This special issue of *Energies* focuses on numerical heat transfer and fluid flow, featuring a collection of papers that address various aspects of these fields. The issue is edited by Artur Bartosik from Kielce University of Technology, Poland, and includes contributions from researchers around the world. The papers cover a wide range of topics, including: 1. **Hydraulic Transients in Viscoelastic Pipeline Systems**: Kubrak et al. investigate the effects of sudden cross-section changes in high-density polyethylene (HDPE) pipeline systems on pressure oscillations during water hammer events. They use numerical and experimental methods to validate their models. 2. ** Effects of Bent Outlet on Fluidic Oscillators**: Kim and Kim study the impact of bent outlets on the characteristics of fluidic oscillators with and without external flow. Their findings highlight the sensitivity of flow control to the pitch angle of the fluidic oscillators. 3. **Natural Convection of Van Der Waals Gases**: Avramenko et al. present an integral method for natural convection of van der Waals gases over a vertical plate, considering the effects of van der Waals parameters on heat transfer. 4. **Numerical Modelling of Heat Transfer in Fine Dispersive Slurry Flow**: Bartosik proposes a mathematical model for heat transfer in turbulent flow of fine-dispersive slurry, including a wall damping function. 5. **Heat Pipe Heat Exchangers**: Górecki et al. conduct experimental and numerical studies on heat pipe heat exchangers with individually finned heat pipes, demonstrating their efficiency in small air conditioning systems. 6. **Heat Transfer Analysis for Non-Contacting Mechanical Face Seals**: Blasiak uses a variable-order derivative approach to analyze heat transfer in non-contact mechanical face seals, particularly during startup. 7. **Modeling Transient Pipe Flow in Plastic Pipes**: Urbanowicz et al. develop a modified discrete bubble cavitation model to simulate pressure and velocity waveforms in plastic pipes, including cavitation and retarded strain effects. 8. **Optimization of Air-Cooling Systems for Electric Vehicle Battery Packs**: Widyantara et al. optimize the design of air-cooling systems for electric vehicle battery packs to meet optimal operating temperatures, using numerical models and CFD simulations. 9. **Enhancement of Heat Transfer in Oil-Cooled Motors**: Zhao et al. study the influence of vortex generator arrangement on heat transfer enhancement in oil-cooled motors, finding that a 45° attack angle provides the best results. 10. **Hydrostatic Transmission Control**: Bury et al. analyze the impact of control signals on the performance of hydrostatic transmissions, optimizing the start-up process to reduce energy consumption and noise. These papers collectively contribute to the advancement of numerical heat transfer and fluid flow, providing valuable insights and practical solutions for various engineering applications.This special issue of *Energies* focuses on numerical heat transfer and fluid flow, featuring a collection of papers that address various aspects of these fields. The issue is edited by Artur Bartosik from Kielce University of Technology, Poland, and includes contributions from researchers around the world. The papers cover a wide range of topics, including: 1. **Hydraulic Transients in Viscoelastic Pipeline Systems**: Kubrak et al. investigate the effects of sudden cross-section changes in high-density polyethylene (HDPE) pipeline systems on pressure oscillations during water hammer events. They use numerical and experimental methods to validate their models. 2. ** Effects of Bent Outlet on Fluidic Oscillators**: Kim and Kim study the impact of bent outlets on the characteristics of fluidic oscillators with and without external flow. Their findings highlight the sensitivity of flow control to the pitch angle of the fluidic oscillators. 3. **Natural Convection of Van Der Waals Gases**: Avramenko et al. present an integral method for natural convection of van der Waals gases over a vertical plate, considering the effects of van der Waals parameters on heat transfer. 4. **Numerical Modelling of Heat Transfer in Fine Dispersive Slurry Flow**: Bartosik proposes a mathematical model for heat transfer in turbulent flow of fine-dispersive slurry, including a wall damping function. 5. **Heat Pipe Heat Exchangers**: Górecki et al. conduct experimental and numerical studies on heat pipe heat exchangers with individually finned heat pipes, demonstrating their efficiency in small air conditioning systems. 6. **Heat Transfer Analysis for Non-Contacting Mechanical Face Seals**: Blasiak uses a variable-order derivative approach to analyze heat transfer in non-contact mechanical face seals, particularly during startup. 7. **Modeling Transient Pipe Flow in Plastic Pipes**: Urbanowicz et al. develop a modified discrete bubble cavitation model to simulate pressure and velocity waveforms in plastic pipes, including cavitation and retarded strain effects. 8. **Optimization of Air-Cooling Systems for Electric Vehicle Battery Packs**: Widyantara et al. optimize the design of air-cooling systems for electric vehicle battery packs to meet optimal operating temperatures, using numerical models and CFD simulations. 9. **Enhancement of Heat Transfer in Oil-Cooled Motors**: Zhao et al. study the influence of vortex generator arrangement on heat transfer enhancement in oil-cooled motors, finding that a 45° attack angle provides the best results. 10. **Hydrostatic Transmission Control**: Bury et al. analyze the impact of control signals on the performance of hydrostatic transmissions, optimizing the start-up process to reduce energy consumption and noise. These papers collectively contribute to the advancement of numerical heat transfer and fluid flow, providing valuable insights and practical solutions for various engineering applications.