Modeling thermosyphon and heat pipe performance for mold cooling applications
Abstract
Thermosyphons are enhanced heat transfer devices that can continuously transfer very large amounts of heat rapidly over long distances with small temperature differences. The high heat transfer rate is achieved through simultaneous boiling and condensation of the working fluid and the continuous heat transfer is achieved through recirculation of the working fluid in its liquid and vapor phase. A potentially important application of the thermosyphons has been towards reducing the cycle times of the mold cooling processes which would provide economic incentives to the automotive industry.
Different operational and geometrical parameters such as the input heating power, fill ratio (FR), aspect ratio (AR) and orientation influence the thermodynamic state of the working fluid and the boiling and condensation thermal resistances. The goal of the present work is to develop a more rigorous understanding of the thermal performance of thermosyphons as a function of their design parameters and operating conditions.
A thermodynamic model is developed to predict the thermodynamic state of the working fluid and its related properties inside a thermosyphon and heat pipe at each stage of the thermodynamic cycle. The model can predict the operating temperature of a thermosyphon and a heat pipe under a given geometrical and operating condition.
The existing predictive heat transfer correlations for estimating the boiling and condensation thermal resistances for thermosyphons are inadequate and lack consistency. Using non-dimensional analysis based on the Buckingham-pi theorem, a series of analytical expressions capable of predicting the boiling and condensation thermal resistance under different operational parameters are developed.
The thermal performance of ten distilled-water copper thermosyphons is experimentally investigated by varying the input heating power between 6.68 [W] and 32.98 [W], fill ratio between 0.35 - 0.75 and aspect ratio between 5.33 and 12.43 to understand the relationship between these operating parameters and the boiling and condensation thermal resistances.
The present experimental data when compared to the new thermal resistance expressions showed excellent agreement (± 15%). The previous experimental data, however, showed a slightly worse agreement (± 30%), which could be attributed to the overall energy balance and the uncertainties in the reported fill ratio. The results from this work can be useful for selecting thermosyphons for specific applications and optimize the existing design of thermosyphons.