格拉斯霍夫数物理意义的一种解释方法

Another Explanation of the Physical Meaning of Grashof Number

格拉斯霍夫数是将影响对流传热膜系数因素进行量纲分析得到的无量纲数。国内外几乎所有《化工原理》教材均将其解释为表征自然对流的影响,但解释过程均较为模糊。本文以一垂直平板与大空间液体的自然对流传热过程为例,将流体在垂直平板附近的热边界层内流动等效到圆管内流动,直接用泊稷叶公式计算出热边界层内流体流动为层流时的环流速度,取等效圆管的当量直径为特征尺寸,明确得到格拉斯霍夫数是雷诺数的一种变形,表征自然对流时惯性力与黏性力之比。

Grashof number is a dimensionless number derived from the dimensional analysis of parameters influencing convective heat transfer which in all the textbooks of Principles of Chemical Engineering is re garded as a parameter characterizing the influence of natural convection on heat transfer, while the expla- nations of the physical meaning of it are rather vague. In this work, the natural convective heat transfer from a vertical plate to large space fluid is investigated, the flow in the thermal boundary layer of the ver- tical plate is equivalent to the flow in the circular tube, and the circulating velocity in the thermal bounda ry layer is calculated based on Poiseuille equation with the characteristic dimension being the equivalent diameter of the equivalent circular tube. The results show that the Grashof number is a variant of the Reynolds number, characterizing the ratio of inertial force to viscous force for natural convection.