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Although supercritical CO2 (sCO2) heat transfer has been employed in industrial process since the 1960s, the underlying transport phenomenon in high-flux microscale geometries, as could be employed in concentrating solar receivers, is poorly understood. To date, nearly all experimental studies and simulations of supercritical convective heat transfer have focused on large diameter vertical channel and tube bundle flows, which may differ dramatically from microscale supercritical convection. Computational studies have primarily employed Reynolds averaged (RANS) turbulence modeling approaches, which may not capture effects from the sharply varying property trends of supercritical fluids. In this study, large eddy simulation (LES) turbulence modeling techniques are employed to study heat transfer characteristics of sCO2 in microscale heat exchangers. The simulation geometry consists of a microchannel of 750×737 μm cross-section and 5 mm length, heated from all four sides. Simulation cases are evaluated at reduced pressure P_r = 1.1, mass flux G = 1000 kg/m^2-s, heat flux q'' = 1.7 − 8.9 W/cm^2 , and varying inlet temperature: 20 − 100℃. Computational results reveal thermal transport mechanisms specific to microscale sCO2 flows. Results have been compared with available supercritical convection correlations to identify the most applicable heat transfer models for engineering of microchannel sCO2 heat exchangers.
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VALIDATION OF HIGH-DEFINITION FIBER OPTIC SENSORS FOR TEMPERATURE AND LOCAL HEAT TRANSFER COEFFICIENT MEASUREMENTS OF SYMMETRICALLY HEATED, SINGLE-PHASE INTERNAL FLOW
The objective of this paper is to use fiber optic sensors embedded in a tube wall to measure local convective heat transfer coefficients of a single-phase fluid. By using Rayleigh backscatter and an interferometer technique, mechanical changes in a fiber sensor that are proportional to temperature can be detected. This allows the location and magnitude of the temperature along the fiber to be measured. Using these fibers, we can measure axial profiles of the wall temperature in a heated tube with an internal fluid. By using multiple sensors spaced circumferentially around the tube, we can then generate axial and circumferential temperature maps of the tube wall. When combined with a known uniformly applied heat flux, these measurements can be used to determine the local heat transfer coefficients for single-, two-phase, and supercritical flows. In this study we consider a horizontal tube with internal diameter of 4.57 mm and heated length of 0.4 m. Using the fibers, wall temperature is measured every 0.7 mm in the streamwise direction at eight evenly spaced axial locations with an uncertainty of 2 °C. Co-located, calibrated thermocouples will verify the fiber temperature readings. Electric heaters provide a heat flux up to 20 W/cm2. Using this setup, heat transfer coefficients in the developing and fully developed region are obtained for water in laminar flow regimes and compared with established convective heat transfer correlations and models. The measured heat transfer coefficients in agreement with what is expected. In future work, this test section will be used to study nearcritical carbon dioxide convective heat transfer in both steady and transient conditions.
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- Award ID(s):
- 2235356
- PAR ID:
- 10508702
- Publisher / Repository:
- Begellhouse
- Date Published:
- Page Range / eLocation ID:
- 8
- Format(s):
- Medium: X
- Location:
- Cape Town, South Africa
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1615/IHTC17.260-50
