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对高粘度液体在正弦形波纹壁面上的自由降落和蒸发建立了分析模型。对控制微分方程及边界条件作无量纲处理,引入流函数,采用摄动展开得到了0级近似和一级近似的微分方程组,讨论了液膜的流动和传热特性与壁面之间的关系。 相似文献
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Yu. Ya. Trifonov 《Thermophysics and Aeromechanics》2014,21(3):319-336
The wavy downflow of a viscous liquid film in the presence of the turbulent gas flow was analyzed theoretically. Two-dimensional stationary running waves are calculated in a wide range of Reynolds numbers of liquid and gas. Hydrodynamics of liquid is calculated based on complete Navier-Stokes equations. The wave interface surface is considered as a small perturbation and equations in gas are linearized near the main turbulent flow. Different optimal downflow regimes are determined, and the main wave characteristics are compared in detail with and without the co- and counter-current gas flows. It is shown that at high velocities of the co-current gas flow, the calculated waves correspond to ripples observed in experiments. 相似文献
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对饱和蒸发状态下的低雷诺数的液膜在小波幅正弦型波纹壁面上的自由降落进行了理论分析.通过无量纲化、引入流函数、采用摄动展开对数学模型进行处理,得到了这种情况下液膜流动的分析模型,计入了饱和蒸发压力的影响,得到了近似分析解.讨论了壁面波纹的波幅、波数、液膜表面张力和蒸发压力对液膜波动的影响. 相似文献
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A technique far measuring the thickness of liquid films was developed and successfully tested. The feasibility of this technique was demonstrated in stagnant liquid films as well as in liquid jets. A procedure for in-situ calibration of the thickness probe was developed, allowing the adaptation of the probe to measurements of wavy liquid films. The thickness probe was constructed from a platinum-rhodium wire that was stretched across the film. A constant DC current was supplied through the probe wire, and film thickness was determined from variations in the probe voltage drop resulting from the large differences in the electrical resistances of the wetted and unwetted segments of the wire. Unlike electrical admittance thickness probes, the new probe did not require dissolving an electrolyte in the liquid, making the new probe well suited to studies involving sensible heating of a film of pure dielectric liquid that is in duvet contact with a current-carrying wall. Also presented is a composite probe that facilitated simultaneous measurements of temperature profile across a wavy liquid film and film thickness. Experimental results demonstrate a strong influence of wariness on liquid temperature in a film of deionized water falling freely on the outside wall of a vertical, electrically heated tube for film Reynolds numbers smaller than 10,000. 相似文献
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Spray impingement wall film breakup by wave entrainment 总被引:1,自引:0,他引:1
Xuesong Li Hujie Pan Xue Dong David Hung Min Xu 《Proceedings of the Combustion Institute》2019,37(3):3287-3294
Fuel spray impingement on engine wall and piston in the spark-ignition direct-injection (SIDI) setting has been considered a major concern in the aspect of engine emission and combustion efficiency. Excess wall film will result in deterioration of engine friction, incomplete combustion, and substantial cycle-to-cycle variations. These effects are more pronounced during engine cold-start process. Therefore, the formation of wall film on engine wall/piston and the dynamic process of the wall film interacting with impinging spray and spray-induced gas flow are of great significance for reducing wall film mass. However, the dynamic process of wall film was not investigated thoroughly in existing literatures. This work will present a high-speed, simultaneous measurement of a single-hole spray structure, as well as wall film geometry and thickness, via Mie scattering and volumetric laser-induced fluorescence, respectively. Quantitative film thickness measurement was achieved via fluorescence intensity signal calibration with a known, wedge-shape liquid film apparatus. Remarkable wall film droplet entrainment at the leading edge of the liquid film waves was revealed in the measurement, which has not been adequately depicted or analyzed in existing spray impingement studies. A considerable amount of liquid droplets detaches from the liquid film via liquid film fingering, during which process the quantity of liquid mass on the wall is decreased. Quantitative analysis of such phenomenon is performed and we estimated that a liquid mass equivalent to 30–40% of the residual liquid film mass is detached from the liquid film via wave entrainment. Furthermore, through the comparative study of the side view of the spray and the liquid film caused by spray impingement, it is shown that non-uniform spray structure is likely the cause of liquid film wavy motions. These observations suggest that wave entrainment should be considered by numerical models and experimental designs to accurately predict spray impingement phenomenon. 相似文献
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1引言降膜式蒸发器由于具有很高的换热强度和易操作性,在化工、食品、制冷和海水淡化等工业中得到了广泛的应用。对于垂直降膜的流动和换热特性,前人已经进行了很多的研究[‘-’]。但是绝大部分的研究工作都局限于单侧液膜的传热与流动问题,缺少对实际工程上采用的降膜蒸发器在同时考虑双侧传热时的实验结果。同时,大部分实验研究中采用恒热流的电加热方式,而对于工程上常用的蒸汽加热的恒壁温情况实验结果较少。此外,管壁材料对蒸发器换热性能的影响也需要进一步的研究。本文采用模拟工程上的竖直管降膜式蒸发实验装置,对不锈钢… 相似文献
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采用正交实验方法考察了具有不同结构参数的三维周期波纹流道中的流体性能,并采用Webb评价方法对其进行性能评价。比较了不同波纹宽度的波纹流道的阻力因子ef、传热因子eNu和能效因子η的值,结果表明三者都随Re的增大而增大,波纹宽度最小时能效因子η最大。流体在波纹流道中垂直于主流方向的横截面上产生二次流,随着Re增大,二次流增强,阻力增大,温度边界层减薄,温度等值线分布变得不均匀,传热增强。采用拉格朗日粒子跟踪技术分析了不同Re下,流体粒子在波纹流道内的运动轨迹,绘制了不同周期出口流体粒子的庞加莱截面图,结果表明流体粒子在波纹流道中被反复拉伸和折叠,增加了流体粒子的接触面积,提高混合效率,强化了传热。 相似文献
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In the present contribution, a numerical treatment is provided to describe unsteady nanofluid flow near a vertical heated wavy surface. A memorable feature of the present work is the investigation of nanofluid flow associated with thermal radiation that acts as a catalyst for heat transfer rates. Likewise, the effectiveness of variable viscosity is examined as it controls fluid flow as well as heat transfer. It is necessary to study heat and mass transfer for complex geometries because predicting heat and mass transfer for irregular surfaces is a topic of fundamental importance, and irregular surfaces frequently appear in many applications, such as flat-plate solar collectors and flat-plate condensers in refrigerators. A simple coordinate transformation from the wavy surface into a flat one is employed. The non-dimensional boundary layer equations that governing both heat transfer and nanofluid flow phenomena along the wavy surface are solved via a powerful numerical approach called the implicit Chebyshev pseudospectral (ICPS) method with Mathematica code. A comparison graph of the current numerical computation and the published data shows a perfect match. Figures depict the effect of various physical parameters on nanofluid velocities, temperature, salt concentration, nanoparticle concentration, skin friction, Sherwood, nanoparticle Sherwood, and Nusselt numbers. According to the numerical results, increasing the variable viscosity parameter value causes a drop in the local skin friction coefficient value and an increase in the steady-state axial nanofluid velocity profile near the wavy surface. Furthermore, as heat radiation is increased, the local Nusselt number decreases but the nanoparticle Sherwood number increases. 相似文献
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气-液两相流设备的性能受限于临界热流密度,开展流动微液膜动力学特性及其稳定性的相关研究是深入理解沸腾危机及临界热流密度机理的关键。采用光学玻璃制成的矩形通道作为实验段,使用微流量齿轮泵驱动去离子水,使其在实验通道入口处与在其上部流动的压缩空气接触形成同向流动的分层流。利用共轭光学探测器对流动微液膜的厚度进行了测量,利用高速摄像机对气-液两相分层流波动特性进行了可视化观测。研究表明,在绝热情况下,当液速一定时,液膜的平均厚度随着气速增加而减小,当气速增加到某一阈值时会导致液膜破裂。 相似文献
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Flow characteristics of a liquid film flowing over a smooth surface and structured surface with the Reynolds number range from 10 to 1121 are studied. The mixture of R21 and R114 refrigerants is used as the test liquid. The 3D transient simulations are taken to capture the liquid film’s dynamic characteristics and spatial distribution. Effects of the inlet dimension, inlet flow rates, surface tension, and surface structuring on the wettability, average velocity, and film thickness are studied systematically. The obtained results show that surface tension is essential for an accurate simulation, while inlet width has no effect on the liquid film parameters in the steady-state flow regime. For low flow rates, wetting area and film thickness both are small, and a suggested range of Reynolds number is chosen to simulate further heat transfer in order to balance the film thickness and dry spots generation. It is shown that a ripple surface structure hinders the liquid film movement, reflected in a lower velocity and a larger film thickness compared to the smooth surface. Lateral movement of a liquid film can also be observed at the structured surface. 相似文献
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Numerical investigations on thermo-hydraulic performance and mechanisms of flow and heat transfer in a square channel heat exchanger inserted with right triangular wavy surfaces are examined. The influence of the flow attack angles (30°, 45° and 60°) is investigated for laminar flow (Re = 100–2000). The configurations of the right triangular wavy surfaces are varied as inclined and V-shaped wavy surfaces (the pointing of V-tip with downstream and upstream called “V-downstream” and “V-upstream”, respectively). The insertions of the wavy surfaces in the channel heat exchanger are divided into two types: middle and diagonal insertions. The computational results reveal that the maximum thermal enhancement factor, TEF, is around 2.31 for the 30° V-downstream wavy surface with diagonal insertion at Re = 2000. 相似文献
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毛细管内薄液膜轮廓和传热特性研究 总被引:2,自引:0,他引:2
本文认为毛细管的相变传热机理为液膜的导热和表面蒸发;表面蒸发受蒸汽温度、汽液界面的温度以及汽液压力差的共同控制。汽液流动机理为流动受脱离压力梯度、毛细力梯度支配。汽液相互作用机理为存在由于蒸发导致的动量转移切应力和由于汽液流速不同产生的摩擦切应力。提出的物理模型中较为全面地考虑了毛细管内传热、汽液流动及其相互作用。对毛细管半径和传热功率对薄液膜轮廓和传热特性影响程度的计算结果表明,随着毛细管半径的减小、传热功率的增大,蒸发界面区的长度会有所减小,这是针对微小空间得出的不同于常规情况的结论。 相似文献
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A mathematical model is developed to investigate the dynamics of vapor bubble growth in a thin fiquid film, movement of the interface between two fluids and the surface heat transfer characteristics. The model takes into account the effects of phase change between the vapor and liquid, gravity, surface tension and viscosity. The details of the multiphase flow and heat transfer are discussed for two cases: (1) when a water micro-droplet impacts a thin liquid film with a vapor bubble growing and (2) when the vapor bubble grows and merges with the vapor layer above the liquid film without the droplet impacting. The development trend of the interface between the vapor and liquid is coincident qualitatively with the available literature, mostly at the first stage. We also provide an important method to better understand the mechanism of nucleate spray cooling. 相似文献
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We investigate the stability of a thin liquid film flowing down an inclined wavy plane using a direct numerical solver based on a finite element/arbitrary Lagrangian Eulerian approximation of the free-surface Navier-Stokes equations. We study the dependence of the critical Reynolds number for the onset of surface wave instabilities on the inclination angle, the waviness parameter, and the wavelength parameter, focusing in particular on mild inclinations and relatively large waviness so that the bottom does not fall monotonously. In the present parameter range, shorter wavelengths and higher amplitude for the bottom undulation stabilize the flow. The dependence of the critical Reynolds number evaluated with the Nusselt flow rate on the inclination angle is more complex than the classical relation (5/6 times the cotangent of the inclination angle), but this dependence can be recovered if the actual flow rate at critical conditions is used instead. 相似文献
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本文对垂直上升光管中环状流流动沸腾的理论模型进行了分析,以液膜紊流的动量方程和能量方程为基础,推导了环状流的数学模型,通过求解动量方程和能量方程,获得了流动沸腾换热系数的预测模型,并对该预测模型进行了数值求解,将预测的换热系数同实验值作了比较。 相似文献