Sung Jin Kim

The lattice Boltzmann method (LBM) is utilized to numerically investigate the permeability and effective thermal conductivity of simple cubic and body-centered truss structures. The key objective of this paper is to analyze how different geometric parameters affect the macroscopic properties of these truss structures that are increasingly used in advanced engineering applications due to their unique thermal-fluid characteristics. Simple cubic and body-centered cubic (SC-BCC) lattice structures are modeled, and the simulations are performed to determine their permeability and effective thermal conductivity. The findings highlight that the rod diameter and simple-cubic diameter significantly influence porosity, permeability, and thermal conductivity. Larger rod diameters generally result in higher porosity and permeability but may reduce thermal conductivity. Conversely, smaller simple-cubic diameters tend to enhance thermal conductivity. These results can optimize the design and application of truss structures in heat exchangers, cooling systems, and other areas where efficient thermal management and fluid flow are critical. The study concludes that LBM is an effective tool for predicting the thermal-fluid behavior of complex porous structures, providing valuable insights for the engineering design of advanced materials.

In topology optimization, the optimal layout of the structure is obtained through an iterative update of the geometry. Since this update procedure utilizes the gradient of the objective function, it is important to select an appropriate objective function in order to obtain a meaningful geometry. From this point of view, the mathematical definition and physical meaning of the objective functions for topology optimization of heat sinks are reviewed in this article. For this, studies from conductive heat transfer problems to recent computational fluid dynamics (CFD)-based conjugate heat transfer problems are covered. In most of the studies, the objective functions are defined in order to minimize the thermal resistance of the heat sink or to minimize the maximum temperature of the heat source. In place of thermal resistance, thermal compliance is commonly used as the objective function. However, it is worth noting that minimization of the thermal compliance may lead to considerable deviation of the design from that based on the minimum thermal resistance. This is because the thermal compliance is linearly proportional to the average temperature of the heat source in the computational domain, and the average temperature of the heat source is usually different from the maximum temperature of the heat source. In heat conduction problems, the difference between the maximum temperature and the average temperature becomes large when the thermal conductivity of the solid material becomes low. In convection problems, the difference becomes large when the streamwise fluid temperature variation becomes large due to the low heat capacity rate of the fluid.

Since their introduction in the early 1990s, pulsating heat pipes (PHPs) have received a lot of attention due to their obvious advantages such as their geometrical simplicity, and their potential for high-heat flux applications even without power consumption. Although numerous investigators have studied PHPs over the last three decades, there still exist a few controversial issues on fundamental characteristics and several technical problems in practical applications. To put the finishing touches to the controversial issues and to shed light on the technical problems, recent advances in PHPs are critically reviewed in this paper. The results of this critical review are classified into two categories: (i) fundamental aspects of PHPs and (ii) practical aspects of PHPs. The first category focuses on reviewing the current state-of-the-art fundamental characteristics of PHPs. The second category summarizes the technical problems that are resolved for utilizing PHPs in practical applications. This review paper would help researchers or engineers who are working on or utilizing PHPs.