Study on dry-out occurrence in inclined loop thermosyphon for CPU cooling

Abstract

A loop thermosyphon is known to have a higher heat transfer performance without a pump. In addition, there is demand for the reduction of the electricity needed for the fan used to cool the CPUs in a rack mount server. We more efficiently designed a new loop thermosyphon that is thinner and applicable to rack mount servers. Thermosyphon uses gravity to return the refrigerant from the condensation part to the boiling part of the system. Therefore, when it is in operation, the water level in the condenser should be higher than that in the evaporator. However, it is slightly more difficult to ensure the water level differences in such a thin loop thermosyphon than the thick one. The dry-out which is the phenomenon that refrigerant does not return to the boiling part is easy to occur, when the loop thermosyphon is inclined and there is less refrigerant in it. Therefore, it is important to clarify the relationship between the angle needed to start the dry-out and the refrigerant enclosure quantity. For this purpose, we conducted experimental measurements while changing the conditions, and showed that the dry-out angles could be estimated by using a geometric water surface model, which is combined with the calculation of the pressure drop in a vapor tube and the calculation of the steam temperature based on the water surface level in the condensation tube. As a result of using this model, we confirmed that the length of the vapor tube and the volumetric capacity of the bottom header affect the dry-out angle.

Keywords

• thermosyphon
• boiling
• condensation
• dry-out
• heat transfer
• cpu cooling