## 2 Special Issue Findings
It was found that a MgO-DW nanofluid with good dispersion stability, high thermal conductivity, and low viscosity could be generated by controlling the ultrasonic time and/or volume concentration. * 12. The authors of the twelfth article in this Special Issue ([PERSON]; [PERSON]; [PERSON]; [PERSON]; and [PERSON], from the Technical University of Darmstadt, Germany, and Kermanshah University of Technology, Iran) provided a review article on the possible application of nanofluids in CO\({}_{2}\) absorption [17]. Fossil fuel conversion (e.g., combustion or gasification) and industrial processes are now the main sources of CO\({}_{2}\) emissions, along with forestry and land use, including livestock. Reducing greenhouse gas emissions is, therefore, essential to mitigate climate change and its devastating environmental consequences. Carbon capture and storage (CCS) or carbon dioxide utilisation (CCU) technologies are one of the most environmentally friendly options for reducing anthropogenic greenhouse gas emissions. However, these technologies are currently associated with significant efficiency losses, resulting in higher CO\({}_{2}\) avoidance costs. Owing to the advent of nanotechnology and its unique advantages in various fields, a new approach was introduced using suspended particles in a base fluid (suspension) to achieve maximum absorption efficiency and minimum energy consumption through CO\({}_{2}\) capture. In this review article, the performance of nanofluids, preparation methods, and their stability, which is one of the most important factors to prevent sedimentation of nanofluids, were discussed. The objective of this article was to present the factors contributing to CO\({}_{2}\) absorption by nanofluids in an easy-to-understand manner, focusing on the role of base fluids and the reasons for their selection. Based on research in the literature, the authors claimed that nanofluids are an effective way to increase CO\({}_{2}\) absorption compared with the base fluid, which can reduce host plant energy consumption. * 13. The thirteenth article in this Special Issue focused on evaluating the thermal performance of a tubular heat exchanger equipped with combined basket-twisted tape inserts. Here, the authors, [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON]; and [PERSON], from the Technical University of Darmstadt, Germany, and the University of Technology, Iraq, experimentally investigated the properties of tubular heat exchangers [18]. For this purpose, a test rig was built, equipped with novel inserts, and finally operated under different conditions. Air was used as the working medium, and a constant wall heat flux was provided for turbulent discharge conditions (6000 \(\leq\) Re \(\leq\) 19,500). Two inserts were used to improve heat transfer: First, bascule turbulators were inserted into the heat exchanger with a constant pitch ratio (PR = 150 mm), and second, basket turbulators together with a twisted tape were installed in the core of the basket turbulators. The results showed that the Nusselt numbers were 131.8%, 169.5%, 187.7%, and 206.5% higher than those of a plain heat exchanger for basket turbulators and the combined basket-twistedtape inserts with y/w = 6, 3, and 2, respectively. On average, the maximum thermal efficiency of the elevated tubular heat exchanger was 1.63 times higher than that of the simple heat exchanger, which is due to a binary basket-shaped band for a twist ratio y/w equal to 2 at constant pump power. Finally, practical correlations for the Nusselt number and friction characteristics were developed and presented. * The fourteenth article in this Special Issue dealt with the microstructural analysis and mechanical characteristics of hybrid nanocomposites of aluminium (Al), ferric oxide (Fe\({}_{2}\)O\({}_{3}\)), and silver (Ag). The aluminium alloys can be used in many fields such as electronics, aerospace, and automotive due to their properties such as lightness, strength, wear, and corrosion resistance.