LBM Numerical Simulation for Contact Angle Behavior on Gradient Sodium-philic Structures

Heat pipe-cooled reactors are a significant research focus within the nuclear energy domain, distinguished by their inherent advantages of high safety, straightforward operational characteristics, and ease of modularization. High-temperature alkali metal heat pipes, emerge as the preferred choice for integration into such reactor systems, primarily due to their exceptional isothermal behavior and superior heat transfer efficiency. The performance of these heat pipes critically depends on their core component, the wick, where surface wettability directly governs both the capillary force driving the working fluid and the overall heat transfer efficiency. However, conventional wick designs currently face a challenge in simultaneously achieving both high capillary force and high permeability. To address this limitation and enhance heat pipe efficiency, a novel composite wick incorporating gradient sodium-philic nanostructures was proposed. Despite this potential, the majority of existing research focuses on conventional working fluids, and consequently, investigations into the flow and spreading mechanisms to alkali metal working fluids, within high-temperature heat pipe wicks remain notably limited. This study aims to systematically investigate the influence exerted by both flat and inclined surfaces engineered with gradient sodium-philic nanostructures on the contact angle of liquid sodium, employing the lattice Boltzmann method (LBM) as the computational tool. The LBM was specifically selected over alternative simulation approaches because it offers a more accurate description of multiphase flow interface characteristics while also requiring significantly shorter computational time. Nevertheless, the stability of the fundamental LBM model is often inadequate. To substantially enhance model stability, this work adopted an improved multi-relaxation-time (MRT) collision model coupled with a pseudo-potential interaction force model. Geometric parameters defining the wick micro-structure, including micropillar height, micropillar spacing, and micropillar width, were incrementally adjusted. Furthermore, inclined surfaces and gradient sodium-philic nanostructures were incorporated into the computational domain. The impact of each of these parameters and configurations on the spreading contact angle of liquid sodium was then individually analyzed. This approach successfully achieved stable and robust simulations of liquid sodium wetting and spreading dynamics on both flat and inclined surfaces featuring diverse micropillar geometries. The results indicate that on inclined surfaces, liquid sodium droplets exhibit faster spreading in the direction of the inclination; Upon reaching equilibrium, the droplet’s bottom spreading profile becomes asymmetric, and the gradient structure and the inclined surface work synergistically to promote the directional wetting and spreading of the liquid. When the micropillar height exceeds 3 lu, increasing the micro-pillar spacing causes the liquid sodium contact angle to increase, potentially leading to a transition of the surface from sodium-wetting to sodium-repelling. The variation trend of micropillar width is similar to that of spacing; However, beyond 10 lu, the surface can approximate a flat surface, and the contact angle tends to stabilize. Within the height range of 2-7 lu, increasing the micropillar height decreases the liquid sodium contact angle, thereby enhancing wettability. These findings provide a theoretical basis for optimizing wick surface design and are significant for enhancing the thermal performance of heat pipes.