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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 accounted for by using “enhanced energy transfer”. The energy transfer requires a second dye that allows re-absorption of stimulated emission and thus enables a shift of dye-lasing to higher wavelengths. However, records of the droplet’s internal temperature field reveal a nonphysical inhomogeneity that is based on locally changing dye excitation intensity and locally changing efficiency of the energy transfer. Dynamics of the inhomogeneity effect are studied extensively by imaging and spectroscopy. Results are used for method optimization.
Error fields of micro-droplets during pulsed 2D-2cLIF-EET as a function of dye concentration and laser excitation intensity. Results can be grouped: (i) noise-based error pattern; (ii) negligible error pattern; and (iii) strong error with a central maximum accompanied by to minimum. In general, errors becomes stronger wither higher dye concentration Cj\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$C_j$$\end{document} (concentrations of both dyes (j\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm {j}$$\end{document} = PM597 or OBN) remain equal) and higher excitation intensity I0\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$I_\mathrm {0}$$\end{document}. Droplet conditions: EtOH, Tliq\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T_\mathrm {liq}$$\end{document} = 298 K, d ≈\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\approx$$\end{document} 82 μ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upmu$$\end{document}m, v ≈\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\approx$$\end{document} 17.5 m/s
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Horizontal (a) and vertical (b) inhomogeneity errors of droplets as function of dye concentration Cj\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$C_j$$\end{document} (concentrations of both dyes (j\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm {j}$$\end{document} = PM597 or OBN) remain equal) and excitation intensity I0\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$I_\mathrm {0}$$\end{document}. Error quantification reveals an amplified influence of laser excitation intensity for higher dye concentration. For a moderate dye concentration, errors show lowest values. Droplet conditions: EtOH, Tliq\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T_\mathrm {liq}$$\end{document}= 298 K, d ≈\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\approx$$\end{document} 82 μ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upmu$$\end{document}m, v ≈\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\approx$$\end{document} 17.5 m/s
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Average normalized spectral image of a droplet (a) next to a droplet ratio image (b). For larger wavelengths, spectral differences between droplet surface (top, P1, bottom, P3) and droplet center (P2) become visible. Increased intensity in the droplet volume is caused by additional spontaneous emission of OBN that is superimposed on PM597 emission. Droplet conditions: EtOH seeded with 20 μ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upmu$$\end{document}M PM597 and OBN, d ≈\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\approx \,$$\end{document} 82 μ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\upmu$$\end{document}m, v ≈\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\approx \,$$\end{document} 17.5 m/s, Tliq\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$T_\mathrm {liq} \,$$\end{document} = 289 K, I0,CW\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$I_\mathrm {0,CW }\,$$\end{document} = 59 MW/cm2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^2 $$\end{document}
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... 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.
... 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. ...
Article
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.
... 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] . ...
Article
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
... 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.
... 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
... 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.
... 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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The fluorescence spectra of dye solutions change their spectral signature with temperature. This effect is frequently used for temperature imaging in liquids and sprays based on two-color laser-induced fluorescence (2cLIF) measurements by simultaneously detecting the fluorescence intensity in two separate wavelength channels resulting in a temperature-sensitive ratio. In this work, we recorded temperature-dependent absorption and fluorescence spectra of solutions of five laser dyes (coumarin 152, coumarin 153, rhodamine B, pyrromethene 597, and DCM) dissolved in ethanol, a 35/65 vol.% mixture of ethanol/2-ethylhexanoic acid, ethanol/hexamethylsiloxane, o-xylene, and 1-butanol to investigate their potential as temperature tracers in evaporating and burning sprays. The dissolved tracers were excited at either 266, 355, and 532 nm (depending on the tracer) for temperatures between 296 and 393 K (depending on the solvent) and for concentrations ranging between 0.1 and 10 mg/l. Absorption and fluorescence spectra of the tracers were investigated for their temperature dependence, the magnitude of signal re-absorption, the impact of different solvents, and varying two-component solvent compositions. Based on the measured fluorescence spectra, the tracers were analyzed for their 2cLIF temperature sensitivity in the respective solvents. Coumarin 152 showed for single-component solvents the overall best spectroscopic properties for our specific measurement situation related to temperature imaging measurements in spray-flame synthesis of nanoparticles as demonstrated previously in ethanol spray flames [Exp. Fluids61, 77 (2020)10.1007/s00348-020-2909-9].
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The present study deals with the solvent dependent morphology-dependent resonances (MDR) in the LIF-signal (LIF: laser-induced fluorescence) of monodisperse gasoline droplets (30 µm – 60 µm) generated with a droplet generator. To investigate the influence of ethanol addition to gasoline and the respective LIF-signal of the dye nile red dissolved in these fuel blends, a reference gasoline fuel is blended with various ethanol concentrations from E0 (gasoline) to E100 (pure ethanol). A spectral fluorescence characterization of the investigated fuel mixtures at various concentrations is carried out in a micro cell in order to identify the dye and ethanol concentration influence of the respective fuel mixtures. The absorption and emission spectra of the fuel mixtures show a Stokes shift with increasing ethanol concentration towards larger wavelengths. The coefficient of variation (COV) of the fluorescence signals of spherical droplets was utilized to characterize the MDR effects within the droplet LIF images. The investigations revealed an increase of MDR contribution in terms of the COV of LIF signals with larger droplet diameters. For small droplets, no monotonic trend was found for contribution of MDR in the LIF signal as a function of the ethanol concentration. For larger droplets (e.g. 50 µm-60 µm), a lower contribution of MDR in LIF signals was observed with increasing ethanol content. For E80 and most of the studied ethanol blends, the normalized integrated COV values exhibited maxima at certain droplet sizes (40 µm, 47.5 µm and 55 µm), which indicate the presence of distinct MDR-effects.
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Heat transfers at the impact of a droplet on a hot solid surface are investigated experimentally. Millimeter-sized water droplets impinge a flat sapphire window heated at 600 °C. The time evolution of the droplet temperature is characterized using the two-color laser-induced fluorescence technique. For that, a Q-switched Nd:YAG laser is used for the excitation of the fluorescence to obtain instantaneous images of the droplet temperature. Water is seeded with two fluorescent dyes, one sensitive to temperature (fluorescein disodium) and the other not (sulforhodamine 640). Owing to a wavelength shift between the dyes’ emissions, the fluorescence signal of the dyes can be detected separately by two cameras. The liquid temperature is determined with a good accuracy by doing the ratio of the images of the dyes’ fluorescence. A critical feature of the method is that the image ratio is not disturbed by the deformation of the impacting droplet, which affects the signals of the dyes almost identically. Experiments are performed in the conditions of film boiling. A thin vapor film at the interface between the droplet and the solid surface prevents the deposition of liquid on the hot solid surface. Measurements highlight some differences in the rate of heat transfers and in the temperature distribution within the droplet between the bouncing and splashing regimes of impact.
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In this work, a new measurement system is presented for studying temperature of micro-droplets by pulsed 2-color laser-induced fluorescence. Pulsed fluorescence excitation allows motion blur suppression and thus simultaneous measurements of droplet size, velocity and temperature. However, high excitation intensities of pulsed lasers lead to morphology-dependent resonances inside micro-droplets, which are accompanied by disruptive stimulated emission. Investigations showed that stimulated emission can be avoided by enhanced energy transfer via an additional dye. The suitability and accuracy of the new pulsed method are verified on the basis of a spectroscopic analysis and comparison to continuously excited 2-color laser-induced fluorescence.
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In imaging, the detection of light originating from multiple scattering, indirect reflections and surrounding backgrounds are known to produce errors especially in intensity-ratio based measurements. SLIPI (Structured Laser Illumination Planar Imaging) is an imaging technique that significantly reduces the impact of such issues. In this study, SLIPI is combined with the two-color LIF (Laser Induced Fluorescence) ratio thermometry approach for measuring water temperature in both a cuvette and a hollow-cone spray. By removing the unwanted background interferences using SLIPI, we observe both significant improvements in terms of temperature sensitivity as well as more pronounced temperature gradients within the spray.
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The temperature dependence of the fluorescence emission of certain organic dyes such as rhodamine B has been widely utilized for measuring the temperature in liquid flows. Measurements are generally based on two assumptions: The fluorescence signal is proportional to the intensity of the laser excitation, and the temperature sensitivity of the dye is not affected by the laser irradiance. In the ratiometric methods, these assumptions allow justifying that the influence of the laser intensity can be totally eliminated in the intensity ratio of two spectral bands of the fluorescence emission and thus that measurements can be taken with no biases under experimental conditions, where the laser propagation is disturbed by the flow. However, when pulsed lasers are used (mainly in planar LIF measurements), the peak irradiance usually compares or exceeds the saturation intensity of the dyes. The present study assesses the consequences of a saturation of the dye emission on temperature measurements. Tests among fluoresceins and rhodamines reveal that the saturation can be accompanied by a significant loss of temperature sensitivity. The dyes, for which this loss of sensitivity is observed, mainly owe their temperature dependence to the fluorescence quantum yield and have a fluorescence signal decreasing with the temperature. The couple fluorescein/sulforhodamine 640 is finally proposed for an implementation of the ratiometric method, since its relatively high temperature dependence (+3 %/ (Formula presented.)) is not altered at high laser irradiances. The possibility of measuring instantaneous temperature fields with this pair of dyes using a single laser shot is finally demonstrated on a turbulent heated jet injected into quiescent water.
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A monodisperse droplet stream is injected into a high-temperature enclosure supplied with air heated up to 540 °C. The two-color laser-induced fluorescence (2cLIF) is used for measuring the droplet temperature. The liquid fuel is seeded by pyrromethene 597-C8, which is a temperature-sensitive fluorescent dye. Calibration tests are performed for different types of fuels including ethanol and several alkanes and some of their mixtures. Morphology-dependent resonances (MDRs) are identified as a possible adverse effect for temperature measurements. Due to MDRs, lasing of pyrromethene 597-C8 may occur within fluorescent droplets and affect drastically the fluorescence signal upon which temperature measurement relies. The determination of the droplet size and velocity is achieved by means of quantitative shadow imaging. A double cavity PIV laser is focused on a piece of PMMA doped with a fluorescent dye to produce the background illumination of the droplets. A PIV camera is used to capture the drop motion between the pulses of the laser cavities. A large range of initial distance parameters (the ratio between the inter-droplet distance and the droplet diameter) is explored for different liquid fuels (ethanol, isohexane, n-heptane, n-decane, n-dodecane) and their mixtures. To put forward the effects of the interactions between the droplets, size and temperature measurements are compared to the isolated droplet whose evolution can be predicted with the use of classical models. Comparisons reveal that the inter-droplet spacing and also the fuel volatility play an important role in the reduction of the heat and mass transfers for these interacting droplets. Finally, the ability of the 2cLIF techniques to address the case of multicomponent droplet is also demonstrated.
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