Experimental Validation of CFD Predictions for Internal Duct Airflow and Thermal Behaviour in a High-Speed Train Supply Duct Prototype: A Preliminary Stage Towards Carbody Integrated HVAC Testing

Unlike many previous high-speed train Heating, Ventilation, and Air Conditioning (HVAC) studies that combined numerical and experimental analyses within the cabin, this study evaluates the supply duct independently, offering a more targeted assessment of internal duct airflow and thermal performance. The research comprised two numerical stages and an experimental validation. The first stage evaluated four ducting geometry variations and identified the ducting system arrangement of 20 supply diffusers with a 45° air grille blocking angle as optimal for the Kereta Cepat Merah Putih (KCMP) case, keeping air velocity within the thermal comfort range for all simulated passengers. This geometry was then manufactured as a full-scale prototype and assessed in a second numerical stage against experimental measurements. The air velocity yielded a mean absolute deviation (MAD) of 0.57 m/s and a relative mean deviation (RMD) of 34.22%, while the air temperature showed an MAD of 2.13 °C and an RMD of 8.16% at duct planes, and 0.80 °C and 3.06% at duct outlets. These results demonstrate that the proposed approach of testing the supply duct independently of the cabin is able to capture the general internal duct airflow trends, even though, particularly for the temperature parameters, a large temperature discrepancy occurs at downstream locations traced to three factors: uncontrolled ambient thermal conditions in a large-scale indoor facility, simplified thermal boundary conditions in the simulation, and additional uncertainty from the non-standardised temperature measurement method. Nonetheless, it offers a promising preliminary reference for HVAC system development, prior to integrated carbody testing, although further method refinements are still required to strengthen the reliability of this approach.