Nucleate Boiling Heat Transfer Enhancement by Capillary Wicking on Gradient Micropillar Structured Surface

The pool boiling performance on surfaces with gradient micropillar structures is investigated systematically using a three-dimensional phase-change lattice Boltzmann (LB) model. The rationality of boiling enhancement due to the wicking force generated by the gradient structures is validated from the numerical view, and the effects brought by varying structural parameters, including the size difference between neighboring micropillars, micropillar spacing in y-direction, and micropillar height, are mainly discussed. The results indicate that the wicking force from the gradient structures is able to promote bubble growth, merging, and detachment processes effectively, leading to an increase in average heat flux of 14.4% in contrast to the surfaces with uniform structures. By fixing the maximum micropillar size and increasing the size difference between neighboring micropillars, it is found that the boiling performance is closely related to the size of the central pillar. However, the difference in the dimensions of the surface exhibiting optimal boiling performance is fairly minimal. By fixing the minimum micropillar size and increasing the size difference between neighboring micropillars, both the magnitude and range of the wicking force effect are altered accordingly, and the surface with the smallest size difference demonstrates the best boiling performance. The impact of the wicking force increases as micropillar spacing decreases in the y-direction, but the excessive wicking effect can inhibit nucleation. Despite the constant gradient value, the wicking force in the vertical direction is improved remarkably with increasing micropillar height but with an upper limit on enhancing boiling performance due to the presence of flow resistance.