 1.2 ms after start of injection). The liquid spray propagation and SMD are very similar for both fuels indicating similar atomization behavior. Both investigated fuels show comparable cyclic variations of the spray shape. A larger width and slightly larger droplet sizes are observed for the E20 spray when stronger evaporation occurs (at 2 ms). At these later points in time, the PDS-measured droplet sizes differ from the PDA-results. Here the limitation of the PDS-technique becomes obvious as a partial evaporation of the droplets may lead to large systematic errors. A numerical simulation of single droplets is provided for clarification of issues of droplet evaporation in PDS. Graphic abstract
... "Pulsed 2-color Laser-Induced Fluorescence with MDRenhanced energy transfer" (pulsed 2cLIF-EET) was recently introduced for instantaneous temperature imaging of microdroplets (Palmer et al. 2016(Palmer et al. , 2017(Palmer et al. , 2018a. The method is based on conventional 2cLIF thermometry, but uses a pulsed laser for imaging without motion-blur. ...
... Therefore, 2cLIF thermometry capabilities are restored. Some minor remaining spatial artifacts of the EET can be corrected using an "error-mask method" (Palmer et al. 2018a). ...
... Pulsed 2cLIF-EET is based on conventional 2cLIF thermometry theory, where a temperature sensitive fluorescent dye is excited by a laser with an intensity I 0 ( W∕m 2 ). Assuming negligible attenuation (small optical thickness), the non-saturated spontaneous fluorescence intensity I (W) of a measurement volume V is calculated as (Lemoine et al. 1996;Palmer et al. 2018a): where K opt ( ) (−) is an optical calibration constant of the setup and the dye is characterized by its molar attenuation coefficient (L/(mol cm)) at excitation wavelength ( 0 ), quantum yield ( ) and concentration C (mol/L). Factor g is a variable of the pulsed 2cLIF-EET approach that comprises all wavelength and spatially dependent photon density influences with an impact on energy transfer (MDR-EET; self-absorption; attenuation, droplet-lens and shadow-zone effects, etc.). ...
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The relatively new thermometry technique “pulsed 2cLIF with MDR-enhanced energy transfer” is tested for the first time on micro-droplet streams in a heated air flow. The method allows simultaneous measurements of the droplet internal temperature-field, droplet size and droplet velocity. Plausibility of the measurement results is evaluated by comparing results of various experimental conditions among each other and by comparison with analytic modeling. Overall, measurement results are plausible. However, measured temperature-fields are biased by the integral fluorescence signal and initial conditions of the models are limited by the Rayleigh jet breakup that creates the droplet stream. Graphic abstract