Three-dimensional temporal imaging of X-ray CT imaging of dissolution-driven convection in glass beads

The images in this dataset show the mixing of two liquid solutions in a random bead pack as a function of time and in three-dimensions. The working fluids used in this study are solutions of methanol and ethylene-glycol (MEG, fluid 1) and brine (fluid 2). In particular, three mixtures of ethylene-glycol and methanol were prepared that differ in wt% ethylene-glycol, namely 55 wt% (MEG55), 57 wt% (MEG57) and 59 wt% (MEG59). Measurements are conducted using in the regime of Rayleigh numbers, Ra = 2000-5000. X-ray Computed Tomography is applied to image the spatial and temporal evolution of the solute plume non -invasively. The tomograms are used to compute macroscopic quantities including the rate of dissolution and horizontally averaged concentration profiles, and enable the visualisation of the ow patterns that arise upon mixing at a spatial resolution of about (2x2x2) mm3. We observe that the mixing process evolves systematically through three stages, starting from pure diffusion, followed by convection-dominated and shutdown. A modified diffusion equation is applied to model the convective process with an onset time of convection that compares favourably with literature data and an effective diffusion coefficient that is almost two orders of magnitude larger than the molecular diffusivity of the solute. The comparison of the experimental observations of convective mixing against their numerical counterparts of the purely diffusive scenario enables the estimation of a non-dimensional convective mass flux in terms of the Sherwood number, Sh = 0.025Ra. We observe that the latter scales linearly with Ra, in agreement with observations from both experimental and numerical studies on thermal convection over the same Ra regime.本数据集包含的图像展示了随机颗粒充填床内两种液态溶液随时间变化的三维混合过程。本研究采用的工质为甲醇-乙二醇混合溶液（MEG，流体1）与盐水（流体2）。特别地，我们制备了三种乙二醇与甲醇的混合体系，其乙二醇质量分数分别为55%（MEG55）、57%（MEG57）与59%（MEG59）。实验在瑞利数（Rayleigh number, Ra）为2000~5000的区间内开展。我们采用X射线计算机断层扫描（X-ray Computed Tomography）非侵入式地获取溶质羽流的时空演化图像。通过断层扫描图像，我们可以计算溶解速率、水平平均浓度剖面等宏观物理量，并实现空间分辨率约为(2×2×2) mm³的混合流场可视化。研究发现，混合过程系统地经历三个阶段：初始为纯扩散阶段，随后进入对流主导阶段，最终趋于停滞。我们采用修正扩散方程对对流过程进行建模，所得对流起始时间与文献数据吻合良好，且模型得到的有效扩散系数相较溶质分子扩散系数大近两个数量级。将对流混合的实验观测结果与纯扩散场景的数值模拟结果进行对比后，我们可以基于舍伍德数（Sherwood number, Sh）估算无量纲对流通量：Sh=0.025Ra。观测结果表明，该对流通量与瑞利数呈线性缩放关系，这与同一瑞利数区间内热对流相关实验与数值研究的结论一致。