Abstract
The thermohydraulic characteristics of supercritical CO2 flows in a vertical tube at cooling conditions are numerically investigated, and the influence of the heat-flux condition and of the flow direction are evaluated. Constant (i.e., uniform), linearly increasing and linearly decreasing heat-flux conditions are considered as three typical heat-flux distributions over the pipe length. The simulation results show that there exists a maximum heat transfer coefficient at all heat-flux conditions when the fluid bulk temperature is slightly higher than the pseudo-critical temperature, but also that the heat-flux condition has little effect on the peak value of the heat transfer coefficient. From the viewpoint of the second law of thermodynamics, the influence of the heat-flux condition on the local entropy generation can be attributed to the distributed matching between the heat flux and the difference between the wall temperature and the fluid bulk temperature, as a better matching is associated with a higher uniformity of the local entropy generation and reduced overall irreversibilities. Upward and downward flows are considered, along with flows without gravity as a baseline case for comparison purposes, with the field synergy principle employed to explain the different phenomena in these flows. The buoyancy effect laminarises the downward flows and raises the temperature gradient; hence, the heat transfer deteriorates and the irreversibility increases. In the upward flows, the buoyancy effect augments the turbulence and alleviates the variations in temperature and velocity in the core region, consequently reducing the irreversible loss and enhancing heat transfer. The present study provides insights into the mechanisms of supercritical CO2 heat transfer characteristics as well as practical guidance on the design and optimisation of relevant components.
Original language | English |
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Article number | 124628 |
Number of pages | 15 |
Journal | Energy |
Volume | 256 |
Early online date | 11 Jul 2022 |
DOIs | |
Publication status | Published - 1 Oct 2022 |
Bibliographical note
Funding Information:This research has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. [882628]. The research was also supported by the Ministry of Science and Higher Education of the Russian Federation (“Megagrant ” project no. 075-15-2019-1888). Data supporting this publication can be obtained on request from [email protected] . For the purpose of Open Access, the authors have applied a CC BY public copyright licence to any Author Accepted Manuscript version arising from this submission.
Publisher Copyright:
© 2022 Elsevier Ltd
Keywords
- Buoyancy effect
- Entropy generation
- Field synergy principle
- Heat exchanger
- Heat transfer
- Supercritical CO
ASJC Scopus subject areas
- Civil and Structural Engineering
- Modelling and Simulation
- Renewable Energy, Sustainability and the Environment
- Building and Construction
- Fuel Technology
- Energy Engineering and Power Technology
- Pollution
- Mechanical Engineering
- General Energy
- Management, Monitoring, Policy and Law
- Industrial and Manufacturing Engineering
- Electrical and Electronic Engineering