旋流场中PP单球颗粒力学行为实验测试分析

Experimental analysis of mechanical behavior of a single PP sphere in a rotating flow

研究PP(Polypropylene)球形颗粒在旋流场中的力学行为,需要一个比较理想的、稳定的旋流场,为此采用了特殊尺寸的实验转鼓。根据PP球形颗粒在这种强制旋流场中的实验现象、测试数据和颗粒受力平衡分析可知:PP颗粒在雷诺数为180~230范围内的强制旋流场中达到稳定运动状态时,几乎静止在中心水平线附近的某一点,只有颗粒的自转运动;且转鼓转速越高,颗粒的质心位置越靠近旋转中心。依据切向受力平衡条件,利用现有的曳力公式对实验数据进行处理,发现颗粒不能达到受力平衡,且最大误差超过22%。为此,利用实验数据拟合方法,得到了旋流场中颗粒曳力系数的新经验公式,其受力平衡误差均小于0.5%。依据径向受力平衡条件,以径向压力梯度力、附加质量力、浮力与重力之差在径向上的分力之和为基准,与低雷诺数下的Saffman升力和Magnus升力的理论计算结果之和相比较,发现二者也不能达到力平衡条件,最大误差超过了55%。本文根据球形颗粒在旋流场中的升力特征,通过直观的力学分析和量纲分析,定义了颗粒旋流场升力概念,导出了相应的升力系数公式;并利用实验数据拟合得到了升力系数与颗粒雷诺数的经验公式;利用经验公式处理实验数据,其力平衡误差不超过7%。

To study the mechanical behavior of a single PP(Polypropylene)spherical particle in rotating flow,an ideal steady rotating flow field is necessary.A special size experimental rotating drum is used.Based on the experimental phenomena,the test data and the PP sphere’s force balance analysis in the forced rotating flow,the results show that:the PP sphere is almost static at a point near the center horizontal line when it reaches stable motion in the forced rotating flow with Reynolds number of 180~230,but the sphere is only self-spinning.The higher the rotational speed of the drum,the closer the barycenter of the sphere is to the center of rotation.According to the condition of tangential force balance,it is found that the forces balance of the sphere cannot be reached by processing the experimental data with the existing drag coefficient formulas,and the maximum error exceeds 22%.For this reason,a new empirical formula for the particle drag coefficient in the rotating flow is obtained by using the experimental data fitting method,and the force balance errors are no more than 0.5%.According to the radial force balance condition,basing on the sum of the radial pressure gradient force and the added mass force and the radial component of the difference between buoyancy and gravity,compared with the sum of the theoretical calculation results of Saffman lift and Magnus lift at low Reynolds number,it is found that the two cannot reach the force balance condition,and the maximum error exceeds 55%.According to the lift characteristics of spherical particle in the rotating flow,the lift of the particle in the rotating flow is defined by intuitive mechanical analysis and dimensional analysis,and the formula for the corresponding lift coefficient is derived.The empirical formula relating the lift coefficient and the particle Reynolds number has been obtained by fitting experimental data.Using this formula to process experimental data,the errors of force balance are no more than 7%.