Although different types of thermal diodes have different structure, they include evaporative section, adiabatic segment and condensation section.
The working principle of the thermal diode is to evaporate the heat evaporating through the working medium in the evaporating section, flow to the condensing section under the action of the gravity field and the acceleration pressure differential, and release the heat in the condensing section into liquid. Condensate liquid depend on gravity flow back to evaporation section, and then continue to evaporation, condensation, so three type thermal diode structure to carry out from the evaporator to the condenser heat transfer and the cycle of working medium in the pipe.
For thermosiphon  and the separated type heat pipe, due to the evaporation return of condensate from the condenser is rely on gravity, so the two kinds of evaporation of diode must be placed in the condensing section below, so they are unidirectional (from bottom to top) of the heat transfer heat diode. However, the two bidirectional thermal diode concepts proposed by professor zhang renyuan et al.  can change the heat transfer direction by changing the inclination Angle of the adiabatic segment. In addition, there is no contact between the vapor and condensate return liquid in the two-phase bidirectional thermal diode, and there is no shear phenomenon at the interface of the vapor liquid, so there is no heat transfer limit.