Stefan Will’s research while affiliated with Friedrich-Alexander-University Erlangen-Nürnberg and other places

The planar droplet sizing (PDS) method for determining the droplet diameter from the ratio of laser-induced fluorescence (LIF) and Mie scattering signal (Mie) has been extensively applied for right-angle scattering detection. The present work consists of a parametrical study of the Mie signal dependency on the scattering angle and polarization, as well as further experimental investigations in single droplets to ultimately improve the intensity fluctuation characteristics of the signal through a favorable experimental arrangement. Finally, we investigated how these findings impact droplet sizing in a spray on a shot-to-shot basis, as well as averaged across a dataset, via microscopic and macroscopic spray measurements. In the parametrical study, we found that horizontally polarized incident light results in a higher degree of Mie signal fluctuation than vertically polarized incident light at a scattering angle of $\Theta ={90}{^\circ }$, whereas the reduction to $\Theta = {60}{^\circ }$ resulted in the exact opposite observation. Furthermore, the reduction from $\Theta = {90}{^\circ }$ to $\Theta = {60}{^\circ }$ resulted in an overall improvement to the $D^2$-dependency and signal fluctuation of the Mie signal. We observed the effects shown in the parametric study in the experimental droplet measurements. For instance, when observing single droplets, the change from $\Theta = {90}{^\circ }$ to $\Theta = {60}{^\circ }$ resulted in a reduction in signal fluctuation by 46.1% and 17.6% for horizontally and vertically polarized incident light, respectively. An implementation into a spray setup yielded reduced shot-to-shot sizing deviations and closer adherence to the microscopically measured diameter when averaged across the dataset. Graphical abstract

In this work, iron-oxide nanoparticle formation in the spray-flame synthesis (SFS) process of the standardized SpraySyn 2.0 burner was investigated in situ using laser-induced incandescence (LII). For the evaluation of these measurements, prior LII-experiments within iron-oxide aerosols (Fe3O4 and α-Fe2O3) with known primary particle size distribution and morphological properties were performed to determine the thermal accommodation coefficient (TAC) α, which led to approx. α = 0.08. The applicability of the TAC results within the flame was validated using spectrally and temporally resolved measurements in the flame at 65 mm HAB employing a spectrograph. Data for a bimodal particle size distribution, obtained from Transmission Electron Microscopy (TEM), were used in the LII-evaluation. The validated TAC was then used to evaluate the primary particle size evolution from in situ Time-Resolved (TiRe) LII-measurements using PMTs along the centre axis of the burner, ranging from 10 mm to 50 mm HAB. These measurements reveal a relatively constant effective particle diameter along HAB with dp,eff ≈ 300 nm. To further investigate particle formation in SFS, 2-dimensional time-resolved LII-measurements in the SFS flame were performed, showing a clear particle formation region up to approx. 30 mm HAB, from where on a constant particle mass is observed.

Liquid organic hydrogen carrier (LOHC) systems offer a particularly interesting option for chemical hydrogen storage. In order to characterize and understand the endothermal hydrogen release from the carrier liquid and to evaluate suitable catalyst materials, knowledge of the temperature fields in the dehydrogenation reactor is important. One suitable technique for planar temperature sensing in reacting systems is phosphor thermometry. It is based on the excitation of a luminescent material by a laser pulse and detection of the subsequent phosphorescence signal. We investigated the luminescence of the thermographic phosphor (Sr,Ca)SiAIN3:Eu²⁺ (“SCASN:Eu²⁺”) dispersed in the H0-DBT / H18-DBT LOHC system in a temperature range from 400 to 600 K. A measurement cell enables repeatable and homogeneous measurement conditions of the hydrogen release reaction. A catalytic plate was put inside the heated LOHC. Temperature fields during the hydrogen release reaction were measured for the first time using the phosphorescence decay time (PDT) and the phosphorescence intensity ratio method (PIR). As expected, a strong cooling at the catalyst surface during the endothermal hydrogen release reaction could be observed, which was quantified to be in the range of 40 K. Graphical abstract

Wide-angle light scattering (WALS) offers the possibility of a highly temporally and spatially resolved measurement of droplets in spray-based methods for nanoparticle synthesis. The size of these droplets is a critical variable affecting the final properties of synthesized materials such as hetero-aggregates. However, conventional methods for determining droplet sizes from WALS image data are labor-intensive and may introduce biases, particularly when applied to complex systems like spray flame synthesis (SFS). To address these challenges, we introduce a fully automatic machine learning-based approach that employs convolutional neural networks (CNNs) in order to streamline the droplet sizing process. This CNN-based methodology offers further advantages: it requires few manual labels and can utilize transfer learning, making it a promising alternative to conventional methods, specifically with respect to efficiency. To evaluate the performance of our machine learning models, we consider WALS data from an ethanol spray flame process at various heights above the burner surface (HABs), where the models are trained and cross-validated on a large dataset comprising nearly 35000 WALS images.

For reliable tomographic measurements the underlying 2D images from different viewing angles must be matched in terms of signal detection characteristics. Non-linearity effects introduced by intensified cameras and spatial intensity variations induced from inhomogeneous transmission of the optical setup can lead, if not corrected, to a biased tomographic reconstruction result. This paper presents a complete correction procedure consisting of a combination of a non-linearity and flatfield correction for a tomographic optical setup employing imaging fiber bundles and four intensified cameras. Influencing parameters on the camera non-linearity are investigated and discussed. The correction procedure is applied to 3D temperature measurements by two-color pyrometry and compared to results without correction. The present paper may serve as a guideline for an appropriate correction procedure for any type of measurement involving optical tomography and intensified cameras.

In this work, the optical properties of soot particles from a Gülder burner fueled with both ethylene or propane gas were investigated in situ using laser-induced incandescence. The particles in the flame were irradiated with four different laser wavelengths, namely 450 nm, 532 nm, 600 nm and 650 nm. The resulting laser-induced emissions were detected spectrally and temporally resolved by means of a spectrograph and an intensified camera at different delay times with respect to the laser pulse. To determine the optical properties of the particles from the data, the emitted spectra were simulated using a spectroscopic model with variable input parameters, and a regression was performed against the measured data. On the basis of an functional approach of the absorption function on wavelength, the dispersion exponent for soot was evaluated for a reference position on the centre axis at 40 mm height above the burner. It was found that the different fuel gases lead to similar values with regard to the absorption function, which can be expressed by a mean dispersion exponent with a value of 1.75 for ethylene and 1.68 for propane.