| 1. |
- Ding, Liang, et al.
(författare)
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A modified dynamic Lee model for two-phase closed thermosyphon (TPCT) simulation
- 2023
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Ingår i: Numerical Heat Transfer, Part B: Fundamentals. - 1040-7790.
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Tidskriftsartikel (refereegranskat)abstract
- A modified model consisting of a dynamic Lee model, volume of fluid model, and continuum surface force model is developed. The modified model investigates heat transfer characteristics of the vapor–liquid phase change process and details of the two-phase flow during operation of a two-phase closed thermosyphon. The mass transfer time relaxation parameters for the Lee phase change model are the most critical coefficients which determine the rate of the vapor–liquid phase change. A dynamic adjustment of the mass transfer time relaxation parameters for the Lee phase change model is realized based on the amount of mass transfer between the vapor and liquid phases and the values of the mass transfer time relaxation parameters become stabilized. The relative error between the modified model and experimental data for the temperature distribution is 5%, representing an acceptable agreement. Compared with the original model, the maximum thermal resistance errors in evaporation section and condensation section are reduced by 19.3% and 107.1%, respectively. These results indicate that the modified model can provide good corrections with high accuracy.
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| 2. |
- Wang, Wei, et al.
(författare)
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Analysis of laminar flow and heat transfer in an interrupted microchannel heat sink with different shaped ribs
- 2019
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Ingår i: Journal of Thermal Analysis and Calorimetry. - : Springer Science and Business Media LLC. - 1388-6150 .- 1588-2926.
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Tidskriftsartikel (refereegranskat)abstract
- A numerical study was conducted to investigate the mechanism of laminar flow and heat transfer enhancement in an interrupted microchannel heat sink (IMCHS) with different shaped ribs at Reynolds number ranging from 100 to 900. The global flow features, heat transfer and friction for IMCHS with no ribs, rectangle ribs, triangle ribs and trapezoid ribs are detailed compared. The results show that the local heat transfer and friction performance of IMCHS with ribs show significant increase at the windward side of the ribs. Additionally, the smaller the chamfer of ribs, the larger average heat transfer and friction performance. For IMCHS with rectangle ribs, the maximum increment of Nu and f can reach to 1.81 and 2.59, respectively. Concerning the overall heat transfer performance (PEC), the trapezoid ribs show the best behavior with PEC = 1.65–1.38 at Re = 100–900.
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| 3. |
- Cui, Yalin, et al.
(författare)
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Unified formula for the field synergy principle
- 2020
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Ingår i: Numerical Heat Transfer, Part B: Fundamentals. - : Informa UK Limited. - 1040-7790 .- 1521-0626. ; 77:4, s. 287-298
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Tidskriftsartikel (refereegranskat)abstract
- The field synergy principle has three criteria to qualitatively describe the essence of single-phase convective heat transfer enhancement. However, in practice these criteria are difficult to be applied to convective heat transfer analysis, because there are no corresponding indicators available to quantitatively describe them. Therefore, a unified formula for the field synergy principle was developed based on these three criteria using probabilistic techniques in this study to overcome these defects. The formula is applicable to incompressible flows with constant properties in both laminar and turbulent flow regimes. The formula contains three categories of non-dimensional indicators corresponding to the three criteria of the field synergy principle, respectively, including domain-averaged cosine of synergy angle, the Pearson linear correlation coefficients between the scalar functions contained in the energy governing equation of convective heat transfer, and the variation coefficients of these functions. The physical meanings of these indicators for the field synergy principle and their connections with the known heat transfer enhancing mechanisms were then discussed. Based on this formula, an improved analytical system for the field synergy principle was proposed. It allows an efficient and quantitative analysis of all single-phase constant-property convective heat transfer phenomena. This new system overcomes the limitation of the conventional field synergy analytical system that mainly analyzes convective heat transfer mechanism from the perspective of synergy angle.
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| 4. |
- Li, Bingxi, et al.
(författare)
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Thaw pond development and initial vegetation succession in experimental plots at a Siberian lowland tundra site
- 2017
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Ingår i: Plant and Soil. - : Springer Science and Business Media LLC. - 1573-5036 .- 0032-079X. ; 420:1-2, s. 147-162
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Tidskriftsartikel (refereegranskat)abstract
- Permafrost degradation has the potential to change the Arctic tundra landscape. We observed rapid local thawing of ice-rich permafrost resulting in thaw pond formation, which was triggered by removal of the shrub cover in a field experiment. This study aimed to examine the rate of permafrost thaw and the initial vegetation succession after the permafrost collapse. Methods In the experiment, we measured changes in soil thaw depth, plant species cover and soil subsidence over nine years (2007–2015). Results After abrupt initial thaw, soil subsidence in the removal plots continued indicating further thawing ofpermafrost albeit at a much slower pace: 1 cm y−1 over 2012–2015 vs. 5 cm y−1 over 2007–2012. Grass cover strongly increased after the initial shrub removal, but later declined with ponding of water in the subsiding removal plots. Sedges established and expanded in the wetter removal plots. Thereby, the removal plots have become increasingly similar to nearby ‘natural’ thawponds. Conclusions The nine years of field observations in a unique shrub removal experiment at a Siberian tundra site document possible trajectories of small-scale permafrost collapse and the initial stage of vegetation recovery,which is essential knowledge for assessing future tundra landscape changes.
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| 5. |
- Vuorinen, Katariina E.M., et al.
(författare)
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Growth rings show limited evidence for ungulates' potential to suppress shrubs across the Arctic
- 2022
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Ingår i: Environmental Research Letters. - : IOP Publishing. - 1748-9318 .- 1748-9326. ; 17:3
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Tidskriftsartikel (refereegranskat)abstract
- Global warming has pronounced effects on tundra vegetation, and rising mean temperatures increase plant growth potential across the Arctic biome. Herbivores may counteract the warming impacts by reducing plant growth, but the strength of this effect may depend on prevailing regional climatic conditions. To study how ungulates interact with temperature to influence growth of tundra shrubs across the Arctic tundra biome, we assembled dendroecological data from 20 sites, comprising 1153 individual shrubs and 223 63 annual growth rings. Evidence for ungulates suppressing shrub radial growth was only observed at intermediate summer temperatures (6.5 °C-9 °C), and even at these temperatures the effect was not strong. Multiple factors, including forage preferences and landscape use by the ungulates, and favourable climatic conditions enabling effective compensatory growth of shrubs, may weaken the effects of ungulates on shrubs, possibly explaining the weakness of observed ungulate effects. Earlier local studies have shown that ungulates may counteract the impacts of warming on tundra shrub growth, but we demonstrate that ungulates' potential to suppress shrub radial growth is not always evident, and may be limited to certain climatic conditions.
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| 6. |
- Wang, Wei, et al.
(författare)
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A review on molten-salt-based and ionic-liquid-based nanofluids for medium-to-high temperature heat transfer
- 2019
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Ingår i: Journal of Thermal Analysis and Calorimetry. - : Springer Science and Business Media LLC. - 1388-6150 .- 1588-2926. ; 136:3, s. 1037-1051
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Tidskriftsartikel (refereegranskat)abstract
- Molten-salt-based nanofluids and ionic-liquid-based nanofluids are developed for thermal storage and heat transfer at relatively high temperatures, in the past few years. Preparation and stabilization techniques are briefly introduced firstly, and then, thermal properties, e.g., specific heat, thermal conductivity and viscosity, are summarized and discussed in detail. The properties are not only affected by the characteristics of nanomaterials and base fluids, but also affected by the synthesis method, such as the sonication intensity and duration. Some of the thermophysical property data are still incomplete, especially the thermal conductivity of molten-salt-based nanofluids, and properties of ionic-liquid-based nanofluids at high temperatures. While several literature works show that the Krieger–Dougherty model can well predict the viscosity, no general models for thermal conductivity and specific heat have been developed yet for both types of nanofluids.
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| 7. |
- Wang, Wei, et al.
(författare)
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Entropy generation analysis of fully-developed turbulent heat transfer flow in inward helically corrugated tubes
- 2018
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Ingår i: Numerical Heat Transfer; Part A: Applications. - : Informa UK Limited. - 1040-7782 .- 1521-0634. ; 73:11, s. 788-805
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Tidskriftsartikel (refereegranskat)abstract
- The entropy generation analysis of fully-developed turbulent heat transfer flow in inward helically corrugated tubes was numerically performed by using a Reynolds stress model. The simulations were conducted for a smooth tube and five cases of corrugated tubes with Reynolds number (Re) ranging from 10,020 to 40,060 at a constant wall temperature condition. The effects of corrugation pitch and height on the flow patterns as well as local thermal and frictional entropy generation are detailed in the near wall region. The results indicate that the local heat transfer entropy generation is significantly evident at the sub-layer region and the detached vortex region, and the local thermal entropy is improved with increases in the secondary flow. Local friction entropy generation is mainly located at the windward of the corrugation and the severely turbulent fluctuation region and is mainly induced by the velocity gradient. The average friction entropy generation exhibits an exponential growth, while the average heat transfer and the total entropy generation display a linear growth trend with increased Re. The average Bejan number (Be) exhibits an exponential decline, and the minimum value can reach 0.69. From a comprehensive viewpoint, it is optimal for the Re to be lower than 30,050. When Re <20,030, higher and dense corrugations are beneficial. When 20,030 < Re <30,050, low and spare corrugations are more optimal. Besides, the case with Hl/D = 0.08 is not recommended.
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| 8. |
- Wang, Wei, et al.
(författare)
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Entropy Study on the Enhanced Heat Transfer Mechanism of the Coupling of Detached and Spiral Vortex Fields in Spirally Corrugated Tubes
- 2021
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Ingår i: Heat Transfer Engineering. - : Informa UK Limited. - 0145-7632 .- 1521-0537. ; 42:17, s. 1417-1431
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Tidskriftsartikel (refereegranskat)abstract
- The present work numerically studied the enhanced heat transfer mechanism of the coupled fields of detached and spiral vortices in symmetrical and asymmetrical spirally corrugated tubes. The heat transfer and viscous dissipation were analyzed by evaluating the entropy combined with the flow patterns, and the global entropy generation rate was analyzed by considering the local Nusselt number and friction factor. The results indicated that, both the leeside and windward corrugation angles had obvious effects on the strength of detached vortex and spiral flow, and also the location of the vortex. The maximum values of the heat transfer entropy were located at the boundary layers, and the heat transfer entropy of the secondary flow region was more distinguished than in other parts of the main flow region. The maximum values of friction entropy generation were located at both the boundary layers and the core of detached vortex. The performance evaluation criterion (PEC) presents nearly the same values for the four cases, when the Reynolds number (Re) is less than 6,300. In addition, the PEC of symmetrical spirally corrugated tube with corrugation angle equal to 25° showed the best performance, when Re is greater than 6,300. To keep the PEC above 1, Re should not exceed 33,000.
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| 9. |
- Wang, Wei, et al.
(författare)
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Investigation of complex flow and heat transfer mechanism in multi-tube heat exchanger with different arrangement corrugated tube
- 2021
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Ingår i: International Journal of Thermal Sciences. - : Elsevier BV. - 1290-0729. ; 167
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, the enhanced heat transfer performance and mechanism of a multi-tube heat exchanger (MTHX) with outward helically corrugated tubes (HCTs) are investigated by a numerical approach. Three types of tube arrangement (circle, square and triangle, indicated as CTA, STA and TSA, respectively) and four different tube spacings (1.25, 1.375, 1.5 and 1.625) are compared for Reynolds (Re) number ranging from 8900 to 89,400. Details of the coupled relationship among the multiple spiral detached vortices and turbulent pulsations are presented to explore the complex flow features. In addition, the variation trends of the average Nusselt number, friction factor, and performance index (PEC) are compared to obtain the optimum geometry parameter of the MTHX with HCTs. The results show that the rotational flow with opposite direction presents a strong overlay effect, and the circumferential velocity can reach up to 20% of the mean velocity. The non-directional turbulent pulsation only presents an overlap and strengthening effect. Besides, the secondary flow causes better heat transfer in the sub-layers, the turbulent pulsation has better heat transfer between the sub-layer and main flow region, while the rotational flow has a minor inhibiting effect on heat transfer performance. The TTA presents better interaction at the intervals of the tubes, and the Nu, f and PEC index of the TTA are all outstanding compared to the CTA and STA. The SL/Φt = 1.375–1.5 of the MTHX-TTA are suggested from the findings, and the Re should not exceed 40, 000.
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| 10. |
- Wang, Wei, et al.
(författare)
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Large eddy simulation of turbulent flow and heat transfer in outward transverse and helically corrugated tubes
- 2019
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Ingår i: Numerical Heat Transfer; Part A: Applications. - : Informa UK Limited. - 1040-7782 .- 1521-0634. ; 75:7, s. 456-468
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Tidskriftsartikel (refereegranskat)abstract
- Turbulent flow and heat transfer in outward transverse and helically corrugated tubes are performed with large eddy simulation by the ANSYS Fluent software. The prediction accuracy is validated by comparison with experimental data and empirical correlations for a wavy surface wall and smooth tube, respectively. The turbulent flow patterns, local heat transfer, and friction factor are discussed. The results show that the secondary and turbulent eddies are inhibited by the spiral flow. Otherwise, the flow impact of the wall is the key factor for heat transfer enhancement, and the spiral flow has of small effect on heat transfer performance, however it can decrease the flow resistance significantly. The overall heat transfer performance for the helical corrugated tube is 1.23, which is superior to the value of 1.18 for the transverse corrugated tube.
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