| 1. |
- Akiyama, Kazunori, et al.
(författare)
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First Sagittarius A* Event Horizon Telescope Results. II. EHT and Multiwavelength Observations, Data Processing, and Calibration
- 2022
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8213 .- 2041-8205. ; 930:2
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Tidskriftsartikel (refereegranskat)abstract
- We present Event Horizon Telescope (EHT) 1.3 mm measurements of the radio source located at the position of the supermassive black hole Sagittarius A* (Sgr A*), collected during the 2017 April 5-11 campaign. The observations were carried out with eight facilities at six locations across the globe. Novel calibration methods are employed to account for Sgr A*'s flux variability. The majority of the 1.3 mm emission arises from horizon scales, where intrinsic structural source variability is detected on timescales of minutes to hours. The effects of interstellar scattering on the image and its variability are found to be subdominant to intrinsic source structure. The calibrated visibility amplitudes, particularly the locations of the visibility minima, are broadly consistent with a blurred ring with a diameter of similar to 50 mu as, as determined in later works in this series. Contemporaneous multiwavelength monitoring of Sgr A* was performed at 22, 43, and 86 GHz and at near-infrared and X-ray wavelengths. Several X-ray flares from Sgr A* are detected by Chandra, one at low significance jointly with Swift on 2017 April 7 and the other at higher significance jointly with NuSTAR on 2017 April 11. The brighter April 11 flare is not observed simultaneously by the EHT but is followed by a significant increase in millimeter flux variability immediately after the X-ray outburst, indicating a likely connection in the emission physics near the event horizon. We compare Sgr A*'s broadband flux during the EHT campaign to its historical spectral energy distribution and find that both the quiescent emission and flare emission are consistent with its long-term behavior.
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| 2. |
- Zheng, Shao Fei, et al.
(författare)
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Fluid flow and heat transfer in a rectangular ribbed channel with a hierarchical design for turbine blade internal cooling
- 2022
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Ingår i: Applied Thermal Engineering. - : Elsevier BV. - 1359-4311. ; 217
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Tidskriftsartikel (refereegranskat)abstract
- For internal cooling of a turbine blade, various advanced rib turbulators can markedly contribute to the heat transfer enhancement while suffering a great increase in pressure loss. In those designs, ribs with the same configuration are periodically and evenly mounted on the channel wall. In this context, this work proposes a hierarchical design concept to optimize the rib arrangement with the desired reduction in pressure loss. In terms of the rib height, this new design concept is implemented to construct three new rib configurations. Based on an established turbulence model, three-dimensional (3D) numerical simulations are entirely adopted to verify the feasibility of the new configuration in a wide Reynolds number range. The numerical results demonstrate that the optimal configuration with a linearly decreasing rib height can greatly reduce the pressure loss with a slight heat transfer deterioration. The negligible reduction in the heat transfer performance results from the enhanced fluid impingement on the reattachment region because of the lowering effect of the mainstream, although small ribs weaken the fluid impingement. The marked pressure drop reduction comes from the combination of the lowering effect and small ribs which constrains the separation vortex behind ribs. Furthermore, the comparison of the overall thermal performance is carried out considering a wide range of the Reynolds number, pitch ratios, and aspect ratios. The optimal configuration can greatly enhance the overall thermal performance up to 138.3% for the factor (Nu/Nu0)/(f/f0) and up to 32.5% for the factor (Nu/Nu0)/(f/f0)1/3. Eliminating the entrance effect of developing flow, the increment in the overall thermal performance is considerably reduced but still keeps at a high level. Finally, it is significantly highlighted that as a simple but effective improvement, the hierarchical design concept presents great potential in developing high-performance internal cooling of turbine blades.
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| 3. |
- Zheng, Shao Fei, et al.
(författare)
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The condensation characteristics of individual droplets during dropwise condensation
- 2022
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Ingår i: International Communications in Heat and Mass Transfer. - : Elsevier BV. - 0735-1933. ; 131
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Tidskriftsartikel (refereegranskat)abstract
- Recently, nonwetting surfaces have attracted explosive attention in the community of dropwise condensation. Prediction models have been used to improve the fundamental understanding of dropwise condensation heat transfer. However, the multiscale heat transfer characteristics of individual droplets and the quantitative heat transfer evaluation of the droplet growth on different condensing surfaces are rarely carried out for dropwise condensation, which are focused on in this work. Based on the droplet heat transfer models, we consider three respective groups of nonwetting surfaces (hydrophobic surfaces, structured superhydrophobic surfaces, and slippery surfaces) in a pure vapor environment, as well as the presence of non-condensable gas (NCG). We first elucidate the dynamic roles that the thermal resistances have in the intrinsically multiscale droplets during condensation. The resulting heat transfer characteristics of droplets are understood simultaneously. We highlight that two critical sizes of the droplet significantly characterizes the condensation behaviors of droplets, and three regions are defined to characterize the dependence of the droplet size on the dominant thermal driving loss. Subsequently, the droplet size distribution is considered to further understand the role of dynamically growing droplets on the total thermal resistance. In the presence of NCG, over the whole size range of droplets, the dynamic roles of the thermal resistances and the heat transfer characteristics are significantly changed due to the resulting diffusion resistance.
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