Computational Fluid Dynamics Simulation of Lattice Structures Toward Computational Design of Heat Sink Structures Applicable to Additive Manufacturing

Thermal management has become an important issue in automobiles and electronic devices. Heat sinks dissipate heat generated by electric devices, resulting in extending their life. Conventional manufacturing processes of heat sinks, including extrusions, forging, and machining have limited the manufacturable shapes to fin or pin. Recently, additive manufacturing (AM) has expanded the freedom of manufacturable shapes and enabled complex architected materials including lattice structures. Lattice structures are composed of periodically arranged unit cells and have a large surface area compared to fin or pin heat sinks. Therefore, more efficient heat transfer will be expected by using lattice structures as heat sinks instead of fin or pin structures.<br/>Heat sinks are used under natural or forced convection. When heat sinks are used under forced convection, fluid is supplied to the heat sinks by using a fan or rotating the heat sinks themselves. A small pressure loss generated by the heat sinks should be preferable to minimize the energy required to supply the fluid. In addition, heat transfer characteristics are affected not only by surface area but also by the flow around the solids, which varies with the structure of heat sinks. A good balance between low-pressure loss and high heat transfer rate is practically important for heat sinks. In order to overcome current limitations, it is required to identify structural features dominating pressure loss and heat transfer in lattice structures. In this study, the heat transfer characteristics and pressure loss of various lattice structures and conventional heat sinks (fin and pin) were investigated numerically using computational fluid dynamics (CFD) to clarify dominant structural features for heat transfer and pressure loss.<br/>The structures used in this study were BCC, Kelvin, Cubic, Lotus, fin, and pin with a fixed solid volume fraction of 20%. The number of unit cells was varied in the range of 1³–3³. A finite element analysis software (Femtet) was used for the CFD calculations. The analytical model consists of fluid (air), base, and heat sink structures and has an overall dimension of 30 × 150 × 30 mm³. The dimensions of the base and heat sink structures are 30 × 30 × 10 mm³ and 30 × 30 × 30 mm³, respectively. The thermal conductivity of the base and heat sink structures was set at 110 W･m^-1･K^-1, assuming laser powder bed fused AlSi10Mg alloy. Flow velocity of 1.0–4.0 m･s^-1 was applied to the fluid inlet surface, whereas natural outflow was set at the fluid outlet surface. A slip wall with zero flow velocity only in the normal direction was set at the other surfaces of the model. A heat of 4.5 W was applied to the bottom of the base, and the temperature rise of the base and pressure loss at steady state were calculated.<br/>In the balance between low-temperature rise and low-pressure loss, Kelvin and Lotus were superior under low-pressure loss, and BCC was superior under high-pressure loss. Under a constant flow velocity, BCC and Kelvin exhibited a low-temperature rise of the base and high-pressure loss, whereas Lotus and fin exhibited high-temperature rise and low-pressure loss. Based on Newton’s cooling law and the Darcy-Weisbach equation, surface area and hydraulic diameter are important for the low-temperature rise and low-pressure loss, respectively. However, even when the heat transfer coefficient and Darcy friction factor were evaluated to exclude the effects of surface area and hydraulic diameter, the same tendencies as the temperature rise and pressure loss were retained. To clarify the dominant structural features, local heat flux and pressure were analyzed. Heat transfer was most active at the inlet of the lattice structures, while the rear parts did not contribute significantly to heat transfer. In addition, pressure significantly dropped at flow paths with locally narrow areas or greatly varied areas. These results could guide the design of suitable structures for heat sinks.