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Mechanical and Civil Engineering Seminar

Thursday, October 5, 2023
11:00am to 12:00pm
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Gates-Thomas 135
Experimental Investigation on Heat Transfer Characteristics of Supercritical CO2 Flows Through Microtubes
Hojin Ahn, Professor, Mechanical Engineering Department, Yeditepe University,

Mechanical and Civil Engineering Seminar Series

Title: Experimental Investigation on Heat Transfer Characteristics of Supercritical CO2 Flows Through Microtubes

Abstract: Experimental study on convective heat transfer through microtubes encounters many challenges, such as measuring the wall temperature of the tube with a reasonable uncertainty, relatively high heat loss from the outer surface of the tube, and difficulty in measuring the fluid bulk temperature at the tube outlet. These issues become exacerbated with flow at low Reynolds numbers. The present study presents a new method to measure the tube wall temperature with a small solder cast where a thermocouple is embedded. This method significantly reduced the thermal resistance between the thermocouples and the tube wall and was successfully validated with laminar water flows through a horizontally configured microtube, 0.501 mm in inner diameter. The method developed in this study was applied to supercritical CO2 (sCO2) flows through microtubes, 0.317, 0.509, and 0.847 mm in diameter, under heating conditions. Rapid changes in sCO2 properties near the pseudo-critical region resulted in thermal acceleration and buoyancy effects. For both high and low mass flow rates, a local maximum of the convection heat transfer coefficient was observed where the film temperature was close to the pseudo-critical temperature. Heat transfer characteristics of flows with Reynolds number less than 1000 at the tube inlet were dependent on flow configurations (vertically upward or downward, or horizontal flow), implying that flows were strongly affected by the buoyancy force. Furthermore, the heat transfer rate in the horizontal configuration was considerably higher than those in the vertical configurations at the same operating conditions. For flows with Reynolds number greater than 5000 at the inlet, on the other hand, different flow configurations had no significant effect on heat transfer characteristics.

Bio: Hojin transferred as a sophomore from Seoul National University in Korea to Caltech in 1982. Subsequently, he obtained his B.S. in 1985, M.S. in 1986 (under Prof. Rolf Sabersky), and Ph.D. in 1989 (under Prof. C.E. Brennen), all at Caltech. After working on liquid rocket engines for seven years at The Aerospace Corporation, El Segundo, he moved to Turkey, teaching at Marmara University and then at Yeditepe University. He has lived in Istanbul, Turkey, since 1996.

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