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Mechanism Study of Heat Transfer Enhancement Using Twisted Hexagonal Tube with Slurry from Biogas Plant
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science. State Key Laboratory of Material-Oriented Chemical Engineering, Nanjing Tech University.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.ORCID iD: 0000-0002-0200-9960
Key Laboratory of Material and Chemical Engineering, Nanjing Tech University, Nanjing .
State Key Laboratory of Material-Oriented Chemical Engineering, Nanjing Tech University, Nanjing .
2017 (English)In: Energy Procedia, ISSN 1876-6102, Vol. 142, p. 880-885Article in journal (Refereed) Published
Abstract [en]

Waste-heat recovery from discharged slurries is important to improve the biogas production efficiency but still remains challenge duo to the special properties of slurries in anaerobic digestion process. In this work, numerical study was carried out to investigate the flow field, and heat transfer performance of slurry from biogas plant in the twisted hexagonal and other twisted tubes was simulated with computer fluid dynamic (CFD) for the first time. The numerical method was validated with experimental data from the literature. The heat transfer performance and flow resistance of twisted hexagon tube were calculated and compared with other types of twisted tubes. The enhancement factor of the twisted hexagonal tube reached to 2 and kept optimum at turbulence flow region compared to the twisted tubes with square and elliptical cross section. Meanwhile, the mechanism of heat transfer enhancement with different twisted tubes was further studied, and the optimal field synergy and minimum local circulation flow near the wall are the main reasons for the high performance and low flow resistance of the twisted hexagonal tube.

Place, publisher, year, edition, pages
Elsevier, 2017. Vol. 142, p. 880-885
National Category
Energy Engineering
Research subject
Energy Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-67537DOI: 10.1016/j.egypro.2017.12.141ISI: 000452901601007Scopus ID: 2-s2.0-85041495126OAI: oai:DiVA.org:ltu-67537DiVA, id: diva2:1180731
Conference
9th International Conference on Applied Energy, Cardiff UK, August 21-24, 2017
Note

Konferensartikel i tidskrift

Available from: 2018-02-06 Created: 2018-02-06 Last updated: 2023-08-28Bibliographically approved
In thesis
1. Heat-transfer Enhancement for Slurries from Biogas Plants− Properties, processes, and thermal systems
Open this publication in new window or tab >>Heat-transfer Enhancement for Slurries from Biogas Plants− Properties, processes, and thermal systems
2022 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

 Biomethane production from renewable residues with anaerobic digestion gains increasing attention as a crucial alternative to petroleum fuels. It has been vigorously developed, but the large amounts of subsidy from the government indicate that the process efficiency needs to be further improved. For biomethane production, on the one hand, a great amount of heat needs to be used for heating the feeding slurry, sanitation of slurry, and maintaining the temperature in the large-scale reactors. On the other hand, a large amount of thermophilic effluent slurries brings a huge amount of waste heat, which can be recovered. This makes it important to study how to increase production by improving the thermal efficiency of biogas plants with novel heat exchangers. 

The working fluids in the biogas plants are the non-Newtonian and high-viscous slurries, and the conventional heat exchangers in biogas plants always show much lower performance compared to those in other industries. Normally, the slurries in the biogas plant consist of different substrates, including straw, manure, food waste, municipal sludge, and their mixtures, and various factors such as the amount and type of solids, particle size, shear rate, and temperature impact the rheological properties of the slurries, which makes the complexity in the rheological properties and the difficulty in developing novel heat exchangers.

The development of heat exchangers calls for the rheological properties of slurries. However, to the best of our knowledge, only the rheology of manure slurry was systematically determined and modeled considering the effect of temperature. The lack of the rheological properties of slurries further hinders the design and development of novel geometries to enhance the heat transfer of the slurries. Correspondingly, the quantitative contribution and potential of the waste-heat recovery from the slurries to production using the enhanced geometry remain unclear. 

    In this thesis work, to design novel geometry with heat-transfer enhancement for different slurries and determine its potential in thermal cycles in the full-scale biogas plants, firstly, the temperature-dependent rheological properties of the slurries, including the corn straw, food waste, and mixed slurries, were tested and modeled. It was found that these slurries possess strong shear-thinning behavior, the temperature has a significant impact on their dynamic viscosity, and the power-law model combined with the Arrhenius equation can describe the rheology well.

    Subsequently, with the reliable models of the rheological properties as the key input,  Computational Fluid Dynamics simulations were conducted to screen different twisted geometries, determine the heat-transfer performance, and reveal the mechanism of the heat-transfer enhancement. Lab- and pilot-scale experiments were also conducted to validate the numerical results. The twisted hexagonal tubes show a positive enhancement factor up to 2.6 compared to normal heat exchangers in a wide range of operating conditions. The continuous and strong near-wall shear effect is the intrinsic reason for achieving a significant heat-transfer enhancement in the twisted hexagonal tubes. Moreover, the generalized engineering equations for predicting the effective shear rate and heat-transfer performance with measurable parameters were established and verified with both numerical and experimental results.

    Finally, the twisted-hexagonal-tube heat exchange was integrated with complete thermal cycles, including waste-heat recovery and external heating processes in the biogas plant, and the potential of increased production and profits were modeled and analyzed combined with the practical operating conditions in a full-scale biogas plant. It was found that for the waste-heat recovery using the twisted hexagonal tubes, the net raw biogas production can increase by up to 17.0 %, and for the external heating process, the increased profit equivalent to 39 % of total production can be achieved owing to energy conservation in external heating using the twisted-hexagonal-tube heat exchangers for a full-scale biogas plant. 

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2022. p. 67
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords
heat-transfer enhancement, slurry, rheological properties, computational fluid dynamics, twisted tubes, biogas production
National Category
Energy Systems Bioprocess Technology Energy Engineering
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-89115 (URN)978-91-8048-026-0 (ISBN)978-91-8048-027-7 (ISBN)
Public defence
2022-03-31, Online (via Zoom) and L-E616A Green room, Lulea University of Technology, Lulea, 10:00 (English)
Opponent
Supervisors
Available from: 2022-02-07 Created: 2022-02-04 Last updated: 2025-02-17Bibliographically approved

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