Dynamic Solidification of Binary Aqueous Solution

本文对二元水溶液的凝固过程进行了研究。本工作的主要目标是建立一个一维近似模型来预测固体层生长和衰变的随时间变化的行为。由于在水溶液中形成的固体层变成糊状（多孔层），我们首先研究了在水饱和多孔介质中的凝固。特别是当冷却温度随时间变化时固体层的响应是我们的主要兴趣。为了预测时间固体层厚度，建立了一维模型。为了验证所提出的一维模型的有效性，我们使用一个充满水饱和玻璃微珠的立方容器进行了实验。为了达到同样的目的，还进行了二维数值分析。结果表明，所提出的一维模型能较好地预测实验和数值结果。接下来，我们将一维模型扩展到二元水溶液的凝固。假设糊状层中固体分数恒定，固流界面处指定热流密度，有效地模拟了界面处对流热流密度，将固体层（糊状层）中随时间变化的热传导方程与界面处的热能平衡相结合，得到了糊状层厚度未知的超越方程。用装硝酸钠的立方容器也进行了实验。采用了两种不同的热边界条件；一个从上面冷却，另一个从下面冷却。前者的特点是在液体区存在强烈的对流，而后者以扩散为主。在两种热边界条件下，一维模型预测的固相层厚度和固相分数与实验结果吻合较好。结果表明，自上而下冷却形成的糊状中固体分数比自下而上冷却形成的糊状中固体分数大。这是由于浆糊中间隙溶液的浓度不同所致。由于质量扩散相对于热扩散非常小，所以当系统从下面冷却时，间隙中的浓度趋于积累。我们还研究了垂直冷却或加热圆柱体的混合对流传热。这对评价地热换热器的换热速率具有重要的应用价值。少

In this study solidification process of binary aqueous solution is investigated. Main goal of the present work is to develop a one-dimensional approximate model to predict the time-dependent behavior of the solid layer growth and decay. Since the solid layer formed in the aqueous solution becomes mushy (a porous layer), we first study the solidification in the water-saturated porous medium. Particularly the solid layer response when the cooling temperature varies with time is our major interest. One-dimensional model is developed in order to predict the temporal solid layer thickness. Experiments using a cubic container filled with water-saturated glass beads are carried out for testing the validity of the proposed one-dimensional model. Two-dimensional numerical analysis is also performed for the same purpose. It is found that the proposed one-dimensional model accurately predicts the both experimental and numerical results. Next we extend our one-dimensional model to solidification o … More f binary aqueous solution. Assuming the constant solid fraction in the mushy layer and the specified heat flux at the solid-fluid interface, which is effectively simulating the convective heat flux at the interface, the time-dependent heat conduction equation in the solid layer (mushy layer) and the thermal energy balance at the interface are combined to yield the transcendental equation for the unknown thickness of the mushy layer. Experiments are also carried out using the cubic container filled with aqueous sodium nitrate. Two different thermal boundary conditions are employed ; one is cooled from above, and the other is cooled from below. The former is characterized by the presence of vigorous convection in the liquid region, while the latter is dominated by diffusion. The solid-layer thickness and the solid fraction in the mush predicted by one-dimensional model again agree well with the experimental results, for the both thermal boundary conditions. It is found that the solid fraction in the mush formed by cooling from above has greater values than those by cooling from below. This is due to the different concentration of the interstitial solution in the mush. Since the mass diffusion is very small relative to thermal diffusion, the concentration in the interstice tends to accumulate when the system is cooled from below. We also investigate mixed convection heat transfer from a vertical cooled or heated cylinder. This has an important application for evaluating heat transfer rate of geothermal heat exchangers. Less