共查询到18条相似文献,搜索用时 187 毫秒
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为验证和优化涔天河水库扩建工程#1泄洪洞“龙落尾”布置方案,首先建立#1泄洪洞泄流数值计算模型,然后根据室内试验翼型掺气坎后的空腔回水结果反馈数值计算模型的合理性,进而基于水力安全和经济综合原则定义了掺气坎体型的优化目标函数,最后利用反馈数值计算模型研究#1泄洪洞掺气设施优化布置。研究表明,采用结构化矩形网格的FAVOR方法、VOF方法、RNGκ-ε湍流模型及精细网格的数值模型进行模拟,其计算结果与室内试验结果在空腔长度和空腔形态上的规律较为接近;#1泄洪洞翼型坎挑坎角度α和坎后底坡坡率θ最优值分别为10%、20%。 相似文献
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在掺气减蚀研究中,掺气设施的掺气保护长度是检验掺气减蚀效果的重要指标,而水流掺气浓度及其分布将决定掺气保护长度。通过反弧段泄槽模型试验,研究了不同流量、不同挑坎高度时掺气浓度与掺气保护长度的变化规律。结果表明,掺气坎后水流掺气浓度沿程分布规律为先增大后减小,而在断面上的分布规律为由水流表面到水流底部逐渐减小;泄槽反弧段与泄槽直段及不同水深处掺气浓度的沿程衰减率不同;在其他条件不变时,掺气浓度与流量大小成反比,与挑坎高度成正比;掺气有效保护长度随流量增大而减小,随掺气坎高的增大而增大。 相似文献
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高速明渠流建筑物中常通过设置掺气坎来避免可能发生的空化空蚀破坏,但当明渠的坡度较缓时,掺气空腔容易回水,从而影响掺气效果,对此提出在小底坡明渠有压进口处设置一个楔形体,将水流分成上、下两股进入明渠。通过模型试验研究这种新型掺气设施的空腔形态、掺气特性。试验结果表明,当有压出口处的水流速度达到2.7m/s时,在两股水流之间就能形成一个稳定的掺气空腔,此时对应的水流弗劳德数约为2.0,空腔内并无回水,掺气效果良好;随着出口流速的增加,掺气空腔不断加长,掺气量也不断增加。试验得到了不同流量下掺气空腔形态的变化规律及沿程断面掺气浓度分布规律,并拟合了计算楔形掺气设施空腔长度的经验公式。研究成果可为优化掺气设施设计和工程应用提供参考。 相似文献
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基于Realizableκ-ε紊流模型和VOF方法,以博贡大型龙抬头泄洪洞直线段为例,对挑坎后加矩型、U型和V型掺气槽的水力特性进行了三维数值模拟.结果表明,U型掺气槽水流冲击底板的压强最大,矩型和V型掺气槽对底板的冲击压强相当,矩形掺气槽形成的侧空腔最长.三种掺气槽后水流流态均平顺,水流强烈紊动形成的涡旋卷吸将空气卷吸入水流中. 相似文献
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针对岸边溢洪道从侧面挑入与其平行布置的已有消力池来消能的布置形式,采用数值模拟方法研究了高程和流量对挑坎(а=45°)及消力池水力特性的影响。结果表明,随着泄流量逐渐增大,挑坎内最大水深、最大压力及消力池最大压力均逐渐增大,挑流水舌入水长度与纵向入水长度均逐渐增大;随着挑坎与消力池高差的减小,挑坎最大水深和最大压力均逐渐增大,而消力池最大压力变化较小。拟合得挑坎最大水深和最大压力的定量计算公式,从而为该型岸边挑坎的工程设计提供了参考。 相似文献
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《International Journal of Heat and Mass Transfer》2007,50(17-18):3638-3648
An experimental investigation on pulsating impinging jets has been performed. The effect of the pulsation on the flow structure and heat transfer have been investigated. Frequency and amplitude were varied separately and the effect of each parameter was examined for different Reynolds numbers and nozzle-to-plate distances.The jet was found to become broader and the core jet length smaller with the pulsation. The reason for this behavior is that pulsation enhanced entrainment of air into the jet, which results in a change of mean velocity of the jet. Nevertheless, the behavior at lower frequencies (up to 140 Hz) is still quasisteady. This means that the amplitude of the pulsation behaves similar to the mean velocity of the jet, that the shapes of the velocity profiles are comparable to steady jets and that the jet behavior is independent of frequency.At moderate frequencies heat transfer is only affected by the pulsation when nozzle-to-plate distance and amplitude are large enough. At small nozzle-to-plate distances enhanced entrainment has no influence and no difference between steady and pulsating jets can be recognized. At large nozzle-to-plate distances entrainment increases and jet velocity reduces. This yields a reduction of heat transfer in the stagnation point of up to 50%.But besides of this effect of enhanced entrainment a theoretical limit could be determined, above which the jet is not anymore quasisteady. Above Sr = 0.2 heat transfer is affected by the pulsation also at small nozzle-to-plate distances. At this frequency boundary layer is also affected by the pulsation. This yields increased heat transfer coefficients at the stagnation point. For larger nozzle-to-plate spacings this effect is superposed by the reduction of heat transfer due to increased entrainment, resulting in a strong decrease of heat transfer coefficient. 相似文献
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《International Journal of Heat and Mass Transfer》2006,49(19-20):3417-3428
Gas entrainment induced by a spray jet can be significantly affected by the spray evaporation rate. In this study, we have directly measured the air entrainment induced by a liquid nitrogen spray jet into an unbounded and stagnant room air. It is realized that the air entrainment is proportional to the axial gradient of oxygen mass flow in a pure nitrogen spray jet. Hence, the air entrainment can be determined by a combined measurement of local cross-sectional distributions of oxygen concentration, gas temperature and gas velocity along the jet path. These measurements are directly obtained using an in situ oxygen concentration analyzer, a thermocouple system, and a Laser Doppler Velocimeter. In order to evaluate the effect of evaporation rate, direct measurements and numerical simulations of the air entrainment by a cold gaseous jet of nitrogen (at a temperature slightly above that of liquid nitrogen) into room air are also performed. Measurements of the entrainment rate and flow similarity of the gaseous jets without droplets compared very well against those from the single-phase jet theories and numerical simulation, which validates our experimental approach and analysis method. Our experimental results indicate rough flow similarities exist in evaporating spray jets with round nozzles. Although the air entrainment by the liquid nitrogen spray is found significantly increased, as compared to that by the cold gaseous jet of nitrogen from the same nozzle and at the same jetting velocity, the increased ratio is far less than the equivalent momentum ratio of the liquid nitrogen spray to the gas nitrogen jet. This experimental finding suggests that the evaporation of spray markedly weakens the gas entrainment. In this study, a parametric model is also developed to provide a theoretical basis of the data analysis for the cross-section averaged spray evaporation rate within the spray jet region. 相似文献
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Kai Cheng 《International Journal of Heat and Mass Transfer》2004,47(23):5139-5148
The entrainment of the ambient air into a turbulent argon plasma jet is studied numerically using a turbulence-enhanced combined-diffusion-coefficient method. Namely, the Navier-Stokes equations and two-equation turbulence model coupled with the turbulence-enhanced combined-diffusion-coefficient approach are employed to predict the turbulent plasma jet characteristics including the evolution of air mole fraction along the plasma jet in air surroundings. Although complicated gas species always exist in the plasma jet due to rather high gas temperatures being involved, it is shown that the entrainment of ambient air into the turbulent argon plasma jet can still be treated simply by the combined turbulent and molecular diffusion between only two different gases (argon and air). Good agreement between the predicted results with corresponding experimental data reported by Fincke et al. [Int. J. Heat Mass Transfer 46 (22) (2003) 4201] demonstrates the applicability of the present modeling approach. 相似文献
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M. Theron 《Combustion and Flame》2006,145(4):688-702
In this paper, we explore the effects of heat release on mixing and flow structure in a high-speed subsonic turbulent H2 jet in an air coflow. Heat release effects are determined from the comparison of nonreacting and reacting jet behavior, boundary conditions being identical in both cases. Experiments are performed in a wind tunnel specifically designed for this purpose. Planar laser induced fluorescence on OH radicals and on acetone (seeded in the hydrogen jet) are used to characterize the cartography of scalars, and laser Doppler velocimetry is used to characterize velocity profiles in the far field of the H2 jet. Results show significant effects of heat release on mixing and flow structure, indicating an overall reduction of mixing and entrainment in the reacting jet compared to the nonreacting jet. First, a change is observed in the orientation of coherent structures originating from Kelvin-Helmholtz type instabilities, and responsible for air entrainment within the jet, which appear “flatter” in the jet flame. Then, the flame length is increased over what would be predicted from the intersection of the mean stoichiometric contour with the centerline of the nonreacting jet. And finally, the longitudinal average velocity decrease along the jet axis is quicker in the nonreacting jet, and nondimensional transverse velocity fluctuations are about half as high in the reacting jet as in the nonreacting jet, indicating a reduction of the turbulence intensity of the flow in this direction in the jet flame. 相似文献
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本文提出了一个适合在准维模型中使用的直喷式柴油机喷雾碰壁与空气卷吸的数学模型。采用紊流射流理论推出了新的壁面射流计算公式,并在正确描述喷注贯穿的基础上建立了有涡流作用的空气卷吸模型。该研究结果用于准维模型的理论计算与实测结果吻合很好,证明了模型的实用性。 相似文献
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In this paper, the similarity form is developed of a hydrogen–air buoyant jet ‘forced plume’ resulting from an unignited small-scale hydrogen leakage in the air. As the domain temperature is assumed to be constant and therefore the density of the mixture is a function of the concentration only, the hydrogen–air mixture is assumed to be of a linear mixing type. The rate of entrainment is assumed to be a function of the plume centerline velocity and the ratio of the mean plume and ambient densities. The local rate of entrainment may be considered to be consisted of two components; one is the component of entrainment due to jet momentum while the other is the component of entrainment due to buoyancy. Consequently, the entrainment coefficient is taken as a variable not constant. The density will be considered as a variable in all terms of the equations of continuity, momentum and concentration. The integral models of the mass, momentum and concentration fluxes are obtained. Non-dimensional transformations known as similarity transformations and used to transform the integral model to a set of ordinary differential equations called similarity equations which are solved numerically. Under various values of Froude number, the hydrogen centerline mass fraction, jet width and centerline velocity are introduced and compared with published experimental results for a slow leak vertical hydrogen jet. 相似文献
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水舌挑距是泄洪消能最基本参数之一,消能形态、冲击区域、冲坑深度、水流衔接均与其密切相关。为研究高海拔地区气压降低对挑射流特性的影响,采用减压模型试验的方法研究了环境气压对水舌挑距的影响程度。结果表明,气压与水舌挑距呈良好的线性关系,气压越低,因空气阻力越小,水舌挑距越远;气压每下降15kPa,对水舌内缘与外缘的最大影响程度分别为1.67%、1.48%;低气压环境下水舌空中扩散与掺气减弱,水舌厚度变薄,水流呈集中趋势。基于气压与空气阻力影响系数关系,修正规范水舌挑距计算公式,为高海拔地区挑射流设计提供参考。 相似文献