Hydrodynamic Stability of a Fluidized Bed
Hydrodynamic Stability of a Fluidized Bed
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流化床的水动力稳定性
DOI：
10.1021/i160013a014
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发表时间：
1965
期刊：
影响因子：
--
通讯作者：
T. Baron
中科院分区：
文献类型：
--
作者：
R. L. Pigford;T. Baron
通过研究固体颗粒和流体通过均匀固体浓度床层的非定常运动方程，发现由于固体和流体的惯性，混合物密度的小扰动将随着它向上通过床层而呈指数增长。因此，均匀的密度分布是不稳定的，不能期望流化床没有小的不规则性，这些不规则性可能会变成”气泡“。“固体浓度波的增长率取决于扰动的波长、空隙率和平均流化速度。波长越短，增长率通常越大，但最短波的增长受到床层抗剪切力的抑制。因此，有一个波的增长速度比任何其他;它的波长大概与在床中所有点自发形成的”气泡”的初始尺寸有关。如果流体逆着重力向上流过离散固体颗粒的床，当流体支撑颗粒并且颗粒自由移动时，床被称为“流化”，除了偶尔的碰撞之外，彼此分离而不接触。在这些条件下，流化床中颗粒之间的自由空间大于其在沉降或固结条件下的自由空间，颗粒的重量由流体中的拖曳力和压力梯度而不是由颗粒与颗粒的接触承受。固体和流体的混合物变得和真正的流体一样移动的。然而，实验表明，固体颗粒在流体中的均匀分散并不是在所有情况下都稳定的。特别是当流体是气体时，流化的均匀性通常很差，气体的袋或气泡自发地形成并上升到床的表面
By examining the equations for unsteady motions of solid particles and fluid through a bed of uniform solids concentration it is found that, owing to the inertia of the solid and fluid, a small disturbance in the mixture density will grow exponentially as it moves upward throughthe bed. A uniform density distribution is therefore unstableand a fluidized bed cannot be expected to be free of small irregularities, which may grow into" bubbles." The rate of growth of the waves of solids concentration depends on the wavelength of the disturbance, on the voidfraction, and on the mean fluidizing velocity. The growth rate is generally greater the shorter the wavelength, but growth of the shortest waves is inhibited by the bed's resistance to shear. As a result, there is a wave that grows faster than any other; its wavelength is presumably related to the initial size of the" bubbles" thatform spontaneously at all points in the bed.Ilf hen a fluid flows upward against gravity through a bed** of discrete solid particles, the bed is said to be “fluidized” when the fluid supports the particles and they move freely, being separated from contact with each other except for occasional collisions. Under these conditions, the freespace between the particles in the fluid bed is greater than it is in the settled or consolidated condition, the weight of the particles being born by the drag forces and the pressure gradient in the fluid rather than by particle-to-particle contact. The mixture of solid plus fluid becomes as mobile as a true fluid. Experiment shows, however, that a uniform dispersion of the solid particles through the fluid is not stable under all circumstances. Especially when the fluid is a gas, the uniformity of fluidization is often poor with pockets or bubbles of gas forming spontaneously and rising to the surface of the bed