The combination of microstructures and mixed wettability for enhancing nucleate boiling has attracted much attention in recent years. However, in the existing experimental and numerical studies, the tops of microstructures are entirely subjected to wettability modification, which makes the influences of mixed wettability dependant on the characteristic length of microstructures. In order to disclose the joint effects of surface structure and mixed wettability on nucleate boiling, in this work we propose an improved type of pillar-textured surface with mixed wettability, in which the tops of square pillars are partially subjected to wettability modification. Numerical investigation of the boiling heat transfer performance on the improved mixed-wettability surface is carried out using a three-dimensional thermal multiphase lattice Boltzmann model. The numerical results show that the width of the wettability-modified region plays an important role in the boiling performance of the improved mixed-wettability surface and the best boiling performance is achieved in the situation that the width of the wettability-modified region is sufficiently large but the bubble nucleated on the pillar top still does not interfere with the coalescence-departure mechanism of the bubbles nucleated around the pillar, which optimizes the joint effects of surface structure and mixed wettability for enhancing nucleate boiling. The influences of the shape of the wettability-modified region are also studied. Among the investigated shapes, the square is found to perform better than the other two shapes.
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