Uncertainty evaluation for velocity–area methods

Velocity–area methods are used for flow rate calculation in various industries. Applied within a fully turbulent flow regime, modest uncertainties can be expected. If the flow profile cannot be described as “log-like”, the recommended measurement positions and integration techniques exhibit larger errors. To reduce these errors, an adapted measurement scheme is proposed. The velocity field inside a Venturi contour is simulated using computational fluid dynamics and validated using laser Doppler anemometry. An analytical formulation for the Reynolds number dependence of the profile is derived. By assuming an analytical velocity profile, an uncertainty evaluation for the flow rate calculation is performed according to the “Guide to the expression of uncertainty in measurement”. The overall uncertainty of the flow rate inside the Venturi contour is determined to be 0.5 % compared to 0.67 % for a fully developed turbulent flow

Time-averaged and time-resolved laser optical temperature measurements in water with Filtered Brillouin Scattering combined with LDV

A laser measuring system was developed and built that allows to optically measure temperature in water using the method of Filtered Brillouin Scattering (FBS). First time-resolved optical temperature measurements were demonstrated. Furthermore, the FBS-system was combined with an LDV to enable simultaneous measurement of flow velocity and therefore the system is also capable to measure the heat flow. Time-averaged temperature values were determined with good accuracy and, as a special highlight, also timeresolved temperature measurements have been demonstrated with temporal resolution in the order of approximately 10 ms, validated by comparison with fast thermocouple measurements. The overarching goal of the research project was to prepare the market introduction of a laser-optical measuring system for spatially point-based and time-resolved measurement of the heat flow in liquids, especially in water. In order to realize this, it was necessary to measure the local velocity and the local temperature in a liquid. The optical measurement of the local velocity has long been possible using the established method of laser Doppler velocimetry (LDV). Therefore, the heat flow measurement method to be developed should be based on this technology. Until now, there was no suitable optical method for measuring the temperature. In recent years, however, the physical phenomenon of Brillouin scattering has become one focus of measurement technology development. If a small volume of liquid is irradiated with light, the molecules in the liquid scatter back part of the light, which is known as Brillouin scattering. The spectrum of the scattered light depends on the local temperature in the liquid; and it turns out that this physical relationship can be exploited to develop a highly accurate, fast, and non-contact method for measuring temperature. In this paper, we explain the Filtered Brillouin Scattering (FBS) method, show a setup for measuring temperature and velocity in water flows and thus a method for determining the heat flow, and demonstrate the measurement accuracy using a calibration test bench. The temperature measurement accuracy achieved is in the order of 1 K

Laseroptische Temperaturmessung in Flüssigkeiten mittels gefilterter Brillouin-Streuung

In diesem Beitrag wird ein neuartiges, laserbasiertes Messverfahren zur Erfassung der Temperatur von Flüssigkeiten vorgestellt. Das Verfahren basiert auf der temperaturabhängigen Brillouin-Streuung von Laserlicht kombiniert mit einem optischen Filter in Form molekularen Joddampfes. Das Verfahren kann beispielsweise zur örtlich und zeitlich aufgelösten Erfassung des Temperaturprofils einer Rohrströmung eingesetzt werden. Der nichtinvasive Charakter laseroptischer Messverfahren verhindert dabei die Beeinflussung des Strömungsprofils durch Sensoreinbauten. Der vorliegende Beitrag führt kurz die physikalischen Grundlagen aus, beschreibt den Einsatz eines Molekularfilters und stellt die verschiedenen Teilkomponenten des Messsystems vor. Anschließend wird mit ausgewählten Messergebnissen für stationäre und dynamische Temperaturzustände die grundsätzliche Funktionsfähigkeit des Messprinzips nachgewiesen

Traceable volume flow rate measurement using high spatial resolution laser Doppler anemometry

Gedruckt erschienen im Verlag Carl Schünemann GmbH, ISBN 978-3-95606-298-8Für die Steigerung der Energieeffizienz und der Ausgangsleistung thermischer Kraftwerke kommt der Reduzierung der Unsicherheit der Volumenstrommessung eine Schlüsselrolle zu. Die derzeitige Unsicherheit von etwa 2 % lässt keine optimale Regelung zu und limitiert durch notwendige Sicherheitsreserven die Ausgangsleistung. Mit dem in dieser Arbeit entwickelten Laseroptischen-Volumenstrom-Normal (LVN) können Volumenstrommessgeräte zukünftig direkt im Kraftwerk mit einer Unsicherheit von 0,15 % kalibriert werden. Dadurch kann die Unsicherheit der Volumenstrommessung reduziert und die Ausgangsleistung angehoben werden. Mit dem LVN werden an mehreren über den Querschnitt der Rohrleitung verteilten Positionen die Fluidgeschwindigkeiten mithilfe der Laser-Doppler-Anemometrie (LDA) gemessen. Daraus wird das Geschwindigkeitsprofil rekonstruiert und integriert, um den Volumenstrom zu berechnen. Bei der LDA liegt im Wandbereich ein Teil des 2000 μm langen Messvolumens außerhalb der Strömung. Dadurch weicht der gemessene Volumenstrom um bis zu 0,63 % ab. Für das LVN wird ein ortsaufgelöstes LDA entwickelt, mit dem diese Abweichung auf 0,04 % reduziert wird. Durch die Überlagerung zweier Messvolumina mit variierendem Interferenzstreifenabstand wird die Ortsauflösung von rund 2000 μm auf 6 μm reduziert. Für die Überlagerung der Messvolumina werden zwei Positioniersysteme entwickelt, mit denen die Positionierunsicherheit von bisher rund 5500 μm auf 67 μm reduziert wird. Es werden zwei Verfahren entwickelt, mit denen die Ausrichtung der LDA-Sonde zum Positioniersystem und zur Rohrleitung erstmalig vermessen wird. Die Winkelabweichungen werden dadurch jeweils von bisher etwa 1 ° auf 0,005 ° und die Positionierunsicherheit von rund 800 μm auf 4 μm reduziert. Durch die Überlagerung der beiden Messvolumina kann das vorwärtsgestreute Licht aus dem Messvolumen detektiert werden, das eine bis zu 1000-fach höhere Intensität als das bisher detektierte, rückgestreute Licht aufweist. So wird das Signal-Rausch-Verhältnis um den Faktor 10 verbessert, die maximale LDA-Datenrate von etwa 400 Hz auf 2500 Hz gesteigert und die Messzeit auf ein 1/10-tel reduziert. Durch diese Verbesserungen konnte die Unsicherheit der Volumenstrommessung mit dem LVN von 4,5 % auf 0,15 % reduziert werden. Abschließend wird eine Vergleichsmessung zwischen dem LVN und der Wärmezählerprüfstrecke (WZP) durchgeführt. Die WZP ist das nationale Normal der Physikalisch-Technischen Bundesanstalt (PTB) zur Darstellung des Volumenstromes mit einer Unsicherheit von 0,04 %. Der Vergleich zeigt eine mittlere Abweichung von weniger als 0,01 % und eine maximale Abweichung von 0,07 %. Damit kann der prognostizierte Unsicherheitswert für das LVN von 0,15 % bestätigt werden.Precise volume flow rate measurements are very important for controlling power plants. Due to the current uncertainty of the volume flow rate measurement of about 2 % the process is not controlled efficiently. Therefore, the maximum power output is reduced. With a newly developed laser optical volume flow rate standard (LVN) it will be possible to calibrate flow meters on site within the power plant to reduce the uncertainty of the flow rate measurement and increase the power output. For the LVN, the velocity profile within the pipe is measured with laser Doppler anemometry (LDA). The profile is integrated to calculate the volume flow rate. Since a conventional LDA has a measurement volume length of about 2000 μm, the near wall region cannot be resolved. This leads to an offset of the volume flow rate of about 0,63 %. To overcome this limitation a high resolution LDA was developed. Therefore, a LDA measurement volume with a constant fringe spacing is overlaid with a LDA measurement volume with a diverging fringe spacing. This way, the position of a particle passing the measurement volume can be determined with a spatial resolution of 6 μm. With the high resolution LDA the influence of the near wall region on the volume flow rate is reduced from 0,63 % to 0,04 %. To implement the high resolution LDA it is necessary to overlay the measurement volumes of two LDA probes within the pipe. This requires a very low uncertainty for the positioning of the LDA measurement volumes. Therefore, two positioning systems are developed to reduce the positioning uncertainty from commonly up to 5500 μm to 67 μm. Furthermore, two measurement methods are developed to precisely align the LDA probes to the positioning system and to the pipe. With these methods the angular deviation of the alignment is reduced from 1 ° to 0,005 °, which reduces the positioning uncertainty for the measurement volume from 800 μm to 4 μm. Additionally, overlapping the two measurement volumes allows to measure the forward scattered light of the particles instead of the conventionally measured backscattered light. The forward scattered light has a 1000 times higher intensity. As a result, the signal-to-noise ratio is 10 times higher, the maximum data rate is improved from 400 Hz to 2500 Hz and the measuring time is reduced to a 10th. With all of the aforementioned improvements it is possible to achieve an uncertainty for the volume flow rate measurement with the LVN of 0,15 % compared to 4,5 % for a conventional LDA setup. A comparison of the LVN with the national primary standard for thermal energy (WZP) at the Physikalisch-Technische Bundesanstalt (PTB) with an uncertainty of 0,04 % shows a maximum deviation of 0,07 % and therefore confirms the predicted uncertainty of the LVN of 0,15

A computational modelling tool for prediction of head reshaping following endoscopic strip craniectomy and helmet therapy for the treatment of scaphocephaly

Endoscopic strip craniectomy followed by helmet therapy (ESCH) is a minimally invasive approach for correcting sagittal craniosynostosis. The treatment involves a patient-specific helmet designed to facilitate lateral growth while constraining sagittal expansion. In this study, finite element modelling was used to predict post-treatment head reshaping, improving our comprehension of the necessary helmet therapy duration. Six patients (aged 11 weeks to 9 months) who underwent ESCH at Connecticut Children's Hospital were enrolled in this study. Day-1 post-operative 3D scans were used to create skin, skull, and intracranial volume models. Patient-specific helmet models, incorporating areas for growth, were designed based on post-operative imaging. Brain growth was simulated through thermal expansion, and treatments were modelled according to post-operative Imaging available. Mechanical testing and finite element modelling were combined to determine patient-specific mechanical properties from bone samples collected from surgery. Validation compared simulated end-of-treatment skin surfaces with optical scans in terms of shape matching and cranial index estimation. Comparison between the simulated post-treatment head shape and optical scans showed that on average 97.3±2.1% of surface data points were within a distance range of -3 to 3mm. The cranial index was also accurately predicted (r=0.91). In conclusion, finite element models effectively predicted the ESCH cranial remodeling outcomes up to 8 months postoperatively. This computational tool offers valuable insights to guide and refine helmet treatment duration. This study also incorporated patient-specific material properties, enhancing the accuracy of the modeling approach. [Abstract copyright: Copyright © 2024 The Author(s). Published by Elsevier Ltd.. All rights reserved.