Numerical Heat Transfer and Fluid Flow 2021 is a special issue published in Energies, edited by Artur Bartosik. The issue includes 23 articles that explore various aspects of numerical heat transfer and fluid flow, such as hydraulic transients in viscoelastic pipelines, effects of bent outlets on fluid oscillators, natural convection of Van Der Waals gases, heat transfer in slurry flows, and heat exchanger designs. The articles present both numerical simulations and experimental studies, covering topics like heat transfer enhancement, fluid dynamics, and energy systems. The issue highlights the importance of accurate modeling and simulation in engineering applications, emphasizing the role of computational fluid dynamics and experimental validation. The articles discuss challenges in heat transfer and fluid flow, including turbulence, cavitation, and the impact of material properties on system performance. The special issue also addresses practical applications in energy systems, such as heat recovery, cooling systems, and fluid transport. The contributions provide insights into improving efficiency and reliability in various engineering fields, demonstrating the significance of numerical methods and experimental research in advancing heat transfer and fluid flow technologies.Numerical Heat Transfer and Fluid Flow 2021 is a special issue published in Energies, edited by Artur Bartosik. The issue includes 23 articles that explore various aspects of numerical heat transfer and fluid flow, such as hydraulic transients in viscoelastic pipelines, effects of bent outlets on fluid oscillators, natural convection of Van Der Waals gases, heat transfer in slurry flows, and heat exchanger designs. The articles present both numerical simulations and experimental studies, covering topics like heat transfer enhancement, fluid dynamics, and energy systems. The issue highlights the importance of accurate modeling and simulation in engineering applications, emphasizing the role of computational fluid dynamics and experimental validation. The articles discuss challenges in heat transfer and fluid flow, including turbulence, cavitation, and the impact of material properties on system performance. The special issue also addresses practical applications in energy systems, such as heat recovery, cooling systems, and fluid transport. The contributions provide insights into improving efficiency and reliability in various engineering fields, demonstrating the significance of numerical methods and experimental research in advancing heat transfer and fluid flow technologies.