Date of Award
2024-05-01
Degree Name
Doctor of Philosophy
Department
Mechanical Engineering
Advisor(s)
Md Mahamudur Rahman
Abstract
The enhancement of boiling heat transfer is critical because it has the ability to solve thermal management issues that are seen across all engineering and manufacturing applications. The focus of this research is first to characterize the critical heat flux (CHF) on cylindrical surfaces during high pressure pool boiling where fundamental boiling parameters were characterized such as bubble diameter, bubble growth, wait time, and bubble departure frequency as well as an approximation for nucleation site density. The results were compared with 4 different models t in investigate if the pressure, engineered surface or other factor dictated the value of CHF.A custom-built heater was designed to test cylindrical samples under pool boiling capable to deliver radial heat flux to challenge all the engineering samples tested on pool boiling up to CHF. The active heating area on the cylindrical tubes had a dimension of 9.5 mm outer diameter and 10.5 mm length. A custom-built cylindrical heater was designed using a nichrome wire coil of 30 AWG with a resistance of 19.57 Ω/inch of coil to provide joule heating to the cylindrical tubes. The electrical wire was insulated from the copper heater using a thin layer of alumina paste. The research detailed here is based on the characterization of pool boiling heat transfer enhancement on cylindrical tubes with circumferential micro-channels, nanostructures, and hierarchical surfaces where they were compared with a cylindrical copper tube with smooth surface as reference using saturated water at different pressure conditions: 1 bar, 2 bar and 3 bar pressure. Three engineered copper tubes with circumferential microchannel dimensions of 300 μm, 600 μm, and 900 μm fin width and a fixed 400 μm channel width with 410 μm channel depth were machined using CNC. Additionally, nano-leaves CuO surface were fabricated on a plain copper tube copper to create nanostructures. Hierarchical surfaces were fabricated with the three circumferential microchannel of 300 μm, 600 μm, and 900 μm fin width and the same technic of CuO nano-leaves. The plain cylindrical copper tube reached a CHF value of 72 W/cm2 at 1 bar, 98 W/cm2 at 2 bar, and 120.5 W/cm2 at 3 bar pressure. The cylindrical tubes with circumferential microchannel geometries tested at atmospheric pressure reached a CHF value between 131-144 W/cm2, which corresponds to 82% to 100% enhancement compared to the plain cylindrical tube at 1 bar. CHF on cylindrical tubes with circumferential microchannel reached values in the range of 172 -185 W/cm2 at 2 bar pressure and for 3 bar pressure CHF values in the range of 193 â?? 201 W/cm2 where all these values of CHF represent an enhancement over 100%. The nanostructure reached 110.22 W/cm2 that corresponds to 53% enhancement compared with plain cylindrical tube. The nanostructured tube reached CHF of 125.28 W/cm2 that corresponds to 27.8% enhancement compared with plain cylindrical tube at 2 bar and 74% enhancement compared with plain cylindrical tube at atmospheric pressure. Nanostructure tube reached CHF of 145 W/cm2 at a wall superheat of 17.23 with a HTC of 86 kW/m2K. The hierarchical tubes were tested at atmospheric pressure, 2 bar and 3 bar where they reached a CHF value between 133-151 W/cm2, 175-194 W/cm2, and 217-235 W/cm2 for atmospheric, 2 bar and 3 bar pressure, respectively. Also, a flow boiling setup was developed to work at a range of atmospheric to 10 bar pressure at a volumetric flow rate of 1000 kg/m2s and between a temperature range of ambient temperature to 180 °C. To validate the setup a test was performed at 10 K subcooled with distilled water and titanium that was deposited with a sputtering on an infrared radiation transparent flat sapphire. Flat heaters were built on a substrate of optical grade sapphire, where the heater consists of a substrate of sapphire with dimensions of 20 Ã? 20 mm2 with 1 mm thickness. A thin layer of 20 nm of ITO is deposited by thermal evaporation that works as the Joule heating element, and gold pads around the ends of the sapphire are deposited for electrical connection as well as definition of the tested area of 10 Ã? 10 mm2. The results obtained with water as working fluid were compared with literature data that showed a good similarity with the results for flow boiling under same conditions. 10 K infrared thermometry was used during 10 K subcooled flow boiling and pool boiling with FC-72 at atmospheric pressure and 4 bar to capture fundamental boiling parameters such as bubble departure diameter, bubble growth time as well as wall superheat and heat flux. An initial calibration test has been performed for the infrared camera. The flow boiling tests will be performed at atmospheric pressure under a mass flux of 1000 kg/m2s with a hydraulic diameter of 1.5 cm. The high-speed infrared thermometry technique will measure the local time-dependent 2-D temperature and heat flux distribution on the boiling surface. However, the bubble side and departure time of the bubbles on FC-72 did not allow us to obtain the fundamental boiling parameters. On the other hand, boiling curves were obtained for pool and flow boiling at atmospheric and 4 bar pressure with a subcooled of 10 K.
Language
en
Provenance
Received from ProQuest
Copyright Date
2024-05
File Size
118 p.
File Format
application/pdf
Rights Holder
Omar Hernandez Rodriguez
Recommended Citation
Hernandez Rodriguez, Omar, "High Pressure Boiling Enhancement Using Micro/nano Structures" (2024). Open Access Theses & Dissertations. 4103.
https://scholarworks.utep.edu/open_etd/4103