## 6 Limitations of the TPCT  
The limitations of the TPCT includes its own limitations and problems that arise in applications. First section: Although TPCTs are effective transfer devices, there are some limits, including viscous, sonic, dry-out, boiling, flooding, and geyser boiling. 1. The sonic limits are mentioned in theory, but never observed. 2. The viscous limit is that the pressure difference is smaller than the viscous force, the vapor cannot move and the viscous limit is reached [82]. 3. The dryout limit occurs at a small working fluid. For the continuous circulation of the two phases, there is a minimal mass of working fluid. When the available mass is smaller than the minimum, the bottom of the evaporator will dry out [83; 84]. 4. The boiling limit: when it occurs, there is a transition from nucleate boiling to film boiling, which will greatly increase the risk of the TPCT damage. 5. The flooding limit: the latent heat of vaporization of the working fluid is necessary in controlling the flooding limit. Once the axial heat fluxes are higher than the radial heat fluxes, the upward vapor will prevent the liquid from going back to the evaporator. Meanwhile, the evaporator will dry out [85; 86]. 6. The geyser boiling limit: this is an unstable phenomenon [87]. [PERSON] et al. [88] examined geyser boiling in two TPCTs with different diameters (15 and 25 mm) and a length of 1000 mm. The results of the experiment showed that the period of geyser boiling decreased as the heat load and aspect ratio increased. Considering the above limit, when using TPCTs, it is more important to pay attention to the conditions of its use and choose the appropriate optimal design ratio to meet the higher engineering needs. Second section: the TPCTs were successful in the highways, and can significantly cool the roadbeds and enhance the permafrost thermal stability. However, there are several limits listed below. 1. The cooling range of the TPCTs is limited, and the effective cooling range is about 2.4 m [89]. 2. The asymmetric geotemperature distribution under the roadbeds caused by the TPCTs can affect their stability [14]. The area is near the TPCTs where the temperature is \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reference** & **Structure** & **Methods** & **Performance** \\ \hline
[52] & Vertical TPCT & 1, 3 & Cooling is evident but causes pavement cracks. \\
[58] & Vertical TPCT & 1, 3 & Threats the stability and develops deformation. \\
[11] & Vertical TPCT & 3 & Decreases the temperature, increases mechanical strength. \\
[77] & Vertical TPCT & 3 & Adjusts the temperature of embankment with shady-sunny effect. \\
[66] & Vertical TPCT & 1 & Cools the embankment, keeps the thermal stability. \\
[78] & L-shaped TPCT & 3 & Effective in cooling permafrost, but duration limited. \\
[38] & L-shaped TPCT & 2 & Keeps the thermal stability with the combined measures. \\
[79] & L-shaped TPCT & 3 & Works in cold seasons, cools embankment core efficiently. \\
[12] & L-shaped TPCT & 1, 3 & Extends the horizontal cooling extent under embankment. \\
[80] & L-shaped TPCT & 3 & Controls permafrost degradation rate by reducing heat absorption \\
[68] & L-shaped TPCT & 1 & Improves the long-term service performance. \\
[71] & L-shaped TPCT & 1 & Cools the embankment but may cause longitudinal cracks. \\
[81] & L-shaped TPCT & 3 & Considers the installing position of the TPCT. \\
[20] & Inclined TPCT & 3 & Eases permafrost degeneration, cools down permafrost. \\
[13] & Inclined TPCT & 3 & Reveals the cause of longitudinal crack on TPCT. \\
[16] & Inclined and L-shaped TPCT & 2 & Cools down the embankment center, prevent it thawing. \\
[14] & Horizontal TPCT & 1, 2 & Reduces horizontal temperature difference, increases heat control \\
[69] & Horizontal TPCT & 1, 3 & Quables the contact surface, reduces heat flux difference. \\
[54] & Heating TPCT & 3 & Reduces the freezing depth and evaluates the railway. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The structure and evaluation method of the TPCTs.