Effects of surface morphologies on boiling heat transfer in droplet impingement on superheated surfaces

A phase change spray cooling system is an important engineering application of droplet impingement on superheated surfaces. This work studies the impacts of different morphologies on boiling heat transfer during droplet impingement on superheated surfaces using a three-dimensional hybrid approach: the multiphase pseudopotential lattice Boltzmann method for multiphase flows and the finite difference method for heat transfer. Simulations are conducted by varying boiling number (Bo) from 0.0023 to 0.0460 at an initial Reynolds number of 100 for four morphologies: single-concave, rectangular-groove, wavy-groove, and multi-concave. For single-concave morphology, the ratio of concave diameter to droplet diameter (Dc/d) is examined with values of 1.0, 2.0, 3.0, and 3.3. In the other morphologies, cross sections are evaluated with two widths: 0.333d and 0.667d, with identical depths. The results show that the thermal performance of the single-concave morphology is mainly affected by Dc/d. The curved geometry gives the single-concave morphology superiority in boiling heat transfer compared to other morphologies studied in the range 0.0023<Bo<0.0389 at Dc/d=2.0. The curved surface controls the bounce of droplets at high Bo, allowing them to deposit smoothly with a large exposed contact area, and achieve an efficient cooling effect. However, for 0.0389≤Bo, superiority in boiling heat transfer is achieved by the multi-concave morphology, where full film boiling does not occur. The thermal performance of other morphologies is primarily influenced by the cross-sectional width. At a width of 0.667d, the wavy-groove morphology provides comparable performance to the multi-concave morphology within 0.0023<Bo<0.0184, while the multi-concave morphology achieves higher boiling heat transfer at 0.0184≤Bo. Conversely, a smaller width of 0.333d significantly reduces heat transfer. This occurs because the rapid surface isolation hinders droplet access to the heated surface base. Furthermore, the rectangular-groove morphology provides the worst thermal performance due to the restrictions against penetration and smooth deposition over the superheated surface. Thermal and hydrodynamic analysis discovers the significance of the single-concave morphology in enhancing the boiling heat transfer in spray cooling systems.