We applied CFD to a UV disinfection reactor to understand the combined effects of convective heat transfer, radiation and multiphase behaviour on dose delivery and hydraulic efficiency. The model incorporated realistic hydraulic boundaries, lamp arrangements and internal baffles to predict flow patterns, residence time distribution and UV intensity fields. Study objectives were mapped to measurable metrics such as dose uniformity, risk of short circuiting and pressure loss to make design trade offs explicit. We investigated changes to lamp placement and baffle geometry and quantified their impact on iso‑dose distribution and head loss at target flow rates. Verification steps and sensitivity checks improved confidence in relative performance between concepts. Results were communicated through annotated iso‑dose maps, streamlines and summary charts that link physical intuition with KPIs engineers use for reactor selection. Recommendations prioritised geometry refinements that improved disinfection uniformity with minimal penalty to pressure drop, together with an implementation plan for prototyping and validation. The work reduced the risk of downstream testing and provided a reproducible basis for future operational updates.