Cooling of power electronic devices using rectangular flat heat pipes with externally and internally cooled condenser regions

In traditional heat pipes employing external cooling, the internal flow pattern undergoes a transition to annular flow. In this stage, vapor moves through the central region of the heat pipe, while a liquid film develops on the inner wall surface. This liquid film acts as a thermal barrier, impeding the condensation of vapor within the vapor core area, which reduces the coolant's ability to condense vapor effectively. To address this issue, a rectangular flat heat pipe with an internal condenser (RFHP-IC) has been designed and tested for heat loads up to 360 W with different fill ratios. The RFHP-IC incorporates a tube-in-tube structure within the condenser section of the heat pipe, facilitating the condensation of the working fluid while maintaining a compact form factor. Results indicate that employing an internal condenser reduces thermal resistance and evaporator wall temperature by 67.5% and 20.1% respectively when compared to an external condenser. Furthermore, the RFHP-IC allows vapor to make direct contact with the cold surface of the internal condenser, increasing the condensation rate by 8.5% in comparison to an external condenser under the same heat load. The findings of this study can improve the performance, capacity, and reliability of RFHPs, making them more suitable and efficient solutions for thermal management applications.