We prove that under the condition of closed boundary to mass flux, pure advection is not a valid mechanism to make a practical thermal diode. Among the various designs of thermal diodes, many of them involve circulating fluid flow, such as in thermosyphons. However, those designs often employ natural convection, which is basically a nonlinear process. It thus remains unclear how the pure advection of temperature field induced by a decoupled velocity field influences the symmetry of heat transfer. Here we study three typical models with pure advection: one with open boundary, one with closed boundary at unsteady state, and one with closed boundary at steady state. It is shown that only the last model is practical, while it cannot become a thermal diode. Finally, a general proof is given for our claim by analyzing the diffusive reciprocity.

我们证明，在质量通量的封闭边界条件下，纯平流不是制造实用热二极管的有效机制。在热二极管的各种设计中，其中许多涉及循环流体流动，例如在热虹吸管中。然而，这些设计通常采用自然对流，这基本上是一个非线性过程。因此，由解耦速度场引起的温度场的纯平流如何影响传热对称性仍不清楚。在这里，我们研究了三种典型的纯平流模型：一种具有开放边界，一种在非稳态时具有封闭边界，一种在稳态时具有封闭边界。结果表明，只有最后一个模型是实用的，而不能成为热二极管。最后，通过分析扩散互易性，对我们的主张给出了一般证明。