Numerical Study on the Effect of Various Working Fluids on the Performance of Loop Heat Pipes Based on Two-Dimensional Simulation

The computational fluid dynamics (CFD) method is often used to study heat and mass transfer mechanisms in loop heat pipes (LHPs). However, few numerical studies have been conducted on the entire loop heat pipe system. In this work, a two-dimensional simulation model of an LHP was developed. The wicking process was described using a capillary pressure model, implemented through user-defined functions (UDFs). The flow, phase change, and heat transfer processes vary with the change of the working fluid. Therefore, the effects of ammonia, propylene, and R245fa on the heat transfer performance of the LHP were analyzed in the simulation. According to the simulation results, the LHP charged with ammonia showed the best heat transfer performance, followed by the LHP charged with propylene, and the LHP charged with R245fa showed the worst heat transfer performance. The simulation results align with the predictions from figures of merit (FOMs). Moreover, the model provides detailed insights into the temperature field and vapor–liquid distribution during LHP operation. Experiments using an LHP with the three working fluids were also conducted to verify the model's accuracy. Regarding the influence of the working fluid and the heat load on LHP performance, the simulation model aligns with the experimental results.