Weiterentwicklung und Validierung eines quasistationären Modells für die solare Direktverdampfung im Hinblick auf eine höhere örtliche und zeitliche Auflösung

Die Stromerzeugung aus regenerativen Energiequellen hat in den letzten Jahren stark zugenommen. Um den zukünftigen Ausbau so effizient und wirtschaftlich wie möglich zu gestalten, werden leistungsfähige Simulationsprogramme zur Abbildung regenerativer Kraftwerke und Prozesswärmeanlagen benötigt. Ein solches Programm ist die am Deutschen Zentrum für Luft- und Raumfahrt entwickelte Software greenius, welche die Anlagenplanung unter Berücksichtigung sowohl technischer als auch ökonomischer Parameter unterstützt. Im Rahmen dieser Arbeit wurde die Simulation der solaren Direktverdampfung in greenius optimiert. Neben einer Reduktion der Rechenzeit wurde ein Vergleich zweier quasistationärer Modellierungsansätze durchgeführt sowie der Einfluss einer höheren örtlichen und zeitlichen Auflösung analysiert. Das verwendete Modell erfordert die Lösung eines nichtlinearen Gleichungssystems, welches mit dem Newton-Verfahren gelöst wird. Durch die Optimierung des numerischen Lösungsverfahrens und einer effizienteren Berechnung der Jacobi-Matrix, gelang es die Rechenzeit um ca. 85% zu reduzieren. Die verglichenen Ansätze unterscheiden sich bezüglich der Berechnung der lokalen Massenströme. Der rein quasistationäre Ansatz verwendet einen über den Loop konstanten Massenstrom. Der modifizierte Ansatz berücksichtigt lokale Abweichungen, die aufgrund von Dichteänderungen im Absorberrohr auftreten. Die Validierung mit einem dynamischen Modell zeigt, dass der modifizierte Ansatz bezüglich der Abbildung von Zustandsgrößen, Rechenzeit und Robustheit überlegen ist. Die Analyse des Einflusses höherer Auflösungen belegt, dass eine Erhöhung der örtlichen Auflösung sich positiv auf die Robustheit des Systems auswirkt. Eine hohe zeitliche Auflösung ermöglicht eine deutlich verbesserte Abbildung der Enthalpie- und Massenströme im transienten Bereich, aber wirkt sich negativ auf die Robustheit des Modells aus

Thermal optimisation of the stator vent design for a synchronous generator

An appropriate cooling of electrical machines is essential for safe, reliable and efficient operation and to address the growing demand for highly power-dense machines. Conventional thermal design optimisation procedures have traditionally relied on simple analytical correlations, which are not well suited to investigate complex and novel designs. Computational Fluid Dynamics (CFD) enables a more accurate prediction of fluid flow and heat transfer. But to date, the high complexity of the geometry, mesh and solver setup have required the model setup to be updated manually when the machine design is changed. Within this thesis, a novel methodology for the thermal optimisation of electrical machines is developed based on a 3-D Conjugate Heat Transfer CFD model of a synchronous generator. It enables the automated optimisation of multiple design parameters without requiring any additional user input after the initial model setup. The impact the design changes have on the electromagnetic performance was taken into account through analytical correlations developed from electromagnetic 2-D FEA. By implementing these correlations into the thermal optimisation procedure, an optimal machine design considering the thermal as well as the electromagnetic performance can be achieved. The developed methodology was applied to improve the stator cooling of an air-cooled synchronous generator with a power rating of several hundred kVA used for continuous industrial power generation. Initially, a model of the current machine design was created to gain insight into its thermal performance. As the machine hot spot is located in the stator core windings, venting the stator was identified as a beneficial design change. Subsequently, five vent design parameters were optimised, reducing peak and average stator winding temperatures. Based on the outcome of the design optimisation, a vented stator-casing arrangement was built and tested experimentally. In comparison to the original machine design, the peak stator winding temperature rise was reduced by 20.1 %, while the average stator winding temperature rise decreased by 12.3 %. The investigated generator and the designed prototype were validated experimentally by torque, mass flow, rotor and stator winding temperature measurements. The temperature measurements were taken at various loads, including full load, while the machine was connected to the grid. This enabled an accurate prediction of the generator's thermal performance under normal operating conditions. Good agreement between CFD and experimental data was found, validating the design methodology

A CFD and experimental investigation into a non-intrusive method for measuring cooling air mass flow rate through a synchronous generator

This paper presents a detailed methodology for a non-intrusive measurement of cooling air mass flow rate that enables an overall machine evaluation. This approach enables the simultaneous measurement of air mass flow with shaft torque at differing operating points, while minimising the change in air flow introduced by the measurement system. The impact of geometric parameters in the designed system are investigated using a detailed 180° CFD model. Special attention was paid to minimising their influence on pressure drop, mass flow rate through the machine and measurement uncertainty. Based on the results of this investigation, the system was designed and manufactured and the experimentally measured data was used to validate the CFD predictions. For the as optimal identified configuration, the flow rate is predicted to decrease by 2.2 % relative to unrestricted operation. The achieved measurement uncertainty is ±2.6 % at synchronous speed

CFD optimisation of the thermal design for a vented electrical machine

Optimisation algorithms hold the potential to dramatically reduce computational time whilst ensuring the optimal solution is found. Within this paper, the feasibility of using this novel approach on complex 3-D Computational Fluid Dynamics models, which are required for thermal management of electrical machines, is proven. A model of a simplified generator is parameterised with the aim of minimising the peak stator temperature by varying the axial location of a single stator vent. By generating a single parameterised case, and automating the optimisation, the simulations are run independently after initial setup, hence reducing both computational and user time. Locating a vent in the optimal position reduced the peak stator temperature by 9.4 K. A sensitivity study linking peak temperature to vent position has been carried out developing a polynomial relationship between them for the aforementioned geometry. Mass flow and pressure distribution in the vent have been analysed in detail

A CFD and experimental investigation into a non-intrusive method for measuring cooling air mass flow rate through a synchronous generator

This paper presents a detailed methodology for a non-intrusive measurement of cooling air mass flow rate that enables an overall machine evaluation. This approach enables the simultaneous measurement of air mass flow with shaft torque at differing operating points, while minimising the change in air flow introduced by the measurement system. The impact of geometric parameters in the designed system are investigated using a detailed 180° CFD model. Special attention was paid to minimising their influence on pressure drop, mass flow rate through the machine and measurement uncertainty. Based on the results of this investigation, the system was designed and manufactured and the experimentally measured data was used to validate the CFD predictions. For the as optimal identified configuration, the flow rate is predicted to decrease by 2.2 % relative to unrestricted operation. The achieved measurement uncertainty is ±2.6 % at synchronous speed

Thermal optimisation of the stator vent design for a synchronous generator

An appropriate cooling of electrical machines is essential for safe, reliable and efficient operation and to address the growing demand for highly power-dense machines. Conventional thermal design optimisation procedures have traditionally relied on simple analytical correlations, which are not well suited to investigate complex and novel designs. Computational Fluid Dynamics (CFD) enables a more accurate prediction of fluid flow and heat transfer. But to date, the high complexity of the geometry, mesh and solver setup have required the model setup to be updated manually when the machine design is changed. Within this thesis, a novel methodology for the thermal optimisation of electrical machines is developed based on a 3-D Conjugate Heat Transfer CFD model of a synchronous generator. It enables the automated optimisation of multiple design parameters without requiring any additional user input after the initial model setup. The impact the design changes have on the electromagnetic performance was taken into account through analytical correlations developed from electromagnetic 2-D FEA. By implementing these correlations into the thermal optimisation procedure, an optimal machine design considering the thermal as well as the electromagnetic performance can be achieved. The developed methodology was applied to improve the stator cooling of an air-cooled synchronous generator with a power rating of several hundred kVA used for continuous industrial power generation. Initially, a model of the current machine design was created to gain insight into its thermal performance. As the machine hot spot is located in the stator core windings, venting the stator was identified as a beneficial design change. Subsequently, five vent design parameters were optimised, reducing peak and average stator winding temperatures. Based on the outcome of the design optimisation, a vented stator-casing arrangement was built and tested experimentally. In comparison to the original machine design, the peak stator winding temperature rise was reduced by 20.1 %, while the average stator winding temperature rise decreased by 12.3 %. The investigated generator and the designed prototype were validated experimentally by torque, mass flow, rotor and stator winding temperature measurements. The temperature measurements were taken at various loads, including full load, while the machine was connected to the grid. This enabled an accurate prediction of the generator's thermal performance under normal operating conditions. Good agreement between CFD and experimental data was found, validating the design methodology

Pancreatic surgery outcomes: multicentre prospective snapshot study in 67 countries

Background: Pancreatic surgery remains associated with high morbidity rates. Although postoperative mortality appears to have improved with specialization, the outcomes reported in the literature reflect the activity of highly specialized centres. The aim of this study was to evaluate the outcomes following pancreatic surgery worldwide.Methods: This was an international, prospective, multicentre, cross-sectional snapshot study of consecutive patients undergoing pancreatic operations worldwide in a 3-month interval in 2021. The primary outcome was postoperative mortality within 90 days of surgery. Multivariable logistic regression was used to explore relationships with Human Development Index (HDI) and other parameters.Results: A total of 4223 patients from 67 countries were analysed. A complication of any severity was detected in 68.7 percent of patients (2901 of 4223). Major complication rates (Clavien-Dindo grade at least IIIa) were 24, 18, and 27 percent, and mortality rates were 10, 5, and 5 per cent in low-to-middle-, high-, and very high-HDI countries respectively. The 90-day postoperative mortality rate was 5.4 per cent (229 of 4223) overall, but was significantly higher in the low-to-middle-HDI group (adjusted OR 2.88, 95 per cent c.i. 1.80 to 4.48). The overall failure-to-rescue rate was 21 percent; however, it was 41 per cent in low-to-middle-compared with 19 per cent in very high-HDI countries.Conclusion: Excess mortality in low-to-middle-HDI countries could be attributable to failure to rescue of patients from severe complications. The authors call for a collaborative response from international and regional associations of pancreatic surgeons to address management related to death from postoperative complications to tackle the global disparities in the outcomes of pancreatic surgery (NCT04652271; ISRCTN95140761)