Analysis of Liquid Film Evaporation in Porous Particles: Toward Optimal Wick Parameters for Heat Transfer in Heat Pipes

Evaporation of working fluids inside capillary wicks determines the heat transfer capability of heat pipes. However, the relationship between wick parameters and evaporative heat transfer remains unclear. To establish a correlation between wick parameters of sintered porous particles and evaporation characteristics, a boundary condition model was developed, incorporating wick parameters such as particle radius (R), particle distance (d), apparent contact angle (θ_a), and initial liquid height (H). In the absence of a significant size effect, the profile of the liquid–vapor interface was determined using the boundary model by numerically solving the augmented Young–Laplace equation. Ammonia was used as an example to investigate evaporation characteristics. The curvature radius of the intrinsic meniscus (R_e) was found to serve as a bridging factor between these wick parameters and evaporation characteristics. When R_e exceeded 40.3 μm, a limitation in evaporative heat transfer within the thin film region was observed. The relationship between R, d, θ_a, and H was quantitatively described based on this evaporative heat transfer limit. Furthermore, a nondimensional analysis of the governing equation for the evaporating liquid film profile was conducted, yielding an influencing factor (λ) that governed the thin film profile. The proposed model and its outcomes could offer valuable theoretical insights for the structural design of sintered porous particles, the optimization of surface modification levels, and the determination of the appropriate working fluid charging ratio during the manufacturing process of heat pipes.