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51.
Convective heat transfer coefficients were measured experimentally for a tube immersed vertically in a circulating fluidized bed. Circulating fluidized beds operate in the dilute transport regime of two-phase (solid/gas) flow. The dominant mechanism for heat transfer to surfaces is particle-induced convection. In this study, experiments were carried out in a circulating fluidized bed of15 cm diameter and 11 m height. An instrumented tube of 9.5 mm diameter and 1.3 m length was placed vertically at the centeriine of the fluidized bed to measure convective heat transfer coefficients at several different elevations in the bed. Three types of particles, with mean diameters ranging from 68 to 2S1 urn, were used in the experiments at superficial gas velocities in the range of 1.3 to 8.2 m/s. Results showed that the convective heat transfer coefficients with solid/gas two-phase circulation were two to three times greater than those for single-phase gas convection at the same velocity. For a given gas velocity, the coefficients increased with increasing solid mass flux, but decreased with elevation. It was demonstrated that the heat transfer coefficients for the immersed tube and for the bed wall could be correlated with different functional dependence on the two-phase suspension density. 相似文献
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The particle fluctuation velocities of a horizontal self-excited gas–solid two-phase pipe flow with soft fins near MPD (minimum pressure drop) air velocity are first measured by high-speed PIV in the acceleration and fully-developed regimes. Then orthogonal wavelet multi-resolution analysis and power spectrum are used to reveal multi-scale characteristics of particle fluctuation velocity. It is observed that the pronounced peaks of the spectra of axial and vertical fluctuation velocities appear in the range of low frequency near the bottom of pipe. These peaks of spectra become larger and their frequencies decrease by using fins. In the range of low frequencies (3–25 Hz), the wavelet components of the fluctuating energy of axial particle velocity make the main contribution accounting for 87% and 93% respectively for non-fin and using fins near the bottom of pipe. In the range of relatively high frequency (50–400 Hz), however, the wavelet components of using fins, accounting for about 49%, become smaller than that of non-fin, accounting for about 72%, in the suspension flow regime near the top of pipe. The skewness factor of axial particle fluctuation velocity indicates that the wavelet components follow the Gaussian probability distribution as the central frequency decreases. 相似文献
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在药型罩上粘附铝隔板是一种新的形成尾翼型爆炸成型弹丸(Explosively Formed Projectile,EFP)的实验方法。通过实验和数值模拟对该方法进行了分析,探讨了形成尾翼的机理。利用有限元分析软件LS-DYNA,对大锥角型、球缺型和弧锥结合型3种不同药型罩在采用新方法时形成的尾翼型EFP进行了三维数值模拟分析,并将模拟结果与实验结果进行对比。研究结果表明:3种药型罩均能形成比较明显的尾翼,数值计算结果与实验结果较吻合。该研究为设计尾翼型EFP提供了一种新的参考方法。 相似文献
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提出一种新的针肋结构,采用直径为0.25 mm的圆柱型针肋,在一定的错排和顺排相结合的优化布置方式下,能获得较高的换热效率.在三维空间上的对流换热模拟表明,铜制的针肋结构的换热系数要高出百叶窗翅片24%~34%,而流阻仅高出10%~16%.总换热量取决于翅片材料的导热性能,导热系数越高,针肋的强化作用约明显.本文提出的针肋翅片结构可以用来制造紧凑性更高的换热器. 相似文献
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The incipient separation induced by the shock wave/turbulent boundary layer interaction at the sharp fin is the subject of the present study. Existing theories for the prediction of incipient separation, such as those put forward by McCabe (1966) and Dou and Deng (1992), can thus far only predic the direction of surface streamline and tend to overpredict the incipient separation condition based on the Stanbrook’s criterion. In this paper, the incipient separation is first predicted with Dou and Deng (1992)’s theory and then compared with Lu and Settles’ (1990) experimental data. The physical mechanism of the incipient separation as induced by the shock wave/turbulent boundary layer interactions at sharp fin is explained via surface flow pattern analysis. Furthermore, the reason for the observed discrepancy between the predicted and experimental incipient separation conditions is clarified. It is found that when the wall-limiting streamlines behind the shock wave becomes aligned with one ray from the virtual origin as the strength of the shock wave increases, the incipient separation line is formed at which the wall-limiting streamline becomes perpendicular to the local pressure gradient. The formation of this incipient separation line is the beginning of the separation process. The effects of Reynolds number and Mach number on incipient separation are also discussed. Finally, a correlation for the correction of the incipient separation angle as predicted by the theory is also given.
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59.
Mohammad Ghalambaz Hassan Shirivand Kasra Ayoubi Ayoubloo S.A.M. Mehryan Obai Younis Pouyan Talebizadehsardari Wahiba Yaïci 《Molecules (Basel, Switzerland)》2021,26(6)
A latent heat thermal energy storage (LHTES) unit can store a notable amount of heat in a compact volume. However, the charging time could be tediously long due to weak heat transfer. Thus, an improvement of heat transfer and a reduction in charging time is an essential task. The present research aims to improve the thermal charging of a conical shell-tube LHTES unit by optimizing the shell-shape and fin-inclination angle in the presence of nanoadditives. The governing equations for the natural convection heat transfer and phase change heat transfer are written as partial differential equations. The finite element method is applied to solve the equations numerically. The Taguchi optimization approach is then invoked to optimize the fin-inclination angle, shell aspect ratio, and the type and volume fraction of nanoparticles. The results showed that the shell-aspect ratio and fin inclination angle are the most important design parameters influencing the charging time. The charging time could be changed by 40% by variation of design parameters. Interestingly a conical shell with a small radius at the bottom and a large radius at the top (small aspect ratio) is the best shell design. However, a too-small aspect ratio could entrap the liquid-PCM between fins and increase the charging time. An optimum volume fraction of 4% is found for nanoparticle concentration. 相似文献
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通过三维层流数值模拟,与平片计算结果对比,设计了适用于低流速和Re数(迎面风速1-3 m/s,对应Re数 901-2702)下的空调蒸发器用管翅式换热器的开缝翅片形式。研究表明:在上述迎面风速和Re数范围内,开缝翅片性能曲线和平片性能曲线不可避免地有一个交叉点,对应Re数称为转折Re数,在交叉点前的速度和Re数范围内,平片换热器的综合性能优于开缝翅片,而在高于转折Re数后,随着迎面风速的增大,开缝翅片的综合换热性能将越来越优于平片;依据“前疏后密”原则适当减少开缝翅片换热器开缝的条数,可以有效降低转折Re数,并显著改善低流速和Re数下换热器的综合性能。另外,再次验证了场协同理论-温度场和速度场的协同性与换热器换热量间的必然联系。 相似文献