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High-Resolution Temperature Measurement during Forced Convective Heat Transfer at a Wall with a Dimple Structure

Su, Bo (2015)
High-Resolution Temperature Measurement during Forced Convective Heat Transfer at a Wall with a Dimple Structure.
Technische Universität Darmstadt
Ph.D. Thesis, Primary publication

Abstract

A dimple structure is a concave surface, processed onto a heat transfer surface to promote convective heat transfer. Benefits, such as heat transfer enhancement with moderate flow resistance, less fouling, etc., have been reported by many researchers. Due to the limitations of the experimental method, heat transfer on dimpled surfaces in turbulent flow, which is complex in distribution and dynamic over time, has not yet been fully understood. With the aid of Infrared (IR) Thermography, details of the Nusselt number distributions on these surfaces in high resolution were investigated in this study. Three surfaces, including a plane surface, a single-dimpled surface, and a double-dimpled surface were observed in turbulent water flow within a dimple Reynolds number range of Re_d=1×10^4-3.5×10^4 and a flow temperature range of T_in=25-43 ℃. All surfaces have the same dimple structure with a printing diameter D_d=15.3 mm and a relative dimple depth t⁄D_d=0.26. These surfaces were coated using physical vapor deposition (PVD) technology with an indium tin oxide (ITO) layer, a SiO2 layer, and copper layers onto the dimpled surfaces of calcium fluoride (CaF2) glass substrates. During the heating experiment, the ITO layer was charged with direct current (DC) power providing a constant heat flux (q) up to 53 kW⁄m^2 from the structured surface through Joule heating. Wall temperature distributions on the heating surface were recorded through the CaF2 substrate by an IR camera with a spatial and a temporal resolution of 283 μm and 50 Hz, respectively. Before the measurement for each surface, in-situ calibration was conducted to determine the relationship between wall temperature and raw IR intensity from the camera. These IR images include the averaged images (averaged over 10 s) and image sequences over 115 s. Reattaching zone, recirculating zone, and wake were observed in the wall temperature distribution on dimpled surface. At Re_d=2×10^4, a maximum heat transfer region was found in the reattaching zone close to the rear dimple’s edge with a Nusselt number ratio (Nu_d⁄Nu_pl) of around 2, while the rate was found to be around 1 in the recirculating zone. Considering the influence of three-dimensional distributions of heat transfer, the spanwise-averaged and area-averaged Nusselt number ratios (Nu_span,d⁄Nu_span,pl and Nu_d⁄Nu_pl) showed maximum values of 1.6 and 1.3, respectively. In the wake, the heat transfer drops sharply from the higher value near the rear edge to that on the plane surface (Nu_d⁄Nu_pl=1) with a distance around 1.5D_d. The experiment on the double-dimpled surface showed that the maximum Nusselt number ratio in the second dimple was significantly larger than in the dimple upstream. The increase was around 10% at Re_d=2×10^4. Heat transfer on dimpled surfaces increased with rising Reynolds number. However, the Nusselt number ratio was observed to decrease with increasing of the Reynolds number. On double dimpled surface, the influence of the upstream dimple in heat transfer became weaker at higher Reynolds numbers. Finally, other experimental parameters, such as the inlet flow temperature and the heat flux, had limited influence on the heat transfer over the dimpled surfaces. Time resolved temperature distributions showed that a switching mode of the wall temperature between two symmetric states would repeat several times in low frequency as Re_d>1×10^4. The comparison showed that the switching mode influenced six temperature subzones, which were symmetrically distributed in and downstream of the dimple.

Item Type: Ph.D. Thesis
Erschienen: 2015
Creators: Su, Bo
Type of entry: Primary publication
Title: High-Resolution Temperature Measurement during Forced Convective Heat Transfer at a Wall with a Dimple Structure
Language: English
Referees: Stephan, Prof. Dr. Peter ; Kornev, Prof. Dr. Nikolai
Date: 22 March 2015
Refereed: 9 December 2014
URL / URN: http://tuprints.ulb.tu-darmstadt.de/4463
Abstract:

A dimple structure is a concave surface, processed onto a heat transfer surface to promote convective heat transfer. Benefits, such as heat transfer enhancement with moderate flow resistance, less fouling, etc., have been reported by many researchers. Due to the limitations of the experimental method, heat transfer on dimpled surfaces in turbulent flow, which is complex in distribution and dynamic over time, has not yet been fully understood. With the aid of Infrared (IR) Thermography, details of the Nusselt number distributions on these surfaces in high resolution were investigated in this study. Three surfaces, including a plane surface, a single-dimpled surface, and a double-dimpled surface were observed in turbulent water flow within a dimple Reynolds number range of Re_d=1×10^4-3.5×10^4 and a flow temperature range of T_in=25-43 ℃. All surfaces have the same dimple structure with a printing diameter D_d=15.3 mm and a relative dimple depth t⁄D_d=0.26. These surfaces were coated using physical vapor deposition (PVD) technology with an indium tin oxide (ITO) layer, a SiO2 layer, and copper layers onto the dimpled surfaces of calcium fluoride (CaF2) glass substrates. During the heating experiment, the ITO layer was charged with direct current (DC) power providing a constant heat flux (q) up to 53 kW⁄m^2 from the structured surface through Joule heating. Wall temperature distributions on the heating surface were recorded through the CaF2 substrate by an IR camera with a spatial and a temporal resolution of 283 μm and 50 Hz, respectively. Before the measurement for each surface, in-situ calibration was conducted to determine the relationship between wall temperature and raw IR intensity from the camera. These IR images include the averaged images (averaged over 10 s) and image sequences over 115 s. Reattaching zone, recirculating zone, and wake were observed in the wall temperature distribution on dimpled surface. At Re_d=2×10^4, a maximum heat transfer region was found in the reattaching zone close to the rear dimple’s edge with a Nusselt number ratio (Nu_d⁄Nu_pl) of around 2, while the rate was found to be around 1 in the recirculating zone. Considering the influence of three-dimensional distributions of heat transfer, the spanwise-averaged and area-averaged Nusselt number ratios (Nu_span,d⁄Nu_span,pl and Nu_d⁄Nu_pl) showed maximum values of 1.6 and 1.3, respectively. In the wake, the heat transfer drops sharply from the higher value near the rear edge to that on the plane surface (Nu_d⁄Nu_pl=1) with a distance around 1.5D_d. The experiment on the double-dimpled surface showed that the maximum Nusselt number ratio in the second dimple was significantly larger than in the dimple upstream. The increase was around 10% at Re_d=2×10^4. Heat transfer on dimpled surfaces increased with rising Reynolds number. However, the Nusselt number ratio was observed to decrease with increasing of the Reynolds number. On double dimpled surface, the influence of the upstream dimple in heat transfer became weaker at higher Reynolds numbers. Finally, other experimental parameters, such as the inlet flow temperature and the heat flux, had limited influence on the heat transfer over the dimpled surfaces. Time resolved temperature distributions showed that a switching mode of the wall temperature between two symmetric states would repeat several times in low frequency as Re_d>1×10^4. The comparison showed that the switching mode influenced six temperature subzones, which were symmetrically distributed in and downstream of the dimple.

Alternative Abstract:
Alternative abstract Language

Eine Dellenstruktur besteht aus konkaven Wölbungen, welche in die Oberfläche eines Wärmeübertragers eingebracht sind, um den konvektive Wärmeübergang zu verbessern. Vorteile wie ein verbesserter Wärmeübergang bei moderatem Strömungswiderstand, wenigerem Fouling etc. wurden von vielen Forschern bestätigt. Die Wärmeübertragung an Dellenoberflächen in turbulenter Strömung ist aufgrund der räumlichen Verteilung und der zeitlichen Dynamik sehr komplex. Durch die begrenzten experimentellen Möglichkeiten ist dieser Mechanismus noch nicht vollständig verstanden. Mithilfe von Infrarot-Thermographie wird in dieser Studie die Verteilung der Nusselt-Zahl auf Oberflächen mittels hoher räumlicher und zeitlicher Auflösung untersucht. Es werden drei Oberflächen in turbulenter Wasserströmung untersucht: Eine ebene Oberfläche, eine Oberfläche mit einer Delle und eine Oberfläche mit zwei in Strömungsrichtung aufeinander folgende Dellen. Die auf den Dellendurchmesser bezogene Reynolds-Zahl beträgt zwischen Re_d=1×10^4 und 3.5×10^4 bei einer Strömungs-Einlasstemperatur zwischen T_in=25 und 43 ℃. Alle Oberflächen weisen die gleichen Dellen auf mit einem Durchmesser von D_d=15.3 mm und einer relativen Tiefe von t⁄D_d =0.26. Diese Oberflächen werden unter Verwendung des physikalischen Gasphasenabscheidungs-Verfahrens (engl. physical vapor deposition, PVD) beschichtet. Dünne Schichten Indium-Zinn-Oxid (engl. indium tin oxide, ITO), SiO2 und Kupfer werden auf den mit Dellen versehenen Glas-Träger aus CaF2 aufgedampft. Während des Experiments wird die ITO-Schicht mit Gleichtstrom beaufschlagt. Aufgrund des Jouleschen Widerstandes der ITO-Schicht stellt sich eine konstante Wärmestromdichte von bis zu q=53 kW/m^2 ein. Die Temperaturverteilung auf der beheizten Oberfläche wird durch das CaF2-Glas hindurch mittels einer IR-Kamera mit einer räumlichen und zeitlichen Auflösung von 283 μm und 50 Hz gemessen. Um die Verteilung der Wandtemperatur aus den Kamera-Rohdaten zu ermitteln, wird vor Messungen eine In-Situ-Kalibrierung durchgeführt. Es wurden Sequenzen über eine Zeitspanne von 115 s sowie über 10 s gemittelte Bilder aufgenommen. Die Verteilung der Wandtemperatur wird in der Wiederanlegezone, dem Rezirkulationsbereich und in der Nachlaufströmung untersucht. Bei Re_d=20000 befindet sich der Bereich maximaler Wärmeübertragung mit einem Nusselt-Quotienten Nu_d/Nu_pl von etwa 2 in der Wiederanlegezone der Strömung nahe der hinteren Dellenkante. Zum Vergleich beträgt dieser Quotient im Rezirkulationsbereich etwa 1. Zur Berücksichtigung des Einflusses der dreidimensionalen Verteilung des Wärmetransports werden die quer zur Strömungsrichtung und über die Fläche gemittelten Nusselt-Zahlen-Quotienten Nu_span,d⁄Nu_span,pl und Nu_d⁄Nu_pl ermittelt. Diese besitzen Maximalwerte von 1.6 und 1.3. Im Nachlaufbereich fällt der Wärmeübergang stark ab vom erhöhten Wert an der hinteren Dellenkante auf den Wert der flachen Oberfläche. Bei der Oberfläche mit zwei Dellen beträgt der Dellenabstand in der Kanalmitte etwa 1.5D_d. Dieses Experiment zeigt, dass der maximale Nusselt-Quotient in der in Strömungsrichtung hinteren Delle signifikant größer ist als in der vorderen Delle. Bei Re_d=2×10^4 beträgt diese Vergrößerung rund 10%. Der Wärmeübergang auf der Dellenoberfläche verbessert sich mit steigender Reynolds-Zahl. Der Nusselt-Quotient jedoch verringert sich mit steigender Reynolds-Zahl. Des Weiteren zeigt die Auswertung der Experimente, dass der Einfluss der vorderen Delle mit steigender Reynolds-Zahl sinkt. Andere Parameter, wie die Einlasstemperatur und die Wärmestromdichte, zeigen nur einen begrenzten Einfluss auf die Wärmeübertragung der Dellenoberfläche. Die zeitliche Verteilung der Temperatur zeigt, dass bei Reynolds-Zahlen größer als 10000 ein hochfrequenter Wechsel zwischen zwei symmetrischen Strömungskonfigurationen erfolgt. Ein Vergleich zeigt, dass dieser Wechsel sechs symmetrisch verteilte Temperatur-Teilbereiche in der Delle und stromabwärts beeinflusst.

German
URN: urn:nbn:de:tuda-tuprints-44632
Classification DDC: 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: 26 Apr 2015 19:55
Last Modified: 26 Apr 2015 19:55
PPN:
Referees: Stephan, Prof. Dr. Peter ; Kornev, Prof. Dr. Nikolai
Refereed / Verteidigung / mdl. Prüfung: 9 December 2014
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