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Error corrected ratio fields (a) and temperature fields (b) of isothermal droplets (each based on ca. 1200 droplets). Maximum temperature deviations account for approximately ±5 K. Droplet conditions see Fig. 8

Error corrected ratio fields (a) and temperature fields (b) of isothermal droplets (each based on ca. 1200 droplets). Maximum temperature deviations account for approximately ±5 K. Droplet conditions see Fig. 8

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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 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 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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... 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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... 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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... 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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... 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