传统的能量方程为三维的，但是诸多文献证实温度场在轴向方向上是梯度几乎为零的，此时，三维可以降为二维，方程求解更加简便。-The traditional three-dimensional energy equation, but many literature confirm temperature gradient in the axial direction is almost zero, in which case, three-dimensional can be reduced, mo