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为了研究低温泵内的空化流动,考虑热力学效应,对现有Zwart空化模型进行了修正,并通过NACA0015水翼的空化实验验证了修正后模型的可行性。同时采用数值模拟的方法分析低温泵在输送液氮介质时所产生的空化流动现象,获得了低温泵的空化性能、叶片表面中间流线上的载荷分布情况和流道内的温度分布规律。研究结果对准确预测低温泵的空化性能,提高低温泵的抗空化能力具有意义。 相似文献
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氦气作为增压气体排出贮罐内液氢过程的CFD分析 总被引:1,自引:0,他引:1
《低温工程》2015,(4)
基于计算流体力学方法,数值模拟了用常温氦气作为增压气体压出贮罐内液氢过程的流动和传热传质特性。构建了基于二维轴对称的VOF多相流以及包含氢气和氦气组分流动的气相多组分数值模型,液氢界面相变传质基于Hertz-Knudsen方程计算。分析了排出过程贮罐内压力、温度、液位及液氢相变率随时间的变化,重点考察气相出现在贮罐出口时间,以及此时气相中氦气含量。发现刚开始增压时,高温氦气和低温氢气传热只发生在氦气进口附近,贮罐内压力增加较慢,液氢界面不存在蒸发现象。随着进入氦气增加,贮罐内气相温度逐渐形成分层,在一定时刻,液面上气体温度开始上升,触发沸腾蒸发,导致压力快速增加。由于贮罐出口液体外流导致的减压效应远小于气相空间的压力增速,贮罐压力急剧增加并超过氦气入口,部分低温气体混合物从入口倒流出贮罐,同时使氦气入口处温度降低。由于贮罐内压力增加,底部液氢出口流量随时间呈线性增加。计算结果揭示了液氢贮罐增压流出过程复杂的流动和传热传质特性,对低温液体的储运有实际工程指导意义。 相似文献
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为了研究泵内压降和水力损失耦合诱导泵内液氮空化,采用Zwart空化模型和RNG k-ε湍流模型,并使用CEL语言将饱和蒸气压随温度变化函数关系式导入CFX软件中进行求解,对不同流量下低温泵的空化特性曲线进行分析。研究结果表明,低温泵内压力、温度和空泡体积分数分布与空化的发展程度有关,由于水力损失的作用,小流量工况下,泵内会出现涡状流,从而对叶轮内空化产生影响。 相似文献
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搭建了一套液氮非稳态空化可视化实验装置,成功测量得到空化区前后的压力、温度和液体速度;使用高速摄像仪拍摄不同空化数下液氮在文氏管内的空化过程,图像处理得到空化区长度及脱落频率,并基于B因子理论推导得到低温空化体积分数经验计算公式。随着空化数的减小,空化区周期性脱落频率变小,空化区长度、最大温降和气相含量变大。基于空化区长度计算得到液氮空化的斯特劳哈数(St)平均值为0.378,其大于传统水空化的0.2—0.3区间。经分析,液氮较大的韦伯数和气液密度比使得空化区更易于断裂成小空化云团,从而更易于从壁面上脱落,这也和实验现象吻合。 相似文献
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《低温学》2018
Thermodynamic effects on cryogenic cavitating flow is important to the accuracy of numerical simulations mainly because cryogenic fluids are thermo-sensitive, and the vapour saturation pressure is strongly dependent on the local temperature. The present study analyses the thermal cavitating flows in liquid nitrogen around a 2D hydrofoil. Thermal effects were considered using the RNG k-ε turbulence model with a modified turbulent eddy viscosity and the mass transfer homogenous cavitation model coupled with energy equation. In the cavitation model process, the saturated vapour pressure is modified based on the Clausius-Clapron equation. The convection heat transfer approach is also considered to extend the Zwart-Gerber-Belamri model. The predicted pressure and temperature inside the cavity under cryogenic conditions show that the modified Zwart-Gerber-Belamri model is in agreement with the experimental data of Hord et al. in NASA, especially in the thermal field. The thermal effect significantly affects the cavitation dynamics during phase-change process, which could delay or suppress the occurrence and development of cavitation behaviour. Based on the modified Zwart-Gerber-Belamri model proposed in this paper, better prediction of the cryogenic cavitation is attainable. 相似文献
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Pure and 0.25 wt% MgO-doped alumina powder compacts were sintered using a radio-frequency induction-coupled argon plasma.
The effects of additive oxygen, hydrogen, nitrogen and water were investigated by injecting varying amounts of each gas during
sintering. The addition of diatomic gases or water vapour to argon gas during plasma sintering increased the temperature and
sintered density of the specimens. Water vapour showed the strongest effect, followed by hydrogen, nitrogen and then oxygen.
The presence of MgO dopant resulted in greater density and a lower sample temperature than those of the pure material. 相似文献
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Kjellsson L. Svensson L. E. Sundberg R. Dunlop G. L. 《Journal of Materials Science》1978,13(11):2441-2448
High temperature stress relief intergranular cavitation and subsequent room temperature embrittlement of an aluminium-bearing-brass has been studied metallographically. The behaviour of a cast susceptible to cavitation has been compared to one which does not exhibit intergranular cavitation during stress relief, and which is subsequently more ductile at room temperature. A number of micro-analytical techniques (SIMS and EDX) failed to reveal any difference in the grain boundary chemistry between a cast susceptible to intergranular cavitation and one which was not, but it is suspected that the combined action of dissolved gases (e.g. hydrogen) and trace element impurities plays a major role in cavitation. The cavities formed during stress relief were often polyhedral in shape and it is considered that this occurs by the diffusion of matter around the cavity surface to attain a lower energy surface configuration. Second phase particles were found to play only a minor role in the nucleation of cavities. Room temperature intergranular fracture surfaces of material, in which cavities had formed during stress relief, were interpreted in terms of high temperature cavity formation and coalescence combined with low temperature plastic void growth and interlinkage. 相似文献
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《低温学》2018
Large eddy simulation on unsteady cavitating flow of liquid hydrogen over a three-dimensional NACA0015 hydrofoil with the attack angle (α) of 6° are carried out to investigate the dynamic features of cavity with the existence of thermal effects. The numerical model considers the compressibility of both liquid and vapor phase, and is validated by comparing the results with the available experimental data. Special emphasis is put on analyzing the frequency characteristics of cavitation cloud. Strouhal number (St) is plotted against σ/2α (σ is cavitation number), and the water cavitation data reported by Andrt et al. are also used as a reference. It is found that the St number for LH2 cavitation is much smaller than the water, in which the thermal effects are generally not considered, at the same σ/2α value when it is greater than about 2.0, while it returns to the same level as water when σ/2α decreases to below 2.0. The reason is primarily ascribed to the thermal effects, and the detailed explanations are given based on the recognitions that the shedding mechanism of cavitation clouds is predominated by the combined action of the vortex flow and thermal effects. While, when σ/2α decreases to a critical value, the relative effect of the thermal effects on the cavitation dynamics is greatly weakened compared with the mechanism due to the vortex flow, like those in isothermal cavitation flow in traditional fluids. The results provide a deeper understanding of the cryogenic fluid cavitation flow. 相似文献
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Cavitation is the formation of vapor bubbles within a liquid where flow dynamics cause the local static pressure to drop below the vapor pressure. This paper presents the steady computational fluid dynamic (CFD) results of cavitation in liquid nitrogen flow through hydrofoils and ogives with so-called “full cavitation model”. The model is reexamined to assess the performance prediction from the standpoint of cryogenic fluids with the assumption of thermal equilibrium between vapor phase and liquid phase. The fluid thermodynamic properties are specified along the saturation line using the “Gaspak 3.2” databank. The thermal effects and accompanying property variations due to phase change are modeled rigorously. The thermodynamic cavitation framework is validated against experimental data of NASA hydrofoil and ogive. The global sensibility of the cavitation solution with respect to the cavitation model coefficients and the free-stream velocity is investigated in detail and the choking phenomenon is reported with high Mach number. The full cavitation model with the default coefficients is applicable for cavitation prediction in liquid nitrogen, taking into account of the thermodynamic effects. 相似文献
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研究了20 kHz的圆柱形料腔中超声空化效应的形成及其空间分布特性。应用柱贝塞尔函数,推导获取了柱形声场内超声传播的声能密度的分布,并采用有限元方法进行仿真分析。针对频率为20 kHz的功率超声实验,结合声学测量方法和鲁米诺声致化学发光方法,对理论分析结果进行了验证对照。结果表明:料腔半径R=50 mm,20 kHz谐振液位高度H=90 mm时,若功放电流<40 mA,超声空化效应出现在变幅杆端部区域;若40 mA≤功放电流≤80 mA,空化效应显著增强,空化效应的空间分布与场内声压分布一致,空化效应受声模态影响,形成远场空化效应的分布特性;若功放电流>80 mA,受非线性因素影响,谐振液位时,空化效应在声流作用下呈柱形拖尾状分布,并在底部壁面边界形成平铺状分布;非谐振液位高度等于75 mm时,超声空化效应随功率增加仅在变幅杆端部区域出现,且呈现局域空化分布特性。 相似文献
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由于夹层真空度下降、绝热保温材料珠光砂下沉,造成4个液氮贮罐的冷损大幅增加,通过对夹层补充珠光砂、重新抽真空,有效提高了液氮贮罐的绝热保冷效果,降低冷损。 相似文献
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《低温学》2018
The cavitating flow of cryogenic liquid through a spray nozzle is influenced by many factors, such as unique thermophysical properties of cryogenic liquid, the inflow temperature and the complicated geometrical structure of the spray nozzle. The geometrical parameters of liquid nitrogen spray nozzles have a profound impact on cavitating flow which in turn affects spray atomization characteristics and cooling performance. In present study, CFD simulations are performed to investigate influence of the nozzle geometry on the liquid nitrogen cavitating flow. The mixture model is used to describe the liquid-vapor two phase flow, and both the cavitation and evaporation are considered for the phase change. The predictions of mass flow of liquid nitrogen spray are validated against experimental results. The effects of geometric parameters, including the outlet orifice diameter and the length of nozzle, the inlet edge angle of orifice, the inlet corner radius of orifice, the orifice shape and different positions of swirl vanes, are investigated under a wide range of pressure difference and inflow temperature. The results show that the effects of geometric parameters on cavitating flow show different trends under subcooled conditions compared with saturated temperature conditions. The flow characteristics are more affected by the changes of the inlet edge angle, the inlet corner radius, and the orifice shape. The insert of swirl vanes has an effect on the distribution of the cavitated vapor within the orifice, but it has little influence on flow characteristics. The results could enrich our knowledge of liquid nitrogen cavitating flow in spray nozzles of various shapes. 相似文献