Natural Convection Heat Transfer of Suspension

对矩形容器内悬浮液的自然对流换热进行了研究。容器的一个垂直壁被加热，相对的垂直壁被冷却。将平均粒径为3~6μm的SiO_2微粒分散到水或二甲基亚砜水溶液中。颗粒大小分布窄。DMSO溶液的折射率与SiO_2颗粒的折射率相匹配，并用粒子图像测速仪测量了自然对流的速度。采用水-SiO_2悬浮液，在不同温差、初始颗粒浓度、颗粒平均直径、容器宽度等条件下，测量了悬浮液中的温度、颗粒浓度分布以及垂直壁上的平均换热系数。比较了悬浮液自然对流的平均努塞尔数和水的平均努塞尔数。颗粒的沉降影响悬浮液的自然对流。当初始颗粒浓度较大、温差较小时，自然对流受到抑制，努塞尔数变小。随着容器的宽度变宽，努塞尔特数减小。初始颗粒浓度较小、温差较大时，对流单体分层，自然对流活跃，努塞尔数为水的80%以上。当对流胞个数为2个时，当对流胞间的界面位置接近容器高度的一半时，努塞尔数最大。

Natural convection heat transfer of suspension in a rectangular vessel was investigated. A vertical wall of the vessel was heated and the opposite vertical wall was cooled. SiO_2 fine particles with 3 or 6 μm of mean diameters were dispersed into water or Dimethyl Sulfoxide (DMSO) aqueous solution. The particle size distributions were narrow. The refractive index of the DMSO solution was matched to that of the SiO_2 particles and the velocity of the natural convection was measured with Particle Image Velocimetry. By using the water-SiO_2 suspension, the temperature and particle concentration distributions in the suspension and the average heat transfer coefficient on the vertical wall were measured under various conditions of the temperature difference, the initial particle concentration, the mean diameter of particle, and the width of the vessel. The mean Nusselt number of the natural convection of the suspension was compared with that of water. Sedimentation of particles affected the natural convection of suspension. When the initial particle concentration was large and the temperature difference was small, the natural convection was depressed and the Nusselt number became small. As the width of the vessel became wider, the Nusselt number decreased. When the initial particle concentration was small and the temperature difference was large, convection cells were stratified and the natural convection in the cells was active and the Nusselt number was more than 80% of that of water. If the number of the convection cells was two, the Nusselt number had the maximum when the position of the interface between the convection cells was near half of the vessel height.