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One Dimensional Simulation of Water Boiling in an Electrically Heated Pipe
No full textIndustrial heating processes account for nearly 28% of global CO2 emissions, and fossil-
fueled boilers predominate in high temperature applications. Industrial electrification could be
an effective, especially when combined with renewable energy sources. However, any electric
heating design requires accurate modeling of the heat transfer phenomena that occurs during
phase change to ensure efficiency and safety. This paper presents a one-dimensional numerical
model that simulates water boiling considering heat transfer and pressure drop in an electrically
heated pipe using subcritical water. The model integrates subcooled liquid, two-phase, and
superheated vapor regions, applying the Gnielinski correlation for single-phase heat transfer,
a set of established flow boiling correlations for two-phase heat transfer, and the Friedel
correlation for pressure drop. These results show the efficiency and precise control provided by
electric heating, proving its ability to improve thermal performance while reducing emission
Scaling of an existing 60 kN LOX/LCH4 combustion down to a liquid 2.5 kN HTP/Kerosene combustion of an innovative hyperboloid rocket engine in Ansys Fluent
No full textFeasibility Demonstration of Electric Pump-Fed Systems Powered by Green Propellants
No full textThis document presents the work performed in the frame of the ASCenSIon project towards the feasibility demonstration of electric pump feeding, providing a comprehensive overview of the activities conducted to advance the technology’s maturity level.
The suggested pressurization system is proposed in combination with green propellants as an alternative to the current state of the art pressure-fed systems powered by MMH/NTO. The premise is grounded on the advancements in battery and manufacturing technologies, advancements that could position electric pump feeding as a competitive alternative to pressure feds in the small to medium thrust level (from hundreds to a few kN thrust).
Specifically, the study focuses on the 400 N level, with a particular emphasis on exploring the
use of hydrogen peroxide and ethanol as baseline green propellant combination.
Studying the feasibility of a technology involves providing a clear understanding of its development potential and associated challenges. The technical feasibility is addressed during the study by identifying requirements, potentialities, challenges and engineering constraints.
To assess this potential, a dedicated set of tools was developed based on global sensitivity and Monte Carlo analyses. The aim is to enhance the agility of early-stage electric pump-fed studies by effectively identifying the key design drivers and based on that, delivering a competitive
system specification. This information is essential to perform well-informed tradeoffs with respect to the state of the art, i.e., pressure-fed systems, way easier to assess due to their higher maturity.
Likewise, the study also identified the challenges that could hinder technology development.
At component level, a detailed discussion on the design of ultra-low specific speed centrifugal and positive displacement pumps is presented.
Regarding the experimental domain, a major challenge stems from the absence of hardware dedicated to the experimental validation of electric pump-fed systems at the thrust level under consideration. To counter this, a test bench has been developed, incorporating a positive
displacement pump to explore the capabilities of these pumps for space propulsion applications. Several tests have been conducted in this context to characterize their dynamics.
Additionally, a first centrifugal pump model specifically tailored for the 400 N thrust level has been additively manufactured, likely standing as one of the few examples at this scale.
From the cost and operational perspective, an assessment of the ArianeGroup former LunaNova kick stage (now OPALS) is presented. Coupled with mission feasibility studies, in particular a delta-v analysis for new kick stages CONOPS and a MDO multi-orbit multi-injection
study, this assessment led to the identification of orbital transfer vehicles as a promising application for the proposed propulsion system.
After detailing the different studies, the conclusions derived from the different activities are drawn. Overall, the integration of electric pump feeding and green propellants is deemed a feasible alternative for the low to medium thrust level, potentially challenging the pressure-fed hydrazine-based solutions currently in use. An electric pump-fed propulsion system powered by green propellants would mark a significant leap towards sustainability within the space industry, representing a practical pathway for transitioning to the use of less hazardous propellants in the analyzed scenarios. Yet, considerable efforts and resources remain necessary to develop the required system components, in particular pumps, tailored for these thrust levels. Finally, a roadmap outlining further actions is provided
Hot streak development in hypersonic boundary layer transition on a blunt cone with cooled walls: shock tunnel experiments and numerical simulations
No full textThe uncertainty of the boundary layer transition location on a hypersonic vehicle and the corresponding uncertainty in the surface heat flux and skin friction present major challenges for sustained hypersonic flight. One source of uncertainty is the mechanism governing breakdown of instabilities in boundary layers on geometries with highly-cooled walls (low ) relevant to flight conditions.
An experimental investigation was carried out using a 7 half-angle, straight cone with a nose radius and a Reynolds number based on nose radius of at angle of attack. The tests were conducted at Mach 7.4 in the High Enthalpy Shock Tunnel Göttingen (HEG) DLR (2018) of the German Aerospace Center (DLR). Dominant primary instabilities, identified as second-mode waves in previous experiments Laurence (2016), were observed as peaks in the power spectral density plot of figure 1 for surface-mounted fast pressure transducers. Broadening of these peaks at the downstream locations on the cone was attributed to nonlinear interactions of instabilities within the boundary layer. Streaks were observed in the nonlinear interaction region, by means of temperature sensitive paint, as shown in figure 2. This suggests nonlinear interaction of the second-mode waves with secondary instabilities and is a unique result from a shock tunnel testing environment.
Accompanying numerical investigations of the linear and nonlinear transition regime were carried out for the wind tunnel conditions and the circular cone geometry of the HEG experiments, in order to understand the role of the secondary instability in the transition process. The primary instability investigations using Linear Stability Theory (LST) and low-amplitude wave packet calculations confirmed that axisymmetric second-mode waves are indeed the dominant primary instability. The dominant frequency range obtained from experiments (\cref{PSD_map}) and numerical calculations are in good agreement. The strongly amplified axisymmetric second-mode waves suggest the possibility of a so-called fundamental resonance, where a large amplitude axisymmetric (primary) disturbance wave interacts (resonates) nonlinearly with a pair of lower amplitude (secondary) oblique disturbance waves of the same frequency. Therefore, fundamental resonance calculations were carried out for a wide range of azimuthal wavenumbers (). The N-factors of the secondary disturbances reveal for which azimuthal wavenumbers the fundamental resonance is "strongest" (see \cref{fig:nfac_secondary}). A strong fundamental resonance gives rise to the nonlinear generation of steady streamwise modes, which were found to be responsible for the generation of "hot" streaks on the surface of a flared cone in the "cold-flow" experiments carried out at the Boeing/AFOSR Mach 6 Quiet Tunnel (BAM6QT) at Purdue University (\cite{Chynoweth2019}). Comparable streaks of high skin-friction have been observed in numerical simulations for the HEG conditions (\cref{fig:streaks_dns}). With the "controlled" simulations a total of 250 streaks around the circumference was obtained, which is in good agreement with the number of streaks observed in the HEG experiments (). For the final version of this paper a detailed comparison between experimental measurements and numerical investigations will be provided for the various transition stages (linear and nonlinear)
Revisiting the Linear Chain Trick in epidemiological models: Implications of underlying assumptions for numerical solutions
No full textIn order to simulate the spread of infectious diseases, many epidemiological models use systems of ordinary differential equations (ODEs) to describe the underlying dynamics. These models incorporate the implicit assumption, that the stay time in each disease state follows an exponential distribution. However, a substantial number of epidemiological, data-based studies indicate that this assumption is not plausible. One method to alleviate this limitation is to employ the Linear Chain Trick (LCT) for ODE systems, which realizes the use of Erlang distributed stay times. As indicated by data, this approach allows for more realistic models while maintaining the advantages of using ODEs.
In this work, we propose an advanced LCT SECIR-type model incorporating eight infection states with demographic stratification. We review key properties of the corresponding LCT model and demonstrate that predictions derived from a simple ODE-based model can be significantly distorted, potentially leading to wrong political decisions. Our findings demonstrate that the influence of distribution assumptions on the behavior at change points and on the prediction of epidemic peaks is substantial, while the assumption has no effect on the final size of the epidemic. With respect to prior findings in literature, we demonstrate that the influence of the number of subcompartments on the timing and size of the epidemic peak is nontrivial and that a general statement cannot be obtained. We, then, show how these age-resolved LCT SECIR-type models capture the spread of SARS-CoV-2 in Germany in 2020. Eventually, we study the implications on the time-to-solution for different LCT models using fixed and adaptive step-size Runge-Kutta methods and provide computational performance for these models in the MEmilio software framework, also using distributed memory parallelism to speed up ensemble runs
The Adaptive Harmonic Linearized Navier-Stokes methodology
No full textThe constant seeking of drag reduction in the aircraft industry has lead to designers to extend the incoming laminar flow beyond the vicinity of the leading edge of aerodynamic surfaces, such as wings, nacelles, and vertical / horizontal stabilizers. However, the detrimental and unavoidable presence of surface imperfections (e.g. steps, gaps, etc.) usually plays a key role in the expected laminar-turbulent transition location. Although current methodologies such as Harmonic Linearized Navier-Stokes (HLNS) and Direct Numerical Simulations (DNS) can handle the presence of those irregularities, the high numerical cost inhibits their practical use for the parametric studies required in the design process.
The Adaptive Harmonic Linearized Navier-Stokes (AHLNS) methodology for compressible quasi three-dimensional boundary-layer instability analysis has been developed and implemented in a numerical code. The AHLNS equations represent a very efficient approach in cases where localized large streamwise gradients are present in the region of study (e.g. due to the presence of the above-mentioned surface imperfections). This efficiency is based on to take advantage of the wave-like character in streamwise direction of the convective flow instabilities along the linear growth stage of the laminar-turbulent transition process.
To illustrate the use of the AHLNS methodology, a relatively simple configuration has been chosen: a compressible flow on a flat plate without any external pressure gradient. Steps (both backward- and forward-facing) and humps of different heights, lengths and shapes (smooth and rectangular) have been studied. The reduced computational resources required by the AHLNS approach enables to increase the range of parametric variations. Therefore, a clearer perspective of the key parameters that have a more decisive influence on the expected laminar-turbulent transition has been achieved.
Finally, the applicability of the actual version of the AHLNS code for realistic large-aircraft geometries under typical transonic flight conditions has been successfully tested in several EC-funded projects: NACOR (for laminar nacelles), HLFC-WIN (for hybrid laminar flow control wings), and BLADE (for natural laminar wings). However, only results from the HLFC-WIN project are allowed to be included in this thesis. The results of the boundary-layer instability analysis computed with AHLNS suggest that the initial tolerances for the joint between different elements on the wings (in the form of superficial gaps) could be relaxed without compromising the expected transition location of the incoming flow.
To sum up, this thesis opens up the possibility of incorporating the AHLNS methodology in the design of future laminar and/or hybrid laminar wings including the influence of two-dimensional surface imperfections
Spruce canopy cover loss dominates in Germany – A multi-annual comparison among dominant tree species using national-scale remote sensing data
No full textDendroTime: Progressive Hierarchical Clustering for Variable-Length Time Series
No full textMany effective dissimilarity measures for variable-length time series, such as DTW, MSM, or TWED, are expensive to compute because their runtimes increase quadratically with the time series' lengths. When used in hierarchical agglomerative clustering algorithms that need to compute all pairwise time series dissimilarities, they cause slow runtimes and do not scale to large time series collections. However, there are use cases, where fast, interactive hierarchical clustering is necessary. For these use cases, progressive hierarchical clustering algorithms can improve runtimes and interactivity. Progressive algorithms are incremental algorithms that produce and continuously improve an approximate solution, which eventually converges to the exact solution.
In this paper, we present DendroTime, the first (parallel) progressive clustering system for variable-length time series colections. The system incrementally computes the pairwise dissimilarities between the input time series and supports different ordering strategies to achieve progressivity. Our evaluation demonstrates that DendroTime's progressive strategies are very
effective for clustering scenarios with expensive time series dissimilarity computations
Experimental investigation of a windthermal demonstrator with friction based direct wind-to-heat conversion
No full textThis experimental investigation delves into the prospective application of windthermal energy, the direct conversion of wind turbine rotational energy into heat. Therefore, we developed, constructed, and validated a friction-based windthermal demonstrator that consists of a wind turbine and a thermal platform with a hydrodynamic retarder. This paper explains the experimental arrangement of the demonstrator and analyses exhaustively the thermal platform's dynamics, its heat generation performance and auxiliary systems. In a two-step approach, we conducted motor-driven preliminary experiments to validate and optimize the experimental set-up, and then we conducted wind-turbine-driven main experiments. The results confirm that the demonstrator can effectively capture heat from wind energy, with a peak-load of 8 kW and an overall system efficiency of 8.73 %. Besides this low efficiency, we identified challenges like off-design behavior of the hydrodynamic retarder and operational temperature constraints. The efficiency could be improved by optimizing the retarder, refining the shaft system, and enhancing the insulation. Considering these optimization approaches, the demonstrator shows that windthermal energy could potentially contribute to a sustainable heat generation
Integration of Circular Fins into Exhaust Gas Recuperators for Innovative Propulsion Concepts in Aviation
No full textWaste Heat Recovery (WHR) could be a key enabler for future propulsion technologies to meet the defined emission goals by
2050. The recovery of waste heat requires the integration of heat
exchanger into the aero engine’s flow path under consideration
of several constraints such as mass, compactness and transferred
heat. The concept of the Water-Enhanced Turbofan (WET), for
example, integrates an evaporator into the exhaust flow of the
engine to recover exhaust heat by evaporating liquid water. Despite the fact that the WET concept has been deprioritized by the
industry, it still demonstrates the potential of waste heat recovery
and provides an ideal platform for the development of appropriate heat exchanger models considering the interdependencies
between the engine’s components and the heat exchanger itself.
In the evaporator of the WET concept, the thermal resistance of
the exhaust side is the limiting factor for the heat transfer capability of this component. In order to reduce the thermal resistance
of the exhaust side, an annular finned tube bundle setup for the
evaporator of the WET concept is analyzed. Based on 3D Computational Fluid Dynamics Analysis of the finned configuration
local heat transfer and flow behaviour of the finned tube bundle
arrangement are computed. It is found that the present finned
tube bundle setup leads to an overall fin effectiveness of 10 significantly increasing the amount of transferred heat in the heat
exchanger compared to an non-finned tube bundle setup