共查询到19条相似文献,搜索用时 62 毫秒
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本文基于不同流速下变传质强度因子的Lee模型,对低热通量下上升管内流动沸腾进行了数值模拟。通过分析气相分布、局部气泡行为解释了沸腾换热特性和局部传热恶化。结果表明:低热通量下,不同流速的平均气体体积分数沿管长线性分布,沿径向呈双峰状分布,峰值在壁面附近;高流速时,近壁处气体体积分数的增长速率最大。换热性能受流速影响显著,流速越大,换热系数越大;三种流速下换热系数最小值对应的气体体积分数相同,气相分布不同是换热特性产生差异的直接原因。低中流速下,壁温局部升高位置与换热系数骤降位置是一致的,表明壁面附近局部气泡附着会造成壁温的局部升高,导致传热恶化。 相似文献
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本文利用微液层模型对过冷沸腾的临界热流密度(CHF)进行了理论预测。过冷沸腾的强化换热主要是通过单个气泡的形成和消失造成的对流换热强化而引起的。对等热流面,CHF在高过冷区趋近于常数;对等温面,CHF随过冷度的增加而增加。过冷度增加时,蒸发换热量减少,总热流密度主要由蒸发区外的导热引起。 相似文献
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An experimental investigation was conducted to explore the characteristics of microscopic boiling induced by firing a microsecond pulsed laser beam on a thin platinum (Pt) film that immerged in the liquid nitrogen (LN2) cryostat. High-speed photography aided by a high-voltage lighting system was employed to visually observe the bubble formation and the dynamical boiling process of LN2. A rapid transient temperature-measuring system was designed to record the temperature evolution of the heating surface. Explosive boiling, characterized by bubble cluster, was observed within LN2 at the early stage of laser heating, and conventional boiling followed after a certain time. The transition time, therefore, was introduced for separating these two different boiling modes. The temperature of Pt film rose sharply to its maximum during laser pulse, with a very high rising rate of about 107 K/s, and then dropped rapidly after laser irradiation. A model of bubble cluster was proposed to describe the explosive boiling heat transfer, and the latent heat released by bubble collapse in explosive boiling was explored as an important mechanism considerably influencing the boiling heat transfer. 相似文献
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In this study, condensation of pure refrigerant R134a vapor inside a vertical 18° helical microfin tube was experimentally investigated. Tests were performed at saturation pressure of 5.7–5.9 bar with mass fluxes of 20–100 kg/m2s and heat fluxes of 1.7–5.3 kW/m2. The effects of mass flux and the temperature difference between the refrigerant and tube wall (ΔT) on the heat transfer performance were analyzed throughout experimental data. For experiments in which ΔT is more than 2.5°C, the average condensation Nusselt number showed a tendency to be independent from ΔT. Heat transfer enhancement ratio was found to be 1.59–1.71, which is always higher than the heat transfer area enhancement factor (1.55). Fins always act as a turbulence promoter in the given experimental data range. Finally, the most widely used heat transfer coefficient correlations for condensation inside microfin tubes were analyzed through the experimental data. Best fit was obtained with Yu and Koyama's correlation with an absolute mean deviation of 17% and Kedzierski and Goncalves's correlation with an absolute mean deviation of 19%. 相似文献
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颗粒有序堆积多孔介质对流换热实验研究 总被引:4,自引:0,他引:4
本文采用"瞬态单吹反问题研究方法"对颗粒有序堆积多孔介质内的强制对流换热进行了实验研究。详细研究了颗粒堆积方式变化对多孔介质内对流换热的影响,并对均匀与非均匀颗粒堆积多孔介质内的对流换热特性进行了对比分析。研究表明:通过对颗粒进行合理有序堆积,可以使相应多孔介质内的压降显著降低,其综合换热效率明显提高;通过拟合获得了颗粒有序堆积多孔介质内的宏观流动换热实验关联式,其形式与传统经验公式(Ergun公式和Wakao公式)一致,但部分模型参数值远低于传统经验公式。 相似文献
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"水沸腾"实验是北师大版八年级上册教材第一章"物态及其变化"第四节"汽化和液化"中的一个实验."水沸腾"是日常生活中常见的一种现象,但学生对其具体特征不一定清楚.这又是八年级刚开始学习物理的一个实验,学生对实验很感兴趣,因此成功做好这个实验非常重要. 相似文献
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A simplified arc model based on the integral method is used to study the arc behavior in a supersonic nozzle. Emphasis is placed on the energy balance of the overall arc, which extends to the arc thennal boundary. Similarity rules for aerodynamic and electrical quantities are established, and a quantitative definition of current zero period is given. Computations have been done for two nozzle geometries. The nozzle geometry plays the role of shaping the arc, thereby affecting the axial electric field distribution. Performance curves in terms of the critical rate of rise of recovery voltage (rrrv)c and di/dt at current zero are established. It has been found that (rrrv)c can be seriously affected by the distortion of the current waveform near current zero due to arc-circuit interaction. When experimentally measured current waveform is used as an input, a good quantitative agreement is obtained for the Liverpool orifice arc [1] between theory and experimental results. A satisfactory agreement has also been achieved for the axial electric field distribution without adding a turbulence term into the energy equation. The limitations of the present arc model is also discussed in detail. 相似文献
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A laser model is formulated in terms of quantum harmonic oscillators. Emitters in the low lasing states are usual harmonic oscillators, and emitters in the upper states are inverted harmonic oscillators. Diffusion coefficients, consistent with the model and necessary for solving quantum nonlinear laser equations analytically, are found. Photon number fluctuations of the lasing mode and fluctuations of the population of the lasing states are calculated. Collective Rabi splitting peaks are predicted in the intensity fluctuation spectra of the superradiant lasers. Population fluctuation mechanisms in superradiant lasers and lasers without superradiance are discussed and compared with each other. 相似文献