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 | ||||
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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. |
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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 |
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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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