The performance of a solar collector with wax as a phase change material (PCM) located in a set of staggered pipes configuration was simulated computationally in this work. For the solar radiation of Chile, the accumulation of heat in the PCM system and the heat release at different time intervals were analyzed during the process of energy capture in summer: (a) without wax and with airflow, (b) with wax and without airflow, and (c) with wax and with airflow. Furthermore, the effects of solar radiation (summer and winter) airflow in the collector were analyzed on the performance of the system. The simulation results show that the use of a PCM in a solar air heater allows to store greater amounts of energy and it helps to extend the period of time when the air coming out of the collector has an elevated temperature. By increasing the airflow rate, the efficiency of the system increases and also the energy released to the air to be absorbed.

A numerical study of the combustion of lean methane/air mixtures in a porous media burner is performed using novelty geometry, cylindrical annular space. The combustion process takes place in the porous space located between two pipes, which are filled with alumina beads of 5.6 mm diameter forming a porosity of 0.4. The outer tube diameter of 3.82 cm is isolated; meanwhile the inner tube of 2 cm in diameter is covered by a continuous set of thermoelectric elements (TE) for transforming heat energy into electricity. To achieve and maintain the proper temperature gradient on TE, convective heat losses are considered from the TE. Computer simulations focus on the two-dimensional (2D) temperature analysis and displacement dynamics of the combustion front inside the reactor, depending on the values of the filtration velocity (0.1 to 1.0 m/s), the heat loss coefficient from the internal cylinder (400–1500 W/m²/K), and the fuel equivalence ratio (0.06– 0.5). The conditions that maximized the overall performance of the process of energy conversion are: 0.7 m/s of the filtration velocity, 0.363 of the fuel equivalence ratio and 1500 W/(m^2·K) of the heat transfer coefficient from the internal cylinder, to obtain 2.05 V electrical potential, 21 W of electrical power, and 5.64% of the overall process efficiency. The study shows that the cylindrical annular geometry can be used for converting the energy of combustion from lean gas mixtures into electricity, with a performance similar to the specified by manufacturers of thermoelectric elements (TE).

In recent years, the fast growth of computational power has allowed the application of computational fluid dynamics (CFD) in a wide range of areas of interest, such as gas–solid unit operations. In this context, the multiphase particle-in-cell (MP-PIC) method appears as an option to represent fluid–particle and particle–particle interactions, avoiding the complexity of tracking each particle and the high computational cost derived from this. The MP-PIC method can represent the particles as a group with the same characteristics, allowing the simulation of gas–solid systems at different scales. To achieve this, the particle–particle interactions are simplified using the solid stress tensor to represent them; this does not require explicit expressions. This approach has a low computational cost, allowing the simulation of industrial cases using just workstations. This paper provides a review of the literature on the solid stress tensor and its commercial and non-commercial applications, including its historical and mathematical development in the description of particle–particle interactions. In addition, to consolidate the knowledge and advancing understanding in this crucial aspect of multiphase flow simulations, this review identifies the current challenges and opportunities for future research in multiphase systems based on the solid stress tensor. In addition, this review identifies the current challenges and opportunities for future research in multiphase systems based on the solid stress tensor.

A system for converting combustion heat into electric power is proposed on the basis of gas combustion in a thermoelectric porous element. A lean gas mixture is introduced into the porous medium, where the gas flow rate is changed at a regular interval of time and, as a result, the combustion front is confined within a specific region of the porous medium. A non-steady state temperature distribution was established along the flow direction, resulting in a steep and stable temperature gradient in the thermoelectric porous element. Numerical simulation showed that the total thermal efficiency increases when a leaner mixture is fed to the combustor, besides when the combustion wave propagation velocity increases the global thermal efficiency grows reaching values almost equal to the conversion efficiency of the thermoelectric element itself. Consequently, using the proposed system, the chemical energy can be effectively converted into electric power.

An experimental study on combustion in porous media and thermoelectric generation was performed. The reactor was composed of two types of porous media where flame stabilization was reached at the interface of them. An external thermoelectric module was placed to harvest the thermal energy produced in the system. Maximum values of voltage and current obtained were 503 mV and 150 mA respectively