**3.1 Operation of oscillating heat pipe (OHP)**

When the evaporator section receives heat, the temperature in the evaporator region increases, which will lead to a rise in the vapor pressure in the channel, and causes the growth of bubbles size in the evaporator section, then bubbles move to the condenser zone by the difference between the pressure of the evaporator and condenser. During the movement of the vapor plug to the condenser section, which is under low temperature, so the condensation process occurs, and the vapor plug collapses. Then, the slug-train in the condenser section is pushed to the adjoining vapor plug, and the liquid slug moves to the evaporator section for nucleation.

**Figure 5.** *Types of oscillating heat pipes [12].*

At nucleation locations, dispersed bubbles generate and coalesce to grow in size as bubbles are continuously heated. The formation and growth of dispersed bubbles happen continuously when supplying the heat at the evaporator zone because the temperature of the inner wall of the tube was higher than the saturated temperature of the working fluid [11].

As the condenser cools, the pressure reduces, and a condensation process of bubbles accrues. This process pumps the working fluid again to the evaporator region and continuous between both sections and resulting in oscillating motion. The cycle gets completed in this way, and the same cycle is repeated again and again for heat transportation from the evaporator region to the condenser region.
