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Temperature Field Measurements with High Spatial and Temporal Resolution Using Liquid Crystal Thermography and Laser Induced Fluorescence

Nasarek, Ralph (2010)
Temperature Field Measurements with High Spatial and Temporal Resolution Using Liquid Crystal Thermography and Laser Induced Fluorescence.
Technische Universität Darmstadt
Ph.D. Thesis, Primary publication

Abstract

Temperature field measurements with high spatial and temporal resolution are necessary to understand microscale flow phenomena which occur in various applications like nucleate boiling, evaporation of menisci or heat transfer enhancement through micro-structured surfaces. These phenomena often cannot be fully described by theoretical or numerical models. The two techniques, which are probate for temperature measurements in macro regions, are liquid crystal thermography (LCT) and laser induced fluorescence (LIF). The attraction of the LCT technique is the possibility to simultaneously measure temperature and velocity fields, because the encapsulated TLCs (Thermochromic Liquid Crystals) can be applied as tracer particles for temperature and velocity measurements. If LCT is applicable for high spatial and temporal resolution measurements, it was not clarified at this point, though. In principal, the LIF method can be applied for high spatial and temporal resolution measurements due to its physical principle. On this account, the measurement methods LCT and LIF were characterized regarding their spatial and temporal resolutions and applicability. Moreover parameters which affect these measurement techniques were identified, and possibilities to minimize the error sources were presented. As the analysis of the measurement data plays an important role in the enhancement of the measurement accuracy, it was investigated in detail and new processing methods were developed. To determine the measurement accuracy of the LCT method, measurements of homogeneous temperature distributions and thermal induced convection were conducted in a cubic facility and compared with numerically calculated ones. The influence of the angle between the illumination and the camera axis was investigated and compared with results from literature. Due to diverse statements in literature concerning hysteresis effects caused by overheating of the TLCs, adequate measurements were conducted. It was found, that hysteresis occurs but it does not depend on the grade of overheating. Further, the hysteresis disappears, when the TLCs are cooled beyond the start temperature of the activity range. The main restriction of the temporal resolution is assumed in the thermal response time of the encapsulated TLCs. Therefore, measurements concerning the response characteristics of TLCs were performed and a thermal response time of less than 10 ms at high temperature gradients was observed. Several image analysis algorithms were developed and applied which leaded to an enhancement of the measurement accuracy. Volume illumination was employed for the first time in a channel flow, and it was shown that extraction of temperature fields by applying special image analysis methods is possible. Moreover, the optical setup was extended and scanning of the channel flow was realized so that 3D temperature fields could be achieved. Similar to the LCT method, the applicability of the LIF method is investigated regarding its accuracy and good results concerning the accuracy and spatial resolution were found. To overcome the distinctive dependency on the dye concentration and the illumination, two-color-LIF was employed. It was found that fluorescence dyes used in literature for two-color-LIF are not appropriate for the use with a Nd:YAG laser which again is useful for high spatial resolution measurements. For this reason, spectral investigations on eight different dyes were conducted. As a result, Pyridine 1 and Rhodamine 6G were found as suitable dyes for two-color-LIF which provide a good signal to noise ratio and therefore potential for high resolution measurements. Further, it was proved that the two-color-LIF method significantly reduces the influence of inhomogeneities in illumination and in concentration. When reckoning the TLC capsule diameter, the thermal response time and the measurement uncertainty, the limitation in the spatial and temporal resolution of LCT becomes clear. Nevertheless, due to the simplicity of the measurement setup and the possibility for simultaneous measurements of temperature and velocity fields, LCT is attractive for applications like static flows. A further advantage is the possibility to resolve small temperature differences of hundredth of Kelvin when applying TLCs with small measurement ranges. TLCs also offer an important advantage for applications in micro flows: volume illumination can be applied instead of a light sheet. The LIF technique exhibits the possibility of very high spatial and temporal resolution and the limitation is mainly caused by the optical equipment like the laser and the camera. Further, the basis for high temporal and spatial resolution measurements using two-color-LIF was established. Employing an Nd:YAG laser as a powerful illumination source in combination with Pyridine 1 and Rhodamine 6G enables high acquisition rates (up to 500 Hz with the employed equipment) and high spatial resolution.

Item Type: Ph.D. Thesis
Erschienen: 2010
Creators: Nasarek, Ralph
Type of entry: Primary publication
Title: Temperature Field Measurements with High Spatial and Temporal Resolution Using Liquid Crystal Thermography and Laser Induced Fluorescence
Language: English
Referees: Stephan, Prof. Dr.- Peter ; Wereley, Prof. Steve
Date: 24 March 2010
Refereed: 6 July 2009
URL / URN: urn:nbn:de:tuda-tuprints-20965
Abstract:

Temperature field measurements with high spatial and temporal resolution are necessary to understand microscale flow phenomena which occur in various applications like nucleate boiling, evaporation of menisci or heat transfer enhancement through micro-structured surfaces. These phenomena often cannot be fully described by theoretical or numerical models. The two techniques, which are probate for temperature measurements in macro regions, are liquid crystal thermography (LCT) and laser induced fluorescence (LIF). The attraction of the LCT technique is the possibility to simultaneously measure temperature and velocity fields, because the encapsulated TLCs (Thermochromic Liquid Crystals) can be applied as tracer particles for temperature and velocity measurements. If LCT is applicable for high spatial and temporal resolution measurements, it was not clarified at this point, though. In principal, the LIF method can be applied for high spatial and temporal resolution measurements due to its physical principle. On this account, the measurement methods LCT and LIF were characterized regarding their spatial and temporal resolutions and applicability. Moreover parameters which affect these measurement techniques were identified, and possibilities to minimize the error sources were presented. As the analysis of the measurement data plays an important role in the enhancement of the measurement accuracy, it was investigated in detail and new processing methods were developed. To determine the measurement accuracy of the LCT method, measurements of homogeneous temperature distributions and thermal induced convection were conducted in a cubic facility and compared with numerically calculated ones. The influence of the angle between the illumination and the camera axis was investigated and compared with results from literature. Due to diverse statements in literature concerning hysteresis effects caused by overheating of the TLCs, adequate measurements were conducted. It was found, that hysteresis occurs but it does not depend on the grade of overheating. Further, the hysteresis disappears, when the TLCs are cooled beyond the start temperature of the activity range. The main restriction of the temporal resolution is assumed in the thermal response time of the encapsulated TLCs. Therefore, measurements concerning the response characteristics of TLCs were performed and a thermal response time of less than 10 ms at high temperature gradients was observed. Several image analysis algorithms were developed and applied which leaded to an enhancement of the measurement accuracy. Volume illumination was employed for the first time in a channel flow, and it was shown that extraction of temperature fields by applying special image analysis methods is possible. Moreover, the optical setup was extended and scanning of the channel flow was realized so that 3D temperature fields could be achieved. Similar to the LCT method, the applicability of the LIF method is investigated regarding its accuracy and good results concerning the accuracy and spatial resolution were found. To overcome the distinctive dependency on the dye concentration and the illumination, two-color-LIF was employed. It was found that fluorescence dyes used in literature for two-color-LIF are not appropriate for the use with a Nd:YAG laser which again is useful for high spatial resolution measurements. For this reason, spectral investigations on eight different dyes were conducted. As a result, Pyridine 1 and Rhodamine 6G were found as suitable dyes for two-color-LIF which provide a good signal to noise ratio and therefore potential for high resolution measurements. Further, it was proved that the two-color-LIF method significantly reduces the influence of inhomogeneities in illumination and in concentration. When reckoning the TLC capsule diameter, the thermal response time and the measurement uncertainty, the limitation in the spatial and temporal resolution of LCT becomes clear. Nevertheless, due to the simplicity of the measurement setup and the possibility for simultaneous measurements of temperature and velocity fields, LCT is attractive for applications like static flows. A further advantage is the possibility to resolve small temperature differences of hundredth of Kelvin when applying TLCs with small measurement ranges. TLCs also offer an important advantage for applications in micro flows: volume illumination can be applied instead of a light sheet. The LIF technique exhibits the possibility of very high spatial and temporal resolution and the limitation is mainly caused by the optical equipment like the laser and the camera. Further, the basis for high temporal and spatial resolution measurements using two-color-LIF was established. Employing an Nd:YAG laser as a powerful illumination source in combination with Pyridine 1 and Rhodamine 6G enables high acquisition rates (up to 500 Hz with the employed equipment) and high spatial resolution.

Alternative Abstract:
Alternative abstract Language

Räumlich hochauflösende Temperaturfeldmessungen sind wichtig, um mikroskopische Wärmetransport-vorgänge zu bestimmen, welche beispielsweise beim Blasensieden, bei verdampfenden Menisken oder beim Wärmeaustausch an mikro-strukturierten Oberflächen zu finden sind. Diese Vorgänge können aufgrund ihrer Komplexität oftmals nicht durch numerische oder theoretische Modelle beschrieben und müssen demnach experimentell ermittelt werden. Die zwei gebräuchlichen Messmethoden zur Bestimmung von Temperaturfeldern in Flüssigkeiten sind die Flüssigkristall Thermographie (LCT) und die Laserinduzierte Fluoreszenz (LIF). Eine Besonderheit der LCT-Methode ist die mögliche gleichzeitige Erfassung von Geschwindigkeits- und Temperaturfeldern, was deren Einsatz besonders attraktiv erscheinen lässt. Ob die Messmethode jedoch für Messungen mit hoher räumlicher und zeitlicher Auflösung anwendbar ist, war zu diesem Zeitpunkt nicht bekannt. Die Messmethode LIF verspricht aufgrund ihres physikalischen Prinzips eine sehr hohe Auflösung. Aus diesem Grund wurden beide Messmethoden hinsichtlich ihrer räumlichen und zeitlichen Auflösung sowie ihrer Anwendbarkeit charakterisiert. Darüber hinaus wurden Parameter identifiziert, welche die Messgenauigkeit beeinflussen und darauf basierend Verfahren entwickelt, um die Einflüsse zu minimieren. Um die Messgenauigkeit der LCT-Methode zu bestimmen, wurden Temperaturfelder mit homogener Temperaturverteilung und thermisch induzierter Konvektion gemessen und mit numerischen Ergebnissen verglichen. Der Einfluss des Beleuchtungswinkels wurde für den besonderen Fall der Volumenbeleuchtung ermittelt und mit Ergebnissen aus der Literatur verglichen. Da es viele widersprüchliche Aussagen bezüglich des Verhaltens von Flüssigkristallen (TLCs) bei Überhitzung gibt, wurde dies experimentell untersucht. Es wurde herausgefunden, dass Hystereseerscheinungen in dem Temperatur-Farbwertverlauf auftreten, diese jedoch unabhängig von dem Grad der Überhitzung sind und bei ausreichender Unterkühlung wieder verschwinden. Die zeitliche Auflösung der Messmethode LCT ist letztendlich durch das thermische Antwortverhalten der TLCs beschränkt. Da auch hier unterschiedliche Aussagen in der Literatur zu finden sind, wurden diesbezüglich Experimente durchgeführt, welche eine thermische Antwortzeit von 10 ms bei höheren Temperaturgradienten hervorbrachten. Im Hinblick auf die Verbesserung der Messgenauigkeit wurden verschiedene Bildverarbeitungsalgorithmen entwickelt und angewandt. Weiterhin wurde gezeigt, dass Volumenbeleuchtung in Kombination mit entsprechender Bildverarbeitung eine Möglichkeit darstellt, Temperaturfelder in Mini-Strömungen zu messen. Eine Erweiterung des optischen Aufbaus erlaubte darüber hinaus die Messung von 3D-Temperaturverteilungen durch Scannen der Strömung. Analog zu der LCT-Methode wurde die LIF-Methode hinsichtlich ihrer Anwendbarkeit und Messgenauigkeit untersucht und Potential zur hohen räumlichen Auflösung festgestellt. Um die Abhängigkeit von Beleuchtung und Konzentration zu minimieren, wurde die Zwei-Farb-LIF Methode angewandt. Da ein Nd:YAG Laser aufgrund seiner hohen Pulsenergie und schnellen Pulswiederholrate eingesetzt werden soll, musste eine geeignete Farbstoffpaarung gefunden werden. Aus diesem Grund wurden verschiedene Farbstoffe spektral untersucht und Pyridine 1 und Rhodamine 6G als adäquate Farbstoffe identifiziert. Diese wurden in Zwei-Farb-LIF Messungen eingesetzt und ein gutes Signal-Rausch-Verhältnis wurde erzielt. Ferner konnte gezeigt werden, dass der Einfluss von Beleuchtung und Konzentration erheblich gesenkt wird. Die Größe der TLC Kapsel, die thermische Antwortzeit und die hohe Messungenauigkeit sind Faktoren, welche die Auflösung der Messmethode LCT einschränken. Eine gleichzeitig räumlich und zeitlich hochauflösende Messung ist daher nicht möglich. Dahingegen hat die Methode LCT entscheidende Vorteile. Es ist möglich mit entsprechenden TLCs Temperaturen im Bereich von hundertstel Kelvin aufzulösen. Ferner können TLCs in Kombination mit Volumenbeleuchtung eingesetzt werden, was von großem Vorteil für Strömungsvisualisierung im Mikrobereich ist. Mit der Messmethode LIF können sehr große räumliche und zeitliche Auflösungen erzielt werden. Die im Rahmen dieser Arbeit gefundene Farbstoffkombination und entwickelte Messaufbau ermöglichen den Einsatz eines leistungsstarken Nd:YAG Lasers und somit zeitlich hochauflösende Messungen bis hin zu 500 Hz mit der verwendeten Messtechnik.

German
Uncontrolled Keywords: PIV, microPIV, LIF, microLIF, LCT, TLC, PTV, temperature measurement, velocity measurement, temperature field, velocity field, 3D velocity, high spatial resolution, high temporal resolution, measurement accuracy, free convection, channel flow, capillary flow, meniscus, contact angle, lightsheet, Nd:YAG
Classification DDC: 500 Science and mathematics > 530 Physics
600 Technology, medicine, applied sciences > 620 Engineering and machine engineering
Divisions: 16 Department of Mechanical Engineering > Institute for Technical Thermodynamics (TTD)
16 Department of Mechanical Engineering
Date Deposited: 15 Apr 2010 07:15
Last Modified: 05 Mar 2013 09:33
PPN:
Referees: Stephan, Prof. Dr.- Peter ; Wereley, Prof. Steve
Refereed / Verteidigung / mdl. Prüfung: 6 July 2009
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