Lower limit of internal diameter for oscillating heat pipes: A theoretical model

• Developing a theoretical model to predict the lower limit of internal diameter (LLID) for oscillating heat pipes (OHPs).
• The LLID decreased with increasing the operating temperature of an OHP.
• A large ID greater than the ULID is required for the steady operation at horizontal orientation or micro-gravity conditions.
• Both of R123 and R141b are favorable for ultra-compact micro-OHPs.

The upper limit of internal diameter (ULID) of oscillating heat pipes (OHPs) based on Bond number is widely accepted and considered as a primary criterion to define OHPs, whereas the lower limit of internal diameter (LLID) is still unclear. In this paper, an analytical model based on the operating characteristics and design criterion of OHPs is developed to predict the LLID. Also, a visualization investigation of glass OHP was performed to verify the reliability of the model. Comparison of theoretical LLID and actual ID for successful startup and robust operation of OHP shows a good agreement between them. In addition, the influence of several parameters on the LLID were numerically analyzed and discussed, particularly inclination angle and working fluid. This work provides useful information and understanding of traditional capillary tube OHPs (both at normal gravity and micro-gravity environment) and ultra-compact micro-OHPs to better design them.