In recent years, safety concerns surrounding electric vehicles have intensified due to incidents involving battery thermal runaway (TR). Computational modelling has emerged as an effective tool for studying vented TR events. However, few studies have examined the behaviour of multiphase flow and heat transfer in vented ejecta within realistic confined battery module or pack environments. This study presents a numerical model that has been developed and validated to investigate the multiphase dynamics of vented ejecta in a confined configuration. The model captures both the gas and particle phases, and its accuracy is verified using independently published experimental data. The results show that the ejecta undergoes a two-stage process involving an initial rapid spreading phase that heats the internal void space, followed by a stable cooling phase. Solid particles have been found to have a significant impact on ejecta dispersion, temperature distribution and gas mixing. Key particle characteristics, including residence time and dispersion distance, strongly affect the thermal response. Sensitivity analyses were conducted on particle size, mass loading, the specific heat-to-density ratio (Cp/ρ), restitution coefficient and wall stick probability. The findings indicate that larger particles, a lower Cp/ρ ratio, greater wall stick probability and smaller restitution coefficients are factors that reduce thermal and gas-related hazards. Conversely, smaller particle mass loading results in fewer thermal hazards but more gas hazards. This study advances the understanding of ejecta behaviour during TR and provides guidance for the design of safer battery systems and the development of future safety standards.