## 1 Introduction
\begin{table}
\begin{tabular}{p{128.0 pt} p{128.0 pt} p{128.0 pt} p{128.0 pt} p{128.0 pt}} \hline \hline
**Year** & **Objectives** & **Advantages** & **Limitations** & **Results** \\ \hline \multirow{3}{*}{2012 [7]} & PHP with different working fluids & Working fluids with different properties were simulated. & Dynamic contact angles were assumed to be zero. & Good agreement between numerical results and experimental data \\ \hline \multirow{3}{*}{2012 [6]} & PHPs with various working fluids, & \multirow{3}{*}{Local resistance of the inclination angles, and number of turns} & \multirow{3}{*}{Local resistance of the bond was considered.} & Capillary force on liquid slugs was not calculated. & The performance of PHP was affected by local pressure losses of bends. \\ \cline{3-5}  & & & & \\ \hline \multirow{3}{*}{2013 [9; 10]} & Models with different fitting and design parameters & \multirow{3}{*}{Effect of new bubble generation on PHP’s performance was analyzed.} & New bubbles were assumed to generate at given nucleate sites. Local resistance and gravity were ignored. & Simulation with a bubble generation showed that most of the heat was transferred by the sensible heat (74\%). \\ \cline{3-5}  & PHP under various gravity levels & Heterogeneous and homogeneous phase changes were calculated & Dynamic contact angles were assumed to be zero, and liquid films were neglected. & The model showed good prediction capability at both normal and hyper-gravity conditions. \\ \hline \multirow{3}{*}{2017 [13]} & PHP in horizontal and vertical positions & The dynamics of the liquid film was considered. & Local pressure drop and capillary resistance were ignored. & Film dynamics had an important effect on the performance of horizontal PHP. \\ \hline \multirow{3}{*}{2017 [14]} & Analysis on oscillation waveform for PHP & Local resistance of the bend was considered. & Thickness of liquid film was assumed to be constant. Capillary resistance was ignored. & The propagation of energy was a reason for pressure propagation in PHP. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Typical numerical models in recent 10 years. It is well known that the performance of the PHP is influenced by many parameters, such as the thermophysical properties of the working fluids, the inclination angles, the structure parameters of the PHP, etc. The adiabatic section length, as one of the structure parameters, has a significant effect on the practical application of the PHP, because the adiabatic section length of PHP was determined by the distance between the heat source and the heat sink. To ensure that the PHP can operate efficiently in practical situations, the influence of the adiabatic section length on the heat transfer performance of the PHP was investigated. For example, [PERSON] et al. [22] experimentally investigated the effect of the adiabatic length on the heat transfer performance of the PHP, and the results showed that the startup performance of the PHP with shorter adiabatic section length was better than that with longer adiabatic section length. In addition, with the increase of the adiabatic section length, the dry-out heat input of the PHP was also increased. Thus, the adiabatic section length had a significant effect on the performance of the PHP in practical applications. However, few models have been validated for transient operations of the PHP with various adiabatic section lengths. Only [PERSON] et al. [5] developed a CFD model, using Ansys/Fluent, to investigate the effect of the adiabatic section length on the performance of the PHP. The results showed that, as the adiabatic section length increased, the start-up time of the PHP reduced and the anti-dry-out ability was weakened. Based on the existed research, a new transient numerical model was proposed to investigate the transient flow of the working fluid and the transient heat transfer in the PHP with various adiabatic section lengths (60-240 mm) in this work.