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Subject: search.filters.subject.620Start date: 2020Institute: search.filters.UBSInstitut.Institut für Thermodynamik der Luft- und RaumfahrtAuthor: search.filters.author.Lamanna, Grazia

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Now showing 1 - 8 of 8
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    Droplet velocity and diameter distributions in flash boiling liquid nitrogen jets by means of phase Doppler diagnostics
    (2020) Rees, Andreas; Araneo, Lucio; Salzmann, Heiko; Lamanna, Grazia; Sender, Joachim; Oschwald, Michael
    Due to current and future environmental and safety issues in space propulsion, typical propellants for upper stage or satellite rocket engines such as the toxic hydrazine are going to be replaced by green propellants like the combination of liquid oxygen and hydrogen or methane. The injection of that kind of cryogenic fluids into the vacuum atmosphere of space leads to a superheated state, which results in a sudden and eruptive atomization due to flash boiling. For a detailed experimental investigation of superheated cryogenic fluids, the new cryogenic test bench M3.3 with a temperature controlled injection system was built at DLR Lampoldshausen. After a first test campaign with high-speed shadowgraphy of flash boiling liquid nitrogen sprays, a laser-based Phase Doppler system was set-up to determine the spatial distributions of droplet velocities and diameters in highly superheated sprays. The spatial distributions revealed a core region with high mean velocities close to the injector orifice. With increasing distance from the injector orifice, the sprays develop a more and more monodisperse pattern. These distributions also showed that atomization due to flash boiling generates finer sprays with growing degrees of superheat. In certain spray regions, two droplet populations varying in their direction of motion, velocity and diameter due to possible recirculation zones were observed. The experimental data of flash boiling liquid nitrogen generated within this study provide a comprehensive data base for the validation of numerical models and further numerical investigations.
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    Characterisation of the transient mixing behaviour of evaporating near-critical droplets
    (2023) Steinhausen, Christoph; Gerber, Valerie; Stierle, Rolf; Preusche, Andreas; Dreizler, Andreas; Gross, Joachim; Weigand, Bernhard; Lamanna, Grazia
    With technical progress, combustion pressures have been increased over the years, frequently exceeding the critical pressure of the injected fluids. For conditions beyond the critical point of the injected fluids, the fundamental physics of mixing and evaporation processes is not yet fully understood. In particular, quantitative data for validation of numerical simulations and analytical models remain sparse. In previous works, transient speed of sound studies applying laser-induced thermal acoustics (LITA) have been conducted to investigate the mixing behaviour in the wake of an evaporating droplet injected into a supercritical atmosphere. LITA is a seedless, non-intrusive measurement technique capable of direct speed of sound measurements within these mixing processes. The used setup employs a high-repetition-rate excitation laser source and, therefore, allows the acquisition of time-resolved speed of sound data. For the visualisation of the evaporation process, measurements are accompanied by direct, high-speed shadowgraphy. In the present work, the measured speed of sound data are evaluated by applying an advection-controlled mixing assumption to estimate both the local mole fraction and mixing temperature. For this purpose, planar spontaneous Raman scattering results measured under the same operating conditions are evaluated using an advection-controlled mixing assumption with the perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state. Successively, the resulting concentration–temperature field is used for the estimation of local mixture parameters from the detected speed of sound data. Moreover, models using the PC-SAFT equation of state and the NIST database for the computation of the speed of sound are compared. The investigations indicate a classical two-phase evaporation process with evaporative cooling of the droplet. The subsequent mixing of fluid vapour and ambient gas also remains subcritical in the direct vicinity of the droplet.
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    On the potential and challenges of laser-induced thermal acoustics for experimental investigation of macroscopic fluid phenomena
    (2020) Steinhausen, Christoph; Gerber, Valerie; Preusche, Andreas; Weigand, Bernhard; Dreizler, Andreas; Lamanna, Grazia
    Mixing and evaporation processes play an important role in fluid injection and disintegration. Laser-induced thermal acoustics (LITA) also known as laser-induced grating spectroscopy (LIGS) is a promising four-wave mixing technique capable to acquire speed of sound and transport properties of fluids. Since the signal intensity scales with pressure, LITA is effective in high-pressure environments. By analysing the frequency of LITA signals using a direct Fourier analysis, speed of sound data can be directly determined using only geometrical parameters of the optical arrangement no equation of state or additional modelling is needed at this point. Furthermore, transport properties, like acoustic damping rate and thermal diffusivity, are acquired using an analytical expression for LITA signals with finite beam sizes. By combining both evaluations in one LITA signal, we can estimate mixing parameters, such as the mixture temperature and composition, using suitable models for speed of sound and the acquired transport properties. Finally, direct measurements of the acoustic damping rate can provide important insights on the physics of supercritical fluid behaviour.
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    Editorial - physics of droplets
    (2024) Planchette, Carole; Lamanna, Grazia; Pan, Kuo-Long
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    Micro-PIV study on the influence of viscosity on the dynamics of droplet impact onto a thin film
    (2024) Schubert, Stefan; Steigerwald, Jonas; Geppert, Anne K.; Weigand, Bernhard; Lamanna, Grazia
    This work presents a systematic experimental study of droplet impact onto a wet substrate. Four different silicone oils are used, covering a range of Reynolds number between 116270. This is not observed at lower Re numbers due to the increased pressure losses caused by the extensional (normal) strain during the radial spreading of the lamella. To validate these findings a holistic approach is chosen, which combines numerical results, analytical solutions and experimental data from literature. In particular, by using the continuity equation, it is shown that the experimental decay of the wall film height can be reconstructed from the velocity measurements. Consilience of results from different approaches provides a robust validation of the micro-PIV data obtained in this work.
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    On the feasibility of visualizing transient droplet dynamic process with event based imaging
    (2025) Steinhausen, Christoph; Essig, Nicklas; Lamanna, Grazia; Geppert, Anne
    Event-based cameras (EBCs) offer the possibility of overcoming the weaknesses of frame-based cameras (FBCs) due to their high dynamic range, low illumination intensity, low costs, and direct edge detection. Accordingly, EBCs are well suited for the investigation of transient processes. EBCs have independently controllable pixels that detect events when the relative change of the detected voltage exceeds (ON-event) or drops (OFF-event) below a set value. In addition, only the detected events together with their spatial and temporal position are stored, leading to a high memory efficiency. Compared to FBCs, the inherent edge detection of EBCs eliminates the need for a threshold based post-processing, while the memory efficiency allows capturing events with different temporal dynamics. In the present study, the feasibility of using EBCs to investigate transient process is assessed. As an evaluation case, droplet impact on a thin film is selected. The experimental setup consists of a side and a bottom perspective. While the bottom perspective utilises a background illumination with an EBC, the side perspective consists of a shadowgraphy setup with a FBC and an EBC for direct comparison. The present study shows that EBCs are suited for visualizing droplet film interaction with a similar or even higher temporal resolution than FBCs. Moreover, the data structure of EBCs offers additional possibilities for data analysis, such as a presentation in the x,y,t-space. With this, the temporal development of the crown, the time of the crown bottom breakdown as well as the trailing edge of the droplet are clearly shown.
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    Novel post-processing methods for infrared measurements on porous surfaces
    (2025) Härter, Julian; Lamanna, Grazia; Poser, Rico
    Infrared (IR) thermography is a key diagnostic tool for non-invasive measurements of surface temperatures. However, for porous materials, particularly those employed in self-pumping transpiration cooling systems, conventional IR techniques suffer from significant inaccuracies due to local emissivity variations caused by multiple materials, surface roughness, and presence of liquid in the porous media. This study presents and compares three post-processing methods to enhance the accuracy of IR temperature measurements on saturated porous surfaces. A constant emissivity approach, a blackbody radiator method, and a novel differential method are applied to an Isopropanol filled porous sintered bronze plate under controlled experimental conditions. The results reveal that the conventional constant emissivity method leads to the largest errors and broadest temperature distributions. The blackbody radiator method improves accuracy by introducing a reference emissivity point but retains significant uncertainties. The differential method, leveraging a distribution of reference temperatures, achieves the highest precision, effectively resolving local emissivity variations and minimizing deviations from thermocouple reference measurements. This method enables a detailed spatial characterization of surface temperatures, particularly capturing the thermal gradients associated with evaporation in self-pumping cooling regimes. Furthermore, the effect of liquid in metallic porous samples could be demonstrated and shows clear differences to ordinary metallic surfaces such as the border of the porous sample. These findings provide a refined framework and experimental validation for accurate thermal diagnostics of porous surfaces under two-phase flow conditions.
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    Modelling of heat addition to near-critical and supercritical fluids
    (2025) Lamanna, Grazia; Steinhausen, Christoph
    This work presents a coupled flow model to describe heat addition to near-critical and supercritical fluids. In a first approximation only forced convection is considered, while buoyancy forces, viscous and volume expansion losses are neglected. The steady, one-dimensional balance equations are solved simultaneously through an iterative procedure, taking into account real gas effects. The analysis shows that the intrinsic high compressibility of near-critical fluids strongly reduces the region of validity for the incompressible flow assumption. Indeed, depending on the initial conditions, compressible flow effects may occur at Mach numbers below 0.1. For a given mass flux, the occurrence of heat transfer deterioration (HTD) correlates directly with the maximum amount of heat ( ) that a compressible flow can absorb at a specific local Mach number. For near-critical fluids, depends upon two parameters, namely the heat flux to mass flux ratio and the thermal dilatation. The latter induces not only the early inception of compressibility effects, but it also poses an additional volumetric constraint to . Indeed, in confined flows the enhancement of the thermal dilatation parameter strongly limits the capability of the flow to increase the local mass flux by expanding the flow, which eventually decreases the value. These findings provide a generalization of the well-known dependence of upon the local Mach number for an ideal gas. The local enhancement of the thermal dilatation also explains the counterintuitive cooling of the fluid upon heat addition. Overall, our analysis advocates for a more comprehensive flow analysis in the design of regenerative cooling systems.