There is an internal heat source on the adiabatic side of the flat wall exposed to the fluid. Why is it meaningful to calculate the insulation side according to symmetry when calculating the temperature distribution? In any heat transfer problem, the definition of adiabatic is that the heat flux density is zero, and for the wall, it also means that the temperature gradient is zero, that is, the boundary condition is dT/dx = 0. The gradient is zero, that is, if the adiabatic surface is regarded as a mirror, the temperature gradient on the mirror side of its symmetry is also zero. Therefore, adiabatic can be written as symmetry in mathematical model. However, if the heat flow is not zero, it cannot be calculated by symmetry, because if dT/dx = C > 0, then dt/dx =-C.