## 4 Simulation Results
### Loop Heat Pipe as a Heat Switch  
In this section, we show transient simulation results of the developed lumped parameter model, which clarify the operation of the loop heat pipe functioning as a heat switch. Figures 6-8 provide valuable insights into the behaviour of a loop heat pipe as a heat switch, clarifying its internal temperature, mass balance, and pressure dynamics.  
The numerical simulation begins from an initial stationary state at an ambient temperature of \(T_{ext}=23\)\({}^{\circ}\)C, without any heating or cooling power applied. After a few seconds, the evaporator (Node 8) and the condenser (Nodes 3-5) are subjected to a heat power input of \(Q_{evap}=20\) W and \(\dot{Q}_{CC}=40\) W, respectively, with a duration of 60 and 12 min as  
Figure 5: \(T_{8}\) and \(T_{2-3}\) response comparisons between modeling results and experimental data. (\(Q_{evap}=10W\), \(T_{ext}=23\)\({}^{\circ}\)C).  
shown in Figure 6. This initiates the temperature increase in the evaporator (\(T_{8}\)) and the compensation chamber (\(T_{CC}\)), resulting in the natural evaporation of liquid from the Wick (\(M_{7-1}\)). The liquid inside the compensation chamber (\(M_{6-7}\)) also shows a reduction, as in Figure 7. During this initial period, the pressure differential between the compensation chamber (\(P_{6-7}\)) and the condenser (\(P_{4-5}\)), denoted as \(\Delta P=P_{6-7}-P_{4-5}\), shows a positive value. This means that there is no liquid flowing from the condenser (\(M_{4-5}\)) to the compensation chamber (\(M_{6-7}\)), which results in a dry-out of the liquid in \(M_{6-7}\), as shown in Figure 7. This dry-out phenomenon first occurs in the compensation chamber (\(M_{6-7}\)) at approximately 0.1 h, followed by the Wick (\(M_{7-1}\)) at approximately 0.65 h.  
Once the Wick and the compensation chamber have undergone dry-out, i.e., \(M_{6-7}=M_{7-1}=0\), the loop heat pipe is considered in an off-state, and its temperature increases linearly, as illustrated in Figure 6, from 0.65 h to 1 h. In this model, there are no significant heat leaks that could cause the temperature of the evaporator (Node 8) and the compensation chamber to cool down and restart the loop heat pipe operation. This means that it forces the device to a start-up.  
To demonstrate the functionality of the loop heat pipe as a heat switch device, a cooling effect of \(\dot{Q}_{CC}=-70\) W is imposed on the compensation chamber with a duration of 12 min. This restarts the device in an operational mode. In order to initiate the operation, it is crucial to establish a negative pressure differential between the compensation chamber (\(P_{6-7}\)) and the condenser (\(P_{4-5}=P_{3-4}\)), i.e., \(\Delta P=P_{6-7}-P_{4-5}<0\) Pa. This pressure differential is essential for facilitating the suction of liquid into compensation chamber \(M_{6-7}\) and Wick \(M_{7-1}\), as illustrated in Figure 8 at 1.1 h.  
During the 12 min of \(\dot{Q}_{CC}=-70\) W, the compensation chamber temperature reduces linearly to \(T_{CC}\approx 0\)\({}^{\circ}\)C. At 1.1 h, the liquid inside Wick (\(M_{7-1}\)) and compensation chamber (\(M_{6-7}\)) starts to increase and evaporator temperature \(T_{8}\) starts to decrease.  
Finally, without any input heat load, the loop heat pipe returns to the equilibrium ambient temperature of \(T_{ext}=23\)\({}^{\circ}\)C.  
Figure 6: Chart with changing in temperatures in evaporator and compensation chamber (\(T_{8}\) and \(T_{CC}\)), and heating/cooling loads (\(Q_{evap}\) and \(Q_{CC}\)) over time.