对流传热传质中的液滴蒸发过程数值模拟

液滴蒸发在空调、传热强化及农药高效利用等领域均有重要意义。以往理论研究主要基于静止空气,忽略了气流的影响。本文建立了存在空气对流传热传质的二维固着液滴蒸发数值模型,并通过COMSOL求解;反映了流动空气对液滴内部流动、温度分布及蒸发速率的影响并进行了实验验证。结果表明,液滴顶部的蒸发速率最大,从而产生了由底部向上的温度梯度,引发了液滴内部的Marangoni流动。由于气流促进了液滴蒸发,与背风侧相比,迎风侧液滴表面的蒸发量更大,其内部流动也更为剧烈。提高风速可以降低液滴周围空气的平均相对湿度,从而增强传热传质能力;提高空气与液滴之间的温差或湿度差能够提高两者之间的饱和水汽分压差,加快水分转移。

青藏铁路建设工程中热桩的应用——物理与国家重点工程专题之四

热桩(热管)是一种气液两相对流循环的导热系统.它是一根密封的管,里面填充一定量的工质(如氮、丙烷、CO2等),上端为散热片组成的冷凝器,下端为装有工质的蒸发器,中间为绝热段(图1).

Entropy Analyses of Droplet Combustion in Convective Environment with Small Reynolds Number

This paper analyzes the entropy generation rate of simple pure droplet combustion in a tempera-ture-elevated air convective environment based on the solutions of flow, and heat and mass transfer between the two phases. The flow-field calculations are carried out by solving the respective conservation equations for each phase, accounting for the droplet deformation with the axisymmetric model. The effects of the temperature, velocity and oxygen fraction of the free stream air on the total entropy generation rate in the process of the droplet combustion are investigated. Special attention is given to analyze the quantitative effects of droplet deformation. The results re-veal that the entropy generation rate due to chemical reaction occupies a large fraction of the total entropy generated, as a result of the large areas covered by the flame. Although, the magnitude of the entropy generation rate per volume due to heat transfer and combined mass and heat transfer has a magnitude of one order greater than that due to chemical reaction, they cover a very limited area, leading to a small fraction of the total entropy generated. The en-tropy generation rate due to mass transfer is negligible. High temperature and high velocity of the free stream are advantageous to increase the exergy efficiency in the range of small Reynolds number (<1) from the viewpoint of the second-law analysis over the droplet lifetime. The effect of droplet deformation on the total entropy generation is the modest.

Laminar Forced Convection Heat and Mass Transfer of Humid Air across a Vertical Plate with Condensation

Condensation of humid air along a vertical plate was numerically investigated, with the mathematical model built on the full boundary layer equations and the film-wise condensation assumption. The velocity, heat and mass transfer characteristics at the gas-liquid interface were numerical analyzed and the results indicated that it was not reasonable to neglect the condensate film from the point of its thickness only. The condensate film thickness, interface temperature drop and the interface tangential velocity affect the physical fields weakly. However, the subcooling and the interface normal velocity were important factors to be considered before the simplification was made. For higher wall temperature, the advective mass transfer contributed much to the total mass transfer. Therefore, the boundary conditions were the key to judge the rationality of neglecting the condensate film for numerical solutions. The numerical results were checked by comparing with experiments and correlations.

Prediction of Fluid Velocity Distribution near a Rising Bubble

Flow and concentration fields of liquid phase in a gas-liquid contacting system are simulated to showthe Rayleigh convection by utilizing the finite-element method. The Schlieren images in CO2-ethanol system provided direct visual verification of the present simulation, and the simulated results were well consistent with theexperimental observation. The influence of the Rayleigh convection on mass transfer is analyzed qualitatively andquantitatively based on the simulated and the experimental results.